Tribological Characteristics of Coated Vane Surfaces Under Mixed Environments of Various Oils and CO₂ as a Refrigerant

The CO₂ is investigated as an alternative refrigerant to replace HFC refrigerant in air conditioning systems due to the environmental concerns. Because new compressors with CO₂ are going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To develop new compressor, especially rotary type, the friction and wear characteristics of sliding surfaces between a vane and a roller in the rotary compressor were evaluated in this paper. Several hard coatings, such as TiN, CrN, WC/C and two types of nitridings, were applied on vane surfaces in order to improve the tribological characteristics, and their performances were evaluated experimentally. Two types of lubricants were used, namely POE (polyol ester) oil and PAG (polyalkylene glycol) oil. The vane-on-disk type sliding tests were performed with an exclusive high pressure wear tester. From the tests, coefficient of friction and wear volume of vane surfaces applied various coatings were compared. Test results showed that WC/C coatings showed good tribological properties. TiN and CrN coated vanes showed good wear resistance properties but produced high friction. Also, the results of the sliding tests showed that using PAG oil has an advantage over POE oil in CO₂ environment.