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Effect of rubber on tribological behaviors of polyamide 66 under dry and water lubricated sliding
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Wear 265 (2008) 361–366
Effect of rubber on tribological behaviors of polyamide 66 under dry and water lubricated sliding Sirong Yu a,∗ , Haixia Hu a , Jian Yin b a
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China b College of Materials Science and Engineering, Changchun University of Technology, Changchun 130012, China Received 9 February 2007; received in revised form 15 September 2007; accepted 20 November 2007 Available online 11 January 2008
Abstract The friction and wear behaviors of polyamide 66 (PA 66) and rubber-filled PA 66 (PA 66/SEBS-g-MA) composites were investigated on a blockon-wheel model friction and wear tester under dry sliding and water lubricating conditions. In order to further understand the wear mechanisms, the worn surfaces and scraps of samples were analyzed by scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). The experimental results indicated that the wear mass loss and the friction coefficient of PA 66 decreased with the addition of rubber particles. The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition are lower than those under dry sliding condition, but the wear mass losses are higher than those under dry sliding condition. The main wear mechanisms under dry sliding condition are the plastic deformation and mechanical microploughing. Whereas the main wear mechanisms under water lubricating condition are the mechanical microploughing and abrasive wear. © 2007 Elsevier B.V. All rights reserved. Keywords: Polyamide 66; Rubber; Composites; Friction and wear
1. Introduction Polyamide possesses high tensile and impact strengths, superior wear resistance and self-lubricating characteristic and is widely used in the fields of aerospace, automotive, and chemical engineering. These excellent properties result from the presence of hydrogen bonds in molecular chains of the polyamide [1]. Therefore, polyamide is preferable for many engineering parts undergoing friction and wear, such as bearing and gear [2]. Polyamide 66 (PA 66) is a kind of important engineering plastics and has been widely investigated by many researchers in recent years [3,4]. Chen et al. obtained immiscible, partially miscible and miscible blends of PA 66 and high-density polyethylene (HDPE) by changing compatilizer concentrations and studied their tribological properties [3]. It was found that the addition of compatilizer improved significantly the tribological properties of PA 66 and HDPE. Zhang et al. investigated the
∗
Corresponding author. Tel.: +86 431 85095862; fax: +86 431 85095876. E-mail address: [email protected] (S. Yu).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.11.006
tribological properties of PA 66 composites filled with TiO2 nano-particles, short carbon fibers, and graphite flakes on a pin-on-disk apparatus, and proposed that the rolling effect of nano-particles between material pairs contributed to the remarkable improvement of the load carrying capacity of polymer nanocomposites [4]. It is well known that rubber is an organic macromolecule elastic compound, and it possesses favorable fatigue and wear resistance. Available investigation indicated that soft rubber particles dispersed in polymer matrix were very effective to improve the wear resistance of polymers [5]. Bearings are frequently required to operate in an aqueous medium. Water either is deliberately introduced as a coolant (e.g. rolling-mill bearing) or presents as a working fluid (e.g. the submerged pumps and the journal bearings of rolling-mills) [6]. Most investigations published on the friction and wear behaviors of polymers have been focused on the sliding against steels in dry sliding condition, and only several results with water lubricant are available [7–10]. G. Srinath et al. [10] found that the specific wear rate of PA 6 nanocomposite and neat PA 6 under water lubricating condition increases with the applied load and
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S. Yu et al. / Wear 265 (2008) 361–366
is higher than that under dry sliding condition. The presence of water disrupts the formation of the transfer layer and promotes direct contact between the sample and counterface. In this work, the tribological properties of PA 66 and rubberfilled PA 66 (PA 66/SEBS-g-MA) composites sliding against the steel wheel under dry sliding and water lubricating conditions were investigated, and the wear mechanisms were discussed. It is helpful to a better understanding of the role of rubber particles in polymer composites. 2. Experimental 2.1. Preparation of materials PA 66 with a trade name of Vydyne® 21PC was supplied by Monsanto. Styrene–ethylene/butylenes–styrene triblock copolymer grafted with 1.84 wt% of maleic anhydride (SEBSg-MA) was supplied by Shell Chemical Company under the trade name of Kraton FG 1901X. The ratio of styrene to ethylene/butylene in the triblock copolymer was 28/72 by wt% and the glass transition temperature of the SEBS-g-MA was −42 ◦ C. Prior to blending, PA 66 was dried at 90 ◦ C under vacuum for 12 h. PA 66 composites were prepared in a Werner & Pfleiderer ZSK-30 twin-screw extruder (L/D = 30, L = 0.88 m) at 260–280 ◦ C and a screw speed of 300 rpm. SEBS-g-MA was mixed with PA 66 to form PA 66/SEBS-g-MA (80/15) composites. The extrudate was pelletized at the die exit. After dried, they were moulded into rectangular bars with an injection-molding machine (SZ-160/80 NB, China). The temperatures at the barrel and the mould were maintained at 270 ◦ C and 70 ◦ C, respectively. 2.2. Friction and wear test Friction and wear tests were carried out with an M-200 model block-on-wheel friction and wear tester produced by Xuanhua Testing Machineries Works in China under dry sliding and water lubricating conditions, respectively. The schematic diagram of the block-on-wheel friction and wear tester under the water lubricating condition is shown in Fig. 1. GCr15 bearing steel wheel, whose composition is C 0.95–1.05 wt%, Si 0.15–0.35 wt%, Mn 0.20–0.40 wt%, Cr 1.30–1.65 wt%, and Fe balance, with a bulk hardness of HRC65 ± 5 was used as the counterpart. The sizes of specimen and steel wheel are 10 mm × 10 mm × 14 mm and Φ40 mm × 10 mm, respectively. The rotating speed of the steel wheel was 200 rpm, which resulted in a sliding speed of 0.42 m/s in the wear track. The wearing time was 120 min, and the applied loads were 49 N, 98 N, 147 N, and 196 N, respectively. Prior to wear testing, the specimen was dried at 60 ◦ C for 1 h to remove the moisture and cooled to ambient temperature in a desiccator. The surfaces of the specimen and counterpart steel wheel were abraded with No. 1000 water-abrasive sandpaper, and the surface roughness, Ra, of about 0.1–0.4 m is obtained. Then the surfaces of the specimen and steel wheel were cleaned with acetone-dipped cotton and dried in air. The environmental temperature of the laboratory was about 12 ◦ C, and the relative humidity was about 80%.
Fig. 1. Schematic diagram of the block-on-wheel model friction and wear tester under the water lubricating condition.
During the test, the temperature of the steel wheel was monitored by an IR-77L model Infrared Thermometer produced by Shenzhen Everbest Machinery Industry Co. Ltd., China. The water lubricating condition was realized through continuous dropping of the distilled water onto the rotating steel wheel surface at a rate of 55–65 drops/min. The wear mass loss of specimen was measured using an electronic balance of 0.01 mg accuracy. The friction coefficient was calculated with the following equation: μ=
T RP
(1)
where μ refers to the friction coefficient, T the friction torque, R the radius of the steel wheel, and P the applied load. For each specimen, the friction and wear test was carried out three times, and the average of three testing data was used as the finally reported result so as to minimize the error of data. 2.3. Physical test After wear tests, the surfaces of specimens were coated with a thin layer of gold, and then the surface morphologies were observed with a scanning electron microscopy (SEM) (Model JSM-5600, Japan) at an accelerating voltage of 20 kV to understand the wear deformation and damage mechanisms. Differential scanning calorimeter (DSC) experiments were carried out using a PerkinElmer DSC-7 differential scanning calorimeter (USA) in a dry nitrogen atmosphere and working with 8–10 mg samples in aluminum pans. The heating rate was 10 ◦ C/min. The testing temperature range was from 50 ◦ C to 180 ◦ C. The first scan was ignored, and the second scan was used to determine the glass transition temperatures (Tg ) of PA 66 and PA 66/SEBS-g-MA composites.
S. Yu et al. / Wear 265 (2008) 361–366
Fig. 2. Changes in friction coefficients of PA 66 and PA 66/SEBS-g-MA composites with increasing applied loads under dry sliding (a) and water lubricating (b) conditions.
3. Results and discussion 3.1. Friction and wear behavior The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites at different applied loads under dry sliding and water lubricating conditions are shown in Fig. 2. It can be seen that the friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition increase initially with increasing applied load and then decrease when the applied load is more than 98 N (Fig. 2(a)). The changes of the friction coefficients of PA 66 and PA 66/SEBS-g-MA composites with increasing applied load under water lubricating condition are very small (Fig. 2(b)). The friction coefficient under water lubricating condition is lower than that under dry sliding condition at the same load. The friction coefficient of PA 66/SEBS-g-MA composites is lower than that of PA 66 at the same load. The tribological behaviors of material are related to the transfer film on the counterpart surface. A uniform and continuous
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transfer film can decrease the friction coefficient and improve the wear resistance of material. Under dry sliding condition, when the applied load is small, the stress in contact region between the sample and the steel wheel is small, the friction heat is less, and the surface of the steel wheel is smooth, i.e., the transfer film is not easy to form. With the increase of the applied load, the friction heat increases, and a part of wear debris adheres on the steel wheel and forms a transfer film, but this initial transfer film is noncontinuous and increases the friction coefficient. When the applied load increases further, the friction heat is much more, more wear debris adheres on the steel wheel, and a complete transfer film is quickly formed. The contact between the sample and the steel wheel turns into the contact between polymer and polymer, and the friction coefficient decreases. In this work, when the applied load was lower than 98 N, the transfer film was not continuous, and the friction coefficient increased with increasing applied load. When the applied load was more than 98 N, the transfer film became more and more complete, and the friction coefficient became less and less with increasing applied load. So there are the biggest values in the friction coefficient curves of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition (Fig. 2(a)). Under water lubricating condition, the chemical affinity between polymer surface molecules and the steel wheel was reduced on account of the presence of a water film. In addition, water is of cooling and washing action for the surfaces of sample and steel wheel, therefore, a transfer film like that under dry sliding condition on the steel wheel surface failed to form, and the slide occurred direct between polymer sample and the steel wheel. Consequently, the change of the friction coefficient with the increase of the applied load was very small. On the other hand, water acted as a lubricant and could form a lubricant film in the contact region during sliding friction, which led to that the friction coefficients under water lubricating condition were lower than those under dry sliding condition. Fig. 3 shows that the wear mass losses of PA 66 and PA 66/SEBS-g-MA composites increase with increasing applied load under dry sliding and water lubricating conditions. In addition, the wear mass loss of PA 66/SEBS-g-MA composites is lower than that of PA 66. According to Ref. [11], the impact strength of PA 66 sample is 62.5 ± 3.3 J/m, whereas the impact strength of PA 66/SEBS-g-MA composites sample is 303.4 ± 56.2 J/m. This indicates that the toughness of PA 66 was improved greatly by the addition of SEBS-g-MA, which led to low wear mass loss. It is also found that the wear mass losses under water lubricating condition are higher than those under dry sliding condition at the same load (Table 1). The main reason is that the transfer film on steel wheel surface failed to form and the slide occurred direct between the polymer sample and hard steel wheel during sliding. 3.2. Morphologies of the worn surfaces SEM micrographs of the worn surfaces of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition at the applied load of 196 N are given in Fig. 4. It can be seen that
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Fig. 3. Changes in wear mass losses of PA 66 and PA 66/SEBS-g-MA composites with increasing applied loads under dry sliding (a) and water lubricating (b) conditions.
some apparently plastic flow traces and plowed grooves on the worn surfaces of PA66 and PA 66/SEBS-g-MA composites are parallel to the sliding direction, which suggests that the wear process was governed by the plastic deformation and mechanical microploughing. At the same time, the uniform and continuous transfer films formed on the surfaces of the steel wheels. As for Table 1 Wear mass losses of PA 66 and PA 66/SEBS-g-MA composites at different applied loads under dry sliding and water lubricating conditions Materials
Loads (N)
Wear mass losses (g) Dry sliding
Water lubricating
PA 66
49 98 147 196
0.00185 0.00275 0.00685 0.00820
0.00280 0.00385 0.00730 0.01055
PA 66/SEBS-g-MA
49 98 147 196
0.00130 0.00180 0.00300 0.00520
0.00170 0.00235 0.00325 0.00570
Fig. 4. SEM photographs of worn surfaces of PA 66 (a) and PA 66/SEBS-g-MA composites (b) under dry sliding condition at the applied load of 196 N.
the role of transfer films, it is widely believed that the transfer film can prevent the polymer specimen from directly contacting the hard metal counterface, decrease the intensely plowing damage, and improve the wear resistance of sample. Fig. 5 shows the SEM micrographs of worn surfaces of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition at the applied load of 196 N. The worn surfaces are smooth, and there exist some ploughed grooves parallel to the sliding direction. This indicates that the wear process was governed by the mechanical microploughing and abrasive wear mechanisms. 3.3. Thermal analyses Typical DSC plots of PA 66 and PA 66/SEBS-g-MA composites in nitrogen are shown in Fig. 6. It can be seen that the glass transition temperature (Tg ) of PA 66 is about 137.20 ◦ C while Tg of PA 66/SEBS-g-MA composites is around 119.67 ◦ C. The decrease of Tg of PA 66/SEBS-g-MA composites can be attributed to the presence of some amount of the dissolved rubber, which was also found in varied rubber modified polymers [12,13].
S. Yu et al. / Wear 265 (2008) 361–366
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Fig. 5. SEM photographs of worn surfaces of PA 66 (a) and PA 66/SEBS-g-MA composites (b) under water lubricating condition at the applied load of 196 N.
During the sliding test, the temperature of the steel wheel can rise due to the frictional heat [14]. Fig. 7 shows the temperature changes of the steel wheels rubbing against PA 66 and PA 66/SEBS-g-MA composites at the applied load of 196 N under
Fig. 6. DSC plots of PA 66 and PA 66/SEBS-g-MA composites.
Fig. 7. Temperature rises of the steel wheels against PA 66 and PA 66/SEBS-gMA composites under dry sliding (a) and water lubricating (b) conditions at the applied load of 196 N.
dry sliding and water lubricating conditions. It can be found that the temperature rises of the steel wheels under dry sliding condition (Fig. 7(a)) were higher than those under water lubricating condition (Fig. 7(b)). Clearly, this is the result of the cooling action of water. In addition, the temperature rises of the steel wheels rubbing against PA 66/SEBS-g-MA composites were lower than those against PA 66. This is because the friction coefficient was small when the PA 66/SEBS-g-MA composites slide over the steel wheel, and the heat generated was also less. Due to the high contact temperature during sliding friction, PA 66 was softened, and its wear resistance was weakened. So, it can be drawn that the reason of the enhancement on the wear resistance of PA 66/SEBS-g-MA composites was mostly the low contact temperature during sliding friction, which abated the degradation of the mechanical properties of polymer matrix [4]. If the service temperature reaches up to Tg , the surfaces of the polymer samples would become seriously soft or even produce the plastic flow when there is a force on the surfaces, which possibly happens under dry sliding and high applied load
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condition. Correspondingly, the friction coefficient of samples would seriously increase because of the degraded mechanical properties [15]. Furthermore, the chemical reaction, such as thermal decomposition, would occur. According to Fig. 7, only the contact temperature of steel wheel rubbing against PA 66 at the applied load of 196 N under dry sliding condition is more than the glass transition temperature of PA 66, i.e., the surface of PA 66 sample could be seriously softened, so the friction coefficient of PA 66 sample under dry sliding condition is big (Fig. 2(a)). Whereas the contact temperatures of steel wheels rubbing against PA 66/SEBS-g-MA composites under dry sliding condition or steel wheels sliding under water lubricating condition were not more than the glass transition temperatures of samples, so the serious softening and thermal decomposition of the samples surfaces did not occur. 4. Conclusions (1) The wear mass loss and the friction coefficient of PA 66 decrease with the addition of SEBS-g-MA particles. (2) The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition are lower than those under dry sliding condition, but the wear mass losses are higher than those under dry sliding condition. (3) The main wear mechanisms under dry sliding condition are the plastic deformation and mechanical microploughing. Whereas the main wear mechanisms under water lubricating condition are the mechanical microploughing and abrasive wear. Acknowledgements The project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. This work was supported by Program for New Century Excellent Talents in University and “985 Project” of Jilin University of China. Sirong Yu thanks Prof. Y.-W. Mai and Dr. Zhongzhen Yu of the Centre for Advanced Materials
Technology (CAMT) in Sydney University for their great helps in this work. References [1] K. Yamanaka, Y. Ohmori, Effect of boron on transformation of low-carbon low-alloy steels, Trans. AIME 17 (1977) 93–101. [2] J.H. Byett, C. Allen, Dry sliding wear behavior of polyamide 66 and polycarbonate composites, Tribol. Int. 25 (1992) 237–246. [3] Z.B. Chen, T.S. Li, Y.L. Yang, Y. Zhang, S.Q. Lai, The effect of phase structure on the tribological properties of PA66/HDPE blends, Macromol. Mater. Eng. 289 (2004) 662–671. [4] L. Chang, Z. Zhang, H. Zhang, A.K. Schlarb, On the sliding wear of nanoparticle filled polyamide 66 composites, Compos. Sci. Technol. 66 (2006) 3188–3198. [5] Z. Zhang, K. Friedrich, Tribological characteristics of micro- and nanoparticle filled polymer composites, in: K. Friedrich, S. Fakirov, Z. Zhang (Eds.), Polymer Composites—From Nano- to Macro-Scale, Springer, 2005, pp. 169–185. [6] S.S. Kim, H.G. Lee, D.G. Lee, The tribological behavior of polymer coated carbon composites under dry and water lubricating conditions, Compos. Struct. 77 (2007) 364–372. [7] J.H. Jia, H.D. Zhou, S.Q. Gao, J.M. Chen, A comparative investigation of the friction and wear behavior of polyimide composites under dry sliding and water-lubricated condition, Mater. Sci. Eng. A 356 (2003) 48–53. [8] Y. Yamamoto, T. Takashima, Friction and wear of water lubricated PEEK and PPS sliding contacts, Wear 253 (2002) 820–826. [9] R. Prehn, F. Haupert, K. Friedrich, Sliding wear performance of polymer composites under abrasive and water lubricated conditions for pump applications, Wear 259 (2005) 693–696. [10] G. Srinath, R. Gnanamoorthy, Sliding wear performance of polyamide 6clay nanocomposites in water, Compos. Sci. Technol. 67 (2007) 399–405. [11] A. Dasari, Y. Zhong-Zhen, Y. Mingshu, Q.-X. Zhang, X.-L. Xie, Y.-W. Mai, Micro- and nano-scale deformation behavior of nylon 66-based binary and ternary nanocomposites, Compos. Sci. Technol. 66 (2006) 3097–3114. [12] D. Ratna, A.K. Banthia, Toughened epoxy adhesive modified with acrylate based liquid rubber, Polym. Int. 49 (2000) 281–287. [13] D. Ratna, Phase separation in liquid rubber modified epoxy mixture, relationship between curing conditions, morphology and ultimate behavior, Polymer 42 (2001) 4209–4218. [14] K.S. Surjeet, Wear failure of plastics, in: W.T. Becker, R.J. Shipley (Eds.), ASM Handbook: Failure Analysis and Prevention, vol. 11, ASM International, Ohio, 2002, p. 1020. [15] M. Watanabe, H. Yamaguchi, The friction and wear properties of nylon, Wear 110 (1986) 379–388.
Wear 265 (2008) 361–366
Effect of rubber on tribological behaviors of polyamide 66 under dry and water lubricated sliding Sirong Yu a,∗ , Haixia Hu a , Jian Yin b a
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China b College of Materials Science and Engineering, Changchun University of Technology, Changchun 130012, China Received 9 February 2007; received in revised form 15 September 2007; accepted 20 November 2007 Available online 11 January 2008
Abstract The friction and wear behaviors of polyamide 66 (PA 66) and rubber-filled PA 66 (PA 66/SEBS-g-MA) composites were investigated on a blockon-wheel model friction and wear tester under dry sliding and water lubricating conditions. In order to further understand the wear mechanisms, the worn surfaces and scraps of samples were analyzed by scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). The experimental results indicated that the wear mass loss and the friction coefficient of PA 66 decreased with the addition of rubber particles. The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition are lower than those under dry sliding condition, but the wear mass losses are higher than those under dry sliding condition. The main wear mechanisms under dry sliding condition are the plastic deformation and mechanical microploughing. Whereas the main wear mechanisms under water lubricating condition are the mechanical microploughing and abrasive wear. © 2007 Elsevier B.V. All rights reserved. Keywords: Polyamide 66; Rubber; Composites; Friction and wear
1. Introduction Polyamide possesses high tensile and impact strengths, superior wear resistance and self-lubricating characteristic and is widely used in the fields of aerospace, automotive, and chemical engineering. These excellent properties result from the presence of hydrogen bonds in molecular chains of the polyamide [1]. Therefore, polyamide is preferable for many engineering parts undergoing friction and wear, such as bearing and gear [2]. Polyamide 66 (PA 66) is a kind of important engineering plastics and has been widely investigated by many researchers in recent years [3,4]. Chen et al. obtained immiscible, partially miscible and miscible blends of PA 66 and high-density polyethylene (HDPE) by changing compatilizer concentrations and studied their tribological properties [3]. It was found that the addition of compatilizer improved significantly the tribological properties of PA 66 and HDPE. Zhang et al. investigated the
∗
Corresponding author. Tel.: +86 431 85095862; fax: +86 431 85095876. E-mail address: [email protected] (S. Yu).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.11.006
tribological properties of PA 66 composites filled with TiO2 nano-particles, short carbon fibers, and graphite flakes on a pin-on-disk apparatus, and proposed that the rolling effect of nano-particles between material pairs contributed to the remarkable improvement of the load carrying capacity of polymer nanocomposites [4]. It is well known that rubber is an organic macromolecule elastic compound, and it possesses favorable fatigue and wear resistance. Available investigation indicated that soft rubber particles dispersed in polymer matrix were very effective to improve the wear resistance of polymers [5]. Bearings are frequently required to operate in an aqueous medium. Water either is deliberately introduced as a coolant (e.g. rolling-mill bearing) or presents as a working fluid (e.g. the submerged pumps and the journal bearings of rolling-mills) [6]. Most investigations published on the friction and wear behaviors of polymers have been focused on the sliding against steels in dry sliding condition, and only several results with water lubricant are available [7–10]. G. Srinath et al. [10] found that the specific wear rate of PA 6 nanocomposite and neat PA 6 under water lubricating condition increases with the applied load and
362
S. Yu et al. / Wear 265 (2008) 361–366
is higher than that under dry sliding condition. The presence of water disrupts the formation of the transfer layer and promotes direct contact between the sample and counterface. In this work, the tribological properties of PA 66 and rubberfilled PA 66 (PA 66/SEBS-g-MA) composites sliding against the steel wheel under dry sliding and water lubricating conditions were investigated, and the wear mechanisms were discussed. It is helpful to a better understanding of the role of rubber particles in polymer composites. 2. Experimental 2.1. Preparation of materials PA 66 with a trade name of Vydyne® 21PC was supplied by Monsanto. Styrene–ethylene/butylenes–styrene triblock copolymer grafted with 1.84 wt% of maleic anhydride (SEBSg-MA) was supplied by Shell Chemical Company under the trade name of Kraton FG 1901X. The ratio of styrene to ethylene/butylene in the triblock copolymer was 28/72 by wt% and the glass transition temperature of the SEBS-g-MA was −42 ◦ C. Prior to blending, PA 66 was dried at 90 ◦ C under vacuum for 12 h. PA 66 composites were prepared in a Werner & Pfleiderer ZSK-30 twin-screw extruder (L/D = 30, L = 0.88 m) at 260–280 ◦ C and a screw speed of 300 rpm. SEBS-g-MA was mixed with PA 66 to form PA 66/SEBS-g-MA (80/15) composites. The extrudate was pelletized at the die exit. After dried, they were moulded into rectangular bars with an injection-molding machine (SZ-160/80 NB, China). The temperatures at the barrel and the mould were maintained at 270 ◦ C and 70 ◦ C, respectively. 2.2. Friction and wear test Friction and wear tests were carried out with an M-200 model block-on-wheel friction and wear tester produced by Xuanhua Testing Machineries Works in China under dry sliding and water lubricating conditions, respectively. The schematic diagram of the block-on-wheel friction and wear tester under the water lubricating condition is shown in Fig. 1. GCr15 bearing steel wheel, whose composition is C 0.95–1.05 wt%, Si 0.15–0.35 wt%, Mn 0.20–0.40 wt%, Cr 1.30–1.65 wt%, and Fe balance, with a bulk hardness of HRC65 ± 5 was used as the counterpart. The sizes of specimen and steel wheel are 10 mm × 10 mm × 14 mm and Φ40 mm × 10 mm, respectively. The rotating speed of the steel wheel was 200 rpm, which resulted in a sliding speed of 0.42 m/s in the wear track. The wearing time was 120 min, and the applied loads were 49 N, 98 N, 147 N, and 196 N, respectively. Prior to wear testing, the specimen was dried at 60 ◦ C for 1 h to remove the moisture and cooled to ambient temperature in a desiccator. The surfaces of the specimen and counterpart steel wheel were abraded with No. 1000 water-abrasive sandpaper, and the surface roughness, Ra, of about 0.1–0.4 m is obtained. Then the surfaces of the specimen and steel wheel were cleaned with acetone-dipped cotton and dried in air. The environmental temperature of the laboratory was about 12 ◦ C, and the relative humidity was about 80%.
Fig. 1. Schematic diagram of the block-on-wheel model friction and wear tester under the water lubricating condition.
During the test, the temperature of the steel wheel was monitored by an IR-77L model Infrared Thermometer produced by Shenzhen Everbest Machinery Industry Co. Ltd., China. The water lubricating condition was realized through continuous dropping of the distilled water onto the rotating steel wheel surface at a rate of 55–65 drops/min. The wear mass loss of specimen was measured using an electronic balance of 0.01 mg accuracy. The friction coefficient was calculated with the following equation: μ=
T RP
(1)
where μ refers to the friction coefficient, T the friction torque, R the radius of the steel wheel, and P the applied load. For each specimen, the friction and wear test was carried out three times, and the average of three testing data was used as the finally reported result so as to minimize the error of data. 2.3. Physical test After wear tests, the surfaces of specimens were coated with a thin layer of gold, and then the surface morphologies were observed with a scanning electron microscopy (SEM) (Model JSM-5600, Japan) at an accelerating voltage of 20 kV to understand the wear deformation and damage mechanisms. Differential scanning calorimeter (DSC) experiments were carried out using a PerkinElmer DSC-7 differential scanning calorimeter (USA) in a dry nitrogen atmosphere and working with 8–10 mg samples in aluminum pans. The heating rate was 10 ◦ C/min. The testing temperature range was from 50 ◦ C to 180 ◦ C. The first scan was ignored, and the second scan was used to determine the glass transition temperatures (Tg ) of PA 66 and PA 66/SEBS-g-MA composites.
S. Yu et al. / Wear 265 (2008) 361–366
Fig. 2. Changes in friction coefficients of PA 66 and PA 66/SEBS-g-MA composites with increasing applied loads under dry sliding (a) and water lubricating (b) conditions.
3. Results and discussion 3.1. Friction and wear behavior The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites at different applied loads under dry sliding and water lubricating conditions are shown in Fig. 2. It can be seen that the friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition increase initially with increasing applied load and then decrease when the applied load is more than 98 N (Fig. 2(a)). The changes of the friction coefficients of PA 66 and PA 66/SEBS-g-MA composites with increasing applied load under water lubricating condition are very small (Fig. 2(b)). The friction coefficient under water lubricating condition is lower than that under dry sliding condition at the same load. The friction coefficient of PA 66/SEBS-g-MA composites is lower than that of PA 66 at the same load. The tribological behaviors of material are related to the transfer film on the counterpart surface. A uniform and continuous
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transfer film can decrease the friction coefficient and improve the wear resistance of material. Under dry sliding condition, when the applied load is small, the stress in contact region between the sample and the steel wheel is small, the friction heat is less, and the surface of the steel wheel is smooth, i.e., the transfer film is not easy to form. With the increase of the applied load, the friction heat increases, and a part of wear debris adheres on the steel wheel and forms a transfer film, but this initial transfer film is noncontinuous and increases the friction coefficient. When the applied load increases further, the friction heat is much more, more wear debris adheres on the steel wheel, and a complete transfer film is quickly formed. The contact between the sample and the steel wheel turns into the contact between polymer and polymer, and the friction coefficient decreases. In this work, when the applied load was lower than 98 N, the transfer film was not continuous, and the friction coefficient increased with increasing applied load. When the applied load was more than 98 N, the transfer film became more and more complete, and the friction coefficient became less and less with increasing applied load. So there are the biggest values in the friction coefficient curves of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition (Fig. 2(a)). Under water lubricating condition, the chemical affinity between polymer surface molecules and the steel wheel was reduced on account of the presence of a water film. In addition, water is of cooling and washing action for the surfaces of sample and steel wheel, therefore, a transfer film like that under dry sliding condition on the steel wheel surface failed to form, and the slide occurred direct between polymer sample and the steel wheel. Consequently, the change of the friction coefficient with the increase of the applied load was very small. On the other hand, water acted as a lubricant and could form a lubricant film in the contact region during sliding friction, which led to that the friction coefficients under water lubricating condition were lower than those under dry sliding condition. Fig. 3 shows that the wear mass losses of PA 66 and PA 66/SEBS-g-MA composites increase with increasing applied load under dry sliding and water lubricating conditions. In addition, the wear mass loss of PA 66/SEBS-g-MA composites is lower than that of PA 66. According to Ref. [11], the impact strength of PA 66 sample is 62.5 ± 3.3 J/m, whereas the impact strength of PA 66/SEBS-g-MA composites sample is 303.4 ± 56.2 J/m. This indicates that the toughness of PA 66 was improved greatly by the addition of SEBS-g-MA, which led to low wear mass loss. It is also found that the wear mass losses under water lubricating condition are higher than those under dry sliding condition at the same load (Table 1). The main reason is that the transfer film on steel wheel surface failed to form and the slide occurred direct between the polymer sample and hard steel wheel during sliding. 3.2. Morphologies of the worn surfaces SEM micrographs of the worn surfaces of PA 66 and PA 66/SEBS-g-MA composites under dry sliding condition at the applied load of 196 N are given in Fig. 4. It can be seen that
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Fig. 3. Changes in wear mass losses of PA 66 and PA 66/SEBS-g-MA composites with increasing applied loads under dry sliding (a) and water lubricating (b) conditions.
some apparently plastic flow traces and plowed grooves on the worn surfaces of PA66 and PA 66/SEBS-g-MA composites are parallel to the sliding direction, which suggests that the wear process was governed by the plastic deformation and mechanical microploughing. At the same time, the uniform and continuous transfer films formed on the surfaces of the steel wheels. As for Table 1 Wear mass losses of PA 66 and PA 66/SEBS-g-MA composites at different applied loads under dry sliding and water lubricating conditions Materials
Loads (N)
Wear mass losses (g) Dry sliding
Water lubricating
PA 66
49 98 147 196
0.00185 0.00275 0.00685 0.00820
0.00280 0.00385 0.00730 0.01055
PA 66/SEBS-g-MA
49 98 147 196
0.00130 0.00180 0.00300 0.00520
0.00170 0.00235 0.00325 0.00570
Fig. 4. SEM photographs of worn surfaces of PA 66 (a) and PA 66/SEBS-g-MA composites (b) under dry sliding condition at the applied load of 196 N.
the role of transfer films, it is widely believed that the transfer film can prevent the polymer specimen from directly contacting the hard metal counterface, decrease the intensely plowing damage, and improve the wear resistance of sample. Fig. 5 shows the SEM micrographs of worn surfaces of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition at the applied load of 196 N. The worn surfaces are smooth, and there exist some ploughed grooves parallel to the sliding direction. This indicates that the wear process was governed by the mechanical microploughing and abrasive wear mechanisms. 3.3. Thermal analyses Typical DSC plots of PA 66 and PA 66/SEBS-g-MA composites in nitrogen are shown in Fig. 6. It can be seen that the glass transition temperature (Tg ) of PA 66 is about 137.20 ◦ C while Tg of PA 66/SEBS-g-MA composites is around 119.67 ◦ C. The decrease of Tg of PA 66/SEBS-g-MA composites can be attributed to the presence of some amount of the dissolved rubber, which was also found in varied rubber modified polymers [12,13].
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Fig. 5. SEM photographs of worn surfaces of PA 66 (a) and PA 66/SEBS-g-MA composites (b) under water lubricating condition at the applied load of 196 N.
During the sliding test, the temperature of the steel wheel can rise due to the frictional heat [14]. Fig. 7 shows the temperature changes of the steel wheels rubbing against PA 66 and PA 66/SEBS-g-MA composites at the applied load of 196 N under
Fig. 6. DSC plots of PA 66 and PA 66/SEBS-g-MA composites.
Fig. 7. Temperature rises of the steel wheels against PA 66 and PA 66/SEBS-gMA composites under dry sliding (a) and water lubricating (b) conditions at the applied load of 196 N.
dry sliding and water lubricating conditions. It can be found that the temperature rises of the steel wheels under dry sliding condition (Fig. 7(a)) were higher than those under water lubricating condition (Fig. 7(b)). Clearly, this is the result of the cooling action of water. In addition, the temperature rises of the steel wheels rubbing against PA 66/SEBS-g-MA composites were lower than those against PA 66. This is because the friction coefficient was small when the PA 66/SEBS-g-MA composites slide over the steel wheel, and the heat generated was also less. Due to the high contact temperature during sliding friction, PA 66 was softened, and its wear resistance was weakened. So, it can be drawn that the reason of the enhancement on the wear resistance of PA 66/SEBS-g-MA composites was mostly the low contact temperature during sliding friction, which abated the degradation of the mechanical properties of polymer matrix [4]. If the service temperature reaches up to Tg , the surfaces of the polymer samples would become seriously soft or even produce the plastic flow when there is a force on the surfaces, which possibly happens under dry sliding and high applied load
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condition. Correspondingly, the friction coefficient of samples would seriously increase because of the degraded mechanical properties [15]. Furthermore, the chemical reaction, such as thermal decomposition, would occur. According to Fig. 7, only the contact temperature of steel wheel rubbing against PA 66 at the applied load of 196 N under dry sliding condition is more than the glass transition temperature of PA 66, i.e., the surface of PA 66 sample could be seriously softened, so the friction coefficient of PA 66 sample under dry sliding condition is big (Fig. 2(a)). Whereas the contact temperatures of steel wheels rubbing against PA 66/SEBS-g-MA composites under dry sliding condition or steel wheels sliding under water lubricating condition were not more than the glass transition temperatures of samples, so the serious softening and thermal decomposition of the samples surfaces did not occur. 4. Conclusions (1) The wear mass loss and the friction coefficient of PA 66 decrease with the addition of SEBS-g-MA particles. (2) The friction coefficients of PA 66 and PA 66/SEBS-g-MA composites under water lubricating condition are lower than those under dry sliding condition, but the wear mass losses are higher than those under dry sliding condition. (3) The main wear mechanisms under dry sliding condition are the plastic deformation and mechanical microploughing. Whereas the main wear mechanisms under water lubricating condition are the mechanical microploughing and abrasive wear. Acknowledgements The project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. This work was supported by Program for New Century Excellent Talents in University and “985 Project” of Jilin University of China. Sirong Yu thanks Prof. Y.-W. Mai and Dr. Zhongzhen Yu of the Centre for Advanced Materials
Technology (CAMT) in Sydney University for their great helps in this work. References [1] K. Yamanaka, Y. Ohmori, Effect of boron on transformation of low-carbon low-alloy steels, Trans. AIME 17 (1977) 93–101. [2] J.H. Byett, C. Allen, Dry sliding wear behavior of polyamide 66 and polycarbonate composites, Tribol. Int. 25 (1992) 237–246. [3] Z.B. Chen, T.S. Li, Y.L. Yang, Y. Zhang, S.Q. Lai, The effect of phase structure on the tribological properties of PA66/HDPE blends, Macromol. Mater. Eng. 289 (2004) 662–671. [4] L. Chang, Z. Zhang, H. Zhang, A.K. Schlarb, On the sliding wear of nanoparticle filled polyamide 66 composites, Compos. Sci. Technol. 66 (2006) 3188–3198. [5] Z. Zhang, K. Friedrich, Tribological characteristics of micro- and nanoparticle filled polymer composites, in: K. Friedrich, S. Fakirov, Z. Zhang (Eds.), Polymer Composites—From Nano- to Macro-Scale, Springer, 2005, pp. 169–185. [6] S.S. Kim, H.G. Lee, D.G. Lee, The tribological behavior of polymer coated carbon composites under dry and water lubricating conditions, Compos. Struct. 77 (2007) 364–372. [7] J.H. Jia, H.D. Zhou, S.Q. Gao, J.M. Chen, A comparative investigation of the friction and wear behavior of polyimide composites under dry sliding and water-lubricated condition, Mater. Sci. Eng. A 356 (2003) 48–53. [8] Y. Yamamoto, T. Takashima, Friction and wear of water lubricated PEEK and PPS sliding contacts, Wear 253 (2002) 820–826. [9] R. Prehn, F. Haupert, K. Friedrich, Sliding wear performance of polymer composites under abrasive and water lubricated conditions for pump applications, Wear 259 (2005) 693–696. [10] G. Srinath, R. Gnanamoorthy, Sliding wear performance of polyamide 6clay nanocomposites in water, Compos. Sci. Technol. 67 (2007) 399–405. [11] A. Dasari, Y. Zhong-Zhen, Y. Mingshu, Q.-X. Zhang, X.-L. Xie, Y.-W. Mai, Micro- and nano-scale deformation behavior of nylon 66-based binary and ternary nanocomposites, Compos. Sci. Technol. 66 (2006) 3097–3114. [12] D. Ratna, A.K. Banthia, Toughened epoxy adhesive modified with acrylate based liquid rubber, Polym. Int. 49 (2000) 281–287. [13] D. Ratna, Phase separation in liquid rubber modified epoxy mixture, relationship between curing conditions, morphology and ultimate behavior, Polymer 42 (2001) 4209–4218. [14] K.S. Surjeet, Wear failure of plastics, in: W.T. Becker, R.J. Shipley (Eds.), ASM Handbook: Failure Analysis and Prevention, vol. 11, ASM International, Ohio, 2002, p. 1020. [15] M. Watanabe, H. Yamaguchi, The friction and wear properties of nylon, Wear 110 (1986) 379–388.