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CHAPTER 5

STUDIES ON MECHANICAL, THERMAL, WEAR AND

MORPHOLOGICAL BEHAVIOURS OF MOLYBDENUM

DISULPHIDE FILLED NYLON 66/CARBON

BLACK/COMPOSITES

In this study, the effect of different weight fractions of MoS2 viz., 0, 0.5, 1.0,

2.0 and 3.0 wt. % content on the nylon 66/carbon black (CB) composites are studied.

With an objective to investigate the influence of molybdenum disulphides (MoS2) in

the presence of CB on wear characteristics of nylon 66. Nylon 66 was compounded

with MoS2 and CB in a co-rotating twin screw extruder. The fabricated composites

were evaluated for the surface friction, wear, laser etching resistance along with

physico-mechanical, thermal and morphological characteristic. The wear behaviour of

nylon 66/CB/MoS2 composites were investigated under dry sliding conditions at

different normal loads, sliding distances and sliding velocities at room temperature.

Additional techniques such as SEM and optical microscopy were performed to probe

the wear mechanism. It was found that the introduction of MoS2 in the presence of CB

has certainly reduced the friction, wear behaviour of nylon 66 with improvement in

laser etching resistance. MoS2 could increase the adhesion between the transfer film

and the counterface surface. The ability of the synergistic fillers in helping the

formation of thin, uniform and continuous transfer film would contribute to the

increase in wear resistance of nylon 66 composites.

5.1. Introduction

Polymer composites have been increasingly used for numerous tribological

purposes such as seals, gears and bearings, providing light weight alternatives to

metallic components. The feature that makes polymer composites so promising in

industrial applications is the possibility of tailoring their properties with special fillers,

low cost of materials and quantum of production. Due to the low coefficient of

friction and also the ability to maintain loads, some specific grades of polymer are

used in place of the traditional metal based materials in recent times [1,2].

Furthermore polymer gears and bearings can accommodate shock loading, shaft

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misalignment and bending better than metal parts. Polyamides (PAs) and polyacetals

are widely used thermoplastic polymers for such applications. Polyamides such as

PA6 and PA66 are engineering thermoplastics which have found great attractions in

such applications due to their desirable mechanical properties, suitable tribological

characteristics and ease of processing.

It was reported that the friction and wear behaviour of nylons was fairly

satisfactory under dry sliding conditions and lubrication at higher speeds. However, in

order to keep pace with the modern technological innovations, ever increasing

demands are being placed on tribo-materials for enhanced performance for operating

under stringent conditions of loads, speeds, temperatures and hazardous

environments. In order to enhance the tribological characteristics of nylons efficiently,

solid lubricants may be added into the polymer matrix. A solid lubricant is defined as

a material that provides lubrication, under essentially dry conditions, to two surfaces

moving relatively to each other. The solid lubricants often lead to decrease of friction

coefficient and wear rate through the reduction in adhesion with the counterface or

creation a transfer film with a low shear strength at the interface [3,4]. A number of

scientists are reported on the improvement in wear resistance of various nylons such

as nylon 66, nylon 6, nylon 11, nylon 46, etc., with solid lubricants such as

polytetrafluoroethylene (PTFE), graphite (Gr) and MoS2 [5,6]. These are the pre-

dominant materials used as solid lubricants in thermoplastics.

Among these solid lubricants, MoS2 is a common dry lubricant in the form of

black crystalline powder and it has a layered structure. In its appearance and feel,

MoS2 is similar to graphite. It is a dark blue-grey or black solid, which feels slippery

or greasy to the touch. It has hexagonal layer lattice. It is widely used as a solid

lubricant because of its low friction properties, sometimes to relatively high

temperatures. MoS2 with particle sizes in the range of 1–100 µm is a common dry

lubricant. MoS2 is often a component of blends and composites where low friction is

sought. When added to plastics, MoS2 forms a composite with improved strength as

well as reduced friction. Polymers that have been filled with MoS2 include nylon.

MoS2 and graphite have a layered structure. The important effect is that the materials

can shear more easily parallel to the layers than across them. They can therefore

support relatively heavy loads at right angle to the layers while still being able to slide

easily parallel to the layers. This property is being effectively used for lubrication

process. The coefficient of friction is more or less equal to the shear stress parallel to

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the layers divided by the yield stress or hardness perpendicular to the layers. Because

the low friction only occurs parallel to the layers, it follows that these solid lubricants

will only be effective when their layers are parallel to the direction of sliding. It is

also important that the solid lubricant should adhere strongly to the bearing surface;

otherwise it would be easily rubbed away and gives very short service life.

Many researchers have studied the friction and wear behaviour of MoS2 filled

polymers. Yinping Ye et al [7] have reported that high load and frequency promote

the formation of a compact transfer films. The compact transfer films are believed to

be the predominant mechanism giving rise to high load-carrying capacity, and

excellent wear-resistance performances of the bonded MoS2 solid film lubricants

[7,8]. Liu et al [9] pointed out that MoS2 was not very effective for reducing friction

and caused an increase in wear of nylon 6, while Steinbuch [10] reported that MoS2

filled nylon could reduce the wear rate but not its friction coefficient. Bijwe et al [11]

proposed that the addition of MoS2 alone did not impart a good wear resistance to

PTFE, especially during severe conditions of sliding. However, addition of lubricating

MoS2 in PTFE matrix and alone with reinforcing phase (glass fibers) had the potential

to reduce the abrasive wear by maintaining a low friction transfer film. This film

would have been disrupted by the glass fiber if only PTFE were present [12].

The objective of the present work was to look at the effects of the transfer film

and the tribo-chemical reactions between MoS2 filler and carbon black on the

tribological properties of nylon, in order to find out the important factors for reducing

wear and friction of nylon 66. Nylon 66 was compounded with MoS2 and CB. Carbon

black is a form of amorphous carbon that has a high surface area to volume ratio. It is

used as pigment and filler in rubber products, especially tires. Compounding of CB

with rubber improves the tensile strength and wear resistance, but it is not much

explored in the tribological performances of thermoplastics like nylon. Under these

circumstances, nylon 66 was compounded with varying amounts of, i.e., 0.5, 1.0, 2.0

and 3.0 wt%, MoS2 with one weight percentage of CB. Thus the fabricated nylon

66/CB/MoS2 composites have been evaluated for friction, wear, scratches and

morphological characteristics along with thermal and physico-mechanical properties.

The wear characteristics of the composites were evaluated using pin-on-disc

equipment.

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5.2 Compounding and specimen preparation

CB powder (1 wt %) was premixed with varying amounts viz., 0.5, 1, 2 and 3

wt % of MoS2 powder and then mixed with nylon 66 in tumbling mixer for 15 min,

after pre-drying in hot air oven at 80°C for 8 h duration, and then melt blended using

a co-rotating intermesh twin screw extruder at a screw speed of 175 rpm with barrel

temperature ranging from 200 to 280 ºC. The extruder consists of nine nozzles and the

temperature zones maintained at each of the nozzles are different and lay in the range

200-280 o

C. The extrudate strand was pelletized and stored in sealed packs containing

desiccant. The test specimens for tensile behaviours, impact strength, and water

absorption were prepared using an R.H. Windsor India, SD-75 automatic injection

molding machine with 70 ton clamping pressure at 260-285 oC and an injection

pressure of 80 bars. After molding, the test specimens were conditioned at 23 ± 2 oC

and 50 ± 5% RH for 40 h according to ASTM D 618 prior to testing.

5.3 Results and Discussion

5.3.1 Physico - mechanical properties

The prepared nylon 66/CB/MoS2 composites were characterized for physico –

mechanical properties according to ASTM methods. The measured physico-

mechanical properties such as water uptake, density, void content and surface

hardness for nylon 66/CB/MoS2 are given in Table 5.1.

5.3.1.1 Water uptake behaviour

The results shown in Table 6.1 indicates that the incorporation of MoS2 has

reduced the water uptake from 2.7 (0.5% MoS2) to 2.1% (3.0% MoS2). When working

with nylons, it is important to examine the water absorption, because it severely

affects its mechanical and thermal properties.

5.3.1.2 Density

The density measurements were performed on all composites of nylon

66/CB/MoS2. The density values of nylon 66/CB/MoS2 composites falls in the range

1.136– 1.148g/cc. Table 5.1 shows that the density of composites increased linearly

with increase in MoS2 content. MoS2 being a higher dense material, the composite

material density increases with increase in MoS2 content.

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Table 5.1. Physical properties of nylon 66/CB/ MoS2composites

MoS2 (%) Water

uptake (%)

Density (g/cc) Void content

(%)

Surface

hardness

(Shore D) Expt. Theo.

0 2.7 1.136 1.144 0.72 79

0.5 2.6 1.140 1.149 0.75 73

1.0 2.4 1.142 1.153 0.96 69

2.0 2.3 1.144 1.162 1.55 64

3.0 2.1 1.148 1.171 1.98 62

5.3.1.3 Surface hardness

Surface hardness is a measure of resistance to indentation. Surface hardness

indicates the degree of compatibility to certain extent. The surface hardness values of

nylon 66/CB/MoS2 composites falls in the range 79 - 62 shore D (Table 5.1). From the

table it is noticed that a significant decrease in surface hardness values with increase

in MoS2 content.

Table 5.2. Mechanical properties of nylon 66/CB/ MoS2composites

MoS2

content(%)

Tensile

strength (σ)

(MPa)

Tensile

modulus

(MPa)

Elongation

at break,

(e) (%)

Product

Parameter

(σ x e)

Impact

strength

(J/m)

0 65 2670 10.3 669 37.2

0.5 68 2712 13.2 897 38.4

1.0 73 2814 16.7 1219 41.4

2.0 76 2944 20.8 1581 42.5

3.0 78 2975 22.5 1755 43.2

5.3.1.4 Tensile behaviour

From the Table 5.2, it is noticed that tensile strength and percentage

elongation at break increases with increasing in MoS2 content. The tensile strength of

nylon 66/CB/MoS2 lays in the range 65 – 78 MPa. Similarly, percentage elongation at

break lies in the range 10.3 - 22.5 % for MoS2 filled nylon 66/CB composites. The

tensile modulus of the composites increased from 2670 to 2975 MPa. These results

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indicate that the tensile properties increased significantly with increase in MoS2

content in nylon 66/CB matrix.

5.3.1.5 Impact strength

The impact strength of the composites depends upon many factors like

toughness properties of the polymers, the degree of miscibility and phase

morphology. The nature of the interface region is of extreme importance in

determining the toughness of the composites. Table 5.2 reveals increase in izod

impact strength from 37.2 to 43.2 J/m.

5.3.2 Thermal behaviours

5.3.2.1 Differential scanning calorimetry

The thermal properties of the nylon 66/CB/MoS2 composites were

investigated by DSC technique to analyze the effect of MoS2 content on the

crystallization melting temperature, enthalpy of melting and percentage of

crystallisation and shown in Figure 6.1.

Figure 5.1. DSC thermograms for nylon 66/ CB/MoS2 composites

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The data obtained from DSC analysis are given in Table 6.3. Table reveals

that the onset of Crystallization temperature (To), peak melting temperature of

crystallization (Tm) and completion of crystallization temperature (Tc) are decreased

linearly with increase in MoS2 content. Similarly the enthalpy of crystallization (∆H)

and percentage crystallinity (χc) also decreased from 75.1 to 58.6 J/g and from 38.3 to

29.9% respectively with increase in MoS2 content. The above results clearly show that

the incorporation of MoS2 particles gone in-between the molecular layers of nylon 66

and reduced the crystalline behaviour of nylon 66. The reduction in ∆H and χc values

is due to the plasticization effect of MoS2.

Table 5.3. Thermal data (glass transition temperatures) obtained from DSC

thermograms for nylon 66/ CB/MoS2 composites

Composition

of MoS2 in

nylon 66/CB/

MoS2

composites

(%, wt/wt)

To

(ºC)

Tm

(ºC)

Tc

(ºC)

H (J/g) Crystallinity* χc

(%)

Exp. Cal. Dev.

(%) Exp. Cal.

Dev.

(%)

0 243 265 278 75.1 - - 38.3 - -

0.5 242 264 277 73.0 74.7 -2.4 37.2 38.1 -2.4

1.0 241 263 272 70.6 74.3 -5.3 36.0 37.9 -5.3

2.0 240 263 271 65.4 73.6 -12.5 33.4 37.6 -12.5

3.0 239 262 270 58.6 72.8 -24.3 29.9 37.2 -24.3

*The heat of fusion value of 100% crystalline nylon 66 is 196 J/g.

5.3.2.2 Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) is one of the most appropriate methods

to study viscoelastic behaviour and relaxations in polymeric materials. The Tg is a key

process in most polymers and influences use and processability of the material,

possibly more than any other factor. This technique provides very revealing

information about these relaxations through the tan δ vs. temperature data. Tan δ is an

important parameter characterizing material’s viscoelastic behaviour [36]. The same

experiment also yields the stiffness (modulus) of the material versus temperature. The

DMA properties such as storage modulus (E'), loss modulus (E'') and loss tangent

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(tanδ) are recorded as a function of temperature from 25 to 200°C are shown in

Figures 5.2, 5.3 and 5.4 respectively.

Figure 5.2. Plots of storage modulus versus temperature for nylon 66/CB/MoS2

composites

From the thermograms, it can be noticed that, the incorporation of MoS2 into

nylon, causes remarkable increase of E’ and E'' values as compared to pure nylon 66.

This indicates that the increase in MoS2 content in nylon 66 increases the energy of

dissipation. The obtained tan δ values (both predicted and experimental) from DMA

thermograms for nylon 66/CB/MoS2 composites along with Tg which was represented

by the peak temperature of the tan δ curve is addressed in Table 5.4.

Table 5.4. Data obtained from DMA analysis for nylon 66/CB/MoS2 composites

MoS2

wt. %

Tan δ Tg

Storage modulus (MPa)

Exp. Cal. Glassy region Rubbery region

0 0.088 - 64.8 903 361

0.5 0.087 0.087 65.1 954 387

1.0 0.085 0.086 65.6 1011 405

2.0 0.082 0.085 63.7 1095 406

3.0 0.080 0.084 63.0 1127 441

The storage modulus of nylon 66 measured at 40 °C was 903 MPa and it

increases after incorporating MoS2 into nylon matrix. The maximum storage modulus

value was noticed to be 1127 MPa for 3 % MoS2 loaded nylon 66/CB composites,

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which is about 25 % higher than that of nylon 66. The results obtained in this study

are comparable to the literature data [37]. Table 5.4 also lists the average values of Tg

for different wt. % of MoS2. The measured Tg values of nylon 66/CB/MoS2

composites are in the range 65-63°C.

Figure 5.3. Plots of loss modulus versus temperature for nylon 66/CB/MoS2

composites

Figure 5.4. Plots of loss tangent versus temperature for nylon 66/CB/MoS2

composites

5.3.2.3 Thermo gravimetric analysis

TG analysis was carried out in order to understand the influence of MoS2

addition on the thermal stability of the nylon 66/CB/MoS2 composites. The TGA

thermograms of MoS2 loaded nylon 66/CB composites are shown in Figures 5.5 along

with inset thermograms. The temperature range of thermal degradation was analyzed

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from the TGA thermograms and is given in Tables 5.5. TGA thermograms of the

composites indicate only single stage thermal degradation process. The decomposition

temperature of composites was started at 334°C and takes place upto 710°C, which

corresponds to the weight loss ranging from 95.9 to 98.2 %. From TGA curves it can

be clearly observed that the thermal stability markedly improved with increase in

MoS2 content.

Figure 5.5. TGA thermograms for nylon 66/ CB/MoS2 composites

Some characteristics TGA data related to the temperature corresponding to

weight loss such as T0 (temperature of onset decomposition), T10 (temperature for 10

% weight loss), T20 (temperature for 20% weight loss), T50 (temperature for 50 %

weight loss) and Tmax (temperature for maximum weight loss) are the main criteria to

indicate their thermal stability of the composites (Table 5.5). Higher the values of T10,

T20, T50 and Tmax higher will be the thermal stability of the composites [13].

Figure 5.5 and Table 5.5 data reveals that the initial stage thermal degradation

process pattern is almost same for all nylon 66/CB/MoS2 composites. Higher the

values of oxidation index (OI), higher will be the thermal stability [13-16]. From the

table it was observed that the OI values increases with increase in MoS2 content and it

lies in the range 0.178 - 0.476 (Table 5.5). This data indicates that the nylon

66/CB/MoS2 composites are more thermally stable.

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Table 5.5. Thermal data obtained from TGA thermograms of nylon 66/

CB/ MoS2 composites

MOS2 content in nylon

66/CB composites

(%, wt.)

Temperature at different weight loss (± 2ºC) Oxidation

Index

(OI) T0 T10 T20 T50 Tmax

0 334 407 423 440 641 0.178

0.5 335 408 424 443 692 0.239

1.0 336 409 430 450 694 0.288

2.0 339 410 432 452 701 0.389

3.0 349 413 436 457 710 0.476

5.3.3 Wear studies

In order to improve the friction and wear behaviour of polymeric materials,

one typical concept is to reduce their adhesion to the counterpart material and to

enhance their hardness, stiffness and compressive strength. This can be achieved quite

successfully by using special fillers. To reduce the adhesion, internal lubricants are

frequently incorporated. One of the mechanisms of the corresponding reduction in the

coefficient of friction is the formation of transfer film on the surface of the

counterpart [17]. The wear resistance is increased when fillers decompose and

generate reaction products which enhance the bonding between the transfer film and

the counterface [18], MoS2 is one such filler and lubricant that will decompose and

produce MoO3, FeS, FeSO4 and Fe2(SO4)3 during sliding. These compounds could

increase the adhesion between the transfer film and the counterface surface.

Normally the matrix should possess a high temperature resistance to withstand the

high heat generated during the frictional dry sliding. Additional fillers that enhance

the thermal conductivity are often of great advantage, especially if effects of

temperature enhancement in the contact area must be avoided in order to prevent an

increase in the specific wear rate. Carbon black is one such conductive filler which

forms conductive pathways in the polymer matrix to dissipate the heat due to

frictional sliding. Here we have utilized both MoS2 and CB to have synergistic effect

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in formation of thin, uniform and continuous transfer film would contribute to the

increase in wear resistance of nylon composites.

5.3.3.1 Wear loss

The plots of wear loss as a function of MoS2 content (0, 0.5, 1, 2, and 3 wt %)

at different applied loads (50, 100 and 150N) and sliding distances (1000, 1500 and

2000m) for all nylon 66/CB/MoS2 composites are shown in Figures 5.6(a)-(c). The

plots of wear loss as a function of loads, sliding distances and sliding velocities are

shown in Figures 5.7-5.9 respectively. All the plots indicate that wear loss decreases

with increase in MoS2 content. That means pristine nylon 66 has more wear loss than

blends containing MoS2 content for all loads investigated. It is clearly evident from all

the plots that the MoS2 content has significant influence on the wear behaviour of the

composites.

Figure 5.6. Weight loss as a function of MoS2 content for nylon 66/CB

composites at varying sliding distances and at varying loads; (a) 50 N, (b) 100 N

and (c) 150 N

Figure 5.7. Weight loss as a function of load for MoS2 filled nylon 66/CB

composites at sliding distances of 1000 m, 1500 m and 2000 m

153

Figure 5.8. Weight loss as a function of sliding distance for nylon 66/CB/ MoS2

composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 0.5 1 2 3

Wei

ght

loss

(g)

MoS2 (%)

50 N 5 m/s

7

9

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 0.5 1 2 3

Wei

ght

loss

(g)

MoS2 (%)

100 N 5 m/s

7

9

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 0.5 1 2 3W

eigh

t lo

ss (g

)

MoS2 (%)

150 N 5 m/s

7

9

Figure 5.9. Weight loss as a function of MoS2 content for nylon 66/CB

composites at varying sliding velocities and at varying loads; (a) 50 N, (b) 100 N

and (c) 150 N

The improvement in wear resistance is due to the presence of solid lubricant

(MoS2) dispersed in the polymer matrix. These MoS2 dispersed in the polymer matrix

acts as barrier and also prevent large scale fragmentation of nylon 66/CB matrix. This

behaviour is clearly observed from SEM pictures. The MoS2 also acts as reinforcing

element, bears the load and reduces the wear rate. The wear resistance is maximum

for the inorganic MoS2 content of 3 wt. %. The wear loss of MoS2 filled nylon 66/CB

composites exhibiting an inverse relationship with all the parameters as the inorganic

filler content MoS2 goes on increases in the composites. It was reported that addition

of MoS2 with the base polymers lowers the wear rate when tested under unlubricated

dry conditions [7-8, 19]. When polymers slide against metal counterfaces, transfer

films are formed and the wear behaviour of a polymer in dry sliding condition is

strongly influenced by its ability to form a transfer film on the counterface [20-21].

The transfer films provide a shielding of the soft polymer surface from the hard metal

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asperities [21-26]. This study indicates that anti-wear properties of nylon 66/CB

composites are improved markedly after incorporation of MoS2.

5.3.3.2 Specific wear rate

Specific wear rate as a function of MoS2 content (0, 0.5, 1, 2, and 3 wt %) at

different applied loads (50, 100 and 150N) and sliding distances (1000, 1500 and

2000m) for all nylon 66/CB/MoS2 composites are shown in Figures 5.10(a)-(c).

Specific wear rate as a function of loads, sliding distances and sliding velocities are

given in 5.11-5.12(a)-(c) respectively. In all these plots, specific wear rate is inversely

proportional to MoS2 content, load and sliding distance. It is evident that the increase

of MoS2 content from 0 to 3 wt % led to a remarkable decrease of specific wear rate.

This kind of variation has been reported for glass fabric reinforced epoxy composites

[27]. It is evident that the increase of MoS2 content from 0.5 to 3 wt % in nylon led to

a remarkable reduction in specific wear rate of the composites. Similar kind of

variation has been reported elsewhere for glass fabric reinforced epoxy composites

[28].

Figure 5.10. Specific wear rate as a function of MoS2 content for nylon 66/CB

composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N

Figure 5.11. Specific wear rate as a function of sliding distance for nylon 66/CB/

MoS2 composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N

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Figure 5.12. Specific wear rate as a function of load for nylon 66/CB/

MoS2composites at sliding distances of 1000 m, 1500 m and 2000 m

The order of wear resistance behaviour of composites is as follows; 3 > 2 > 1

> 0.5 % by weight of MoS2. Also, it was noticed that 3 wt % MoS2 filled composites

exhibited the higher wear resistance under all sliding distances/loads. The reduction in

specific wear rate with increase in MoS2 content in nylon 66 matrix is due to the

transfer film formed on the counterface, which act as effective barrier to prevent

large-scale fragmentation of polymer. It is well known that the wear behaviour of a

polymer sliding against a metal is strongly influenced by its ability to form a transfer

film on the counterface [6].

5.3.3.3 Co-efficient of friction

The 'coefficient of friction' (COF), µ, is a dimensionless scalar value which

describes the ratio of the force of friction between two bodies and the force pressing

them together. The coefficient of friction depends on the materials used. A common

way to reduce friction is by using a lubricant, the science of friction and lubrication is

called tribology. One of the mechanisms of the corresponding reduction in the

coefficient of friction is the formation of a transfer film on the surface of the

counterpart [17]. It is also important that the solid lubricant should adhere strongly to

the bearing surface; otherwise it would be easily rubbed away and gives very short

service life. Yinping Ye et al [7] have reported that high load and frequency promote

the formation of a compact transfer films. The compact transfer films are believed to

be the predominant mechanism giving rise to high load-carrying capacity, and

excellent wear-resistance performances of the bonded MoS2 solid film lubricants

[7,8]. The variation in coefficient of friction as a function of MoS2 at 5 m/s sliding

velocity for MoS2 filled nylon 66/CB composites is tabulated in Table 5.6. The co-

156

efficient of friction decreases with increase in MoS2 content. This behaviour is

obviously relevant to the lubricating effect of MoS2 which can reduce the adhesion

between the composite with the metallic counterpart. MoS2 is composed of sheets and

layers. The layers themselves are strong but the bonding between the layers is weak.

Consequently MoS2 is strong in compression but weak in shear. This is advantageous

for producing low friction [29-34]. As the real area of contact and shear strength of

polymer substrate changes during sliding, the coefficient of friction increases with

increase in sliding load. Similar trends were observed at other sliding distances and

velocities investigated during the current studies.

Table 5.6. Coefficient of friction for nylon 66/ CB / MoS2 composites at 5 m/s

velocity

Load

(N)

Sliding

length

(m)

Co-efficient of friction for nylon 66/ CB / MoS2 composites

with varying MoS2 (%)

0 0.5 1.0 2.0 3.0

50

1000 0.169 0.157 0.154 0.144 0.132

1500 0.200 0.190 0.185 0.179 0.170

2000 0.219 0.207 0.199 0.188 0.181

100

1000 0.185 0.179 0.168 0.155 0.145

1500 0.231 0. 220 0.211 0.199 0.189

2000 0.245 0.238 0.229 0.216 0.199

150

1000 0.211 0.194 0.181 0.174 0.165

1500 0.248 0.237 0.220 0.215 0.210

2000 0.264 0.250 0.242 0.229 0.221

5.3.3.4 Surface morphology of worn surfaces

Scanning electron microscopic images are used for correlating the wear data.

Figures 5.13.(a)-(e) indicate the positive effect of MoS2 content on the worn surface

of the nylon 66/CB composites. The worn surface of 0 wt. % MoS2 filled nylon 6/CB

composites is relatively rough with more matrix damage and wear tracks compared to

other loadings (1.0, 2.0 and 3.0 wt MoS2). It can be seen from Figure 5.13(a) that the

ploughed marks appeared on the rubbing surface of composite. Figure 5.14 shows the

SEM images of MoS2 filled nylon 66/CB composites at 1000 m sliding distance, 50

N load at (a) 5 m/s and (b) 9 m/s.

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Figure 5.13. SEM images of nylon 66/CB composites at 5 m/s sliding velocity for

1000 m sliding distance and with varying amounts of MoS2 content;

(a) 0 %, (b) 0.5 %, (c) 1.0 %, (d) 2 % and (e) 3 %.

A close examination of SEM images reveals that even though both are

looking similar, but more surface damage and wear track is oberserved for higher

speed (9 m/s) than that is observed for lower speed (5 m/s). Similarly more sliding

distance (2000 m) shows more damage to the wear surface and breaking of

delaminated layers than that is observed for lower sliding distance as seen in Figure

5.15 (a) -(b). Similar results are observed at higher loads (150N) (Figure 5.16(a)–(b)).

From above four SEM images it can be conclueded the addition of MoS2 protects the

wear damage to the polymeric surface by forming a barrier film, however, both

a b

d c

e

158

increase in load and sliding distances does not prevent the surface damage or surface

delamination

Figure 5.14. SEM images for MoS2 filled nylon 66/CB composites at 1000 m

sliding distance, 50 N load at (a) 5 m/s and (b) 9 m/s.

Figure 5.15. SEM images for MoS2 filled nylon 66/CB composites at 5 m/s

sliding velocity, 100 N load (a) 1000 m and (b) 2000 m.

Figure 5.16. SEM images of nylon 66/CB/MoS2 composites at 5 m/s sliding

velocity, 150 N load (a)1000 m and (b) 2000 m.

b

a b

a

159

5.3.4 Regression analysis

The wear model for the tested materials was developed based on the applied

load, sliding velocity and sliding distance. The process parameters for the purpose of

analysis are shown in Table 5.7 Furthermore regression analysis and analysis of

variance (ANOVA) are employed to investigate the characteristics of the materials.

The dry sliding wear of composites depend on several parameters such as size, shape,

contents, environment and test conditions such as load, speed and temperature A

mathematical model will be developed by using analysis techniques such as ANOVA

and regression analysis whereby the mathematical model (Table 5.8) shows the

relationship between the input parameters and the input responses.

5.3.4.1 Process parameters

Table 5.7 Process parameters employed for analysis of variance

Levels Load (N) Sliding velocity (m/s) Sliding distance (m)

1 50 5 1000

2 100 7 1500

3 150 9 2000

Table 5.8. Regression equations for nylon 66/CB/MoS2 composites

%MoS2 Regression Equation

0 0.000897 + 0.000014 L + 0.000292 Sl.Vel +0.0036D

0.5 0.000064 + 0.000010 L + 0.000158 Sl.Vel +0.00028D

1.0 0.000117 + 0.000007 L + 0.000150 Sl.Vel +0.00030D

2.0 0.000019 + 0.000006 L + 0.000125 Sl.Vel +0.00025D

3.0 0.000139 + 0.000005 L + 0.000117 Sl.Vel +0.00024D

where, L = Load, Sl. Vel = Sliding velocity and D = Sliding distance

The analysis of variance for nylon 66/PTFE composites is shown in Table 5.9.

The Durbin- Watson statistic is used to establish the correlation amongst the

variables. If the Durbin–Watson statistic is substantially less than 2, there is evidence

of positive serial correlation. As a rough rule of thumb, if Durbin–Watson is less than

1.0, there may be cause for alarm. Small values of d indicate successive error terms

are, on average, close in value to one another, or positively correlated. If d > 2,

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successive error terms are much different in value with one another, i.e., negatively

correlated. In the table DW values for all the compositions are less than 2 indicating

the presence of positive serial correlation which can be further implied that the

material’s resistance to wear.

Table 5.9. Analysis of variance (ANOVA) for nylon 66/CB/MoS2 composites

Source % MoS2 in nylon 66/CB

0% 0.5% 1% 2% 3%

DOF 2 2 2 2 2

SS 5.12E-06 2.20E-06 1.35E-06 8.57E-07 6.53E-07

MS 2.56E-06 1.10E-06 6.73E-07 4.28E-07 3.27E-07

F 32.55 126.57 121.2 136.06 220.5

P 0.105 0.698 0.387 0.843 0.075

S 0.00028 0.00009 0.00007 0.00006 0.00004

R-SQ(%) 91.6 97.7 97.6 97.8 98.7

Coefficient 0.000897 0.000064 0.000117 0.000019 0.000139

SE Coeff 0.000471 0.000157 0.000125 0.000094 0.000065

T 1.90 0.41 0.93 0.21 2.15

R. Error 4.72E-07 5.22E-08 3.33E-08 1.89E-08 8.89E-09

DW Statistic 1.93 2.34 1.87 1.88 2.13

DOF = Degree of freedom; SS = Sum of variance; MS = Mean square; P = %

contribution; S = Standard deviation; D-W Statistics = Durbin-Watson statistics;

R. Error = Residual error, T = No. of observations.

5.3.5 Laser assisted etching behaviour

The effect of MoS2 content and power of laser on the etching of the surface of

the nylon composites has been studied. The laser etched surface has been

characterized by surface roughness in z-direction. The surface roughness values (Ra)

of laser etched specimens at different laser parameters are tabulated in Table 5.10.

The Ra values showed decreasing trend with increase of MoS2 content in the nylon

66/CB composites. That shows surface roughness decreases with increasing MoS2

content. However, between 50% and 100% power, 100% power has more roughness

than 50% power. This result indicates that MoS2 content control the etching behaviour

of the composite. Also the increase in power of etching increases the surface

roughness of the specimens.

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Table 5.10. Surface roughness results for laser etched nylon 66/CB/MoS2/

composites

MoS2 content in

nylon 66/CB (wt. %)

Surface Roughness (Ra)

500 mm/s, 5 kHz

50 % power 100 % power

0 4.9 5.6

0.5 4.2 5.0

1.0 3.9 4.2

2.0 3.7 3.8

3.0 3.2 3.5

5.3.5.1 Surface morphology of laser etched surfaces

Figure 5.17(a)-(e) shows the optical photomicrographs of laser etched surfaces

of nylon 66/CB/MoS2 composites for, (a) 0.5 %, (b) 1.0 %, (c) 2.0 % and (d) 3.0%

MoS2 at 50 % power, 5 kHz frequency for 500 mm/s velocity and this figure reveals

that the surface roughness decreases and co-hesive strength increases with increase in

MoS2 content.

Figure 5.17. Photomicrographs of laser etched nylon 66/ CB/ MoS2 composites

with (a) 0%, (b) 0.5%, (c) 1.0%, (d) 2.0% and (e) 3.0 % MoS2 content at 5 kHz

frequency, 50% power and 500 mm/s velocity

a

b c

d e

162

Figure 5.18(a)-(e) shows the optical photomicrographs of laser etched surfaces of

nylon 66/CB/MoS2 composites for (a) 0.5%, (b) 1.0%, (c) 2.0% and (d) 3.0% MoS2 at

100 % power, 5 kHz frequency for 500 mm/s velocity. At the outlook, these pictures

look similar in roughness; however a close examination reveals that smoothness

increases with increase in MoS2 content.

Figure 5.18. Photomicrographs of laser etched nylon 66/ CB/ MoS2 composites

with (a) 0%, (b) 0.5%, (c) 1.0%, (d) 2.0% and (e) 3.0 % MoS2 content at 5 kHz

frequency, 100% power and 500 mm/s velocity

5.4. Conclusions

With the objective to improve the physico-mechanical and tribological

performance of the nylon 66 matrix, the MoS2 was incorporated as filler along with

CB. The MoS2 content was varied selectively from 0.5 to 3.0 wt % keeping CB

content constant at 1 wt %. Thus, the wear resistance of resultant nylon 66/CB/MoS2

composite increased with increase in MoS2 content, but decreased with increase in

load or sliding distances. Similarly the specific wear rate and friction of nylon

66/CB/MoS2 decreased with increase in MoS2 content, load and sliding distances. The

a

b c

d e

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impact strength of the nylon 66 matrix increased from 37.2 to 43.2 J/m for the

incorporation of MoS2 content. The tensile properties- tensile strength, tensile

elongation and tensile modulus, increased linearly with increase in MoS2 content. The

addition of MoS2 renders this material somewhat stiffer and dimensionally more

stable than nylon 66. MoS2-filled nylon provides a degree of self-lubrication suited to

applications where external lubrication is impractical, contaminating, or difficult to

maintain leading to an improvement in wear. This, combined with lower water

absorption extends the range of applications that MoS2-filled nylon. All of the above

research findings suggest, that incorporation of two or more filler materials each

having a distinct functionality (one act as a lubricant and another act as heat

dissipater), can result in a composite with the potential of enhancing tribological

performance.

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