tribology of elastomeric seal materials991521/... · 2016-10-19 · tribology of elastomeric seal...

DOCTORA L T H E S I S

Department of Applied Physics and Mechanical EngineeringDivision of Machine Elements

Tribology of elastomeric seal materials

Mohammadreza Mofidi

ISSN: 1402-1544 ISBN 978-91-86233-26-6

Luleå University of Technology 2009

Moham

madreza M

ofidi T

ribology of elastomeric seal m

aterials

ISSN: 1402-1544 ISBN 978-91-86233-XX-X Se i listan och fyll i siffror där kryssen är

Luleå University of Technology

Tribology of elastomeric seal materials

Mohammadreza Mofidi

Luleå University of TechnologyDepartment of Applied Physics and Mechanical Engineering

Division of Machine Elements

Tryck: Universitetstryckeriet, Luleå

ISSN: 1402-1544 ISBN 978-91-86233-26-6

Luleå

www.ltu.se

http://www.ltu.se

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ iii

Preface

The work presented in this thesis has been carried out at the Division of Machine Elements, Department of Applied Physics and Mechanical Engineering at Luleå University of Technology (LTU) in Luleå, Sweden.

I would like to express my deep gratitude to my supervisors, Professor Braham Prakash and Professor Elisabet Kassfeldt, for their wholehearted support and guidance throughout this work. I have learnt a lot from the various courses I attended at this university and would like to thank all my teachers, especially Professor Braham Prakash and Professor Roland Larsson.

My sincere thanks also go to my previous teachers at BSc and MSc levels especially, Professor Mansour Nikkhah Bahrami at the University of Tehran, Dr. Mohammad Reza Ghazavi and Professor Gholamhosein Liaghat at Tarbiat Modarres University.

I wish to thank the “Ministry of Science, Research and Technology of IRAN” for awarding me the scholarship to pursue research at Luleå University of Technology. This work would not have been possible without this support and I am really grateful to the Government of Iran for this.

All my colleagues at the Division of Machine Elements, especially Dr. Marika Torbacke, Gregory F. Simmons, Jens Hardell and Donald McCarthy have been very helpful whenever I had any difficulty and I sincerely acknowledge their support.

A special thanks to my wife, Sedigheh and my son Aref for their support and patience. My sincere gratitude goes to my parents-in-law for their affection and kindness who passed away during the last three years. I would like to extend my appreciation to all my brothers and sisters for their profound kindness.

Finally, I am deeply indebted to my parents, Parviz and Soghra, and feel a tremendous sense of appreciation for their genuine support, care, encouragement, patience and eternal dedication.

Luleå, April 2009

Mohammadreza Mofidi

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ iv

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ v

Abstract

Elastomers are the most commonly used materials for various sealing applications owing to their low modulus of elasticity, large elongation-at-break, and high Poisson’s ratio. Most seals operate in the presence of lubricants, therefore the sealing elastomer-oil interaction plays an important role in determining the tribological performance of elastomers. Furthermore, at times, such as during start-up periods, the seals may also operate under dry conditions and the seal material can be affected by high friction coefficient and wear.

In this work, the tribological behaviour of different sealing elastomers has been studied. The influence of the ageing of a sealing elastomer in different lubricants on its tribological behaviour has been investigated. Further studies pertaining to the influence of lubrication on the abrasive wear of a sealing elastomer have also been carried out.

The results show that the friction coefficient of an elastomer in lubricated sliding against a hard counterface, at low contact pressure depends on the surface topography of the elastomer but at high contact pressure, it is mainly a result of the viscoelastic deformation of the rubber by the counterface surface asperities. Even if the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness, which may make the dominant contribution to rubber friction. Ageing of the nitrile rubber in ester base fluids leads to a reduction of the friction coefficient.

In unidirectional dry sliding of an elastomer against a counterface, the friction coefficient decreases during the running-in period. The longest running-in periods have been observed when the elastomers slide against relatively smooth surfaces.

Depending on the elastomer-lubricant compatibility, abrasive coarseness, geometry of sliding contact area and contact pressure, the two-body abrasive wear of elastomers may increase or decrease in the presence of lubricants.

Ageing of nitrile rubber in lubricating fluids increases the abrasive wear both in dry and lubricated conditions. The abrasive wear of nitrile rubber in ester base fluids and rapeseed oil is higher than that in the mineral oils.

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ vi

Appended papers

A. G. F. Simmons. M. Mofidi. B. Prakash. Friction evaluation of elastomers in lubricated contact: a comparison of different test methodologies. Submitted for publication.

B. M. Mofidi, B. Prakash, Influence of counterface topography on sliding friction and wear of some elastomers under dry sliding conditions, Proc. Inst. Mech. Eng. Part J.-J. Eng. Tribol. 222(5) (2008) 667-673.

C. M. Mofidi, E. Kassfeldt, B. Prakash, Tribological behaviour of an elastomer aged in different oils, Tribol. Int. 41 (2008) 860-866.

D. M. Mofidi, B. Prakash, B. N. J. Persson, O. Albohr, Rubber friction on (apparently) smooth lubricated surfaces, J. Phys.-Condes. Matter 20 (2008) 085223.

E. M. Mofidi, B. Prakash, The influence of lubrication on two-body abrasive wear of sealing elastomers under reciprocating sliding conditions. Submitted for publication (A part of this paper was presented at NordTrib 08, Tampere, June 2008 and published in the conference proceedings).

F. M. Mofidi, B. Prakash, Two body abrasive wear and frictional characteristics of sealing elastomers under unidirectional lubricated sliding conditions. Submitted for publication.

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ vii

List of symbols

Variable Meaning Units of Measurement

E( ) Complex dynamic modulus Pa

E1 The real part of E( ) Pa

E2 The imaginary part of E( ) Pa

Oscillation frequency s-1

Poisson’s ratio -

wavelength of the surface roughness m

q The wavevector corresponding to the wavelength = 2 /q m-1

q0 The low-wavevector cut-off to the wavelength 0 m-1

q1 The large-wavevector cut-off to the wavelength 1 m-1

C(q) The power spectrum of the surface roughness 1/m

A The contact area observed at the highest magnification M2

A0 The nominal or apparent contact area M2

p Mean normal pressure Pa

v Sliding velocity ms-1

Dynamic viscosity Pa s

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ viii

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ ix

Table of contents

Preface .............................................................................................................................................. iii Abstract ...............................................................................................................................................v Appended papers .............................................................................................................................. vi List of symbols ................................................................................................................................. vii Table of contents .............................................................................................................................. ix 1. Introduction................................................................................................................................... 1 2. Seals and sealing........................................................................................................................... 2

2.1. Seal Classification............................................................................................................................ 2 2.1.1. Static & semi-static seals ....................................................................................................... 2 2.1.2. Rotary Seals........................................................................................................................... 3 2.1.3. Reciprocating seals ................................................................................................................ 4

2.2. Elastomeric seal materials .............................................................................................................. 4 2.3. Failure of elastomeric seals ............................................................................................................. 7

3. Tribology of elastomers................................................................................................................. 8 3.1. Oil- elastomer interaction ............................................................................................................... 8 3.2. Friction ........................................................................................................................................... 10 3.3. Wear 12

4. Objectives and Limitations.......................................................................................................... 17 4.1. Objectives of the research............................................................................................................. 17 4.2. Limitations ..................................................................................................................................... 17

5. Experiments................................................................................................................................. 18 5.1. Lubricated sliding friction ............................................................................................................ 18

5.1.1. High frequency short stroke reciprocating machine (Optimol SRV)................................... 18 5.1.2. Low frequency, long-stroke reciprocating test rig (Cameron-Plint) .................................... 18 5.1.3. Low frequency, long stroke with O-rings (Cameron-Plint) ................................................. 19

5.2. Dry sliding friction ........................................................................................................................ 20 5.3. Two body abrasive wear ............................................................................................................... 21

5.3.1. Two body abrasive wear in reciprocating sliding ................................................................ 21 5.3.2. Two body abrasive wear in unidirectional sliding............................................................... 22

6. Summary of important results .................................................................................................... 23 6.1. Friction evaluation of elastomers in lubricated contact (Paper A)............................................ 23 6.2. Friction and wear behaviour of selected sealing elastomers under dry sliding conditions

(Paper B) ........................................................................................................................................ 24 6.3. Influence on tribological behaviour from ageing of an elastomer in different oils (Paper C) 25 6.4. Rubber friction on (apparently) smooth lubricated surfaces (Paper D)................................... 26 6.5. The influence of lubrication on two-body abrasive wear of selected sealing elastomers in

reciprocating sliding (Paper E)..................................................................................................... 29 6.6. The influence of lubrication on two-body abrasive wear of selected sealing elastomers in

unidirectional sliding (Paper F).................................................................................................... 31

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ x

7. Conclusions ................................................................................................................................. 34 8. Suggestions for future work ....................................................................................................... 35 9. References ................................................................................................................................... 36 Paper A ............................................................................................................................................ 41 Paper B ............................................................................................................................................ 57 Paper C ............................................................................................................................................ 67 Paper D ............................................................................................................................................ 77 Paper E ............................................................................................................................................ 87 Paper F .......................................................................................................................................... 101

Tribology Of Elastomeric Seal Materials ____________________________________________________________________________ 1

1. Introduction

A seal is a component which prevents the leakage of fluid or gas from a machine and prevents contamination from entering the machine. Elastomers have quite unique properties which enable them to function reliably as seal materials. Many seals (dynamic seals) slide against a sealing surface during their operation and have to be optimized to reduce leakage, friction and wear. Friction of a seal sliding against a sealing surface has to be minimised to increase the overall efficiency of machines and simultaneously the thickness of lubricant film has to be reduced to minimise the leakage. Leakage can be avoided by surface patterning on the elastomeric seal (which may be formed during the production or operation) but in some seals, such as reciprocating seals, the leakage can be prevented by increasing the contact pressure and decreasing the lubrication film thickness. In such situations there exists a high risk of insufficient lubrication and direct contact between the elastomeric seal and sealing surface. This risk may be further aggravated by several other factors such as side loads, vibrations and inadequate surface finish.

Seals may fail through different mechanisms resulting in leakage or contamination entering the lubricant. The most important types of seal failure are abrasion, thermal degradation, chemical degradation, compression set, plasma degradation, over compression, extrusion and extraction. Further, high seal friction impairs the efficiency of machines. High friction also results in an increase in temperature and accelerates the failure of a seal through different mechanisms such as thermal degradation, chemical degradation, abrasion and so on. Most seals operate in lubricated conditions but may also occasionally operate in dry conditions or with insufficient lubrication such as during running-in periods. Thus, understanding the frictional behaviour and wear mechanisms of sealing elastomers is important in determining their performance and service life. While the frictional behaviour of elastomers has been investigated extensively, most of the previous studies pertain to their behaviour under either dry conditions or in the full film and elastohydrodynamic lubrication regimes. The tribological behaviour of sealing elastomers in boundary or mixed lubrication has been investigated scantily. An enhanced understanding of the tribological behaviours of elastomers can potentially help a designer to choose the best material with suitable properties for a specified application.


2. Seals and sealing

Sealing is the control of fluid interchange between two regions sharing a common boundary. Some structural or design issues or tolerance considerations may necessitate a relatively large gap between two surfaces which cannot therefore perform the sealing function autonomously. Such gaps can be reduced to small dimensions by introducing “seals” as additional components [1].

2.1. Seal ClassificationSeals can be subdivided into static and dynamic seals. Static seals provide sealing function between surfaces which do not move relative to each other while dynamic seals provide sealing function between surfaces in relative motion. Dynamic seals can be subdivided into rotary and reciprocating seals [2, 3]. A general classification of seals is shown in Figure 1. Tribological aspects are significant in dynamic seals owing to their sliding against sealing surfaces.

Figure 1: Seals classification [3]

2.1.1. Static & semi-static seals

Static seals are used where there is no relative motion between the mating surfaces being sealed. In semi-static seals some motion is possible through the elastic deflections of the seal. Figure 2 shows an O-ring as a static seal. As pressure is applied to the seal, the flexibility of the elastomer material and its resistance to volume change, transfers the pressure to the sealing surface. O-rings are also used in reciprocating seals and occasionally in rotary seals. The same mechanism is prevalent when an O-ring is used in reciprocating seal applications [4].


Figure 2: The sealing action of an elastomeric O-ring: (a) with no applied pressure; (b) when pressure is applied [4].

Given that in static seals and semi-static seals there exists no relative motion between the sealing surfaces, the tribological issues are not critical.

2.1.2. Rotary Seals

A rotary seal provides sealing between a rotating shaft and an outer surface, such as a groove or housing bore.

Rotary lip seals are the most commonly used dynamic seals. The sealing surface is lubricated by a very thin layer of the sealed fluid. At very low speed and during the running-in period, mixed lubrication occurs [3].

Figure 3: Schematic of a rotary lip seal (left) and the mechanism of reverse pumping in rotary lip seals (right) [3].

Figure 3 shows the region in the vicinity of the sealing zone. A thin liquid film separates the lip from the shaft surface. This film is of the order of 1 μm in thickness and 0.05 to 1 mm in length [3]. A mechanism which is called “reverse pumping” prevents leakage from the oil side to the air side. Several reasons have been identified to explain the reverse pumping. The most important reason is the effect of shear deformation. As shown in Figure 3, when the shaft rotates, shear stresses in the film cause the asperities to deform into vane-like shapes which pump the fluid from the air side of the seal to the oil side [3-7]. Extensive modelling and numerical calculations have been done by Salant and his co-workers on the lubrication of rotary lip seals [e. g. 8-15].

Other types of rotary seals that are available include labyrinth seals, viscoseals, mechanical face seals and grooved seals.


2.1.3. Reciprocating seals

A reciprocating seal provides sealing in relative reciprocating motion along the shaft axis between the inner and outer elements (Figure 4). Reciprocating seals are sub-divided into rod seals, piston seals and wipers.

Figure 4: Schematic figure of typical reciprocating seals

The behaviour of rod seals and piston seals are similar to that presented in Figure 2 for static seals. Significant effort has been made to develop the applied materials, technology, shape and geometry, accuracy and reliability of reciprocating seals to improve their operating characteristics, such as reducing leakage and frictional losses, reducing the required housing volume, increasing the seal life and so on. Since O-rings and other elastomeric reciprocating seals with symmetrical cross-sections usually have poor performance, V-rings, U-rings or a combination of several seals are used in many applications. Some reciprocating seals are reinforced with harder materials such as metals or hard polymers to improve their performance [4, 16]. When a U-ring is used instead of a V-ring, the required housing volume decreases and the efficiency increases. This change in the value of the variables is taken a stage further when a compact composite seal is used [16].

The friction of a reciprocating seal is very high at the start of its operation and depends significantly on the preceding down-time [5]. When the seal slides against the sealing surface from the oil-side to the dry-side (in-stroke), the film thickness is very thin and the friction is high but when the seal is moving from the dry-side to the oil-side (outstroke), the film thickness is much higher and friction is lower. The leakage of a reciprocating seal is proportional to the film thickness occurring mainly during the outstroke when a full film builds up in the contact [17, 18].

2.2. Elastomeric seal materials Depending on the application, metals, plastomers, elastomers and composite materials can be used as seal materials. However elastomers are the most popular seal materials in general application.

Elastomers are a class of polymeric materials that possess the quality of elasticity, i.e., the ability to regain shape after deformation. Elastomer comes from two terms, “elasto” which indicates the ability of a material to return to its original shape and “mer” which comes from polymer. A polymer is a substance comprising of repeating structural units, or monomers, connected by covalent chemical bonds. Elastomers refer to all the polymeric materials with high elasticity including crosslinked rubbers. However, a


distinction is made between raw rubber and crosslinked rubber. The former is completely deformable in a plastic-like manner, particularly at high temperatures, because it does not have a rigid network structure. In contrast, the crosslinked rubber does not have a plastic transition zone due to its networks which restrain the movement of macro-molecular chain molecules [19].

Elastomers show a phenomenon of time-dependent strain, called viscoelasticity. It exhibits both viscous and elastic characteristics when undergoing deformation. Viscous materials resist against shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantly when a stress is applied and return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, exhibit time dependent strain. The deformation of an amorphous material does not involve atomic displacements on specific crystallographic planes, as is the case in crystalline metals [20]. Figure 5 shows a schematic drawing of elastomer chains in stretched and unstretched situations.

Figure 5: A schematic drawing of stretched and unstretched elastomer chains. The dots stand for cross-links

When a material is loaded by a small oscillatory strain and the resulting stress is measured, purely elastic materials have stress and strain in phase, such that the response of one caused by the other is immediate. In purely viscous materials, strain lags stress by a 90 degree phase lag. Viscoelastic materials exhibit behaviour somewhere in the middle of these two types of material (some lag in strain). The complex dynamic modulus E= E1+iE2 can be used to represent the relationship between the oscillating stress and strain. The real (E1 = Re E) and the imaginary part (E2 = Im E) of E( ) as well as the loss tangent E2 /E1 are shown in Figure 6.


Figure 6: (a) The viscoelastic modulus E( ) = E1 + iE2 of a typical rubber-like material, and (b) the loss tangent E2/E1 (schematic). [21]

At low frequencies, the material is in the rubbery region where Re E( ) is relatively small and approximately constant. At very high frequencies (glassy region) the material is elastically very stiff and Re E is again nearly constant but much larger (typically by three to four orders of magnitude) than in the rubbery region. In the intermediate frequency range (transition region) the loss tangent is very large. An increase in the temperature shifts the viscoelastic spectrum to higher frequencies [21].

Besides the main polymer backbone, numerous chemicals, fillers and additives are used in an elastomeric product. Mastication and peptizers facilitate providing a homogeneous dispersion of the compounding ingredients. Vulcanization is the conversion of rubber molecules into a network by the formation of crosslinks. Vulcanization agents are necessary for crosslink formation. The most common vulcanization agents are sulfur and peroxides. Ageing protectors are used to increase the elastomer resistance to heat, oxidation, ozone cracking, fatigue ageing and so on. Fillers are particles added to an elastomer to lower the consumption of the polymer backbone and consequently the cost of the product and/or to improve some properties of the elastomer. The most common fillers are carbon black, silica, kaolin clay, mica, etc. Organic and inorganic pigments, which are insoluble in the elastomer, are used to color elastomeric compounds. Plasticizers, which are usually mineral oils or esters, are used to improve the filler dispersion in the compound, improve the flow of the compound during processing and consequently conserve energy and improve some properties of the elastomer such as, elongation, lowering of the glass transition temperature and so on. Blowing agents are used in the production of some porous elastomers such as sponge. In some elastomeric products, a rubber may be used as a coating for metal. In some other cases, wires or other metal shapes may be used to reinforce the elastomeric product. In such cases, a


strong bond between the elastomer and metal is needed. Bonding agents provide good adhesion between the elastomer and metal in such products. Other ingredients of elastomeric products are odour improving agents, antimicrobials, surface modifiers and so on [19, 22].

Some of the advantages of elastomers as seal material are as follows:

They have low modulus of elasticity; they can be heavily deformed without giving high contact stresses;

They can easily be stretched to fit into housing and piston grooves;

They are resilient and thus able to follow irregularities and vibration of the sealed surface;

They have a high Poisson's ratio (close to 0.5), therefore the material behaves in a manner similar to a liquid under pressure, transferring any applied pressure hydrostatically, enabling an elastomeric seal to create its own sealing force automatically in proportion to the pressure.

Elastomers are reasonably inexpensive; even expensive special elastomer seals can give a low cost for the total seal system in comparison with the seals which are designed and produced from other materials.

Elastomeric seals also have the following disadvantages:

They can have friction characteristics which are not always predictable;

Their chemical and temperature resistance is poor compared with many other engineering materials;

Elastomers under pressure readily extrude into even quite fine clearances, owing to the high Poisson's ratio and low modulus of elasticity [14, 17].

2.3. Failure of elastomeric seals Many factors usually combine to cause seal failure. The most common factors are: design, size, elastomer-oil interaction, abrasion, environmental conditions, installation, and loading conditions.

Many of these failure types are related to tribological issues. Frictional heating may increase the temperature and accelerate the chemical and/or thermal degradation of elastomeric seals. The high temperature may also decrease the hardness of the elastomeric seal which may result in other types of failure such as spiral failure, extrusion, and explosive decomposition. A non-uniform distribution of friction may cause an O-ring to be twisted resulting in spiral failure. The non-uniform friction may be due to eccentric components, wide clearance combined with side loads, uneven surface finish or inadequate lubrication [4, 23-26].


3. Tribology of elastomers

Tribology is the science and technology of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Many elastomeric components, such as seals, car tires, windscreen wipers, belts… interact with other surfaces in relative motions and in view of the distinctive properties of elastomers, specialised studies pertaining to their tribological characteristics are an important task in science and technology.

3.1. Oil - elastomer interaction Oil-elastomer interaction and the heat resistance of elastomers have a very significant influence on the performance, life and reliability of sealing systems. When an elastomer and oil are brought into contact with each other, the elastomer material may absorb the oil or the oil may extract soluble constituents of the elastomer. The oil may also react with the elastomer [27, 28]. A solubility parameter (Hildebrand parameter), generally denoted by , is defined to estimate the degree of interaction between materials, particularly for non-polar materials such as many polymers. This parameter is defined as the square root of the internal energy of vaporisation divided by the molar volume, referred to as the cohesive energy density. Materials with similar values of are likely to be miscible [29]. The presence of polar side-groups in the backbone chain of an elastomer increases the oil resistance of the polymer [30]. Crosslinking also limits the degree of polymer swelling by providing tie points (constraints) that limit the amount of solvent that can be absorbed into the polymer [30].

Elastomers may show progressive change in their physical properties due to exposure to heat. Three types of changes have been observed: additional crosslinking resulting in higher crosslink density and an increase in hardness, chain scission leading to reduction in chain length and average molecular weight and consequently softening of the elastomer, and chemical alternation of the polymer chain by formation of polar or other groups [31]. Figure 7 shows the oil and heat resistance of different elastomers [30].

Figure 7: Oil resistance (%swell in ASTM oil #3) [30]


Nitrile rubber (NBR) is a copolymer of acrylonitrile and butadiene and provides a low-cost elastomer with good mechanical properties in sealing applications. The concentration of acrylonitrile in the copolymer has a considerable influence on the polarity and swell resistance of the vulcanizate in non-polar solvents. The greater the acrylonitrile content, the lower the amount of the swell in motor fuels, oils, fats, etc [19]. Carbon black is the major filler for NBR compounds because of the properties’ improvement that it imparts to the compound. These include tensile and tear strength, abrasion resistance, chemical resistance, resilience, low compression set and good processing properties. The main types of non-black fillers used with nitrile rubber are silica, silicate, clay, talc, and calcium carbonate, barium sulphate, titanium dioxide, aluminium trioxide, antimony trioxide, magnesium hydroxide, zinc oxide. Because of the polarity of nitrile rubber, polar plasticizers need to be used with this elastomer. Highly aromatic mineral oils can be used in limited quantities with NBR having acrylonitrile (ACN) content under 28% as a means of reducing cost. As the ACN level of the nitrile rubber increases, its polarity increases and consequently it is less compatible with plasticizer. Therefore only smaller amounts can be incorporated without bleeding or exuding to the surface of the vulcanizate. Usually two or even three types of ester plasticizers are used in an NBR compound to ensure their compatibility. More polar plasticizers are recommended to be used with NBR with a higher level of ACN [19, 32].

Hydrogenated Nitrile rubber (HNBR) is produced by the catalytic hydrogenation of nitrile rubber (NBR). The HNBR materials are much more resistant to oxidation and sulphur attack at higher temperatures in comparison with NBRs and provide the flexibility and toughness of NBRs with improved temperature and chemical resistance. HNBR and NBR can be compounded with many of the same plasticizers and softeners. Carbon black is also the major filler for HNBR compounds. The plasticizer used with HNBR should not be so volatile that it detracts from the overall heat resistance of the compound [22, 32].

Acrylic rubber (ACM) is a type of synthetic rubber containing acrylonitrile. It is a copolymer of two major components: the backbone (95-99%) and the reactive cure site (1-5%). The outstanding property of ACM rubber is its resistance to hot oil. It is more heat resistant than NBR. Its resistance to weather, ozone and natural ageing is also higher than NBR but it has less resistance to wear and oil swelling [19]. The fillers used in ACM should be neutral or basic in order to avoid an interference with the basic vulcanization reaction. Active blacks and silicates are used as fillers in ACM to improve its mechanical properties. Silica in combination with Al-silicates or silane-treated clays is recommended as well. Softeners are normally not used in ACM compounds but some low volatility plasticizers may be used to improve its low temperature flexibility [19].

Fluoroelastomers are typically used in harsh environments where other elastomers fail. Chemical resistance and heat resistance are the two main attributes that make fluoroelastomers attractive for sealing applications. FKM is the designation for a large family of fluoroelastomers containing vinylidene fluoride as a monomer. Fluoroelastomers are more expensive than acrylic rubber or nitrile rubber. Non-reinforcing blacks and mineral fillers are used to obtain good processibility, desired hardness and to reduce the compound cost. Conventional plasticizers, such as ester plasticizers, are not compatible with FKM. Special compounds (particularly low molecular weight polymers) can be used to improve the processibility of fluoroelastomers [19].


3.2. FrictionThe coefficient of friction of a rubber surface against a hard counterface can be expressed in terms of the contribution of adhesion, deformation (hysteresis), viscous and cohesion (tearing) components [33, 34]. Adhesion (Figure 8) is generally recognized to consist of the making and breaking of junctions at a molecular level [33, 35]. Hysteretic friction (Figure 8) is a consequence of energy loss associated with internal damping within the viscoelastic body [33, 36]. The cohesive component of friction is the contribution of wear to the bulk losses and the viscous component is the viscous drag under wet conditions [34]. Most texts have considered only two terms for friction components since the deformation component can represent both the hysteresis and tearing component whereas the viscous component of friction can be a subset of the adhesion component [33]. Recent studies show that the independency of the adhesion and deformation components of friction is only a simplified assumption. It has been assumed that the adhesive force per unit area should be constant during any deformation while the surface free energy is a function of both internal energy and entropy, and so it should change if the internal energy and/or entropy change due to any bulk deformation [37].

Figure 8: Adhesion and hysteresis components of elastomeric friction [33]

The contribution of adhesion and hysteresis friction depends on the temperature, sliding velocity, geometry and cleanliness of the mating surfaces [38, 39]. The adhesion component is significant when a rubber is sliding on very clean, dry and smooth counterfaces [39 - 41]. It can also be significant at low loads, even in lubricated conditions [42], because of the significance of the attractive Van der Waals’ forces in temporary bonds between the surfaces in comparison to the normal load [43].

The frictional force of rubber sliding at various velocities and temperatures on a given surface can be expressed by a single master curve and the glass transition temperature of the material [44]. This transform agrees closely with the William-Landel-Ferry (W.L.F.) transform [45] and thus shows that the friction is viscoelastic in nature.

Previous studies on rubber friction have mostly focused on the hysteretic component which is the most significant component in many real applications. A theory which describes the energy dissipation in rubber sliding on a hard counterface has been developed by Persson [46]. The hysteresis friction of rubber sliding against a hard counterface depends on the ratio of the amplitude to wavelength of the surface roughness. If this ratio is constant, the different surface roughness (of varying length scales) may contribute equally to the friction force. However the shortest wavelength which can contribute to the friction force may be limited by some parameters such as surface contamination [46].

P v

FHYST.

v

FADH.


When rubber slides against a hard, rough surface with roughness on the length scales , it will be exposed to fluctuating forces with frequencies ~ / . Since a real surface has a wide distribution of length scales, a correspondingly wide distribution of frequency components in the Fourier decomposition of the surface roughness acts on the sliding rubber block. The contribution of surface roughness with the length scale to the friction coefficient will be at a maximum when / 1/ , where 1/ is the frequency where Im E( )/|E( )| is at a maximum. This point is located in the transition region between the rubbery region (low frequencies) and the glassy region (high frequencies) [46]. The contribution of different length scales of surface roughness has been shown schematically in Figure 9.

Figure 9: Rubber sliding on a hard corrugated substrate. The magnitude of the contribution to the friction from the internal damping in the rubber is the same in (a) and (b) because the ratio between the amplitude and the wavelength of the corrugation is the same. (c) shows the ( ) curves for the roughness profiles in (a) and (b) schematically [46].

As the rubber slides against a hard counterface, frictional heating results in an increase in the temperature and consequently the viscoelastic spectrum shifts to higher frequencies. Since in most applications the perturbing frequencies are mostly below 1 (where Im E( )/|E( )| is at a maximum), the increase in the temperature (due to the frictional heating) results in a decrease in friction coefficient [21].

When a soft rubber slides against a hard track, or a hard slider slides against a soft rubber track, the relative motion between the two frictional members is often due to ‘waves of detachment’ crossing the contact area at high speed from front to rear. These waves, which move much faster than the two bodies in sliding [47], are known as ‘Schallamach waves’ named after the researcher who first described them. The Schallamach waves appear at a critical sliding speed whose value depends on the adhesive properties of the interface, the geometrical characteristics of the contact, elastic properties of the rubber-like material, normal load and temperature [48, 49]. Figure 10 shows the Schallamach waves generated on the surface of a rubber by a hard, spherical slider.


Figure 10: Schallamach waves on the surface of a rubber generated by a hard sphere at a sliding speed of 0.43 mm/s (8 frames at 1/32 s intervals) [47]

Lubrication decreases the real contact area between the rubber and hard counterface resulting in a decrease in friction coefficient. The presence of fluid between the rubber and hard substrate not only reduces the adhesion but also the hysteretic component of friction. On a lubricated substrate the valleys turn into fluid pools which are sealed off and effectively smoothen the substrate surface (Figure 11). This smoothening reduces the viscoelastic deformation caused by the surface asperities and thus reduces rubber friction [50, 51].

Rubber

Hard counterface

Fluid

Figure 11: Smoothing the substrate in presence of lubricant [50]

3.3. WearThree different mechanisms of wear can be identified when an elastomer slides against a hard substrate. During sliding against a hard countersurface with a sharp texture, abrasive wear takes place as a result of tearing of the elastomer sliding surface. Fatigue wear is another mechanism of wear which occurs on the surface of an elastomer sliding against blunt projections on a hard substrate. When a highly elastic elastomer slides against a smooth surface, roll formation occurs. In this type of wear the high frictional force shears a projection on the rubber surface, tears and then rolls the tongue along the direction of sliding [32]. A critical value of shear stress can be defined for each rubber such that if the shear stress is higher than the critical shear stress, roll formation occurs. For shear stresses lower than the critical value, wear is mainly due to fatigue. Thus the friction coefficient is one of the most important properties of rubber governing the type


of wear [52]. Figure 12 shows a schematic diagram of the friction and wear mechanisms in elastomers.

Figure 12: Schematic diagram of the friction and wear mechanisms in elastomers [32, 53].

In practice, a combination of three forms of wear occurs and it is difficult to separate the contribution of each mechanism to the overall wear [32].

When rubber is abraded with no change in sliding direction, sets of parallel ridges are often found on the surface of the samples at right angles to the direction of motion. These have been called “abrasion patterns” [54]. Their intensity increases with increasing coarseness of the track and with decreasing stiffness of the compound. Figure 13 shows some typical abraded surfaces of natural rubbers. The surfaces of elastomers abraded by fatigue wear exhibit pitting marks while the surfaces of harder elastomers, sliding against sharp asperities, exhibit scratches parallel to the direction of sliding [55]. The scratches parallel to the sliding direction occur on the surface of elastomers sliding in point contacts with sharp asperities. Figure 14 shows a typical abraded surface of hydrogenated nitrile rubber characterised by scratches parallel to the sliding direction.


Figure 13: Abrasion patterns on two different carbon black-filled vulcanizates of natural rubber, (a) and (c) Hard rubber, (b) and (d) Soft rubber, (a) and (b) Fine abrasive track, (c) and (d) Coarse abrasive track [54]

Figure 14: Scratches on abraded surface of filled HNBR vulcanizate at 25 ºC [54].

The elastomer surface is pulled in the direction of sliding and fails in tension behind the contact perpendicular to the tensile stress field [56] - see Figure 15(a). The formation of ridges starts by initiation of cracks at the rear of the contact region due to the high shearing stress and continues by expansion of the cracks under repetitive loading [57, 58] - see Figure 15(b).


Figure 15: Mechanism of scratch and ridge formation on the sliding surface of an elastomer; (a) scratch formation [56, 57], (b) ridge formation [57, 58].

Grosch and Schallamach found that on sharp tracks, such as abrasive paper, linear wear rate as a result of tensile failure was proportional to the ratio between frictional energy dissipation and energy density at break [59]. Abrasion of rubber surfaces in line contact has been investigated extensively, but most of the previous studies were focused on the dry abrasion of rubber [e.g. 58-71]. Southern and Thomas studied abrasion of rubber surfaces by a razor blade in line contact and formulated a theory which describes the correlation between the wear rate and frictional force as well as the crack growth characteristics of the rubber. They also mentioned that the pattern spacing depends on the abrading force and test temperature [58]. Zhang and Yang have introduced a theoretical wear equation of rubber abrasion in a line contact from the viewpoint of energy on the basis of experimental results [60, 61].

Another classification introduces the wear of elastomers as a result of two processes: local mechanical rupture (tearing) and decomposition of the molecular network to a low molecular weight (smearing) [63]. The oily decomposition product which forms during smearing protects the underlying rubber from tearing and thus decreases the rate of wear [64]. Experiments show that the rate of wear during smearing decreases through the introduction of antioxidants [64, 72].

Polymers are soluble in many organic fluids and there can be a synergistic effect between an aggressive solvent and the polymer resulting in significant wear. If the solvent can penetrate the surface of the polymer it will have a detrimental effect on the polymer’s behaviour. The rapid wear which results is believed to occur as a result of aggravated cracking of the solvent weakened polymer during contact with the counterface [73-75]. This is schematically illustrated in Figure 16. The wear rate is believed to reach a maximum when the solubility parameter of the polymer and the solvent are the same [74, 75].


Penetration and softening of polymer surface by solvent

Aggravated cracking and wear in softened layer

Figure 16: Synergism between wear of polymer and damage by a solvent [73].

Muhr et al. have studied the influence of lubrication on the abrasion of rubber [70, 71 and 76]. They observed that when a lubricant is applied in the abrasion of rubber by a blade in line contact, a much finer pattern develops and the rate of abrasion is much lower but the horizontal force on the blade does not decrease as dramatically [70, 71]. However, when a blunt abrader slides against a rubber surface, the horizontal force decreases significantly in the presence of a lubricant [71, 76]. Chandrasekaran and Batchelor have studied the friction and wear of butyl rubber sliding on abrasive paper as a function of temperature and load. They conducted dry and lubricated unidirectional sliding tests and reported that the presence of lubricant reduced the coefficient of friction but accelerated wear due to chemical degradation of the rubber [77].

Solvent

Polymer

Sliding

Counterface


4. Objectives and Limitations

Many dynamic seals are ideally expected to work in the full film or elastohydrodynamic lubrication regimes [1, 2, and 3]. In view of this, extensive studies have been conducted on the lubrication mechanisms of different seals [e.g. 3 -19]. However, the lubrication may occasionally fail in some seals [1, 3] which can result in many other problems such as high friction, heating, severe wear and so on. Starved or inadequate lubrication may occur during start up of the machine or at unexpectedly high pressures. Full film or elastohydrodynamic lubrication does not occur in some seals, at least during periods of their operation (e.g. the end of the in-stroke for reciprocating seals). In such situations, the friction increases significantly and the high friction affects the overall efficiency of the machine. Thus, understanding the frictional behaviour of sealing elastomers is an important issue in seal design.

Abrasive wear is a common type of seal failure. It may come from contamination and hard particles which are embedded in the sealing surfaces. It can also be caused by rough sealing surfaces (which are roughened by corrosion, erosion or damage sustained as a result of poor installation procedures) [1, 2]. Abrasive wear can become more critical at higher temperatures and in interaction with some lubricating fluids.

4.1. Objectives of the research The purpose of this research is to study and develop knowledge pertaining to the tribological behaviour of sealing elastomers, especially in lubricated conditions. The specific objectives of this research are:

To investigate the influence on lubricated frictional behaviour of different lubricants and the influence of ageing of sealing elastomers in these lubricants.

To investigate the mechanisms of abrasive wear together with the effects of lubrication and the influence on their abrasive wear of ageing sealing elastomers in lubricants.

To study the tribological behaviour of elastomers sliding against sealing surfaces in dry conditions during run-in periods.

4.2. LimitationsAlthough the elastomeric compounds are normally referred to by the name of the base polymer, this does not fully define the material. The details of compounding and processing affect the material properties significantly but these details are generally not revealed by manufacturers.


5. Experiments

To carry out research in accomplishing the objectives mentioned in Section 4.1, several items of test equipments were used. Two different test rigs have been used to perform two-body abrasive wear experiments and three test rigs have been used to conduct experiments on frictional behaviour of elastomers in dry and lubricated conditions. Other equipments used in these studies include a 3D optical profilometer (to measure the surface roughness), a high resolution semi-microelectronic weighing balance (to quantify wear), an optical microscope and a universal tensile testing machine.

5.1. Lubricated sliding friction An evaluation of reciprocating testing machines, including high frequency short-stroke and low frequency long-stroke friction and wear machines has been conducted to assess their usefulness. Several specimen configurations have been used and their deficiencies discussed with the aim of helping a laboratory experimenter to overcome many of the pitfalls associated with testing of elastomers in lubricated conditions. The lubricated frictional behaviour of selected elastomers and the influence of ageing an elastomer in different lubricants on its frictional behaviour have also been studied.

5.1.1. High frequency short stroke reciprocating machine (Optimol SRV)

An Optimol SRV machine has been used to measure the friction coefficient of elastomeric discs against a steel cylinder in lubricated reciprocating sliding conditions. The machine uses reciprocating upper cylindrical specimen loaded against a stationary lower specimen. The sliding direction of the cylinder is along its principal axis. The friction force is measured by a pair of piezoelectric force sensors. Temperature, normal force, frequency of motion and stroke length can be controlled during the tests. The diameter of the cylinder is 15 mm and its length 22 mm. The edges of the cylindrical slider are chamfered/ rounded off with an aim to minimise the edge effect. The test configuration is shown in Figure 17.

Figure 17: High frequency short stroke reciprocating machine

5.1.2. Low frequency, long-stroke reciprocating test rig (Cameron-Plint)

A series of experiments were conducted on a low frequency, long-stroke test rig. As with the Optimol SRV machine, the friction force is measured by using a piezoelectric sensor. Unlike the high frequency rig, there are no significant geometrical constraints in the low frequency test rig and sample holders can be designed to suit almost any


reciprocating contact. Considering this adaptability, several sample geometries have been used in these experiments. These are shown in Figure 18.

Figure 18: Low frequency, long-stroke reciprocating test rig

5.1.3. Low frequency, long stroke with O-rings (Cameron-Plint)

A series of experiments were conducted using the Cameron-Plint test machine and commercially available O-ring specimens. In these experiments, O-ring specimens were placed in an oscillating contact against a steel plate perpendicular to the O-ring axis. Lubricant was then applied liberally to the steel plate. The test geometry is shown in Figure 19.


Figure 19: Low frequency, long stroke with O-rings

5.2. Dry sliding friction The friction and wear behaviour of some sealing elastomers in dry sliding conditions have been studied using a Micro-Tribometer UMT-2 in block-on-ring configuration (Figure 20). In these experiments, a rubber specimen, attached to a metal backing plate, was pressed against a rotating ring. The normal and frictional forces were measured by using strain gauge force sensors. Three sets of bearing steel rings with different surface roughness values were used with an aim to study the effect of surface roughness on friction and wear. The surface topographies of the rings are shown in Figure 21. Each test was run for a duration of 12 hours (43200 sec). The rubber specimens’ dimensions were 16 mm × 4 mm × 2 mm (the width of the contact area was 4 mm). The counterface bearing steel rings were Ø35mm and 8 mm thick. The rubber specimens were washed in industrial petroleum using an ultrasonic cleaner, dried in an oven at 40 ºC and then weighed. The same procedure was repeated after running the test for each specimen in order to quantify wear. Each ring was washed in industrial petroleum for 3 minutes using an ultrasonic cleaner and dried before the test and used only in one test. All the tests have been performed at room temperature (22 ± 2 ºC).

Elastomer block (16 mm×4 mm×2 mm)

Steel ring Ø35 mm

Load Backing support plate

Figure 20: Micro -Tribometer UMT-2


Smooth surface (0.15<Ra<0.3 μm)

Medium surface (0.35<Ra<0.55 μm)

Rough surface (0.5<Ra<0.7 μm)

Figure 21: Surface topographies of steel rings

5.3. Two body abrasive wear Two different machines were used to study the two-body abrasive wear of elastomers in reciprocating and unidirectional sliding. The machines have some advantages and disadvantages. The reciprocating abrasive tester provides a relatively long sliding distance with fresh abrasives sliding against the elastomer surface. The long sliding distance gives the opportunity to have stable conditions, good reproducibility (even at low normal loads), and good distinction between different elastomers and lubricants could be attained. However, this machine does not measure the frictional force. In reciprocating sliding, the tearing mechanisms of the elastomer’s surface are more complicated and investigations into the contributing wear mechanisms from analysis of the worn surfaces is difficult compared to that in unidirectional sliding.

5.3.1. Two body abrasive wear in reciprocating sliding

The two-body abrasive wear of some sealing elastomers in dry and lubricated conditions has been studied using an abrasive wear tester (Figure 22). The influence on abrasive wear from ageing an elastomer in different lubricating fluids has also been investigated.

This test equipment consists of a reciprocating platform on which the sample is mounted and held against an abrasive paper wrapped around the circumferential surface of a wheel (Ø50 mm × 12 mm thick). The wheel is turned through a small angle at the end of each stroke so as to enable rubbing of the elastomer against a fresh abrasive surface. All abrasive wear tests have been performed at room temperature (22 ± 2 ºC). In lubricated tests, the oil was injected into the counterface by using a syringe. The rubber test specimens were washed in industrial petroleum by an ultrasonic cleaner, dried in an oven and then weighed. The same procedure was repeated after running the test for each specimen to quantify the abrasive wear.


Figure 22: Abrasive wear tester

5.3.2. Two body abrasive wear in unidirectional sliding

Two-body abrasive wear of selected elastomers was studied using the Micro-Tribometer UMT-2 in block-on-ring configuration (Figure 23). The rubber specimen, attached to a metal backing plate, was pressed against an abrasive tape glued to a rotating steel ring and the normal and frictional forces were recorded by piezoelectric sensors. The rubber specimens’ dimensions were 16mm × 6mm × 2mm (the width of contact was 6 mm). The steel rings were Ø 60 mm and 10 mm thick. The rubber specimens were washed in industrial petroleum using an ultrasonic cleaner, dried in an oven and then weighed. The same procedure was repeated after running the test for each specimen to quantify the abrasive wear. All tests were performed at room temperature (22 ± 2 ºC). In lubricated tests, the oil was injected into the interface by using a syringe. The worn rubber surfaces were examined using an optical microscope. Test parameters are shown in Table 1.

Figure 23: The test configuration for abrasive wear tests

Table 1: Test parameters

Test parameters

Abrasive grit number, # (Nominal particle size, μm)

Speed mm/s

Load N

Contact pressure, KPa

Level 1 500 ( 18) 3.1 10 379 Level 2 320 ( 34) 31.4 20 536


6. Summary of important results

In regard to the objectives of this research (Section 4.1.), Papers A, C and D discuss the frictional behaviours of elastomers in lubricated conditions. In Paper B the frictional behaviours of some selected sealing elastomers in the dry condition has been studied. In Papers E and F, the influence of lubrication on the abrasive wear of some sealing elastomers has been studied in reciprocating and unidirectional sliding respectively.

6.1. Friction evaluation of elastomers in lubricated contact (Paper A) In paper A, the friction testing of elastomers in lubricated contacts is discussed with focus on developing suitable experimental methods that can produce useful results. The experiments were performed using two reciprocating friction test machines with high and low frequency (SRV Optimol and Cameron-Plint, respectively). Several test configurations were utilized with the Cameron-Plint machine to study the practical issues unique to friction testing of elastomers in lubricated conditions including: contact mechanics, material response to loading, contact edges, oil absorption, cleaning and specimen geometry in an effort to find suitable solutions. The frictional behaviour of some specified sealing elastomers and lubricants has also been studied.

Some problems with the tribological testing of elastomers, such as thermal degradation and effects of edges, have been discussed. Typical tested samples with thermal degradation and edge effects are shown in Figures 24 and 25 respectively.

Figure 24: Typical thermal effects for materials in high frequency tests after 30 minutes at 26°C

Figure 25: A typical worn surface of a sample (FKM) tested in cylinder on flat (perpendicular sliding) configuration, Normal load= 100 N, Oscillation frequency= 5 Hz, Stroke length= 1.4 mm


Of the testing arrangements discussed, the low frequency test rig using O-ring material proved to produce the most repeatable, worthwhile results and testing using the curve on curve geometry had good potential to be a suitable test given future refining. While these results allowed for selection of materials in specific applications, they were not able to provide significant deeper understanding of elastomers and their behaviour in lubricated contacts. However, the development of these tests does highlight the greater need in elastomer testing to eliminate unknown parameters and take into account material behaviour, sealing effects, edge effects and thermal degradation than in similar standardized tests with non-viscoelastic materials. Finally a comparison of elastomers tested using environmentally adapted lubricants demonstrated that the performance of the synthetic and mineral based lubricants were comparable.

6.2. Friction and wear behaviour of selected sealing elastomers under dry sliding conditions (Paper B)

In Paper B, the friction and wear behaviour of four sealing elastomers including acrylonitrile butadiene rubber (NBR), hydrogenated acrylonitrile butadiene rubber (HNBR), acrylate rubber (ACM) and fluoroelastomer (FKM) have been studied in dry, unidirectional sliding conditions using a block-on-ring configuration with the Micro-Tribometer UMT-2 (Section 5.2.). Table 2 shows the tested elastomers and some of their properties. The experiments were conducted at three levels of sliding velocity and two different contact pressures using three sets of steel ring with different surface roughness.

Table 2: Tested elastomers and their properties

Elastomeric materials Hardness (Shore A)

Tensile strength (MPa)

Elongation at break (%)

Density (g/cm3)

Nitrile rubber , (NBR) 76.1 25.4 466 1.31 Hydrogenated nitrile rubber, (HNBR) 71.3 17.5 303 1.24

Acrylate rubber, (ACM) 73.4 7.8 171 1.49 Fluoro rubber, (FKM) 72.8 - - 2.03

The results show that at low contact pressure, the friction coefficients drop during running-in periods to steady-state values. The longest running-in periods were observed during sliding against smooth surfaces. Apart from results for FKM, the steady-state friction coefficient is seen to increase as the surface roughness decreases. The surface roughness has the least and greatest effect on the steady-state friction coefficient of ACM and HNBR respectively. At low contact pressure, no severe wear occurred but at high contact pressure, the FKM was severely worn through roll formation. Although roll formation occurred on the surface of HNBR, the amount and sizes of the wear particles were much smaller than those for FKM (Figure 26). Consequently, the worn mass of HNBR was much lower than that of FKM. The surface of ACM was also worn and fine wear particles were produced on the sliding surface but the surface of NBR was torn locally and the worn mass was low. Overall, the results have shown that under dry sliding and other operating conditions for load and speed, the ACM elastomer displays superior frictional characteristics, whereas the NBR is characterised by superior wear performance vis-a-vis the other elastomeric materials (Figure 27). More investigation in lubricated conditions and considering the oil-elastomer compatibility, lubrication


regime, and operating environment are needed to choose the proper elastomeric seal material.

Figure 26: Wear particles of the tested rubbers (normal load: 10 N, speed: 10 r/min, duration: 12 h, surface roughness of rings, Ra: 0.35–0.55 )

Figure 27: Worn mass of elastomers (normal load: 10 N, speed: 10 r/min, duration: 12 h, surface roughness Ra: 0.35–0.55 m)

6.3. Influence on tribological behaviour from ageing of an elastomer in different oils (Paper C)

In Paper C, the influence on frictional behaviour and two-body abrasive wear of lubrication of an elastomer with several different lubricants as well as the ageing of the same elastomer in these lubricants has been studied. The friction and two-body abrasive wear experiments were performed using an SRV Optimol test rig (Section 5.1.1.) and abrasive wear tester (Section 5.3.1), respectively. The elastomer which has been studied is acrylonitrile butadiene rubber (NBR) with an acrylonitrile content of 28% vulcanized by sulphur. The surface of the elastomer was examined in a Wyko 3D optical surface profilometer. The elastomer surface was characterised by parallel grooves (Figure 28) caused by the moulding of elastomeric sheets in a steel mould.

The lubricating fluids used in these experiments included several mineral oils, synthetic fluids and a rapeseed oil without any additives.

Ageing of the elastomer samples was carried out by immersing the fresh samples in the oils at 120 °C for one week (168 hours). All friction tests have been conducted in two directions, parallel and perpendicular to the direction of grooves on the elastomeric samples.


Figure 28: Surface topography of the fresh elastomer sample (Ra=0.344 m, Rq=0.438 m)

The results revealed that ageing the nitrile rubber in the synthetic ester base fluids leads to a reduction of friction coefficient. This effect, especially in perpendicular sliding to the initial lay on the surface, is more considerable for the sample aged in polyol ester. The presence of the base fluids increases the abrasive wear of the tested nitrile rubber. Ageing the nitrile rubber in the lubricating fluids increases the abrasive wear in both dry and lubricated conditions.

6.4. Rubber friction on (apparently) smooth lubricated surfaces (Paper D)

In Paper D, the lubricated friction of rubber on an apparently smooth surface was investigated with the experimental results compared to theoretical calculations. The elastomer studied is acrylonitrile butadiene rubber (NBR) with an acrylonitrile content of 28% vulcanized by sulphur. The lubricating fluids used in these experiments include several mineral oils, synthetic fluids and a rapeseed oil without any additives. Further lubricants containing friction modifiers were also used. Friction experiments were performed using an SRV Optimol (Section 5.1.1.). The surface height was measured over different surface areas using atomic force microscopy and an optical method (Wyko NT 1100, 3D optical surface profilometer in vertical scanning interferometry mode). The viscoelastic modulus E ( ) was measured (using Eplexor 150) for a rectangular rubber block measuring 5 × 2 × 30 mm (Figure 29).

Figure 29: The logarithm of (a) the real part and (b) the imaginary part of the viscoelastic modulus of the elastomer (NBR) as a function of the logarithm of the frequency for the temperatures T = 50 and 80º C.


The experimental results show that at low contact pressure, friction coefficient is affected by the surface texture of the elastomers. This is due to the hydrodynamic effects [78]. However, at high contact pressure, the hydrodynamic effects are not significant (Figure 30). The drop in friction for large load is most likely due to the increase in temperature caused by the frictional heating.

Results show that the friction coefficient is not significantly affected by the friction modifiers in the lubricants (Figure 31). This is due to the negligible contribution of the adhesive component in comparison with the large contribution of the hysteresis component of friction.

Figure 30: Friction coefficient as a function of load at the background temperature T = 25 C.

Figure 31: Friction coefficients for one base fluid with different additive packages, T = 40 and 80 C. Normal load FN = 100 N.

Using the surface roughness measurements, the power spectrum is calculated by:

xdehhqC i 2.)()()( xq0x (6-4-1)


Where <…> stands for ensemble averaging. Here h(x) is the surface height at the point x, where we have assumed <h(x)> = 0. The longest and the shortest wavelength roughness included in the analysis is 0 = 2 /q0 0.3 mm and 1 = 2 /q1 6 nm. Figure 32 shows the power spectrum of the surface roughness of the steel surface. In the calculation of friction, a linear approximation (green line) was used.

Figure 32: The power spectrum of the surface roughness of the steel surface.

Neglecting the flash temperature and using the linear approximation of power spectrum of the surface roughness (green line in Figure 32), the hysteresis component of friction coefficient is calculated by:

2

0 23

)1()cos(Imcos)()(

21 *

1

*0

qvEdqPqCqdqq

q, (6-4-2)

Where: G

erfqGxx

xdxqP2

1)(expsin2)( 2

0 , (6-4-3)

And: 2

0

2

23

)1()cos()(

81)(

*1

*0

qvEdqCqdqqGq

q, (6-4-4)

Where is the average normal pressure (the load divided by the nominal contact area), is Poisson’s ratio which is close to 0.5 for rubber-like materials and E ( ) is the

complex viscoelastic modulus of the rubber. Figure 33 shows the calculated hysteresis friction coefficient.


Figure 33: The steady state kinetic friction coefficient calculated using the measured surface roughness power spectrum (Figure 32) and the measured viscoelastic modulus of the rubber (Figure 29) for background temperatures 50 and 80º C and nominal squeezing pressure p = 1 MPa.

A rough estimation showed that the contribution of the viscous component of friction is very small (of the order 0.06). Comparison of experimental data with theoretical calculations showed that the friction coefficient of an elastomer sliding against a hard counterface (in lubricated conditions) at high contact pressure is mainly a result of the viscoelastic deformation of the rubber by the counterface surface asperities. Even if the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness, which may produce the dominant contribution to rubber friction.

6.5. The influence of lubrication on two-body abrasive wear of selected sealing elastomers in reciprocating sliding (Paper E)

An abrasive wear tester (Section 5.3.1) has been used to study the two-body abrasive wear of some selected sealing elastomers in reciprocating sliding and the influence of lubrication. Some of the properties of the tested lubricant and elastomers are shown in Tables 3 and 4 respectively.

Table 3: Properties of lubricants

Base fluid Density (kg/m3) Viscosity@40ºC (cSt) NPI Naphthenic base oil 896 30.0 —

Monoester 864 (20 °C) 8.5 102 Poly alpha olefin 790 5.5 —


Elastomeric materials

Hardness (Shore A)

Tensile strength (MPa)


Density (g/cm3)

NBR - type A 76.7 12.5 378 1.35 NBR - type B 76.1 25.4 466 1.31

HNBR - type A 71.3 17.5 303 1.24 HNBR - type B 79.9 17.9 340 1.3

ACM 73.4 7.8 171 1.49 FKM - type A 72.8 - - 2.03 FKM - type B 81.2 - - 2.19


The results show that in most conditions, lubrication increases the abrasive wear of elastomers (Figure 34) which is presumably due to weakening of the elastomers in the presence of the oils. However, for one tested elastomer (NBR-B) sliding against a coarse abrasive, the abrasive wear in the lubricated condition was lower than that in the dry condition.

Figure 34: Abrasive wear of different elastomers in dry and lubricated conditions. Normal load: 4 N, Abrasive grit size: #320

Examination of the wear particles and surfaces showed that in most dry sliding the wear particles were aggregated (Figure 35). Such aggregation can reduce direct contact between the elastomer and abrasive particles. Although the wheel rotates after each stroke to enable the elastomeric sample to slide against fresh abrasive, but during each individual stroke, the aggregated wear particles may be stuck on the abrasive paper and reduce the amount of direct contact between the abrasives and the elastomer, consequently decreasing the abrasive wear. Lubrication generally prevents the wear particles from becoming aggregated and stuck on the abrasive paper. The prevention of the aggregation of worn particles might ease the worn particles exit from the contact and provide more direct contact between the abrasive particles and elastomer surface. Results show that the influence of lubrication on the increase in abrasive wear is more significant when the elastomer slides against finer abrasives. With the exception of acrylic rubber, the influence of synthetic ester on the increase in abrasive wear of the elastomers is slightly more pronounced than that for the mineral oil and polyalphaolefin. For the acrylic rubber, the influence of mineral oil and polyalphaolefin on the increase in abrasive wear was greater than that from the synthetic ester.


Figure 35: Wear particles of three tested elastomers in dry and lubricated conditions, Normal load: 4 N, Abrasive grit size #320, Lubricant: Naphthenic oil

6.6. The influence of lubrication on two-body abrasive wear of selected sealing elastomers in unidirectional sliding (Paper F)

In Paper F, the abrasive wear and friction of some selected sealing elastomers were studied under dry and lubricated unidirectional sliding condition. The experiments were conducted using the Micro-Tribometer UMT-2 in the block-on-ring configuration (Section 5.3.2.). The experiments have been performed with two levels of abrasive grit size, two levels of sliding velocity and two levels of normal load (contact pressure). The lubricant used in these experiments was a monoester (Table 3) and the elastomers were two nitrile rubbers (NBR), an acrylic rubber (ACM) and a fluoro rubber (FKM). Some properties of the elastomers have been shown in Table 4. The oil absorption and influence of the oil on tear strength of the elastomers have also been measured after immersing the elastomers in the oil for 2 weeks.

Results show that the wear of the ACM and FKM sliding in lubricated conditions was significantly higher than that in dry sliding. The wear of the NBR-A sliding under lubricated conditions was slightly higher than that in dry sliding but the influence of the oil on wear of NBR-B depends on the sliding velocity, abrasive grit size and contact pressure. The wear particles of elastomers in dry sliding conditions were aggregated but for the lubricated conditions, the presence of the oil in the contact dispersed the wear particles and prevented aggregation.

Investigation of the worn surfaces showed that both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) were formed on the worn surfaces of the NBRs. The worn surfaces in the dry conditions are characterised by more ridges but in the lubricated conditions are defined by more scratches (Figure 36). In the dry condition, the shear stress is distributed more uniformly over the area resulting in more uniform tensile stress at the rear of the apparent contact area and consequently the formation of ridges (Figure 15.b). The worn surfaces of ACM and FKM were characterised by scratches (parallel to the direction of sliding). As shown in Figure 36, aggregated wear particles stuck to the surface of the tested ACM.


Figure 36: Worn surfaces of the elastomers, in dry (left) and lubricated (right) condition, Normal load: 20 N, speed: 31.42 mm/s, Abrasive grit size: #320


In both dry and lubricated conditions, the friction coefficient decreased with increasing of the contact pressure and increased with increasing sliding velocity. Apart from FKM, the presence of oil resulted in a decrease in the friction coefficient as well as the tear strength of the elastomers (Figure 37), especially for ACM and NBR-A. Among all the elastomers tested, FKM and ACM have absorbed the least and the most oil respectively, after immersion in the oil (Figure 37).

Figure 37: Change in tear strength and weight of the elastomers after ageing in the oil


7. Conclusions

The tribological behaviour of some elastomeric seal materials has been studied under different operating conditions. The influences of lubrication as well as ageing of an elastomer (NBR) on the friction and two-body abrasive wear have been investigated. The tribological behaviour of some selected sealing elastomers in dry conditions has also been studied.

The salient conclusions from this work are as follows:

Conclusions pertaining to friction:

The friction coefficient of an elastomer in lubricated sliding against a hard counterface at high contact pressure is mainly due to the viscoelastic deformation of the rubber by the counterface surface asperities. Even if the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness which may make the dominant contribution to rubber friction. The friction coefficient of an elastomer sliding against a hard counterface at low contact pressure, where the hydrodynamic effects are significant, is affected by the surface topography of the elastomer but at high contact pressure, the friction coefficient is not affected significantly by the surface topography of the elastomer.

Ageing of the nitrile rubber in ester base fluids reduces the friction coefficient.

The longest running-in periods were observed during sliding against fine surfaces. As the roughness of the hard sliding surface increases, the mechanical action exerted on the elastomer surface increases and consequently the extraction of the elastomer’s constituents. This causes formation of the layer on the counterface to accelerate leading to a rapid decrease in friction.

Conclusions pertaining to abrasive wear:

Depending on the mechanism of wear and the oil-elastomer compatibility, the presence of lubricant may decrease or increase the abrasive wear of elastomers. However, in most cases, lubrication increased the abrasive wear.

In unidirectional abrasive wear, both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) may be observed on the worn surface of elastomers. The ridges on the worn surfaces are formed close to the zone of maximum contact pressure where the elastomer penetrates into more voids and the contact pressure is more distributed on the surface of the elastomer. Increasing the contact pressure and/or using finer abrasives increases ridge formation but the presence of lubricant in the contact decreases the formation of ridges.

Ageing the NBR in any of the tested fluids increases the abrasive wear. Ageing the NBR in ester base fluids and rapeseed oil leads to the highest increase in abrasive wear.


8. Suggestions for future work

The unexpected practical issues such as the influence of the lack of lubrication or starved lubrication, frictional heating and the influence of lubricant additives on tribological behaviour of elastomers are the interesting topics for further investigations.

Studying the lubricated abrasive wear of elastomers is a complicated task. The influence of different parameters, such as contact pressure, abrasive coarseness and test periods on the wear mechanisms and different forms of tearing resulting in abrasive wear of different sealing elastomers should also be studied.

In spite of the important role of three-body abrasive wear of sealing elastomers on the seal performance, this subject has been studied very scantily. In view of this, three-body abrasive wear of elastomers should also be a part of any future work.


9. References

[1] T. A. Stolarski, Tribology in Machine Design, Butterworth-Heinemann, Oxford, 2000.

[2] Friction, lubrication, and wear technology. In ASM handbook, vol. 18, 1992 (American Society for Metals, Materials Park, Ohio).

[3] A. V. Beek, Machine lifetime performance and reliability, TU Delft, Delft, 2004.

[4] R. Flitney, Seals and Sealing Handbook, 5th edition, Elsevier Ltd., United Kingdom, 2007.

[5] H. K. Muller, B. S. Nau, Fluid sealing technology, Marcel Dekker, New York, 1998.

[6] E. T. Jagger, Study of the lubrication of synthetic rubber rotary shaft seals, Proc. Conf. on Lubrication and Wear, Institution of Mechanical Engineers, London (1957) 409–415.

[7] H. K. Muller, Concepts of sealing mechanism of rubber lip type rotary shaft seals, Proc. 11th International Conference on Fluid Sealing, BHRA (1987) 698–709.

[8] R. F. Salant, A. L. Flaherty, Elastohydrodynamic Analysis of Reverse Pumping in Rotary Lip Seals With Microasperities, J. Tribol.-Trans. ASME 117 (1995) 53-59.

[9] R. F. Salant, Numerical Analysis of the Flow Field Within Lip Seals Containing Microundulations, J. Tribol.-Trans. ASME 114 (1992) 485-491.

[10] K. Tønder, R. F. Salant, Non-Leaking Lip seals: A Roughness Effect Study, J. Tribol.-Trans. ASME 114 (1992) 595-599.

[11] R. F. Salant, Modelling rotary lip seals, Wear 207 (1997) 92-99.

[12] R. F. Salant, Analysis of the Transient Behavior of Rotary Lip Seals-Fluid Mechanics and Bulk Deformation, Tribol. Trans. 41(4) (1998) 471-480.

[13] F. Shi, R. F. Salant, A Mixed Soft Elastohydrodynamic Lubrication Model with Interasperity Cavitation and Surface Shear Deformation, J. Tribol.-Trans. ASME 122 (2000) 308- 316.

[14] D. Shen, R. F. Salant, Elastohydrodynamic analysis of the effect of shaft surface finish on lip seal behaviour, Tribol. Trans. 46 (2003) 391-396.

[15] D. Shen, R. F. Salant, An unsteady mixed soft EHL model, with application to a rotary lip seal, Tribol. Int. 40(4) (2007) 646-651.


[16] S. Bisztray- Balku, Design development and Tribology of reciprocating hydraulic seals, Periodica Polytechnica Mechanical Engineering 47 (1) (2004) 163–178.

[17] R. K. Flitney, Reciprocating seals, Tribol. Int. 15(4) (1982) 219-226.

[18] A. Karaszkiewicz, Hydrodynamics of rubber seals for reciprocating motion, lubricating film thickness, and outleakage of O-seals, Ind. Eng. Chem. Res. 26 (11) (1987) 2180–2185.

[19] W. Hofmann, Rubber Technology Handbook, Hanser, Munich, 2001.

[20] M. A. Meyers, K. K. Chawla, Mechanical Behavior of Materials, Cambridge University Press, New York, 2008.

[21] R. C. Klingender, Handbook of specialty elastomers, CRC Press, Taylor & Francis Group, US, 2008.

[22] Information obtained from the website: http://www.plantengineering.com/article/CA378911.html#Weather%20or%20ozone%20cracking .

[23] Information obtained from the website: http://www.thesealman.com/pages/oring_handbook/pdf_files/epm_oring_hbpt1.pdf, last access: 2009-02-17.

[24] Information obtained from the website: http://www.simritna.com/catalog/o-ring/Contents.htm , last access: 2009-02-17.

[25] Information obtained from the website: http://www.afmusa.com/doc_generator.asp?doc_id=2357, last access: 2009-02-17.

[26] M. Torbacke, A. Johansson, Seal Material and Base Fluid Compatibility: An Overview, J. Synthetic Lubrication 22(2) (2005) 123-142.

[27] G. F. Ogorodnikova, V. V. Sinitsyn, Mechanism of rubber swelling in oils and greases, Chemistry and Technology of Fuels and Oils 18 (1982) 306-308.

[28] M. F. Myntti, Comparing fuel and oil resistance properties, Rubber World 228 (2003) 38-45.

[29] A. O. Patil, T. S. Coolbaugh, Elastomers: A literature review with emphasis on oil resistance, Rubber Chem. Technol. 78 (2005) 516-535.

[30] Z. Tao, N. Viriyabanthorn, B. Ghumman, C. Barry, J. Mead, Heat resistance elastomers, Rubber Chem. Technol. 78(3) (2005) 489-515.

[31] R. B. Simpson, Rubber Basics, Rapra Technology Limited, UK, 2002.

[32] D. F. Moore, The Friction and Lubrication of Elastomers, Pergamon Press, 1972.

http://www.plantengineering.com/article/CA378911.html#Weather%20or%20oz

http://www.thesealman.com/pages/oring_handbook/pdf_files/epm_oring_hbpt1

http://www.simritna.com/catalog/o-ring/Contents.htm



http://www.afmusa.com/doc_generator.asp?doc_id=2357


[33] K. C. Ludema, Physical factors in tyre traction, Physics in Technology 6(1) (1975) 11-17.

[34] D. F. Moore, A review of adhesion theories for elastomers, Wear 22 (1972) 113-141.

[35] D. F. Moore, A review of hysteresis theories for elastomers, Wear 30 (1974) 1-34.

[36] K. Maeda, A. Bismark, B. Briscoe, Effect of bulk deformation on rubber adhesion, Wear 263 (2007) 1016-1022.

[37] A.D. Roberts, Theories of dry rubber friction, Tribol. Int. 9(2) (1976) 75-81.

[38] B. N. J. Persson, On the theory of rubber friction, Surf. Sci. 401 (1998) 445-454.

[39] B. N. J. Persson, Volokitin, A. I., Rubber friction on smooth surfaces, Eur. Phys. J. E 21(1) (2006) 69-80.

[40] K. N. G. Fuller, D. Tabor, The Effect of Surface Roughness on the Adhesion of Elastic Solids, Proc. R. Soc. Lond. A.345 (1975) 327-342.

[41] J. A. Greenwood, D. Tabor, The Friction of Hard Sliders on Lubricated Rubber: The Importance of Deformation Losses, Proc. Phys. Soc. 71 (1958) 989-1001.

[42] K. L. Johnson, K. Kendall, A. D. Roberts, Surface energy and the contact of Elastic Solids, Proc. R. Soc. Lond. A. 324 (1971) 301-313.

[43] K. A. Grosch, The Relation between the Friction and Visco-Elastic Properties of Rubber, Proc. R. Soc. Lond. A. 274 (1963) 21-39.

[44] M. L. Williams, R. F. Landel, J. D. Ferry, The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-forming Liquids, J. Am. Chem. Soc. 77(14) (1955) 3701–3707.

[45] B. N. J. Persson, Theory of rubber friction and contact mechanics, J. Chem. Phys. 115 (2001) 3840-3861.

[46] B. N. J. Persson, Rubber friction: role of the flash temperature, J. Phys.-Condes. Matter 18 (2006) 7789–7823.

[47] A. Schallamach, How does rubber slide?, Wear 17 (1971) 301-312.

[48] M. Barquins, Friction and wear of rubber-like materials, Wear 160 (1993) 1-11.

[49] M. Barquins, Sliding Friction of Rubber and Schallamach Waves – A Review, Mater. Sci. Eng. 73 (1985) 45 -63.

[50] B. N. J. Persson, U. Tartaglino, O. Albohr, E. Tosatti, Sealing is at the origin of rubber slipping on wet roads, Nat. Mater. 3(12) (2004) 882-885.

[51] B. N. J. Persson, U. Tartaglino, O. Albohr, and E. Tosatti, Rubber friction on wet and dry road surfaces: The sealing effect, Phys. Rev. B 71, 035428 (2005).


[52] D. I. James, M., E. Jolley, Abrasion of rubber, MacLaren & Sons Ltd., London, 1967.

[53] D. F. Moore, Friction and wear in rubbers and tyres, Wear 61 (1980) 273-282.

[54] A. Schallamach, Friction and abrasion of rubber, Wear 1 (1958) 384-417.

[55] P. Thavamani, D. Khastgir, A. K. Bhowmick, Microscopic studies on the mechanisms of wear of NR, SBR and HNBR vulcanizates under different conditions, J. Mater. Sci. 28 (1993) 6318-6322.

[56] A. Schallamach, Abrasion of rubber by a needle, J. Polymer Sci. 9 (1952) 385-404.

[57] B. J. Brisco, S. K. Sinha, Wear of polymers, Proc. Inst. Mech. Eng. Part J.-J. Eng. Tribol. 216(6) (2002) 401-413.

[58] E. Southern, A. G. Thomas, Studies of rubber abrasion, Rubber Chem. Technol. 52(4) (1979) 1008-1018.

[59] K. A. Grosch, A. Schallamach, Relation between abrasion and strength of rubber, Rubber Chem. Technol. 39 (1966) 287-305.

[60] S. W. Zhang, Tribology of Elastomers, Elsevier, Netherlands, 2004.

[61] S. W. Zhang, Z. Yang, Energy theory of rubber abrasion by a line contact, Tribol. Int. 30(12) (1997) 839-843.

[62] S. W. Zhang, Mechanisms of rubber abrasion in unsteady state, Rubber Chem. Technol. 57 (1984) 755-768.

[63] A. N. Gent, C. T. R. Pulford, Mechanisms of rubber abrasion, J. Appl. Polym. Sci. 28 (1983) 943-960.

[64] C. T. R. Pulford, Antioxidant Effect during Blade Abrasion of Natural Rubber, J. Appl. Polym. Sci. 28 (1983) 709-713.

[65] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 1: Abrasion pattern formation in natural rubber vulcanizate, Wear 171 (1994) 195-202.

[66] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 2. General rule in abrasion pattern formation in rubber-like materials, Wear 178 (1994) 109- 116.

[67] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 3: how is friction linked to fracture in rubber abrasion?, Wear 188 (1995) 19-26.

[68] Y. Uchiyama, Y. Ishino, Pattern abrasion mechanisms of rubber, Wear 158 (1992) 141-155.

[69] T. Iwai, Y. Uchiyama, K. Shimosaka, K. Takase, Study on the formation of periodic ridges on the rubber surface by friction and wear monitoring, Wear 259 (2005) 669-675.


[70] A. H. Muhr, T. J. Pond, A. G. Thomas, Abrasion of rubber and the effect of lubricants, J. Chim. Phys. 84 (1987) 331-334.

[71] A. H. Muhr, A. D. Roberts, Rubber abrasion and wear, Wear 158 (1992) 213-228.

[72] A. Schallamach, Abrasion, Fatigue, and Smearing of Rubber, J. Appl. Polym. Sci. 12 (1968) 281-293.

[73] G. W. Stachowiak, A. W. Batchelor, Engineering Tribology (2nd edition.), Butterworth-Heinemann, Boston, 2001.

[74] D. C. Evans, Polymer – fluid interactions in relation to wear, Proc. 3rd Leeds-Lyon Symp. on Tribology, Wear of Non-Metallic Materials, Sept. 1976, Publ.(1978) London 47-71.

[75] M. K. Omar, A. G. Atkins, J. K. Lancaster, The role of crack resistance parameters in polymer wear, J. Phys. D-Appl. Phys. 19 (1986) 177-195.

[76] A. H. Muhr, Lubrication of model asperities on rubber, Proc. 17rd Leeds-Lyon Symp. on Tribology, Vehicle Tribology, Sept. 1990, Publ. (1991) Amsterdam 195-204.

[77] M. Chandrasekaran, A. W. Batchelor, In situ observation of sliding wear tests of butyl rubber in the presence of lubricants in an X-ray microfocus instrument, Wear 211 (1997) 35-43.

[78] N. Patir, H. S. Cheng, An average Flow Model for Determining Effects of Three- Dimensional Roughness on Partial Hydrodynamics Lubrication, J. Lubrication Technol. 100 (1978) 12-17.


Paper A

Friction evaluation of elastomers in lubricated contact _______________________________________________________________________ 43

Friction evaluation of elastomers in lubricated contact: a comparison of different test methodologies

G. F. Simmons1, M. Mofidi2, B. Prakash1

E-mails: [email protected], [email protected], [email protected]

1 Division of Machine Elements, Luleå University of Technology, Luleå SE-971 87 Sweden

ABSTRACTFriction testing of elastomers in lubricated contact is discussed with a focus on developing experimental arrangements that can produce worthwhile results. Practical issues unique to elastomers are covered as well as their solutions including contact mechanics, material response to loading, contact edges, oil absorption, cleaning, and specimen geometry. A critique of reciprocating laboratory testing machines, including high frequency short stroke and low frequency long stroke friction and wear machines, for their usefulness is conducted as is critical analysis of a wide variety of specimen configurations with the aim of helping the laboratory experimenter to overcome many of the pitfalls associated with testing of elastomers in lubricated conditions. Results from experiments using various testing arrangements are analyzed and it is found that the synthetic ester and mineral oil used produced similar results.

Keywords: Friction, Elastomer, lubrication

INTRODUCTION While significant research has focused on the Tribological characteristics of elastomers in relation to automobile tires in the dry condition, little effort has been focused on the characteristics of elastomers in relation to seals and sealing systems, especially under lubricated conditions. Some earlier work however has been accomplished investigating lubricated face seals [1]. Other work has focused on the effects of temperature changes on the wear of rubber in contact with abrasive paper under lubricated conditions, finding that the wear increased with higher temperatures due to the increased level of thermal degradation. More recent work has investigated the friction characteristics when steel is in lubricated contact with elastomers [2, 3].

Previous work in relation to emulsions of water and oil in soft-EHL contact determined that the tribological properties are generally dominated by the more viscous of the emulsion’s components [4]. Further study in soft-EHL contact revealed that the sliding friction was highly dependent on the speed to viscosity ratio in the contact such that at low values, friction was dominated by adhesion between the asperities and at high values, friction was dominated by the Couette flow [5].

Other research has been conducted on the wear of metal surfaces by elastomer materials in the presence of water. It was found that, when abrasive particles are present, wear of the metal counterface is primarily due to the embedment of the

mailto:[email protected]



Friction evaluation of elastomers in lubricated contact _______________________________________________________________________44

abrasive in the elastomer [6]. Without the presence of water, it was found that a chemical reaction occurred on the surface of the metal causing a surface layer to form. This surface then broke away, becoming embedded in the elastomer and increasing the wear of the metal [7]. In both cases, it was found that without any abrasive, the wear of the steel was very small or negligible.

Most recently, testing has been conducted on rubber friction in lubricated sliding against surfaces that appear to be smooth and polished [8]. In these studies, it was found that even the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness which may make the dominant contribution to rubber friction. It was also found that the friction was reduced by ageing the elastomer in lubricants which may be due to the diffusion of oil in the elastomer resulting in a decrease in internal friction of the elastomer. In addition, when the rubber block is pressed against the counterface, oil may be squeezed out from the rubber matrix, giving a thicker oil film at the interface and thus decreasing the friction.

Very little has been reported regarding the tribological compatibility of base oils and elastomers beyond the proprietary knowledge of seal and lubricant manufacturers. With the advent of and conversion to environmentally adapted lubricants (EALs), much of the basic experimental research into compatibility is necessary both to ensure its relevance and to further support and validate the use of EALs. Basic testing of elastomer seal material physical characteristics were conducted in [9] to determine the effects of ageing in various EALs but studies were limited to swelling and changes in hardness and tensile strength.

Unfortunately, very little has been reported regarding testing of elastomers in lubricated contact and fewer reliable testing arrangements have been reported. Thus while the aim of this work is to determine the basic performance of several commonly used seal materials lubricated with EALs, the bulk of the work conducted to accomplish this task focuses on development of testing conditions suitable to elastomers in lubricated contact.

CHALLENGES TO ELASTOMER TESTING

Contact Mechanics

One of the primary issues in performing tests on elastomers is the large difference in module of elasticity between the elastomer and the counter surface which causes the elastomer to undergo significant deformation under relatively low loading, thus limiting the contact pressure. When lubricant is added to the contact, the limited contact pressure causes the contact to more readily enter the hydrodynamic lubrication regime, thus hampering any efforts to investigate the tribological behaviour of elastomer in boundary or mixed lubrication. Further compounding this is that friction of rubber is affected by the surface roughness of hard counter face on a large variation of length scale down to the nano-scale. The friction of rubber sliding against a hard counter face depends on the ratio of the amplitude to wavelength of the surface roughness and if this ratio is constant, the different surface roughness (of varying length scales) may contribute equally to the friction force [10].


Response to frictional loading

Another issue that can occur with elastomers is elastic deformation of the specimen under loading due to the frictional forces. While deformation is expected in the contact region under loading, with most materials, deformation is not expected far from the contact region or throughout the specimen as a whole. However, elastomers exhibit significant deformations outside the contact area (Figure 1).

Figure 1: Deformation of elastomeric samples in reciprocating tests

The simplest way to reduce the effects of this phenomenon is to avoid using short stroke lengths in reciprocating tests. Using longer stroke lengths ensures that friction measurements are actually measuring the friction in the contact as opposed to the force required to deform the entire elastomer specimen.

The edges of the contact

In lubricated contacts with elastomers and hard surfaces, the elastomer behaves in much the same manner as the windscreen wipers on an automobile do to push fluid away from the contact. Likewise, the elastomer can form a seal around the edges of the contact, trapping lubricant inside the contact and thus causing greatly different friction mechanisms across the contact area. Clearly care must be taken to ensure that lubricant is not wiped clear from or excessively trapped within the contact. Through experiments it was found that any sharp edges that were perpendicular to the direction of motion tended to wipe lubricant out of the contact. On the contrary, edges parallel to the direction of motion tended to act as walls, trapping lubricant within the contact area and causing stress concentrations in the elastomer as it deformed around the hard surface’s edge.

The simplest measure to reduce these effects is to ensure that contact geometries are free from sharp edges, or in the worst case, that any sharp edges in the contact are on the elastomer side.

Oil absorption and cleaning

Surprisingly, one of the most difficult issues to deal with when performing tests with elastomers is not actually the testing itself, but is instead the issue of cleaning and


preparing the specimens. The difficulty with elastomers is that different elastomers may absorb lubricants and cleaning agents or the solvents may extract elastomers’ constituents at different rates. Further compounding this issue is that some elastomers react to most cleaning agents, thus a sample mass can change significantly through the process of cleaning alone. The question then is how rigorously the samples need to be cleaned to get realistic values for wear when the worn mass is on the same magnitude as the oil absorption.

Concerning friction testing, any interaction or oil absorption at the surface of the elastomer clearly has an effect on its physical properties and consequentially, its frictional behaviour. If cleaning solvents interact with rubber additives (e.g. plasticizers which are often oil-based [11]), potential exists for dramatic changes in surface makeup. Extracting the plasticizers by cleaning agent can result in a change in the elasticity and low temperature flexibility of the rubber [11] and consequently, the friction coefficient.

This issue of elastomer - oil interaction becomes even greater when one wishes to perform tests on aged elastomer specimens. When elastomers age in oil, they can absorb much more lubricant than during a typical test and then to further complicate matters, when they are tested after ageing, the ageing process can affect how much more oil they release or absorb during the test. Because of these complex interactions and the major uncertainties they introduce, it is generally not very useful or reasonable to attempt to measure wear rates of aged elastomer specimens other than to get an idea of the oil absorption properties of the elastomers.

Specimen geometry

The vast majority of tribological testing on metal specimens utilizes some sort of mechanical device to hold the specimen in place. In the case of elastomers, this becomes a real issue because the clamping force used to hold the specimen causes large deformations of the specimen itself. These large deformations can greatly affect the tribological characteristics of the material, and thus the experimental results. For mechanical holding to function and produce adequate results, it works best to replicate the holding system used in actual applications. This causes the specimen deformations to be similar to those of the application and thus allows for results that can be applied to the selected application. One major advantage to mechanical holding of the specimens is that worthwhile wear measurements can be taken assuming an effective cleaning regime is followed.

Several methods that have been experimented with to bond elastomers to a metal backing include using quick drying super glue, contact cement and, slow curing epoxy. Super glue seemed to work quite well to hold the specimen to its backing in most of the tests, but in a few higher frequency lubricated tests, the lubricant was able to creep in between the metal and elastomer and break the super glue bond. A similar problem occurred when epoxy was used, except that the epoxy bond failed in nearly every test with lubricant. At higher temperatures, the reliability of bonding the specimens becomes even more questionable as the strength of many common adhesives at higher temperatures is greatly diminished. The greatest drawbacks to bonding specimens for testing is that useful wear measurements are difficult to perform on account of the added mass to the specimen from the adhesive and backing material as well as the inherent interaction between the bonding and cleaning agents.


Another method that has been explored is to hold the specimen with double sided tape. While this method works quite well in dry contacts, it has not been successful for lubricated contacts due to the lubricant breaking down the bond between the tape and the specimen allowing the specimen to slip.

Specimen geometry can have large effects on the thermal aspects of the test as well. As mentioned earlier, because elastomers are visco-elastic, significant amounts of heat energy are produced through friction in the contact and internal damping of friction induced motion in the specimen itself. Further compounding the problem of heat generation is that unlike most metals, elastomers have quite good insulating properties which cause the heat generated in the specimen to stay within the specimen, heating it up. While lubricant in the contact does help to transfer heat away from the specimen, using the right specimen geometry can also significantly help. In this regard, specimens with small cross-sections and large surface areas contacting the holding device are preferred.

EXPERIMENTAL

High frequency short stroke reciprocating machine

A large series of tests were conducted using a high frequency reciprocating test machine in hopes of determining wear characteristics of elastomers at higher frequencies. The test configuration and parameters have been shown in Figure 2 and Table 1.

Figure 2: Test configuration (high frequency tests)


Table 1: Test configuration and parameters (high frequency tests) Cylinder Length 22 mm Cylinder Diameter 15 mm

Upper Specimen

Cylinder Roughness (Ra) 60 nm Lower Specimen Specimen Thickness 6 mm

Load 100 N Mean Contact Pressure 1.8 MPa Stroke Length 2.5 mm Oscillation Frequency 40 Hz Mean Sliding Speed 0.20 m/s Test Duration 30 min

Test Parameters

Temperature 26°C and 80°C Lubricants Uncontaminated Polyol ester

Uncontaminated Complex ester Polyol ester w/ 5% H2O content Aged Polyol ester w/ 5% H2O content

Test Materials

Elastomers NBR, HNBR, FKM

While much was learned in terms of elastomer testing from these tests, very little worthwhile data was produced. The reason for this was a combination of almost everything discussed earlier in regards to developing effective testing routines for elastomers. Due to limitations in the testing machine, a friction value of 1 could not be exceeded during the test which necessitated that a high load of 200 N be used. This high loading caused very high wear in some specimens, such as FKM and HNBR, but interestingly caused minimal wear on the NBR specimens. These results were the opposite of what one would expect based on the material properties and it is believed that the primary reason for this reversal is that the varying visco-elasticity characteristics of the materials played a role in their response to the short stroke motion. It is thought that in the case of NBR, the entire specimen was deformed from side to side during each stroke. This minimized the apparent stroke length and sliding distance and thus, the total observed wear.

The most heavily worn areas of many samples were close to the contact area edges (Figure 3-b) which could have been related to the sealing effects. In the center of the contact, it is theorized that the lubricant was held in place by the sealing effect discussed in [12, 13], and thereby the wear was greatly reduced.

Thermal effects were also a significant player with this testing arrangement due to the thick specimens, high frequency and loading, and lack of fresh lubricant in the contact (Figure 3).

Figure 3: Typical thermal effects for materials in HIGH FREQUENCY tests after 30 minutes

at 26°C


In many cases, the thermal effects added with the lubricant to break the specimen’s bond with its holder causing that specific test to be thrown out.

The final and most significant problem with this test however was the geometry of the contact region. The contact consisted of a steel cylinder which slid longitudinally along the rubber specimen in reciprocating motion. This contact geometry produced sharp stress concentrations and lubricant starvation at the ends of the cylinder which caused large sections of elastomer to be torn and dug up at the ends of the contact. The unstable nature of this process caused instability in the friction curves for the specimens, and in the end led to a dataset that did not allow for comparison between the individual elastomers’ performance.

Low frequency, long stroke reciprocating test rig

Using the lessons learned from the high frequency test rig, a series of experiments were conducted on a low frequency, long stroke test rig in an effort to produce more useful results. The low frequency test rig is similar to the high frequency rig in that the sample reciprocates against a base material however, unlike the high frequency rig, there are no significant geometrical constraints and sample holders can be crafted to suit almost any reciprocating contact. This adaptability is indispensable for aiding in the development of suitable test geometry.

In the development of a final testing configuration, several sample and countersurface arrangements were tested with widely varying results. The test configurations and parameters have been shown in Figure 4 and Table 2.

Figure 4: Test configuration in low frequency tests, (a) Cylinder on disc (sliding parallel to

the axis), (b) Cylinder on disc (sliding perpendicular to the axis), ( c) Ball on disc, (d) Cylinder on cylinder


Table 2: Test configuration and parameters in low frequency tests Test Conf. (a) (b) (c) (d)

Length 10mm 10 mm - 22 mm Diameter 10 mm 10 mm 10 mm 15 mm

Upper Specimen

Roughness (Ra) 200 nm 200 nm 130 nm 160 nm Thickness 6 mm 6 mm 6 mm 6 mm Lower

Specimen Curvature None None None 100 mm radius Load 100 N 100 N 100 N 100 N and 150 N Stroke Length 1.4 mm 1.4 mm 1.4 mm 6 mm Oscillation Frequency 5 Hz 5 Hz 5 Hz 5 Hz Mean Sliding Speed 0.014mm/s 0.014mm/s 0.014mm/s 0.06 m/s Test Duration 30 min 30 min 30 min 60 min

Test Parameters

Temperature 26 °C 26 °C 26 °C 26 °C Lubricants Uncontaminated Polyol ester

Uncontaminated Complex ester Polyol ester w/ 5% H2O content Aged Polyol ester w/ 5% H2O content

Test Materials

Elastomers NBR, HNBR, FKM

First, a cylinder was oscillated longitudinally against an elastomer specimen (Figure 4-a). As expected, the results were similar to the high frequency results with wear scars mostly occurring where the ends of the cylinder contacted the elastomer. Additionally, the wear and damage to the elastomer were significantly lower due to the fact that the frequency was much lower and the stroke much longer, allowing for better cooling of the specimen through the lubricant and the counter surface. Figure 5 shows a typical worn surface tested in this configuration.

Figure 5: A typical worn surface of a sample (FKM) tested in cylinder on flat (parallel

sliding) configuration, Normal load= 100 N, Oscillation frequency= 5 Hz, Stroke length= 1.4 mm

To try and keep the cylinder ends from the contact, the cylinder was next turned so that it oscillated perpendicularly, like a roller (Figure 4-b). This arrangement was tested both with specimens that were narrower and wider than the length of the cylinder and neither produced worthwhile results. When the specimen was wider than the cylinder, the ends of the cylinder produced high shear stress concentrations in the elastomer and fluid starvation in the contact causing longitudinal slices to form in the specimen (Figure 6). With specimens narrower than the contact cylinder, the elastomer specimen deformed to such a degree that it was not possible to effectively hold it in place during testing.


Figure 6: A typical worn surface of a sample (FKM) tested in cylinder on flat (perpendicular sliding) configuration, Normal load= 100 N, Osillation frequency= 5 Hz, Stroke length= 1.4

mm

A steel ball against a flat elastomer specimen (Figure 4-c) seemed to be the logical next test, however it was found that with this contact the elastomer fatigued and split longitudinally along the contact (Figure 7). This appeared to be most likely due to fatigue caused by the tensile stresses in the material in the center of the contact as opposed to actual wearing of the material.

Figure 7: A typical worn surface of a sample (FKM) tested in ball on flat configuration,

Normal load= 100 N, Osillation frequency= 5 Hz, Stroke length= 1.4 mm

In an effort to eliminate all sharp edges and to provide a contact that would minimize the tensile fatigue seen in other tests, it was deemed that using a steel cylinder against a curved elastomer specimen could potentially provide an adequate test (Figure 5-d). The reasoning behind this configuration is that firstly, the end geometry of the cylinder does not contact the elastomer, thus removing the stress concentrations seen earlier. Secondly, and equally important is that the gross deformation of the elastomer


remains relatively constant throughout the course of the test. This eliminates the gross fatiguing of the material due to the tensile stresses along the center of the contact allowing for study of the tribological characteristics of the contact instead of the mechanical characteristics of the materials.

Tests were performed using this configuration and the results (Figure 9) allowed for a differentiation between the various elastomer compounds. It was also possible to draw some minor conclusions about the performance of the various lubricants, but the experimental error was such that it was not possible to make any confident conclusions in this regard. It is felt, that given further refining, this test method could provide valuable insights into the more fundamental aspects of lubricated elastomer friction and the effects of surface roughness of the counter face.

Low frequency, long stroke with O-rings

To more accurately simulate an elastomer application, a test was developed utilizing the low frequency test machine and commercially available O-ring specimens. This test involved holding an O-ring specimen and oscillating it perpendicularly against a steel plate as seen in Figure 10 with lubricant applied liberally to the steel plate. The arrangement of this test allowed for complete and consistent control of the experimental parameters. The steel counter surfaces were finished using one grinding pass giving them nearly identical surface roughness structures while the elastomer specimens were fitted and clamped into their holder using methods similar to those used in hydraulic cylinders.

Figure 8: Test configuration in low frequency tests (with O-rings)

Table 3: Low Frequency Rig O-Ring on Flat Experimental Conditions

Oscillation Frequency 1 Hz Average speed 3 cm/s Test Duration 60 min Contact Load 100 N Temperature 26 °C Stroke Length 1.5 cm

Test Parameters

Contact Width 3 cm Upper Specimen

O-Ring Materials Molded NBR, Extruded NBR, Molded FKM and Extruded FKM

Plate Material Steel Lower Specimen Plate roughness 1.1 μm and 0.2 μm


The results from this test demonstrated, with very little uncertainty, the differences in the performance of the elastomers and the oils to a degree not seen in the other tests as shown in Figure 11. Similarly low uncertainty was attained when these tests were repeated using materials aged in their respective lubricants. The primary drawback to this test is that the deformation that occurs in the elastomer through the course of each stroke and the round shape of the elastomer which tends to draw lubricant into the contact make it difficult to develop further fundamental understanding of the behaviour of elastomers in lubricated contact. However, this drawback is eclipsed by the fact that the results are readily applicable to elastomer applications because of their small uncertainty and the similarities between the test conditions and those in industry.

RESULTS AND DISCUSSION

High frequency short stroke reciprocating machine

As mentioned earlier, the results from the high frequency short stroke reciprocating machine in the cylinder on flat arrangement are quite difficult to draw any firm conclusions from due to the large number of parameters that affected the test results. However, some basic remarks can be made from Figure 9. The primary point of interest is that in nearly all cases, the water contaminated polyol ester produced lower friction than the uncontaminated and contaminated and aged polyol ester base lubricants. Additionally, the aged lubricant generally produced higher friction than its un-aged counterpart. These observations are primarily of interest as they are quite different from the results of later testing under different conditions, thus reinforcing the complexity of testing elastomers.

Figure 9: Coefficient of friction from high frequency tests, 200 N load, 2.5 mm stroke, 40 Hz

oscillation frequency, 30 minute long tests

Low frequency, long stroke reciprocating test rig

The results from the low frequency long stroke test rig in the cylinder on curve allowed for clear comparison between the performance of the different elastomers, but in most cases did not allow for comparison between the lubricants (Figure 10). Of note is that the HNBR produces much lower friction in all tests than the other elastomers. This is thought to be primarily a result of the material properties of the


HNBR which could have allowed it to more readily approach full film lubrication under the same levels of loading as the other materials.

NBR produced the highest friction of the group which contrasts greatly with the results from the high frequency tests. This contrast is believed to be at least in part caused by the fact that NBR experienced significant deformations in the high frequency tests due to the short stroke. In the case of the low frequency tests and NBR, the stroke length was long enough that sliding dominated the contact instead of material deformations.

Figure 10: Coefficient of friction from low frequency tests, 100 N and 150 N loads, 1 MPa and 1.5 MPa contact pressures, 6 mm stroke, 5 Hz oscillation frequency, 60 minute long

tests

Low frequency, long stroke with O-rings

Testing of the O-ring materials using the low frequency test rig allowed for a clear distinction between the various elastomer and lubricant combinations. Of note is that the extruded NBR consistently produced lower friction than the other elastomer materials both against smooth and rough surfaces. Also of interest is that the mineral oil produces slightly lower friction in all cases than the synthetic ester. This is believed to be caused by the variation in viscosity of the two lubricants at the testing temperatures. While the two lubricants have the same viscosity at operating temperatures, the synthetic has slightly lower viscosity at the testing temperature. This higher viscosity of the mineral oil is believed to have caused the contact to be slightly more towards hydrodynamic lubrication and thus have slightly lower friction than in the case of the synthetic lubricant. It is believed that this minor difference would become negligible at operating temperatures when viscosities of the two lubricants are equal.


Figure 11: The results of the experiments with low frequency rig O-Ring on Flat, Normal load= 100 N, Plate roughness= 1.1 μm (Rough surface) and 0.2 μm (Smooth surface), Test

duration= 60 min, Frequency= 1 Hz, Average speed= 3 cm/s

CONCLUSIONThis work has made an effort to identify the pitfalls associated with performing tribological tests of elastomers in lubricated conditions. Of the testing arrangements discussed, the low frequency test rig using O-ring material proved to produce the most repeatable, worthwhile results and testing using the curve on curve geometry had good potential to be a suitable test, given future refining. While these results allowed for selection of materials in specific applications, they were not able to provide significant deeper understanding of elastomers and their behaviour in lubricated contact. However, the development of these tests does highlight the greater need in elastomer testing to eliminate unknown parameters and take into account material behaviour, sealing effect, edge effects, thermal degradation than in similar standardized tests with non-viscoelastic materials. Finally a comparison of elastomers tested using environmentally adapted lubricants demonstrated that the performance of the synthetic and mineral based lubricants were comparable.

ACKNOWLEDGEMENTS The authors thank Dr. Marika Torbacke of Statoil Lubricants, Mr. Alf Johansson of Trelleborg Forsheda and Dr. Jan Ukonsaari of Vattenfall Research and Development for providing lubricant and elastomer samples for these tests.

References 1. McClune C, Tabor D. An interferometric study of lubricated rotary face seals.

Tribology International 1978; 11: 219-227

2. Vicente J, Stokes J, Spikes H. Rolling and sliding friction in compliant, lubricated contact. Proc. IMechE Part J Engineering Tribology 2006; 220: 55-63.


3. Mofidi M, Kassfeldt E, Prakash B. Tribological behaviour of an elastomer aged in different oils. Tribology International 2008; 41: 860-866.

4. Vicente J, Spikes H, Stokes J. Viscosity Ratio Effect in the Emulsion Lubrication of Soft EHL Contact. Journal of Tribology 2006;128: 795-800.

5. Vicente J, Spikes H, Stokes J. Behaviour of complex fluids between highly deformable surfaces: isoviscous elastohydrodynamic lubrication. FRC Unilever R & D Culworth. Available at: http://www.mariecurie.org/annals/volume4/phy1.pdf, Last access: 2009-02-06

6. King R, Lancaster J. Wear of metals by elastomers in an abrasive environment. Wear 1980; 61: 341-352.

7. Zhang SW, Liu H, He R. Mechanisms of wear of steel by natural rubber in water medium. Wear 2004; 256: 226-232.

8. Mofidi M, Prakash B, Persson BNJ, Albohr O. Rubber friction on (apparently) smooth lubricated surfaces. Journal of Physics: Condensed Matter 2008; 20-8: 085223.

9. Torbacke M, Johansson A. Seal material and base fluid compatibility: an overview. Journal of Synthetic Lubrication 2005; 22-2: 123–142.

10. Persson BNJ. Theory of rubber friction and contact mechanics. Journal of Chemical Physics 2001; 115-8: 3840-3861.

11. Hofmann W. Rubber Technology Handbook.

12. Hanser Publishers: Munich, 1989.

13. Persson BNJ, Tartaglino U, Albohr O. Tosatti E. Rubber friction on wet and dry road surfaces: The sealing effect. Physical Review B 2005; 71-3: 035428.

14. Persson BNJ, Tartaglino U, Albohr O, Tosatti E. Sealing is at the origin of rubber slipping on wet roads. Nature Materials 2004; 3: 882 - 885.

http://www.mariecurie.org/annals/volume4/phy1.pdf


Paper B

667

Influence of counterface topography onsliding friction and wear of some elastomersunder dry sliding conditionsMMofidi and B Prakash∗

Division of Machine Elements, Luleå University of Technology, Luleå, Sweden

The manuscript was received on 30 October 2007 and was accepted after revision for publication on 14 April 2008.

DOI: 10.1243/13506501JET392

Abstract: In this work, the friction and wear behaviour of acrylonitrile butadiene rubber (NBR),hydrogenated acrylonitrile butadiene rubber (HNBR), acrylate rubber (ACM), and fluoroelas-tomer (FKM)against steel surfacesunderunidirectional dry sliding conditionshavebeen studied.The influenceof surface roughness of the steel counterfaceon friction andwearwas studiedusinga block-on-ring test configuration. At low load, the friction coefficient decreased after a running-in period and the wear was insignificant, especially for the ACM and FKM. The running-in timein terms of achieving a stable dry friction for the different elastomers, from longest to shortest,is in the order HNBR, NBR, FKM, and ACM, with an exception in case of FKM sliding against asmooth steel counterface. At higher contact pressure, powdery worn particles on the ACM and adecrease in friction coefficient were observed, but for FKM and HNBR, worn particles with rollshapeswere produced.Thewornparticles of FKMwere significantly larger than those of the othertested materials, and a considerably higher wear in FKMwas observed.

Keywords: elastomer, friction, wear

1 INTRODUCTION

Elastomers are characterized by low Young’s mod-ulus, large elongation-to-break, and high Poisson’sratio. These properties make them suitable for vari-ous sealing applications. Friction andwear are the twoimportant factors in seal performance and the over-all efficiency of the machine. Most seals operate inthe presence of oils during their service life but attimes, such as the starting-up of the machine, theyoperate in dry conditions. Although such periods areshort, the seal may exhibit high-friction coefficientand wear. Wear may affect the sealing ability. There-fore, the friction and wear behaviour of seal materialin dry conditions may play an important role in sealperformance.

∗Corresponding author: Division of Machine Elements, Luleå

University of Technology, Luleå SE-971 87, Sweden. email:

[email protected]

1.1 Friction

The coefficient of friction of rubber during slidingagainst a hard counterface may be attributed to thecontribution of adhesion, deformation (hysteresis),viscous, and tearing components [1, 2]. However;some researchers considered only two terms of fric-tion components. They considered that the tearingand viscous components can be represented by adhe-sive and deformation components, respectively [2–4].Adhesion is generally recognized to consist of themaking and breaking of junctions at a molecularlevel. Several theories have been proposed to describethe adhesion component of friction. These studieshave shown that the adhesive contribution to fric-tion of an elastomer against a hard surface decreaseswith decreasing Young’s modulus and it is a func-tion of the viscoelastic properties of the elastomer,which in turn depend on temperature and slidingvelocity [2, 5].The deformation component of friction is caused by

theflowingactionof theelastomerover the rigidasper-ities of the mating counterface and has been termed

JET392 © IMechE 2008 Proc. IMechE Vol. 222 Part J: J. Engineering Tribology


668 MMofidi and B Prakash

as hysteretic friction. The occurrence of hystereticfriction is a consequence of dampingwithin the visco-elastic body [6]. Similar to the adhesion componentof friction, the hysteretic component of friction is afunction of viscoelastic properties of the elastomer.Unlike the adhesive friction, the hysteretic frictionincreases with the decrease in Young’s modulus of theelastomer [2]. If the applied pressure is high, then therubber is squeezed into complete contact with thesubstrate. The magnitude of the hysteretic compo-nent of friction depends on h/λ (the ratio of asperityheight amplitude to the wavelength of the rough-ness) [3]. Thus, if the ratio between the amplitudeand wavelength is constant, the surface roughness ofdifferent length scales may contribute equally to thefriction force [3]. The minimum length scale, whichcontributes to the friction force, is limited by themicrostructure of rubber and the contamination ofcontact area [4].The contribution of adhesion and hysteresis to fric-

tion depends on the geometry and cleanliness ofthe mating surfaces. In many cases, especially inlubricated conditions, the hysteresis is the dominantcomponent of rubber friction. Even if the hard sur-face appears smooth to the naked eye, it may exhibitshort-wavelength roughness, which may make thedominant contribution to rubber friction [7]. Theadhesion component is dominant on very clean andsmooth surfaces [8–10]. It can alsobe significant at lowloads, even in lubricated conditions [11] because ofthe significance of the attractive van derWaals’ forcesbetween the surfaces in comparison with the normalload [12].

1.2 Wear

Wear of elastomers occurs as a result of two processes:localmechanical rupture (tearing) anddecompositionof the molecular network to a low molecular weight(smearing) [13]. The mechanical rupture of rubberagainst smooth hard substrates can be due to fatigueor frictional wear. The strength of rubber has a con-siderable effect on the wear resistance. A critical valueof shear stress can be defined for each rubber, abovewhich roll formation occurs and below which wear ismainly due to fatigue. Thus, the friction coefficientis one of the most important parameters governingthe type of wear [14]. Fatigue wear, as a result ofrepeated deformation cycles, takes place when rub-ber slides against hard and blunt projections on thehard surface at low frictional force [15]. The surface ofrubber, which is worn by frictional wear or roll for-mation, is characterized by ridges perpendicular tothe direction of sliding, but in fatigue wear, the wornsurface does not bear any visible ridges except pittingmarks [16].

1.3 Running-in

The determination of friction of rubber requires thespecification of the history of the two rubbing sur-faces. Wear of rubber, through tearing or smearing,can result in changes in the geometry and propertiesof the surface contact area and consequently the fric-tion force.When an unlubricated rubber slides againstthe same counterface repeatedly, a decrease in fric-tion may occur until a complete layer of material isdeposited from the rubber onto the opposing surface.However, when the rubber slides continuously againsta fresh counterface, the friction force increases [17].The metal may also wear during running-in period.

When a rubber slides against a metallic counterface,molecular segments of the freshly ruptured rubbermay adhere to the metallic surface under the actionof van der Waals’ forces, forming a lubricating layerof rubbery material. The free radicals of segments inthe rubbery layer react with the metallic oxide surfaceandproduce ametal oxide–polymer complex,which isweaker than the metal oxide itself and detaches moreeasily from the surface [18]. This process results in thewear of themetallic countersurface and the formationof a layer which has properties quite different fromboth the rubber and the metal affecting the frictionforce.In particular situations, such as starting periods,

seals may operate in dry conditions, which can affecttheirperformance significantly.Understanding the tri-bological behaviour of seal materials during the initialstart-up and run-in periods is relevant for predictingthe performance of the seal. Regarding the applica-tion, the contact pressure between an elastomeric sealand a sealing surface may vary from a few tens ofkPa to a few tens of MPa [19]. The aim of this studyis to investigate the friction and wear behaviour offour sealing elastomers in dry sliding conditions. Theinfluences of surface topography of countersurface,contact pressure, and sliding speed on the friction andwear characteristicsduring the run-inperiodhavealsobeen studied.

2 EXPERIMENTALWORK

The experiments were carried out using Micro-Tribometer UMT-2. A rubber specimen glued to ametal backing plate was pressed against a rotatingsteel ringcounterface.Thenormaland frictional forceswere recorded by piezoelectric sensors. The schematicof the test configuration is shown in Fig. 1. Threesets of bearing steel rings with different ranges ofsurface roughness were used to study the effect of sur-face roughness on friction and wear. Figure 2 showsthe typical surfaces of the three sets of the steelrings. The dimensions of the rubber specimen were

Proc. IMechE Vol. 222 Part J: J. Engineering Tribology JET392 © IMechE 2008

Influence of counterface topography on sliding friction and wear 669

Fig. 1 Test configuration

16 × 4 × 2mm (the width of the contact area was4mm). The rubber samples were cut from sheets of2mm thickness. The counterface bearing steel ringswere of 35mm (outer diameter) and 8mm thick.The rubber specimens were washed in industrialpetroleum for 3min using an ultrasonic cleaner, driedin an oven for 10min at 40 ◦C, and then weighed. Thesame procedure was repeated after running the teston each specimen in order to quantify wear. Each ringwas washed in industrial petroleum for 3min by usingthe ultrasonic cleaner and dried before the test. A newring was used for each test.The elastomers studied during this work are com-

monly used seal materials, acrylonitrile butadienerubber (NBR), hydrogenated acrylonitrile butadienerubber (HNBR), acrylate rubber (ACM), and fluoroe-lastomer (FKM). All the elastomers have a module ofelasticity of about 10MPa at very low speed and roomtemperature. The nominal hardness, tensile strength,elongation at break, and material densities of theseelastomers are given in Table 1. The surfaces of thebearing steel rings were characterized by a Wyco 3Doptical surface profilometer. Figure 2 shows the typi-cal surface topographies of the three sets of rings andthe range of average surface roughness (Ra).The experiments were carried out at two contact

pressures. The wear and frictional behaviour of thematerial (running-in and steady-state friction) werestudied. At low-contact pressure, the influence of sur-face roughness was also investigated. Using the Hertzcontact theory, the average contact pressure at lowload (1.5N) is estimated to be about 240 kPa, which

Table 1 Experimental elastomers and their properties

TensileElastomeric Hardness strength Elongation Densitymaterials (Shore A) (MPa) at break (%) (g/cm3)

Nitrile rubber(NBR 3143)

76.1 25.4 466 1.31

Hydrogenatednitrile rubber(HNBR 7611)

71.3 17.5 303 1.24

Acrylic rubber(ACM)

73.4 7.8 171 1.49

Fluoro rubber(FKM 7327)

72.8 – – 2.03

is of the order of the contact pressure on a new elas-tomeric radial lip seal. At higher load (10N), anaveragecontact pressure of 620 kPa is expected, which is inthe range of the contact pressure between an elas-tomeric O-ring and an actuator rod. Each test was runfor 12h duration at a speed of 10 r/min. Some shortduration tests were also conducted at speeds of 1 and100 r/min, respectively. All the tests were performed atroom temperature (22 ± 2 ◦C) and repeated twice.

3 RESULTS ANDDISCUSSION

3.1 Friction

Friction coefficients of the elstomers were affected bychanges in the contacting surfaces. These results arepresented and discussed below.

3.1.1 Frictional behaviour at high contact pressure

Figure 3 shows the worn surfaces of different elas-tomers tested at a high contact pressure of 620 kPa.As shown in the figure, although the worn particles ofbothHNBRandFKMhavea roll shape, thewearmech-anisms are different. The ridges on theworn surface ofHNBR show that the worn particles of HNBR initiallymay be small but eventually agglomerate along thecontact area. Theworn surface of FKMhowever showsthat the worn particles are relatively large (Fig. 4). Asshown in the figure, a white powdery layer has beenformed on the surface of ACM and the surface of NBRwas torn locallybutdidnot indicateany roll formation.

Fig. 2 Surface topographies of steel rings



Fig. 3 Worn surfaces of the tested rubbers (normal load:10N, speed: 10 r/min, duration: 12 h, surfaceroughness of rings: 0.35–0.55μ)

Figure 5 shows the friction coefficient of the testedrubbers against the bearing steel rings of mediumroughness. The highest friction coefficient has beenobserved in tribological pairs involving FKM followedbyHNBR, whichmay, in part, be due to the energy dis-sipated in the tearing and roll formation. The frictioncoefficient of ACM increases gradually during the test,which may be due to the changes in the propertiesand/or the dimensions of the particles in the powderylayer between the surfaces.

3.1.2 Frictional behaviour at low contact pressure

Figures 6 to 9 show the friction coefficients of NBR,HNBR, FKM, and ACM, respectively. The friction coef-ficientsdropduring running-inperiods to steady-statevalues, with the exception for FKM sliding against finecountersurfaces. The longest running-in periods havebeen observed during sliding against fine counter-surfaces. The shortest running-in time was observedfor FKM. The decrease in friction coefficient during

Fig. 4 Worn particles of NBR, HNBR, and FKM

Fig. 5 Friction coefficient versus time (normal load:10N, speed: 10 r/min, surface roughness, Ra:0.35–0.55μ)

Fig. 6 Friction coefficient of NBR versus time (normalload: 1.50N, speed: 10 r/min)



Fig. 7 Friction coefficient of HNBR versus time (normalload: 1.50N, speed: 10 r/min)

Fig. 8 Friction coefficient of FKM versus time (normalload: 1.50N, speed: 10 r/min)

Fig. 9 Friction coefficient of ACM versus time (normalload: 1.50N, speed: 10 r/min)

running-in periods may be due to the deposition ofa layer of filler and low-molecular components fromthe rubber onto the surface of the ring. Smearing as aresult of the decomposition of the molecular networkto low molecular components may be another reasonfor the decrease in friction coefficient.The average value of the friction coefficient was

calculated from the steady-state friction-coefficientvalues during the last 5000 s of the test (Fig. 10). Theerror bars in the figure indicate the difference betweenthe results of repeated tests. The steady-state values

Fig. 10 Steady-state friction coefficient (average valueof friction coefficient)

of friction coefficients increase as the surface rough-ness decreases, with exception for FKM. The surfaceroughness has theminimumandmaximumeffects onthe steady-state friction coefficient of ACMandHNBR,respectively.A low friction layer, comprising fine particles, has

been formed on the sliding surface of ACM, and thedeformation of the surface of the rubber is moreaffected by the dimensions of the particles than thesurface roughness of the ring. Thus, the hysteresiscomponent (which is the most dominant componentof steady-state friction) is not much affected by theroughness of the ring.The steady-state friction coefficient of FKM against

the rough surface is higher than that against the sur-face with medium surface roughness. Further inves-tigations, taking into account the various materialproperties and the surface roughness of the ring atnanoscale, are required in order to clearly explain thiseffect.The slight local increase in friction coefficients seen

in some cases (ACM in Fig. 5, NBR-fine counterfacein Fig. 6, and FKM-medium counterface in Fig. 8) issomewhat unclear, and further investigations may berequired to explain this behaviour. Some possible rea-sons for this behaviourmay be the formation/removalof the transfer layer on/from the interacting surfacesduring sliding. For example, the transfer layer in thecontact between the FKM and the hard counterfacemay form quickly but its properties may be modi-fied during sliding, resulting in higher friction. But inthe case of HNBR, the transfer layer may form rathergradually and consequently the friction decreasesmonotonically.

3.1.3 Frictional behaviour at different speeds

Figure 11 shows the friction coefficient of the differ-ent elastomers during sliding against steel rings ofmedium roughness at three different sliding veloci-ties. These results in Fig. 11 show that the running-in



Fig. 11 Friction coefficients of elastomers at different speeds (normal load: 1.50N, surfaceroughness, Ra: 0.35–0.55μ)

duration increases as the sliding velocity decreases. Itmay however be noted that in these figures the slidingdistances at each speed differ for a specific test dura-tion. As shown in Figs 6 to 8, the running-in periodsfor NBR and HNBR and FKM are longer than 50minwhen the sliding is 1 and 10 r/min. Thus, the frictioncoefficient of NBR and HNBR and FKM (Fig. 11) didnot reach the steady-state values at low speeds, but athigh speed (100 r/min), the friction coefficients seemto reach the steady-state values, which are close tothose in Figs 6 and 7.In general, there is no clear trend in the variation

of coefficients of friction as a function of speed, butfriction tends to decrease when the sliding speed isincreased to 100 r/min in all the elastomers exceptthose in case of ACM. The results for ACM show thatthe steady-state value of friction coefficient increaseswith the sliding velocity.

3.2 Wear

At low contact pressure, no severe wear occurred onthe surface of elastomers, and the lost mass of thetested elastomers is very small (<0.5mg). Figure 12shows the worn mass of the tested elastomers at highcontact pressure. Roll formation occurred on the sur-face of FKM, resulting in severe wear (Fig. 4). Althoughthe roll formation occurred on the surface of HNBR,the amount and dimensions of the worn particles are

Fig. 12 Worn mass of elastomers (normal load: 10N,speed: 10 r/min, duration: 12 h, surface rough-ness Ra: 0.35–0.55μ)

much smaller than those of FKM. Consequently, thewornmass ofHNBRwasmuch lower than that of FKM.The surface of ACM was also worn and fine worn par-ticles were produced on the sliding surface, but thesurface of NBR was torn locally and the worn masswas low.Overall, these results (Figs 6 to 9) have shown

that under dry sliding and other operating conditionsof load and speed, the ACM elastomer has shown



superior frictional characteristics, whereas the elas-tomerNBR has resulted in superior wear performancevis-a-vis the other elastomeric materials. Further, itmay also be noted that the running-in characteris-tics (i.e. the magnitude of friction coefficient as wellas the running-in time to steady-state friction) of theNBR elastomer are superior to those of the other elas-tomericmaterials. Itmayhowever be kept inmind thatother than friction, wear, and running-in behaviour,the choice of a sealmaterial will depend upon the typeof lubricant, lubricant-elastomer compatibility, lubri-cation regime, and the operating environment, andfurther research is required inorder to enable selectionof the most appropriate material for a specific sealingapplication.

4 CONCLUSIONS

The frictional characteristics of four different sealmaterials (NBR, HNBR, ACM, and FKM) during slid-ing against steel counterfaces of varying roughnessvalues have been investigated. At low contact pres-sure, the results show that the friction coefficientsdrop during running-in periods to steady-state val-ues. The longest running-in periods were observedduring sliding against smooth surfaces. Apart fromFKM, the steady-state friction coefficient increasesas the surface roughness decreases. The surfaceroughness has the least and most effects on thesteady-state friction coefficient of ACM and HNBR,respectively.

ACKNOWLEDGEMENTS

All elastomeric materials and steel rings used in thiswork were supplied by Mr Stellario Barbera (SKFSealing Solutions, Italy) and Mr Joop Vree (SKF Engi-neering Research Centre, The Netherlands) and theauthors thankfully acknowledge their support. Theauthors express their gratitude to Dr Richard Schaake(SKF ERC) for his useful suggestions concerning thismanuscript. Finally, the authors also thank the SKFERC management for their permission to publish thiswork.

REFERENCES

1 Ludema, K. C. Physical factors in tyre friction. Phys.Technol., 1975, 6, 11.

2 Moore, D. F. The friction and lubrication of elastomers,1st edition, 1972 (Pergamon Press, NewYork).

3 Persson, B. N. J. Theory of rubber friction and contactmechanics. J. Chem. Phys., 2001, 115–8, 3840–3861.

4 Persson,B. N. J.On the nature of surface roughness withapplication to contact mechanics, sealing, rubber fric-tion and adhesion. J. Phys. Condens. Matter, 2005, 17,R1–R62.

5 Moore,D. F.A reviewof adhesion theories for elastomers.Wear, 1972, 22, 113–141.

6 Moore, D. F. A review of hysteresis theories for elas-tomers.Wear, 1974, 30, 1–34.

7 Mofidi,M., Prakash, B., Persson, B. N. J., and Albohr, O.Rubber friction on (apparently) smooth lubricated sur-faces. J. Phys. Condens. Matter, 2008, 20(8), 085223.

8 Persson, B. N. J. On the theory of rubber friction. Surf.Sci., 1998, 401, 445–454.

9 Persson, B. N. J. and Volokitin, A. I. Rubber friction onsmooth surfaces. Eur. Phys. J., 2006, E 21–1, 69–80.

10 Fuller,K. N. G. andTabor,D.The effect of surface rough-ness on the adhesion of elastic solids. Proc. R. Soc. Lond.Ser. A,Math. Phys. Sci., 1975, 345(1642), 327–342.

11 Greenwood, J. A. and Tabor, D. The friction of hard slid-ers on lubricated rubber: the importance of deformationlosses. Proc. Phys. Soc., 1958, 71, 989–1001.

12 Johnson, K. L., Kendall, K., and Roberts, A. D. Surfaceenergy and the contact of Elastic Solids. Proc. R. Soc.Lond. Ser. A,Math. Phys. Sci., 1971, 324(1558), 301–313.

13 Gent, A. N. Pulford, mechanisms of rubber abrasion.J. Appl. Polym. Sci., 1983, 28, 943.

14 James, D. I. and Jolley, M. E. Abrasion of rubber, 1964(MacLaren and Sons Ltd., London).

15 Zhang, S. W. Tribology of elastomers, 1st edition, 2004(Elsevier, Netherlands).

16 Thavamani,P.,Khastgir,D.,andBhowmick,A. K.Micro-scopic studies on the mechanisms of wear of NR,SBR, andHNBR vulcanizates under different conditions.J. Mater. Sci., 1993, 28, 6318.

17 Cooper, R. P. and Ellis, B. The effect of run-in on rubberfriction. J. Appl. Polym. Sci., 1982, 27, 4735.

18 Ab-Malek, K. and Stevenson, A. On the lubrication andwear of metal by rubber. J. Mater. Sci., 1984, 19, 585.

19 Friction, lubrication, andwear technology. InASMhand-book, vol. 18, 1992 (American Society for Metals, Materi-als Park, Ohio).



Paper C

Tribology International 41 (2008) 860–866

Tribological behaviour of an elastomer aged in different oils

Mohammadreza Mofidi�, Elisabet Kassfeldt, Braham Prakash

Department of Applied Physics and Mechanical Engineering, Lulea University of Technology, Lulea SE-971 87, Sweden

Received 31 January 2007; received in revised form 13 November 2007; accepted 22 November 2007

Available online 11 January 2008

Abstract

This paper presents the influence of aging the nitrile rubber, the most popular seal material, in various base fluids on sliding friction

and abrasive wear. The lubricants used are synthetic esters, natural esters, different types of mineral base oils, poly-a-olefins and very

high viscosity index oils. Friction has been studied for two directions of motion with respect to lay on the elastomer sample by using the

SRV Optimol test machine. These findings show that as compared to all other lubricant formulations, ageing the elastomer in polyol

ester leads to the maximum reduction of friction coefficient especially in perpendicular sliding to the initial lay on the surface. The

abrasive wear studies were carried out by using a two-body abrasive wear tester against dry and lubricated elastomer. It was interesting to

note that two-body abrasive wear of elastomeric material was higher during rubbing in presence of the fluids as compared to that in dry

condition. Further, aging the elastomer in these base fluids especially in ester base fluids, results in more abrasive wear.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Elastomers; Lubricating fluids; Friction; Abrasive wear

1. Introduction

Elastomers have some very useful properties such as lowYoung’s modulus, large elongation-to-break and highvalue of Poisson’s ratio which make them suitable formany sealing applications. Seal is a component whichprevents the leakage of fluids or gas from the machine andcontamination entering the machine. Most seals operate inpresence of oils during their service life. Friction and wearare two important factors in seal performance and theoverall efficiency of the machine. Therefore, the interactionbetween oils and elastomer and its influence on friction andwear behaviour of elastomer has an important role in sealperformance. The aim of this study is to investigate theinfluence of aging nitrile rubber in different oils on itstribological behaviour.

1.1. Friction

The coefficient of friction of a rubber surface duringsliding against a hard surface in lubricated conditionscan be expressed in terms of the contribution of liquid,adhesion and deformation (hysteresis) components. Thecontribution of adhesion component is related to theasperity peaks where the fluid film is extremely thin and hasproperties distinct from the bulk lubricant in the voids. Thelubricant film at asperities peaks has some of the propertiesof draped elastomer and shear strength at these areas isconsiderably higher than those of other areas [1]. Adhesionis generally recognised to consist in the making andbreaking of junctions at a molecular level. Several theorieshave been proposed to describe the adhesion component offriction. These studies have confirmed that the adhesionfriction of elastomer against hard surface decreases withthe decrease in the Young’s modulus and it is a function ofviscoelastic properties of elastomer which in turn dependon temperature and sliding velocity [2,3].In contrast to the ploughing action of metal-on-metal

friction, the sliding elastomer flows readily over the rigidasperities of the mating counterface and conform to their

ARTICLE IN PRESS

www.elsevier.com/locate/triboint

0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.triboint.2007.11.013

�Corresponding author. Tel.: +46920 491038; fax: +46 920 491047.

E-mail addresses: [email protected] (M. Mofidi),

[email protected] (E. Kassfeldt), [email protected]

(B. Prakash).

http://www.elsevier.com/locate/triboint




contours. The deformation component of friction pro-duced by such flowing action is called hysteresis. Theexistence of hysteresis friction is a consequence of energyloss associated with internal damping within the viscoelas-tic body [4]. Like the adhesion component of friction, thehysteresis component of friction is also a function ofviscoelastic properties of the elastomer, but unlike theadhesion friction, the hysteresis friction increases withdecrease in Young’s modulus of elastomer [1].

The fraction of contribution of adhesion and hysteresisfriction depends on the geometry and cleanliness of themating surfaces. The adhesion component is importantonly for very clean and smooth rubber surfaces [5,6]. Themain source of friction in well lubricated sliding arises fromdeformation [7,8]. Greenwood and Tabor performed sometests with spherical and conical specimens sliding againstrubber. They stated that the sliding friction of sphericalspecimen, in a well lubricated condition at high loads, is thesame as rolling friction but at low loads the sliding frictionis larger than rolling friction. They have concluded that asthe load is reduced, the shearing term becomes moreimportant [7]. The significance of adhesion friction at lowload can be interpreted as the attractive forces as demon-strated by Johnson et al. [9]. Presence of fluid between rubberand hard substrate reduces not only the adhesion but also thehysteresis component of friction. On a lubricated substratethe valleys turn into fluid pools which are sealed off andeffectively smoothen the substrate surface. Smootheningreduces the viscoelastic deformation from the surfaceasperities, and thus reduces rubber friction [10,11].

1.2. Abrasion

Abrasion occurs as a result of local mechanical rupture(tearing) and/or general decomposition of the molecularnetwork to a low-molecular-weight material (smearing)[12]. If the hard sliding specimen against rubber is sharp,the abrasion results from tensile failure and if it is blunt,the abrasion results from fatigue failure. Schallamach hasstudied the rubber abrasion in dry condition and reportedthat the abrasion is proportional to normal load andproportional to the mean radius of asperity curvature ifthey can be approximated to hemisphere and independentof particle size if the particles are polyhedral [13]. Thespacing of the abrasion pattern is proportional to the cuberoot of the normal load, proportional to the two-thirdspower of the particle size of the abrasive with polyhedralparticles, and directly proportional to the size of abrasivehemispherical particles [13]. Later on Grosch and Schalla-mach found that on sharp tracks such as abrasive paperlinear wear rate as a result of tensile failure, wasproportional to the normal stress, friction coefficient andinversely proportional to the energy density at break [14].Southern and Thomas studied abrasion of rubber surfaceby a razor blade in line contact and formulated a theoryrelating the rate of abrasion is to the crack growthcharacteristics of the rubber, the angle of crack growth

and the frictional force on the blade [15]. Zhang and Yanghave introduced a theoretical wear equation of rubberabrasion in a line contact from the viewpoint of energy onthe basis of experimental results [16]. Muhr et al. havestudied the influence of lubrication on the abrasion of rubberby blade in line contact. They observed that when a lubricantis applied, a much finer pattern develops and the rate ofabrasion is much lower but the horizontal force on the bladedoes not decrease so dramatically [17,18]. Chandrasekaranand Batchelor have studied the friction and wear of butylrubber sliding on abrasive paper as a function of temperatureand load. They conducted dry and lubricated unidirectionalsliding tests and reported that the presence of lubricantreduced the coefficient of friction but accelerated wear due tochemical degradation of rubber [19].

1.3. Seal-oil compatibility

Elastomers can swell and/or degrade in chemical sealenvironments through reactions with the polymer back-bone and crosslink system, or by reactions with the fillersystem [20]. Presence of the polar side-groups in thebackbone chain increases the oil resistance of the polymer[21]. Crosslinking also limits the degree of polymer swellingby providing tie-points (constrains) that limit the amountof solvent that can be absorbed into the polymer [22].Nitrile rubber is a copolymer of acrylonitrile and buta-diene. NBR is a low-cost elastomer with good mechanicalproperties. The concentration of acrylonitrile in thecopolymer has a considerable influence on the polarityand swell resistance of the vulcanizates in non-polarsolvents. The greater the acrylonitrile content, the lessthe swell in motor fuels, oils, fats, etc. [22]. However, theelasticity and low temperature flexibility also becomepoorer. The mechanical properties of elastomers areaffected by oils. Van der Waal [23] and Torbacke andJohansson [24] have studied the influence of different basefluids on the changes in mechanical properties of elasto-mers. Generally, the influence of ester base fluids ondeterioration of nitrile rubber is more significant incomparison to that of mineral oils and PAOs [25]. Someelastomers are sensitive to polar compounds. The differ-ence between the chemical structure of ester base fluids andmineral oils and PAOs is the existence of more carboxylicgroups in ester base fluids which are polar groups.

2. Experimental work

Two series of experiments, including friction and two-body abrasion tests have been performed. The influences ofageing the elastomer in different types of base oils on thefriction coefficient and abrasive wear have been investigated.

2.1. Friction tests

The friction tests have been carried out by usingOptimol SRV machine. The machine reciprocates an upper

ARTICLE IN PRESSM. Mofidi et al. / Tribology International 41 (2008) 860–866 861

cylindrical specimen loaded against lower specimen. Themotion of cylinder is parallel to its axis. The friction forceis measured by piezoelectric force sensors. Temperature,normal force, frequency of motion and stroke length can becontrolled during the tests. The diameter of the cylinder is15mm and its length is 22mm. The edges of the slider arechamfered/rounded off with a view to minimise the edgeeffect. Fig. 1 shows the test configuration and the surfacetopography of the slider.

The rubber specimens used in friction tests were washedin industrial petroleum for 3min by using an ultrasoniccleaner and then dried for 10min.

2.2. Abrasion tests

The two-body abrasion tests were conducted by usingan abrasive wear tester (Fig. 2). It consists of a table thatholds the specimen and reciprocates it against an abrasive

paper wrapped around the circumferential surface of awheel (+ 50mm� 12mm thick). The wheel is turned by afraction of one rotation at the end of each stroke so as toenable the rubbing of elastomer against fresh abrasivesurface. The frequency of reciprocation of the test speci-men was 100 cpm and the stroke length was 30mm. Allabrasive wear tests were run for a total of 160 cycles. Allthese tests have been done at room temperature (2272 1C).The rubber test specimens were washed in industrial

petroleum for 3min by an ultrasonic cleaner, dried in anoven for 20min at 45 1C and then weighed. The sameprocedure was repeated after running the test for eachspecimen to quantify abrasive wear.

2.3. Test materials and lubricants

The elastomer which has been studied is acrylonitrilebutadiene rubber (NBR). The content of acrylonitrile inthe tested elastomer is 28%, which is common for oilapplications. This elastomeric material is vulcanised bysulphur. The polymeric content is 44% and the remainingpart consists of different types of additives. The rubbersamples used in these studies were in the form of sheets of4mm thickness. The initial hardness of the elastomer was7575 IRHD (international rubber hardness degrees). Thesurface was examined in a Wyko 3D optical surfaceprofilometer. The elastomer surface was characterised byparallel grooves (Fig. 3) and are caused during moulding ofelastomeric sheets in steel mould.The test specimens for tribological studies were cut out

from the elastomeric sheets. For friction studies, discs of+ 25 and 4mm thickness were used. In abrasive weartests, rectangular sheet specimens of 40mm� 20mm and4mm thickness were used.The lubricating fluids used were paraffinic, naphthenic,

two PAOs, two VHVI, monoester, diester, polyolester,complex ester and rapeseed oils. The base fluids used inthese studies and their properties are listed in Table 1.The aged elastomer samples were prepared by immersing

them in different base fluids at 125 1C for one week.

3. Results and discussion

Fig. 4 shows the average values of friction coefficientsduring sliding of bearing steel cylinder against the non-aged rubber sample in the presence of different base fluids.It shows that there is no correlation between the friction

coefficient and viscosity. It means that the sliding occurringmainly is in boundary or mixed lubrication regime. Whenthe slider reciprocates parallel to the direction of lay on therubber surface, the friction coefficient is marginally higherthan that in perpendicular sliding. In perpendicular sliding,the oil can be trapped within the grooves which may resultin some hydrodynamic effects and subsequently lower thefriction coefficient. It confirms the Patir and Cheng’s [25]results for rubber.

ARTICLE IN PRESS

Fig. 1. Test configuration for friction studies under reciprocating sliding

conditions by using Optimol SRV machine (left); surface topography of

the slider, Ra ¼ 80 nm, Rq ¼ 107 nm (right).

Fig. 2. Test configuration for abrasive wear studies under reciprocating

sliding.

M. Mofidi et al. / Tribology International 41 (2008) 860–866862

Fig. 5 shows the average values of friction coefficients onaged rubbers in different base fluids. Comparison offriction results in Figs. 4 and 5 shows that aging therubber in synthetic esters leads to decrease in frictioncoefficient.Investigation of the surfaces of the tested specimens

shows that the worn surface areas of the aged specimensare considerably less than those of non-aged specimens.The two-body abrasive wear results of aged and non-

aged rubbers in dry and lubricated conditions have beenshown in Fig. 6. It shows that the abrasive wear of bothaged and non-aged nitrile rubber in lubricated condition ishigher than that in dry condition.

ARTICLE IN PRESS

Fig. 3. Surface topography of the fresh elastomer sample (Ra ¼ 0.344mm, Rq ¼ 0.438mm).

Table 1

The properties of lubricants

Base fluid Density (kg/m3) Viscosity at 40 1C (cSt) NPI

Naphthenic base oil 896 30.0 –

PAO2 830 28.5 –

PAO1 790 5.5 –

VHVI2 830 26.0 –

VHVI1 822 12.0 –

Paraffinic base oil 870 34.1 –

Diester 910 (20 1C) 26.1 82

Complex ester 980 46.0 80

Polyol ester 900 35.5 170

Monoester 864 (20 1C) 8.5 102

Rapeseed oil 910 (30 1C) 34.0 190

Fig. 4. Coefficients of friction of non-aged samples in different base oils

(temperature: 40 1C, load: 100N, frequency: 50Hz, stroke: 1mm, test

duration: 15min).

Fig. 5. Coefficients of friction of aged samples in different base oils

(temperature: 40 1C, load: 100N, frequency: 50Hz, stroke: 1mm, test

duration: 15min).

M. Mofidi et al. / Tribology International 41 (2008) 860–866 863

Aging the rubber in any base fluid, especially in esterbase fluids and rapeseed oil leads to more abrasive wear aswell, but the influence of presence of oil is more significantthan that of aging especially for mineral oils. Use of themonoester base fluid has resulted in highest abrasive wearof non-aged and aged rubber samples.

The changes in the mechanical properties of the samecompound of nitrile rubber in the same base fluids wereinvestigated previously by Torbacke and Johansson [24].Our results have not shown any clear correlation between

the changes in friction coefficient and changes in mechan-ical properties due to aging in different lubricatingfluids (Fig. 7). It seems that the changes in the physico-chemical properties of the surface of exposed rubber,rather than changes in mechanical properties may play amore significant role in determining the frictional beha-viour. Fig. 8 shows the correlation between the abrasivewear and the changes in mechanical properties. Thedecrease in tensile strength of rubber due to aging inmonoester fluid seems to cause relatively higher abrasivewear.Fig. 9 shows the abrasive wear and friction coefficient

as a function of the non-polarity index of oils (NPI).It can be seen that in both aged and non-aged samplesabrasive wear is maximum when the monoester with non-polarity index 102 is used. However, non-polarity indexdoes not seem to have any influence on frictionalbehaviour.

4. Conclusion

Ageing the nitrile rubber in the synthetic ester base fluidsleads to reduction of friction coefficient. This effect inreducing the friction coefficient, especially in perpendicularsliding to the initial lay on the surface, is more considerablefor the sample aged in polyol ester. The presence of thebase fluids increases the abrasive wear of tested nitrilerubber. Ageing the nitrile rubber in the lubricating fluidsincreases the abrasive wear in both dry and lubricated

ARTICLE IN PRESS

Fig. 6. Abrasive wear of aged and non-aged elastomer by using different

base fluids (abrasive grit size: # 500, load: 500 g, speed: 100 cpm, abrasive

wheel rotation: 1/200, total cycles: 160).

Fig. 7. Friction coefficient vs. the changes in mechanical properties of aged rubber in different base fluids.

M. Mofidi et al. / Tribology International 41 (2008) 860–866864

conditions. The maximum change in abrasive wear due toaging is observed for the samples aged in monoester. Dryabrasive wear of the aged nitrile rubber in ester base fluids

is higher than that in the mineral oils. The influence ofpresence of oil on increasing the abrasive wear is moresignificant than that of aging.

ARTICLE IN PRESS

Fig. 8. Abrasive wear vs. the changes in mechanical properties of aged rubber in different base fluids.

Fig. 9. Abrasive wear and friction coefficient vs. the non-polarity index (NPI) of oils.

M. Mofidi et al. / Tribology International 41 (2008) 860–866 865

References

[1] Moore DF. The friction and lubrication of elastomers. Pergamon

Press; 1972.

[2] Moore DF. A review of adhesion theories for elastomers. Wear

1972;22:113–41.

[3] Persson BNJ. On the theory of rubber friction. Surf Sci

1998;401:445–54.

[4] Moore DF. A review of hysteresis theories for elastomers. Wear

1974;30:1–34.

[5] Persson BNJ, Volokitin AI. Rubber friction on smooth surfaces. Eur

Phys J E 2006;21(1):69–80.

[6] Fuller KNG, Tabor D. The effect of surface roughness on the

adhesion of elastic solids. Proc R Soc London. Ser A, Math Phys Sci

1975;345(1642):327–42.

[7] Greenwood JA, Tabor D. The friction of hard sliders on lubricated

rubber: the importance of deformation losses. Proc Phys Soc

1958;71:989–1001.

[8] Trachman EG, Williams R, Sheng P. Orientation effects in the

friction of a hard ellipsoid sliding on rubber. J Appl Phys

1977;48(8):3270–3.

[9] Johnson KL, Kendall K, Roberts AD. Surface energy and the contact

of elastic solids. Proc R Soc London. Ser A, Math Phys Sci

1971;324(1558):301–13.

[10] Persson BNJ, Albohr O, Tartaglino U, Volokitin AI, Tosatti E. On

the nature of surface roughness with application to contact

mechanics, sealing, rubber friction and adhesion. J Phys Condens

Matter 2005;17(1):R1–R62.

[11] Persson BNJ, Tartaglino U, Albohr O, Tosatti E. Sealing is at the

origin of rubber slipping on wet roads. Nat Mater 2004;3(12):882–5.

[12] Gent AN, Pulford CTR. Mechanisms of rubber abrasion. J Appl

Polym Sci 1983;28:943–60.

[13] Schallamach A. On the abrasion of rubber. Proc Phys Soc 1954;B

67:883–91.

[14] Grosch KA, Schallamach A. Relation between abrasion and strength

of rubber. Rubber Chem. Technol 1966;39:287.

[15] Southern E, Thomas AG. Studies of rubber abrasion. Rubber Chem

Technol 1979;52(4):1008–18.

[16] Zhang SW, Yang Z. Energy theory of rubber abrasion by a line

contact. Tribol Int 1997;30(12):839–43.

[17] Muhr AH, Roberts AD. Rubber abrasion and wear. Wear

1992;158:213–28.

[18] Muhr AH, Pond TJ, Thomas AG. Abrasion of rubber and the effect

of lubricants. J Chim Phys 1987;84:331.

[19] Chandrasekaran M, Batchelor AW. In situ observation of sliding

wear tests of butyl rubber in the presence of lubricants in an X-ray

microfocus instrument. Wear 1997;211:35–43.

[20] Information obtained from the website: /http://www.pspglobal.com/

prop-chemical-compatibility.htmlS.

[21] Patil AO, Coolbaugh TS. A literature review with emphasis on oil

resistance. Rubber Chem Technol 2005;78(3):516.

[22] Hofmann W. Rubber technology handbook. Munich: Hanser; 2001.

[23] Van der Waal G. The relationship between the chemical structure of

ester base fluids and their influence on elastomer seals, and wear

characteristics. J Synth Lubr 1984;1–4:35–47.

[24] Torbacke M, Johansson A. Seal material and base fluid compat-

ibility: an overview. J Synth Lubr 2005;22(2):123–42.

[25] Patir N, Cheng HS. An average flow model for determining effects of

tree-dimensional roughness on partial hydrodynamics lubrication.

J Lubr Technol 1978;100:12–7.

ARTICLE IN PRESSM. Mofidi et al. / Tribology International 41 (2008) 860–866866

http://www.pspglobal.com


Paper D

IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 20 (2008) 085223 (8pp) doi:10.1088/0953-8984/20/8/085223

Rubber friction on (apparently) smoothlubricated surfacesM Mofidi1, B Prakash1, B N J Persson2 and O Albohr3

1 Division of Machine Elements, Lulea University of Technology, Lulea SE-97187, Sweden2 IFF, FZ-Julich, 52425 Julich, Germany3 Pirelli Deutschland AG, 64733 Hochst/Odenwald, Postfach 1120, Germany

Received 19 December 2007, in final form 17 January 2008Published 7 February 2008Online at stacks.iop.org/JPhysCM/20/085223

AbstractWe study rubber sliding friction on hard lubricated surfaces. We show that even if the hardsurface appears smooth to the naked eye, it may exhibit short-wavelength roughness, whichmay make the dominant contribution to rubber friction. That is, the observed sliding friction ismainly due to the viscoelastic deformations of the rubber by the counterface surface asperities.The results presented are of great importance for rubber sealing and other rubber applicationsinvolving (apparently) smooth surfaces.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Rubber friction on smooth surfaces is a topic of great practicalimportance, e.g., for rubber sealing, wiper blades or for thecontact between a tire and the metal rim [1]. For perfectlysmooth surfaces rubber friction is believed to be due to periodiccycles of pinning, elastic deformation, and rapid slip of rubbermolecules [2–4] or, more likely, small patches [5] of the rubberat the sliding interface. In a recent publication, Vorvolakos andChaudhury [6] (see also [7, 8]) have studied rubber frictionfor a silicone elastomer sliding on extremely smooth Si wafer,with the root-mean-square roughness ≈0.5 nm, covered byinert self-assembled monolayer films. The observed frictionas a function of the sliding velocity exhibit a bell-like shapeas expected from theory [2, 5]. However, a surface whichappears smooth to the naked eye may exhibit strong surfaceroughness at short length scales, e.g., at the micrometer andnanometer length scale. This is true even for highly polishedsurfaces which may appear perfectly smooth to the nakedeye. When a rubber block slides on a hard surface withsurface roughness, a large contribution to the friction forcemay arise from the time-dependent, substrate asperity-induceddeformations of the rubber surface. That is, during sliding thesubstrate asperities give rise to pulsating deformations of therubber, which will result in energy dissipation because of theinternal friction of the rubber. This is believed to be the majorcontribution to the tire-road friction [9, 10]. In this paper wewill show that the roughness of a highly polished steel surfacemay also give the dominant contribution to the friction, evenfor lubricated surfaces. This result is very important for rubber

sealing applications [11], in particular at low sliding velocitiesand low temperatures.

2. Rubber friction: experimental results

Friction tests have been carried out using a reciprocatingtribometer where a steel cylinder (diameter D = 1.5 cm andlength L = 2.2 cm) is squeezed against the substrate (rubberblock, thickness 4 mm), see figure 1. The steel cylinderperforms longitudinal oscillations against the rubber blockwith a stroke a = 1 mm and frequency f = 50 Hz. This givesthe average slip velocity v ≈ 0.1 m s−1. The rubber specimens(acronitrile butadiene rubber (NBR)) have been washed inindustrial petroleum for 3 min by using an ultrasonic cleanerand then dried for 10 min. The rubber surface has the root-mean-square roughness ≈0.4 μm, and has parallel groovescaused during molding of elastomer sheets in steel mold. Thesteel cylinder has a root-mean-square roughness of ≈0.1 μm.

Figure 2 shows the power spectrum of the surfaceroughness of the steel surface. The power spectrum is definedby [12]

C(q) =∫

d2x〈h(x)h(0)〉eiq·x (1)

where 〈..〉 stands for ensemble averaging. Here h(x) is thesurface height at the point x, where we have assumed 〈h(x)〉 =0. The surface height was measured over different surface areasusing atomic force microscopy and an optical method (3Doptical surface profiler (Wyko NT 1100) in vertical scanninginterferometry mode), and figure 2 was obtained from three

0953-8984/08/085223+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

J. Phys.: Condens. Matter 20 (2008) 085223 M Mofidi et al

Figure 1. Test configuration for friction studies under reciprocatingsliding conditions.

4 6 8

-22

-26

-30

-34

log q (1/m)10

log

C

(m

)

410

Figure 2. The power spectrum of the surface roughness of the steelsurface. The root-mean-square surface roughness is about 0.1 μm.The straight line has a slope corresponding to the fractal dimensionDf ≈ 2.66.

different measurements involving different resolution. Thestraight (green) line has a slope corresponding to the fractaldimension Df ≈ 2.66. In the calculations of the frictionpresented below we have used the this linear approximationand included the surface roughness power spectra over the fullwavevector range shown in the figure. Thus the longest andthe shortest wavelength roughness included in the analysis isλ0 = 2π/q0 ≈ 0.3 mm and λ1 = 2π/q1 ≈ 6 nm.

The experimental results presented in figures 5 and 6 wereobtained for the load FN = 100 N and with a test durationof 15 min. Since the oscillation stroke is very small (1 mm)one expects that most of the oil is squeezed out from the steelcylinder–rubber contact region.

The viscoelastic modulus E(ω) has been measured (usingEplexor 150) using a rectangular rubber block 5×2×30 mm3.The measurements were done in tension with 8% of prestrainand 1.3% of dynamic strain amplitude. Figure 3 showsthe logarithm of the real part of the viscoelastic modulusof the acronitrile butadiene rubber used in the present study,as a function of the logarithm of the frequency ω, for thetemperatures T = 50 and 80 ◦C.

The diameter d of the contact region between the steelcylinder and the rubber substrate can be estimated using theHertz contact theory for bodies with cylinder geometry, seefigure 4. For elastic solids, the diameter d of the contact areais given by [13]

d = 2

(2FN D

π L E∗

)1/2

, (2)

where E∗ = E/(1 − ν2) (where E is the Young modulus and

6.6

7

7.4

7.8

8.2

log ω (1/s)10

log

(R

e E

) (P

a)10

T = 50 C

80 C

-10 0 10

6

7

8

log

(

Im E

) (P

a)10

T = 50 C

80 C

(a)

(b)

Figure 3. The logarithm of (a) the real part and (b) the imaginarypart of the viscoelastic modulus as a function of the logarithm of thefrequency ω for the temperatures T = 50 and 80 ◦C. For acronitrilebutadiene rubber.

D

d

FN

rubber

steel

Figure 4. Steel cylinder squeezed in contact to a rubber substrate.

ν the Poisson ratio). The average pressure in the contact regionis

p = 1

2

(π FN E∗

2L D

)1/2

. (3)

For FN = 100 N and for T ≈ 50 ◦C we have (see figure 3)E∗ ≈ 10 MPa (where we have assumed the frequency ω ≈10−3 s−1, corresponding to the contact time ∼1000 s) givingd ≈ 0.4 cm and p ≈ 1 MPa.

Figure 5 shows the measured friction coefficients for thesteel cylinder sliding against non-aged rubber in 11 differentlubrication oils with very different viscosities. Thus, forexample, the PAO1 and PAO2 oils have the viscosities (at T =40 ◦C) 4.4 × 10−3 and 22.8 × 10−3 Pa s, respectively. Inspite of the large difference in viscosities, the rubber frictioncoefficients are nearly equal. This indicates that the rubberfriction is not (mainly) due to shearing a thin viscous layer,but due to the internal friction of the rubber (see below).

2


Figure 5. Coefficient of friction of non-aged samples in different base oils. For the load FN = 100 N.

Figure 6. Coefficient of friction of aged samples in different base oils. For the load FN = 100 N.

Figure 6 shows the measured friction coefficients for agedrubber. The aged rubber samples were prepared by immersingthem in different base fluids at T = 125 ◦C for one week.NBR rubber has polar nitrile groups and non-polar oils suchas naphthenic have nearly no effect on the properties of NBRrubber, and this explained why rubber aged in naphthenicexhibits nearly the same friction as for non-aged NBR rubber(compare figure 5 with figure 6). However, oils with polargroups, e.g. polyol ester, will diffuse into the rubber whichmay reduce the internal friction of the rubber. In addition,when the rubber block is squeezed against the counterface,oil may be squeezed out from the rubber matrix, giving athicker oil film at the interface and thus lower the friction (asimilar effect is believed to contribute to the extremely lowfriction exhibited by human joints [16]). We believe that botheffects may contribute to why NBR rubber aged in polyol esterexhibits much smaller friction than the non-aged rubber.

Figures 7–9 show the friction coefficient for differentloads and temperatures. Here the temperature refers to thebackground temperature, which was varied by contactingthe back-side of the rubber block to a metal block withthe given temperature. (The temperature in the slidingcontact is not known, but will be higher due to the frictionalheating.) Note that as the temperature increases the frictiondecreases. This cannot result from the change in viscosity ofthe lubricant oil since we already know from above that thelubricant viscosity has a negligible influence on the friction,at least for the squeezing force FN = 100 N, see figure 5.However, we will show in section 3 that the temperaturedependence of the sliding friction can be understood fromthe temperature dependence of the internal friction of therubber. Thus, when the temperature increases the rubberbecomes more elastic (less viscous) and the internal frictiondecreases.

3


Figure 7. Friction coefficient as a function of load at the backgroundtemperature T = 25 ◦C.


The dependence of the rubber friction on the load can beunderstood as follows. For very small load (FN = 20 N)the average pressure in the contact area (see equation (3))is relative low and the grooves on the rubber surface willnot be (fully) elastically flattened, and will trap lubricantoil, which may be pulled into the contact area during eachoscillation. This will result in an oil film which is thickenough to reduce the rubber–steel asperity contact and hencelower the viscoelastic contribution to the friction. This dragof lubricant fluid into the contact area is particularly largewhen the oscillation direction is perpendicular to the grooveson the rubber surface [14], and this explains why the frictionfor small load is much lower for perpendicular sliding thanparallel sliding. However, for high load (FN � 100 N) thereis negligible difference between parallel and perpendicularsliding, indicating that the lubricant has a negligible directinfluence on the friction.

The drop in the friction for large load is most likely due tothe increase in the temperature caused by the frictional heating.This effect becomes more important as the load increases, andexplains why the friction decreases for high load. At lowersliding velocity (or oscillation frequency) the heating effectsbecome less important (because of heat diffusion) and in thiscase one expects a smaller drop in the friction coefficient withincreasing load. We plan to test this prediction experimentally.


Figure 10 shows the friction coefficients (for the loadFN = 100 N) at T = 40 and 80 ◦C for the same base oilbut with different additives. As expected, there is negligibledependence of the friction on the additives. The reason for thisis the same as before: the observed friction is mainly due to theinternal friction of the rubber which does not change betweenthe different experiments. That is, although the additives in thebase oil may adsorb on the solid surfaces and act as boundarylubricants, the result of the study above indicates that such(mono) layers have negligible influence on the friction.

3. Rubber friction: theory

We have calculated the dependence of the rubber friction on thesliding velocity and the temperature using the theory presentedin [9]. The theory assumes that the friction is entirely due tothe viscoelastic deformation of the rubber, which results fromthe pulsating deformations from the substrate asperities. Theonly inputs in the calculations are the counterface roughnesspower spectrum (see figure 2) and the rubber viscoelasticmodulus. We have measured the viscoelastic modulus E(ω)

of the rubber as a function of frequency (and temperature). Inthe calculations we do not take into account the lubrication oildirectly (but it influences the friction indirectly by reducing (orremoving) the adhesion between the solid walls [15]). We haveassumed the nominal contact pressure of 1 MPa.

Neglecting the flash temperature, the friction coefficient isgiven by [9]

μ = 1

2

∫dq q3C(q)P(q)

∫ 2π

0dφ cosφ Im

E(qv cosφ)

(1 − ν2)σ

where

P(q) = 2

π

∫ ∞

0dx

sinx

xexp

[−x2G(q)] = erf

(1/2

√G

)

with

G(q) = 1

8

∫ q

0dq q3C(q)

∫ 2π

0dφ

∣∣∣∣ E(qv cosφ)

(1 − ν2)σ

∣∣∣∣2

where σ is the perpendicular pressure (the load divided by thenominal contact area).

4


Figure 10. Friction coefficient for one base oil with several different additives and for T = 40 and 80 ◦C. For the load FN = 100 N.

0

0.2

0.4

0.6

-8 -4 0-6 -2

μ k

log v (m/s)10

T = 50 C

80 C

Figure 11. The steady state kinetic friction coefficient calculatedusing the measured surface roughness power spectrum (fromfigure 2) and the measured viscoelastic modulus of the rubber. Forthe background temperatures 50 and 80 ◦C, and the nominalsqueezing pressure p = 1 MPa.

Figure 11 shows the steady state kinetic friction coefficientcalculated using the measured surface roughness powerspectrum (from figure 2) and the measured viscoelasticmodulus of the rubber. Results are presented for thebackground temperatures 50 and 80◦C. Note that themagnitude of the calculated friction coefficient at the slidingvelocity ∼0.1–1 m s−1 is similar to what is observedexperimentally, and also the temperature dependence is in goodagreement with the measurements (see section 2).

In figure 12 we show (a) the friction coefficient μk, and (b)the logarithm of the (normalized) contact area A/A0 (whereA is the contact area observed at the highest magnification,and A0 is the nominal or apparent contact area), as afunction of the logarithm of the large-wavevector cut-off q1

(in the calculations we only include surface roughness withwavevectors q0 < q < q1).

Results are presented for two different temperatures T =50 and 80 ◦C and for the sliding velocity v = 1 m s−1.The figure shows that the long-wavelength roughness gives anegligible contribution to the friction. The reason for why only

2 3 4 5

0

0.2

0.4

0.6

0

-0.4

-0.8

-1.2

-1.6

log

(A

/A

)10

0

μ k T = 50 C

80 C

50 C

80 C

(a)

(b)

log (q /q )10 1 0

Figure 12. The friction coefficient μk (a) and the logarithm of the(normalized) contact area A/A0 (b), as a function of the logarithm ofthe large-wavevector cut-off q1 (in units of the low-wavevectorcut-off q0). In the calculations we only include surface roughnesswith wavevectors q0 < q < q1. Results are presented for twodifferent temperatures T = 50 and 80 ◦C and for the sliding velocityv = 1 m s−1.

the short-wavelength roughness is important in the present caseis the large fractal dimension (Df ≈ 2.7) of the steel surface,which implies that the ratio between the amplitude and thewavelength of the surface roughness strongly increases as thewavelength decreases4, and this makes the short-wavelength

4 For a self affine fractal surface the ratio between the height h(λ) andwavelength λ of the surface roughness component with wavevector q = 2π/λ

is h/λ ∼ λ2−Df . Thus the larger the fractal dimension Df > 2, the faster theratio h/λ and will increase as the wavelength decreases, and this will tend toincrease the importance of the short-wavelength roughness.

5


h(t)

p

hard solid

hard solid

fluid

d

Figure 13. A block squeezed against a substrate in a fluid. Theseparation between the bottom surface of the block and the topsurface of the substrate is denoted by h(t).

roughness much more important than the long-wavelengthroughness.

4. Squeeze-out

We have argued above that the observed rubber friction canbe explained as resulting from the viscoelastic deformationsof the rubber by the countersurface asperities. In this sectionwe briefly address the role of the lubrication oil. We first notethat the oil will effectively eliminate the adhesive interactionbetween the rubber and the countersurface [15]. Most of theoil will be squeezed out from the steel–rubber contact area, buta molecular thin layer may remain even after long squeezingtime.

Consider first a flat rigid rectangular block squeezedagainst a flat hard countersurface with the nominal (or average)pressure p in a lubrication fluid with the viscosity η. Theseparation between the surfaces after the time t is (seefigure 13) [16]

h(t) ≈ (η/2pt)1/2d. (4)

Here d is the width of the bottom surface of the block andwe assume that d L, where L is the length of the bottomsurface of the block. With d ≈ 0.4 cm, p ≈ 1 MPa and witht = 1000 s we get with the typical viscosity η ≈ 0.01 Pa s,h(t) ≈ 4 nm. For surfaces with roughness the squeeze-outfrom asperity contact regions is even faster, but in this casesome liquid may get ‘trapped’ in sealed off regions [17]. Fornon-aged rubber the trapped islands may disappear because ofdiffusion of lubricant oil into the rubber matrix, see figure 14.The shear stress developed in a fluid film with thickness h isσ = ηv/h. In the present case, if v = 0.1 m s−1 and h =10 nm we get σ = 0.1 MPa which would give a contributionto the friction coefficient of order σ/p ≈ 0.1. However, thethickness of the oil film will be very non-uniform, and in manyregions (cavity regions) at the interface the film may be muchthicker than 10 nm (see below), and shearing the lubricantfilm in these regions will give a negligible contribution to thefriction. In other regions, where the steel asperities make directcontact with the rubber, the local squeezing pressure is muchhigher than the average pressure, and in these regions at most afew monolayers of oil film will remain trapped. Nevertheless,since the region of direct wall–wall contact is only a smallfraction of the nominal contact area, the contribution to the

rubber

hard solid

oil

Figure 14. A rubber block squeezed against a substrate in an oil. Theoil is partly squeezed out at the external boundaries of the nominalcontact area and partly transfered to (or from) the rubber matrix.

0 40 8020 600

0.01

0.02

0.03

u (nm)

_ P

(1/n

m)

u

Figure 15. The calculated probability distribution Pu of surfaceseparations u.

friction from shearing the confined thin layers appears to benegligible (see section 2).

Figure 15 shows the probability distribution Pu of surfaceseparations u. This function has been calculated as outlinedin [18]. In the calculation we have assumed a rubber elasticmodulus E = 100 MPa which correspond to the temperatureT = 40 ◦C and the perturbing frequencies ω ≈ 106 s−1 (seefigure 2), which is a typical perturbing frequency (ω = qv)from surface roughness with wavevector q = 107 m−1 andsliding velocity v = 0.1 m s−1. In the calculation we haveneglected the direct influence of the lubrication oil, but it isaccounted for indirectly by neglecting the adhesive interactionbetween the rubber and the steel surface. Using Pu we can givea more accurate estimate of the contribution from the oil filmto the shear stress. We get the viscous shear stress

σ ≈ ηv

∫ ∞

uc

duPu

u(5)

where uc is a cut-off length of order ∼1 nm since molecularthin lubrication films cannot be described by the continuumtheory of fluid mechanics [17]. We note that Pu has a deltafunction at the origin u = 0, but in the present case this carriesthe weight A(ζ1)/A0 ≈ 0.01 and the contribution from the areaof real contact to the friction force can be neglected. Using thecalculated Pu (see figure 15), and assuming η = 0.01 Pa sand v = 0.1 m s−1, equation (5) gives σ ≈ 0.06 MPa sothe contribution to the friction from the lubricant film is verysmall, of order 0.06 (where we have assumed the normal stressp = 1 MPa). We note that this is likely to be an overestimation

6


of the contribution of the oil film to the friction coefficient,as the oil film may tend to slightly increase the separationbetween the walls, and also because we have not accountedfor the roughness on the rubber surface in the analysis.

5. Discussion

When a block of a viscoelastic solid, such as rubber, is slidingon a hard rough countersurface, the largest contribution tothe sliding friction is usually derived from the time-dependentdeformations of the rubber by the countersurface asperities.This is the case, for example, for the tire-road contact. Herewe have shown that even a highly polished countersurface,which may appear mirror-smooth to the naked eye, may haveenough surface roughness at short length scale to give a largecontribution to rubber friction. This result has many importantapplications, e.g., in the context of rubber sealing.

In many rubber sealing applications the rubber surfaceand the (lubricated) countersurface are squeezed together fora long time between the slip events. Furthermore, during theonset (and stop) of sliding the slip velocities will be very small.This may result in nearly complete squeeze-out of the lubricantfilm. Thus, at some point in time slip will occur between whatis effectively unlubricated surfaces. This may result in highfriction and large wear, and perhaps failure of the seal withpotentially serious consequences.

Note that with respect to sliding friction there is anasymmetry between roughness on the countersurface andon the rubber block. Thus, only roughness on the hardcountersurface will contribute to the friction force. Roughnesson the rubber surface may in fact lower the sliding friction bytrapping lubrication fluid. On the other hand, with respect tostationary contact mechanics, roughness on the two surfacesplays a similar role [13, 19].

There is an important difference between rubber frictionon very rough surfaces, such as a road surface, andrubber friction on smoother surfaces with only short-wavelength roughness. On very rough surfaces, as themagnification increases we observe smaller and smallerrubber-countersurface asperity contact regions, and the localstress and temperature will rapidly increase until the limit ofstrength of the rubber has been reached. For tread rubber incontact with road surfaces this limit is reached at the lengthscale (or resolution) λc ≈ 1–10 μm, and at this length scaleduring slip strong wear processes occur. The rubber frictionon road surfaces can be explained by including the viscoelasticdeformations of the rubber from road surface roughness withwavelength down to λc. On the other hand, for surfaces withmainly short-wavelength roughness, such as the steel surfaceused in the present study, it may be necessary to includeroughness with wavelength down to the molecular length scale,e.g., the distance between cross links in the rubber whichtypically is of order a few nanometers. This may result indifferent wear mechanisms and wear rates than on surfaceswith large long-wavelength roughness.

The results presented in this paper may also be relevantfor the adhesion and locomotion of some animals on roughsubstrates. Thus, some animals, such as grasshoppers

and tree frogs, have smooth attachment pads built froma (non-compact) material which is highly viscoelastic (likerubber) [20]. Furthermore, the toe pad–substrate contactregion is wet (lubricated) with a liquid injected into thecontact area by the animal. The liquid viscosity, the nominalsqueezing pressure, and the size and shape of the contactarea differ from the lubricated rubber–counterface contactproblem studied above, but some of the results presented abovemay nevertheless be relevant for the animal toe pad–substrateinteraction problem [21, 22].

6. Summary and conclusion

We have presented a combined experimental–theoretical studyof rubber sliding friction against hard lubricated surfaces. Wehave shown that even if the hard surface appears smooth to thenaked eye, it may exhibit short-wavelength roughness, whichmay give the dominant contribution to rubber friction. Thepresented results may be of great importance for rubber sealingand other rubber applications involving (apparently) smoothsurfaces.

References

[1] Moore D F 1972 The Friction and Lubrication of Elastomers(Oxford: Pergamon)

[2] Schallamach A 1963Wear 6 375Schallamach A 1971Wear 17 301

[3] Chernyak Y B and Leonov A I 1986Wear 108 105[4] Filippov A E, Klafter J and Urbakh M 2004 Phys. Rev. Lett.

92 135503[5] Persson B N J and Volokitin A I 2006 Eur. Phys. J. E 21 69[6] Vorvolakos K and Chaudhury M K 2003 Langmuir 19 6778

See also Casoli A, Brendle M, Schultz J, Philippe A andReiter G 2001 Langmuir 17 388

[7] Grosch K A 1963 Proc. R. Soc. A 274 21Grosch K A 1974 The Physics of Tire Traction: Theory and

Experiment ed D F Hays and A L Browne (New York:Plenum) p 143

[8] Baumberger T and Caroli C 2006 Adv. Phys. 55 279Baumberger T, Caroli C and Ronsin O 2001 Eur. Phys. J. E

11 85Ronsin O and Coeyrehourcq K L 2001 Proc. R. Soc. A

457 1277[9] Persson B N J 2001 J. Chem. Phys. 115 3840

Persson B N J 2006 J. Phys.: Condens. Matter 18 7789Persson B N J and Volokitin A I 2002 Phys. Rev. B 65 134106Persson B N J 1998 Surf. Sci. 401 445

[10] Kluppel M and Heinrich G 2000 Rubber Chem. Technol. 73578

Le Gal A and Kluppel M 2005 J. Chem. Phys. 123 014704[11] Salant R F, Maser N and Yang B 2007 J. Tribol. 129 91[12] Persson B N J, Albohr O, Tartaglino U and Tosatti E 2005

J. Phys.: Condens. Matter 17 R1[13] Johnson K L 1985 Contact Mechanics (Cambridge: Cambridge

University Press)[14] Patir N and Cheng H S 1978 J. Lubr. Technol. 100 12[15] Zappone B, Rosenberg K J and Israelachvili J 2007 Tribol. Lett.

26 191[16] See, e.g. Persson B N J 2000 Sliding Friction: Physical

Principles and Application 2nd edn (Heidelberg: Springer)[17] Persson B N J and Mugele F 2004 J. Phys.: Condens. Matter

16 R295Persson B N J, Tartaglino U, Albohr O and Tosatti E 2004 Nat.

Mater. 3 882

7


[18] Yang C and Persson B N J 2008 at press[19] See, e.g. Persson B N J 2006 Surf. Sci. Rep. 61 201[20] Goodwyn P P, Peressadko A, Schwarz H, Kastner V and

Gorb S 2006 J. Comp. Physiol. A 192 1233

[21] Persson B N J 2007 J. Phys.: Condens. Matter19 376110

[22] Federle W, Barnes W J P, Baumgartner W, Drechsler P andSmith J M 2006 J. R. Soc. Interfaces 3 689

8


Paper E

The influence of lubrication on two body abrasive wear of sealing elastomers under reciprocating sliding conditions

_____________________________________________________________________________89

The influence of lubrication on two body abrasive wear of sealing elastomers under reciprocating sliding conditions M. Mofidi1, B. Prakash1

E-mails: [email protected], [email protected] 1 Division of Machine Elements, Luleå University of Technology, Luleå SE-971 87 Sweden

ABSTRACTElastomeric seals are prone to failure caused by abrasion during sliding against rough surfaces. In this research, the two body abrasive wear of some selected sealing elastomers (nitrile rubber, hydrogenated nitrile rubber, acrylic rubber and fluoro rubber) in dry and lubricated conditions has been studied. A two body abrasive wear tester was used to investigate the abrasive wear of the elastomers. The experiments have been carried out at varying normal load and abrasive particle size in dry and lubricated conditions. The influence of three different oils (synthetic ester, polyalphaolefin and mineral oil) on the abrasive wear has been studied. The results show that, depending on the material, lubricant, abrasive size and normal load, the abrasive wear of elastomers may increase or decrease in the presence of lubricants and in most cases, the abrasive wear in the lubricated condition is higher than those in the dry condition. The influence of lubricant on the increase in abrasive wear is more significant during sliding against fine abrasives. Apart from the acrylic rubber, increase in the abrasive wear in presence of the synthetic ester is higher than those in presence of the mineral oil and polyalphaolefin.

Keywords: Abrasive wear; Elastomer; Lubrication

1 INTRODUCTION Abrasive wear of a sealing material is an important factor that often limits the seal life. It may result from many causes including particulate suspended in lubricating oil, wear debris from inadequate lubrication, products of corrosion, airborne dust or a rough surface finish [1, 2]. Such particulates may move freely to abrade both surfaces by a three body abrasive wear mechanism. They may also become partially imbedded in one of the surfaces and act as a cutting tool resulting in two body abrasion of the other mating surface [1]. The metallic sealing surface can also be worn and roughened by a corrosive wear mechanism and then it abrades the sealing material. Since most seals operate in the presence of lubricants during their service life, the influence of lubricants on wear mechanisms of elastomers has an important role in seal performance.

Three different mechanisms of wear, including abrasive wear, fatigue wear, and roll formation, can be identified when an elastomer slides against a hard counterface [3].




_____________________________________________________________________________90

When an elastomer slides against a hard counterface with a sharp texture, abrasive wear takes place as a result of tearing of the sliding surface of the elastomer. Fatigue wear occurs on the surface of an elastomer sliding against blunt projections on the hard counterface. When a highly elastic elastomer slides against a smooth surface, roll formation occurs. In this type of wear, the high frictional force shears a projection on the rubber surface, tears and then rolls the tongue along the direction of sliding [4]. In practice, a combination of all three forms of wear occurs but it is generally difficult to separate the contribution of each mechanism to the overall wear [3].

When a rubber is abraded in unidirectional sliding, sets of parallel ridges are often formed on the surface of the elastomer at right angles to the direction of motion which have been called “abrasion patterns” [5]. The surfaces of harder elastomers, sliding against sharp asperities, exhibit scratches parallel to the direction of sliding [6]. The mechanism of abrasion leading to the ridge formation has been studied extensively [5, 7-16]. Most of previous experiments on the mechanism of ridge formation have been carried out using a line contact configuration.

Another classification introduces the wear of elastomers as a result of two processes; local mechanical rupture (tearing) and decomposition of the molecular network to a low molecular weight (smearing) [17]. The oily decomposition product which forms during smearing protects the underlying rubber from tearing and thus decreases the rate of wear [18]. Experiments show that the rate of wear during smearing decreases by introducing antioxidants [17, 18].

Oil and heat resistance of elastomers has an important role in sealing applications. When an elastomer and a base fluid are brought in contact with each other, the elastomer material may absorb the base fluid or the base fluid may extract soluble constituents of the elastomer. The base fluid may also react with the elastomer [19, 20]. A solubility parameter (Hildebrand parameter), generally denoted by , is defined to estimate the degree of interaction between materials, particularly for non polar materials such as many polymers. This parameter is defined as the square root of the internal energy of vaporisation divided by the molar volume, which is referred to as the cohesive energy density. Materials with similar values of are likely to be miscible [21, 22]. Presence of the polar side-groups in the backbone chain of the elastomer increases the oil resistance of the polymer [23]. Crosslinking also limits the degree of polymer swelling by providing tie points that limit the amount of solvent that can be absorbed into the polymer [23]. Torbacke and Johansson have studied the seal material compatibility of some environmentally adapted fluids and compared it with that of some mineral and synthetic base fluids. Their results show that the mechanical properties of the sealing elastomers deteriorated especially in presence of ester base fluids [19]. If the lubricant can penetrate the surface of the polymer, it will have a detrimental effect on its wear behaviour [24 - 27]. The rapid wear is believed to occur through aggravated cracking of the solvent weakened polymer during contact with the counterface. The wear rate reaches a maximum when the solubility parameter of the polymer and the solvent are the same [26, 27].

In the abrasion of rubber by a blade in line contact, when a lubricant is applied, a much finer pattern develops and the rate of abrasion decreases but the horizontal force on the blade does not decrease as dramatically [28, 29]. However, when a blunt abrader slides


_____________________________________________________________________________91

against a rubber surface, the horizontal force decreases significantly in presence of a lubricant [29, 30]. Chandrasekaran and Batchelor have studied the friction and wear of butyl rubber sliding against an abrasive paper as a function of temperature and load. They conducted dry and lubricated unidirectional sliding tests and reported that the presence of lubricant reduced the coefficient of friction but accelerated wear due to chemical degradation of rubber [31].

Acrylonitrile butadiene rubber (NBR), hydrogenated acrylonitrile butadiene rubber (HNBR), fluoro rubber (FKM) and acrylic rubber (ACM) are commonly used elastomers in lubricated applications. FKM has the highest heat and oil resistance but NBR has limited heat resistance and ACM occupy an intermediate position between NBR and FKM. HNBR is produced by partially or fully hydrogenating NBR. The hydrogenating process saturates the polymeric chain and improves the heat resistance and overall mechanical properties of the elastomer [32].

Abrasion of sealing elastomers is an important factor in seal failure and reduces the life and sealing ability of seals [33]. The effect of lubrication on the mechanisms of abrasion of elastomer surfaces has not been adequately studied and this research thus aims at investigating the effect of lubrication on the two body abrasion of some selected sealing elastomers such as nitrile rubber, hydrogenated nitrile rubber, acrylic rubber and fluoro rubber.

2 EXPERIMENTAL The materials used in this study were seven different types of sealing elastomers including two type of acrylonitrile butadiene rubber (NBR), two types of hydrogenated acrylonitrile butadiene rubber (HNBR), two types of fluoro rubber (FKM) and an acrylic rubber (ACM). In this study, rectangular elastomeric sheet specimens of 40 mm × 20 mm and 2 mm thickness were used (Table 1). Three different lubricating oils have been used to investigate the influence of lubrication on the abrasive wear (Table 2). The hardness of abrasive particles was HV 30-800.


Elastomericmaterials

Hardness (Shore A)

Tensilestrength (MPa)


Density (g/cm3)

NBR - type A 76.7 12.5 378 1.35 NBR - type B 76.1 25.4 466 1.31 HNBR - type A 71.3 17.5 303 1.24 HNBR - type B 79.9 17.9 340 1.3 ACM 73.4 7.8 171 1.49 FKM - type A 72.8 - - 2.03 FKM - type B 81.2 - - 2.19


_____________________________________________________________________________92

Table 2: The properties of lubricants

Base fluid Density (kg/m3) Viscosity@40ºC (cSt) NPINaphthenic base oil 896 30.0 — Monoester 864 (20 °C) 8.5 102 Poly alpha olefin 790 5.5 —

An abrasive wear tester (Figure 1) has been used to study dry and lubricated abrasive wear of seven type of sealing elastomers. This tester consists of a platform on which the sample is mounted and reciprocated against a silicon carbide abrasive paper wrapped around the circumferential surface of a wheel (ø50 mm × 12 mm thick). The wheel is turned by a small angle at the end of each stroke so as to ensure the rubbing of the elastomer against a fresh abrasive surface. The frequency of reciprocation of the test specimen was 60 cpm and the stroke length was 30 mm. The tests were run for a total of 160 cycles at room temperature (22 ± 2 ºC). In lubricated tests, the oil was injected into the counterface using a syringe. The rubber test specimens were washed in industrial petroleum for 3 minutes by an ultrasonic cleaner, dried in an oven for 20 minutes at 45 ºC and then weighed using a precision semi-mciro electronic weighing balance (with a resolution of hundredth of a mg). The same procedure was repeated after running the test for each specimen to quantify abrasive wear. The test parameters have been shown in Table 3. The worn surfaces and the collected worn particles were examined by using an optical microscope.

Figure 1: Abrasive wear tester


Testparameters

Load, N

Average contact pressure, MPa


Level 1 4 180 KPa 120 ( 106)

Level 2 8 260 KPa 320 ( 34)

Level 3 - - 500 ( 18)


_____________________________________________________________________________93

3 RESULTS AND DISCUSSION Figure 2 shows the abrasive wear of the elastomers in dry and lubricated conditions. The figure shows that, in most cases, the abrasive wear of elastomers in lubricated condition is higher than that in the dry condition which is presumably due to the weakening the elastomer by the lubricant [25, 27]. The results show that ACM and NBR-B are the most and least affected by the lubricant, respectively. The abrasive wear of ACM in presence of the mineral oils is about seven to eight times more than that in dry condition. The ACMs are generally known to be less oil compatible than NBRs and FKMs [23]. The different behaviours of the two NBRs might be due to different degrees of cross linking, different types or amount of fillers, plasticizers or other additives and especially different percentage of the acrylonitrile in the elastomer [23, 32].

Figure 2: Abrasive wear of different elastomers in dry and lubricated conditions, Normal load: 4

N, Abrasive grit size: #320

Figure 3 shows the wear particles of three tested elastomers. The wear particles of all elastomers, with the exception of NBR-B, in dry condition have been aggregated but the aggregation of wear particles did not occur in the lubricated condition. Although the abrasive tape (attached on the wheel) rotates after each stroke to provide a fresh surface sliding against the elastomer, it seems that the worn particles during each stroke stick together and form roll shapes between the contacting surfaces. Such aggregated worn particles in the contact might lessen the direct contact between the abrasive particles and the surface of elastomer and consequently reduce the abrasive wear of elastomer. Presence of lubricant in the contact prevented the wear particles to be aggregated and therefore, more abrasive particles can come into contact with the surface of elastomer and contribute in the abrasive wear. As shown in Figure 3, such aggregation of wear particles did not occur in case of NBR-B, even in dry condition, and consequently its abrasive wear in lubricated condition is not significantly higher than that in dry condition (Figure 2). As shown in Figure 2, the abrasive wear of ACM in presence of a mineral oil is higher than that in presence of the monoester, but the abrasive wear of the


_____________________________________________________________________________94

other elastomers in presence of the monoester is higher than that in presence of mineral oil.

Figure 3: Wear particles of three tested elastomers in dry and lubricated conditions, Normal

load: 4 N, Abrasive grit size #320, Lubricant: Naphthenic oil

Figure 4 shows the worn surfaces of the same elastomers as those in Figure 3. As shown in the figure, the aggregated worn particles, which are stuck on the worn surface of ACM in the dry condition, reduced the direct contact between the abrasives and the surface of the elastomer. Comparison of the worn surfaces of ACM and NBR-A in dry and lubricated conditions, shows that the scratches on the worn surface in the lubricated condition are more continuous than those on the worn surfaces in the dry condition.

Figure 4: Worn surfaces of three tested elastomers in dry and lubricated conditions, Normal

load: 4 N, Abrasive grit size #320, Lubricant: Naphthenic oil


_____________________________________________________________________________95

The abrasive wear of three elastomers has been investigated under different conditions and the results have been shown in Figure 5. The results show that the effect of the monoester oil on the increase in abrasive wear of the selected elastomers, with the exception of that of ACM, is slightly greater than that of the naphthenic oil. The influence of oil on the increase in abrasive wear of the tested elastomers is more significant when the elastomers are sliding against finer abrasives. When an elastomer and a lubricant interact, a very thin layer of elastomer may be weakened by the diffusion of lubricant into the elastomer.

Figure 5: Abrasive wear of three different elastomers in dry and lubrication conditions


_____________________________________________________________________________96

As shown in Figure 5, even the abrasive wear of the NBR- B sliding against finer abrasives increases in presence of lubricants. Figure 6 and 7 show the wear particles of the NBR-B in sliding against fine and coarse abrasives, respectively. Figure 6 shows that when the NBR-B slides against fine abrasives in the dry condition, the worn particles are aggregated but as shown in Figure 7, such aggregation does not occurr when the NBR-B slides against coarse abrasives in dry or in lubricated condition.

Figure 6: Worn particles of NBR–B in dry and lubricated conditions, Normal load: 4 N,

Abrasive grit size #500, Lubricant: Naphthenic oil

Although the abrasive wear of NBR-B in sliding against coarse abrasives (#120), in the lubricated condition is higher than that in dry condition, the wear particles in the lubricated condition are significantly larger than those in the dry condition (Figure 7).


_____________________________________________________________________________97

Figure 7: Worn particles of NBR–B in dry and lubricated conditions, Normal load: 4 N,

Abrasive grit size #120, Lubricant: Naphthenic oil

An illustration of the mechanisms of abrasion of elastomers in point contact and line contact have been shown in Figure 8. When an elastomer is sliding in point contacts with sharp asperities, the elastomer surface is pulled in the direction of sliding and fails in tension behind the contact perpendicular to the tensile stress field (Figure 8-a) but when an elastomer is sliding in line contact with an abrader, the tears are generated perpendicular to the direction of sliding at the rear of the contact region (Figure 8-b) [34]. In the present experiments,with concentrated stress acting on the abrasive particles tips, the wear occurs through a combination of the two mechanisms shown in Figure 8. When a lubricant is applied in the contact, the frictional shearing stress is decreased and concentrated at the tips of the abrasive particles. The concentrated shearing stress causes the scratches on the surface of the elastomer parallel to the direction of sliding but it hinders breaking of the tongues formed on the surface, thereby enlarging the wear particles during lubricated sliding.

Figure 8: stress distribution and tearing the surface of elastomer in two different mechanisms of

abrasion (point and line contact)


_____________________________________________________________________________98

4 CONCLUSIONS The two body abrasive wear of sealing elastomers and the influence of lubrication have been studied. The results show that in most conditions, lubrication increases the abrasive wear of elastomers which is presumably due to the weakening of the elastomers in presence of the oils; however, in one tested elastomer (NBR-B) sliding against coarse abrasive, the abrasive wear in lubricated condition was lower than that in dry condition. Examination of the wear particles and surfaces showed that in most dry sliding of elastomers against abrasives, the worn particles were aggregated. Such aggregation can reduce the direct contact between the elastomer and abrasive particles. Lubrication generally prevented the wear particles to be aggregated. The prevention of the aggregation of worn particles might ease the worn particles to be removed from the contact and provide more direct contact between the abrasive particles and surface of elastomer. The results show that the influence of lubrication on the increase in abrasive wear is more significant when the elastomer slides against finer abrasives. Apart from the acrylic rubber, increasing the abrasive wear in presence of the synthetic ester is slightly more pronounced than that in presence of the mineral oil and polyalphaolefin. The increase in abrasive wear of acrylic rubber in presence of the polyalphaolefin and mineral oil is greater than that in presence of the synthetic ester.

REFERENCES[1] T. A. Stolarski, Tribology in Machine Design, Butterworth-Heinemann, Oxford,

2000.


[3] D. F. Moore, The Friction and Lubrication of Elastomers, Pergamon Press, 1972.

[4] D. I. James, M. E. Jolley, Abrasion of rubber, MacLaren & Sons Ltd., London, 1967.




[8] E. Southern, A. G. Thomas, Studies of rubber abrasion, Rubber Chem. Technol. 52 (1979) 1008-1018.

[9] S. W. Zhang, Investigation of abrasion of nitrile rubber, Rubber Chem. Technol. 57 (1984) 769-778.



_____________________________________________________________________________99

[11] S. W. Zhang, Z. Yang, Energy theory of rubber abrasion by a line contact, Tribol. Int. 30 (1997) 839-843.


[13] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 1: Abrasion pattern formation in natural rubber vulcanizate, Wear 171 (1994) 195-202.

[14] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 2. General rule in abrasion pattern formation in rubber-like materials, Wear 178 (1994) 109- 116.

[15] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion, Part 3: how is friction linked to fracture in rubber abrasion?, Wear 188 (1995) 19-26.

[16] A. N. Gent, C. T. R. Pulford, Mechanisms of Rubber Abrasion, J. Appl. Polym. Sci. 28 (1983) 943-960.



[19] M. Torbacke, A. Johansson, Seal Material and Base Fluid Compatibility: An Overview, J. Synthetic Lubrication 22 (2005) 123-142.

[20] G. F. Ogorodnikova, V. V. Sinitsyn, Mechanism of rubber swelling in oils and greases, Chemistry and Technology of Fuels and Oils 18 (1982) 306-308.

[21] M. F. Myntti, Comparing fuel and oil resistance properties, Rubber World 228 (2003) 38-45.

[22] H. J. Vandenburg, A. A. Clifford, K. D. Bartle, R. E. Carlson, J. Carroll, I. D. Newton, A simple solvent selection method for accelerated solvent extraction of additives from polymers, Analyst 124 (1999) 1707-1710.


[24] M. Mofidi, E. Kassfeldt, B. Prakash, Tribological behaviour of an elastomer aged in different oils, Tribol. Int. 41 (2008) 860-866.

[25] G. W. Stachowiak, A. W. Batchelor, Engineering Tribology (2nd edition.), Butterworth-Heinemann, Boston, 2001.

[26] D. C. Evans, Polymer – fluid interactions in relation to wear, Proc. 3rd Leeds-Lyon Symp. on Tribology, Wear of Non-Metallic Materials, Sept. 1976, Publ., 1978, London, 47-71.

[27] M. K. Omar, A. G. Atkins, J. K. Lancaster, The role of crack resistance parameters in polymer wear, J. Phys. D: Appl. Phys. 19 (1986) 177-195.


_____________________________________________________________________________100



[30] A. H. Muhr, Lubrication of model asperities on rubber, Proc. 17rd Leeds-Lyon Symp. on Tribology, Vehicle Tribology, Sept. 1990, Publ., 1991, Amsterdam, 195-204.


[32] W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich, 1989.

[33] Information obtained from the website: http://www.simritna.com/catalog/o-ring/993Failures.htm, last access: 2008-09-04.

[34] B. J. Brisco, S. K. Sinha, Wear of polymers, Proc. Inst. Mech. Eng. Part J.-J. Eng. Tribol. 216- 6 (2002) 401-413.

http://www.simritna.com/catalog/o-ring/993Failures.htm




Paper F

Two body abrasive wear and frictional characteristics of sealing elastomers under unidirectional lubricated sliding conditions _________________________________________________________________________________

103

Two body abrasive wear and frictional characteristics of sealing elastomers under unidirectional lubricated sliding conditions

M. Mofidi1, B. Prakash1

E-mails: [email protected], [email protected] 1 Division of Machine Elements, Luleå University of Technology, Luleå SE-971 87 Sweden

AbstractSince abrasion is a common cause of seal failures, understanding the mechanisms of abrasion of an elastomer in the presence of lubricants is of importance in sealing applications. In this research a block on ring configuration was used to study the influence of lubrication on the two-body abrasion of several commonly used sealing elastomers (two acrylonitrile butadiene rubbers,an acrylic rubber and a fluoro rubber). The friction force and the abrasive wear of the samples were meassured and the worn surfaces and wear particles were investigated using an optical microscope. The tear strength of the elastomers before and after immersion in monoester oil as well as the oil absorption has been measured. Both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) were observed on the worn surfaces of nitrile rubbers but the surfaces of acrylic and fluoro rubber were characterised by scratches only. The worn surfaces of nitrile rubbers were defined with more continuous ridges at lower sliding velocity and the presence of a lubricant in the contact reduced the continuous ridges. Examination of the wear particles shows that the wear particles (particularly for acrylic rubber) in dry sliding condition were aggregated but the lubricant dispersed the wear particles and prevented aggregation. In most cases, abrasive wear of the elastomers in the lubricated condition is higher than that in the dry condition. The results show that the friction coefficient increased with increasing sliding velocity and decreased with contact pressure. Apart from the fluoro rubber, the friction coefficient as well as the tear strength of the elastomers decreased significantly in the presence of lubricant, particularly for acrylic rubber.

Keywords: Abrasive wear; Elastomer; Lubrication

1. Introduction Abrasive wear is a common type of wear mechanism which reduces the sealing ability and the service life of seals. It may result from many causes including particulates suspended in lubricating oil, wear debris generated during starved lubrication conditions, products of corrosion, airborne dust or a rough surface finish [1, 2 and 3]. Such particulates may move freely to abrade both surfaces by a three body abrasive wear mechanism. They may also




104

become partially embedded in one of the surfaces and act as a cutting tool resulting in two body abrasion of the other mating surface [1]. The metallic sealing surface can also be worn and roughened by a corrosive wear mechanism which abrades the sealing material through the two-body abrasive wear process. Since most seals operate in the presence of lubricants during their service life, the influence of lubricants on friction and wear mechanisms of elastomers has an important role in seal performance.

The friction coefficient of an elastomer surface sliding against a hard counterface can be expressed in terms of the contribution of adhesion, deformation (hysteretic), viscous and cohesive (tearing) components [4, 5]. However, most texts consider only two terms for friction components. They suggest that the tearing and viscous components can be represented by deformation and adhesive components respectively [4]. Adhesion is generally recognized as the making and breaking of junctions at a molecular level [6]. Hysteretic friction is a consequence of energy loss associated with internal damping within the viscoelastic body [7]. The cohesion component of friction is the contribution of wear to the bulk losses and the viscous component is the viscous drag under wet conditions [4]. The previous studies on friction of elastomers have focused on the hysteretic and adhesive components and the contribution of the tearing component in friction has not been studied well. Since the contribution of the tearing component is significant when severe wear occurs, it is of less interest in real applications; however, elucidation of this component can provide an enhanced understanding of the wear mechanisms. The contribution of adhesion and hysteresis to friction depends on the geometry, cleanliness of the mating surfaces and contact pressure. In many applications, especially in lubricated conditions, the hysteretic friction is the dominant component. Even if the hard surface appears smooth to the naked eye, it may exhibit short-wavelength roughness, which may make the dominant contribution to rubber friction in lubricated condition [8]. The adhesive component is dominant on very clean and smooth surfaces [9 - 11]. It can also be significant at low loads, even in lubricated conditions [12] because of the significance of the attractive Van der Waals’ forces between the surfaces compared to the normal load [13].

Presence of fluid between rubber and hard substrate reduces not only the adhesion but also the hysteretic and tearing components of friction. The lubrication decreases the real contact area between the rubber and hard counterface resulting in a decrease in friction coefficient. This effect is more pronounced at higher velocities due to hydrodynamic effects. On a lubricated surface, the valleys turn into fluid pools which are sealed off and thus make the surface smooth. This smoothening reduces the viscoelastic deformation caused by the surface asperities, and reduces rubber friction [14, 15]. The lubricant – elastomer interaction can also affect the elastomer properties and consequently the hysteretic and tearing component of friction.

Any estimation of abrasion of rubber needs to take into account the mechanism of wear. Three different mechanisms of wear, including abrasive wear, fatigue wear and roll formation, can be identified when an elastomer slides against a hard counterface [1]. During sliding against a hard countersurface with a sharp texture, abrasive wear takes place as a result of tearing of the sliding surface of the elastomer. Fatigue wear is another mechanism of wear which occurs on


105

the surface of an elastomer sliding against blunt projections on the hard counterface. When a highly elastic elastomer slides against a smooth surface, roll formation occurs. In this type of wear, the high frictional force shears a projection on the rubber surface, tears and then rolls the tongue along the direction of sliding [1]. A critical value of shear stress can be defined for each rubber above which roll formation occurs, and below which wear is mainly due to fatigue. Thus the friction coefficient is one of the most important properties of rubber governing the type of wear [16]. In practice, a combination of three forms of wear occurs and it is difficult to separate the contribution of each mechanism to the overall wear [1].

Another classification of wear of elastomers introduces two mechanisms of wear of elastomers sliding against hard counterface. Mechanochemical decomposition of the molecular network to a low molecular weight leading to a tar-like wear product (smearing) and cohesive rupture (tearing) [17]. The oily decomposition product which forms during smearing protects the underlying rubber from tearing and thus decreases the rate of wear [18]. Experiments show that the rate of wear during smearing decreases by introducing antioxidants [17, 18].

The worn surface of an elastomer may exhibit different appearance. When rubber is abraded without any change in sliding direction, sets of parallel ridges, perpendicular to the sliding direction, are often found on the worn surface [19]. The surfaces of elastomers worn by fatigue wear exhibit pitting marks and the surfaces of harder elastomers, sliding against sharp asperities (multiple point contacts), exhibit scratches parallel to the direction of sliding [20]. When an elastomr slides against sharp asperities, the elastomer surface is pulled in the direction of sliding and fails in tension behind the contact perpendicular to the tensile stress field [21, 22]. The formation of ridges starts by initiation the cracks, perpendicular to the sliding direction, at the rear of the contact region, due to the high shearing stress, and continues by growing the cracks under repetitive loading [23]. The mechanism of abrasion leading to the ridge formation, in dry condition, has been studied extensively [19, 23 -34]. Most of previous experiments on the mechanism of ridge formation have been carried out using a line contact configuration.

Muhr et al. have studied the influence of lubrication on the abrasion of rubber by a blade in line contact. They observed that when a lubricant is applied, a finer pattern develops and the rate of abrasion is reduced, but the tangential force on the blade does not decrease as dramatically [35, 36]. Chandrasekaran and Batchelor have studied the friction and wear of butyl rubber sliding on abrasive paper as a function of temperature and load. They conducted dry and lubricated unidirectional sliding tests and reported that the presence of lubricant reduced the coefficient of friction but accelerated wear due to chemical degradation of the rubber [37].

When an elastomer and oil are brought in contact with each other, the elastomer material may absorb the base fluid, the base fluid may extract soluble constituents of the elastomer or the base fluid may react with the elastomer [38]. Sealing elastomers must be able to withstand the fluids they are sealing. If the physical properties of the elastomer degrade too far, the seal will fail. Presence of the polar side-groups in the backbone chain increases the oil resistance of the


106

polymer [39]. Crosslinking also limits the degree of polymer swelling by providing tie points (constrains) that limit the amount of solvent that can be absorbed into the polymer [39]. If oil penetrates the surface of the polymer, it will have a detrimental effect on its wear resistance. The rapid wear is believed to occur through aggravated cracking of the solvent weakened polymer during contact with the counterface [40, 43].

Nitrile rubber (NBR) is a copolymer of acrylonitrile and butadiene and provides a low-cost elastomer with good mechanical properties in sealing applications. The concentration of acrylonitrile in the copolymer has a considerable influence on the polarity and swell resistance of the vulcanizate in non-polar solvents. The greater the acrylonitrile content, the lower the amount of swell in motor fuels, oils, fats, etc [44]. Acrylic rubber (ACM) is a type of synthetic rubber containing acrylonitrile. It is a copolymer of two major components: the backbone (95-99%) and the reactive cure site (1-5%). The outstanding property of ACM rubber is its resistance to hot oil. It is more heat resistant than NBR. Its resistance to weather, ozone and natural aging is also higher than NBR but it has less resistance to wear and oil swelling than NBR [44]. Fluoroelastomers are typically used in harsh environments where other elastomers fail. Chemical resistance and heat resistance are the two main attributes that make fluoroelastomers attractive for sealing applications. FKM is the designation for a large sort of fluoroelastomers containing vinylidene fluoride as a monomer [44].

In spite of the important role of abrasion in seal failure mechanisms and the influence of lubrication on abrasion, it has not been adequately studied. This study thus aims at investigating the effect of lubrication on the mechanisms of two-body abrasion and their contributions to the overall wearing of some selected sealing elastomers, including two types of nitrile rubber, an acrylic rubber and a fluoro rubber.

2. Experimental

2.1 Materials The elastomeric materials studied were two acrylonitrile butadiene rubbers (NBR), an acrylic rubber (ACM) and a fluoro rubber (FKM). The rubber samples were cut out from elastomeric sheets of 2 mm thickness. The nominal hardness, tensile strength, elongation at break and density of the elastomers are given in Table 1.


Elastomer Hardness, Shore A

Tensile strength, MPa

Elongation at break, %

Density, g/cm3

NBR-A 76.7 12.5 378 1.35 NBR-B 76.1 25.4 466 1.31 ACM 73.4 7.8 171 1.49 FKM 81.2 - - 2.19


107

The lubricant used in these experiments was a monoester with a density of 864 (20 °C) kg/m3 and a viscosity of 8.5 cSt at 40 °C.

2.2 Abrasive wear tests The experiments were carried out using Micro-Tribometer UMT-2. The rubber specimen glued to a metal backing plate was pressed against an abrasive tape glued on to the circumferential surface of a rotating ring. The normal and frictional forces were measured by using strain gauge force sensors. The schematic of the test configuration is shown in Fig 1. The rubber specimens’ dimensions were 22 mm×6 mm×2 mm (the width of contact was 6 mm). The rotating ring was of Ø60 and 10 mm thick. The rubber specimens were washed in industrial petroleum for 2 minutes using an ultrasonic cleaner dried in an oven for 10 minutes at 40 ºC and then weighed. The same procedure was repeated after running the test for each specimen to quantify wear. All the tests were performed at room temperature (22 ± 2 ºC). To understand the effect of lubricant on the wear mechanism, the weight loss and friction coefficient were measured and the wear particles and worn surfaces were examined using an optical microscope. All the abrasive wear experiments were performed twice to check for repeatability.

Figure 1: Test configuration

Two sets of SiC abrasive tapes with different grit sizes (#320 and #500) were used in these experiments and the hardness of abrasive particles was HV 30-800. The tests were run at two different loads and speeds. The test parameters are shown in Table 2.


Test parameters


Speed mm/s

Load N

Contact pressure, KPa

Level 1 500 ( 18) 3.14 10 379 Level 2 320 ( 34) 31.42 20 536


108

2.3 Oil absorption To compare the oil absorption of the elastomers, the test specimens immersed in the oil at 120°C for two weeks (336 hours) and their weight changes were measured. The specimens were washed with heptane, dried and weighed before and after immersion in oil using a high resolution semi-micro electronic weighing balance. The specimens’ dimensions for these tests were 2×20×80 mm. To check for repeatability, this procedure was performed on two samples of each elastomer.

2.4 Tear tests To evaluate the influence of oil on the tear strength of the elastomers, the tear strength of aged elastomers in oil as well as that of unaged elastomers was measured using a high resolution (0.01 N) universal tensile testing machine with a trouser test piece configuration (Fig 2). The tearing process continued until complete break and the maximum load per thickness of specimen (Ts = F/d) has been calculated as the tear strength. The tear tests were performed at room temperature (22 ± 2 ºC). The aged samples were prepared in the same procedure as in oil absorption tests (section 2.3).

Figure 2: Tear test configuration (trouser piece form)

3. Results

3.1 Abrasive wear Figs 3, 4, 5 and 6 show the abrasive wear of the elastomers in dry and lubricated conditions. The results show that in most cases the abrasive wear increased in the presence of lubricants. As shown in Figs 3, 5 and 6, the abrasive wear of NBR-A, ACM and FKM increased in the presence of lubricant especially at low speed. Fig 4 shows that, the abrasive wear of NBR-B slightly increased in the presence of lubricants at low speed, but it decreased at high speed. This discrepancy can be explained by the time that the oil had to diffuse in the elastomer. Since the sliding distance is the same in all experiments, at high speed, the oil has less time to diffuse in the elastomer in comparison with that at lower speed. In view of this, the influence of lubrication at lower sliding speed is more significant as compared to that at higher speed.


109

Figure 3: Abrasive wear of NBR-A

Figure 4: Abrasive wear of NBR-B

Figure 5: Abrasive wear of ACM


110

Figure 6: Abrasive wear of FKM

The abrasive wear of these elastomers lubricated with several lubricants have been studied earlier at lower contact pressure and longer sliding distance in reciprocating sliding and similar behaviours were observed in the current study [45]. In those experiments it was seen that the influence of the current lubricant (monoester) on the increase in abrasive wear of these elastomers, with an exception of ACM) was more significant than that of the other lubricants. Furthermore, in the present study the test configuration was simpler and the friction measurements and examination of worn surfaces provided an enhanced understanding of the wear mechanisms.

3.2 FrictionAs shown in Figs 7, 8, 9 and 10, apart from FKM, the friction coefficient in the dry condition was higher than that in lubricated condition. The reduction in friction coefficient can be explained from the adhesive, hysteretic and tearing components of friction. The separating lubricant layer in the contact decreases the adhesive friction and the sealing effect decreases the hysteretic friction [14, 15]. The mechanical properties of the elastomers may vary due to the oil absorption [20]. Any change in mechanical properties, such as the hardness, may affect the adhesion and hysteresis component of friction. It can also affect the mechanism of wear (how the cracks start and propagate in the elastomer) and consequently the friction coefficient.


111

Figure 7: Friction coefficient of NBR-A

Figure 8: Friction coefficient of NBR-B

Figure 9: Friction coefficient of ACM


112

Figure 10: Friction coefficient of FKM

3.3 Oil absorption The weight change after immersion has been compared for all the elastomers. The results are shown in Fig 11. As shown in the figure, the maximum and minimum weight changes are observed for ACM and FKM respectively which is in agreement with the literature [38, 39]. The weight change of the NBR-A is higher than that of NBR-B which can be due to the different ratio of acrylonitrile content and/or different types and amounts of fillers.

Figure 11: Weight change of the elastomers after aging in the oil

3.4 Tear strength Fig 12 shows the change in tear strength of the elastomers after immersion. As shown in the figure, the tear strength of NBRs and ACM reduced due to aging in oil but it is improved for FKM. The maximum reduction in tear strength was observed for ACM.


113

Figure 12: The change in tear strength of the elastomers after aging in the oil

4. Discussion Typical worn surfaces are shown in Figs 13, 14, 15 and 16. It can be observed that a combination of both ridges (perpendicular to the sliding direction) and scratches (along with the sliding direction) were formed on the worn surfaces of NBRs (Figs 13 and 14) but the worn surfaces of ACM (Fig 15) and FKM (Fig 16) are characterised by continuous scratches.

Figure 13: The worn surface of NBR-A in dry (above) and lubricated (below) condition, Normal load:

20 N, speed: 31.42 mm/s, Abrasive grit size: #320


114

Figure 14: The worn surface of ACM in dry (above) and lubricated (below) condition, Normal load: 20

N, speed: 31.42 mm/s, Abrasive grit size: #320

Figure 15: The worn surface of ACM in dry (above) and lubricated (below) condition, Normal load: 20



115

Figure 16: The worn surface of FKM in dry (above) and lubricated (below) condition, Normal load: 20


Fig 17 shows the mechanisms of scratch and ridge formation on the surface of elastomers. When an elastomer slides against sharp asperities in point contacts, the worn surface of elastomer is characterised with scratches parallel to the sliding direction (Fig 17.a) and when an elastomer slides against a hard counterface in a line contact, the worn surface is characterised with ridges perpendicular to the sliding direction (Fig 17.b) [20-22]. In a real situation, a combination of two mechanisms may occur. As the contact pressure is more uniformly distributed, the mechanism shown in Fig 17.b is expected to become more significant and the worn surface is characterised with more ridges and fewer scratches.

Figure 17: Stress distribution and tearing the surface of elastomer in two different mechanisms of

abrasion (point and line contact) [20, 22]


116

The ridges on the worn surfaces of NBRs were concentrated close to the front part of the contact region which is at higher contact pressure (Fig 14). As shown in Figs 13 and 14, the worn surfaces in dry condition are characterised with more ridges but the worn surfaces in lubricated condition are defined with more scratches. The presence of lubricant results in a decrease of the real contact area between the asperities of abrasives and the rubber surface; therefore, the shearing stress is concentrated on the areas in contact with the tip of asperities and consequently the surface of the rubber is pulled in the direction of sliding more locally. The locally concentrated shearing stress results in the formation of scratches parallel to the direction of sliding (Fig 17.a). In the dry condition, the shear stress is distributed more uniformly on the apparent contact area resulting in more uniform tensile stress at the rear of the apparent contact area and consequently causes the formation of ridges (Fig 17.b).

As shown in Figs 15 and 16, the worn surfaces of ACM and FKM, both in dry and lubricated conditions are characterised with continuous scratches and ridges are not formed on these surfaces.

Fig 15 shows that in the dry sliding condition, aggregated worn particles have been stuck on the worn surface of ACM. Such aggregated particles have not been observed on the surfaces which were worn in the lubricated condition.

Investigation of the worn surface of NBRs shows that the worn surfaces at lower speed are characterised with more continuous ridges (Fig 18) which may be due to the viscoelastic behaviour of elastomers. As the elastomer deforms with a higher speed, the imaginary part of its module of elasticity increases and the shearing stress concentration around the point contacts becomes higher and consequently the mechanism shown in Fig 17.a becomes more significant than that in Fig 17.b.

Figure 18: The worn surface of NBR-A in dry condition, Normal load: 20 N, speed: (a) 3.14 mm/s and,

(b) 31.4 mm/s, Abrasive grit size: #520

Fig 19 shows the wear particles of the elastomers in dry and lubricated conditions. The wear particles (particularly those of ACM) in dry sliding have been aggregated but the wear particles in lubricated sliding have been dispersed. The wear particles of ACM in the dry condition have the strongest coherence and the presence of lubricant weakens this coherence;


117

thus, the influence of lubricant on increase in abrasive wear is the most significant for ACM (Figs 3 -6).

Figure 19: The wear particles of elastomers in dry (left) and lubricated (right) condition, Normal load:

20 N, speed: 31.42 mm/s, Abrasive grit size: #320


118

Figs 7 to 10 show that the friction coefficient slightly increases with speed even in the lubricated condition. It means that the hydrodynamic effect is not significant even at high speed. The increase in friction coefficient can be due to increases in hysteretic and tearing components of friction. The tear strength of elastomers at relatively low crack-tip velocities has a similar ranking as the hysteretic friction and both depend on the viscoelastic modulus [46]. Fig 12 shows that the tear strength as well as the friction coefficient of FKM, contrary to those of ACM and NBRs, increased after immersion in the oil. Since the hysteretic and adhesive components of friction are believed to decrease in the lubricated condition, it seems that the increase in friction coefficient of FKM is due to the increase in the tearing component of friction and this component of friction of elastomers sliding against sharp abrasives seems to be a significant component.

5. Conclusion Abrasive wear and friction of two acrylonitrile butadiene rubbers (nitrile rubber, NBR), an acrylic rubber (ACM) and a fluoro rubber (FKM), under dry and lubricated sliding condition were studied. Both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) were observed on the worn surfaces of nitrile rubbers but the worn surfaces of the acrylic rubber and the fluoro rubber were characterised with the scratches (parallel to the direction of sliding). The ridges on the worn surfaces of NBRs were formed close to the zone of maximum contact pressure. Presence of a lubricant in the contact reduced the ridge formation. The wear of the ACM and FKM sliding under lubricated conditions were significantly higher than those in dry sliding. The wear of the NBR-A sliding under lubricated conditions were slightly higher than that in dry sliding but the influence of the oil on wear of NBR-B depends on the sliding velocity, abrasive grit size and contact pressure. The wear particles of elastomers in dry sliding conditions were aggregated but presence of the oil in the contact dispersed the wear particles and prevented aggregation. The friction coefficient decreased with increasing of the contact pressure and increased with increasing of the sliding velocity, both in dry and lubricated condition. Apart from FKM, presence of the oil resulted in a decrease in friction coefficient as well as the tear strength of the elastomers, especially for ACM and NBR-A.

References [1] T. A. Stolarski, Tribology in Machine Design, Butterworth-Heinemann, Oxford,

2000.


[3] Information obtained from the URL http://www.offshoreseals.com/failure-analysis.htm; last access: 2008-09-04.

[4] D. F. Moore, The Friction and Lubrication of Elastomers, 1st ed, Pergamon Press, New York, 1972.

http://www.offshoreseals.com/failure-analysis.htm




119

[5] K. C. Ludema, Physical factors in tyre traction, Physics in Technology 6 (1975) 11-17.

[6] D. F. Moore, A review of adhesion theories for elastomers, Wear 22 (1972) 113-141.

[7] D. F. Moore, A review of hysteresis theories for elastomers, Wear 30 (1974) 1-34.

[8] M. Mofidi, B. Prakash, B. N. J. Persson, O. Albohr, Rubber friction on (apparently) smooth lubricated surfaces, J. Phys.-Condes. Matter 20 (2008) 085223.

[9] B. N. J. Persson, On the theory of rubber friction, Surf. Sci. 401 (1998) 445-454.

[10] B. N. J. Persson, A. I. Volokitin, Rubber friction on smooth surfaces, Eur. Phys. J. E 21(1) (2006) 69-80.

[11] K. N. G. Fuller, D. Tabor, The Effect of Surface Roughness on the Adhesion of Elastic Solids, Proc. R. Soc. Lond. A.345 (1975) 327-342.

[12] J. A. Greenwood, D. Tabor, The Friction of Hard Sliders on Lubricated Rubber: The Importance of Deformation Losses, Proc. Phys. Soc. 71 (1958) 989-1001.

[13] K. L. Johnson, K. Kendall, A. D. Roberts, Surface energy and the contact of Elastic Solids, Proc. R. Soc. Lond. A. 324 (1971) 301-313.

[14] B. N. J. Persson, O. Albohr, U. Tartaglino, A. I. Volokitin, E. Tosatti, On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion, E., J. Phys.-Condes. Matter 17 (2005) R1-R62.

[15] B. N. J. Persson, U. Tartaglino, O. Albohr, E. Tosatti, Sealing is at the origin of rubber slipping on wet roads, Nat. Mater. 3 (2004) 882-885.

[16] D. I. James, M. E. Jolley, Abrasion of rubber, MacLaren & Sons Ltd., London, 1967.





[21] A. Schallamach, Abrasion of rubber by a needle, J. Polymer Sci. 9 (1952) 385-404.

[22] B. J. Brisco, S. K. Sinha, Wear of polymers, Proc. Inst. Mech. Eng. Part J.-J. Eng. Tribol. 216- 6 (2002) 401-413.


120

[23] E. Southern, A. G. Thomas, Studies of rubber abrasion, Rubber Chem. Technol. 52 (1979) 1008-1018.

[24] A. N. Gent, C. T. R. Pulford, Mechanisms of rubber abrasion, J. Appl. Polym. Sci. 28 (1983) 943-960.


[26] S.W. Zhang, Investigation of abrasion of nitrile rubber, Rubber Chem. Technol. 57 (1984) 769-778.


[28] S. W. Zhang, Z. Yang, Energy theory of rubber abrasion by a line contact, Tribol. Int. 30 (1997) 839-843.


[30] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion. I: Abrasion pattern formation in natural rubber vulcanizate, Wear 171 (1994) 195-202.

[31] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion: Part 2. General rule in abrasion pattern formation in rubber-like materials, Wear 178 (1994) 109-116.

[32] Y. Fukahori, H. Yamazaki, Mechanism of rubber abrasion part 3: how is friction linked to fracture in rubber abrasion?, Wear 187 (1995) 19-26.

[33] Y. Fukahori, H. Liang, J. J. C. Busfield, Criteria for crack initiation during rubber abrasion, Wear 265 (2008) 387–395.

[34] H. Liang, Y. Fukahori, A.G. Thomas, J. J. C. Busfield, Rubber abrasion at steady state, Wear 266 (2009) 288–296.




[38] M. Torbake, A. Johansson, Seal material and base fluid compatibility: An overview, J. Synthetic Lubrication 22 (2005) 123-142.


121


[40] G. W. Stachowiak, A. W. Batchelor, Engineering Tribology (2nd edition), Butterworth-Heinemann, Boston, 2001.

[41] M. Mofidi, E. Kassfeldt, B. Prakash, Tribological behaviour of an elastomer aged in different oils, Tribol. Int. 41 (2008) 860-866.

[42] D. C. Evans, Polymer – fluid interactions in relation to wear, Proc. 3rd Leeds-Lyon Symp. on Tribology, Wear of Non-Metallic Materials, Sept. 1976, Publ., 1978, London, 47-71.

[43] M. K. Omar, A. G. Atkins, J. K. Lancaster, The role of crack resistance parameters in polymer wear, J. Phys. D: Appl. Phys. 19 (1986) 177-195.

[44] W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich, 1989.

[45] M. Mofidi, B. Prakash, The influence of lubrication on two body abrasive wear of sealing elastomers, NORDTRIB 2008, 13th Nordic Symposium on Tribology, Tampere, Finland 10-13 JUNE, 2008.

[46] B. N. J. Persson, O. Albohr, G. Heinrich, H. Ueba, Crack propagation in rubber – like materials, J. Phys. - Condens. Matter 17 (2005) R1071-R1142.

tribology of elastomeric seal materials991521/... · 2016-10-19 · tribology of elastomeric seal...

Documents