Ultralow Wear Self-Mated PTFE Composites

Abstract

Remarkably low wear rates were observed in PTFE–PEEK and polytetrafluoroethylene (PTFE)-alpha-alumina composites when evaluated in a “self-mated” configuration, where a polymer pin is slid against a polymer countersample of the same composition. Each composite was tested in a controlled humidity environment on a linearly reciprocating tribometer on two different countersamples: a polymer countersample (self-mated) and a stainless steel countersample for comparison. For all the self-mated PTFE–PEEK composites [polyether ether ketone (PEEK) wt % 10, 20, 30, 40, and 50], the average friction coefficient was reduced, and the steady-state and total specific wear rates were improved when compared to testing against stainless steel. Self-mated PTFE–PEEK (wt % 10–40) achieved ultralow wear rates on the order of 10–9 mm3/Nm and friction coefficients of 0.08–0.14. When compared with samples slid against stainless steel, IR spectroscopy of the sliding surface showed that the self-mated PTFE–PEEK composites accumulate more PEEK at the sliding interface and more expression of a tribochemical carboxylate species, which have been linked with ultralow wear PTFE materials. The PTFE composites slid on steel rely on the formation of transfer films for ultralow wear performance. This is achieved by unidirectional increasing surface energy gradients from the polymer pin to the steel substrate, which dominate the transport and wear of PTFE composites slid on steel. However, the self-mated ultralow wear PTFE-based composites rely only on the formation and stability of tribofilms that consist of tribochemically altered PTFE with new carboxylate end groups as well as accumulated filler (i.e., PEEK or alumina). These films have self-regulating, minimal differences in surface energy. The close match of these low-energy surfaces contributes to low friction and ultralow wear. The self-mated PEEK-filled PTFE outperforms the alumina-filled PTFE primarily because of the ease at which PEEK accumulates at the surface. Additionally, the reinforcement and anchoring of the surface is better for a polymer blend than a particle-reinforced composite.