This article was downloaded by: [Fudan University]On: 09 January 2012, At: 18:12Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Macromolecular Science, PartBPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lmsb20

Investigation on the TribologicalProperties of POM Modified by Nano-PTFETing Huang a , Renguo Lu a , Hongyan Wang a , Yuning Ma a , JianshuTian a & Tongsheng Li aa Key Laboratory of Molecular Engineering of Polymers of Ministryof Education, Department of Macromolecular Science, FudanUniversity, Shanghai, China

Available online: 27 May 2011

To cite this article: Ting Huang, Renguo Lu, Hongyan Wang, Yuning Ma, Jianshu Tian & TongshengLi (2011): Investigation on the Tribological Properties of POM Modified by Nano-PTFE, Journal ofMacromolecular Science, Part B, 50:7, 1235-1248

To link to this article: http://dx.doi.org/10.1080/00222348.2010.503152

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Anysubstantial 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 representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly orindirectly in connection with or arising out of the use of this material.

Journal of Macromolecular Science R©, Part B: Physics, 50:1235–1248, 2011Copyright © Taylor & Francis Group, LLCISSN: 0022-2348 print / 1525-609X onlineDOI: 10.1080/00222348.2010.503152

Investigation on the Tribological Properties of POMModified 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

The tribological properties of polyoxymethylene (POM) modified by nano-polytetrafluorethylene (nano-PTFE) were investigated by a block-on-ring wear tester.For comparison, modified POM with micro-polytetrafluoroethylene (micro-PTFE) wasalso studied. The modified POM with a much lower concentration of nano-PTFE showedthe similar tribological properties compared with POM modified by micro-PTFE. Thefriction coefficient decreased with the increase of nano-PTFE, while the wear rateshowed the lowest value when the concentration of nano-PTFE was 2%. Scanning elec-tron microscope (SEM) micrographs revealed that transfer films played an importantrole in the friction process. The transfer films decreased and stabilized the frictioncoefficient. Comparing to POM/2%nano-PTFE, when the concentration of nano-PTFEreached 4%, the mechanical properties decreased significantly, possibly due to poordispersion of nano-PTFE.

Keywords nano-PTFE, polyoxymethylene, solution treatment, surface topography,tribological properties

Introduction

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, suchas engineering, automotive, bearings, electronic appliances, and building materials.[4] Withthe development of aviation and aerospace, as well as civilian needs, solid self-lubricatingmaterials are being used in ultra-small systems. However, pure POM is limited to beonly applied under the conditions of low sliding speed and low load. Therefore, furtherimprovement is needed to broaden its range of application.

With the development of nanotechnology over the past decades, there have beena number of studies conducted to investigate the role of nanoparticles in tribologicalpolymer nanocomposites.[5–8] Compared with micro-fillers, nanoparticles have the potentialto reduce the abrasion. Because they are of the same size scale as counterface asperities, theymay polish the highest asperities and promote the development of tribologically favorable

Received 18 March 2010; accepted 25 May 2010.Address correspondence to Tongsheng Li, Key Laboratory of Molecular Engineering of Polymers

of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai200433, China. E-mail: lits@fudan.edu.cn

1235

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

1236 T. Huang et al.

transfer films.[9–12] Besides, they can react with macromolecular chains chemically orphysically to enhance the interactions among the macromolecular chains after they areintroduced to polymer composites.[13–16] Consequently, the friction and wear properties ofnanocomposites can be significantly enhanced. Wang and co-workers reported that PEEKfilled by 10.0 wt.% nano-SiC exhibited lower friction coefficient and wear rate than whenfilled with micrometer-scale whiskers or microparticles of SiC.[11,17,19] In addition, it wasreported by Hu and Wang that POM filled by 1 wt.% MoS2 nano-balls presented bettertribological properties than that with micro-MoS2.[19] There are many technologies toprepare nanocomposites, either by the direct incorporation using chemical methods suchas in situ polymerization, or by the application of melting–mechanical mixing.[20,21] Forthermoplastic matrices, solution blending is more efficient to disperse nanoparticles in thepolymer nanocomposites. Recently, inorganic nanoparticles have been used to improve themechanical and tribological properties of POM.[22–24] However, we knew of no study of themechanical and tribological properties of POM modified by organic nanoparticles.

The objective of this paper was to investigate the tribological properties of POM mod-ified by nano-polytetrafluorethylene (nano-PTFE). The experiments were carried out bystudying the effect of concentration of fillers, sliding time, etc. For comparison, the tribo-logical properties of POM modified by micro-PTFE were also studied. The morphologiesof worn surface, wear debris, and transfer films were characterized by scanning electron mi-croscope (SEM). Dynamic mechanical analysis (DMA) and differential thermal scanningcalorimetry (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/cm3; melting point 165◦C) was provided by the Polyplastics Corporation, Japan. Theaverage particle size of the nano-PTFE powder was 50–70 nm, as shown in Fig. 1, suppliedby Shanghai Institute of Applied Physics, Chinese Academy of Science. The averageparticle size of the micro-PTFE powder was 5–20 µm, provided by Daikin Corporation,Japan. N, N-dimethylformamide (DMF) was purchased from ShenXiang Chemical AgentCorporation in Shanghai, China.

Preparation of POM Composites

The Pretreatment of Raw Material. Pure POM and PTFE were dried in a vacuum oven at100◦C for 10 h to remove adsorbed water.

Solution Blending. As a dispersant, 20.0 wt.% DMF (DMF:POM = 20:80) was added togranular POM with mechanical stirring, followed by a melting diffusion treatment. Thenthe mixture was blended with fillers (various contents relative to POM). The compositeswere filtered after soaking in acetone solution, to remove DMF after crystallization bycooling. The dried powder was extruded and granulated for injection-molding. The wholepreparation process is shown in Fig. 2.

Postprocess Treatment. The specimens for the tribological and mechanical properties testswere injection molded from the pelletized POM composites using a Laboratory Mixinginjection molder (Atlas Co., Sweden), which was equipped with a standard mold.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

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 16421-1996 standard, using a universaltesting machine (CMT4104, Shenzhen Sans Testing Machine Ltd., China) at a crossheadspeed of 10 mm/min. The parallel segment of the dumbbell-shaped specimens for tensiletests was 18.50 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, Xu-anhua Machinery Works, China). The contact schematic diagram of the frictional cou-ple is shown in Fig. 3. The dimension of the block specimen was 30 mm × 7 mm ×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.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

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 699-88) 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 andring were ground with abrasive paper of various grit sizes in sequence, such that thesurface roughnesses were controlled to be about Ra 0.20 and 0.10 µm, respectively. Be-fore each friction test, the block specimens and ring were cleaned with acetone-dippedcottons.

All the sliding tests were performed over a period of 120 min at a normal load of200 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 torquemeasuring system. The length of the wear scar (Fig. 3b) on the block specimens wasmeasured with a microscope. The friction coefficients (µ), wear volume (V , mm3), andwear rate (K, mm3/Nm) of the specimens were calculated according to the followingformulas:

µ = M

W × r, (1)

V = B

[r2 arcsin

(b

2r

)− b

2

√r2 − b2

4

], (2)

K = V

P × v × t, (3)

where M refers to the friction force torque (N· mm), r the steel ring radius (mm), W theload (N), b the wear scar length (mm), B the specimen width (mm), ν the sliding speed ofthe steel ring (m/s), t the sliding time (s). The average results of three repeated friction andwear tests are reported here to minimize data scattering.

Characterization

A TS 5136MM SEM (Tescan Co., Czech) was used to observe the morphology of wornsurfaces, wear debris (collected during the friction test), and transfer films. All specimenswere sputtered with gold before the SEM observation.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

Tribological Behavior of POM Modified by Nano-PTFE 1239

The frequency–temperature-dependent viscoelastic tanδ (E′′′/E′) and the storage mod-ulus (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 of10 mm × 4.8 mm. The temperature scans were run from −100◦C to 100◦C at a heatingrate of 5◦C/min. The tests were carried out in dual cantilever mode, frequency being set to1 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 measurementswere conducted by heating in nitrogen up to 200◦C and holding at 200◦C for 5 min to removethe 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 meltingenthalpy.

Results

Properties of POM Composites

The dependence of tribological and mechanical properties of POM composites on weightconcentration of nano-PTFE or micro-PTFE is listed in Table 1. The tensile strength andbreaking elongation rate decreased slightly with the addition of nano-PTFE, but when thecontent of nanoparticles was over 2%, the mechanical properties had a sharp fall with abrittle fracture. This may be due to excessive agglomerated nanoparticles, which weakenedthe 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 highermechanical properties compared with POM modified by 10% micro-PTFE. The resultssuggested 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 arelisted in Table 2. Crystallinity was calculated by the following formula:

Xc = �Hm

�Hom

• 1

W× 100%, (4)

Table 1The tribological and mechanical properties of POM composites

Friction Wear rate Tensile BreakingProperties sample coefficient ×10−6 (mm3/Nm) strength(Mpa) elongation rate (%)

pure POM 0.33 4.11 63.50 49.82POM/1%nano-PTFE 0.27 1.09 60.32 45.02POM/2%nano-PTFE 0.23 0.99 59.04 44.15POM/4%nano-PTFE 0.22 1.11 26.87 19.25POM/2%micro-PTFE 0.28 2.61 57.08 39.89POM/5%micro-PTFE 0.24 1.14 54.02 29.53POM/10%micro-PTFE 0.22 1.07 49.35 28.17

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

1240 T. Huang et al.

Table 2Results of DSC analysis of pure POM and POM/nano-PTFE composites

Sample �Hm (J/g) Xc (%)

Pure POM 144.5 52.9POM/1%nano-PTFE 138.2 51.1POM/2%nano-PTFE 125.9 47.0POM/4%nano-PTFE 119.1 45.4

where �Hm refers to the melting enthalpies of the POM/nano-PTFE composites, �Hom the

melting enthalpy of 100% crystalline copolymer POM, �Hom = 273.1 (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 ofPOM/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 compar-ison to pure POM and the 1% and 2% nano-PTFE samples (Fig. 4a). When the contentof nanoparticles was 2% or less, the storage modulus of POM/nano-PTFE compositesincreased slightly compared with POM (Fig. 4b). However, when the content of nanopar-ticles reached 4%, the excessive agglomerated nanoparticles might have destroyed thecontinuous phase of POM/nano-PTFE composites which lead to the decrease of the storagemodulus.[26–28]

Tribological Properties of POM/Nano-PTFE Composites

Figure 5 reveals the variations of friction coefficient and wear rate of POM/nano-PTFEcomposites 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. Comparedwith pure POM, the friction coefficient of POM/4%nano-PTFE was reduced by 35%.The wear rate (0.99 × 10−6 mm3/Nm) was at its lowest value when the mass fraction ofnano-PTFE in the composites was 2%, being decreased by 74%.

This phenomenon may be explained by the formation of transfer films between theworn surface and the counterface, which improved the self-lubricating properties, dueto the addition of nano-PTFE. In addition, the nanoparticles with small size and highsurface energy filled the roughness and the wear scratches of the counterpart, making thetransfer films uniform and compact. As a result, nanoparticles strengthened the interactionbetween transfer films and the counterpart, which also accounted for the reduction offriction coefficient and wear rate. However, when the content of nanoparticles reached4%, the wear rate (1.11 × 10−6 mm3/Nm) increased slightly. We suggest the main reasonwould be that the excessive agglomerated nanoparticles may block the formation of thetransfer film. Moreover, combining with the analysis of DMA, the excessive content (4%)of agglomerated nanoparticles increased the resistance to the segmental movement of thePOM composites, which resulted in the blocking of POM segmental relaxation and damage

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

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 compositeswith increasing temperature.

of the transfer films, accounting for the decreased wear-resistance of POM/4%nano-PTFEcomposites.

Comparing Fig. 5 with Fig. 6, it was found that POM/2%nano-PTFE compositesshowed 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-PTFEcomposites. In other words, nanoparticles were more effective to improve the tribologicalproperties 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 thePOM/nano-PTFE composites had a lower and more stable friction coefficient in the whole

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

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 POMmodified 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 POMand its composites. Many scratch grooves parallel to the sliding direction were clearlyobserved on the worn surface of pure POM when sliding against the steel counterpart. Inaddition, 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 thesurface 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).

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

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 ofnanoparticles, the furrows of the POM/nano-PTFE composites on the worn surface wereobviously reduced, while the adhesive wear still existed. But the wear debris consisted ofrelatively small sheets (Figs. 8f, h, and l). Comparing with pure POM (Fig. 8a), less seriousdamage 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 thevarious POM/nano-PTFE composites. The results suggested that the optimal content ofnanoparticles in the POM/nano-PTFE composites was 2%. Because nanoparticles were ofthe same scale as counterface asperities, they may fill in the highest asperities and promotethe development of tribologically favorable transfer films. Once formed, the transfer filmsprevented the POM/nano-PTFE composites from direct abrasive wear, and resulted in thereduction of the friction coefficient and wear rate.

Figure 9 shows representative SEM micrographs of the counterpart steel and thetransfer films formed by POM/2%nano-PTFE and POM/10%micro-PTFE. It can be seenthat the POM/nano-PTFE composites were peeled off from the worn surface and formedcontinuous, adhesive transfer films (Fig. 9b), spreading homogeneously all over the surfaceof the counterpart steel ring. The wear debris of this POM/nano-PTFE composites consistedof small flakes (Fig. 8h). On the other hand, the transfer films formed by POM/micro-PTFEcomposites were discontinuous (Fig. 9c). Moreover, compared with the wear debris ofPOM/nano-PTFE composites, that of POM/micro-PTFE composites ploughed from theworn surface showed large lamellar particles (Fig. 10a). This phenomenon implied that thePOM modified by 2% nanoparticles tended to form a stable transfer film and small weardebris, 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 ofabout 20 ∼ 300 nm thickness, resulting in self-lubricating properties for PTFE.[29] Thisis because the van der Waals force existing between the molecular chains are weaker

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

1244 T. Huang et al.

than the intramolecular bonds. As a result, macromolecules were easily sheared, whichwas described by Wang.[30] In this study, an even and tenacious transfer film was formedon the counterpart during the friction process of POM modified by nano-PTFE, whichcould 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-PTFEcomposites (load: 200 N, sliding speed: 0.42 m/s).

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

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. Thewear debris of POM/nano-PTFE composites generated were relatively small flakes, whilethose of pure POM were large strips. Thus we conclude that the tribological mechanism ofpure POM was both abrasive and adhesive wear, while that of POM/nano-PTFE compositeswas 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 purePOM, 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 thermalsoftening. Therefore the load-bearing capacity was reduced, resulting in the decrease of thewear resistance of POM. Since DSC and DMA analysis revealed that the thermal behaviorof POM/nano-PTFE composites had little change with the addition of nanoparticles, theimprovement of the wear resistance of the composites is suggested to be due to the decreaseof 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 tribologicalproperties.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

1246 T. Huang et al.

Figure 9. SEM micrographs of counterpart steel ring and the transfer films formed by POM com-posites (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).

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

Tribological Behavior of POM Modified by Nano-PTFE 1247

Conclusions

1. Nano-PTFE was effective to decrease the friction coefficient and wear rate ofmodified POM composites. The wear rate decreased to the lowest value when theconcentration 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 withthe addition of nano-PTFE. When the content of nanoparticles exceeded 2%, themechanical properties decreased sharply with a brittle fracture. This may be dueto excessive agglomerated nanoparticles, which created stress risers and weakenedthe interaction among POM macromolecular chains.

3. POM composites with much lower concentration of nano-PTFE showed similartribological properties compared to POM modified by micro-PTFE. Moreover, thefriction coefficient of POM modified by nano-PTFE was lower and more stablethan that of POM modified by micro-PTFE, possibly due to the formation of theeven and continuous transfer films.

References

1. Benabdallah, H.S. Reciprocating sliding friction and contact stress of some thermoplastics againststeel. J. Mater. Sci. 1997, 32, 5069–5083.

2. Chen, J.Y.; Cao, Y.; Li, H.L. Investigation of the friction and wear behaviors of polyoxymethy-lene/linear low-density polyethylene/ethylene-acrylic-acid blends. Wear 2006, 260, 1342–1348.

3. Sun, L.H.; Yang, Z.G.; Li, X.H. Study on the friction and wear behavior of POM/Al2O3 nanocom-posites. Wear 2008, 264, 693–700.

4. Samyn, P. Wear transitions and stability of polyoxymethylene homopolymer in highly loadedapplications compared to small-scale testing. Tribol. Int. 2007, 40, 819–833.

5. Burris, D.L.; Boesl, B.J.; Bourne, G.R. Polymeric nanocomposites for tribological applications.Macromol. Mater. Eng. 2007, 292, 387–402.

6. Wang, Q.H.; Pei, X.Q. The influence of nanoparticle fillers on the friction and wear behavior ofpolymer matrices. Tribol. Int. Eng. Ser. 2008, 55, 62–68.

7. Chang, L.; Zhang, Z.; Zhang, H. On the sliding wear of nanoparticle filled polyamide 66 com-posites. Compos. Sci. Technol. 2006, 66, 3188–3198.

8. Burris, D.L.; Sawyer, W.G. Improved wear resistance in alumina-PTFE nanocomposites withirregular shaped nanoparticles. Wear 2006, 260, 915–918.

9. Bahadur, S. The development of transfer layers and their role in polymer tribology. Wear 2000,245, 92–99.

10. Bahadur, S.; Schwartz, C.J. The influence of nanoparticle fillers in polymer matrices on theformation and stability of transfer film during wear. Tribol. Int. Eng. Ser. 2008, 55, 17–34.

11. Wang, Q.H.; Xue, Q.J.; Shen, W.C. The friction and wear properties of nanometre SiO2 filledpolyetheretherketone. Tribol. Int. 1997, 30, 193–197.

12. Burris, D.L.; Zhao, S.; Duncan, R. A route to wear resistant PTFE via trace loadings of function-alized nanofillers. Wear 2009, 267, 653–660.

13. Bhimaraj, P.; Burris, D.L.; Action, J. Effect of matrix morphology on the wear and frictionbehavior of alumina nanoparticle/poly(ethylene) terephthalate composites. Wear 2008, 258,1437–1443.

14. Srinath, G.; Gnanamoorthy, R. Two-body abrasive wear characteristics of Nylon clay nanocom-posites: Effect of grit size, load, and sliding velocity. Mater. Sci. Eng. A 2006, 435–436, 181–186.

15. Feng, S.M.; Gu, G.S. Application of nanoparticles. Mater. Rev. 2001, 15, 29–31.16. Hasan, M.M.; Zhou, Y.X.; Mahfuz, H. Effect of SiO2 nanoparticle on thermal and tensile behavior

of nylon-6. Mater. Sci. Eng. A 2006, 429, 181.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12

1248 T. Huang et al.

17. Wang, Q.H.; Xue, Q.J.; Liu, H.W. The effect of particle size of nanometer ZrO2 on the tribologicalbehaviour of PEEK. Wear 1997, 198, 216–219.

18. Xue, Q.J.; Wang, Q.H. Wear mechanisms of polyetheretherketone composites filled with variouskinds of SiC. Wear 1997, 213, 54–58.

19. Hu, K.H.; Wang, J. Tribological properties of MoS2 nano-balls as filler in polyoxymethylene-based composite layer of three-layer self-lubrication bearing materials. Wear 2009, 266,1198–1207.

20. Rong, M.Z.; Zhang, M.Q. Irradiation graft polymerization on nano-inoranic particles: an effectivemeans to design polymer-based nanocomposites. J. Mater. Sci. Lett. 2000, 19(13), 1159–1161.

21. Cho, M.H.; Bahadur, S. Study of the tribological synergistic effects in nano CuO-fiiled andfiber-reinforced polyphenylene sulfide composites. Wear 2005, 258, 835–845.

22. Sun, L.H.; Yang, Z.G.; Li, X.H. Mechanical and tribological properties of polyoxymethylenemodified with nanoparticles and solid lubricants. Polym. Eng. Sci. 2008, 48(9), 1824–1832.

23. Wang, J.; Hu, K.H.; Xu, Y.F. Structural, thermal, and tribological properties of intercalatedpolyoxymethylene-molybdenum disulfide nanocomposites. J. Appl. Polym. Sci. 2008, 110,91–96.

24. Wacharawichanant, S.; Thongyai, S.; Phutthaphan, A. Effect of particle sizes of zinc oxide onmechanical, thermal and morphological properties of polyoxymethylene zinc oxide nanocom-posites. Polym. Test. 2008, 27, 971–976.

25. Kong, Y.; Hay, J.N. The measurement of the crystallinity of polymers by DSC. Polymer 2002,43, 3873–3878.

26. Wei, L.P.; Liu, Y.Z. Research on repeatability it of polymer viscoelastic parameters measurementsby DMA. Eng. Plast. Appl. 2007, 35–75.

27. Kontou, E.; Niaounakis, M. Thermo-mechanical properties of LLDPE/SiO2 nanocomposites.Polymer 2006, 47, 1267–1280.

28. Xu, X.M.; Li, B.J.; Lu, H.M. The interface structure of nano-SiO2PA66 composites and itsinfluence on material’s mechanical and thermal properties. Appl. Surf. Sci. 2007, 254, 1456–1462.

29. Wang, C.H. Plastic tribology-theory and practice of plastic friction: Wear and lubrication. Beijing,China: China Machine Press, 1994, p. 163.

30. Chang, L.; Zhang, Z.; Ye, L. Tribological properties of epoxy nanocomposites III. Characteristicsof transfer films. Wear 2007, 262, 699–706.

Dow

nloa

ded

by [

Fuda

n U

nive

rsity

] at

18:

12 0

9 Ja

nuar

y 20

12