Dry friction and wear characteristics of rare-earthMoSi2 composite

Tribological behavior of PTFE nanocomposite films reinforcedwith carbon nanoparticlesJeong-Yeop Lee,Dong-Phill Lim,Dae-Soon Lim*Department of Materials Science and Engineering,Korea University,Seoul 136-701,South KoreaReceived 5October 2006;accepted 22December 2006Available online 12January 2007AbstractCarbon-based nanoparticles synthesized by heat treatment of nanodiamond in the temperature range of 1000–1900°C were added to PTFE film to investigate the structural effect of the carbon particles on the tribological properties of PTFE composite film.Carbon-based nanoparticles were prepared by milling with micron sized beads in chemically treated water before their addition to PTFE film.The wear and frictional properties of PTFE nanocomposite film were measured by the ball on plate type wear test.The wear resistance of PTFE film was found to be enhanced by the addition of 2wt%of carbon nanoparticles.The wear coefficient of PTFE film was decreased from 16.2to 3.5·10À6mm 3/N m by the addition of carbon-based nanoparticles heat-treated at 1000°C.Increasing the heating temperature of the nanodiamonds caused the extent of aggregation and particle size to increase.The wear resistance of PTFE nanocomposite film was enhanced by the addition of nanodiamonds heat-treated at 1000°C,but decreased when the heat treatment temperature of carbon nano-particles was further increased.Tribological behavior of PTFE nanocomposite films depending on the types of carbon nanoparticles were explained based on the structural,physical and chemical modification of carbon nanoparticles.Ó2007Elsevier Ltd.All rights reserved.Keywords:A.Particle-reinforcement;B.Surface properties;E.Heat treatment;B.Wear;PTFE film1.IntroductionPTFE (polytetrafluoroethylene)is a well-known mate-rial which has a high potential for anti-wear applications due to its various advantages,such as its low frictional coefficient,good chemical resistance and good thermal sta-bility.However,its anti-wear applications have been some-what limited by its poor wear resistance which has led to the failure of anti-wear parts and films.Therefore,many researchers have attempted to reinforce PTFE using vari-ous fillers.Glass fibers,carbon fibers,and nonferrous metallic and ceramic powders were studied as potential fill-ers for the enhancement of the wear resistance of PTFE,but it was proven that these fillers induced a large frictional coefficient and abrasion of the fractured fiber [1–5].Recently,several researchers adopted nanometer size inor-ganic powders as a filler and demonstrated their ability to enhance the wear properties of the PTFE [6–10].However,these fillers inevitably increased the frictional coefficient.It has been shown that nanodiamond (ND)particles consti-tute very good reinforcement filler having a low friction coefficient [11].The addition of 2wt%of ND particles to PTFE was found to increase its wear resistance by 48%.ND particles consist of 4–6nm sized crystalline diamonds produced by a detonation method [12].ND particles have superior thermal conductivity and stability,excellent mechanical properties and outstanding tribological proper-ties.Nowadays,carbon-based nanoparticles containing carbon onions can be produced by the annealing of NDs and used as low friction materials [13,14].In this study,the wear coefficient and frictional coeffi-cient of a nanocomposite with PTFE and various carbon-based nanoparticles were tested,in order to examine their structural effect on the tribological properties of the PTFE1359-8368/$-see front matter Ó2007Elsevier Ltd.All rights reserved.doi:10.1016/positesb.2006.12.006*Corresponding author.Tel.:+82232903272;fax:+8229295344.E-mail address:dslim@korea.ac.kr (D.-S.Lim)./locate/compositesbComposites:Part B 38(2007)810–816film.The various forms of these carbon-based nanoparti-cles were mixed with polyacrylic acid in water and milled with300l m zirconia balls for the purpose of dispersing the agglomerates of the heat-treated particles after the heat treatment of the ND in the temperature range of1000–1900°C in an N2gas environment.2.ExperimentalThe ND powders were purchased from Shenzhen, China.They were synthesized by explosive detonation using trinitrotoluene and hexogene in an inert gas environ-ment following the procedure described in the literature [12].The ND powders were heat-treated at temperatures ranging from1000to1900°C in an N2gas environment for1h.For the dispersion of the carbon-based nanoparti-cles,we prepared a mixture of1.5g of carbon nanoparti-cles,250ml of distilled water,and 1.5g of polyacrylic acid(MW:5000,Wako Pure Chemical Ind.,Japan)as an anionic surfactant,in order to improve the dispersion and solubility of the carbon-based nanoparticles in water. The pH of the mixture was maintained at pH10using ammonia water.For attritional milling,the mixture was poured into an attritional milling machine consisting of a stainless steel vessel with a ceramic coated agitator.As the milling medium,300l m size zirconia balls(95%zirco-nia)purchased from Toray Co.in Japan,were loaded tofill 70%of the vessel.After closing the vessel,the agitator was rotated at1000rpm,which was found to be the optimum speed through practice considering the increase in temper-ature.The attritioner was alternatingly agitated for10min and cooled for5–10min,while being constantly cooled with a circulatory cooling system using chilled water.After milling,the zirconia balls were separated byfiltration,and the particle size and zeta potential of the OLC slurry were analyzed.After milling for120min,2wt%of the carbon-based nanoparticles were added to the PTFE and coated on an aluminum substrate.The thickness of the coating was 10l m.The sliding wear test was carried out using 12.7mm steel balls on a plate at room temperature under dry conditions.Fig.1shows the schematic diagram of the wear tester which was designed to evaluate the friction coefficient and wear loss.The variation of the load during the wear test was detected by a load cell and the frictional coefficient was calculated by a PC connected to the load cell of the wear tester.The sliding speed was4cm/s and the load was33.3N.The wear coefficient of the PTFEfilms containing the ND particles and heat-treated carbon nano-particles were calculated after measuring the profile of the wear track.XRD(Rigaku D/Max–2A),Raman spectros-copy(Jobin–Yvon LabRam HR),HRTEM(JEOL JEM-4010)and FTIR(PerkinElmer Co.GX spectrophotometer) were used to determine the structure of the prepared nano-particles.Surface profile analysis and FESEM(Hitachi S-4700)were used to observe the wear track and scars on the worn specimen after the wear test and the cross sections after fracturing the coating with liquid nitrogen.The wear coefficient was derived by calculating the relation between the wear volume and load.Prior to the FESEM observa-tion,the specimens were coated with Pt.3.Results and discussionFig.2shows the XRD patterns of the carbon-based nanoparticles obtained after heat treating them in the tem-perature range of25–1900°C.It can be seen that the peaks at2h=43.5°and75°correspond to the(111)and(220) planes indicating the nanodiamonds and the peaks at 2h=25.8°and43.5°correspond to the(002)and(100) planes indicating the graphite phases,respectively.The NDs retain their structure until1000°C and then begin to transform into graphitic sheets when the temperature reaches1300°C.The temperature region of1600–1900°C shows the completion of their graphitization,due to the absence of the(220)peak.The transformation is sup-ported by the Raman spectroscopy and HRTEM results, as shown at Figs.3and4,respectively.J.-Y.Lee et al./Composites:Part B38(2007)810–816811In the Raman spectra of the NDs at room temperature shown in Fig.3,the diamond peak is present at1326cmÀ1 and when they are heated to over1300°C,a‘disordered’(D)peak at1340cmÀ1and a‘graphite’(G)peak at around 1580cmÀ1are observed.Researchers believed that the rel-ative intensity of the D to G bands(I D/I G)can be used to represent the relative degree,although the quantitative value is hard to be calculated[15,16].The I D/I G decreases with increasing the temperature over1600°C relative to temperature under1300°C.Fig.4shows the HRTEM images of the heat-treated carbon-based nanoparticles.The carbon nanoparticles maintained their diamond structure at1000°C,but trans-formed into carbon onions and polyhedral carbon at 1600°C and1900°C,respectively.The non heat-treated NDs and NDs heat-treated at1000°C have several parallel layers of diamond d-spacings,as shown at Fig.4a and b, respectively.After the heat treatment at1300°C,several spherical layers were formed as the outer layer of the ND particles and the small particles under3nm changed into onion like carbon.The ND particles completely changed into spherical onions at1600°C,and as the temperature was further increased to1900°C,polyhedrons were obtained.Tomita et al.reported that the structural trans-formation of carbon is caused by the localization of the p-electrons[17].Moreover,during continued heat treat-ment at1900°C,the p-electrons are delocalized and this causes the formation of polyhedrons.Fig.5shows the friction coefficient of the PTFEfilled with the carbon nanoparticles.The friction coefficient of the PTFEfilled with the carbon nanoparticles is about 0.1,which is the same as that of pure PTFE.The low fric-tion coefficient of PTFE is caused by the tribo-film trans-ferred to the contact surface on the counterpart[6,10]. Generally,filling the PTFE with particles increases its fric-tion coefficient,because thefillers have a much higher fric-tion coefficient than the PTFE matrix and act as abrasives [1–10].However,in this experiment,the addition of carbon nanoparticles to the PTFE did not increase its friction coef-ficient,because the nanodiamonds and carbon onions behave like a solid lubricant itself having a low friction coefficient.Fig.6shows the wear coefficient of the PTFEfilled with the carbon nanoparticles.The addition of the NDs reduced the wear coefficient from16.2to5.05·10À6mm3/N m. The carbon nanoparticles heat-treated at1000°C resulted in the lowest wear coefficient(3.35·10À6mm3/N m). However,the wear coefficient of the PTFE increased when the temperature exceeded1300°C due to the partial forma-tion of carbon onions.Fig.4.HRTEM(400.0kV)images showing(a)nanocrystalline diamondparticles,(b)nanodiamond particles heat-treated at1000°C,(c)carbononions after heat treatment at1600°C and(d)polyhedral carbon structureafter heat treatment at1900°C.812J.-Y.Lee et al./Composites:Part B38(2007)810–816In order to understand the wear behaviors of the PTFE composite with the carbon-based nanoparticles,the parti-cle sizes of the NDs and carbon-based particles were mea-sured.Fig.7shows the average particle size of the NDs and carbon-based nanoparticles.An attritional mill with 300l m zirconia balls was used to attack the weak points of the intermediate aggregates and the particles were dis-persed in PAA added aqueous medium.The particle size of the carbon nanoparticles was the smallest(33nm)when the heating temperature was1000°C and increased over (300nm)when temperature is over1300°C.Kru¨ger et al. reported the dispersion of NDs in terms of the primary par-ticle size and classified the distribution of the particles into the primary particles(4.4nm),core aggregates(100nm), intermediate aggregates(2000nm),and agglomerates (20000nm)[18].They reported that the effect of milling with100l m size silica balls was to loosen the core struc-tures by producing cracks at the weak portions of the core aggregates.Of course,in the present experiment,the parti-cle size of the NDs and carbon-based nanoparticles was similar to that of the core aggregates(100nm),except for the particles heat-treated at1000°C.The best dispersion of the ND particles heat-treated at1000°C which are smal-ler than the core aggregates can be explained by the forma-tion of disorder.While ND heated at1000is completely nanocrystalline diamond to be scrutinized from XRD and HRTEM,Raman spectra showed that ND particles are disordered at1000°C.It is thus believed that the disor-der is formed in the grain boundaries of the ND at that temperature.The formation of disorder means the produc-tion of defects different from diamond structure in the grain boundaries and the resulting energy concentration results in the failure of the bonds in the grain boundaries during the attrition milling.Therefore,the heat treatment of the ND lessens the bonding strength between the grains and the ND heat-treated at1000°C has the smallest parti-cle size.The formation of bigger particles when the carbon-based nanoparticles are heated at temperatures over 1300°C can be explained from the viewpoint of the func-tional groups on the surfaces of the particles.J.-Y.Lee et al./Composites:Part B38(2007)810–816813Fig.8shows the FT-IR spectra of the NDs and carbon-based nanoparticles.In the spectra of the ND and carbon-based nanoparticles heated at 1000°C,the broad peak at around 3427cm À1is assigned to the hydroxyl group and the small peak at 2931cm À1is assigned to the alkyl group [19,20].Hydroxyl groups are easily adsorbed on NDs in air due to the many dangling bonds on the diamond structuredsurface.The signals at 1738,1630and 1125cm À1indicate the presence of carboxyl and hydroxyl groups [19,20].When the heating temperature exceeds 1300°C,the func-tionality on the surfaces of the carbon-based nanoparticles is reduced and small peaks appear at around 2100–2400cm À1indicating the presence of nitrile (C …N)and alkynyl (C …C)moieties.The termination of thedanglingFig.9.Worn surfaces of the nanocomposite of PTFE and carbon nanoparticles:(a)pure PTFE,PTFE nanocomposite with ND at (b)room temperature,(c)1000°C,(d)1300°C and (e)1900°C.The sliding direction is marked by arrows.814J.-Y.Lee et al./Composites:Part B 38(2007)810–816bonds on the ND surface during the formation of the graphite structure causes the reduction of the functionality. From the FT-IR spectra,the unheated ND and ND heated at1000°C have hydrophilic hydroxyl and carboxyl groups on their surfaces,but these hydrophilic functional groups were eliminated with further heat treatment at tempera-tures over1300°C.Even though the added PAA supplies hydrophilic carboxyl groups,the graphite-like carbon sur-face has insufficient branches to bind with hydroxyl and carboxyl groups to be adsorbed as compared to the ND. Therefore,the distribution of the carbon-based nanoparti-cles is affected by the bond strength and the surface condi-tion of the particles.The wear behavior is affected by the size effect following the influence of surface properties explained above.The reason that the particle size affects the wear behavior sensi-tively,even though the carbon particles are comparatively small,is that the carbon particles can act as abrasives.It can be inferred that the aggregation of the carbon onions and their own graphite-like structure reduce the wear resis-tance of the PTFE nanocomposite and are detrimental to the reinforcement effect.The wear resistance of the PTFE was maximized byfilling it with the ND particles heat-trea-ted at1000°C,because the smallest particles can be well dispersed in the PTFE matrix and these particles have the strongest diamond structure.However,when temperature increased over1300°C,the surface properties of the car-bon particles was changed due to the transformation of the morphology from nanodiamonds to carbon onions. Despite the structure of the carbon onions which makes them easy to fracture,it is difficult to prevent the aggrega-tion of the particles because of the weak repulsion between the particles in the aqueous medium,so that the particle size increases with increasing heat treatment temperature over1300°C.The micrographical images in Fig.9show the worn sur-faces of the PTFE with the ND and carbon-based nanopar-ticles obtained using FESEM.The worn surfaces of the PTFE nanocomposites after the wear test support the wear behaviors shown above.By adding the non heat-treated ND and ND heat-treated at1000°C,the amount of dam-age on the worn surfaces of the PTFE composites was reduced.However,when the heating temperature of the ND was further increased,the amount of damage on the worn surfaces of the PTFE with the carbon-based nano-particles increased and became similar to that on the sur-face of the pure PTFE when the temperature attained 1900°C.4.ConclusionsND particles and carbon onions were prepared by heat treatment at temperatures of up to1900°C,in order to determine their reinforcement effect on the tribological behavior of PTFE nanocomposite coatings.The enhance-ment of the tribological properties with the addition of a small amount of carbon-based nanoparticles is afforded by the uniform dispersion of the nanoparticles when the heating temperature is1000°C.At this temperature,the smallest particle size was observed,due to the decrease in the bond strength between the grains,and the particles retain their good mechanical properties,due to their role as afiller.However,when the heating temperature is increased to over1300°C,the carbon nanoparticles become aggregated and the wear coefficient of the nano-composite produced with them is increased.It was dem-onstrated that the failure of the wear behavior is caused by the aggregation of the particles,due to the formation of carbon onions.Finally,it is found that the wear behav-ior of the PTFE with the carbon nanoparticles 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