The tribological response of PTFE-based composites for mechanical seals can be significantly affected by the nature of the counterface. In this work, glass fibre (GF) and carbon fibre (CF) reinforced PTFE-matrix composites were slid, in a pin-on-disc configuration, against AISI 304 stainless steel, either uncoated or coated with plasma sprayed Cr2O3. When mated with uncoated steel, GF-PTFE composites tended to wear down more, but also produced a thicker and more continuous tribofilm than did CF-PTFE ones. The pulverized glass debris adsorbed humidity and formed a cohesive “skeleton” to keep the PTFE debris in place, thus resulting in comparatively lower friction coefficients of ≈0.20–0.27. On the other hand, the polished Cr2O3 surface, with smooth plateaus and numerous valleys of various sizes, facilitated the retention of transfer material. Therefore, it produced less friction than uncoated steel, both against GF-PTFE and CF-PTFE, although the CF-PTFE pin showed a limited capacity to develop a tribofilm. CF-PTFE, however, suffered more severe wear than did GF-PTFE against Cr2O3, because the carbon fibres fractured more frequently. The bulk temperature of the composites never increased by more than 12 °C during the tests, and the rise in temperature depended more on the friction coefficient than on the thermal conductivity and diffusivity of the mated surfaces.
