Friction and wear of the lubricated vane and roller materials in a carbon dioxide refrigerant

Abstract

Due to environmental concerns, carbon dioxide (CO 2), a natural refrigerant, is an attractive alternative refrigerant to replace the current HFC refrigerants that are used in air conditioning systems. This paper investigated the friction and wear characteristics of sliding surfaces between a vane and an orbiting roller in a compressor in order to develop a new compressor, and especially a rotary type. The sliding tests were performed using various sliding speeds and refrigerant pressures. Two types of lubricants were used in these tests, namely POE (polyol ester) oil and PAG (polyalkylene glycol) oil. The PAG oil under the CO 2 refrigerant had better lubricity than did the POE oil. It is believed that the amounts of the CO 2 refrigerant dissolved in the POE oil were larger than those dissolved in the PAG oil. As a result, the viscosity of the PAG oil was less reduced than that of the POE oil due to the dissolved CO 2 refrigerant. The reduction in the oil viscosity was associated with the formation of the relatively thin lubricating films on the sliding surfaces. Therefore, this thin film was associated with higher friction forces and rates of wear. Surface damage during the CO 2/POE tests was one of the reasons that the wear volume was increased in this specimen. The formation of a thicker oxide layer caused less wear on the surface of the pin. It is possible that as the pressure increased, the viscosity of the POE oil dropped and the lubricant film on the sliding surface became thinner. As a consequence, the friction and the wear increased as the real area of the metallic contact increased. However, the friction and wear were nearly constant regardless of the CO 2 pressure in the CO 2/PAG mixture environment.