Tribological investigation of cast iron air-conditioning compressor surfaces in CO2 refrigerant

Tribological investigations of air-conditioning compressors have been a topic of great interest in recent years and gray cast iron has been a commonly used material by various compressor manufacturers. The scope of this paper is to determine the role of oxygen and in particular carbon dioxide refrigerant (R744) in cast iron samples tribologically tested using an Ultra High Pressure Tribometer that is suitable for tribological testing of compressor contact interfaces that operate with carbon dioxide refrigerant. A series of experiments was performed in environments of air, nitrogen (N2), oxygen (O2) and carbon dioxide (CO2). While it was found that the presence of oxygen is beneficial, CO2 has a more positive effect on the surfaces than in the case of pure O2 suggesting that the use of CO2 promotes a different wear mechanism. Also, it was found that CO2 has better tribological performance over a range of pressures between 100 psi (0.69 MPa) to 600 psi (4.1 MPa), compared to lower pressures. Furthermore, CO2 was compared with tetrafluorethane (R134a), a common hydrofluorocarbon refrigerant and found to have superior tribological performance. Two surface chemical analysis techniques were utilized to examine the surfaces after tribological testing. Auger electron spectroscopy (AES) was used to track changes in the elemental composition while X-ray photoelectron spectroscopy (XPS) was utilized to detect the different chemical states resulting from compound formation on the tribologically tested surfaces. It was found that CO2 leads to better tribological performance of the interface due to the formation of carbonates on the surface, which reduce friction and prevent wear.