The significance of tribochemistry on the performance of PTFE-based coatings in CO2 refrigerant environment

This work reports on the tribological performance of three commercially available PTFE-based coatings (PTFE/pyrrolidone (C1), PTFE/pyrrolidone (C2) and PTFE/MoS2 (C3)) deposited onto engineered disk samples (substrates) made out of three different commonly used materials (gray cast iron, sintered iron and Al390-T6). Controlled oscillatory experiments were conducted to simulate the operating conditions of piston-type compressors in the presence of environmentally friendly CO2 refrigerant. It was found that the substrate played a major role on the tribological performance of the coatings and one of the coatings (PTFE/MoS2) was found to significantly improve the scuffing performance of Al390-T6 substrate. Furthermore, the tribological performance was found to improve with increasing CO2 pressure since a thicker patchy PTFE transfer layer was formed at the substrate–pin interface. XPS analysis showed that metal fluorides have a beneficial role on the tribological performance of these coatings.