On sliding friction and wear of PEEK and its composites

On sliding friction and wear of PEEK and its composites

WEAR ELSEVIER Wear 181-183 (1995) 624-631 On sliding friction and wear of PEEK and its composites Z.P. Lu, K. Friedrich Institute for Composite Mat...

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Wear 181-183 (1995) 624-631

On sliding friction and wear of PEEK and its composites Z.P. Lu, K. Friedrich Institute for Composite

Materials Ltd,

University of Kaiserslautem,

67663 Kaisedautem,


Received 11 May 1994; accepted 11 October 1994

Abstract The friction and wear behaviour of different molecular weight virgin poly(ether ether)ketones (PEEK) and of PEEK blends with polytetrafluoroethylene (PTPE), as well as of PEEK composites with short carbon fibres was studied under dry sliding conditions against hard steel on a pin-on-disc apparatus. As characterised by the melt viscosity, and similar crystallinities after heat treatment! the high molecular weight PEEK had a better wear resistance than low molecular weight PEEK. The effects were greatest when the pv-level was high. Opposite to the wear rate, the coefficient of friction was not clearly affected by the pv combination term. The spherulite size and hardness of the PEEK play an important role in both friction and wear performance. Concerning the friction and wear properties of PEEK-PTPE blends, the inclusion of PTPE reduced the friction of PEEK. The lowest friction coefficient was measured for the blend with a PTPE volume fraction of 15%. The wear rates of the blends with PTPE volume fraction varying from S-85% were lower than that of virgin PEEK. A minimum value in wear rate existed at a PTPE volume fraction of 5%. Wear of the PEEK-composites was always lower than for the neat PEEK matrix, but was increased with increasing temperature. ,v.&Fi,-~~nt th,wc.a ~.,.a.= .a” r\e.t;m,,m ,.,xntnnt r\f fihva. ot .,ha..t ln .ml UL C.,r m:n;m.,m fr:rti,,n I”1 llll‘llll‘“lll Il‘~U”11 WCIII~.CII& Ln‘rlr “LlP CL.‘ “puIBJ”~u b”II%CII& “I 11”lGO c&s LI”“UI A” “Vi. I”. Keywords:


Short carbon fibre; Polymer blend; Thermoplastic composite; Sliding

1. Introduction

High performance materials, such as poly(ether ether)ketone (PEEK) and its composites are becoming more widely used as bearing and slider materials [l]. The friction and wear properties of these materials, therefore, become of greater interest and importance, which in turn encourages research and development efforts in this iieid. PEEK is a high performance, semicrystalline thermoplastic. Its relatively stiff backbone gives excellent high-temperature stability. It has a high glass transition temperature and high melting point and a high continuous setvice temperature with the advantages of easy processability by injection mouldjng and other techniques common to thermoplastic polymers [2-4]. Many

investigations on the friction and wear properties of PEEK and its composites have been carried out. Cirino et al. [5] reported of the abrasive wear behaviour, Voss et al. [6] investigated the sliding as well as abrasive wear behaviour at room temperature, and Friedrich et al. [7] examined effects of the counterface roughness and te_mnemt~re frirtinn _‘__~ reviewed __ .__.. _=--- *___ nn ___ __* ______ and weari Thev

0043-1648/95/$09.500 1995 Elsevier Science S.A. All rights reserved SSDI 0043-1648(94)07051-2

also the tribological properties of PEEK materials for sliding applications [8]. Briscoe et al. [9] described the friction and wear of PEEK-PTFE blends over a wide composition range under several testing conditions. Schelling and Kausch [lo] discussed the effect of crystallinity of PEEK on the friction and wear in both reciprocating and sliding dry friction tests. Friction and wear data of different molecular weight virgin poly(ether ether)ketones (PEEK) and of PEEK blends with polytetrafluoroethylene (PTFE), as well as of PEEK composites with short carbon fibres are presented here. In particular, it was of interest how friction coefficient and specific wear rate are influenced under dry sliding conditions against hard steel.

2. Experimental


2.1. Materials The experiments were carried out with unfilled PEEK (Table 1) and with PTFE- or short fibre-filled PEEK versions 21. -All -_-_-__- (Table -- these ____L_mat&& \-----,--- of

were delivered

Z.P. Ly Table

K. Friedrich / Wear 181-183

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Neat PEEK with different Code No.

Mp2/3 Mp2/5 Mp2/6 Mp2t7 Mp2/8 Mp2/10 Mp2i26


Melt viscosity



and its annealing

(kN s m-‘)

0.174 0.226 0.293 0.419 0.439 0.622 0.625






265 270 275 282 285 290 290


1.304 1.304 1.304 1.303 1.302 1.300 1.301



X’, a

X”b E

3s 34 33 33 34 32 32

30 30 30 29 28 27 28

’ X’,, by DSC-measurement. ’ X”,, by density method.

Table 2 Composition

and properties

of the PEEK composites





of filler (vol.%)


PEEK-1SOPF RX-L-89-338 RX-L-89-339 RX-L-89-340 RX-L-89-341 RX-G89-342 RX-L89-343 PTFE-7C RX-G90666 RX-L-90667 RX-L-90668 RX-L-90669

5 of PRFE 15 of PTFE 40 of PTFE 60 of PTFE 85 of PTFE 95 of PTFE 100 of PTFE 5 of carbon fiber 10 of carbon fiber 20 of carbon fiber 30 of carbon fiber

as injection-moulded plates or bars, except that the PEEK-PTFE blends with more than 40 vol.% of PTFE were delivered as compression-moulded discs. The cross section of the test samples cut from those plates amounted to 20 x 25 mm2 and is equal to the apparent contact area in the sliding experiments. It is known for semicrystalline thermoplastics that the bulk mechanical properties are dependent on the morphology of the material, such as crystallinity and spherulite size [11,12]. In order to get the same crystallinity effects on friction and wear, all the materials used here were annealed before wear testing. In order to control the same degree of crystallinity (30%) in the neat PEEK samples of various molecular weight, different annealing temperatures were determined. The time of annealing was always between 60 and 70 min. After annealing the density of the samples was measured. According to the relationship between crystallinity and density for PEEK (Fig. 1 [2]), this led to degrees of crystallinity prior to wear testing as shown in Table 1. In a similar way, Table 2 illustrates the relationship between density and volume fraction of PTFE in the PEEK/PTFE-blends. The annealing treatment for PEEK composites with short carbon fibre was


(g cmm3)


1.30 1.33 1.3s 1.59 1.80 2.04 2.12 2.13 1.33 1.35 1.39 1.43

28 2s 21 11 6 5 4 3 36 39 42 50



I 0


I 40

I 60

Crystallinity Fig. 1. Relationship


of crystallinity



. 100


and density of PEEK [2].

performed at 320 “C for 1.5 h followed by an isothermal heat treatment at 260 “C for 50 h which also caused an increase in crystallinity [13]. This results in an increase in the material’s hardness without reducing the toughness (expressed in terms of the product of strength and strain to failure). In addition, the coefficient of friction remained almost the same. In order to reduce the time of “running-in” period, specimens were “pre-worn” on grinding paper (Grid


Z.P. Ly

IC Friedrich I Wear 181-183

800) prior to the actual sliding wear testing. With this method the roughness of the specimens before testing was always the same, and so was the apparent area of contact with the steel ring because of better parallelism between the two mating surfaces.

2.2. Sliding wear testing procedure

Sliding wear studies of polymer composites against steel counterparts are usually carried out at different pressure and velocity conditions. The latter can be maintained by the use of rotating steel rings against which polymer composite pins are pressed by the use of a dead weight loading device. Determination of the wear rates is usually performed under multiple pass sliding conditions, in a time regime where the system performs under steady state sliding conditions. This regime is reached after a running-in period during which fibres in the composite usually smoothen the rough profile of the steel counter-face due to their abrasiveness. At the same time a transfer film is formed on the surface of the steel counterpart. Both effects result quite often, but not necessarily, in a lower slope of the wear volume versus time curve in the steady state regime relative to the initial testing period [14-161. Besides these effects, the actual wear rate achieved is dependent on the initial roughness, R,, and the counterpart material itself [17]. In this paper the roughness of the steel rings was R, = 0.2-0.3 ,um, the corresponding R, values were 2.5-3.5 pm. The evaluation of the friction and wear performance of materials at temperatures below and above room temperature is possible by using a pin-on-disc type testing machine, which allows to run pins of the material to be tested against stationary steel rings (100 Cr 6, HRC 62) in a temperature chamber. The temperature of the steel rings, as measured in a distance of 1 mm beneath the direct contact surface of the two partners, can be controlled by a cooling or heating device. For high temperature studies it is of great importance to .allow the system to adjust to an equilibrium condition, so that the measured wear rates (given by changes in the ‘specimen height) are not wrongly affected by ‘thermal expansion effects of the specimens in the warming-up period. Continuous monitoring of the torque allows to determine the coefficient of friction, p. In this work three or more samples with an apparent. contact area-‘of about, 4x5 m2 for each test conditions were used and the average value with scatter of less than 26% were obtained, Further design details, of the wear testing procedures were published in [7,18-201. Worn surface of the materials were investigated, after gold sputtering, by scanning. electron microscopy (SEM) using a JSM 5400 microscope.

(1995) 624-631

3. Results and discussion 3.1. Effect of molecular weight It is well known that molecular weight can be characterised by melt viscosity for a polymer. the molecular weight. The PEEK used in our work have the values of melt viscosity from 0.174 to 0.625 (kN s me2) the melt viscosity, meaning the weight average molecular weight (&,) between 14 000 and 56000 [21]. After annealing DSC scans at 10 “C mine1 was carried out and are shown in Fig, 2. The annealed materials, curves 2-6, have the similar general endothermic response seen in annealing study made by Cebe [22], consisting of a small endotherm just above the treatment temperature, and the high-temperature endotherm whose position increases slightly with decreasing of the molecular weight. The degrees of crystallinity measured by density measurement as well with the DSC crystallinity based on the measured melt enthalpy of the samples remain constant, although the spherulite perfection of the PEEK used here with different treatment temperature can be not identical [23]. With regard to the effect of molecular weight on friction and wear, PEEK of different molecular weights, after heat treatment, were worn under different conditions i.e. under loads varying from 0.2-S MPa and sliding velocities from l-3 m s-l, at ambient temperature. In this case, the temperature of the steel rings was under the control of the cooling device at 20-25 “C. Fig. 3 shows the effect of melt viscosity (and therefore molecular weight) on the specific wear rate at a sliding velocity of 1 m s-l and different apparent contact pressures. Under these conditions there was a value of molecular weight below which the specific wear rate remained almost constant for a certain load. Above this value of molecular weight, however, the wear rate decreased with increasing molecular weight; this effect

0,626 0,622 0,439 0,293 0,226 0,174


(as- received) (annealed on (annealed on (annealed on (annealed on (annealed on


290°C) 265°C) 275°C) 270°C) 265°C)


Temperature Fig. 2. DSC-thermograms weight after annealing.

of the PEEK


[“Cl with



Z.P. Ly

K. Friedrich I Wear 181-183

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~Melt Viscosity z

[kN s






‘9 0 c







s E ? 0 c











Melt Viscosity





[kN s mm21

Fig. 3. Specific wear rates plotted against under different apparent pressures.

the melt viscosity


Fig. 5. Effect of molecular as a function of p-factor.


z e

weight on the specific wear rates of PEEK



0.6 =L

*: ‘p



0 c

0.2 0.1

0 0.1




Melt Viscosity Fig. 4. Specific wear rates plotted under different sliding velocities.





[kN s mF2] the melt viscosity


was greater when the load was higher. It was surprising that the load did not affect the wear rate in a linear way, but only after an increase from 4-g MPa; the wear rates measured under 1 and 4 MPa were almost on the same level. The effect of sliding velocity on the wear properties of PEEK is shown in Fig. 4. Here it becomes evident that the sliding velocity has a more significant effect on the wear properties of PEEK than the apparent contact pressure. This is especially true for velocities higher than 1 m s-l, for which the specific wear rates clearly decreased with increasing molecular weight, and this effect was greater at lower molecular weight. A plot of the specific wear rate as a function of pvproducts showed, that the effects of molecular weight were greatest when the p-level was high (Fig. 5). For comparison, a higher velocity at the cost of lower pressure resulted in higher values of the specific wear rates than higher pressure/lower velocity conditions. This means that the sliding velocity have stronger effect on the wear behaviour of PEEK than the pressure The specific wear rate (same meaning as wear coefficient)




Melt Viscosity Fig. 6. Effect of molecular as a function of p-factor.




[kN s mm21

weight on the friction



is constant, only when the pv-limit is less than 1 MPa m s-l. While the specific wear rate is also constant with a pressure of more than 3 MPa, if the sliding velocity is 1 m s-l. It was believed that each polymer would have a unique pv-limit as measured by some test. The p-limit for polymers is often thought to be based on thermal behaviour of the material, but this is not a valid assumption. The pv-limit as a criterion (for the application of polymers is properly misunderstand and misused. According to the results shown in Fig. 5 it could be not the same, for the behaviour of a polymer at a middle pv-condition, to use high pressure and low sliding velocity or low pressure but high velocity although the product pv could remain constant. Therefore, when using pv as a performance indicating parameter, it should be more specific about the values of individual components of this parameter. Results of friction measurements on annealed PEEK under different conditions are shown in Fig. 6 as a function of melt viscosity. It is apparent that there is no significant trend in the frictional coefficient for PEEK with different molecular weight, which changed from 0.367-0.529 at an apparent contact pressure, varying


Z.P. Ly

K Friedrich I Wear 181-183

from l-8 MPa in a range of sliding speeds between 1 and 3 m s-l. In a similar condition ofp = 0.5 MPa and v = 0.25-0.5 m s-l, Schelling and Kausch [lo] have studied the reciprocating friction of PEEK with a melt viscosity of 0.45 (kN s m-‘) on a stainless steel. In their results the friction coefficient increased from 0.35-0.55 with increasing of the sliding velocity. It is should be indicated that the interface temperature in their test was not controlled at a constant temperature but increased also from 35-75 “C with the increasing of the sliding velocity. One can consider that the interface temperature seems to be a major influencing factor on the increasing of the friction coefficient. The amorphous phase in PEEK has a more important effect on the wear behaviour than the crystalline phase, because its portion of the PEEK morphology amounts to about 70%. In this region the higher molecular weight causes a higher degree of chain entanglements, so that the chain motion is more difficult [23]. This can explain why the wear rate of PEEK with the higher molecular weight is generally lower. The spherulite size and the bonding between the spherulites of the PEEK plays also an important role in both friction and wear performance. In the PEEK with high molecular weight there are more interspherulitic links, so that local cracking between the spherulites is highly prevented. But if it occurs during the wear process (as one possible micro-wear mechanism) it will absorb more energy than in the case of the PEEK with higher molecular weight. This means that under a high apparent pressure the PEEK with high molecular weight has a stronger wear resistance. For PA and PBT materials Erhard [14] also confirmed these results. Because the surface energy of polymers is almost independent on the molecular weight [24], it can be understood that the friction coefficient of PEEK with different molecular weight did not change significantly under the adhesive sliding condition applied here.

(1995) 624-631







PTFE [vii. %] Fig. 7. Optimum range of PTFE content in PEEK composites with respect to the tribological behaviour. PEEK in PTFE

PTFE in PEEK -------

Lubrication (a)

--------r”M”‘ec I Crystal layer

-30 nm


Undeformed region

= = -------------_---_-----

= ---





Fibrillar J crystals


Sliding direction



__t (b)

Fig. 8. Schematic representation of the lubricating of PTFE (a) two extreme sliding cases for PEEIWTFE blends and (b) shear deformation of PTFJZ with smooth chain structure.

3.2. Effect of solid lubricants PTFE in PEEK Fig. 7 describes the tribological behaviour of PEEK-PTFE blends conducted with 1 m s-’ sliding velocity and 1 MPa apparent pressure. Both the specific wear rate and the coefficient of friction possess a minimum range for blends containing about lO-20% PTFE. It is therefore considered that for this system the optimum volume fraction of PTFE is about 15%. The reduction of friction at low PTFE contents is assumed to be due to the increasing efficiency of creating a lubricating transfer film of PTFE on the steel countersurface (Fig. 8) [9,25]. The rise of the coefficient of friction above 20 vol.% PTFE can be explained by the microstructure of the materials. Some micrographs of the materials are shown in Fig. 9. The micrographs of

samples with a PTFE content up to 15 vol.% show a continuous phase of PEEK with dispersed PTFE particles. With 40 vol.% PTFE content the appearance is different. PEEK and PTFE are present as two dispersed phases. With a higher PTFE content this phase becomes continuous so that the poor wear resistance of PTFE and its low coefficient of friction finally dominate the performance of the blends. Similar tribological investigations have been performed by Briscoe and coworkers [9], who performed their tests at lower pu-products and with PEEK/PTFE blends that were produced in a different way than the ones of this study. Therefore their results are slightly different although the general trends are pointing in the same direction. In their work, the wear increase

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L_0.6 CL

(b) 15 Vol -% PTFE

(a) 5 Vol -% PTFE







Fiber Content [vol. %] Fig. 11. Friction coefficient of CF-PEEK content at room temperature.

ic) 40 Vol -% PTFE

id) 95 Vol.-%

composites against fibre


Fig. 9. Transmission light micrographs of PEEK-PTFE 5 vol.%PTFE, (b) 15 vol.%PTFE, (c) 40 vol.%PTFE vol.%PTFE.

blends. (a) and (d) 95


10 -4




ZE ;


10 -s i 0

k .3




Temperature [“Cl


Fig. 12. Effect of temperature on the specific wear rate of PEEK composites with different volume fraction of short carbon fibres. 10” 0






steadily with increasing PTFE content. The resulting friction exhibited a strong drop at low PTFE volume fractions, up to about 20%, and beyond this point a further but slight reduction.

was observed (Fig. 11). Analogous runs in the specific wear rate were reported for other short carbon fibre (CF) reinforced thermoplastic systems such as CF/PTFE [26], CF/PA-66 and CF/PES [18], CF/PEN [27], as well as CF/LCP [28]. This beneficial effect as a result of fibre reinforcement is mainly due to a reduced ability for ploughing, tearing and other non-adhesive components of wear [29].

3.3. Effects of short carbon fibres and temperature on friction and wear of CF-PEEK composites

Effect of apparent contact temperature The specific wear rates of unfilled

Fiber Content Fig. 10. Specific wear rate of CF-PEEK content at room temperature.

[vol. %] composites against fibre

Effects of short carbon fibres

Figs. 10 and 11 show clearly how fibres incorporated in a PEEK matrix can improve the wear resistance and the friction coefficient at room temperature. The specific wear rate was reduced by more than one order of magnitude when at least 10 vol.% of short carbon fibres was added. Above this fibre content, however, further improvement was only slightly occurring (Fig. 10). With respect to the coefficient of friction, a similar trend

PEEK and its carbon fibre reinforced composites are shown in Fig. 12 as a function of apparent contact temperature. Only slight changes in wear rates were detected for the unfilled PEEK within the range of testing temperatures, whereas a clear response of the specific wear rate to the testing temperature was found for the PEEK composites. The wear rate of the PEEK composite with different fibre volume increased with increasing temperature, in particular, by about 1.5 orders of magnitude within the temperature range from 20-220 “C.

Z.P. Lu, K. Friedrich / Wear 181-183


The friction coefficients of these materials are shown as a function of temperature in Fig. 13. For the friction of unfilled PEEK a maximum at about 150 “C exists. The run of the curve for PEEK with short carbon fibres shows a minimum at a contact temperature of 150 “C, i.e. near the T, region for PEEK. At the temperature of 220 “C the friction coefficients of the neat matrix and the composites were almost on the same high level of p = 0.4. It was associated with a stick-slip behaviour, especially in case of higher fibre fractions. Studies of the worn surface by SEM [30,31] indicated that an effect of additional third-body abrasives caused by the fibre fragments was clearly stronger at this high temperature. It explains the higher friction coefficient, the stick-slip phenomenon, and mainly the higher wear rate. A combination of the optimum conditions, found out here for the PEEK matrix, the PEEIQPTFE blend and the CF/PEEK-composite should lead to an overall good friction and wear performance within broad pv- and testing temperature ranges. Such materials, as schematically illustrated in Fig. 14 (with additional inclusion of graphite as a further internal lubricant), are in fact produced for tribological applications with high re-






0.0 0




Temperature [“Cl

(1995) 624-631

quirement of long service life under extreme loading and environmental conditions [31].

4. Conclusions The following conclusions can be drawn from this study: (i> The PEEK with high molecular weight had a better wear resistance than with low molecular weight. These effects were greatest when thepv-level was high. The sliding velocity have stronger effect on the wear behaviour of the PEEK than the pressure. (ii) Opposite to the wear rate, the coefficient of friction was not clearly affected by thepv combination term. (iii) The inclusion of PTFE reduced the friction of PEEK. The lowest friction coefficient was measured for the blend with a PTFE volume fraction of 15%. The wear rates of the blends with PTFE volume fraction varying from 5-85% were lower than that of virgin PEEK. A minimum value in wear rate existed at a PTFE volume fraction between 5 and 40%. As an optimum range for PTFE in PEEK, lO-20% is recommended. 69 Over a wide range of temperature, short carbon fibre reinforcement improves the wear resistance of PEEK. In addition, the friction coefficient of PEEKis reduced if short fibres are added. However, more than 2Ovol.% carbon fibres can cause stick-slip behaviour, especially at very high testing temperatures (e.g. 220 “C). A (4 combination of the optimum conditions, found out here should lead to an overall good friction and wear performance within broadpv and testing temperature-ranges. This have been used in fact for tribological applications with high requirement of long service life under extreme loading and environmental conditions.

Fig. 13. Variation in friction coefficient with testing temperature for PEEK composites with different volume fraction of short carbon fibres.

Acknowledgements Graphite Flakes (10%) PTFE Particles Short Carbon


high molecular

Fig. 14. Schematic of PEEK composite graphite flakes and PTFE particles.



we ight)



Thanks are due to ICI Wilton in the UK and ICI/ LNP in the USA who kindly provided the testing materials. Parts of the experimental works were financially supported by the Arbeitsgemeinschaft Industrieller Forschungsvereinigung (AIF-FKM Nr. 691572). Dr. Lu would like to thank the Deutscher Akadamischer Austauschdienst (DAAD) for his fellowship. Prof. Friedrich gratefully acknowledges the help of AGARD, Project G85, Prof. Paipetis and Prof. Phillippidis, University of Patras, Greece for support and collaboration in this research field.

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K. Fried&h

I Wear 181-183




PI [31



[61 [71









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