Tribological Study Comparing PAG and POE Lubricants Used in Air-Conditioning Compressors under the Presence of CO₂

Abstract

Polyalkylene glycol (PAG) and polyolester (POE) synthetic lubricants are good candidates for air-conditioning systems that work with alternative refrigerants such as carbon dioxide (CO₂). Both synthetic lubricants are widely used in air-conditioning compressors and have been optimized for use with hydrofluorocarbon (HFC) refrigerants. However, it is still not clear which lubricant is more suitable for use in compressors operating with CO₂ as a refrigerant. This study compares the performance of PAG and POE lubricants of the same viscosity (ISO VG 68) used in air-conditioning compressors. The materials used were Al390-T6 disks and hardened steel SAE 52100 pins. The tests were performed using a high pressure tribometer (pin-on-disk configuration) in the presence of CO₂. The results showed that scuffing and wear resistance of Al390-T6 tested with PAG were superior compared to the samples tested with the POE lubricant. Chemical analysis using X-ray photoelectron spectroscopy showed that PAG tends to promote the formation of carbonate layers on the surface, leading to improvement in the tribological performance of the interface.

KEY WORDS:

• Compressors
• Scuffing
• Wear
• Synthetic Lubricants
• Carbon Dioxide
• XPS

ACKNOWLEDGEMENT

This research work was supported by the 29 member companies of the Air-Conditioning and Refrigeration Center, an Industry-University Cooperative Research Center at the University of Illinois at Urbana-Champaign and the Cyprus Fulbright Commission via a 2007 Fellowship to Dr. K. Polychronopoulou. XPS was performed at the Center for Microanalysis of Materials at the University of Illinois, which is supported by the U.S. Department of Energy under Grant DEFG02-96-ER45439.

Review led by Lois Gschwender