Tribological performance of PTFE-based coatings for air-conditioning compressors

Research on coatings in environments simulating compressor conditions, especially in a carbon dioxide (CO2) environment, has been limited. In this study, polymeric, PTFE-based coatings were investigated for use in air-conditioning compressors. Tribological experiments were performed to simulate piston-type compressors as well as swash plate compressors. Specifically, the tribological contact at the connecting “rod-wrist” pin interface of a piston-type compressor and the “shoe-disk” interface of a swash plate compressor were simulated. Machined 52100 steel wrist pins from a piston-type compressor and 52100 rounded steel pins, referred to as shoes, from a swash plate compressor were used. In both cases the counterparts were gray cast iron disks coated with three commercially available PTFE-based coatings, namely, DuPont® 958-303, DuPont® 958-414 and Whitford Xylan® 1052. These coatings were tested in CO2 environment, while testing was also performed in ambient air, and R410a, a common hydrofluorocarbon refrigerant, and the results were compared with a diamond-like-carbon (DLC) coating. The PTFE-based coatings showed low friction characteristics and high load carrying capacity and it was found that unlike the DLC coating they were not greatly affected by the testing environment. There was a significant increase in wear of the coatings under high contact pressures, however, it was shown that the wear debris generated acted as a third-body lubricant with a beneficial role in the overall wear performance. Due to their excellent tribological behavior under a range of experimental conditions, the coatings tested in this work could find use in compressor systems and other engineering applications.