STUDIES ON MECHANICAL, THERMAL, WEAR AND MORPHOLOGICAL BEHAVIOURS OF NYLON 66/POLYTETRA FLUORO ETHYLENE COMPOSITES

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1 CHAPTER 3 STUDIES ON MECHANICAL, THERMAL, WEAR AND MORPHOLOGICAL BEHAVIOURS OF NYLON 66/POLYTETRA FLUORO ETHYLENE COMPOSITES This chapter presents the mechanical, thermal, sliding wear and morphological behaviours of nylon 66/polytetrafluoroethylene (PTFE) composites. Nylon 66/PTFE composites with varying amounts viz., 0, 5, 10, 15 and 20 % of PTFE have been prepared by melt mixing technique. The physico-mechanical properties such as density, surface hardness, tensile behaviours, and impact strength have been evaluated for nylon 66/PTFE. A slight improvement in the impact strength was noticed after incorporation of PTFE and it lies in the range J/m. The thermal characteristics of nylon 66/PTFE composites have been performed using DSC, TGA and DMA. DSC thermograms showed that PTFE had no significant effect on the melting temperature (T m ) and heat of melting ( H m ) of nylon 66. Sliding wear experiments were conducted using a pin-on-disc wear tester under dry contact condition. The effect of PTFE content, load, sliding velocities and sliding distances on wear loss, specific wear rate and coefficient of friction of composites have been evaluated. Worn surfaces were examined with optical microscope images to have better insight of the wear mechanism. The surface erosion resistance by laser etching of nylon 66/PTFE composites have been performed using surface roughness (Ra) optical photomicrographs. 3.1 Introduction The good wear resistance and self-lubricating characteristics as well as good strength and stiffness of nylons make these thermoplastics promising candidate in the bearing applications where two different bodies are in contact at severe sliding conditions [1-2]. However, further improvement is still required to meet more demanding applications. In order to enhance the tribological characteristics of nylons efficiently, the solid lubricants such as polytetrafluoroethylene (PTFE), graphite, molybdenum disulphide etc., may be added into the nylon matrix. The solid lubricants often lead to decrease of coefficient of friction and wear rate through the reduction in 75

2 adhesion with the counterface or creation a transfer film with low shear strength at the interface [3, 4]. A friction and wear characteristics of unreinforced nylon are widely reported in literature [5-8]. The friction behaviour of commercial polymer based bearing materials were studied by sliding against steel surface [6]. It was reported that the friction and wear behaviour of nylon, an engineering thermoplastic was fairly satisfactory under dry sliding conditions and lubrication was found at higher speeds. Watanabe and Yamaguchi [7] investigated the effect of temperature, load and sliding speed on the coefficient of friction and wear behaviour of nylon 66. In polymer tribology, polymer blending is an effective way to improve the mechanical and tribological properties of polymers [8]. Several researchers have studied the tribological properties of polymer composites and pointed out that the friction and wear properties varied continuously with the compositions for most polymer composites and the optimal properties were found at a certain composition, although some data reported were contradicting [9-14]. To reduce the adhesion, internal lubricants such as PTFE and graphite flakes are frequently incorporated to polymer matrix composites [6-7]. Bijwe et al studied the wear performance of various composites of nylon 66 reinforced with short carbon fibres and lubricated with a solid lubricant, PTFE, under adverse sliding conditions (abrasive wear) [14-15]. The studies revealed that fillers that are very much suitable for adhesive wear applications are detrimental for the abrasive wear mode. It was found that wear performance was greatly influenced by selected experimental parameters. For a very large variety of reasons these systems have been very extensively studied, not least because of the historically practical and academic interest in the wear of the PTFE, and the sister polymers such as the linear polyethylene, in particular the high molecular weight polyethylene (HMWPE); the latter have found great favour in orthopaedic replacement components. The class is also one that shows a wide range of applications - PTFE and high-temperature polymer composites for bearings and compressor piston seals, nylons as common automobile bearings, polyethylene in marine environments and polyvinylidenefluoride (PVDF) in electrical shielding [16-17]. Polyamide (PA) is a widely used engineering plastic. It possesses an outstanding combination of properties such as low density, easy processing, good strength and solvent resistance. However, its heat distortion temperature is low and because of the presence of 76

3 amide groups in the molecular chain, it easily absorbs water which deteriorates its mechanical properties and dimensional stability. PTFE is an engineering plastic has a low friction, scratch resistance, high thermal stability, chemically inert and biocompatible material. This unique combination of properties makes PTFE indispensable in a number of extreme applications. It has, however, a lower elongation to break, a higher cost and difficult to process. The superior wear resistance of PA compared with other polymers has been attributed to its ability to form an adhesive transfer film when sliding against metal counterface. Unfortunately, its application at extreme environmental conditions, solid lubricant has been preclude by a prohibitively high wear rate [18-21]. Numerous hard fillers can abate severe wear of PTFE, but typically this comes at the expense of its other beneficial properties (most notably friction) [19-20, 22-26]. PTFE is a wellknown anti-adhesive material for tribological applications [27]. However, due to its poor processability with the commercial processing equipments such as extruder and injection moulding machines and relatively weak mechanical properties, the widespread usage of PTFE as antiwear materials is restricted. Alternatively, PTFE can act as an efficient solid lubricant when it is added into the nylon matrix so that the nylon 66/PTFE composites take the advantages of good mechanical properties and processability of the nylons and very low friction coefficient and good wear resistance of PTFE. The roughness and the lay orientation influence the wear rate of polymers. With the enlargement of application fields of engineering plastics, it is essential to study the tribological behaviours as well as its mechanisms. So, nylon 66 and PTFE which are promising tribo-materials are selected in this investigation. Addition of PTFE will affect the friction and wear properties of nylon 66 and the effect needs to be clearly understood for optimum utilization and widening the applications of nylon 66. In this chapter the effect of PTFE content on sliding wear characteristics of nylon 66/PTFE composites have been studied. 3.2 Fabrication of composites Nylon 66 was mixed with PTFE using a co-rotating intermesh twin screw extruder (Nanjing Rubber & Plastics Pvt. Ltd., China) at a screw speed of 175 rpm with barrel temperature ranging from 278 to 280 ºC for 30 min to achieve a reasonably uniform dispersion. The extrudates were pelletized in the form of granule. Prior to compounding, the polymers were first dried in a vacuum oven at 70 ºC for 8 h to 77

4 remove the possible moisture. The machine consists of nine nozzles and the temperature zones maintained at each of the nozzles are different and lies in the range o C. Composites have been prepared with varying amounts viz., 0, 5, 10, 15 and 20 % by wt., of PTFE. The specimens for mechanical testing were prepared using injection molding machine (Windsor SD-75, India). The barrel temperature of molding machine was between o C, the screw speed was 60 rpm and the injection pressure was 80 bars. 3.3 Results and Discussion Physico-mechanical properties The mechanical properties, among all the properties of plastic materials, are often the most important properties. The values of measured mechanical properties such as density, surface hardness, tensile properties, and impact strength of the nylon 66/PTFE composites were given in Table Density Density is a very important physical property of the material. It has great significance in comparing polymeric materials when combined with strength and stiffness of the materials. Density also depends upon the composition of the composites and their physical as well as chemical properties. Density is a significant indicator of end use of the polymeric materials, especially for thermoplastics. The theoretical density of the composites was calculated by weight additive principle, which states that; ρ = w 1 ρ 1 + w 2 ρ 2 (3.1) where, ρ is the density of the composite, w 1 and w 2 are the weight fractions of the constituents, ρ 1 and ρ 2 are the corresponding densities. The nylon 66 and PTFE had average density values of and 2.1 g/cc respectively. The measured density values of nylon 66/PTFE composites lies in the range g/cc. The measured density values of the nylon 66/PTFE composites were lower than the calculated value. The disagreement between the theoretical and measured values could be due to the formation of voids at the interface between the two components and also immiscible nature of the nylon 66/PTFE composites. The density values increases as increase in PTFE content in the composites, due to the incorporation of high dense PTFE in nylon 66 matrix. 78

5 Water uptake behaviour The equilibrium water uptake behaviour is an important quantitative measure in the practice of moisture sensitive products or the final products which are come in contact with water. Water uptake in case of polymeric systems depends on the chemical structure and morphological behaviours. Nylons are highly hygroscopic in nature because the presence of polyamide linkage in the chain. There is a need to address the composites of polyamides to make it suitable for engineering application. The measured equilibrium water uptake content is used to predict the hygroscopic nature of the composites. From Table 3.1 it was noticed that the water absorption value of pure nylon 66 is high (2.7 %) and it decreases from 2.4 to 1.2 % with increase in PTFE content from 5 to 20 % in the nylon 66/PTFE composites. This can be attributed to the increase in water repellent or hydrophobic PTFE component in the nylon 66 matrix. This result clearly indicates that the significant improvement in the water resistance behaviour of the composites 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 3.1 reveals that void content of this composite material increased from to % with increase in PTFE powder in composite. This shows the increase in free volume in the composites with increase in PTFE content. This may be due to an entrapment of air bubble during mixing particulate PTFE powder in crystalline nylon 66 matrix Surface hardness The surface hardness is an important property of the material, which may be defined as the resistance of a material against deformation, indentation and scratching. Hardness test can differentiate the relative hardness of different grades of thermoplastics. Also hardness reveals the dimensional stability of the composites which depends on the nature of composition of the components. The surface hardness values of nylon 66/PTFE composites lies in the range shore D and there is a slight reduction in hardness with the composition of PTFE content in the composites. 79

6 Tensile behaviours Tensile test is a measure of the ability of a material to withstand forces that tend to pull it apart and to determine to what extent the material stretches before breaking. The basic understanding of stress-strain behaviour of polymer material is of utmost importance to design engineers. Tensile properties were evaluated from the stress verses strain curves and the measured tensile properties of nylon 66/ PTFE composites are shown in the Table 3.2. After inclusion of PTFE component into nylon 66 reduces the elongation at fracture of the composites. Table 3.1. Physical properties of the nylon 66/PTFE composites PTFE content (%) Water uptake (%) Experimental Density (g/cc) Theoretical Void content (%) Surface hardness (Shore D) From the Table 3.2 it can be seen that a decrease in tensile strength from 78 to 64 MPa and tensile modulus from 2765 to 2613 MPa with increase in PTFE content from 0 to 20% in the composites. This is because tensile strength, percentage elongation at break and modulus of the PTFE is very low as compared to pristine nylon 66, hence reduction in tensile behaviour was noticed after incorporation of PTFE into nylon 66. This makes difficult the analysis of interaction between the nylon 66 and PTFE based on the tensile properties. However, incorporation of PTFE can improve the mechanical properties of nylon 66 at wet conditions because of the reduction in the water absorptivity of nylons [18]. Alternatively, quantitative analysis of the tensile properties can provide much profound insight about the interaction properties in this system. The tensile strength has been evaluated using (Voigt upper bound model) the rule of mixtures [29, 30] as follows: E c = V f E f + (1-V f ) E m (3.2) 80

7 where, V f is the fraction of the filler in the matrix, E c, E f and E m are the moduli of the composite, filler and matrix respectively. V f is the volume fraction of the filler in the matrix. In Reuss model (inverse of rue mixtures), the stress is applied in the transverse direction of the fibre. The modulus in the transverse direction is given by [28, 29]; 1/E T = [V f /E f ] + [1- V f ]/E m (3.3) where, E T is the modulus along the transverse direction. The Voigt and Reuss models can be used irrespective of the filler shape and contain only three parameters-filler volume fraction and moduli of the filler and the matrix. Figure 3.1. Plots of experimental and theoretical (Voigt and Reuss models) tensile strength as a function of PTFE content Figure 3.1 compares the experimental and theoretical values of the tensile strength and tensile modulus. It is to be noted that the yield stress and elastic modulus of PTFE were taken to be 33 MPa [3] and 340 MPa [30] respectively. As can be deduced from Figure 3.2, the tensile strength values of nylon 66/PTFE are lower than the theoretically calculated values for all compositions. This can be attributed to the lack of interaction between the nylon 66 and PTFE, as expected due to the lack of thermodynamic affinity, as well as the occurrence of severe agglomeration. It is deduced that the tensile strength and elastic modulus decreases with incorporation of PTFE in nylon matrix. 81

8 Figure 3.2. Plots of experimental and theoretical (Voigt and Reuss models) tensile modulus as a function of PTFE content Impact strength The impact strength becomes very important property because cracks due to sudden loads are very common in service conditions. Forces (loads) 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 impact strength of the composites depends upon many factors like toughness 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. The impact strengths of the composites are higher than that of nylon 66 and found to gradually increase with increase in PTFE dosage in the composites. The results of mechanical behaviours of the composites clearly indicate that the poor interaction between nylon 66 and PTFE. This is because nylon is hydrophilic and PTFE is strongly hydrophobic polymer and hence, their composites are immiscible in nature. The higher impact strength of PTFE loaded nylon composites can also be hypothesized due to the severe fibrilization of PTFE which can inhibit the growth of micro cracks resulting in improved impact strength. When PTFE is loaded into nylon 66, it results in increase in toughness with a marginal increase in the impact strength and decrease in tensile strength. 82

9 PTFE content (%) Table 3.2. Mechanical properties of the nylon 66/PTFE composites Tensile strength (σ) (MPa) Tensile modulus (MPa) Elongation at break load (e) (%) Product parameter (σ x e) Impact strength (J/m) + 1.5% Thermal analysis The thermal characteristics of nylon 66/PTFE composites have been characterized by using HDT, DSC, DMA and TGA. The results of thermal behaviours of the composites briefly interpreted in the forthcoming section Heat distortion temperature The performance of nylon 66 exposed to heat depends on the specific thermal properties of the resin concerned, time of exposure, the nature of heat source and the mechanical load as well as design of moulding. The measured heat distortion temperature (HDT) value for pristine nylon 66 is in the range o C. Nylon 66 is a crystalline thermoplastic with a low linear thermal expansion co-efficient. The decrease in HDT values is observed after incorporation of PTFE content in nylon 66. The HDT values of 20 % PTFE loaded nylon 66 composite is around o C Differential scanning calorimetry The thermal properties of nylon 66/PTFE composites were investigated by DSC technique to analyze the effect of PTFE content in nylon 66 on T g, T m, heat of fusion and crystallinity. The DSC thermograms of nylon/ptfe composites were shown in Figure 3.3. The thermal data obtained from DSC curves of neat nylon 66 and PTFE filled nylon 66 are summarized in Table

10 Table 3.3. Transition temperature (melt characteristics) data obtained from DSC thermograms for nylon 66/PTFE composites Compositio n of nylon 66/PTFE (%, wt/wt) *The heat of fusion value of 100% crystalline nylon J/g. The values of H m provide important information about the crystallinity of the composites. Based on the DSC results and considering the PTFE proportions, it is possible to conclude the following: T o ( º C) T m ( º C) T c ( º C) If H m of the composites is lower than 80% of the value for pure nylon 66, it means that its interaction with the polymer matrix decreasing its crystallinity; If H m of the composites is larger than 80% of the value pure nylon 66, it means that its interaction with the polymer matrix increasing its crystallinity; If H m of the composites is 80% of the value for pure nylon 66, it means that it do not interact with the polymer matrix. H (J/g) Crystallinity* χ c (%) Exp. Cal. Dev. (%) Exp. Cal. 100/ Dev. (%) 95/ / / / From the results obtained for H m (Table 3.3), it was observed that 20% PTFE filled composite has larger deviation, +3.8 % from the expected. While 5, 10 and 15 % filled composites have lower H m value of +1.9%, +1.6% and 3.0 % deviations from expected data. The crystallinity values for the composites were calculated using the crystallinity of 100% nylon 66 as a parameter, 196 J/g, [28] and considering the proportion of PTFE in the composites. 5% PTFE filled composite showed lower degree of crystallinity than expected, while 10, 15 and 20 % filled composites showed still lower, crystallinity values than pristine nylon 66 crystallinity value. It can be inferred that the melting temperature (T m ) and heat of melting ( H m ) do not change considerably by the incorporation of PTFE content into nylon 66. In other words, this indicates that PTFE does not affect the crystalline structure of the nylon 66. This phenomenon may be explained by the increase of crystallinity provided by the PTFE 84

11 that is acting as a nucleating agent, due to the trans-crystallinity effect provided by the strong interaction between the PTFE and the matrix. Figure 3.3. DSC thermograms of nylon 66/PTFE composites Such behaviours for nylon 66/PTFE composites reveal that the extent of interaction is not such a high value to influence crystallinity of nylon 66; this is because they are immiscible composites. Similar kind of observation was made for nylon with lower dosage of PTFE systems by many scientists [31-32] have reported that PTFE loading in nylon 66 had no effect on the type of melt and enthalpy of crystallisation 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. The dynamic modulus indicates the inherent stiffness of the material under dynamic load conditions. The dynamic mechanical behaviours of polymers are usually studied over a wide temperature range (25 o C to 200 o C). In the region where the dynamic modulus temperature curve has an inflection point and tan delta curve goes through a maximum and this transition is called as T g region. The DMA analysis is carried out to study the effect of incorporation of PTFE powder in nylon 66. The effect of PTFE content on the storage modulus of composites is shown in Figure 3.4. All the specimens showed a glassy state that is followed by the rubbery state. In general, the storage modulus, E' is found to increases with increasing PTFE 85

12 content below and above the glass transition temperature. This indicates that incorporation of the PTFE has improved the stiffness of the nylon 66/PTFE composites and the dependence of E' on PTFE loading is more pronounced around the glassy region. The drop in the storage modulus with temperature during the transition from the glassy to the rubbery state occurs at around C [33, 34] as shown in Figure 3.4. The measured storage modulus of the composites at glassy region and rubbery region is given in Table 3.4. The effect of temperature on loss modulus (E'') of PTFE loaded nylon 66 is shown in Figure 3.5. From the figure, 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 incorporation of PTFE into nylon 66 phase, dissipation of energy also increases. Similar type of observation reported by Hamdan et al for barium ferrite filled thermoplastic natural rubber [35]. The damping property (tan δ), as the ratio of the dynamic loss modulus (or viscous) to the dynamic storage modulus (or elastic), is related to the molecular motions and phase transitions. Tan δ is therefore sensitive to all molecular movement occurring in polymers. Loss tangent (tan δ) as a function of temperature for all composites is shown in Figure 3.6. The obtained tan δ values (both predicted and experimental.) along with T g which was represented by the peak temperature of the tan δ curve is summarized in Table 3.4. From the results it can be seen that the incorporation of PTFE to nylon/ptfe, leads to a slight change in the T g at the same time tan δ values decreases with increase in PTFE content as expected. The glass transition temperature of the nylon 66/PTFE composites increases with increase in PTFE content. The effect of small amounts of PTFE on the free volume of nylon 66 is significant to influence the T g of virgin nylon, which may be attributed to the interactions between the phases. These interactions enhance the rigidity of the soft segments and limit the movement or motions of the soft segments. Therefore, the introduction of PTFE results in a slight increase in phase transition temperatures of the soft segments. Since the damping behaviour of polymer composites deserves much importance industrially and academically their predictive methods will be beneficial for polymer technologists. The incorporation of rigid components usually decreases the damping as expressed by tan δ to an extent predicted by rule of mixture equation [36], Tan δ c = V f Tan δ f + V m Tan δ m (3.4) 86

13 But in the case of rigid inclusions, the first term can be neglected and therefore the equation (3.6) becomes [37], Tan δ c = V m Tan δ m (3.5) where, the subscripts c and m represents composites and matrix, V m is the volume fraction of nylon. Figure 3.4. Plots of storage modulus versus temperature for nylon 66/PTFE composites Table 3.4. Data obtained from DMA analysis for nylon 66/PTFE composites PTFE wt. % Tan δ Storage modulus (MPa) T g ( o C) Exp. Theo. Glassy region Rubbery region

14 Figure 3.5. Plots of loss modulus versus temperature for nylon 66/PTFE composites Figure 3.6. Plots of loss tangent versus temperature for nylon 66/PTFE composites The effect of PTFE content on the tan δ is given in Table 3.4. From the table it can be observed that the experimental values are slightly high or almost identical to the theoretically calculated values Thermogravimetric analysis The thermogravimetric analysis (TGA) is a useful technique to determine the quantitative degradation based on the weight loss of a composite material as a function of temperature. The typical TGA and their derivative thermograms for nylon 66/PTFE 88

15 composites are shown in Figures 3.7 (a)-(e) and TGA thermogram for all the samples is shown in Figure 3.8. From this figure it was observed that the, decomposition of the composites when viewed as a whole is a complex process to follow. The multi-stage decomposition observed for composites is due to the scission of chemically different segments in the polymer chain. The thermograms obtained during the TGA scans were analyzed to give the percentage weight loss as a function of temperature. T o (temperature of onset decomposition), T 10, T 20 and T 50 (temperature for 10, 20 and 50 % weight loss) are the main criteria to indicate the heat stability of the composites. The relative thermal stability of nylon 66/PTFE composites was evaluated by comparing decomposition temperatures at various percent weight losses (Table 3.5). Table 3.5. Thermal data obtained from TGA thermograms of nylon 66/PTFE composites Composition of nylon 66/PTFE (%, wt./wt.) Temperature at different weight loss (± 2ºC) Oxidation index (OI) T o T 10 T 20 T 50 T max 100/ / / / / Onset decomposition values increases noticeably as increase in PTFE content in nylon. That means the moisture content in the nylon 66/PTFE composites depends on PTFE content present in nylon 66. The temperature for maximum weight loss values increases markedly as increase in PTFE content. This is because PTFE is thermally more stable and its thermal degradation rate is very slow as compared to nylon 66. TGA data reveals that as the PTFE content increases the thermal stability of nylon 66/PTFE composites increases. From the thermograms, it is found that the nylon 66/PTFE composites are stable up to 360 o C and the maximum thermal degradation has occurred at around 615 o C. The TGA thermogram of composites showed single step thermal degradation process (457 C) with a weight loss of 99.9 %. It was also noticed that all PTFE loaded nylon composites undergo two step thermal degradation processes. The temperature range and percentage weight loss for different thermal degradation 89

16 steps of all nylon 66/PTFE composites are shown in Table 3.5. The first step weight loss occurred in the temperature range C, which is due to the decomposition of nylon. (a) (b) (c) (d) (e) Figure 3.7. TGA and derivative thermograms of nylon 66/PTFE composites (a) 0 %, (b) 5 %, (c) 10 %, (d) 15% and (e) 20% PTFE content The weight loss which occurred in first step decreases from 80 to 92 %. with increase in PTFE content in nylon matrix. This result clearly indicates that the weight loss in the first step is significantly dependent on the nylon 66 content. The weight loss 90

17 that occurs in the temperature range C is called second stage thermal decomposition in which the weight loss lies in the range %. The weight loss in this step may be due to the degradation of PTFE component. From Table 3.6 it was observed that the ash content of the composites lies in the range %. But there is no systematic variation in ash content with the composition of the composites. Table 3.6. Thermal data obtained from derivative TGA curves for nylon 66/PTFE composites Composition of nylon 66/PTFE composites (%, wt/wt) Degradation stage Temperature ( С ±2 ) T o T p T c Weight loss (%) 100/0 95/5 90/10 85/15 80/ Ash Ash Ash Ash Ash From TGA measurements, the char yield for 5% PTFE filled nylon 66 composites was found to be 0.5 %. On the other hand, the char yield for all filled systems increases with increase in PTFE content and it lies in the range % (Table 3.6). From this data oxidation index (OI) was calculated and the obtained values are very low ( ). Higher the values of oxidation index, higher will be the thermal stability. Based upon the mass carbonaceous char, it is concluded that, nylon/ptfe composites with higher PTFE contents are not good flame-retardants as evident by their lower OI values. 91

18 Figure 3.8. TGA thermograms for nylon 66/PTFE composites Sliding wear behaviour The effect of PTFE content, applied load, sliding velocity, and sliding distance on the wear characteristics of the nylon 66/PTFE composites have been described in the forthcoming sections Wear loss The sliding wear data of nylon 66/PTFE composites at different applied loads shown in Figures 3.9(a)-(c) are considered for interpretation. All the plots indicate that wear loss decreases with increase in PTFE content. That means pristine nylon 66 has more wear loss than composites containing PTFE content for all loads investigated. From Figure 3.9(a)-(c) it was noticed that the wear loss of nylon 66/PTFE composites increases as increase in applied load (Figures 3.10(a)-(c)). It is established that PTFE exhibits significantly low coefficient of friction when sliding against steels. The variation of wear loss as a function of sliding distance for all composites is shown in Figure From the figure it was noticed that wear loss increases as increase in sliding velocity expected. It was, observed that the wear loss decreases as increase in PTFE content in the composites as shown in Figures 3.11(a) - (c). That means composite with 5 % PTFE has more wear loss than composites containing higher PTFE content for all loads investigated. It is clearly evident from all the plots that the PTFE content has significant influence on the wear behaviour of the composites. The effect of sliding velocity on the wear characteristics of nylon 66/PTFE composites are shown in 92

19 Weight loss (g) Weight loss (g) Weight loss (g) Weight loss (g) Weight loss (g) Weight loss (g) Figure From the figure it was noticed that as the sliding velocity increases wear loss of the composites increases as expected. Figures 3.10 to 3.12 clearly indicate that the wear loss increases with increase in applied load, sliding distance and sliding velocity. This can be attributed to, when PTFE rubbed against a hard surface; the polymer chain undergoes scission, creating active groups which chemically react with the counterface. This results in strong adhesion and a coherent transfer film. Further interaction between the bulk polymer and the transfer film gives rise to anisotropic deformation of the unit cell, which results in closeness of adjacent chains and easy shear between chains. Sliding wear brings about growth in as well as reorientation of crystallites situated in a very thin subsurface region of the bulk polymer. Such structural rearrangement facilitates the joining of adjacent aligned crystallites to form films and ribbons which emerge as debris N 1000m 1500m 2000m N 1000m 1500m 2000m N 1000m 1500m 2000m PTFE (%) PTFE (%) PTFE (%) Figure 3.9. Weight loss as a function of PTFE content for nylon 66/PTFE composites at varying sliding distances for (a) 50 N, (b) 100 N and (c) 150 N load m m m N 100N 150N Load (N) N 100N 150N Load (N) N 100N 150N Load (N) Figure Plots of weight loss as a function of load for the composites at (a) 1000 m, (b) 1500 m and (c) 2000 m sliding distance 93

20 Weight loss (g) Weight loss (g) PTFE (%) N PTFE (%) N Slinding distance (m) Sliding distance (m) Figure Weight loss as a function of sliding distance for the composites at 5 m/s for loads of, (a) 50 N, (b) 100 N and (c) 150 N Figure Weight loss as a function of PTFE content for nylon 66/PTFE composites at varying sliding velocities for (a) 50 N, (b) 100 N and (c) 150 N load Specific wear rate The plots of specific wear rate as a function of abrading loads, PTFE content and sliding distances at sliding velocity of 5 m/s are shown in Figures 3.13, 3.14 and 3.15 respectively. Figures 3.13(a)-(c) indicates that, the specific wear rate decreases as increase in abrading load as expected. The specific wear rate of the composites increases with an increase in sliding distances, as the wear loss is proportional to the sliding distances. The specific wear rate was calculated from the material wear loss measurements. Figures 3.14 (a)-(c) shows the effect of PTFE content on the specific wear rate of nylon 66 and nylon 66/PTFE composites. Unfilled nylon has the highest specific wear rate while the PTFE loaded nylon 66 composites has the lowest specific wear rate at all the conditions investigated. 94

21 Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) Specific wear rate (g/n-m) 2.5E E m E E m E E m E E E E E E E E E E N 100 N 150 N Load (N) 0.0E N 100 N 150 N Load (N) 0.0E N 100 N 150 N Load (N) Figure Specific wear rate as a function of load for the composites at (a) 1000 m, (b) 1500 m and (c) 2000 m sliding distance 2.5E E N 1000m 1500m 2000m 2.5E E N 1000m 1500m 2000m 2.5E E N 1000m 1500m 2000m 1.5E E E E E E E E E E PTFE (%) 0.0E PTFE (%) 0.0E PTFE (%) Figure Specific wear rate as a function of PTFE content for nylon 66/PTFE composites at 5 m/sec for varying loads of (a) 50 N, (b) 100 N and (c) 150 N 2.5E E N 2.5E E N 2.5E E N E E E E E E E E Sliding distance (m) E E Sliding distance (m) E E Sliding distance (m) Figure Specific wear rate as a function of sliding distance for the composites at 5 m/s for loads of (a) 50 N, (b) 100 N and (c) 150 N 95

22 The reduction in specific wear rate is about 45 % for the composite containing 20 % PTFE filled nylon. From the Figures 3.14(a)-(c) it was evident that the increase of PTFE content from 0 to 20 wt % led to a remarkable reduction in specific wear rate of the composites. Figures 3.15(a)-(c) indicate the effect of sliding distance on the specific wear rate of nylon 66 and nylon 66/PTFE composites. The increase in specific wear rate is observed as increase in sliding distance. Similar kind of variation has been reported elsewhere for glass fabric reinforced epoxy composites [13]. 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]. The correlations of wear volume with selected mechanical properties such as re factor (where, σ is the ultimate tensile strength and e is the ultimate elongation), hardness (H) have been reported for single-pass studies of polymers with out fillers and composites [38, 39]. Lancaster [38] stated that the product re is a very important factor which controls the abrasive wear behaviour of composites. Generally, fiber/filler reinforcement increases the tensile strength (σ) of neat polymer, they usually decrease the ultimate elongation (e) and hence the product re may become smaller than that of neat polymer. Hence, reinforcement usually leads to deterioration in the abrasive wear resistance. The model proposed by Ratner et al [39] states that the rate of material removal is inversely proportional to the product of stress and strain at rupture. In the present work for nylon/ptfe composites, the weight loss decreased with decrease in hardness and σ e factor Coefficient of friction The variation in coefficient of friction as a function of PTFE compositions at 5 m/s sliding velocity for nylon 66/PTFE composites is tabulated in Table 3.7. Table 3.7 reveals that coefficient of friction increases with increase in abrading load and sliding distance. A significant reduction in coefficient of friction was noticed with increase in PTFE content from 0 to 20%. 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 investigated during the current studies. 96

23 It is evident that the friction coefficient reduces continuously by increasing the content of PTFE in the composites, so that the friction coefficient at 20 wt % loading PTFE reduces to almost 28% of the pristine nylon 66. This behaviour is obviously relevant to the self lubricating effect of PTFE which can reduce the adhesion between the composite with the metallic counterpart [40]. The coefficient of friction of nylon is in the range ~ and is in agreement with the published data [5, 9, 10, 14]. Gong et al concluded that the wear reduction of PTFE composite is not due to the increased adhesion between the transfer film and the counterface but to the increased cohesion of the transfer film itself [41]. Table 3.7. Coefficient of friction for nylon 66/PTFE composites at 5 m/s velocity Load (N) Sliding Co-efficient of friction for nylon 66/PTFE composites (%) distance (m) 100/0 95/5 90/10 85/15 80/ The low coefficient of friction results from the ability of its extended chain linear molecules, (CF 2 CF 2 ) n, to form low shear strength films upon its surface and mating counterfaces during sliding [42-43]. However, this repetitive formation and destruction of the film occurs at a high rate and results in unacceptable high rates of wear as reported by Steijn [43]. However, in the present study improvement in wear behaviour is due to synergetic effect of nylon 66 and PTFE Wear mechanism Early attention to the complex tribology of PTFE was drawn by Professor David Tabor and his co-workers at the Cavendish Laboratory in the early 1960s. They rubbed PTFE against glass plates and observed that in the steady state the rubbing interaction is confined to the transfer film and a very thin slice of the pin surface. They also noted that this interaction is highly sensitive to sliding speed and inferred that the 97

24 participation of the amorphous phase in the shear process activates a viscoelastic response which dominates the speed effect. a b c d e Figure Photomicrographs of nylon 66/PTFE 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 sliding distance To correlate the wear data better, Figures 3.16 (a)-(e) indicates the effect of PTFE content on the worn surface morphology of the nylon/ptfe composites. The worn-out surface of low PTFE content loaded nylon 66 is relatively rough with more matrix damage at all sliding distances and sliding velocities. There is an evidence of more matrix removal and deep furrows (marked as arrow in Figure 3.16 (a)) in the direction of abrasion due to the ploughing action of sharp counterface. The worn surface of 20 % PTFE loaded composite (Figure 3.16 (e)) becomes smooth and smooth indicating that the wear volume loss is less because of more and more PTFE particles adhered on the surface of the specimens hinders the abrasion of matrix. It can be concluded from the photomicrographs taken through metallurgical microscope that 98

25 inclusion of PTFE as a solid lubricant in nylon 66 matrix by blending is beneficial from the abrasion resistance point of view. a b Figure Photomicrographs for nylon 66/PTFE (95/5) composites at 5 m/s sliding velocity, 100 N load for (a) 1000 m and (b) 2000 m a b Figure Photomicrographs for nylon 66/PTFE (90/10) composites at 1000 m sliding distance, 50 N load for (a) 5 m/s and (b) 9 m/s As the sliding distance increases from 1000 m to 2000 m, the worn surface (Figure 3.17) becomes rough and which indicates that the wear volume loss is more. The worn surface images of higher sliding distance (2000 m) specimens exhibited severe matrix damage. The deep furrows and some wider cracks in the direction of abrasion are evident from Figure 3.17(b). Figures 3.18 (a)-(b) shows the abrasive wear surfaces of 90/10 nylon 66/PTFE composites at low and high sliding velocities. Figure 3.18(b) shows the matrix is more damaged at higher sliding distance indicating more material removal during abrasion process. The wear loss data shown in Figures 3.19 (a)-(c) supports this observation. Deep furrows in the direction of abrasion were observed from the optical microscope images (Figure 3.19(b)-(c)). The effect of abrading loads on the worn surface morphology of nylon 66/PTFE (90/10) composite is shown in Figures 3.19(a)-(c). The optical photomicrographs indicate that rougher surface at lower load abraded surface (Figure 3.19(a)). The smooth surface, deep furrows and more wear loss at higher sliding load was noticed (Figure 3.19(c)). 99

26 a b c Figure Photomicrographs for nylon 66/PTFE (90/10) composites for 5 m/s sliding velocity (1500 m) varying loads of, (a) 50 N, (b) 100 N and (c) 150 N a b c Figure Transfer layer formed on the counterface of nylon 66/PTFE composites during wear testing at 5 m/s for (a) 50 N, (b) 100 N and (c) pristine nylon 66 When polymers slide against metal counterface, transfer films are formed and the wear characteristics of a polymer in dry sliding condition are strongly influenced by its ability to form a transfer film on the counterface [44]. The transfer films formed by PTFE present in the composites provide a shielding of the soft polymer surface from the hard metal asperities [45]. Figures 3.20(a)-(c) shows the continuous transfer layer formed on the PTFE filled nylon test specimens counterface after wear tests. Generally during sliding, pristine nylon 66 also forms a transfer film. In the present case, weak transfer film formed on the counterface by pristine nylon is removed due to continuous sliding (mechanical action) and high heat generation during sliding. Sinha [46] also reported that the transfer film formed by nylon during dry sliding is weak and is easily removed by high interfacial temperatures and dynamic actions during sliding. The results have indicated that the PTFE decreases friction coefficient and wear rate of the nylons while the mechanical properties are reduced due to the weaker mechanical properties of PTFE with respect to neat nylons [47-49] Regression analysis The experimentations were conducted so as to investigate which design parameter significantly affects the dry sliding wear for the selected combinations of 100

27 load, sliding speed and sliding distance. Linear regression techniques have been widely used in engineering analysis. These techniques consist of plan of experiments with objective of acquiring data in a controlled way, executing these experiments in order to obtain information about the behaviour of a given process. Two levels of each of the three factors were used for the statistical analysis. The levels for the three factors are entered in Table 3.8 and the treatment combinations for the two levels and three factors are tabulated in Table 3.9. Based on the experimental results the correlation between the wear parameters is obtained using linear regression (Anova) technique. In the equations shown in the Table 3.10, it is observed that the values associated with load and sliding velocity parameter decreases with increasing PTFE content in nylon 66 indicating that wear decreases as PTFE composition in nylon 66 increases. Table 3.8. Process parameters for Anova Load Sliding Sliding Levels (N) Velocity(m/s) Distance(m) Table 3.9. Analysis of variance (Anova) for nylon 66/PTFE composites Source % PTFE in nylon 66 0% 5% 10% 15% 20% DOF SS 1.52 E E E E E-06 MS 7.62 E E E E E-06 F P S R-SQ (%) Coefficient SE Coeff T R. Error 4.32 E E E E E-07 DW Statistic(d) 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. 101

28 The wear model for the tested composite materials was developed based on the applied load, sliding velocity and sliding distance. The process parameters for the purpose of analysis are given in Table 3.9. 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 [51-52]. 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 [53]. Table Regression equations for nylon 66/PTFE composites % PTFE Regression Equation L Sl. Vel D L Sl. Vel D L Sl. Vel D L Sl. Vel D L Sl. Vel D The data obtained from analysis of variance for nylon 66/PTFE composites is given in Table 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 such as 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. In the table DW statistic 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 Laser assisted etching behaviour A study has been conducted using a 75 W pulsed Nd-YAG laser to etch the surface of nylon 66/PTFE specimens. To probe the possibility of this type of laser providing a solution to the problem of etching materials, machining and subsequent 102

29 analyses like morphological study using optical microscopic images and surface roughness. Tagliaferri [54] conducted an experimental study to determine the surface finish characteristics of carbon and aramid fibre-reinforced plastics (CFRP, AFRP) by CO 2 laser. The Heat Affected Zone (HAZ) depends strictly on the feed rate. The higher the speed of laser beam, the smaller the volume of damage and better the cut finish. Graphite reinforced composites are found to be less suitable for laser cutting due to high fibre conductivity and vaporization temperature [54-56]. Caprino [55] developed a simple one-parameter thermal model, predicting the maximum feed rate as a function of beam power. Table Surface roughness results for laser etched nylon 66/PTFE composites PTFE content in nylon 66 (wt. %) Surface roughness before Surface roughness (R a ) 500 mm/s, 5 khz etching (R a ) 50 % 100 % power power The surface roughness values (R a ) of nylon 66/PTFE composite specimens before and after laser etching at different laser parameters are tabulated in Table The R a values showed a decreasing trend with increase of PTFE content in the nylon 66/PTFE composites. Also it is observed that R a values exhibits increasing trend with increasing power from 50 % to 100 % for all the composites. This result indicates that PTFE content controls the surface damage by laser etching behaviour of the composites. Also the increase in power of etching increases the surface roughness of the specimens Morphology of laser etched surfaces The optical photomicrographs of laser etched surfaces of nylon 66/PTFE composites at 50% and 100% power are shown in Figures 3.21 and 3.22 respectively. The laser surfaces of the composites are characterized by rough surfaces on account of 103

30 etching by laser. The photomicrographs of nylon 66/PTFE composites showed decreased trend in surface roughness with increasing PTFE content at 50 % power, 5 khz frequency for 500 mm/s velocity as shown in Figures 3.21 (a)-(e). The increase in laser etching power from 50 to 100 %, increases the surface roughness of the composites as shown in Figures 3.22(a)-(e). The high surface roughness with more matrix damage of surface for nylon 66 as compared to nylon 66/PTFE systems. a b c d e Figure Photomicrographs of laser etched nylon 66/PTFE 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 104

31 a b c d e Figure Photomicrographs of laser etched nylon 66/PTFE composites with PTFE content, (a) 0 %, (b) 5 %, (c) 10 %, (d) 15 % and (e) 20 % at 5 khz frequency, 100 % power and 500 mm/s velocity 3.4. Conclusions From the tribological study on nylon 66 and nylon 66/PTFE composites, the following conclusions were drawn: The tensile behaviours data clearly indicates that there is no benefit by incorporation of PTFE in nylon matrix. However, a slight improvement in impact strength in the composites was noticed, because nylon 66/PTFE composites are immiscible in nature. Additionally, according to the mechanical data, it was revealed that the extent of interaction between nylon and PTFE is not considerable. This latter claim was supported by the DSC results in which no influence of PTFE on the T m and H m was observed. 105