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ACRC Technical Report 243PDF


Title:Tribology of Protective Hard Coatings for Oil-Less Compressors
Author(s):Solzak, T.A.; Polycarpou, Andreas A.
oil-less compressors
Abstract: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, natural ferrous chloride layers on iron surfaces have been eliminated. To recover lost wear resistance with the transition to oil-less operation, greater protection is sought through the application of hard coatings with enhanced tribological properties. Controlled shoe-on-disk experiments simulating the interface in a swashplate compressor were performed using a High Pressure Tribometer under unlubricated conditions. Specimens used for these experiments included 52100 steel shoes and coated aluminum A390-T6 disks. Coatings were provided by two leading manufacturers and consisted of two different single layer WC/C coatings (one from each manufacturer), multi-layer WC/C + DLC and multi-layer TiAlN + WC/C. To help increase load bearing ability on the relatively soft aluminum, a CrN underlayer was deposited on half of the disks. The performance of the coatings was evaluated using surface profilometry and scanning electron microscopy. In some cases, the CrN underlayer increased friction due to increased roughness, while it also significantly increased the load bearing capability in most applications. One of the CrN + WC/C multilayered coatings exhibited friction coefficients similar to lubricated experiments with scuffing loads roughly ten times larger than uncoated, unlubricated tests. Controlled, reciprocating pin-on-disk experiments imitating the wrist pin-connecting rod interface were also performed under unlubricated conditions. Specimens used for these experiments included coated, cylindrical 52100 steel wrist pins and uncoated cast iron disks. Coatings used were the same as those for the swashplate simulation tests but without the CrN underlayer. Analysis of experiments investigating temperature effects, frequency variation, and performance in various refrigerants including R134a, R410a, and R600a, was completed using scanning electron microscopy, energy dispersive x-ray microanalysis, and surface profilometry. 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. Tests conducted in R410a produced the lowest wear, while those in R600a had the lowest friction coefficients. Tests in R134a performed the worst, but better than tests conducted in dry nitrogen. Based on the research presented in this work, hard coatings have the potential to replace oil in future compressors.
Issue Date:2006-06
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center TR-243
Genre:Technical Report
Sponsor:Air Conditioning and Refrigeration Project 171
Date Available in IDEALS:2009-06-22

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