Tribological composition optimization of chromium-carbide-based solid lubricant coatings for foil gas bearings at temperatures to 650 °C

Abstract In this paper a test program is described to determine the tribologically optimum composition of chromium-carbide-based solid lubricant coatings using a foil gas bearing test apparatus. The coatings contain a wear-resistant chromium carbide “base stock” with the lubricant additives silver and BaF 2 -CaF 2 eutectic. The optimum amounts of each constituent were determined in a previous study using a pin-on-disk test apparatus. The coating composition optimization is repeated in this study for air-lubricated foil gas bearings from 25 to 650 °C. The various coating compositions were prepared by powder blending, then plasma sprayed onto Inconel 718 test journals and diamond ground to the desired coating thickness and surface finish. The journals were operated against preoxidized Ni-Cr alloy foils. The test bearings were subjected to repeated start-stop cycles under a bearing unit of 14 kPa (2 lbf in -2 ). Sliding contact between the coated journal and the smooth foil occurs during bearing start-up before lift-off or hydrodynamic lubrication by the air film is achieved and during bearing coast-down. The bearings were tested for 9000 start-stop cycles or until specimen wear reached a predetermined failure level. In general, the addition of silver and eutectic to the chromium carbide “base stock” significantly reduced foil wear and increased journal coating wear. The optimum coating composition, PS212 (70 wt.% metal-bonded Cr 3 C 2 , 15 wt.% Ag, 15 wt.% BaF 2 -CaF 2 eutectic), reduced foil wear by a factor of 2 and gave rise to coating wear well within acceptable limits.