Tribology of WC/C coatings for use in oil-less piston-type compressors

Compressors are being required to operate at increasingly severe conditions in order to increase efficiency, and with the transition from CFC to HFC and natural refrigerants, protective ferrous chloride films have been eliminated. To recover lost wear resistance and transition to oil-less operation, greater protection is sought through the application of hard coatings with enhanced tribological properties. Controlled reciprocating pin-on-disk experiments imitating the wrist pin-connecting rod interface were performed using a High Pressure Tribometer under unlubricated conditions. Specimens used for these experiments included WC/C coated cylindrical 52100 steel wrist pins and uncoated cast iron disks. The performance of WC/C with temperature variation as well as running-in characteristics were evaluated using surface profilometry and scanning electron microscopy. It was found that steady-state friction coefficients decreased with increasing temperature while friction coefficients increased during the running-in period. Also, wear decreased from testing in room temperature up to 80 °C and beyond that, increased due to unstable running-in. Based on the research presented in this work, hard coatings have the potential to replace oil in future compressors.