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1 This article was downloaded by: [Fudan University] On: 09 January 2012, At: 18:12 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Macromolecular Science, Part B Publication details, including instructions for authors and subscription information: Investigation on the Tribological Properties of POM Modified by Nano- PTFE Ting Huang a, Renguo Lu a, Hongyan Wang a, Yuning Ma a, Jianshu Tian a & Tongsheng Li a a Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai, China Available online: 27 May 2011 To cite this article: Ting Huang, Renguo Lu, Hongyan Wang, Yuning Ma, Jianshu Tian & Tongsheng Li (2011): Investigation on the Tribological Properties of POM Modified by Nano-PTFE, Journal of Macromolecular Science, Part B, 50:7, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Journal of Macromolecular Science R, Part B: Physics, 50: , 2011 Copyright Taylor & Francis Group, LLC ISSN: print / X online DOI: / Investigation on the Tribological Properties of POM Modified by Nano-PTFE TING HUANG, RENGUO LU, HONGYAN WANG, YUNING MA, JIANSHU TIAN, AND TONGSHENG LI Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai, China Introduction The tribological properties of polyoxymethylene (POM) modified by nanopolytetrafluorethylene (nano-ptfe) were investigated by a block-on-ring wear tester. For comparison, modified POM with micro-polytetrafluoroethylene (micro-ptfe) was also studied. The modified POM with a much lower concentration of nano-ptfe showed the similar tribological properties compared with POM modified by micro-ptfe. The friction coefficient decreased with the increase of nano-ptfe, while the wear rate showed the lowest value when the concentration of nano-ptfe was 2%. Scanning electron microscope (SEM) micrographs revealed that transfer films played an important role in the friction process. The transfer films decreased and stabilized the friction coefficient. Comparing to POM/2%nano-PTFE, when the concentration of nano-ptfe reached 4%, the mechanical properties decreased significantly, possibly due to poor dispersion of nano-ptfe. Keywords nano-ptfe, polyoxymethylene, solution treatment, surface topography, tribological properties As a type of engineering plastics with excellent performance, polyoxymethylene (POM) exhibits low friction coefficient, low wear rate, and good fatigue and creep resistance. [1 3] Consequently, POM has been widely used as self-lubricating materials in many fields, such as engineering, automotive, bearings, electronic appliances, and building materials. [4] With the development of aviation and aerospace, as well as civilian needs, solid self-lubricating materials are being used in ultra-small systems. However, pure POM is limited to be only applied under the conditions of low sliding speed and low load. Therefore, further improvement is needed to broaden its range of application. With the development of nanotechnology over the past decades, there have been a number of studies conducted to investigate the role of nanoparticles in tribological polymer nanocomposites. [5 8] Compared with micro-fillers, nanoparticles have the potential to reduce the abrasion. Because they are of the same size scale as counterface asperities, they may polish the highest asperities and promote the development of tribologically favorable Received 18 March 2010; accepted 25 May Address correspondence to Tongsheng Li, Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai , China. lits@fudan.edu.cn 1235

3 1236 T. Huang et al. transfer films. [9 12] Besides, they can react with macromolecular chains chemically or physically to enhance the interactions among the macromolecular chains after they are introduced to polymer composites. [13 16] Consequently, the friction and wear properties of nanocomposites can be significantly enhanced. Wang and co-workers reported that PEEK filled by 10.0 wt.% nano-sic exhibited lower friction coefficient and wear rate than when filled with micrometer-scale whiskers or microparticles of SiC. [11,17,19] In addition, it was reported by Hu and Wang that POM filled by 1 wt.% MoS 2 nano-balls presented better tribological properties than that with micro-mos 2. [19] There are many technologies to prepare nanocomposites, either by the direct incorporation using chemical methods such as in situ polymerization, or by the application of melting mechanical mixing. [20,21] For thermoplastic matrices, solution blending is more efficient to disperse nanoparticles in the polymer nanocomposites. Recently, inorganic nanoparticles have been used to improve the mechanical and tribological properties of POM. [22 24] However, we knew of no study of the mechanical and tribological properties of POM modified by organic nanoparticles. The objective of this paper was to investigate the tribological properties of POM modified by nano-polytetrafluorethylene (nano-ptfe). The experiments were carried out by studying the effect of concentration of fillers, sliding time, etc. For comparison, the tribological properties of POM modified by micro-ptfe were also studied. The morphologies of worn surface, wear debris, and transfer films were characterized by scanning electron microscope (SEM). Dynamic mechanical analysis (DMA) and differential thermal scanning calorimetry (DSC) analyses were also used for understanding the related mechanism. Experimental Details Materials Chemically pure granular POM copolymer (designated as M90 44; specific gravity = 1.41 g/cm 3 ; melting point 165 C) was provided by the Polyplastics Corporation, Japan. The average particle size of the nano-ptfe powder was nm, as shown in Fig. 1, supplied by Shanghai Institute of Applied Physics, Chinese Academy of Science. The average particle size of the micro-ptfe powder was 5 20 µm, provided by Daikin Corporation, Japan. N, N-dimethylformamide (DMF) was purchased from ShenXiang Chemical Agent Corporation in Shanghai, China. Preparation of POM Composites The Pretreatment of Raw Material. Pure POM and PTFE were dried in a vacuum oven at 100 C for 10 h to remove adsorbed water. Solution Blending. As a dispersant, 20.0 wt.% DMF (DMF:POM = 20:80) was added to granular POM with mechanical stirring, followed by a melting diffusion treatment. Then the mixture was blended with fillers (various contents relative to POM). The composites were filtered after soaking in acetone solution, to remove DMF after crystallization by cooling. The dried powder was extruded and granulated for injection-molding. The whole preparation process is shown in Fig. 2. Postprocess Treatment. The specimens for the tribological and mechanical properties tests were injection molded from the pelletized POM composites using a Laboratory Mixing injection molder (Atlas Co., Sweden), which was equipped with a standard mold.

4 Tribological Behavior of POM Modified by Nano-PTFE 1237 Figure 1. The average particle size of nano-ptfe. Mechanical Properties Tests Tensile tests were carried out according to GB/T standard, using a universal testing machine (CMT4104, Shenzhen Sans Testing Machine Ltd., China) at a crosshead speed of 10 mm/min. The parallel segment of the dumbbell-shaped specimens for tensile tests was mm 4.80 mm 1.65 mm. Friction and Wear Tests The friction and wear tests were conducted on a block-on-ring wear tester (M-2000, Xuanhua Machinery Works, China). The contact schematic diagram of the frictional couple is shown in Fig. 3. The dimension of the block specimen was 30 mm 7mm 6 mm, and the working face was 30 mm 7 mm. The mated ring was carbon steel Figure 2. Preparation process of the POM composites by solution mixing.

5 1238 T. Huang et al. Figure 3. Contact schematic diagram of the friction couple (a) and worn surface of specimen (b) (unit: mm). (No.45, GB ) with an inner diameter of 16 mm, an outer diameter of 40 mm, and a width of 10 mm. Before each test, the surfaces of both the block specimens and ring were ground with abrasive paper of various grit sizes in sequence, such that the surface roughnesses were controlled to be about R a 0.20 and 0.10 µm, respectively. Before each friction test, the block specimens and ring were cleaned with acetone-dipped cottons. All the sliding tests were performed over a period of 120 min at a normal load of 200 N and a sliding speed of 0.42 m/s under ambient conditions (temperature: 23 C ± 2 C, relative humidity: 50 ± 10%). The friction force torque was determined by a torque measuring system. The length of the wear scar (Fig. 3b) on the block specimens was measured with a microscope. The friction coefficients (µ), wear volume (V, mm 3 ), and wear rate (K, mm 3 /Nm) of the specimens were calculated according to the following formulas: µ = M W r, (1) [ ( ) b V = B r 2 arcsin b ] r 2r 2 2 b2, (2) 4 K = V P v t, (3) where M refers to the friction force torque (N mm), r the steel ring radius (mm), W the load (N), b the wear scar length (mm), B the specimen width (mm), ν the sliding speed of the steel ring (m/s), t the sliding time (s). The average results of three repeated friction and wear tests are reported here to minimize data scattering. Characterization A TS 5136MM SEM (Tescan Co., Czech) was used to observe the morphology of worn surfaces, wear debris (collected during the friction test), and transfer films. All specimens were sputtered with gold before the SEM observation.

6 Tribological Behavior of POM Modified by Nano-PTFE 1239 The frequency temperature-dependent viscoelastic tanδ (E /E ) and the storage modulus (E ) of the specimens were measured by a DMA-242 dynamic mechanical analyzer (Netzsch Co., Germany), using a 1.75-mm-thick rectangular block with the dimensions of 10 mm 4.8 mm. The temperature scans were run from 100 C to 100 C at a heating rate of 5 C/min. The tests were carried out in dual cantilever mode, frequency being set to 1 Hz, vibration amplitude 120 µm, static compressive stress 4 N. The crystallinity of samples was detected by DSC (DSC204, Netzsch Co., Germany). The samples (2 3 mg) were stacked in aluminum pans with pierced lid. The measurements were conducted by heating in nitrogen up to 200 C and holding at 200 C for 5 min to remove the heating history. The temperature was then dropped to 80 C at a cooling rate of 10 C/min, and once again heated up to 200 C at a heating rate of 10 C/min to measure the melting enthalpy. Results Properties of POM Composites The dependence of tribological and mechanical properties of POM composites on weight concentration of nano-ptfe or micro-ptfe is listed in Table 1. The tensile strength and breaking elongation rate decreased slightly with the addition of nano-ptfe, but when the content of nanoparticles was over 2%, the mechanical properties had a sharp fall with a brittle fracture. This may be due to excessive agglomerated nanoparticles, which weakened the interaction among POM macromolecular chains. Moreover, it can be seen that POM modified by 2% nano-ptfe (and also 1% nano- PTFE) showed much lower friction coefficient and wear rate than POM modified by 2% micro-ptfe. In fact, POM/2%nano-PTFE had similar tribological properties and higher mechanical properties compared with POM modified by 10% micro-ptfe. The results suggested nano-ptfe was more effective to enhance the tribological properties. Crystallinity and Dynamic Mechanical Analysis of POM/nano-PTFE Composites The analytic results of the heating curves of POM with the addition of nanoparticles are listed in Table 2. Crystallinity was calculated by the following formula: X c = H m 1 Hm o W 100%, (4) Table 1 The tribological and mechanical properties of POM composites Friction Wear rate Tensile Breaking Properties sample coefficient 10 6 (mm 3 /Nm) strength(mpa) elongation rate (%) pure POM POM/1%nano-PTFE POM/2%nano-PTFE POM/4%nano-PTFE POM/2%micro-PTFE POM/5%micro-PTFE POM/10%micro-PTFE

7 1240 T. Huang et al. Table 2 Results of DSC analysis of pure POM and POM/nano-PTFE composites Sample H m (J/g) X c (%) Pure POM POM/1%nano-PTFE POM/2%nano-PTFE POM/4%nano-PTFE where H m refers to the melting enthalpies of the POM/nano-PTFE composites, H o m the melting enthalpy of 100% crystalline copolymer POM, H o m = (J/g), and W the content of POM in POM/nano-PTFE composites. [25] As shown in Table 2, the filling with nanoparticles decreased the crystallinity of POM/nano-PTFE composites. The interaction between POM and nano-ptfe was poor, therefore it was hard for POM to be adsorbed and nucleated on the PTFE nano-particles. The peak height of tanδ of POM/4%nano-PTFE was obviously enhanced in comparison to pure POM and the 1% and 2% nano-ptfe samples (Fig. 4a). When the content of nanoparticles was 2% or less, the storage modulus of POM/nano-PTFE composites increased slightly compared with POM (Fig. 4b). However, when the content of nanoparticles reached 4%, the excessive agglomerated nanoparticles might have destroyed the continuous phase of POM/nano-PTFE composites which lead to the decrease of the storage modulus. [26 28] Tribological Properties of POM/Nano-PTFE Composites Figure 5 reveals the variations of friction coefficient and wear rate of POM/nano-PTFE composites with the mass fraction of nano-ptfe. With the increase of the content of nano- PTFE, the friction coefficient showed a downward trend, as did the wear rate. Compared with pure POM, the friction coefficient of POM/4%nano-PTFE was reduced by 35%. The wear rate ( mm 3 /Nm) was at its lowest value when the mass fraction of nano-ptfe in the composites was 2%, being decreased by 74%. This phenomenon may be explained by the formation of transfer films between the worn surface and the counterface, which improved the self-lubricating properties, due to the addition of nano-ptfe. In addition, the nanoparticles with small size and high surface energy filled the roughness and the wear scratches of the counterpart, making the transfer films uniform and compact. As a result, nanoparticles strengthened the interaction between transfer films and the counterpart, which also accounted for the reduction of friction coefficient and wear rate. However, when the content of nanoparticles reached 4%, the wear rate ( mm 3 /Nm) increased slightly. We suggest the main reason would be that the excessive agglomerated nanoparticles may block the formation of the transfer film. Moreover, combining with the analysis of DMA, the excessive content (4%) of agglomerated nanoparticles increased the resistance to the segmental movement of the POM composites, which resulted in the blocking of POM segmental relaxation and damage

8 Tribological Behavior of POM Modified by Nano-PTFE 1241 Figure 4. Changes of tanδ (a) and storage modulus (b) of POM and POM/nano-PTFE composites with increasing temperature. of the transfer films, accounting for the decreased wear-resistance of POM/4%nano-PTFE composites. Comparing Fig. 5 with Fig. 6, it was found that POM/2%nano-PTFE composites showed similar friction coefficient and wear rate as POM/10%micro-PTFE composites, while it had a much lower friction coefficient and wear rate than POM/2%micro-PTFE composites. In other words, nanoparticles were more effective to improve the tribological properties of POM composites than microparticles. Figure 7 shows the variations of friction coefficient with sliding time for POM/2%nano- PTFE and POM/2%micro-PTFE. Compared with POM/micro-PTFE composites, all of the POM/nano-PTFE composites had a lower and more stable friction coefficient in the whole

9 1242 T. Huang et al. Figure 5. Effect of concentration of nano-ptfe on friction coefficient and wear rate of POM/nano- PTFE composites (load: 200 N, sliding speed: 0.42 m/s). process. It took more than 60 min to reach the steady-state friction coefficient of POM modified by microparticles. That s to say, the running-in time exceeded 60 min. Worn Surfaces, Wear Debris, and Transfer Films Observations Figure 8 shows SEM micrographs of the worn surfaces and wear debris of pure POM and its composites. Many scratch grooves parallel to the sliding direction were clearly observed on the worn surface of pure POM when sliding against the steel counterpart. In addition, the phenomenon of thermal softening was obviously present on the worn surface (Fig. 8a). For pure POM, the friction heat was hard to dissipate, which increased the surface temperature, inducing the adhesive wear and the plastic deformation. Deformed Figure 6. Effect of concentration of micro-ptfe on friction coefficient and wear rate of POM/micro- PTFE composites (load: 200 N, sliding speed: 0.42 m/s).

10 Tribological Behavior of POM Modified by Nano-PTFE 1243 Figure 7. Variation of friction coefficient with sliding time for POM/2% nano-ptfe and POM/10% micro-ptfe (load: 200 N, sliding speed: 0.42 m/s). POM particles were peeled off from the worn surface (Fig. 8b). With the addition of nanoparticles, the furrows of the POM/nano-PTFE composites on the worn surface were obviously reduced, while the adhesive wear still existed. But the wear debris consisted of relatively small sheets (Figs. 8f, h, and l). Comparing with pure POM (Fig. 8a), less serious damage was observed on the worn surface of all of the POM/nano-PTFE composites. Furthermore, the POM/2%nano-PTFE composite had the smoothest worn surface of the various POM/nano-PTFE composites. The results suggested that the optimal content of nanoparticles in the POM/nano-PTFE composites was 2%. Because nanoparticles were of the same scale as counterface asperities, they may fill in the highest asperities and promote the development of tribologically favorable transfer films. Once formed, the transfer films prevented the POM/nano-PTFE composites from direct abrasive wear, and resulted in the reduction of the friction coefficient and wear rate. Figure 9 shows representative SEM micrographs of the counterpart steel and the transfer films formed by POM/2%nano-PTFE and POM/10%micro-PTFE. It can be seen that the POM/nano-PTFE composites were peeled off from the worn surface and formed continuous, adhesive transfer films (Fig. 9b), spreading homogeneously all over the surface of the counterpart steel ring. The wear debris of this POM/nano-PTFE composites consisted of small flakes (Fig. 8h). On the other hand, the transfer films formed by POM/micro-PTFE composites were discontinuous (Fig. 9c). Moreover, compared with the wear debris of POM/nano-PTFE composites, that of POM/micro-PTFE composites ploughed from the worn surface showed large lamellar particles (Fig. 10a). This phenomenon implied that the POM modified by 2% nanoparticles tended to form a stable transfer film and small wear debris, which was related to the stable tribological properties discussed. Discussion Due to the molecular structure of PTFE, it is easily sheared to form transfer films of about nm thickness, resulting in self-lubricating properties for PTFE. [29] This is because the van der Waals force existing between the molecular chains are weaker

11 1244 T. Huang et al. than the intramolecular bonds. As a result, macromolecules were easily sheared, which was described by Wang. [30] In this study, an even and tenacious transfer film was formed on the counterpart during the friction process of POM modified by nano-ptfe, which could protect the surfaces of POM/nano-PTFE composites from direct contact with the Figure 8. SEM micrographs of worn surface and wear debris of pure POM and POM/nano-PTFE composites (load: 200 N, sliding speed: 0.42 m/s).

12 Tribological Behavior of POM Modified by Nano-PTFE 1245 Figure 8. (Continued) counterface, reducing the friction and the formation of furrows on the worn surface. The wear debris of POM/nano-PTFE composites generated were relatively small flakes, while those of pure POM were large strips. Thus we conclude that the tribological mechanism of pure POM was both abrasive and adhesive wear, while that of POM/nano-PTFE composites was mainly adhesive wear. In addition, the adhesive wear of pure POM was more serious than that of POM/nano- PTFE composites, generating sufficient heat to permit its plastic deformation. For pure POM, the friction heat was hard to dissipate due to the poor ability of heat transfer, which enhanced the accumulation of friction heat, aggravating the phenomenon of thermal softening. Therefore the load-bearing capacity was reduced, resulting in the decrease of the wear resistance of POM. Since DSC and DMA analysis revealed that the thermal behavior of POM/nano-PTFE composites had little change with the addition of nanoparticles, the improvement of the wear resistance of the composites is suggested to be due to the decrease of friction coefficient. The formation of transfer films decreased the friction coefficient, inducing the reduction of friction heat (Q = µ v P) and the improvement of tribological properties.

13 1246 T. Huang et al. Figure 9. SEM micrographs of counterpart steel ring and the transfer films formed by POM composites (load: 200 N, sliding speed: 0.42 m/s). Figure 10. SEM micrographs of wear debris of POM/10%micro-PTFE composites (load: 200 N, sliding speed: 0.42 m/s).

14 Tribological Behavior of POM Modified by Nano-PTFE 1247 Conclusions 1. Nano-PTFE was effective to decrease the friction coefficient and wear rate of modified POM composites. The wear rate decreased to the lowest value when the concentration of nano-ptfe was 2%. The main wear mechanism of POM/nano- PTFE composites was adhesive wear. 2. The crystallinity, tensile strength, and breaking elongation decreased slightly with the addition of nano-ptfe. When the content of nanoparticles exceeded 2%, the mechanical properties decreased sharply with a brittle fracture. This may be due to excessive agglomerated nanoparticles, which created stress risers and weakened the interaction among POM macromolecular chains. 3. POM composites with much lower concentration of nano-ptfe showed similar tribological properties compared to POM modified by micro-ptfe. Moreover, the friction coefficient of POM modified by nano-ptfe was lower and more stable than that of POM modified by micro-ptfe, possibly due to the formation of the even and continuous transfer films. References 1. Benabdallah, H.S. Reciprocating sliding friction and contact stress of some thermoplastics against steel. J. Mater. Sci. 1997, 32, Chen, J.Y.; Cao, Y.; Li, H.L. Investigation of the friction and wear behaviors of polyoxymethylene/linear low-density polyethylene/ethylene-acrylic-acid blends. Wear 2006, 260, Sun, L.H.; Yang, Z.G.; Li, X.H. Study on the friction and wear behavior of POM/Al 2 O 3 nanocomposites. Wear 2008, 264, Samyn, P. Wear transitions and stability of polyoxymethylene homopolymer in highly loaded applications compared to small-scale testing. Tribol. Int. 2007, 40, Burris, D.L.; Boesl, B.J.; Bourne, G.R. Polymeric nanocomposites for tribological applications. Macromol. Mater. Eng. 2007, 292, Wang, Q.H.; Pei, X.Q. The influence of nanoparticle fillers on the friction and wear behavior of polymer matrices. Tribol. Int. Eng. Ser. 2008, 55, Chang, L.; Zhang, Z.; Zhang, H. On the sliding wear of nanoparticle filled polyamide 66 composites. Compos. Sci. Technol. 2006, 66, Burris, D.L.; Sawyer, W.G. Improved wear resistance in alumina-ptfe nanocomposites with irregular shaped nanoparticles. Wear 2006, 260, Bahadur, S. The development of transfer layers and their role in polymer tribology. Wear 2000, 245, Bahadur, S.; Schwartz, C.J. The influence of nanoparticle fillers in polymer matrices on the formation and stability of transfer film during wear. Tribol. Int. Eng. Ser. 2008, 55, Wang, Q.H.; Xue, Q.J.; Shen, W.C. The friction and wear properties of nanometre SiO 2 filled polyetheretherketone. Tribol. Int. 1997, 30, Burris, D.L.; Zhao, S.; Duncan, R. A route to wear resistant PTFE via trace loadings of functionalized nanofillers. Wear 2009, 267, Bhimaraj, P.; Burris, D.L.; Action, J. Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly(ethylene) terephthalate composites. Wear 2008, 258, Srinath, G.; Gnanamoorthy, R. Two-body abrasive wear characteristics of Nylon clay nanocomposites: Effect of grit size, load, and sliding velocity. Mater. Sci. Eng. A 2006, , Feng, S.M.; Gu, G.S. Application of nanoparticles. Mater. Rev. 2001, 15, Hasan, M.M.; Zhou, Y.X.; Mahfuz, H. Effect of SiO 2 nanoparticle on thermal and tensile behavior of nylon-6. Mater. Sci. Eng. A 2006, 429, 181.

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