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A study on the friction and wear behavior of polytetrafluoroethylene filled with potassium titanate whiskers
Wear 261 (2006) 1208–1212
A study on the friction and wear behavior of polytetrafluoroethylene filled with potassium titanate whiskers Feng Xin ∗ , Diao Xiaosong, Shi Yijun, Wang Huaiyuan, Sun Shenghua, Lu Xiaohua College of Chemical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, PR China Received 18 January 2005; received in revised form 11 February 2006; accepted 10 March 2006 Available online 2 May 2006
Abstract The friction and wear behavior of polytetrafluoroethylene (PTFE) filled with potassium titanate whiskers (PTW) was studied. It is shown that the friction coefficient of PTW/PTFE composites decreases with the increase of PTW content and with 20 wt% PTW content, the best wear resistance occurs, which is over 1000 times larger than that of pure PTFE. The crystallization of the composite was measured by differential scanning calorimeter (DSC). It reveals that PTW has the ability of inducing heterogeneous nucleation. The degree of crystallization increases initially up to 35 wt% PTW content and then it decreases slightly afterwards. At 35 wt% PTW, the composite has the highest degree of crystallization which is 37% higher than that of pure PTFE. The relationship between the degree of crystallization and the wear behavior was also analyzed which reveals the effect of PTW on PTFE. It is also manifested that the increase of crystallization improves the wear property. Finally, the image of abrasive dust and the worn surface were investigated by optical microscope and SEM, respectively. These observations show that the PTW has the ability to reduce wear by preventing the destruction of PTFE’s strap-like structure. © 2006 Elsevier B.V. All rights reserved. Keywords: Potassium titanate whisker; PTFE; Composite; Friction; Wear
1. Introduction PTFE is widely used as bearing and sealing material with lubricating properties due to its low dry sliding friction coefficient, resistance to chemical erosion and thermal stability. However, the wear resistance of PTFE is very poor which limits its use fields. Therefore, fillers (glass fiber, carbon fiber [1], organic fiber [2]), polymers [3] or nanometer materials [4,5] are added to PTFE in order to improve the wear resistance. Among the numerous inorganic fillers, potassium titanate whiskers (PTW, K2 O·6TiO2 ) has been found to be a promising reinforcer for the wear resistant composites because of its unique properties such as outstanding mechanical performance, hardness (Mohs hardness 4), coefficient of thermal expansion (6.8 × 10−6 K−1 ), and chemical stability. The size of the PTW filler is relatively small which produces micro-reinforcing effect in PTW composites and it is suitable to produce ware with complex shape, high precision and high polished surface [5]. Today, PTW is the only whisker that enjoys wide spread use commer∗
Corresponding author. Fax: +86 25 83588063. E-mail address: [email protected] (X. Feng).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.03.005
cially. Recently, the application of PTW as filler for engineering plastics, for example, nylons [6], has also generated more interests. But only a few studies on PTFE with PTW composites are currently available [7] and a more comprehensive study of effect of PTW on the friction and wear properties of the PTFE composites would be useful. The purpose of this work is to study the friction and wear properties of the PTFE composites reinforced with various amount of PTW under dry sliding conditions. The crystallization of the composites, the abrasive dust and the worn surface were also investigated. It is expected that this research can be useful in promoting the applications of PTFE composites under oil-free lubrication conditions. 2. Experiment The friction and wear tests were carried out on an apparatus showing in Fig. 1. The counter material is a steel ring made of steel 45. Sliding was performed under dry friction condition and ambient conditions (temperature: 25 ◦ C, humidity: 50 ± 5%) at the sliding velocity of 1.4 m/s, normal load of 200 N. The friction duration was 60 min. The running-in friction duration was
X. Feng et al. / Wear 261 (2006) 1208–1212
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Fig. 1. Configuration of the tester: (a) test machine, (b) counter-face and (c) sample.
5min.The counter face and the composite samples were polished at an average roughness of Ra = 0.1–0.2 m with number 800 grit SiC abrasive paper and cleaned with alcohol just before testing. In the experiments, PTFE powder (7A-J, commercial product of Dupont) with an average diameter of 25 m; PTW (synthesized by ourselves) with an average diameter of 1 m and an average length of 20 m. PTW was modified by using amino silane coupling agent KH-550 (commercial product from Nanjing Shuguang). The detailed process of preparing the composites is as follows: the non-agglomerated PTFE powder was mixed completely with the dried PTW. Then the mixtures were molded into a block by compression molding under a pressure of 70 MPa. The resulting PTFE composite block was sintered to 380 ◦ C and cooled to the room temperature step by step. Finally, the sintered block was shaped as shown the Fig. 1(c) with 26 mm in external diameter and 22 mm in inner diameter and 2.5–3 mm in shoulder height. During the experiments, the moment was recorded by a computer at a frequency of 1Hz. The crystallization of PTFE composite was measured by a differential scanning calorimeter (DSC). The samples were heated from the room temperature to 400 ◦ C at 20 ◦ C/min. The micro-structure of the samples’ worn surfaces were investigated a scanning electron microscopy (SEM), and the abrasive dust was examined by an optical microscope.
were determined at an operating condition of 200 N and 1.4 m/s. The results are shown in Figs. 2 and 3. As shown in Fig. 2, the friction coefficient decreases from 0.155 to 0.11 when PTW content is increased from 0 to 40 wt%. The wear rate decreases dramatically as PTW content is increased from 1 to 20 wt%, but the wear rate increases slightly after that (Fig. 3). The lowest wear rate is 8.38 × 10−10 cm3 /Nm at about 20 wt% PTW content, which means that the wear resistant of this PTFE composite is over 1000 times higher than that of pure PTFE (its wear rate is 1 × 10−6 cm3 /Nm). The decreasing friction coefficient with the increasing PTW content might be caused by this reason that the real contact area between PTFE composite and the steel ring is reduced as increasing PTW surface fraction. The same effect was also observed by
3. Results and discussion 3.1. The friction and wear behavior of PTW/PTFE composites with the PTW content For PTW/PTFE composites reinforced with 1, 3, 5, 7, 9, 15, 20, 23, 26, 30, 35, 40 wt% PTW, the friction and wear properties
Fig. 2. The average friction coefficient of PTW/PTFE composites varied with the content of PTW.
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X. Feng et al. / Wear 261 (2006) 1208–1212 Table 1 DSC melting results of pure PTFE and PTW/PTFE composites PTW (wt%)
Ton (◦ C)
Tp (◦ C)
Hm (J/g)
Xm (%)
0 1 3 5 7 9 15 20 23 26 30 35 40
326.348 325.444 326.649 326.43 327.217 327.023 327.895 328.154 328.326 328.273 328.203 328.127 328.036
331.543 330.264 332.070 331.697 333.114 332.956 333.529 333.656 333.764 333.518 333.339 333.368 333.373
43.176 32.571 40.803 41.599 44.534 44.934 46.338 44.487 41.487 42.550 40.432 38.559 32.831
53.97 41.125 52.58 54.74 59.86 61.72 68.14 69.51 70.08 71.88 72.20 74.14 68.40
Fig. 3. The wear rate and degree of crystallization of PTW/PTFE composites varied with the content of PTW.
Ton , the temperature where melting starts; Tp , the temperature at which maximum melting occurs; Hm , heat of fusion; Xm , degree of crystallization.
Chen and co-workers [8] although PTFE was filled with carbon nanotube in that research.
The degree of crystallization decreases initially (at less than 3 wt% PTW), then it increases quickly with increasing PTW content due to the ability of heterogeneous nucleation of PTW. But the degree of crystallization decreases slightly after 35 wt% PTW content (cf. Fig. 3). The composite with 35 wt% PTW has the highest degree of crystallization which is 37% higher than pure PTFE. The melting onset temperature, Ton , and the temperature at which maximum melting occurs, Tp also illustrate the similar behavior. The increase of Ton and Tp is attributed to the increase of integrality of crystallization, owing to the increase of PTW
3.2. The crystallization of PTW/PTFE composites The crystallization of the PTFE composites reinforced with various amount of PTW was examined by DSC and the results are shown in Table 1. The degree of crystallization (Xm ) is estimated by taking the average heat-of-fusion of the sample over that of the complete crystalline PTFE which was assumed to be 80 J/g [4] and the results are also shown in Table 1.
Fig. 4. Abrasive dust of PTW/PTFE composite filled with PTW was observed by an optical microscope: (a) 0 wt% PTW; (b) 9 wt% PTW; (c) 20 wt% PTW; (d) 30 wt% PTW.
X. Feng et al. / Wear 261 (2006) 1208–1212
content. These effects in turn enhance the wear properties. The wear rate and the degree of crystallization of PTFE composite varied with the PTW content as shown in Fig. 3. It is obvious that the relationship between crystallinity and PTW content is similar to that for wear rate. Semicrystalline thermoplastics can be self-reinforced by oriented crystallization, whereby improved tribological and mechanical properties are achieved [9]. The reason that the wear resistance of PTFE is improved remarkably with increasing PTW content is postulated as follows: the increasing wear resistance of the PTW/PTFE may be divided into two parts: one is the presence of PTW which is also type of
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tiny fiber, the modulus and carrying capacity of the PTW/PTFE composite can be enhanced; the other is due to the increasing crystallinity which is also beneficial in improving the strength, modulus, wear resistance of the composites. 3.3. The pattern of abrasive dust and the micro-structure of worn surface of the composites The abrasive dust was observed by an optical microscope in our study. The presence of the abrasive dust can easily be recognized by a microscope and can even been seen by naked
Fig. 5. The micro-structure of a sample’s worn surface. (a) 0 wt% PTW; (b) 1 wt% PTW; (c) 3 wt% PTW; (d) 9 wt% PTW; (e) 20 wt% PTW; (f) 30 wt% PTW.
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X. Feng et al. / Wear 261 (2006) 1208–1212
eyes. Some results from the microscope are shown in Fig. 4. It can easily seen that the size of the dust decreases with increasing amount of PTW. But there the differences of sizes of dust between PTFE composites reinforced with 20 wt% PTW and that with 30 wt% PTW are insignificant (cf. Fig. 4(c and d)). The whiskers and their scraps in abrasive dust can be observed in Fig. 4(d) because the presence of high amount of PTW. In order to understand the effect of PTW on friction and wear properties, the micro-structure of sample surface was analyzed by SEM and results from SEM are shown in Fig. 5. The evidence of destruction of PTFE’s strap-like structure can be easily observed from Fig. 5(a–c), and the scales of destructions of the strap-like structures decrease with increasing PTW content, but the destructions of structures are not evident in Fig. 5(d–f). The abrasive dust and destruction of strap-like structures decreases with increasing amount of PTW can be explained by the fact that PTW has the ability to reduce wear and thereby prevents PTFE’s strap-like structure from being destroyed. This confirms the theory of Tanaka again [10]. If the content of PTW is more than 20 wt%, the binding force between PTW and PTFE decreases gradually, and the PTW can be brushed off easily. Therefore, the whiskers and their scraps can be found as demonstrated in Fig. 4(d) and crackles can be seen at high PTW content (cf. Fig. 5(f)). The free whiskers and their scraps in the interface would actually enhance grain abrasion, which increases the wear rate. 4. Conclusion Friction and wear properties of the PTFE composites reinforced with various amount of PTW under dry sliding conditions were studied. The crystallization of the composite, the patterns of abrasive dust and the micro-structures of the worn surfaces were also investigated. The following conclusions can be made based upon this study: 1. The wear rate of PTW/PTFE composite decrease dramatically when PTW content increases from 1 to 20 wt%, but it decreases slightly when PTW content increases from 20 to 40 wt%. The PTFE composite reinforced with 20 wt% PTW has the best wear resistance, which is over 1000 times higher than that of pure PTFE. The friction coefficient decreases with increasing PTW content. 2. The degree of crystallization decreases initially when the PTW content is <3 wt%, but it increases quickly until the PTW content reaches 30 wt%, and it decreases slightly after-
wards. The composite with 35 wt% PTW has the highest degree of crystallization which is 37% higher than that of pure PTFE. 3. The relationship between crystallinity and wear behavior has been analyzed based on the interaction between PTW and PTFE. The increase of crystallization with increasing PTW content is useful in improving the wear property of the composites. 4. The study of the images of abrasive dust and the worn surfaces has shown that PTW has the ability to reduce wear by preventing the destruction of PTFE’s strap-like structure. Acknowledgements Authors very appreciate the supports by the National Natural Science Foundation of PR China (29925616) through the Outstanding Youth Fund and the National Natural Science Foundation of China (20236010) and National High-tech Research Development Program (863 Program: 2003AA333010) and Dr. Yonggang Chen who gave kind help for polishing this paper. References [1] Q.-J. Xue, Z.-Z. Zhang, W.-M. Liu, et al., Friction and wear characteristics of fiber- and whisker-reinforced PTFE composites under oil lubricated conditions, J. Appl. Polym. Sci. 69 (1998) 1393–1402. [2] A. Bolvari, S. Glenn, R. Janessen, et al., Wear and friction of aramid fiber and polytetrafluoroethylene filled composites, Wear 203–204 (1997) 697–702. [3] G. Theiler, W. H¨ubner, T. Gradt, et al., Friction and wear of PTFE composites at cryogenic temperatures, Tribol. Int. 35 (2002) 449–458. [4] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj, et al., A study on the friction and wear behavior of PTFE filled with alumina nanoparticles, Wear 254 (2003) 573–580. [5] X. Feng, J.Z. L¨u, X.H. Lu, et al., Application of potassium titanate whisker in composite, Acta Mater. Compos. Sin. 16 (1999) 1–7. ¨ X.H. LU, Elastic interlayer toughening of potassium titanate [6] J.Z. LU, whiskers—Nylon66 composites and their fractal research, J. Appl. Polym. Sci. 82 (2001) 368–374. [7] X. Feng, D. Chen, X. Jiang, Friction and wear properties of potassium titanate whiskers reinforced PTFE composites, in: L. Ye, Y.-W. Mai, Z. Su (Eds.), Composites Technologies for 2020, Woodhead Publishing Ltd., England, 2004, pp. 46–50. [8] W.X. Chen, F. Li, G. Han, et al., Tribological behavior of carbon-nanotubefilled PTFE composites, Tribol. Lett. 15 (2002) 275–278. [9] J. Song, G.W. Ehrenstein, Friction and wear of self-reinforced thermoplastics, in: K. Friedrich (Ed.), Advances in Composite Tribology, Elsevier, Amsterdam, 1993, pp. 19–61. [10] K. Tanaka, Effect of various fillers on the friction and wear of PTFEbased composites, in: K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Elsevier, Amsterdam, 1986, pp. 137–174.
A study on the friction and wear behavior of polytetrafluoroethylene filled with potassium titanate whiskers Feng Xin ∗ , Diao Xiaosong, Shi Yijun, Wang Huaiyuan, Sun Shenghua, Lu Xiaohua College of Chemical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, PR China Received 18 January 2005; received in revised form 11 February 2006; accepted 10 March 2006 Available online 2 May 2006
Abstract The friction and wear behavior of polytetrafluoroethylene (PTFE) filled with potassium titanate whiskers (PTW) was studied. It is shown that the friction coefficient of PTW/PTFE composites decreases with the increase of PTW content and with 20 wt% PTW content, the best wear resistance occurs, which is over 1000 times larger than that of pure PTFE. The crystallization of the composite was measured by differential scanning calorimeter (DSC). It reveals that PTW has the ability of inducing heterogeneous nucleation. The degree of crystallization increases initially up to 35 wt% PTW content and then it decreases slightly afterwards. At 35 wt% PTW, the composite has the highest degree of crystallization which is 37% higher than that of pure PTFE. The relationship between the degree of crystallization and the wear behavior was also analyzed which reveals the effect of PTW on PTFE. It is also manifested that the increase of crystallization improves the wear property. Finally, the image of abrasive dust and the worn surface were investigated by optical microscope and SEM, respectively. These observations show that the PTW has the ability to reduce wear by preventing the destruction of PTFE’s strap-like structure. © 2006 Elsevier B.V. All rights reserved. Keywords: Potassium titanate whisker; PTFE; Composite; Friction; Wear
1. Introduction PTFE is widely used as bearing and sealing material with lubricating properties due to its low dry sliding friction coefficient, resistance to chemical erosion and thermal stability. However, the wear resistance of PTFE is very poor which limits its use fields. Therefore, fillers (glass fiber, carbon fiber [1], organic fiber [2]), polymers [3] or nanometer materials [4,5] are added to PTFE in order to improve the wear resistance. Among the numerous inorganic fillers, potassium titanate whiskers (PTW, K2 O·6TiO2 ) has been found to be a promising reinforcer for the wear resistant composites because of its unique properties such as outstanding mechanical performance, hardness (Mohs hardness 4), coefficient of thermal expansion (6.8 × 10−6 K−1 ), and chemical stability. The size of the PTW filler is relatively small which produces micro-reinforcing effect in PTW composites and it is suitable to produce ware with complex shape, high precision and high polished surface [5]. Today, PTW is the only whisker that enjoys wide spread use commer∗
Corresponding author. Fax: +86 25 83588063. E-mail address: [email protected] (X. Feng).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.03.005
cially. Recently, the application of PTW as filler for engineering plastics, for example, nylons [6], has also generated more interests. But only a few studies on PTFE with PTW composites are currently available [7] and a more comprehensive study of effect of PTW on the friction and wear properties of the PTFE composites would be useful. The purpose of this work is to study the friction and wear properties of the PTFE composites reinforced with various amount of PTW under dry sliding conditions. The crystallization of the composites, the abrasive dust and the worn surface were also investigated. It is expected that this research can be useful in promoting the applications of PTFE composites under oil-free lubrication conditions. 2. Experiment The friction and wear tests were carried out on an apparatus showing in Fig. 1. The counter material is a steel ring made of steel 45. Sliding was performed under dry friction condition and ambient conditions (temperature: 25 ◦ C, humidity: 50 ± 5%) at the sliding velocity of 1.4 m/s, normal load of 200 N. The friction duration was 60 min. The running-in friction duration was
X. Feng et al. / Wear 261 (2006) 1208–1212
1209
Fig. 1. Configuration of the tester: (a) test machine, (b) counter-face and (c) sample.
5min.The counter face and the composite samples were polished at an average roughness of Ra = 0.1–0.2 m with number 800 grit SiC abrasive paper and cleaned with alcohol just before testing. In the experiments, PTFE powder (7A-J, commercial product of Dupont) with an average diameter of 25 m; PTW (synthesized by ourselves) with an average diameter of 1 m and an average length of 20 m. PTW was modified by using amino silane coupling agent KH-550 (commercial product from Nanjing Shuguang). The detailed process of preparing the composites is as follows: the non-agglomerated PTFE powder was mixed completely with the dried PTW. Then the mixtures were molded into a block by compression molding under a pressure of 70 MPa. The resulting PTFE composite block was sintered to 380 ◦ C and cooled to the room temperature step by step. Finally, the sintered block was shaped as shown the Fig. 1(c) with 26 mm in external diameter and 22 mm in inner diameter and 2.5–3 mm in shoulder height. During the experiments, the moment was recorded by a computer at a frequency of 1Hz. The crystallization of PTFE composite was measured by a differential scanning calorimeter (DSC). The samples were heated from the room temperature to 400 ◦ C at 20 ◦ C/min. The micro-structure of the samples’ worn surfaces were investigated a scanning electron microscopy (SEM), and the abrasive dust was examined by an optical microscope.
were determined at an operating condition of 200 N and 1.4 m/s. The results are shown in Figs. 2 and 3. As shown in Fig. 2, the friction coefficient decreases from 0.155 to 0.11 when PTW content is increased from 0 to 40 wt%. The wear rate decreases dramatically as PTW content is increased from 1 to 20 wt%, but the wear rate increases slightly after that (Fig. 3). The lowest wear rate is 8.38 × 10−10 cm3 /Nm at about 20 wt% PTW content, which means that the wear resistant of this PTFE composite is over 1000 times higher than that of pure PTFE (its wear rate is 1 × 10−6 cm3 /Nm). The decreasing friction coefficient with the increasing PTW content might be caused by this reason that the real contact area between PTFE composite and the steel ring is reduced as increasing PTW surface fraction. The same effect was also observed by
3. Results and discussion 3.1. The friction and wear behavior of PTW/PTFE composites with the PTW content For PTW/PTFE composites reinforced with 1, 3, 5, 7, 9, 15, 20, 23, 26, 30, 35, 40 wt% PTW, the friction and wear properties
Fig. 2. The average friction coefficient of PTW/PTFE composites varied with the content of PTW.
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X. Feng et al. / Wear 261 (2006) 1208–1212 Table 1 DSC melting results of pure PTFE and PTW/PTFE composites PTW (wt%)
Ton (◦ C)
Tp (◦ C)
Hm (J/g)
Xm (%)
0 1 3 5 7 9 15 20 23 26 30 35 40
326.348 325.444 326.649 326.43 327.217 327.023 327.895 328.154 328.326 328.273 328.203 328.127 328.036
331.543 330.264 332.070 331.697 333.114 332.956 333.529 333.656 333.764 333.518 333.339 333.368 333.373
43.176 32.571 40.803 41.599 44.534 44.934 46.338 44.487 41.487 42.550 40.432 38.559 32.831
53.97 41.125 52.58 54.74 59.86 61.72 68.14 69.51 70.08 71.88 72.20 74.14 68.40
Fig. 3. The wear rate and degree of crystallization of PTW/PTFE composites varied with the content of PTW.
Ton , the temperature where melting starts; Tp , the temperature at which maximum melting occurs; Hm , heat of fusion; Xm , degree of crystallization.
Chen and co-workers [8] although PTFE was filled with carbon nanotube in that research.
The degree of crystallization decreases initially (at less than 3 wt% PTW), then it increases quickly with increasing PTW content due to the ability of heterogeneous nucleation of PTW. But the degree of crystallization decreases slightly after 35 wt% PTW content (cf. Fig. 3). The composite with 35 wt% PTW has the highest degree of crystallization which is 37% higher than pure PTFE. The melting onset temperature, Ton , and the temperature at which maximum melting occurs, Tp also illustrate the similar behavior. The increase of Ton and Tp is attributed to the increase of integrality of crystallization, owing to the increase of PTW
3.2. The crystallization of PTW/PTFE composites The crystallization of the PTFE composites reinforced with various amount of PTW was examined by DSC and the results are shown in Table 1. The degree of crystallization (Xm ) is estimated by taking the average heat-of-fusion of the sample over that of the complete crystalline PTFE which was assumed to be 80 J/g [4] and the results are also shown in Table 1.
Fig. 4. Abrasive dust of PTW/PTFE composite filled with PTW was observed by an optical microscope: (a) 0 wt% PTW; (b) 9 wt% PTW; (c) 20 wt% PTW; (d) 30 wt% PTW.
X. Feng et al. / Wear 261 (2006) 1208–1212
content. These effects in turn enhance the wear properties. The wear rate and the degree of crystallization of PTFE composite varied with the PTW content as shown in Fig. 3. It is obvious that the relationship between crystallinity and PTW content is similar to that for wear rate. Semicrystalline thermoplastics can be self-reinforced by oriented crystallization, whereby improved tribological and mechanical properties are achieved [9]. The reason that the wear resistance of PTFE is improved remarkably with increasing PTW content is postulated as follows: the increasing wear resistance of the PTW/PTFE may be divided into two parts: one is the presence of PTW which is also type of
1211
tiny fiber, the modulus and carrying capacity of the PTW/PTFE composite can be enhanced; the other is due to the increasing crystallinity which is also beneficial in improving the strength, modulus, wear resistance of the composites. 3.3. The pattern of abrasive dust and the micro-structure of worn surface of the composites The abrasive dust was observed by an optical microscope in our study. The presence of the abrasive dust can easily be recognized by a microscope and can even been seen by naked
Fig. 5. The micro-structure of a sample’s worn surface. (a) 0 wt% PTW; (b) 1 wt% PTW; (c) 3 wt% PTW; (d) 9 wt% PTW; (e) 20 wt% PTW; (f) 30 wt% PTW.
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X. Feng et al. / Wear 261 (2006) 1208–1212
eyes. Some results from the microscope are shown in Fig. 4. It can easily seen that the size of the dust decreases with increasing amount of PTW. But there the differences of sizes of dust between PTFE composites reinforced with 20 wt% PTW and that with 30 wt% PTW are insignificant (cf. Fig. 4(c and d)). The whiskers and their scraps in abrasive dust can be observed in Fig. 4(d) because the presence of high amount of PTW. In order to understand the effect of PTW on friction and wear properties, the micro-structure of sample surface was analyzed by SEM and results from SEM are shown in Fig. 5. The evidence of destruction of PTFE’s strap-like structure can be easily observed from Fig. 5(a–c), and the scales of destructions of the strap-like structures decrease with increasing PTW content, but the destructions of structures are not evident in Fig. 5(d–f). The abrasive dust and destruction of strap-like structures decreases with increasing amount of PTW can be explained by the fact that PTW has the ability to reduce wear and thereby prevents PTFE’s strap-like structure from being destroyed. This confirms the theory of Tanaka again [10]. If the content of PTW is more than 20 wt%, the binding force between PTW and PTFE decreases gradually, and the PTW can be brushed off easily. Therefore, the whiskers and their scraps can be found as demonstrated in Fig. 4(d) and crackles can be seen at high PTW content (cf. Fig. 5(f)). The free whiskers and their scraps in the interface would actually enhance grain abrasion, which increases the wear rate. 4. Conclusion Friction and wear properties of the PTFE composites reinforced with various amount of PTW under dry sliding conditions were studied. The crystallization of the composite, the patterns of abrasive dust and the micro-structures of the worn surfaces were also investigated. The following conclusions can be made based upon this study: 1. The wear rate of PTW/PTFE composite decrease dramatically when PTW content increases from 1 to 20 wt%, but it decreases slightly when PTW content increases from 20 to 40 wt%. The PTFE composite reinforced with 20 wt% PTW has the best wear resistance, which is over 1000 times higher than that of pure PTFE. The friction coefficient decreases with increasing PTW content. 2. The degree of crystallization decreases initially when the PTW content is <3 wt%, but it increases quickly until the PTW content reaches 30 wt%, and it decreases slightly after-
wards. The composite with 35 wt% PTW has the highest degree of crystallization which is 37% higher than that of pure PTFE. 3. The relationship between crystallinity and wear behavior has been analyzed based on the interaction between PTW and PTFE. The increase of crystallization with increasing PTW content is useful in improving the wear property of the composites. 4. The study of the images of abrasive dust and the worn surfaces has shown that PTW has the ability to reduce wear by preventing the destruction of PTFE’s strap-like structure. Acknowledgements Authors very appreciate the supports by the National Natural Science Foundation of PR China (29925616) through the Outstanding Youth Fund and the National Natural Science Foundation of China (20236010) and National High-tech Research Development Program (863 Program: 2003AA333010) and Dr. Yonggang Chen who gave kind help for polishing this paper. References [1] Q.-J. Xue, Z.-Z. Zhang, W.-M. Liu, et al., Friction and wear characteristics of fiber- and whisker-reinforced PTFE composites under oil lubricated conditions, J. Appl. Polym. Sci. 69 (1998) 1393–1402. [2] A. Bolvari, S. Glenn, R. Janessen, et al., Wear and friction of aramid fiber and polytetrafluoroethylene filled composites, Wear 203–204 (1997) 697–702. [3] G. Theiler, W. H¨ubner, T. Gradt, et al., Friction and wear of PTFE composites at cryogenic temperatures, Tribol. Int. 35 (2002) 449–458. [4] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj, et al., A study on the friction and wear behavior of PTFE filled with alumina nanoparticles, Wear 254 (2003) 573–580. [5] X. Feng, J.Z. L¨u, X.H. Lu, et al., Application of potassium titanate whisker in composite, Acta Mater. Compos. Sin. 16 (1999) 1–7. ¨ X.H. LU, Elastic interlayer toughening of potassium titanate [6] J.Z. LU, whiskers—Nylon66 composites and their fractal research, J. Appl. Polym. Sci. 82 (2001) 368–374. [7] X. Feng, D. Chen, X. Jiang, Friction and wear properties of potassium titanate whiskers reinforced PTFE composites, in: L. Ye, Y.-W. Mai, Z. Su (Eds.), Composites Technologies for 2020, Woodhead Publishing Ltd., England, 2004, pp. 46–50. [8] W.X. Chen, F. Li, G. Han, et al., Tribological behavior of carbon-nanotubefilled PTFE composites, Tribol. Lett. 15 (2002) 275–278. [9] J. Song, G.W. Ehrenstein, Friction and wear of self-reinforced thermoplastics, in: K. Friedrich (Ed.), Advances in Composite Tribology, Elsevier, Amsterdam, 1993, pp. 19–61. [10] K. Tanaka, Effect of various fillers on the friction and wear of PTFEbased composites, in: K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Elsevier, Amsterdam, 1986, pp. 137–174.