Elsevier

Wear

Volume 269, Issues 3–4, 18 June 2010, Pages 278-284
Wear

Friction and wear of PEEK composites in vacuum environment

https://doi.org/10.1016/j.wear.2010.04.007Get rights and content

Abstract

The sliding performance of PEEK composites was investigated in vacuum environment. Tests were performed with carbon fibre reinforced PEEK composites filled with PTFE, and MoS2 or graphite as further solid lubricant. Polymer samples were tested in a pin-on-disc configuration continuously sliding against CrNi-steel. Depending on sliding speed and temperature, the MoS2 filled composites showed high wear resistance and friction coefficients as low as PVD coatings.

Introduction

For tribological components working under extreme conditions, such as vacuum environment, or broad temperature ranges, it is often a problem to achieve acceptable endurance of their friction parts. For applications in high-vacuum and space-environment, in food-processing machines, or in semiconductor manufacturing equipment, self-lubricating materials prevent high-friction adhesive wear and cold welding that might normally be encountered with conventional all-metal systems. However, the lack of good heat dissipation by conduction with polymers has to be taken in account and requires the use of solid lubricants.

A promising polymeric material for such applications is polyetheretherketone (PEEK) which has a very low vapour pressure. PEEK composites are widely employed for their good tribological characteristics over a large temperature range [1], [2], [3], [4], [5]. For applications in air, optimal performances were obtained with the addition of fillers and fibres such as short carbon fibres (SCF), PTFE, and graphite.

Graphite has excellent lubrication properties in normal atmosphere, but a certain amount of water vapour is necessary for graphite lubrication, since the adsorption of water could reduce the bonding energy between the hexagonal planes [6], or saturate dangling bonds at edge sites [7]. Thus, it is not effective in vacuum environment, but its addition might improve the heat-dissipating characteristics of the polymer composite in some cases [8]. Like graphite, MoS2 has a hexagonal crystal structure but with the intrinsic property of easy shear, which makes it effective in vacuum and the most widely used solid lubricant in vacuum environment [9].

To extend our previous study on reciprocating sliding behaviour of PEEK composite against steel in vacuum between −40 °C and +160 °C [10], this paper presents the tribological properties of PEEK composites filled with either MoS2 or, for comparison, graphite, in rotary motion. Since an exceptional low friction was observed at −40 °C for MoS2 filled PEEK in [10], the experiments are performed here at a lower temperature range, between −80 °C and +20 °C. Furthermore, a particular attention is drawn to the influence of the sliding velocity.

Section snippets

Experiments

The composition of the PEEK materials is presented in Table 1. Materials were compounded by Ensinger GmbH and injection moulded as standard shouldered test bars at the Institute for Composite Materials (IVW, Kaiserslautern). Polymer composites were cut into pins (4 mm × 4 mm × 10 mm), where the contact area is 4 mm × 4 mm. As shown in Table 1, the materials are composed with a PEEK matrix filled with PTFE, short carbon fibres and graphite or MoS2.

The counterbodies are 304 steel discs (diameter 60 mm,) with

Influence of temperature

Fig. 1, Fig. 2 show the friction coefficient and the wear rate of the composites in vacuum between T = −80 °C and 20 °C at 1 and 7 MPa contact pressure. Above room temperature, the friction coefficient of these PEEK composites does not depend on the solid lubricant [10]. At lower temperature, however, the composite with MoS2 has a significantly lower friction coefficient than the composite with graphite, reaching 0.013 at −80 °C (Fig. 1).

Wear rates of the polymer composites are presented in Fig. 2.

On the influence of the solid lubricant

At RT, the wear rate and friction coefficient of PEEK composite in vacuum are relatively low, lower than in air. (The comparison of the tribological properties between air and vacuum was presented previously [11], [16]). The better tribological properties of PEEK composites in vacuum compared to air can suggest that PEEK is sensitive to the amount of water vapour in the environment, as proposed in [17].

Even the graphite filled composite showed relatively good tribological properties in vacuum

Conclusions

Friction and wear behaviour of PEEK composites filled with carbon fibres, PTFE, and graphite or MoS2 were investigated in vacuum in the temperature range between −80 °C and +20 °C in rotary motion. As for previous reciprocating motion, the sliding behaviour of MoS2 filled PEEK shows better tribological performance compared to the composite with graphite but only at low temperature. Surface analyses show that the lower friction coefficient and wear rate of the MoS2 filled composite at −80 °C is

Acknowledgements

The authors are grateful to the BAM colleagues from the division VI.2 and VI.4 who participated to this project. Also many thanks to the Germany Research Association (DFG), which supported this study under the project numbers Gr 1002/6.

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