( 1997) 380-386
Dry sliding friction and wear of short carbon-fiber-reinforced polyetheretherketone (PEEK) at elevated temperatures J. Hanchi, N.S. Eiss, Jr
Abstract ‘~11spaper describes the results from an experimental investigation of the effects of operating temperatureon the dry sliding friction and wear Performance of a short carbon-fiber-reinforced (SCPR) polyetheretherketone(PEEK) composite. The sliding experiments were carded out (pna pin-ondtsc tribometer at selectedtemperaturesin the range of 2&225 ‘C. The contact configuration used was a stationary steel ball on a rotaung polymer flat. As test temperaturesincreased from below to above the glass transition temperature (T,) of the composite. wearand fnctton tmnsmons were seen to occur. These transittons were. however, opposite in oatorc. In effect. a wear intcosification was accompanied by a sharp drop III fricuon. Part~culartv interesting was the fact that in the sliding regime above Ts, SCPR-PEEK exhibited coefficients of fnctlon tha were lower than those exhiblted by neat PEEK by factors of about two to three. The tribological behavior of SCPR-PEEK is dtscossedin terms of the impact of temperatureon st~ctufe and mechanical properties. Additionally, the active friction and wear mechanisms of SCAR-PEEK across the range of test temperaturesare identtfied and dtscussedin relation to those of neat PEEK.
2.1. Materials The material used in this study was 30% (by weight) short carbon-fiber-reinforced
PEEK 380-G. The resin was supplied by ICI in pellet form. Flash-gated plaques measuring 76 X 76 X &iced
machine. The processing temperatures were 360 ‘C zone),
380 “C (middle
390 “C (nozzle).
390 “C (front
Prior to injection molding, the SCFR-PEEK
pellets were dried at 150 “C for 24 h m a vacuum oven. 2.2. Dynamic mechanical them& analysis Dynamic
thermal analysis (DMTA)
was performed on SCFR-PEEK mally activated molecular
in order to investigate tber-
processes and their
Impact on mechanical properties. Mechanical loss factor (tan S) and dynamic modulus were continuously measuredduring a DMTA of 20-225
thermoscan cartiedout witbin the temperatmerange “C. A small rectangular specimen machined from
the injection molded plaques of SCFR-PEEK 15 X 3 X 0J43-16Jll97lS17
00 C 1997
ElsevlerSctrrace S A All ngbtsreserved
was used as DMTA
test sample. DMTA
1. Hanchr. N.S. EMS. Jr/ Wear 203-204 (1997) 385386
measurementswereperformedataheatingrateof3 and a deformation frequency of
The selected test con-
figuration and operating mode were, respectively, singlecantilever
was conducted on a Polymer Laboratories
dynamic mechanical thermal analyzer. 2.3. X-ray diffraction Wide-angle
was used to qual-
natively investigate the development of crystallinity. WAXD scattering patterns for selected SCFR-PEEK
generated and recorded using a fully automatedscintag
2000 X-ray diffractometer.
.4 step scan interval of28=
and a scanning speed of 0.02 (‘20)
s- ’ were selected.
radiation was used.
I Schemancrepresentanon of wear rracksgenerated&err (a) no plasticplowmgoccurs:(b) no removalof matenatoccurs;and tc) both plasticplowmgandremovalof nmenal occur viscoelastic deformation undergone by the test polymersduring frictional interaction. This was accomplished through the evaluation
[ (h,- H)/It,)],with
ttvely, the wear track depths measured from the profilometry traces and at 3000 revolutions.
for reference in
Fig. 2 are typical friction and wear traces as well as the test parameters measured in this study.
3. Results and discussion 3.1. Dynamic mechanical thermat analysis Curves
dynamtc modulus and the mechanical loss factor (tan 6) for as-molded SCFR-PEEK
are shown in Fig. 3; corresponding
curves for as-molded PEEK 7 1 are also shown for reference. Based on the location of the peaks in the tan 8 curves shown in Fig. 3(a). it is apparent that the incorporation of short carbon fibers into PEEK resulted in an I I “C rise m glass transition temperature (T,) from 148 to 159 “C [X6]. In additton, as can be seen in Fig. 3(b) , carbon-fiber remforcement led to an appreciable ntcrease in stiffness. in particular at temperatures below Ts. The considerable difference m intensny between the glass transitions of SCFR and neat PEEK, as evidenced by the difference in height of the tan S peaks in Fig. 3(a). suggested that as-molded PEEK was
I Hmchl. N S Dss. Jr/Wear
203-204 (1997) 3X0-386
amomhous when reinforced and semictvstalline when neat. WAXD scans performed on as-molded specimens of SCFR as well as neat PEEK corroborated this observation. In effect, diffraction patterns in which no crystalline reflections (peaks) were apparent (Fig. 4) attested to the amorphous nature of as-molded SCFR-PEEK. Coupled to its amorphous nature, it may be also noted SCFR-PEEK undergoes a more pronounced drop in modulus, or thermal softening, at the Ts Finally, it is important than its neat counterpart [Fig. 3(b) to point out that the rise in modulus at about 175 “C seen for SCFR-PEEK in Fig. 3(b) evidences the ability of the amorphous matrix of the composite to cold-crystallize at temperatures above Ts 81.
3.2. Tribological evaluation Presented in Fig. 5 are plots showing the temperature dependence of the steady-state coefficient of friction, ctmmlative wear track depth and recovery parameter for SCFRPEEK. For comparative purposes,correspondingdata for neat PEEK 7) are also shown in Fig. 5. It can be seen that at temperatures below Ts. in spite of a wear performance comparable to that of its unreinforced counterpart, SCFR'-*L- --%iecs?h~~~lreat PEEK. PZEKZxhiiiiiZd :ii&~ifIh~~~..,,,,.. Inspection of recovery parameter data [Fig. S(c) together with profilometry traces of wear tracn cross-sections (Figs. 6 and 7) revealed that below Tg. viscoelastic plowing and removal of material in the case of SCFR-PEEK, and viscoelasts-plastic plowing in the case of neat PEEK were the main dissipative processes giving rise to friction and wear during &ding. Thus, the higher coefficients of friction observed for SCFR-PEEK appear to have been derived from the activation of fracture as an interfacial energy dissipation mechanism during the sliding of the composite. In the vicinity of T,, Cxrmal softening associated with the glass transition brought about a nottceable deterioration of
Jr/ Wear 203-204
N S Elrr.
Jr / Wear 203-205
PEEK. was played by a layer of agglomerated wear debris (Fig. 8). cold-crystallized (Fig. 9) and highly strained in the direction of sliding, which covered the wear tracks. Layers of agglomerated wear debris were seen to form at all the temperatures at which SCFR-PEEK was evaluated.
N S Elrs.
Jr / Wear 203-204
However, given the fact that molecular mobility in the glassy state is essentially null, it is highly unlikely that, under the action of the surface tractions, the layers formed at temperatures belaw Ts attained the same degree of orientational anisotropy as those attained by the layers formed at temperatures above T,, wberemolecularmobility isgreatlyenhanced [ IO]. It is believed that a marked difference in degree of orientational anisotropy between the layers formedbelow andabove Ts was the underlying factor giving rise to the friction transition observedfor SCFR-PEEK. Worth noting at this point is that a high degree of molecular orientation in the direction of sliding can lead to decreasesin coefficients of friction as a result of less energy being dissipated in plastic reorientation of molecules Finally, the fact that SCFR-PEEK exhibited lower friction than neat PEEK in the sliding regime above Tg is ascribable to an improved resistance to bulk viscoelastic deformation on the part of the composite. Also attributable to the latter is the improved high-temperature wearperformante of SCFR-PEEK with respect to its neat counterpart. as seen in Fig. 5(b).
f 1997) 38&386
with the processing investigation.
of the test specimens
References [ 11 VtcrrexPEEK Pmp.wes andPmcesstng. Referenceno VKl01069t). ICI Advawed Mater&s. Exton.PA. 1990 121 B 1. Bnscw L H Yao and T Stol~kt. The fnctmn aad wear of polytteuaff~~hylene)-polytetherrtherketone) ccqms,fes. aa m~oalappua~salof II!Z qnmwn composmcm. in K C L&ma (ed ).
Wearufh4rrfenols. ASME. New York. 1985.pi 725-741 131K Fnednch.J Karger-Kocrtsaad2 Lu. Effectsof sfwi cow&&ace ro”8”nessa”d trmperatureon the fnctm,, and wear of PEEK composttesunderdry sbdmgconditmns. Weor. 148 ( 1991) 235-247 141 Z Lu andK Fnednch.On slldq fnclmn andwe;arof PEEK andtts composaes. Wear. IRI-It?3 ( 1995) w-631 [51 R E Wetton.Dynamicmecharucal them& analyasof polynx~sand r&&d systems.m J V Dawkms (ed ). Developmentc m Polymer Chamcrerrw~run. Elsevter. London. 1986 [5J P Gradm. PG Howgate.R. Se&en and R A. Brown, Dywmcmtxhvllwl pmpemes.m C Allen and JC Bevmgton (e&s). Cnmprchenrrve Scrence. Vol 2. Pergamon. Oxford. 1989. pp
J Hanchl and N S Ens. Tnb&qcat bebavror of polyexretherketone.a thermotmp~c lqurd crystallmepolymer.and m-s&~ compwtes basedon tbw bknds under dry sbdm8 condawnsat elevatedtempratures. Wear. 2CfJ ( 19%) 105-121 181 A D’Amore. J M Kenny. L. Nicolats and V Twct. Dynamrcmechvlncalzmddlelectnccharrctenzat~on of PEEK crystalbzat~on. Pdym Engng Set ( 1990) 314-320  V A Belyt. A I Swndenok. M.I. Petmkovetsand V G. Savkm. Fncrronund Wear of Murermls. PerSamcm.Oxford, 1982 [ 101 N G M&urn. C P Buck!ey and C B Bucknzdl. Pnncrpkr of Polymer Engmenn~. Oxford Unvzstty Press.Oxford. 1988 I Hombogenillld K. Schfer. Fncuon and wear 0; tbxawplullc polymm. I” D A Rqney (ed ). Fundamenrak of Fnchrm and War <,f,W,reno/~. ASM. Metal Park. Ohm. 1981. pp 39438 
( 1) Viscoelastic plowing and removal of material appeared to be the main dissipative processes giving rise to friction and wear during the sliding of SCFR-PEEK, as seen in the present study. (2) T6 marked the onset of wear and friction transitions in SCFR-PEEK. As operating temperatures increased from below to above T., the wear performance of the composite was seen to deteriorate. Friction, on the other hand, was seen to decrease appreciably. (3) In the sliding regime above Ts, the fricnon and wear performance of SCFR-PEEK was found to be significantly better than that of neat PEEK. The improved tribological performance of the composite is ascribable to two factors. One is the effectiveness of the reinforcing fibers in terms of counteracting the effects of thermal softening at and above Tr The other is the formation of a strong. cold-crystallized and presumably highly anisotropic layer of agglomerated wear dcbns. which covered the wear tracks and effectively protected the regions beneath the contact interface agamst disruption and extensive deformation.
Acknowledgements The research reported in this paper was supported by the NSF Science and Technology Center for High Performance Polymeric Adhesives and Composites at Vugmia Tech through grant no. DMR-9120004. The authors would ltke to thank Prof. DC. Baird and Dr P. de Souza of the Chemical Engineering Department at VirginiaTech for their assistance
Biographies Norman S. Eiss Jr IS the George R. Goodson Professor of Mechanical Engineertng at VirginiaTech. He hasMechamca1 Engineermg degrees from Rensselaer Polytechnic Institute (B.M.E.) andComellUmverstty (M.S.andPh.D.).Hehas Industrial experience wnh DuPont andCone Aeronautical Laboratory ( 7 years). Since Jointng the faculty at Virgmia Tech m 1966. Dr Eiss has taught kinematics, machme design and upper division and graduate tribology courses. Doctor Eiss’s research uxerests include: friction and wear of polymers. noise caused by frictton instabilities and surface roughness charactenzation. He IS a Fellow of the Society of Tnbologists and Lubrication Engmeers (SHE). Editor-m-Chief for STLE, and was awarded the Society’s highest award, the National Award in 199 I.
( I yec~)
Jorge Hanchi holds graduate
degrees m Mechamcal Engineenng from the University ofMinnesota, TwmCities (MS , 1991) and Vxgmra Tech (Ph.D., 1995). As a doctoral stu-
dent hc conducted research on the tnhology of htgh-temper-
nmse and vibratmn
awre thermoplastic composites. Currently. he IS a Research
nents. He IS a member of the American Society ofMechanical
Assoctate in the Mechamcal
Engineering Department al Vlr-
guua Tech and IS engaged in research on frxrion-induced
and the Society of Tribolog~sts