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CHAPTER 4

STUDIES ON MECHANICAL, THERMAL, WEAR AND

MORPHOLOGICAL BEHAVIOURS OF GRAPHITE

FILLED NYLON 66/POLYTETRAFLUOROETHYLENE

COMPOSITES

This chapter deals with the characterization of polytetrafluoroethylene (PTFE)

filled nylon 66/graphite (Gr) composites. The nylon 66/PTFE/Gr composites were

prepared by melt mixing with different weight fractions of PTFE viz., 0, 5, 10,15 and

20 wt. % and 2% Gr using a co-rotating intermesh twin-screw extruder mixer. The

fabricated nylon 66/PTFE/Gr composites were characterized for physico-mechanical

behaviours such as density, hardness, tensile behaviours and impact strength. The nylon

66/PTFE/Gr composites are evaluated for thermal characteristics using DSC, DMA and

TGA. A slight improvement in thermal stability was noticed for filler loaded

specimens. A pin-on-disc wear testing equipment is employed to investigate the

tribological properties. Morphological features of worn-out surfaces of nylon

66/PTFE/Gr composites have been analyzed using scanning electron microscopy

(SEM). This study also focused on Laser etching technique to ascertain the effect of

PTFE content on the surface abrasion behaviour of nylon 66/Gr composites.

4.1 Introduction

Nylons, also known as polyamides, are preferred materials for gears, cams,

bearing, etc., especially nylon 66 and nylon 6 by virtue of their mechanical strength and

tribo-performance, where two different bodies are in contact at severe sliding

conditions [1,2]. It was reported that the friction and wear behaviour of nylons was

fairly satisfactory under dry sliding conditions and lubrication at higher speeds.

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 [3,4]. A number of research papers are

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available 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 molybdenum sulphide (MoS2) [5,6]. These are the pre-

dominant materials used as solid lubricants in thermoplastics.

PTFE is a commonly used solid lubricant material, which has one of the lowest

coefficients of friction against any solid. Its static coefficient of friction is lower than its

dynamic coefficient, which accounts for the slip/stick properties associated with

PTFE/metal sliding action. PTFE is a non-polar and hydrophobic material, which can

act as an efficient solid lubricant when it is added into the nylon matrix so that the

nylon/PTFE composites take the advantages of good mechanical properties and

processability of the nylon, and very low friction coefficient and good wear resistance

of PTFE. It is a well-known anti-adhesive material for tribological applications [7] and

it is a soft fluorocarbon solid, as it is a high-molecular-weight compound consisting

wholly of carbon and fluorine. It has helical macromolecular structure and its molecules

slip along each other easily similar to lamellar structures of graphite and MoS2.

Similar to PTFE, graphite is an effective solid lubricant and it has lamellar

structure. The lamellas orient parallel to the surface in the direction of motion. Even

between highly loaded stationary surfaces, the lamellar structure is able to prevent

contact. In the direction of motion, the lamellas easily shear over each other resulting in

a low friction. Graphite powders are low-friction, high-temperature solids traditionally

used to lubricate moving metal parts where boundary lubrication is required and it can

be extrusion compounded with a variety of thermoplastics to provide coefficients of

friction and wear factors between those of the base resin and the PTFE/silicone-

lubricated versions. An important use for graphite-lubricated thermoplastics is in

components that operate in aqueous environments.

The importance of the tribological properties of polymeric composites and

blends convinced various researchers to study the wear [8,9] behaviours and to improve

the wear resistance of polymers and fiber reinforced polymeric (FRP) 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 [10, 11]. Solid lubricants, such as PTFE and graphite, have been proved very

112

helpful in developing a transfer film between the two counterparts and can drastically

reduce the wear rate of the polymer composites [12,13]. Bijwe et al studied on the wear

performance of various composites of polyamide (nylon 66) reinforced with short

carbon fibres and lubricated with a solid lubricant, PTFE, under adverse sliding

conditions [14,15]. However, efforts to optimize the combination of these solid

lubricants to boost the strength and tribo-performance of nylons in different wear

modes are not reported and hence required especially in the background of such

literatures available for other polymers and composites [16-20].

In this research investigation, with objective to derive the benefits of both

helical structured PTFE and lamellar structured graphite, nylon 66 composites were

fabricated by varying the concentrations of the PTFE from 5 to 20 wt% with 2wt

%graphite content. The fabricated nylon 66/PTFE/Gr composites have been evaluated

for physico-mechanical properties such as tensile behaviours, impact strength, water

absorption, density and void content. The thermal characteristics of the composites have

been studied by using DSC, DMA and TGA. The effect of PTFE contents, loads,

sliding velocities and sliding distances on wear characteristics of the composites was

evaluated using pin-on-disc equipment.

4.2. Compounding and specimen preparation

Graphite powder (2 wt %) was premixed with varying amounts viz.,0, 5, 10, 15

and 20 wt % of PTFE powder and then mixed with nylon 66 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 automatic injection molding machine with

70 ton clamping pressure at 270-280 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.

113

4.3 Results and Discussion

This section describes the characterization of nylon 66/PTFE/Gr composites for

different properties such as physico-mechanical, thermal, tribological, morphological

and laser etching characteristics. Nylon 66 is a rigid hydrophilic material and

incorporation of soft PTFE powder may affect its mechanical properties in dry

conditions, However, PTFE being hydrophobic in nature, the incorporation of PTFE in

the nylon 66 matrix does not affect its mechanical properties in wet conditions. Two

percentage of graphite (rigid) powder was incorporated in all the compositions

uniformly, the major role of graphite addition is twofold, one is to compensate

reduction in mechanical properties due to the incorporation of soft PTFE powder; it was

reported that hard particulate fillers dramatically improve the mechanical properties

such as wear resistance, even up to three orders of magnitude [24] and second one is to

compensate the great difference in surface energies and polarities of nylon (polar) and

PTFE (non-polar), which is expected to improve the dispersion of PTFE powder in

nylon 66 matrix. In order to prevent poor dispersion of non-polar PTFE powder in the

polar nylon 66 in the melt processing technique, the white PTFE powder, first

thoroughly mixed with black graphite powder and then mixed with nylon 66 granules

and melt compounded to make test specimens.

4.3.1 Physico-mechanical properties

The prepared nylon 66/PTFE/Gr composites were characterized for physico–

mechanical properties according to ASTM methods. The values of measured

mechanical properties such as density, surface hardness, tensile properties, and impact

strength of the nylon 66/PTFE composites were addressed in Table 4.1.

4.3.1.1 Density

Density is a significant indicator of end use of the polymeric materials and it

depends on the composition of the composites and their physical as well as chemical

properties. The measured density values of nylon 66/Gr/PTFE composites lies in the

range 1.143 -1.268 g/cc as shown in table 4.1. The measured density values of the nylon

66/PTFE composites were lower than the calculated values. This could be due to the

formation of voids at the interface between the components of the composites and also

immiscible nature of the nylon 66 and PTFE composites. The density values of the

114

composites increases as PTFE

content in the composites increases, due to the

incorporation of high dense PTFE in nylon 66 matrix.

4.3.1.2 Water uptake behaviour

When working with nylons, it is important to examine the water absorption,

because it severely affects its mechanical and thermal properties. In this investigation,

the results shown in the Table 4.1 indicates that the incorporation of PTFE has reduced

the water absorption from 2.4 (0% PTFE) to 1.5% (20% PTFE).This can be attributed

to the increase in water repellent or hydrophobic PTFE component in the nylon 66/Gr

matrix. This result clearly indicates that the significant improvement in the water

resistance behaviour of the composites.

4.3.1.3 Void content

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

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

Table 4.1 reveals that void content of this composite material increased drastically from

0.539 to 0.785 %. This shows that the increase in free volume with increase in PTFE

content. This is due to greater difference in surface energies and polarities of nylon

(polar/hydrophilic) and PTFE (non-polar/ hydrophobic) materials. This may also be due

to entrapment of air bubble during mixing particulate PTFE in crystalline nylon 66/Gr

matrix.

4.3.1.4 Surface hardness

Table 4.1. Physical properties of the nylon 66/Gr/PTFE composites

PTF

E

(%)

Water

uptake (%)

Density (g/cc) Void content

(%)

Surface hardness

(Shore D) + 1.5 Expt. Cal.

0 2.4 1.143 1.149 0.539 80

5 2.2 1.172 1.179 0.586 79

10 2.0 1.202 1.210 0.677 76

15 1.7 1.234 1.243 0.739 74

20 1.5 1.268 1.278 0.785 72

Hardness test can differentiate the relative hardness of different grades of

thermoplastics. Also hardness reveals the dimensional stability of the composites which

115

depends on the nature of composition of the components. The surface hardness of nylon

66/Gr/PTFE composites decreased from 80 to 72 shore D with incorporation of PTFE

content from 0 to 20 wt%. This can be attributed to soft PTFE materials as compared to

nylon 66.

4.3.1.5 Tensile behaviours

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

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

are given in Table 4.2. From Table 4.2, it is noticed that the tensile strength decreased

from 75 to 66 MPa, tensile modulus increased from 2739 to 2672 MPa and tensile

elongation decreased from 19.5 % to 14.5 % with increase in PTFE content from 0 to

20 wt%. The tensile strength decreased because the presence of soft and friction less

PTFE filler in-between the nylon 66 molecular layers does not resist tensile (stretching)

force, but assist stretching, so, tensile strength and elongation reduced. This is because,

as the filler loading increased, the interfacial area increased, worsening the interfacial

bonding between filler and the matrix polymer, which decreased the tensile strength. In

contrast to the tensile strength and tensile elongation, the tensile modulus increased.

Lancaster [6] 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, at lower filler loading

higher will be the tensile strength (σ) and ultimate elongation (e) of neat polymer, and

hence their product σe also higher. From the table it was noticed that the σe factor

decreases with increase in PTFE content.

In the present investigation wear behaviour is not dependent on the σe values,

that means wear behaviour strongly depend on the volume fraction of solid lubricant in

the composites.

Table 4.2. Mechanical properties of the nylon 66/Gr/PTFE composites

PTFE

(%)

Tensile

strength

(σ) (MPa)

Tensile

modulus

(MPa)

Elongation at

break (e) (%)

Product

parameter

(σ x e)

Impact

strength

(J/m)

0 75 2739 19.5 1463 35.1

5 72 2722 19.1 1375 36.3

10 70 2705 17.3 1211 39.5

15 69 2689 15.2 1049 41.9

20 66 2672 14.5 957 44.6

116

4.3.1.6 Impact strength

The impact strength of composites is even more complex than that of the

unfilled polymers because of the part played by the fillers and the interface in addition

to the polymer. The izod impact strength of composites at different filler loadings are

shown in Table 4.2. The izod impact strength of composites increased from 35.1 to 44.6

J/m with filler loading, which may be due to softness and poor adhesion of PTFE filler

with nylon 66, which dissipates the maximum energy by mechanical friction.

4.3.2 Thermal behaviours

The thermal characteristics of nylon 66/PTFE/Gr composites have been

characterized by using DSC, TGA and DMA. The results of thermal behaviours of the

composites briefly interpreted in the following section.

4.3.2.1 Heat distortion temperature (HDT)

Nylon 66 is a crystalline thermoplastic with a low linear thermal expansion co-

efficient. The measured HDT value of pristine nylon 66 is 205-210 o

C at 0.45 Mpa load.

A significant reduction in HDT values was noticed after incorporation of graphite and

PTFE into nylon 66 matrix and it lies in the range 155-170oC for 20 wt %PTFE loaded

nylon 66/Gr/PTFE composites. This is due to PTFE, being soft component and hence

reinforced nylon 66/Gr matrix undergo little dimensional change when the temperature

changes.

4.3.2.2 Differential scanning calorimetry

DSC was used to study the effect of filler loading on the transition temperature

behaviours of the composites. Figure 4.1 illustrates the DSC thermograms of pristine

PA66 and the composites filled with PTFE. The thermal data obtained from DSC

thermograms therm of PTFE filled nylon 66/Gr composites are tabulated in Table 4.3.

The calculated percent of crystallinity of nylon 66/PTFE/Gr lies in the range 43.7 –

36.2. As it can be inferred, melting temperature (Tm) and heat of melting (Hm) do not

change sensibly by incorporation of both PTFE. The percent deviation in Hm and

degree of crystallinity is less than 2.7. This clearly indicates that PTFE does not affect

the crystalline structure of the PA66. Such behaviour for nylon 66/PTFE/Gr

compounds, where an interfacial interaction was speculated, reveals that the extent of

interaction is not such a high value to influence crystallinity of PA66, probably because

117

of the low degree of functionality or small interfacial area. Thermal transition behaviour

observed here is accordance with crystallization characteristics of PAs/PTFE systems

reported in literature [21-22].

Figure 4.1. DSC thermograms of nylon 66/PTFE/Gr composites

Table 4.3. Thermal characteristics obtained from DSC thermograms for nylon

66/PTFE/Gr composites

*Heat of fusion value of crystalline nylon 66 is 191 J/g

4.3.2.3 Dynamic mechanical analysis

The investigation of dynamic modulus and damping over a temperature range has

proved to be very useful in studying the structure of the polymers and the variation of

properties in relation to performance [23-27]. The dynamic modulus indicates the

PTFE

content in

composite

(% by wt)

To

(ºC)

Tm

(ºC)

Tc

(ºC)

H (J/g) Crystallinity* χc

(%)

Exp. Cal. Dev.

(%)

Exp. Cal. Dev.

(%)

0 245 266 279 85.7 - - 43.7 - -

5 240 265 275 81. 0 81.4 -0.5 41.3 41.5 -0.5

10 239 264 273 77.6 77.1 +0.6 39.6 39.4 +0.6

15 237 263 271 75.1 72.8 +2.7 38.2 37.2 +2.7

20 236 262 269 71.3 68.6 +3.4 36.2 35.0 +3.4

118

inherent stiffness of material under dynamic loading conditions. The mechanical

damping indicates the amount of energy dissipated as heat during the deformation of

the material. The dynamic mechanical properties of polymers are usually studied over a

wide temperature range. In the region where the dynamic modulus-temperature curve

has an inflection point, tan δ curve goes through a maximum. This dissipation is called

Tg region. A few polymeric mixtures are compatible and form one phase systems.

However, most mixtures of polymers form two phases due to incompatibility of the

components.

Figure 4.2. Plots of Storage modulus versus temperature for nylon 66//PTFE/Gr

composites

Figure 4.3. Plots of loss modulus versus temperature for 2% Gr filled nylon

66/PTFE/Gr composites

119

The plots of storage modulus verses temperature curves of nylon 66/PTFE/Gr

composites containing 0, 5, 10, 15 and 20% of the PTFE filler have been shown in

Figure 4.2. The break in modulus curves remain steep, and the modulus is just shifted

on temperature scale in proportion to the relative concentration of PTFE fillers. Here

the DMA studies have been studied to probe the temperature dependence of storage

modulus upon blending with nylon 66/PTFE. For all the compositions, the storage

modulus can be seen increasing in the investigated temperature range, indicating that

the introduction of PTFE increases the storage modulus of nylon 66/Gr composites

proportionately with the composition.

Figure 4.4. Plots of tan δ modulus versus temperature for nylon 66//PTFE/Gr

composites

Table 4.4. Data obtained from DMA analysis for nylon 66/PTFE/Gr composites

PTFE wt. % Tan δ

Tg (oC)

Storage modulus (MPa)

Expt. Cal. Glassy region Rubbery region

0 0.077 - 72.1 924 330

5 0.074 0.072 69.7 938 354

10 0.073 0.066 66.4 1046 363

15 0.072 0.063 66.0 1040 398

20 0.070 0.061 66.2 1224 422

120

The effect of temperature on loss modulus (E'') of PTFE loaded nylon 66 is

shown in Figure 4.3. From the thermogram, it can be noticed that, the incorporation of

PTFE into nylon, causes remarkable increase of E'' value as compared to pure nylon

66. This indicates that the increase in PTFE content in nylon 66 increased the energy of

dissipation.

The plots of loss tangent (tan δ) verses temperature for all composites is shown

in Figure 4.4. The obtained tan δ values (both predicted and experimental) from DMA

thermograms for nylon 66/PTFE/Gr composites along with Tg which was represented

by the peak temperature of the tan δ curve is addressed in Table 4.4.

The experimental tan δ decreased from 0.077 to 0.070 whereas, theoretical

values from 0.072 to 0.061. A noticeable reduction in Tg from 72.1 to 66.2 0C was

noticed with increase in PTFE content from 0 to 20 wt %, which clearly shows that

PTFE particle goes in between the nylon 66 molecular layers and reduced the Tg values.

The storage modulus in the glassy region increased from 924 to 1224 MPa and in the

rubbery region it increased from 330 to 422 MPa (Figure 4.2) as increase in PTFE

content.

4.3.3.4 Thermogravimetric analysis

In order to understand the effect of PTFE on the relative thermal stability of

nylon 66/Gr/PTFE composites, TGA studies have been carried out under nitrogen

atmosphere. TGA is one of the widely used techniques to evaluate thermal stability and

thermal-degradation kinetics of polymeric materials, blends, and composites.

Thermogravimetric technique essentially involves continuous monitoring of the weight

of a sample as a function of temperature using a sensitive microbalance. Typical TGA

traces obtained for the nylon 66/PTFE/Gr composite samples containing 0, 5, 0, 15 and

20% by weight of PTFE are shown in Figure 4.5.

As can be seen from Figure 4.5 that upto 350 oC there is only a minor weight

loss corresponds to the loss of the moisture content from nylon, thereafter the weight

loss begins at 350 oC. The weight loss observed in TGA runs on these samples is given

in Table 4.5. Data generally showed a distinct and consistent non-reversible loss in

weight, attributed to the pyrolysis of nylon 66 in the temperature interval of 350–500

oC. Obviously, the composites degraded in two stages.

121

The TGA thermogram of nylon showed single step thermal degradation

processes (360-498 °C) with a weight loss of 98 %. From the TGA thermogram it was

noticed that nylon/PTFE/Gr undergoes a two-step thermal degradation processes

(Figure 4.5). It was noticed that all PTFE filled nylon composites undergo two step

thermal degradation processes. The temperature range and percent weight loss for

different thermal degradation steps of composites are shown in Table 4.5. The first step

weight loss occurred in the temperature range 339-528 °C. The weight loss which

occurred in first step decreases from 88.0 to 70.0 %. The weight loss in the first step

decreases with increase in PTFE content in nylon 66. This result clearly indicates that

the weight loss in the first step is significantly dependent on the nylon content.

Figure 4.5. TGA thermograms for nylon 66/PTFE/Gr composites

The weight loss that occurs in the temperature range 514-598 °C is called second

stage thermal decomposition in which the weight loss lies in the range 10 – 20 %. From

the table it can be seen that, the ash content of the nylon 66 composites increases with

increase in PTFE content and it lies in the range 2-10 %. The TGA thermograms

obtained were analyzed to give the percentage weight loss as a function of temperature.

T0 (temperature of onset decomposition), T10 (temperature for 10% weight loss) and

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

thermal stability of the composites(Figure 4.6). Higher the values of T0, T10 and Tmax

indicates higher the thermal stability of composites.

The outcome of the TGA analysis clearly reveals (Table 4.4) that the

temperature corresponding to To, T10 or T20 or Tmax are increasing progressively with

increase in PTFE content, which clearly shows the thermal stability of the nylon matrix

122

improved with incorporation of PTFE fillers. This is due to PTFE which has high

thermal stability and Tm than nylon 66.

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Figure 4.6. TGA thermograms along with derivative curves for nylon 66/Gr

composites with varying PTFE (a) 0%, (b) 5%, (c) 10%, (d) 15% and (e) 20%.

It is also seen from Table 4.6 that the oxygen index (OI) values increased with

increase in PTFE content and it lies in the range 0.11 – 0.69 %. Based upon the mass

carbonaceous char, it is concluded that, nylon 66/PTFE/Gr composites are flame-

retardants at higher PTFE loading as evident by their OI values.

(b) (c)

(d) (e)

(a)

123

Table 4.5. Thermal data obtained from derivative TGA curves for nylon

66/PTFE/Gr composites

Table 4.6. Transition temperature data obtained from TGA curves for nylon

66/Gr/PTFE composites

PTFE content

(wt. % )

Temperature at different weight loss (± 4ºC) Oxidation

index (OI) Ti T10 T20 T50 Tc

0 334 395 419 443 503 0.11

5 339 408 431 457 541 0.14

10 341 421 440 460 620 0.35

15 349 423 449 466 625 0.56

20 364 425 451 471 638 0.69

Content of

PTFE in nylon

66/Gr (%,

wt/wt.)

Degradation

stage

Temperature (˚С ±2 )

Weight loss

(%) T0 Tp Tc

0

1 360 457 498 98.0

Ash - - - 2.0

5

1 339 472 526 88.0

2 526 563 586 10.0

Ash ─ ─ ─ 2.0

10

1 341 487 527 78.0

2 527 558 583 17.0

Ash ─ ─ ─ 5.0

15

1 349 489 528 72.0

2 528 552 582 20.0

Ash ─ ─ ─ 8.0

20

1 364 464 514 70.0

2 514 553 598 20.0

Ash ─ ─ ─ 10.0

124

4.3.4 Sliding wear behaviour

4.3.4.1 Wear loss

The wear experiments were conducted with an aim of relating the influence of

PTFE content, sliding distance (D), applied load (L) and sliding velocity with dry

sliding wear characteristics of all nylon 66/PTFE/Gr composites under study. Figure

4.7(a)-(c) shows the plots of weight loss as a function of PTFE content for nylon

66/PTFE/Gr composites at varying sliding distances and at varying loads; (a) 50 N, (b)

100 N and (c) 150 N. It indicates that at all loads (50, 100 and 150N) and at all sliding

distances (1000, 1500 and 2000 m), the wear weight loss decreases, i.e., the wear

resistance of the nylon 66/PTFE/Gr composites improves with increase in PTFE content

from 5 to 20%.

Figure 4.7. Weight loss as a function of PTFE content for nylon 66/Gr composites at

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

Figure 4.8. Weight loss as a function of load for nylon 66/PTFE/Gr composites at

sliding distances of 1000 m, 1500 m and 2000 m

125

The improvement in wear resistance with increase in PTFE content in composite

is due to the formation of thin barrier film on the counterface which retards the rate of

wear. Similarly Figure 4.8(a)-(c) shows the weight loss as a function of load for nylon

66/PTFE/Gr composites at varying PTFE contents and at different sliding distances of

1000, 1500 and 2000 m respectively. From figure it was noticed that for all composites

(0, 5, 10, 15 and 20% PTFE) and at all sliding distances (500, 750 and 1000m) the wear

weight loss increases, with increase in load 50, 100 and 150N.

Figure 4.9(a)-(c) reveals that the weight loss as a function of sliding distance for

nylon 66//PTFE/Gr composites at varying loads. From figure it was noticed that for all

compositions (0, 5, 10, 15 and 20% PTFE) and at all loads (50, 100 and 150 N) the

wear weight loss increases, i.e., the wear resistance of the nylon/PTFE/Gr composites

decreases with increase in sliding distances (1000, 1500 and 2000 m).The effect of

sliding velocity on weight loss of the composites is shown in Figure 4.10(a)-(c).

Figures 4.7 to 4.10 reveals the wear resistance of nylon 66//PTFE/Gr increases

with increase in PTFE content, but wear resistance decreases with increase in applied

load or sliding distances.

Figure 4.9. Weight loss as a function of sliding distance for nylon 66/PTFE/Gr

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

Figure 4.10. Weight loss as a function of PTFE content for nylon 66/Gr composites

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

126

4.3.4.2 Specific wear rate

The plots of specific wear rate as a function of PTFE content for nylon

66/PTFE/Gr composites at varying sliding distances and at varying loads is shown in

Figure 4.11(a)-(c). From the figure it was noticed that at all loads (50, 100 and 150N)

and at all sliding distances (1000, 1500 and 2000 m) the specific wear rate decreases

with increase in PTFE content from 0 to 20%. Similarly Figure 4.12(a)-(c) shows the

specific wear rate as a function of load for nylon 66/PTFE/Gr composites at sliding

distances of 1000, 1500 and 2000 m. It indicates that for all compositions (0, 5, 10, 15

and 20% PTFE) and at all sliding distances (1000, 1500 and 2000 m) the specific wear

rate drastically decreases. Figure 4.13(a)-(c) shows the specific wear rate as a function

of sliding distance for nylon 66//PTFE/Gr composites at varying loads; (a) 50 N, (b)

100 N and (c) 150 N. Figure 4.13 indicates that for all compositions (0, 5, 10, 15 and

20 wt. % PTFE) and at all loads ( 50, 100 and 150N) the specific wear rate gradually

decreases with increase in sliding distance, Figures 4.11 – 4.13 reveals the specific wear

rate of nylon 66//PTFE/Gr decreases with increase in PTFE content, load or sliding

distances.

Figure 4.11. Specific wear rate as a function of PTFE content for nylon 66/Gr

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

A brief discussion of atomic structure of graphite will enhance understanding of

how graphite improves the tribological properties of the polymer composites. In

graphite, the carbon atoms are arranged hexagonally in a planar condensed ring. Also,

the layers are stacked parallel to each other, with the atoms within the rings bonded

covalently, whereas the layers are loosely bonded together by Van der Waal’s forces.

The anisotropic nature of graphite is the result of the two types of bonding acting in

different crystallographic directions. The ability of graphite to form a solid film

lubricant may be attributed to these two contrasting chemical bonds. Also, the weak

127

Van der Waal’s forces govern the bonding between the individual layers, permitting the

layers to slide over one another, making it an ideal lubricant, and resulting in a reduced

coefficient of friction and, hence, wear.

Figure 4.12. Specific wear rate as a function of load for nylon 66/PTFE/Gr

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

Figure 4.13. Specific wear rate as a function of sliding distance for nylon

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

The reduction in specific wear rate with increase in PTFE 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 matrix. 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 [14]. This is also evident from the small size

of the wear debris particles as determined by SEM analysis.

The influence of fibers and/or fillers on the abrasive wear resistance of neat

polymer is more complex and unpredictable and mixed trends are reported [28–32].

Lancaster [28] studied 13 polymers reinforced with 30% short carbon fiber and reported

that reinforcement enhanced the wear performance of seven composites while that of

128

six composites deteriorated. Sole et al [29] 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. [30], 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. Lu et al. [31], for instance,

investigated the abrasion resistance of earth moving equipment components made up of

alumina + PTFE + PPS and reported that the wear performance continuously increased

up to 20% filler concentration. Beyond that it worsened drastically. Ratner et al. [32]

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. Better wear

resistance was obtained due to the presence of graphite filler in the composites. During

sliding, the graphite particles got smeared at the interface and formed a graphite film

that reduced the coefficient of friction and, hence, wear.

4.3.4.3 Co-efficient of friction

The co-efficient of friction data of nylon 66/PTFE/Gr composites are shown in

Table 4.7, which shows that co-efficient of friction decreases with increase in PTFE

content in the nylon 66 matrix. This is due to fact that the PTFE has one of the lowest

coefficients of friction against any solid. Its static coefficient of friction is lower than its

dynamic coefficient. So, naturally the incorporation of PTFE reduces the co-efficient of

friction of nylon 66 matrix. However, the variation in sliding distance and load has

adverse effect on the co-efficient of friction, i.e., the co-efficient of friction increases

with increase in sliding distance (1000, 1500 and 2000m) and load (50, 100 and 150N).

It is evident that the friction coefficient reduces continuously by increasing the content

of both PTFE in the composites. This behaviour is obviously relevant to the self

lubricating effect of PTFE which can reduce the adhesion between the composite with

the metallic counterpart [33].

Similar kind of variation has been reported elsewhere for glass fabric reinforced

epoxy composites [6]. The wear of nylon 66//PTFE/Gr composites consists of two wear

modes: polymer matrix wear, which includes matrix plastic deformation and cracks in

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the matrix and matrix wear. The order of wear resistance behaviour of composite is as

follows; 20 > 15 > 10 > 5> 0 % by weight of PTFE.

Table 4.7 Coefficient of friction for nylon 66/PTFE/Gr composites at velocity of 5

m/s

Load

(N)

Sliding

length (m)

Co-efficient of friction for nylon 66/PTFE/Gr composites

(%)

100/0 95/5 90/10 85/15 80/20

50

1000 0.180 0.172 0.160 0.156 0.155

1500 0.256 0.251 0.236 0.230 0.222

2000 0.263 0.257 0.246 0.235 0.212

100

1000 0.186 0.182 0.171 0.162 0.158

1500 0.282 0. 277 0.249 0.236 0.231

2000 0.309 0.300 0.256 0.229 0.207

150

1000 0.211 0.204 0.189 0.182 0.169

1500 0.298 0.288 0.274 0.252 0.226

2000 0.322 0.315 0.289 0.261 0.229

Also, it was noticed that 20 wt. % PTFE loaded composites exhibited the higher

wear resistance under all abrading distances/loads. The results have indicated that the

PTFE decreases friction coefficient and wear rate of the nylon 66 while the mechanical

properties are reduced due to the weaker mechanical properties of PTFE with respect to

neat nylon 66 [34-37]. However, incorporation of PTFE can improve the mechanical

properties of PAs at wetted conditions because of the reduction in the water absorptivity

of nylon 66 [38].

4.3.4.4 Wear mechanism

SEM images of nylon 66/PTFE/Gr composites with varying PTFE content (a)

0%, (b) 5%, (c) 10%, (d) 15% and (e) 20% at 5 m/s sliding velocity for 1000 m and

2000 m abrading distances is shown in Figure 4.14 and 4.15 respectively. Acording to

SEM images the incorporation of PTFE into nylon 66/Gr matrix leads to fibrilar

morphology. This phenomenon, i.e., fibrillar morphology, is much more pronounced at

high PTFE loadings, i.e., 20 wt % (Figure 4.14(d)), where the long fiber of PTFE can

be observed. Fibrilization of PTFE particles can be a consequence of coalescence of

PTFE micro-powders during the mixing process where hydrophobic PTFE particles

130

might be soft and sticky due to the higher mixing temperature, i.e., close to the melting

temperature of PTFE 326 ºC. Therefore, with the addition of PTFE and graphite

particles, the matrix damage is lessened even at high load and velocity conditions and

led to an enhanced load-carrying capacity of the composite. Further, composites with

higher wt % PTFE shows more stable wear performance under all test conditions. It

seems that although additional lubricants contribute to a stable development of the

transfer film even at extreme sliding conditions, meaning that uniform transfer of PTFE

and graphite from the sample to the counterface is observed.

Figure 4.14. SEM Images of nylon 66/PTFE/Gr composites with varying PTFE

content (a) 0%, (b) 5%, (c) 10%, (d) 15% and (e) 20% at 5 m/s sliding velocity for

1000 m abrading

a b

c d

e

131

Figure 4.15. SEM images of nylon 66/PTFE/Gr composites with varying PTFE

content (a) 0%, (b) 5%, (c) 10%, (d) 15% and (e) 20% at 5 m/s sliding velocity for

2000 m abrading distance

Occurrence and extent of agglomeration is the competition of two opposing

forces acting on the particles during mixing process. These two types of forces include

adhesive force between the particles, which reinforces particle agglomeration, and shear

force exerted on the particles during the mixing, which leads to breakdown of

aggregates [39].

a b

c d

e

132

Figure 4.16. SEM images of nylon 66/PTFE/Gr composites at 5 m/s sliding

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

Figure 4.17. SEM images of nylon 66/PTFE/Gr composites at 1000 m sliding

distance, 50 N load for (a) 5 m/s and (b) 9 m/s

The SEM images of worn surfaces of nylon 66/PTFE/Gr composites subjected

to wearing at 5 m/s sliding velocity at 100 N load is shown in Figure 4.16(a) for 1000 m

sliding distance and Figure 4.16(b) for 2000 m sliding distance. Lower sliding distance

sample shows relatively smooth surface with fine lateral cracks just visible as shown in

Figure 4.16(a). However, at higher sliding distance; 2000 mm, Figure 4.16(b) shows a

pattern of deep cracks spaced between 0.2 and 0.5 mm apart. These appear to be the

result of the high friction forces which produce large tensile stresses at the ends of the

contact. A few patches of PTFE fillers can be seen lying on the surface. The SEM

images of nylon 66/PTFE/Gr composites subjected to wearing at 1000 m sliding

distance at 50 N load is shown in Figure 4.17(a) for 5m/s sliding velocity and Figure

4.17(b) for 9 m/s sliding velocity. The high wear rate and considerable production of

wear debris observed from the beginning of the wear process indicate that the overlying

layer does not protect the substrate effectively.

a b

a b

133

4.3.5 Regression analysis

Table 4.6. Regression equations for nylon 66/PTFE/Gr composites

PTFE

content (%)

Regression Equation

0 - 0.00122 + 0.000016 L + 0.000333 V+ 0.0042D

5 - 0.00114 + 0.000014 L + 0.000317 V + 0.00032D

10 - 0.00117 + 0.000014 L + 0.000267 V + 0.00039D

15 - 0.000292 + 0.000012 L + 0.000275 V + 0.00027D

20 - 0.000569 + 0.000009 L + 0.000142 V + 0.00024D

where, L = Load, V = Sliding velocity and D = Sliding distance

Based on the wear experimental results the correlation between the wear

parameters and wear characteristics is obtained using linear regression technique. In the

equations shown in the Table 4.6., it is observed that the values associated with load

and sliding velocity parameter decreases with increasing PTFE content in nylon 66/Gr

indicating that wear loss decreases as increase in PTFE composition in nylon 66/Gr.

4.3.5.1 Process parameters

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

load, sliding velocity and sliding distance (Table 4.7). 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 experimental parameters such as load, speed and

temperature [40-41].

Table 4.7 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

A mathematical model was 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 [42].

134

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 successive error

terms are, on average, much different in value to one another, i.e., negatively correlated.

From Table 4.8, it was noticed that calculated DW values for all the composites are less

than 2 indicating the presence of positive serial correlation which can be further

implied that the material’s resistance to wear.

The standard deviation values strongly indicate minimum deviation from the

wear values and sum of variance values showed that the variance values show

decreasing trend with increase in PTFE content in nylon 66/Gr. The residual error also

indicative of the error with the composites follow decreasing path with increase in

PTFE content in the composites.

Table 4.8 Analysis of variance (ANOVA) for nylon 66/PTFE/Gr composite

Source PTFE content in composite (wt. %)

0 5 10 15 20

DOF 2 2 2 2 2

SS 6.51E-06 5.35E-06 4.51E-06 2.73E-06 1.61E-06

MS 3.25E-06 2.67E-06 2.25E-06 1.37E-06 8.04E-07

F 42.23 48.77 40.78 46.71 59.9

P 0.470 0.027 0.025 0.031 0.026

S 0.00028 0.00023 0.00024 0.00017 0.00012

R-SQ (%) 93.4 94.2 93.1 94.0 95.2

Coefficient 0.0012222 0.00114 0.00117 0.00081 0.00057

SE Coeff. 0.00047 0.00039 0.00039 0.00029 0.00019

T 2.62 2.9 2.96 2.8 2.93

R. Error 4.62E-07 3.29E-07 3.32E-07 1.76E-07 8.06E-08

DW

Statistic 2.17 1.87 2.15 1.82 1.91

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

contribution, S = Standard deviation, DW Statistics = Durbin Watson statistics, R.

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

135

4.3.6 Laser assisted etching behaviour

The etching parameters like laser power, etching speed, laser pulse duration,

frequency have been optimized by measuring the surface roughness and micro hardness

of laser etched surface. The processing parameters of the machine are marking speed of

250 characters per second with micro pulse duration. The micro hardness of laser

etched surface measured using micro hardness machine. The surface roughness of laser

etched surface was measured by optical photomicrograph. The average roughness, Ra

values were recorded for composites before and after Laser etching. From Table 4.9, it

was noticed that Ra values decreases with increase in PTFE content and increases with

power of etching.

Table 4.9. Surface roughness results for laser etched nylon 66/PTFE/Gr

composites

PTFE content in

nylon 66/Gr (wt. %)

Surface roughness (Ra) 500 mm/s, 5 kHz

50 % power 100 % power

0 4.1 4.8

5 3.8 4.2

10 3.6 3.8

15 3.3 3.5

20 2.9 3.2

Tagliaferri [40] conducted an experimental study to determine the surface finish

characteristics of carbon and aramid fibre-reinforced plastics by CO2 laser. The 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 [41-42]. YAG lasers have low beam power but when

operating in pulsed mode high peak powers enable it to machine even thicker materials.

Caprino [41] developed a simple one-parameter thermal model, predicting the

maximum feed rate as a function of beam power.

4.3.6.1 Morphology of laser etched surfaces

The optical photomicrographs of laser etched surfaces of nylon 66/PTFE/Gr

composites are shown in Figures 4.17- 4.18. The roughness of the surfaces are observed

through optical microscope and the photomicrographs of nylon 66/PTFE/Gr composites

showed decreased trend in roughness with increasing PTFE content at 50 % power, 5

136

kHz frequency for 500 mm/s velocity as shown in Figures 4.18 (a)-(e). The increase in

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

composites (Figures 4.19(a)-(e)). The surface damage increases with increase in power

of etching.

Figure 4.18. Photomicrographs of laser etched nylon 66/PTFE/Gr composites with

(a) 0%, (b) 5%, (c) 10%, (d) 15% and (e) 20 % PTFE content at 5 kHz frequency,

50% power and 500 mm/s velocity

a b

c d

e

137

Figure 4.19. Photomicrographs of laser etched nylon 66/PTFE/Gr composites with

(a) 0%, (b) 5%, (c) 10%, (d) 15% and (e) 20 % PTFE content at 5 kHz frequency,

100% power and 500 mm/s velocity.

4.4 Conclusions

With the objective to improve the physico-mechanical and tribological

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

graphite. The PTFE content was varied selectively from 5 to 20 wt % keeping graphite

content constant at 2 wt%. Thus, the wear resistance of resultant nylon 66/PTFE/Gr

composite increased with increase in PTFE content, but decreased with increase in load,

sliding velocity or sliding distances. Similarly the specific wear rate of nylon

66/PTFE/Gr decreased with increase in PTFE content, load and sliding distances. The

co-efficient of friction decreased with increase in PTFE content. The impact strength of

the nylon 66 matrix increased from 34.8 to 37.3 J/m for the incorporation of PTFE

content. The mechanical property results are not as expected, because of the

incorporation of 2% Gr powder may have not prevented the agglomeration of PTFE

a b

c d

e

138

powder during melt mixing with nylon, due to wide difference in polarity and, secondly

the poor interaction between nylon 66 matrix and PTFE powder leads to an increased in

void formation, in turn reduction in mechanical properties. However, the incorporation

PTFE and graphite improved the wear resistance under different loads. This is due to

synergetic effect of PTFE and graphite. It was observed that the PTFE played a main

role in the wear resistant properties of nylon 66 composites.

139

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