An investigation on the friction of oriented polytetrafluoroethylene (PTFE)

Wear262(2007)1414–1418An investigation on the friction of oriented polytetrafluoroethylene(PTFE) Xiao-Xia Liu,Tong-Sheng Li∗,Xu-Jun Liu,Ren-Guo Lv,Pei-Hong CongKey Laboratory of Molecular Engineering of Polymers,Ministry of Education,Department of Macromolecular Science,Fudan University,Shanghai200433,PR ChinaReceived6April2006;received in revised form25November2006;accepted9January2007Available online12February2007AbstractMeasurements of the friction coefficient of oriented polytetrafluoroethylene(PTFE)have been made and the results indicate that the friction coefficient of drawn PTFE is dependent on the oriented direction.The worn surfaces were observed using an atomic force microscopy(AFM).The crystallinity of the transferfilm was determined using differential scanning calorimeter(DSC).Friction orientation was found to be an important character of PTFE tribology.The friction-induced orientation occurred on the worn surfaces and the transferfilms with high crystallinity were formed on the counterparts,which was one of the causes for the relatively steady friction coefficient in the sliding process.However,the formation of the friction-induced orientation on the worn surfaces and the transferfilm did not change the anisotropy of the friction coefficient for drawn PTFE under our experimental conditions.Although this result meant that the bulk-oriented structures of PTFE play an important role on its friction processes,the formation of better friction-induced orientation could result in the lower friction coefficient.©2007Elsevier B.V.All rights reserved.Keywords:Polytetrafluoroethylene(PTFE);Drawing;Orientation;Friction coefficient1.IntroductionIt has been reported that the molecular chains of linear poly-mer are oriented easily along draw direction,and the oriented structures influence the tribological properties[1].For example, the friction coefficient of polypropylenefiber increased with increasing draw ratio[2].In the case of doubly oriented Nylon 6[3],the wear rate perpendicular to the draw direction was somewhat lower than that parallel with the draw direction.This phenomenon was explained by more energy input to the sur-face via friction motion,which was necessary for molecular reorientation transverse to the draw direction prior to creation of wear debris.We found that polytetrafluoroethylene(PTFE) drawn at its melting point forming more regularfibril structure [4]showed good tribological properties.Pooly,Tabor and co-workers[5–11]attributed the low coefficient of dynamic friction of PTFE to the sliding of an oriented slider.Similar observa-tions for polyethylene(PE)were also reported[5].A study on the worn surfaces of ultra-high molecular weight polyethylene (UHMWPE)also found the friction-induced orientation[12,13].∗Corresponding author.Tel.:+862165642199;fax:+862165640293.E-mail address:lits@(T.-S.Li).Pooly and Tabor attributed this common phenomenon to the ‘smooth’molecular profile of both polymers[5].The static friction was accompanied by transfer of the polymer lumps in several hundred angstroms thick.While sliding,the transfer was in the form of an extremely thinfilm(PTFE)or discrete streaks less than10nm thick(PE)[5].Fusaro[14]noticed that the orien-tation process was time-dependent.Thin belt-like andfiber-like debris of PTFE was observed during sliding process at high tem-peratures.This result indicated that the debris oriented due to friction-induced process[15].Tanaka and Miyata[16]found that the static friction of PTFE was sensitive to pre-rubbing direction of friction surface.It is well accepted that orientation is one of important factors to influence the tribological properties of polymers.However, many important questions in thisfield remain,for example,lit-tle attention has been paid on the relationships among the bulk orientation,friction-induced orientation and the orientation of the transferfilm.In this study,PTFE which is crystallizable and easily oriented was selected as bulk material.The dependence of friction coefficient on oriented direction of drawn PTFE was investigated using a pin-on-disc tester.The analyses of differ-ential scanning calorimeter(DSC),Raman,and atomic force microscopy(AFM)were carried out to give insight into the mechanism how the orientation influences the friction processes0043-1648/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.wear.2007.01.021X.-X.Liu et al./Wear 262(2007)1414–14181415of PTFE.The study will give basic knowledge to understand the tribological behavior of polymer materials,which is very important for the application of polymer materials in industrial fields.2.Experimental2.1.Preparation for samplesPTFE powder was charged into a die and moulded under 30MPa at room temperature.The pressure was held for 2min and was released to atmospheric pressure.The moulded sticks were sintered at 375◦C for about 2h,and then cooled slowly to room temperature.The sintered PTFE was drawn at its melt-ing point of 327◦C using a stretching apparatus.The elongated PTFE was cooled in air to room temperature upon reaching the draw ratio of 4.2.2.Friction testsFriction tests were conducted using a pin-on-disc tester and the schematic diagram of the tribometer is shown in Fig.1.A stainless steel disk (1Cr18Ni9Ti)was used as the counter-part.The diameter of the counterpart is 20mm.PTFE pin is 2.0mm ×1.4mm ×2.5mm.Sliding tests were performed under ambient conditions (temperature:25◦C,humidity:50±10%)at a sliding velocity of 0.42m/s and a normal load of 1.96N.The friction coefficient was recorded automatically by an attached computer.Before each test,the disc surfaces were polished with metallographic abrasive paper to surface roughness Ra of 0.12␮m.Both pin and disc specimens were cleaned in acetone using an ultrasonic bath and then thoroughly dried.The sliding tests of the drawn PTFE were carried out in three different sliding directions.Fig.2shows the draw and friction directions of the specimens.P denotes the sliding direction paral-lel with the draw direction.AP is the sliding direction transverse to the draw direction but also lying in the draw plane.N indicates the end plane,on which the sliding direction is perpendicular to the draw direction.Undrawn PTFE,drawn PTFE and transfer film was expressed as Un,Dr and tf,respectively,in thispaper.Fig.1.Schematic diagram of thetribometer.Fig.2.Schematic diagram of the friction directions for drawn PTFE (P stands for draw direction).2.3.AFM,DSC and Raman analysesWorn surfaces of PTFE were observed using AFM (Nanoscope IV ,USA Digital Instruments Co.)in contact mode.Contact-mode images were obtained in air with silicon nitride V-shaped cantilevers.The crystallinity of samples was detected using DSC (DSC204,NETZSCH Co.).The samples (2∼3mg)were stacked in aluminum pans with pierced lid.The measurements were con-ducted under nitrogen atmosphere at a heating rate of 10◦C/min up to 380◦C.Raman spectra were obtained using Raman spectrometer (LabRam-1B,France Dilor Co.)with a resolution of 2cm −1from 200to 1500cm −1.Polarized He–Ne laser of 632.81nm was used.Two successive measurements were performed:polar-ized light parallel with or perpendicular to the length direction of the samples.In this paper,the draw direction was set as the length direction for drawn PTFE samples.3.Results and discussion 3.1.Raman analysesFig.3shows Raman spectra of undrawn (a)and drawn PTFE (b)bulks.All the spectra are normalized to the intensity of the band at 734cm −1wavelength,which can be attributed to C C stretching vibration,because its intensity is irrelevant to the direction of polarized light.For undrawn PTFE,no obvi-ous difference in the relative intensity of the peaks at the same wave number are observed (Fig.3a),whatever in the direction parallel with or perpendicular to polarized lights.In the case of drawn PTFE,the relative intensity of the peak at 1383cm −1of C–F stretching vibration is markedly stronger in the direction parallel with polarized light than that perpendicular to polarized light.This result indicates that the molecular chains are highly oriented along draw direction for drawn bined with the morphology of fractured surfaces for undrawn and drawn PTFE in our previous paper [4],it can be inferred that the drawn PTFE is highly oriented along the draw direction.3.2.Friction propertiesThe friction coefficient of PTFE specimens is investigated to understand the effect of bulk-oriented structure on the friction1416X.-X.Liu et al./Wear 262(2007)1414–1418Fig.3.Raman spectra of undrawn (a)and drawn (b)PTFE bulks.processes of drawn PTFE.Fig.4shows the friction coefficient of drawn PTFE at different sliding directions with increasing slid-ing time.The data of undrawn PTFE are also presented in Fig.4.It is noteworthy that the friction coefficient is dependent onslid-Fig.4.Friction coefficient vs.sliding time for undrawn PTFE and drawn PTFE at different slidingdirections.Fig.5.Crystallinity of undrawn and drawn PTFE bulks and their transfer films.ing direction,which shows higher values at N and AP directions.These values are higher than that of undrawn PTFE.The friction coefficient at P direction that is parallel with the draw direc-tion is the lowest,which is even lower than that of undrawn PTFE.That is to say,the friction coefficient of PTFE changing with sliding direction,has the following order:P <Un <AP <N.It is also noted that,whatever the samples are,the fric-tion coefficients are relatively stable during the entire sliding process.It is well known that a certain degree of orderly struc-ture must exist at the sliding interface if low friction occurs [5–11].Fig.4shows that the friction coefficient of undrawn PTFE is smaller than that of drawn PTFE at AP and N direc-tions.The result implies that the friction-induced orientation at interface is formed more easily for undrawn PTFE than the drawn PTFE when sliding at AP and N directions.The friction-induced orientation is formed very easily when the slid-ing direction is the same as the arrangement direction of fibrils formed by drawing,which results in the quite small friction coefficient.3.3.DSC analysesThe transfer films formed on the counterpart were collected and analyzed using DSC to explore the structure of transfer substance from undrawn and drawn PTFE.Fig.5shows the analytical results of transfer films and bulk PTFE.The crys-tallinity of bulk drawn PTFE is higher than that of undrawn PTFE by about 20%.It is noteworthy that the crystallinity of all the transfer films is similar and higher than that of bulk drawn PTFE by about 10%.The above results indicate that the draw orientation or friction-induced orientation can increase the crys-tallinity of PTFE.Moreover,the crystallinity of all the transfer films is higher than those of bulks,no matter they are undrawn or drawn PTFE.The result clearly confirms that the crystallinity of the transfer films is high.The formation of transfer films with high crystallinity is one of the causes for low and stable friction coefficient.X.-X.Liu et al./Wear262(2007)1414–14181417Fig.6.(a–d)AFM images of worn surfaces for undrawn PTFE and drawn PTFE at different sliding directions.3.4.AFM analytical resultsFig.6is AFM images of worn surfaces of the undrawn and drawn PTFE.Sliding traces and originally structural charac-ter of undrawn and drawn PTFE can be observed on all the worn surfaces.However,it is clear that the degree of friction-induced orientation on these worn surfaces is different.For the worn surface of AP direction(Fig.6c),the reorientedfib-rils by friction force can be clearly observed,and its direction is transverse to thefibrils formed by drawing.For N direc-tion(Fig.6d),the reorientedfibrils formed by friction force along friction direction are pulled out from the end of the fibrils formed by drawing.The order on the worn surface of undrawn PTFE(Fig.6a)which is worse than that of P direc-tion(Fig.6b)is observed.When the sliding direction is P direction,a better smoothing worn surface occurs(Fig.6b). It can be confirmed that the better friction-induced orienta-tion on the worn surface of P direction is produced because the sliding direction is the same as orientedfibrils formed by drawing.The above AFM observations provide clear evidence on the anisotropy of the friction coefficient of drawn PTFE shown in Fig.4.In the case of P direction,the shear force is parallel with the orientedfibrils,and a steady worn surface is easily formed. The lowest friction coefficient in P direction could be explained based on the formation of steady and smoothing worn surface observed by AFM.4.Conclusions(1)The friction coefficient of drawn PTFE was dependent onthe oriented direction,which showed the following order: P<Un<AP<N.(2)The oriented structure formed by drawing markedly influ-enced the friction processes of PTFE.This influence was not changed by the formation of friction-induced orientation and transferfilm at sliding interface.However,the formation of friction-induced orientation and transferfilm was one of the causes for stable friction coefficient.(3)Friction orientation was an important character of PTFE tri-bology.The formation of better friction-induced orientation resulted in the lower friction coefficient. AcknowledgementsThe authors would like to express their appreciation to the Key Scientific and Technological Project(Contract No.0204) from the Ministry of Education,PR China forfinancial aid.The authors are also thankful to Shanghai R&D Center for Polymeric Materials for carrying out part of the research work in this paper.1418X.-X.Liu et al./Wear262(2007)1414–1418References[1]Y.Yamaguch,Tribology of Plastic Materials,Tribology Series,Elsevier,New York,1990,p.16.[2]B.S.Gupta,Y.E.El-Mogahzy,The effect of hot-wet draw ratio on thecoefficient of friction of wet-spun acrylic yarns,J.Appl.Polym.Sci.38 (1989)899.[3]H.V oss,J.H.Magill,K.Friedric,On the wear behavior ofdoubly-oriented polymers.Nylon6,J.Appl.Polym.Sci.33(1987) 1745.[4]X.X.Liu,T.S.Li,X.J.Liu,R.G.Lv,Study on tribological properties of poly-tetrafluoroethylene drawn uniaxially at different temperature,Macromol.Mater.Eng.290(2005)172.[5]C.M.Pooley,D.Tabor,Friction and molecular structure.The behav-ior of some thermoplastics,Proc.R.Soc.(Lond.)Ser.A329(1972) 251.[6]C.W.Bunn,E.R.Howells,Structures of molecules and crystals offluoro-carbons,Nature174(1954)549.[7]V.A.Bely,V.G.Savkin,A.I.Sviridyonok,Effect of structure on polymerfriction,Wear18(1971)11.[8]C.J.Speerschneider,C.H.Li,A correlation of mechanical properties andmicrostructure of PTFE at various temperatures,J.Appl.Phys.34(1963) 3004.[9]K.M.Sinnott,Mechanical relaxations in single crystals of polyethylene,J.Polym.Sci.14(1966)141.[10]K.R.Makinson,D.Tabor,The friction and transfer of PTFE,Proc.R.Soc.(Lond.)281(1964)49.[11]S.M.Wecker,T.Davidson,D.W.Baker,Preferred orientation of crystallitesin uniaxially deformed PTFE,J.Appl.Phys.43(1972)4344.[12]A.Wang,D.Sun,S.-S.Yau,B.Edwards,M.Sokol,A.Essner,V.Polineni,C.Stark,J.Dumbelton,Orientation softening in the deformation and wearof ultra-high polyethylene,Wear203–204(1997)230.[13]A.Wang,A unified theory of wear for ultra-high molecular weightpolyethylene in multidirectional sliding,Wear248(2001)38.[14]R.L.Fusaro,Molecular relaxations,molecular orientation and the frictioncharacteristics of polyimidefilms,ASLE Trans.20(1975)1.[15]T.S.Li,S.M.Sun,T.Y.Hu,X.L.Li,Study on the wear mechanism ofpolytetrafluoroethylene,Tribology12(1992)222(in Chinese).[16]K.Tanaka,T.Miyata,Studies on the friction and transfer of semicrystallinepolymers,Wear41(1977)383.。