Multi-functionality of Non-asbestos Organic (NAO) Brake Materials

Prof. Jayashree Bijwe

Industrial Tribology Machine Dynamics & Maintenance Engineering Centre (ITMMEC)-

Indian Institute of Technology, Hauz Khas, New Delhi110016 India,

Ph 0091 11 26591280

Email- jbijwe@gmail.com, jbijwe@itmmec.iitd.ac.in

Abstract

1.0 Introduction

1.1 Tribological Situations and Role of Friction and Wear

1.2 Role of Brakes in Automotives

1.3 Evolution in Friction Materials

1.4 Formulation of Friction Materials as a Multi-Criteria Optimization Problem

1.5 Various Classes of Ingredients used in NAO Friction Materials

1.6 Complexity of Composition of Friction Materials

1.7 Complexity Involved in Performance Evaluation of Friction Materials

2. SOME HIGHLIGHTS OF RESEARCH INVESTIGATIONS ABOUT COMPLEX

INFLUENCE OF INGREDIENTS IN NAO FRICTION MATERIALS

2.1 Influence of Size, Shape and Amount of Metals in Friction Materials

2.2 Influence of Amount and Type of Resins in NAO Friction Materials

2.3 Influence of Amount and Type of Fibers in NAO Friction Materials

2.4 Influence of newly developed resins in NAO Friction Materials

3. CONCLUSIONS

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Abstract

“Brake” is a very important part of a moving system and the friction material (FM) is its

heart which is expected to continue its functioning efficiently for a long time in adverse

operating conditions. Reliability, repeatability and serviceability of a FM at an acceptable cost

are of vital importance during braking. The performance requirements from the FMs are so

complex and conflicting that its design and formulation has been well accepted as a multi-criteria

optimization problem. It has to fulfill more than dozen of major performance parameters, which

are most of the times conflicting and so sensitive to the selected type and amount of ingredients

that a small change in one of the ingredients, even in small amount may affect on other

parameters adversely. The performance of such multi- functional FM is achieved by a judicious

choice of right ingredients (generally around 15-20) in right combination and amounts with right

shapes, orientations and distribution in the matrix with right manufacturing process. Hence

industrial formulations are proprietary and designing is still considered as an art rather than

science. The present chapter focuses on the various aspects of this special class of polymeric

composites.

Key words- Friction materials; non-asbestos organic friction materials; friction and wear

performance; fade and recovery; counterface-friendliness

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1.INTRODUCTION

1.1 Tribological Situations and Role of Friction and Wear

Tribology is a science of two interacting surfaces in relative motion and encompasses friction,

wear, lubrication and related design aspects. Friction is a measure of resistance to motion of two

objects and is a joint phenomenon and is expressed as a joint parameter for quantification by

coefficient of friction (µ). Major consequences of friction are; generation of heat leading to

further thermal distortion of components; noise and vibrations. All these lead to wear and hence

wastage of energy and material.

Wear is a loss in weight or change in dimensions of a surface when two surfaces sliding against

each other are in relative motion. It is also a joint phenomenon. However, it is expressed as an

individual parameter for quantification known as wear rate/specific wear rate of each surface.

Consequences of wear are shown in Fig 1.

Figure 1- Consequences of wearing of a component

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Consequences of wearing of a component

Material loss from surface

Deterioration in accuracy & surface

damage

Frequent inspection & replacement of part.

Misalignment & hence induced

vibrations.

Generation & trapping of debris which affects µ

Fatigue & crack, debris generation & hence failure

by rapture/Fracture

µ, W

µW

µW

µ

W

µW

µW

Low Friction, Low WearAntifriction Material-: Bearings, Bushes, Gears, Slide, Seals, Cams, Prosthesis, Dentistry, Turbine Blades etc.

Low Friction, High WearPencil leads, grinding, polishing and Running In High Friction, Low

WearBrakes, Clutches, Tyres, Ignition of fire by stone etc.

High Friction, High WearErasers 

Infinite Friction, Zero WearAdhesives applied to joints 

Figure 2-Various tribo-applications with desired combinations of friction and wear in required

amounts [1]

It is a general misconception that tribological applications need low µ and low wear. As seen in

Fig. 2, there are various combinations of these two tribo-parameters required in different

applications. Amongst these friction material used in brakes and clutches fall in the area where

moderate µ and moderate wear resistance are required [1-7].

1.2 Role of Brakes in Automotives

“Brake” is one of the most important parts of the automobiles, locomotives, aircrafts and other

moving bodies because it is related to the safety of human life and machines. The safety of a

vehicle lies in its efficiency of speed negotiations with the traffic. The basic functions of the

brake systems under various operating conditions are;

Slowing down the speed in congested traffic, typically called as city driving braking.

Stopping the vehicle completely in case of emergency typically known as

emergency/panicky braking.

Holding the vehicle stationary on a slope in a hill.

Brake systems are classified as service brakes and secondary brakes. Service brakes are used for

normal braking. Secondary or emergency brakes are used during partial brake system failure, and

parking [7]. Typical brake systems consist of an energy source, application system, transmission

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system and brake assembly. Energy source (of various types such as muscular driver pedal effort,

brake boost assist systems, power brake systems, surge brakes, drop weight brakes, electric

brakes, spring brakes etc.) is responsible for producing, storing and delivering the energy for

braking. The second subsystem is the ‘energy apply system’ and is used for modulating the level

of braking. The third one is the ‘energy transmission system, (mechanical, hydraulic, pneumatic,

electrical, mixed, etc.) which is used for transferring the energy from apply system to wheel

brakes. Finally ‘brake assembly’ the fourth component is for generating the forces for retarding

the motion of the vehicle. Important facts about brakes are;

Throughput ultimately depends on the emergency braking distance (EBD)

“Brakes should be applied as little as possible but as much as necessary”

Brakes must absorb energy at controlled rates

Ability to stop gives freedom to speed

The friction material (FM) is a sacrificial one and its lining is applied on the sliding part

(pad/shoe/block/strip) which when pressed against the rotating component (disc/drum) fixed on

a wheel, converts kinetic energy into heat energy due to friction process during braking. The heat

thus generated at the sliding interface of the rotor and stator (friction material) is dissipated

primarily by conduction through various components of the brake, by convection to the

atmosphere and by radiation to the atmosphere and adjacent components. It is also absorbed by

chemical, metallurgical and wear processes occurring at the interface. The most important

function of FM is to provide adequate friction with minimal damage to the pad-surface, which

would otherwise affect the tribo-performance in consecutive braking process. Thus, the heart of

the braking device is this friction material, which is expected to continue its functioning reliably

and efficiently for a long time in adverse operating conditions.

The performance expectations from the friction couple, however, have changed

drastically due to advances in the vehicle technology. There have been increasing demands to

produce more powerful vehicles (higher speeds with larger sizes and weights) with higher

performance to power ratio and better aerodynamic properties [8] and hence demands on

performance of FMs are continuously increasing.. Nowadays it is taken for granted that the

brake system must work reliably, despite careless users, extreme speeds and adverse

environment.

1.3 Evolution in Friction Materials

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The primitive friction material i.e. leather was replaced by sintered-metal based, followed by

composites reinforced with asbestos fibers which ruled the world for almost 80 years because of

extremely good performance. However, asbestos, after proven as a health hazard, was replaced

by today’s most popular non-asbestos organic (NAO) friction materials. The drive for eco-

friendliness of friction materials along with the raised expectations for superior performance, led

to the inclusion of multiple fibers based on various classes such as ceramics, mineral, metallic,

organic etc. in FMs. Last three decades have witnessed dramatic changes in the formulations of

the materials as a result of continuous efforts in this direction for more and more improved

performance.

Friction materials are of various types (Fig.3) with their own advantages and limitations. Each

has its own application domain depending on the performance requirements and cost. The

asbestos based composites have become obsolete while semi-metallic composites are also not the

preferred materials because of limitations and problems, such as batch to batch variations in the

properties. Metal matrix composites are used for high speed trains; carbon-carbon composites

(very expensive) are used in air crafts and formula-1 racing cars. Amongst these FMs non-

asbestos based organic (NAO) FMs are quite recent (since mid eighties) and are almost

invariably used in every vehicle. The performance is significantly superior to the asbestos based

materials or semi-metallics [9-12]. The formulations are developed by the industry based on trial

and error basis or with existing expertise and most of these are in patented forms.

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Figure 3- Broad classification of friction materials

1.4 Formulation of Friction Materials as a Multi-criteria Optimization Problem

Unique functional demands on FMs require unique material properties. Brakes must work under

unlubricated sliding contact, while µ must be relatively constant, totally ”reliable” and relatively

high; the wear must be relatively mild and seizure cannot be accepted.This combination of

demands needs a material with multifunctionality. Reliability, repeatability and serviceability of

FMs at an acceptable cost are of vital importance in this area of FMs. Ideal requirements of the

FMs are mostly complex and extremely conflicting [9, 10]. If one of them is being modified,

other gets adversely affected. The friction performance attainment is believed to be a multi-

criteria optimization problem, which further relies upon the multiple objective decision-making

(MODM). Few of them, for example, are as follows.

Desired range of µ ( 0.35-0.45 in general; 0.6 for sports car) depending on the type

of vehicle- If it is higher, then vehicle may get topple up due to sudden locking of

wheels. If it is lower, it will take too long to stop the vehicle at a desired spot which will

be chaotic.

Low sensitivity of µ towards variation in operating parameters (load, speed, temperature

etc.)- Reduction in µ at elevated temperature is called temperature fade while

deterioration in µ due to increased load is called pressure fade. The FM must have fading

tendency as low as possible. Since current friction materials, non-asbestos organic (NAO)

are based on the resins which are sensitive to temperature, with increase in temperature

due to frictional heating or variation in operating pressure, µ invariably reduces to some

extent depending on organic contents of the FM. Revival of µ when the FM is cooled is

called recovery. For ideal FM fade should be minimal and recovery should be high.

Low sensitivity of µ to humidity, water and oils- µ should not be affected due to

contamination of pad surface by water, oils, brake fluids etc.

Moderate Wear resistance (WR)- WR should be moderately low. During use, ingredients

on the top working layers of FM get degraded leading to deterioration in performance.

Generation of carbonaceous material due to charring of the resin is responsible for fade.

Layer on the top surface of FM must be slowly removed for rejuvenating the µ.

Otherwise fade in µ due to accumulation of charred products will be inevitable. Hence

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there must be some reasonable wear of the pad continuously. However, if WR is too low,

then frequent replacement of pads will be required which is not advisable.

Counterface friendliness/rotor compatibility- WR of disc/drum should be high since it is

more expensive. Moreover its surface should not be damaged, scratched due to harshness

of FM since it will in turn, affect overall µ. In short, wear, cracking or grooves on rotor

are not just acceptable and FM should be friendly with rotor.

Generation of noise and vibrations- FM should not produce any type of noise such as

squeal, judder, creep, groan or low frequency vibrations. Comfort in braking without any

noise is essential to the driver as well as for the people nearby.

Metal Pick Up (MPU)- It should not pick up wear debris from the disc since it will affect

overall µ in consecutive braking.

Thermal properties-Thermal conductivity should be adequate. If it is very high, then

brake fluid will be affected and start boiling, leading to “Spongy” brakes. If it is low, heat

accumulation on the pad surface will be more leading to degradation of ingredients and

hence deterioration in µ, which is most dangerous. It is hence very complex task to strike

the right thermal conductivity of the pad.

Thermo-oxidative stability of FM should be very high since during friction flash

temperature of pad is high which tends to degrade the organic constituents directly

resulting in performance deterioration and also decompose heat sensitive ingredients such

as metal sulphides.

Conformability- Friction heating of FM, though undesirable but unavoidable should be

uniform. Hence contact spots should be evenly spread. Hence pad should be conformed

with the disc. It should have low modulus.

Mechanical properties-It should have adequate strength and toughness to take anticipated

load. Adequate compressibility and hardness are also required. If too hard, user will lose

comfort in braking and wear also will be perhaps less than anticipated apart from

undesirable aspects of noise, squeal etc. If it is too soft, it may be easily deformed and

wear will be very high, though braking comfort will be high.

No thermal fatigue or surface cracking. There are three types of stresses encountered on

the FM during friction viz. chemical, thermal and mechanical, which may lead to

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deterioration in the pad surface. Frictional heat should be conducted away without

damaging the disc.

Weight- FM should be light weight from energy saving point of view especially for

aircrafts.

Ease in production, high repeatability and consistency.

It should be environment friendly since wear debris originating from FMs are extremely

fine (including nanometer size) and hence are directly inhaled by the user on the road..

Cost viability etc.

The composition of surface layers on both the interacting surfaces, which in turn is controlled by

very complex friction mechanism and the composition of FM itself, have resulted in the utmost

complexities in tailoring the formulations. It is not surprising that this area of material

development is still regarded as a black art/magic rather than a science [13]. Single ingredient

has never been efficient to satisfy the above performance related issues and hence composite

material is the ideal choice [9,10]. A right combination of ingredients in right amounts, sizes and

shapes is required to optimize these multiple properties.

The rotor material is also expected to have following performance characteristics.

Adequate μ with FMs.

Rigid enough to resist all types of non-homogeneous stresses; but not so rigid to sustain

deformations caused by a tribo-counterpart.

High fatigue strength.

Low thermal capacity.

High thermal conductivity and dissipativity so as dissipate frictional heat from the

surface.

Good conformability with friction composites.

Very high wear resistance.

Very high fatigue resistance, both thermal and mechanical.

No tendency of any cracking.

High thermal stability and light weight.

Good abrasion resistance.

Ease of production.

Generally for drums, perlitic cast iron with Brinell hardness HB 170-280 is used.

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1.5 Various Classes of Ingredients used in NAO Friction Materials

The non-asbestos low metallic fibre reinforced phenolic based composites (NALMFRP) are

currently used in almost all the vehicles on roads or locomotives. The several thousands of

ingredients have been tried so far for developing NAO FMs and can be categorized in four

classes as shown in Fig 4. Many ingredients can be multi-functional also.

Binder –These are polymeric resins (water soluble/oily lq/solid), generally thermosetting

(added in 6-15 % by wt. in general) and are regarded as heart of FMs, which bind all

ingredients firmly so that they can contribute to the final property spectrum. Resin also

contributes to friction, wear and all performance properties. Generally phenolic resins

and their modified forms such as; Oil modified resins- (linseed, castor, soya bin etc.);

Cashew nut shell liquid (CNSL) resins; elastomer- modified resins; Cresylic resins etc.

are used in FMs. These resins have higher µ (0.6-0.7) and lower wear rate as compared to

other polymers. Thermo-reactive binder system of a phenol novolak leads to cross-

linking into a three dimensional network during pad manufacturing. Fully cross-linked

binder network gives the pad strength over a wide temp. range, which is dependent on the

manufacturing conditions such as (curing temp., time, pressure, breathings etc.) and post-

curing (time, temp etc). For final expected pad properties, different

kinds/types/combinations of resins are used. At high temperatures all resins degrade, lead

to fade and deteriorate physical, mechanical and chemical properties.

Fibres – These impart strength, resistance to wear, impact and thermal degradation and

also contribute in other performance properties such as thermal conductivity, porosity,

wear, friction, fade, recovery etc. These are primarily incorporated to improve the

strength, thermal resistance, impact resistance and friction and wear properties. The fibres

can be of ceramic, metallic, inorganic or organic types. The fibres that are most

commonly employed are of glass, steel, aramid, carbon, rock, basalt, cellulose etc.. Most

important parameters during fiber selection are; type, aspect ratio; amount; combination

sizing which leads to resin compatability, uniform distribution etc. Right mixing process

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will preserve its length and flexibility. Some comparison of properties of common fibers

used in FMs is shown in Table 1.

Figure 4. Classification of ingredients of Non-asbestos-organic friction materials [9].

Table 1-Some details of fibers used in NAO materials as a replacement for asbestos

Fibers Service

T (0C)

µ

trends

Composite

strength

Technological

compatibility

Cost Environmental

aspects

Glass 750 G G G G G

Carbon 550 F G F P U

Ceramic 1650 G F F P U

Aramid 500 F G F F U

Asbestos 600 G G E E P

P-Poor; F-Fair; G- Good; E- Excellent; U- Unknown

Ceramic fibers –functions- Sufficient thermal resilience, (high melting point- 1430 °C,

but start to soften at ≈ 600 °C); improves compressive strength and not compressibility;

increases µ and µ-instability; may increase wear resistance; excess amount may abrade

disc; may lead to NVH if in excess.

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Inorganic Fibers-functions-Thermally high stable materials; contribute to the pad

integrity; influence friction level and wear behavior depending on the fiber structure and

chemistry; potential shot works as an abrasive etc.

Organic Fibers –Functions- A special production treatment generate fibrillated fibers

from the filaments; fibrillated fibers offer a process aid to avoid separation of raw

material in a mix during transportation; fiber increases the strength of a pad; a high fiber

content in a mix increases the bulk volume and may increase porosity of FM; at elevated

temperatures organic fibers may degrade and contribute to fade etc.

Cellulose fibers-functions-Quieter product; processing aid; probably gives more

resiliency; increase in fade and wear; cost reduction etc.

Chopped Carbon fibers – Functions- Improvement in thermal conductivity and

lubricity, wear resistance, strength etc.

Aramid (para) Fibers/pulp –functions-Very good strength and temperature resistance;

very good stiffness to weight ratio; imparts good processability; provide dimensional

stability and strength to preforms; stop separation of ingredients after mixing; improves

strength of the final product; stop cracks; increase wear resistance; better friction

stability; capability of damping; reduce noise, vibration and harshness (NVH); affect

other properties porosity, voids, density etc.; not aggressive to the counterface etc. Glass

fibers works only when mixed with Aramid fibers/pulp etc.

PAN (Polyacrylonitrile) fibers-functions-Fibrillated fiber; high surface area acrylic

(PAN) pulp used for combined mix homogeneity and preform strength; offers equal

performance to aramid pulp at a lower cost when used as a processing aid; imparts

excellent frictional stability etc.

Potassium titanate whiskers- functions-Thermally resilient (high m pt≈ 1371 °C); very

hard; imparts good wear resistance etc.

Friction modifiers - These are added to impart the desired friction characteristics to the

friction materials. This class includes the abrasives like alumina, silica, SiC, zirconite,

MgO, chrome oxide etc. to boost the friction level and to clean up the pyrolysed surface

film formed on the counterface which is essential for rejuvenating original friction of FM.

FM also includes lubricants like graphite, MoS2, metal sulphides etc. to moderate and to

stabilize the friction coefficient at ambient or elevated temperatures.

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Fillers – Fillers are of two types. They are the space fillers and the functional fillers.

The space fillers are added mainly to reduce cost and to ease the manufacturing. They

do not contribute to performance properties significantly. Most commonly used inert

fillers are Barite (BaSO4), Calcium carbonate (CaCO3) etc.

The functional fillers are often incorporated to improve some of the specific properties.

For example, vermiculite is added to improve the porosity, while wollastonite is for

reinforcement. Some functional modifiers like cashew dust, brass swarfs, Cu powder, Al

and Zn powders etc. are also included to improve the fade and recovery performance.

Size of fillers has lot of bearing on the performance properties of FMs.

o Finer the size of fillers more is the surface area and more the requirement of resin to

“wet” them and more are the problems associated with resins (fade etc.).

o Bigger particles, however, are easily dug out during braking leading to more wear in spite

of higher strength of a FM.

o Optimum size of the filler is thus necessary for desired performance. Nano-size, however,

shows excellent properties because of improved adhesion with the matrix and fillers apart

from excellent capability of modifying beneficial transfer layer on the counterface [14].

Rubber- Typical rubbers are Nitrile Butadiene (NBR); Styrene butadiene (SBR),

Silicone rubber etc. Major functions are as follows.

o It can be reactive and crosslink during manufacturing or non reactive (already cured)

o These are viscoelastic materials who improve damping behavior of the pad matrix

o At high temp the rubber degrades and may lead to fade

o Friction dust- It is prepared from cashew nut shell liquid (CNSL) by its reaction with

para-Formaldehyde/hexamine/ formaldehyde to produce cross linked polymer which is

then baked in inert atmosphere followed by powdering to produce friction dust (more

thermally stable hard particles). It is moisture resistant, low ash resilient material and

does not adhere well to other ingredients. These particles increase organic contents in FM

without inviting flow problems as in case of resins. These are black in color like coke,

graphite, tire peels etc. and feel hard. At elevated temperature, however, it degrades.

Main functions are; stabilize the friction level; assists FMs to exhibit high performance

over a broad temperature range; influence pad compressibility; improves resistance to

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wear; generates low brake noise; improves skid resistance.; helps in fast heat dissipation;

suppresses brake noise, etc.

1.6 Complexity of Composition of Friction Materials

Friction materials thus encompass the above four classes of constituents making them a

combination of polymeric, carbonaceous, metallic and ceramic phases [9,10]. The performance

of such multi-phased composite materials is determined by the selection of the constituents (in

right amounts and combinations), their shapes, sizes, orientations and uniform distribution in the

matrix. The performance defining attributes of such heterogeneous materials need to be predicted

on the basis of micro-mechanics analyses, while taking into account, some of the design

variables, to a useful level of accuracy [11]. The proportions of the various ingredients are to be

chosen to suit the properties of a desired FM. For example, too little resin reduces its strength

due to inadequate binding leading to excessive wear, where as too much of the same contributes

to friction fade at elevated temperatures [15-17]. Similarly too little of abrasive inclusion makes

the frictional response inadequate where as too much of the same causes rotor incompatibility

and generates severe thermo-elastic-instabilities (TEI) [18,19]. Because of these compositional

and behavioral variations with respect to different operating, environmental and thermal

conditions, it is sometimes said that “FMs are just as variable in behavior as human beings” [13].

By juggling with all these variables of composition and manufacturing, a range of materials with

desired friction and wear properties can be produced.

The numerous brake designs employed on several types of vehicles force another level of

complexity in formulating FMs. For example, the disc pads of a racing car need to withstand

hard, intensive stresses during usage but need to last only at least for the duration of the race,

where as a colliery winder has friction brakes would last for many years and are used only in

emergencies and maneuvering. Similarly, the friction requirement of passenger cars is in the

range of 0.35 to 0.40, whereas for heavy-duty trucks (HCVs) it is in the range of 0.3-0.35.

Hence, in FM development, an attempt to improve one desired feature often proves detrimental

for other due to the interference of the inherently conflicting performance defining attributes

(PDAs). For example, FMs should respond with a stable µ and less fade at elevated

temperatures. Expecting them to be fade resistant at elevated temperatures due to frictional

heating at the braking interfaces and near 100% recovery of the µ as they are cooled down, is

another contradicting performance criterion in ideal materials. Moreover FMs need not have a

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very low wear rate as in the case of bearing materials (anti-friction materials). On the contrary,

they are expected to have a moderate wear rate so that the surface film is removed in the course

of braking and new surface is generated and to attain this abrasives are added which clean up

such films during braking action [10, 14]. However, inclusion beyond a critical level may lead to

grooving of the rotor disc surface. Similarly, the thermal conductivity of these materials is also

expected to be moderate, as too high of the same would lead to “spongy brake” and too low of

the same would accumulate heat on friction surface which is most vital and cause resin

degradation leading to fade. Such delicately balanced conflicting requirements make the problem

of formulation extremely challenging.

The effects of most of the fillers cannot be predicted accurately not only because of

insufficient fundamental knowledge about their influencing patterns in a multi phase

(heterogeneous) composition but also due to their tendency of synergistic/antagonistic effects

apart from very major influence of a third body called “tribo-film” on the disc; whose quality,

composition, thickness and tendency of back-transfer on the pad leading to secondary plateaus

and hence affecting overall performance. Moreover, quality and type of film also depends on

several parameters including compositional and operational [18,19]. Since the influence of

combination of fillers, fibers etc is so complex, that most brake lining formulations are achieved

through trial and error. Thus, the formulation and the composition play a decisive role in the

overall performance of such materials. It is for these complexities that the FM formulation

design and development is believed to be more of an art and less of science [13,20].

Apart from these with the increasing enforcements of the environment-related legislations on the

FM manufacturers and users, they have to be aware of ecological compatibility of ingredients.

There has been always a continuing stress on the manufactures; first asbestos, then Pb, Zn, etc.

and now recently copper to be avoided [21].

Thus the development of FMs is a complex interactive task in which optimized

combination of interdependent properties is sought. Hence the FM performance attainment is

believed to be multi-criteria optimization problem [11, 20], which further rests upon multiple

objective decision- making (MODM). Interestingly there has to be a correlation between the bulk

material and ever changing friction surface and is a necessary requirement in more efficient and

tailored design of friction materials, which is extremely difficult task.

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The successful formulations so far have been confined to the friction industries. It is for this

commercial sensitivity of the materials that the technology remained secluded from the scientific

community for many decades.

1.7 Complexity Involved in Performance Evaluation of Friction Materials

The theme of performance evaluation of developed FMs in general is similar to other

materials viz. laboratory testing for screening the potential on reduced scale prototype followed

by Dynamometer testing under more realistic conditions and finally field testing before

commercialization. However, various available standard schedules have added to the confusion

in research pertaining to FMs. Several researchers have come up with novel set-ups, rigs,

methods and standards for reduced scale testing. Any correlation, however, has not been

necessarily observed in the performance when compared with Dynamometer testing. Among the

testing methods the Chase and the friction assessment and screening test (FAST) inertia

dynamometer are the most popular conforming to the standard SAE J 661 a. The former,

however, is for the evaluation of FMs in the form of drum linings. Based on the detailed studies

it was accepted [22] that the performance evaluation on a chase dynamometer lacks correlation

with that of a FAST dynamometer and did not simulate the realistic conditions of the road,

environment, alpine ascents, descents etc. and hence final outcome of results did not match. In

light of the advent of modern fast moving vehicles, the FM requirements became more complex

in accordance with the applied braking pressure, speed, road conditions, and environmental

conditions. Hence, the old standards became obsolete. Continuously new approaches to brake

testing are being introduced depending on newer vehicles introduced and also realizing that more

realistic condition are necessary for evaluation. JASO C 406, Euro, R-90 (Regulation –90 by

UN) and several others have come up as per the complex vehicle dynamics requirements.

2. SOME HIGHLIGHTS OF RESEARCH INVESTIGATIONS ABOUT COMPLEX

INFLUENCE OF INGREDIENTS IN NAO FRICTION MATERIALS OK

Two approaches are practiced by the researches to study the influence of ingredients on

performance of FMs. Some prefer to develop very simple formulations (binary, ternary or

quaternary) with a view that performance of selected filler will be surfaced out emphatically

while others believe that such reduced scale composites do not represent realistic ones and hence

are unable to take into account of complex interaction (synergistic or antagonistic) and hence

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have no practical significance. They prefer to develop realistic formulations and continue to

study effect of targeted ingredient changing systematically in the selected formulations.

During comprehensive studies undertaken in Author’s laboratory to understand how ingredients

affect the performance of FMs, second approach was preferred. While developing realistic FMs

containing 10-15 ingredients, various themes were selected as follows.

Influence of combination of fibers viz organic, rockwool, PAN, carbon and

cellulose in fixed amount [ 23-26].

Influence of various abrasives (Alumina, SiC, silica and zirconia) in varying

amounts (0, 2, 4 and 6 %) [23].

Influence of varying amount and size (micron and nanometer) of abrasives

(alumina and silica) [27].

Influence of varying amount (10, 12.5 and 15 %) and types of phenolic resins

(straight, alkyl benzene modified, NBR modified, linseed oil modified, CNSL

modified) [15-17].

Influence of varying amount, type (natural and synthetic) and size of graphite

[28].

Influence of varying amount, size, shape and type of metallic contents such as

copper, brass and steel in the form of short fibers and powders (micron and nano

meter-size) [29-32].

Influence of newly synthesized resins etc. [33-36].

In each case parent composition was kept constant and only selected ingredient was changed.

While varying the amount, difference in composition was compensated by adding equivalent

amount of barite (inert filler). The FMs were developed by a particular mixing schedule of

selected ingredients for selected time in a plough shear mixer, followed by compression molding

in the form of brake-pads under selected temperature and pressure along with intermittent

breathings to expel the volatiles and finally post-curing and grinding operations. The

performance evaluation was based on physical (density), chemical (acetone extraction to

investigate uncured resin), mechanical (hardness, tensile strength, flexural strength etc.) and

tribological (reduced scale prototype, Chase machine, Krauss Machine and Brake-inertia

dynamometer as per Industrial schedules).

2.1 Influence of Size, Shape and Amount of Metallic contents in Friction Materials

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Metals such as copper, brass, steel, aluminium etc. are used in FMs in various forms; physical

(powders and fibers of various sizes and shapes) and chemical (as metals or their oxides or

sulphides). Everything affects performance properties significantly. Basically these are added to

enhance thermo-physical properties of FMs (thermal conductivity, diffusivity, specific heat,

thermal expansion etc. and most of the times wear resistance.). Amongst these, especially Cu and

alloys offers lubricating effect at high temperature. Few Efforts were done to investigate

influence of size, shape and amount of Cu in FMs [14, 29-32].

Varying amount of Cu powder (diameter- 280–430 µm) - 0, 10 and 20 wt %

a) Varying shape of Cu powder; micron size 280–430 µm and short fibers (length-2–2.35

mm)

b) Varying size of Cu Powder, micron (280–430 µm) and in combination with nano (70-90

nm).

Two series of composites were developed as follows.

SERIES 1 Composites containing

0, 10 and 20 % Cu Powder

0, 10 and 20 % Cu fibers

SERIES 2- Composites containing

0 and 10 % Cu micro-Powder

0, 10 % (2 % nano +8 % micro) Cu Powder

Developed brake pads were evaluated on Industrial Inertia Dynamometer as per JASO 406

standards. Essence of the results is shown in Figs.5 and 6.

Results for representative series of Copper-based FMs are shown in Fig 5 where wear of FM

with 20% Cu powder showed lowest wear and 20% Cu-fibers led to highest wear. µ due to fibers

was always high with lot of fluctuations (undesirable trend). Ideal material should show these

curves parallel to X axis and with increase in speed there should be very small shift in µ values.

Fibers proved poor in all these respects.

Based on Effectiveness-II (measures the efficiency of a FM to function more reliably under

different pressures and speeds) and F&R-Fade and recovery (highlights the effect of temperature

on µ) studies clear conclusions were drawn as follows.

With addition of Cu (powder and fiber) in FMs

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µ performance, fade resistance, µ recovery and wear resistance of the composites

improved.

From shape point of view, Cu powder proved better than Cu fiber in selected

performance properties.

From amount point of view, 10 wt.% proved better for friction point of view, while 20 %

proved better from wear resistance point of view.

Further studies to investigate effect of nano-sized copper particles, one more FM was developed

which contained 8 % micro-powder and 2 % nano powder and the performance was compared

with that containing 0 % and 10 % micro- powder of Cu. It was concluded that replacement of

just 2 % micro-powder by equivalent amount of nano-particles, performance increased

significantly. (Fig 6) (underlined sentence was OK. Edited is NOT OK)

While conducting in depth studies on three types of metals viz. copper, brass and steel (fibrous

and powdery), several composites were developed as shown in Table 2. Detailed evaluation of

these composites led to following conclusions.

Inclusion of metallic fillers led to significant enhancement in all properties and powdery

fillers led to significant improvement in friction performance as compared to that by their

fibrous forms. Though fibers led to higher enhancement in strength, they deteriorated

friction and wear behavior significantly in general. Mechanical properties showed no

correlation with friction and wear performance.

In almost all the performance properties, copper filler proved best followed by brass,

though the difference was marginal. Iron powder or steel fibers performed poorly except

in case of recovery behavior and counterface friendliness where they excelled. Though

iron powder is the cheapest metallic filler, it is not the proper choice. Thus performance

order was; copper > brass>>> iron which matched with their thermal conductivity order.

Performance was exactly in reverse order of their cost.

For lower µ-pressure-sensitivity, 20 % loading of powders proved best while for µ-

temperature sensitivity, 10 % loading worked best.

In case of FMs with powdery fillers, uniform trends and good correlations were observed

with few properties such as; wear performance and thermal conductivity; TL behavior

with thermal effusivity and compressibility; performance µ of FMs with µ of other metals

in FMs studied in similar conditions etc. For fiber series, no correlation emerged. More

19

heterogeneous structure due to inclusion of fibers could be one of the reasons for the

same.

Table 2- Formulation design for series based on metallic powders and metallic fibers

(a) Metallic powders (Parent formulation 60%)

Powder SeriesSeries name Brass-(BP series) Copper-(CP series) Iron-(IP series)

Designation of composites BP0 BP1 BP2 CP0 CP1 CP2 IP0 IP1 IP2

Selected metal (wt. %) 0 10 20 0 10 20 0 10 20Barite (inert filler) (wt. %) 40 30 20 40 30 20 40 30 20

(b)- Metallic fibers (Parent formulation 60%)Fiber series

Series name Brass-(BF series) Copper- (CF series) Steel- (SF series)Designation of composites BF0 BF1 BF2 CF0 CF1 CF2 SF0 SF1 SF2

Selected metal (wt. %) 0 10 20 0 10 20 0 10 20Barite (inert filler) (wt. %) 40 30 20 40 30 20 40 30 20

Abbreviations used in the Tables-: B- Brass, C- copper, I- iron, S- steel, P-powder, F- fiber and subscripts 0,1and 2 for 0, 10 and 20 wt.% of selected metal in composites respectively. BP 0, CP0,

IP0, BF0 CF0 and SF0 are the same composites and during inter-comparison between all composites a special designation ‘Ref’ was used for these 0 wt. % composites.

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Figure 5- Figures in left column indicate sensitivity of µ towards pressure and speed

(Dynamometer test) while figures in right column indicate results on Krauss test for wear

(top) and µ -temperature relation (bottom).

21

Figure 6-Influence of copper powder in NAO FM performance with varying amount (0-

CRef and 10 % CM) with micro and nano-size (CN contains 8 % micro and 2 % nano-Cu

particles) .

2.2 Influence of Amount and Type of Resins in NAO Friction Materials

Generally phenolic resins are used in FMs as binders. There are several varieties of this class of

materials and studies were taken up to investigate type and amount of resins on performance

properties [15-17]. Five resins viz. straight phenolic (S) , alkyl benzene modified (A); cashew

nut shell liquid (CNSL) modified (C); nitrile-butadiene rubber (NBR) modified (N) and linseed

oil modified (L) were selected and 5 series of composites containing all identical ingredients

except resin were developed. Each of these series contained same resin in three amounts viz. 10,

12.5 and 15 %. In depth studies led to the conclusion that

No FM worked best or worst in all the performance parameters selected as priority wise

such as, performance µ, % fade, fade µ, wear, rise in temperature of disc, % recovery,

recovery µ etc. In general, 12.5 % amount led to better results and proved to be optimum

22

amount. Higher amount led to more fade and very poor friction performance while lower

led to more wear.

Alkyl benzene modified resin proved best from friction related parameters. However, its

wear performance was poorest though strength properties were highest.

On the contrary, linseed oil modified FMs proved best for wear performance & poorest

from strength and friction related aspects.

2.3 Influence of Amount and Type of Fibers in NAO Friction Materials

Fibers are essential for replacing asbestos in FMs. In depth studies were taken up to compare

contribution of various fibers such as Rockwool (Lapinus), PAN-Polyacrylonitrile; Aramid pulp,

Carbon and cellulose [23-26].

Series of composites was developed and tribo-evaluated keeping all ingredients (87%) including

rockwool (10%) as constant and varying these four fibers (3%) in each FM. It was concluded

that extent of contribution by these fibers was as follows.

Magnitude of µ-Cellulose > Aramid >PAN≥ Carbon

Sensitivity of µ to pressure-(lower the better)- PAN > Cellulose > Aramid > Carbon

Sensitivity of µ to pressure-(lower the better)- PAN ≥ Cellulose > Aramid > Carbon

Wear –Aramid ≥carbon ≥PAN >>>>Cellulose

Resistance to fade-(higher the better) -Carbon >>>PAN >Aramid>cellulose

Recovery--(higher the better)-Cellulose>PAN >Carbon>Aramid

Resistance to disc temperature rise- (higher the better)-Carbon>PAN>Aramid>cellulose

Thus no fiber proved best in offering all these performing parameters. Overall, one has to

strike a balance of combination of these fibers to achieve delicate balance of desired

properties.

2.4 Influence of Newly Developed Resins in NAO Friction Materials

Phenolic resins are invariably used for developing FMs although they are associated with

following serious problems including those related to environmental pollution. These include;

Necessity of harsh chemicals as catalysts such as NaOH during its synthesis

Evolution of noxious volatiles (ammonia, formaldehyde) during molding of products leading

to environmental pollution and voids and cracks in the molded products.

Poor shelf life hence problems in transportation and storage.

Shrinkage in molded products

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Keeping this in view new type of benzoxazine resins were developed which do not have above-

mentioned flaws. Four types of FMs containing identical ingredients (90%) and varying four

resins (10 %) in each FM were developed in the laboratory. Fifth FM was developed with

identical ingredients but was based on traditional resin (Straight phenolic resin-10%). In depth

performance evaluation and comparison with FM containing 10 % led to the conclusion that

these resins excelled in all performance properties desired for FMs and performed significantly

better than the conventional resins. Table 3 shows gist of the studies based on performance

parameters of the selected FMs. FM based on new resin showed all properties better than that of

phenolic based FM and also commercial material.

Table 3- Performance parameters of FMs (developed in the laboratory) and commercial

one.

Properties A- FM based on new resin

P-FM based on phenolic resin

C-FM of commercial Brake-pad (reputed company)

μperformance 0.389 0.386 0.430

μRecovery 0.408 0.411 0.471

μFade 0.357 0.329 0.379

% Fade 8 15 12

% Recovery 105 106 109

Wear (x10-6m3) 5.3 6.9 8.8

Temp. rise of disc (°C) 387 456 439

3. CONCLUSIONS

Based on the detailed elaboration on the complexity of simultaneous influence of multiple

ingredients in various sizes, shapes, amounts and in combinations on performance properties of

non-asbestos organic friction materials, it can be concluded that this class of multi-functionality

materials is still a challenge to the researchers and practitioners in the industry. In spite of

extensive efforts to understand the functioning mechanisms of these materials by the researchers,

hardly any commendable knowledge has been accumulated over the several decades. It is still a

weakly researched area; still some type of “black- magic” or an “art” rather than fully revealed

science. It is still a monopoly of practitioners or experts in the Industry who knows the knack of

developing successful formulations or altering the performance parameters as per newer

24

requirements of vehicles as a consequence of innovations in vehicle technology. Nevertheless,

continuous research efforts in this area may bring more transparency in the behavioral patterns of

this class of materials which may use approximately 15-25 ingredients from the array of

thousand of potential ingredients. That needs more systematic efforts on investigating reasons for

synergism or antagonism of typical combination of materials apart from investigating their

complete interaction with the counterpart whose roughness, and texture continuously changes

during braking cycles leading to vicious chain of events of film transfer, back transfer, back-back

transfer, glazing, metal pick up, scratching, scoring etc.

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1. Friedrich, K., editor. Advances in composite tribology”, Composite Materials Series 8, Elsevier

Amsterdam, The Netherlands, (1993)

For Ref 2, there is no editor.

1. A.M. Hager, M. Davies, in K. Friedrich (ed.), Advances in composite tribology,

Composite Materials Series, Vol.8, Elsevier, Amsterdam (1993) ---.

2. K.H. Zum Gahr, “Micro-Structure and Wear of Materials”, Tribology Series, 10, Elsevier

Amsterdam, 1987.

2 Crane, Stephen. The Red Badge of Courage: An Episode of American Civil War.

1895. Ed. Fredson Bowers. Charlottesville: UP of Virginia, 1975. Print.

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