Effect of surface finishing, temperature and chemical ageing on the tribological behaviour of a polyether ether ketone composite/52100 pair

Polyether ether ketone (PEEK) composites and AISI 52100 steel are high-performance materials that are often selected for high stress tribological applications under solid and hydrodynamic lubrication. In this study, the viability of these materials as a pair for boundary-lubricated systems was evaluated. Tribo-tests to evaluate different characteristics were conducted: (i) the effect of metallic surface finish (1 µm polished, 9 µm polished, shot peened and ground); (ii) the effect of temperature (room temperature and 90 °C); and (iii) the effect of long-term PEEK exposure to lubricant oil (i.e., chemical ageing). An AMTI tribometer equipped with a hermetic chamber and a heating system was used for reciprocating sliding tests with a 10 mm stroke, 2 Hz frequency and 2 h duration. The tests used a cylinder-on-disc configuration (polymer on metal), a tetrafluoroethane atmosphere and a 100 N normal force. The wear tracks were analysed using a light interferometer, SEM and optical microscope to assess the wear mechanisms, including transfer film formation. The surface skewness and transfer film establishment were the main factors affecting the friction coefficient. In contrast, the wear of the PEEK composites was mainly affected by the surface bearing index and chamber temperature. Moreover, transfer films were observed only on counterbodies that had a surface roughness (Sq) of at least 0.030 µm. Finally, accelerated ageing of PEEK composites was performed in an autoclave at 180 °C and 3 bar pressure for 7 days in the presence of an ester-based lubricant oil and tetrafluoroethane atmosphere. DSC, DTG and FTIR techniques indicated that the filler/matrix interfaces of the PEEK composites were dissolved by the ester-based lubricant oil. Chemical ageing showed a significant effect on the friction steady state regime. Instead of a stable value, the friction coefficients of the aged composites increased slowly and continuously.