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CHAPTER 8

STUDIES ON MECHANICAL, THERMAL, WEAR AND

MORPHOLOGICAL BEHAVIOURS OF MOLYBDENUM

DISULPHIDE FILLED POLYCARBONATE/CARBON

BLACK COMPOSITES

This chapter covers the influence of molybdenum disulphide (MoS2) as solid

lubricant and filler on the performance of polycarbonate (PC) and carbon black (CB)

composites. PC containing one weight percentage of CB powder was compounded

and extruded with 0.5, 1.0, 2.0 and 3.0 weight percentage of MoS2 powder in a co-

rotating twin screw extruder. Thus, the fabricated PC/CB/MoS2 composites were

characterized for physico-mechanical properties such as, density, void content,

surface hardness, tensile behaviours and impact strength. The thermal characteristics

of the composites have been studied by DSC, DMA and TGA. The effect of MoS2

content, loads, sliding velocities and sliding distances on wear characteristics of the

composites were evaluated using pin-on-disc equipment. Wear, coefficient of friction

and laser etching resistance of PC/CB/MoS2 composites increased with increase in

MoS2 content along with improvement in tensile and impact strength. Worn surfaces

and laser etched surfaces were examined with SEM and optical microscopy

respectively to have better insight of the wear and laser etching mechanism.

8.1 Introduction

Solid lubrication is of great practical interest to mechanical design, being

employed in a wide variety of applications ranging from frying pans to orthopaedic

implants to the deployment mechanisms of communications. It can supplement other

means of lubrication in rolling element bearings, metal forming/cutting, engine valve-

trains and turbine foil air bearings. It can reduce cost, complexity, weight, and

environmental impact by replacing the pumps, filters, pipes and waste management

systems associated with fluid lubrication. Despite the wide and expanding use of solid

lubrication in engineering design, there is an increasing demand for new low friction,

low wear materials to improve efficiency and operational life in various

environments. Polymer based materials are finding increasing use in many

applications owing to their strength, lightness, ease of processing and availability of

215

wider choice of systems [1]. One of the areas where their use has been found to be

particularly advantageous is the situation involving contact wear. Due to the low

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

polymers are used in place of the traditional metal based materials in recent times.

However, further improvement is still required to meet more demanding applications.

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 [2,3]. Solid

lubricants, e.g. PTFE, MoS2 and graphite, have been proved very helpful in

developing a transfer film between the two counterparts and can drastically reduce the

wear rate of the composites [4,5].

The importance of the tribological properties of polymeric composites and

blends convinced various researchers to study the wear [6,7] behaviour and to

improve the wear resistance of polymers and composites. The slide wear of several

polymers sliding against a steel counter surface showed that the wear loss increased

with increasing load/speed and wear rate decreased with sliding distance. The

decrease in wear rate is caused by progressive smoothening of the surface and by the

formation of a protective transfer film of polymer on the steel counter surface [8, 9].

However, efforts to optimize the combination of these solid lubricants to boost the

strength and tribo-performance of polycarbonate (PC) in different wear modes are not

reported and hence required especially in the background of such literatures available

for other polymers and composites [10-14].

The objective of this research investigation is to evaluate the influence of

MoS2 on the mechanical and tribological properties of PC/CB composites. PCs are

high-molecular weight, amorphous engineering thermoplastic that have exceptionally

high impact strength over a wide temperature range and are characterized by an

excellent combination of toughness, transparency, heat and flame resistance and

dimensional stability. The fabricated PC/CB/MoS2 composites have been evaluated

for physico-mechanical properties and thermal characteristics of the composites have

been studied by DSC and DMA. The effect of MoS2 content, loads, sliding velocities

and sliding distances on wear characteristics of the composites were evaluated using

216

pin-on-disc equipment. Worn surfaces and laser etched surfaces examined with SEM

to have better insight of the wear mechanism.

8.2 Compounding and specimen preparation

Carbon black (1 wt %) was premixed with varying amounts viz., 0, 0.5, 1.0,

2.0 and 3.0 wt % of MoS2 powder and then mixed with PC granules in tumbling

mixer for 15 min, after pre-drying in hot air oven at 80°C for 48 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 260 to 280 ºC. The extruder consists of

nine nozzles and the temperature zones maintained at each of the nozzles are different

and lies in the range 260–280 o

C. The extrudate strand was palletized 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 moulding machine with 70 ton clamping pressure at 270-280 oC

and an injection pressure of 80 bars. After moulding, the test specimens were

conditioned at 23 ± 2 oC and 50 ± 5% RH for 40 h according to ASTM D 618 prior to

testing.

8.3. Results and Discussion

8.3.1 Physical properties

The prepared PC/CB/MoS2 composites were characterized for physico –

mechanical properties according to ASTM methods (for details refer Chapter 2). The

measured physical properties such as density, void content and surface hardness are

given in Table 8.1.The mechanical properties such as tensile strength, tensile

modulus, percentage elongation at break, and impact strength are shown in Table 8.1.

8.3.1.1 Density

The density measurements were performed on all composites of PC/CB/MoS2.

The density values of PC/CB/MoS2 composites falls in the range of 1.193– 1.216g/cc.

The densities of the composites were theoretically calculated by the volume additivity

method as described in the earlier chapter. This Table 8.1 shows that the density of

composites increased linearly with increase in MoS2 content. Furthermore actual

density values of these composites are lesser than that of its theoretical values

217

alculated by volume additivity principle. This is due to the formation of voids at the

interface of filler and matrix in the composites.

8.3.1.2. Void content

MoS2 being a higher dense material, the composite material density increases

with increase in MoS2 content. The percentage of void content measured and shown

in Table 8.1 reveals that void content of this composite material increased from 0.915

to 1.292 %. This shows the increase in free volume with increase in MoS2 content.

This may be due to entrapment of air bubble during mixing fine MoS2 and carbon

black powder in high viscous PC melt.

8.3.1.3 Surface hardness

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

indicates the degree of compatibility and crosslink density. Further the surface

hardness values of PC/CB/MoS2 composites falls in the range 81-84 Shore D with

incorporation of MoS2 content from 0 % to 3 wt %. This may be due to increase of

MoS2 filler in PC/CB matrix. From Table 8.1 it is noticed that a slight increase in

surface hardness values with increase in MoS2 content.

Table 8.1. Physical properties of PC/CB/MoS2 composites

MoS2

(%)

Density (g/cc) Void content

(%)

Surface hardness

(Shore D) + 1.5 Expt. Theo.

0 1.193 1.204 0.915 81

0.5 1.197 1.209 0.958 81

1.0 1.201 1.213 1.003 82

2.0 1.210 1.222 1.020 83

3.0 1.216 1.232 1.292 84

8.3.2 Mechanical properties

The effect of MoS2 addition on mechanical properties such as tensile strength,

percentage elongation at break, product parameter, tensile modulus and impact strength

are given in Table 8.2. The notched izod impact strength of composites at different

filler loadings are shown in Table 8.2.

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8.3.2.1 Tensile behaviour

The mechanical properties such as, tensile strength, percentage elongation at

break and tensile modulus were tabulated in Table 8.2. From Table 8.2, it is noticed

that the tensile strength increased from 66.6 to 67.6 MPa, tensile elongation decreased

from 71.0 % to 33.8 % with increase in MoS2 content from 0 to 3 wt %. Generally

filler reinforcement increases the tensile strength (σ) of neat polymer, they usually

decrease the ultimate elongation (e) and hence the product (σe) may become smaller

than that of neat polymer. Lancaster [2] stated that the product of σe factor (where, σ

is the ultimate tensile strength and e is the elongation at fracture) is a very important

factor which controls the abrasive behaviour of composites. In the present

investigation, it was noticed (Table 8.2) that the product parameter (σe) decreased

from 4729 to 2285. The tensile modulus which is indicating stiffness variation of the

material has increased from 2349 to 2369 MPa. The marginal improvement in tensile

strength, and tensile modulus reveals a positive interaction between the carboxyl

groups of PC and sulphide layers of MoS2, even though MoS2 has weak Vander

Waal’s interaction between the sheets of sulphide atoms.

Table 8.2. Mechanical properties of PC/CB/MoS2 composites

MoS2

(%)

Tensile

strength

(σ) (MPa)

+ 2.0

Tensile

modulus

(MPa) +

2.3

Elongation

at break

load

(e) (%)

Product

parameter

(σ x e)

Impact

strength

(J/m)

0 66.6 2349 71.0 4729 67.0

0.5 66.5 2451 50.3 3345 68.9

1.0 67.1 2457 48.0 3221 69.5

2.0 66.4 2365 40.1 2663 70.7

3.0 67.6 2369 33.8 2285 73.0

8.3.2.2 Impact strength

The impact strength becomes very important property because cracks due to

sudden loads are very common in service conditions. Forces of impact are applied so

quickly that the relaxation of the molecular structure does not follow the process,

resulting in fracture which can involve chain breaking and/or interface separation. The

nature of the interface region is of extreme importance in determining the toughness

of the composites. The impact strength of the PC/CB/MoS2 composites is lower than

219

that of nylon and found to gradually increase with increase in MoS2 dosage in the

composites. The notched izod impact strength of composites at different filler

loadings are shown in Table 8.2. The izod impact strength of composites increased

from 67 to 73 J/m with filler loading.

8.3.3 Thermal behaviour

The thermal properties of plastic materials are equally important as the

mechanical properties. Unlike metals, plastics are extremely sensitive to temperature.

The HDT values of PC were 120 -130oC. The HDT values of PC/CB composites

reduced to 115-120oC after the addition of CB into PC. PC/CB/MoS2 composites

show further reduction in HDT values (100-110 o

C) after the inclusion of MoS2 into

the PC/CB composites.

8.3.3.1. Differential scanning calorimetry

The thermal transition behaviour of PC/CB/MoS2 composites was investigated

by DSC technique. The effect of MoS2 filler on glass transition temperature (Tg) of

PC was investigated by DSC technique DSC to probe molecular mobility in the

composites.

Figure 8.1. DSC thermograms for PC/CB/MoS2 composites

Figure 8.1 provides the representative plots heat flow versus temperature of

PC/CB/MoS2 composites with different wt. % of MoS2. From the DSC plots, the

values of Tg, was determined and presented in Table 8.3. The Tg of PC (146 oC) was

found increase linearly from 143 to 138 oC for loading of MoS2 filler content from 0

220

to 3 wt %., which draws to the conclusion that filler loading does not affect the PC

molecular interactions significantly. Results also indicate that the presence of MoS2 in

the PC/CB does not improve the Tg.

Table 8.3. Glass transition temperature data obtained from DSC thermograms

for PC/CB/MoS2 composites

MoS2 content in PC/CB/MoS2

composites (%, wt/wt) Tg (

ºC)

0 143

0.5 141

1.0 140

2.0 139

3.0 138

8.3.3.2 Thermogravimetric analysis

In order to analyze the effect of filler loading on the thermal stability of PC

composite by TGA technique was used. Figure 8.2 provides representative TGA

thermograms for PC and its PC/CB/MoS2 composites containing 0, 0.5, 1.0, 2.0 and 3

wt % of MoS2. It was found that the onset of thermal degradation (To), the

temperature at which partial degradation occurs (Tp) and the temperature at which

complete degradation occurs (Tc) are all set to decrease with increase in filler MoS2

loading. Which clearly shows that, eventhough it is not a significant change, but it

reduces the thermal stability of Polycarbonate materials. Polycarbonate is basically

material of high thermal stability, because the polymer chain is bulked up stable,

thermally tolerant chemical segment. The phenyl ring is a highly stable chemical

entity and plays a prominent role in the thermal satiability.

Table 8.4. Thermal data obtained from TGA thermograms of PC/CB/MoS2

composites

MoS2 content in PC/CB

composites (%, wt.)

Temperature at different weight loss (± 2ºC)

T0 T10 T20 T50 Tmax

0 381 457 474 494 563

0.5 379 444 463 486 560

1.0 377 434 454 480 541

2.0 373 427 448 474 526

3.0 371 424 446 471 513

221

Figure 8.2. TGA thermograms for PC/CB/MoS2 composites

8.3.3.3 Dynamic mechanical analysis

In this study, DMA was used to probe the mechanical properties of the

PC/CB/MoS2 composites. Figures 8.3 and 8.4 are representative DMA curves of

storage modulus and loss modulus of PC/CB/MoS2 composites respectively.

Figure 8.3. Plots of loss modulus versus temperature for PC/CB/MoS2

composites

Storage modulus was found to increase below the Tg (Table 8.5) from 1413 to

1644 MPa with increase of MoS2 content from 0 to 3 wt%. Above the Tg i.e., in the

rubbery region, the storage modulus drops drastically and reaches zero modulus

irrespective of the composition. This is because, in amorphous systems like

polycarbonate, which does not have side chains, but the molecular relaxation is

associated with conjugated motion of large segment of chain backbone, so, glass

222

transition Tg results in loss of load bearing properties, since the entire structure is

affected by relaxation brought about by conjugated motion of large segments of the

chain backbone.

Figure 8.4. Plots of storage modulus versus temperature for PC/CB/MoS2

composites

Figure 8.5. Plots of loss tangent versus temperature for PC/CB/MoS2 composites

Figure 8.5 provides representative plots of tan δ verses temperature for PC/

CB/MoS2 composite. Figure 8.5 and Table 8.5 further reveals that there is little

reduction in tan δ values from 1.358 to 1.300 and Tg values similar to DSC results,

increase from 146 to 149 ºC. One of the important applications of DMA is

measurement of Tg. Amorphous polymers will have different Tg, above which the

material will have rubbery properties instead of glassy behaviour and stiffness of the

223

material will drop dramatically. At Tg the storage modulus decrease dramatically

while loss modulus reach maximum.

Table 8.5. Data obtained from DMA analysis for PC/CB/MoS2 composites

MoS2 wt. % Tan δ

Tg (ºC)

Storage modulus (MPa)

Exp. Cal. Glassy region Rubbery region

0 1.358 - 153.4 1413 0

0.5 1.350 1.338 153.6 1455 0

1.0 1.342 1.323 152.6 1492 0

2.0 1.340 1.310 153.4 1620 0

3.0 1.300 1.296 153.2 1644 0

8.3.4 Wear studies

8.3.4.1 Wear loss

The plots of wear loss as a function of MoS2 content at different applied loads

for all PC/CB/MoS2 composites are shown in Figures 8.6(a)-(c). Figures 8.7(a)-(c)

and Figures 8.8 (a)-(c) reveals that with increase in MoS2 content from 0 to 3 wt %

the wear loss of PC/CB/MoS2 composites decreases under different loads (30, 50 and

70N) and at different sliding distances (750, 1000 and 1250m).

8.3.4.2 Specific wear rate

The plots of specific wear rate as a function of MoS2 content at different

applied loads for all PC/CB/MoS2 composites are shown in Figures 8.9(a)-(c). Figures

8.10(a)-(c) and Figures 8.11(a)-(c) reveals that with increase in MoS2 content from

0.5 to 3 wt % the specific wear rate of PC/CB/MoS2 composites decreases under

different loads (30, 50 and 70N) and at different sliding distances (750, 1000 and

1250m). Figures 8.12(a)-(c) shows the plots of wear loss as function of sliding

velocity at varying loads of (a) 30 N, (b) 50 N and (c) 70 N. It is obvious that the wear

loss of composites decreases with increasing MoS2 content. The improvement in wear

resistance is due to the presence of solid lubricant (MoS2) dispersed in the polymer

matrix. Because the lamellar structured MoS2 readily forms a transfer film on surface,

which acts as a barrier and prevents large scale fragmentation of PC/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

224

for the inorganic MoS2 content of 3 wt. %. However, it is observed that the wear loss

and specific wear rate increase with increase in load and sliding distances.

Sole et al [15] studied the effect of mineral fillers such as talc, CaCO3, BaSO4,

and fly ash on abrasive wear of resistance of polypropylene (PP). They reported that

the addition of mineral fillers to the PP matrix decreases the wear resistance under

severe abrasion conditions. However, under mild abrasion conditions the shape and

size of the reinforcing filler influences the wear performance. Briscoe et al [16],

however, reported mixed trend for the abrasive wear of polyether ether ketone

(PEEK)-filled PTFE and PTFE-filled PPE. Incorporation of PEEK in PTFE reduced

the wear rate of PTFE while the wear rate increased in the later case, though the

extent of influence depended on the polymers and the type of fillers. Ratner et al [17]

have also reported the improvement in wear behaviour of UHMWPE and PP because

of addition of quartz powder of two different sizes and TiO2, respectively.

Figure 8.6. Weight loss as a function of MoS2 content for PC/CB composites at

varying sliding distances and at varying loads; (a) 30 N, (b) 50 N and (c) 70 N

Figure 8.7. Weight loss as a function of load for PC/CB/MoS2 composites at

sliding distances of 750 m, 1000 m and 1250 m

225

Figure 8.8. Weight loss as a function of sliding distance for PC/CB/MoS2

composites at varying loads; (a) 30 N, (b) 50 N and (c) 70 N

Figure 8.9. Weight loss as a function of sliding velocity for PC/CB/MoS2

composites at varying loads; (a) 30 N, (b) 50 N and (c) 70 N

Figure 8.10. Specific wear rate as a function of MoS2 content for PC/CB

composites at varying loads; (a) 30 N, (b) 50 N and (c) 70 N

226

Figure 8.11. Specific wear rate as a function of load for PC/CB/MoS2 composites

at sliding distances of 750 m, 1000 m and 1250 m

Figure 8.12. Specific wear rate as a function of sliding distance for PC/CB/MoS2

composites at varying loads; (a) 30 N, (b) 50 N and (c) 70 N

8.3.4.3 Coefficient of friction

The 'coefficient of friction' (µ), is the measurement of resistance to friction as

related to the effect of how smooth or rough a surface is to prevent material to “slip”

across the surface. The coefficient of friction depends on the materials used. The

variation in coefficient of friction as a function of MoS2 compositions at 5 m/s sliding

velocity for PC/CB/MoS2 composites is tabulated in Table 8.6. The co-efficient of

friction of PC/CB/MoS2 composites at a particular load and sliding distance decreases

with increase in MoS2 loading from 0.5 to 3 wt%. 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 [18–23].

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

227

during the current studies. It is evident that the friction coefficient reduces

continuously by increasing the content of MoS2 in the composites so that 3 wt %

loading MoS2 reduces the coefficient of friction of PC. This behaviour is obviously

relevant to the self lubricating effect of MoS2 which can reduce the adhesion between

the composite with the metallic rotating disc. The MoS2 loaded materials exhibited

lower coefficient of friction as compared with the unfilled PC at all normal loads,

sliding distances and velocities under investigated.

Table 8.6. Coefficient of friction for PC/CB/MoS2 composites at velocity of 5 m/s

Load

(N)

Sliding

length

(m)

Co-efficient of friction for PC/CB/MoS2 composites with

varying amounts of MoS2 (%)

0 0.5 1.0 2.0 3.0

30

750 0.156 0.149 0.144 0.142 0.137

1000 0.169 0.163 0.159 0.155 0.153

1250 0.175 0.169 0.165 0.162 0.158

50

750 0.171 0.165 0.162 0.158 0.159

1000 0.185 0. 181 0.174 0.168 0.157

1250 0.193 0.188 0.181 0.176 0.173

70

750 0.187 0.182 0.176 0.172 0.169

1000 0.203 0.197 0.188 0.174 0.165

1250 0.224 0.215 0.207 0.202 0.197

8.3.4.4 Scanning electron microscopic studies

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

The SEM micrographs of worn out surfaces of PC/CB/MoS2 composites are shown in

Figures 8.13-8.15. Figure 8.13 shows SEM images of worn out surfaces of

PC/MoS2/CB composites at 5 m/s sliding velocity, 30N load 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%. The worn surface of 0 wt. % MoS2 filled PC/CB composites is

relatively rough with more matrix damage. The worn surface of 3% MoS2 filled

PC/CB composites (Figure 8.13(d)) becomes smoother and smoother indicating that

228

the wear volume loss is less because of more and more MoS2 particles adhered on the

surface of the specimens hinders the abrasion of matrix.

It can be concluded from the SEM photomicrographs that inclusion of MoS2

as a solid lubricant in PC/CB matrix by mixing is beneficial from the sliding wear

resistance point of view. Similar is the case with Figures 8.14 and 8.15, which clearly

shows the role played by MoS2 content in reduction of wear loss due to the formation

transfer film, which acts as a barrier and prevents the further damage to the

PC/CB/MoS2 composite surface.

Figure 8.13. SEM images of PC/CB/MoS2 composites at 5 m/s sliding velocity,

30N load 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 wt.%

Figure 8.14 shows SEM images of worn out surfaces of PC/MoS2/CB

composites at 5 m/s sliding velocity, 50N load 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

d

b c

e

229

Figure 8.15 shows SEM images of worn out surfaces of PC/MoS2/CB composites at 5

m/s sliding velocity, 70N load 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 wt%. On close observation of

Figures 8.7, 8.8 and 8.9 reveals that with increase in weight percentage of MoS2

content, the worn out surfaces become relatively smoother and with increase load

from 30 to 70 N, the roughness of the worn out surfaces increases. Figures 8.7(a)-(d)

indicates the positive effect of MoS2 content on the worn surface of the PC/CB

composites.

Figure 8.14. SEM images of PC/ CB/MoS2 composites at 5 m/s sliding velocity, 50

N load 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 wt.%

d e

a

c b

c

230

Figure 8.15. SEM images of PC/CB/MoS2 composites at 5 m/s sliding velocity,

70N load 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%

8.3.5 Regression analysis

Based on the experimental results the correlation between the wear parameters

(Table 8.7) is obtained using linear regression technique. In the equations shown in

the Table 8.8, it is observed that the values associated with load and sliding velocity

parameter decreases with increasing MoS2 content in PC indicating that wear

decreases as MoS2 composition in PC increases.

8.3.5.1 Process parameters

Table 8.7. Process parameters for ANOVA

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

1 30 5 1000

2 50 7 1500

3 70 9 2000

c

d

a

c

b

e

231

Table 8.8. Regression equation for PC/CB/MoS2 composites

MoS2content

% Regression Equation

0 0.000386 + 0.000032 L + 0.000175 Sl.Vel+0.0036D

0.5 0.000133 + 0.000028 L + 0.000183 Sl.Vel+0.0032D

1.0 0.000089 + 0.000025 L + 0.000150 Sl.Vel+0.0030D

2.0 0.000094 + 0.000018 L + 0.000142 Sl.Vel+0.0027D

3.0 0.000183 + 0.000013 L + 0.000117 Sl.Vel+0.0025D

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

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 8.9. 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 [24-

25]. A mathematical model will be developed by using analysis techniques such as

ANOVA and regression analysis whereby the mathematical model shows the

relationship between the input parameters and the input responses [26].

Table 8.9. Analysis of variance (ANOVA) for PC/CB/MoS2 composites

Source % MoS2 in PC/CB

0% 0.50% 1% 2% 3%

DOF 2 2 2 2 2

SS 3.14E-06 2.73E-06 2.04E-06 1.22E-06 7.53E-07

MS 1.57E-06 1.37E-06 1.02E-06 6.08E-07 3.77E-07

F 117.0 1230.0 125.2 93.9 84.8

P 0.065 0.036 0.531 0.459 0.113

S 0.00012 0.00003 0.00009 0.00008 0.00007

R-SQ(%) 97.5 99.8 97.7 96.9 96.6

Coefficient 0.000386 0.000133 0.000088 0.000094 0.000183

SE Coeff 0.000172 0.000049 0.000134 0.000119 0.000099

T 2.25 2.70 0.66 0.79 1.86

R. Error 8.06E-08 6.67E-09 4.89E-08 3.89E-08 2.67E-08

DW Statistic 1.70 2.83 2.25 1.26 1.25

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.

232

8.3.6 Laser assisted etching behaviour

The effect of MoS2 content and power of etching on the laser etched surfaces of

the PC/CB/MoS2 composite specimens was characterized for surface roughness in z-

direction. The surface roughness values (Ra) of laser etched specimens at different

laser parameters are tabulated in Table 8.10. The Ra values show decreasing trend

with increase of MoS2 content in the PC/CB composites. Also it is observed that Ra

values exhibits increasing trend with increasing power from 50 % to 100 % for all the

composites. This result indicates that MoS2 content controls the etching behaviour of

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

of the specimens.

Tagliaferri [27] conducted an experimental study to determine the surface

finish characteristics of carbon and aramide fibre-reinforced plastics (CFRP and

AFRP) by CO2 laser. The HAZ (heat affected zone) depends strictly on the feed rate.

The higher the speed of laser beam, the smaller the volume of damage and the better

the cut finish. Graphite reinforced composites are found to be less suitable for laser

cutting due to high fibre conductivity and vaporization temperature [28].

Table 8.10. Surface roughness results for laser etched PC/CB/MoS2 composites

MoS2 content in

PC/CB (wt. %)

Surface roughness (Ra)

500 mm/s, 5 kHz

50 % power 100 % power

0 4.1 4.7

0.5 3.8 4.5

1.0 3.7 4.3

2.0 3.5 4.0

3.0 3.2 3.7

233

8.3.6.1 Morphology of laser etched surfaces

The optical photomicrographs of laser etched surfaces of PC/CB/MoS2

composites at 50% and 100% power are shown in Figures 8.16 and 8.17 respectively.

The laser surfaces of the composites are characterized by rough surfaces on account

of etching by laser. The photomicrographs of PC/CB/MoS2 composites showed

decreased trend in surface roughness with increasing MoS2 content at 50 % power, 5

kHz frequency for 500 mm/s velocity as shown in Figures 8.16 (a)-(e). The increase

in laser etching power from 50 to 100 %, increases the surface roughness of the

composites. The more matrix damage with higher surface roughness values for laser

etched unfilled PC as compared to MoS2 filled composites.

Figure 8.16. Optical photomicrographs of laser etched PC/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

e

c d

234

Figure 8.17. Optical photomicrographs of laser etched PC/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

8.4 Conclusions

With the objective to study the influence of MoS2 on the friction and wear

behaviour of PC/CB composites, MoS2 was incorporated in 0.5, 1.0 and 2.0 and 3.0

wt %. It was found the MoS2 additions have improved the friction and wear properties

of PC/CB composites linearly with increase in MoS2 content. Further it was found

that tensile and impact properties also increased to a certain extent. DMA analysis

shows the storage modulus of composites increased with increase in MoS2 content.

TG analysis shows reduction in the onset of degradation temperature with increase in

MoS2 content. Tg of PC/CB/MoS2 composites increase to a certain extent for the

addition of MoS2 content.

a b

c d

e

235

8.5 References

1. Y.K.S.Chen, N.Kukureka, C.J.Hooke and M.Rao, J. Mater. Sci., 2000, 35, 1269.

2. J.K. Lancaster, Tribology, 1973, 12, 219.

3. B.B.Jia, X.T.S.Li, J.Liu and P.H. Cong, Wear, 262 (2007) 1353.

4. S.Bahadur, Wear, 245 (2000) 92–99.

5. J.M.Throp, Tribology Intl., 15 (1982) 69–74.

6. Yinping Ye, Jianmin Chen and Huidi Zhou (2009), Wear, 266,859-864

7. Y.Yamaguchi, Tribology of Plastic Materials, Tribology Series 16, Elsevier, New

York, 1990.

8. A.M.Hager and M.Davies, Composite Materials Series Vol. 8, (Elsevier,

Amsterdam, The Netherlands, 1993; vol.8:107-157.

9. S.E.Franklin, Wear 251, 1591(2001).

10. Y.Cohen, D.M.Rein and L.Vaykhansky, Comp. Sci. Tech., 57, (1997), 1149.

11. R.L.Liu, R.D.Amell and J.Tong, Wear 199, (1999)225-229.

12. J. Suwanprateeb, J. Appl. Polym. Sci., 75, (2000)1503.

13. C.X.He, Tribology, 22, (2002) 32.

14. A.A.Cenna, J.Doyle, N.W.Page, A.Beehag and P.Dastoor, Wear, 240(2000)

207.

15. B.M. Sole and A. Ball, Tribol. Intl., 29, 457 (1996).

16. B.J. Briscoe, L.H. Yoo, and T.A. Stolarski, Wear, 108, 357 (1986).

17. S.B. Ratner and I.I. Farberova, Sov. Plast., 8, 51 (1960).

18. J.K. Lancaster, Wear, 22, (1972) 412.

19. W.O.Winer, Wear; 10 (1967) 422–52.

20. R.Holinski and G. Jansheimer, Wear 19 (1972) 329–42.

21. A.I.Brudnyi and A.F. Karmadonov, Wear 33 (1975)243–9.

22. J.P.G. Farr, Wear, 35 (1975)1–22.

23. N.Takahashi and K. Okada, Wear; 33 (1975), 153–67.

24. N.Takahashi, Wear, 124 (1988) 279–89.

25. R.Colaco and R.Villar, Wear, 223 (1998) 131-138.

26. D.P. Mondal, S.Das, A.K.Jha, O.P Modi, R. Das Gupta and A.H. Yogeeswaran,

Composite Part A, 28 (1997), 301-308.

27. K.D.Avanish and Y.Vinod, Int. J. of Mach., Tools & Manuf., 48 (2007)609-628.

28. E. Tagliaferri, R.Sieber and U.Butikofer, Composites, 16 (4) (1985) 317–325.