Enhancing tribological characteristics of PEEK by using PTFE composite as a sacrificial tribofilm-generating part in a novel dual-pins-on-disk tribometer

In this work, polytetrafluoroethylene (PTFE)/bronze composite debris particles with strong tribofilm-forming abilities and slippery characteristics were released into a traditional pure poly-ether-ether-ketone (PEEK)-steel wear system to improve the tribological properties of PEEK. Wear tests were performed at room temperature under dry sliding conditions using a double-pin friction and wear test apparatus. The unfilled PEEK pins were allowed to slide against the steel counterparts with a fixed contact pressure of 4.0 MPa and speed of 0.8 m/s. Different amounts of PTFE/bronze debris particles were released onto the polymer/steel interface by introducing independent PTFE/bronze pins under different contact pressures (0.2-6.0 MPa) in the tribosystem. Results showed that the friction level and wear resistance of unfilled PEEK remarkably improved through releasing the PTFE composite debris onto the wear track. Under the test conditions used in this study, the friction coefficient and wear rate of unfilled PEEK decreased with the increasing contact pressure of the PTFE composite, but the wear rate of the PTFE composite increased. When compared to unfilled PEEK pins working alone, the optimal friction coefficient of PEEK was reduced from 0.45 to below 0.10, and its wear rate was reduced by approximately three orders of magnitude. Analyses of the worn surfaces revealed that the reduction in the friction and wear could be attributed to the formation of the "transfer film" on the counterpart steel surface and the "secondary transfer film" on the unfilled worn PEEK pin surface, both triggered by the PTFE composite debris. The wear mechanism of PEEK then transformed to sliding between the transfer and secondary transfer films, thereby preventing direct contact between the PEEK and steel countersurface.