Effect of lubrication on tribological properties of PEEK and PEEK composites

Abstract

Polymers and polymer-based composites are becoming the preferred materials in many tribological applications due to their advantages such as lightweight, reduced noise and self-lubricating properties. In terms of green technology, reducing weight by replacing metallic parts for polymeric ones is a very promising way to improve fuel efficiency in the automotive industry. Poly-EtherEther-Ketone (PEEK) and its composites have superior mechanical properties and higher thermal stability than other conventional polymers which make them suitable for tribological applications operating under severe conditions. Lubrication has the potential to reduce further friction and wear. However, the tribological behaviour of PEEK and its composites under lubrication has been less reported than under dry conditions, and a solid understanding of the lubrication mechanism is still lacking. Therefore, the aim of this study is to elucidate the effect and mechanism of lubrication on the tribological properties of PEEK and its composites. Tribological tests were mainly performed on PEEK-steel and PEEK composites-steel contacts using a Mini Traction Machine (MTM) which simulates the sliding-rolling contact motion as encountered in gears, one of the main expected applications of this study. In addition to pure PEEK, carbon fibre reinforced (CFR) PEEK and glass fibre reinforced (GFR) PEEK as typical PEEK composites were investigated paired with steel counterparts. Base oils with/without lubricant additives, namely organic friction modifiers (OFMs) and anti-wear (AW) additives, were applied as test lubricants. To further investigate the mechanism of action, after-test specimens were analysed using various surface analysis techniques e.g., 3D surface profilometer, nanoindentation, Electron Probe Micro Analysis (EPMA), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The results indicate that the polymer transfer film that formed on the steel counterparts is the dominant factor controlling the tribological properties of the PEEK-steel and PEEK composites-steel contacts, especially when the steel has a rough surface. The polymer transfer films are assumed to act as protective films, thus preventing the direct contact of the relatively soft polymer surfaces with the hard asperities of the steel surfaces and so reducing the wear of the polymers. Although this contribution of polymer transfer films has been previously found in dry conditions, this study reveals their significance under lubrication and the mechanism of action. Furthermore, the novelty of this study is that essential but little-known knowledge such as the effect and working mechanism of lubricant oil viscosity and lubricant additives (OFMs and AW additives) on tribological properties of the PEEK-steel and PEEK composites-steel contacts, was discussed based on the tribological test results and surface analyses of after-test specimens.

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ORCID for Monica Ratoi: orcid.org/0000-0001-8400-3054

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