Sputtered Silver Films to Improve Chromium Carbide Based Solid Lubricant Coatings for Use to 900°C

Abstract

Thin silver films, 25 to 350 nm thick, were sputtered onto PS200, a plasma sprayed, chromium carbide based, solid lubricant coating, to reduce run-in wear and improve tribological properties. The coating contains bonded chromium carbide as the wear resistant “base stock” with silver and barium fluoride/calcium flouride eutectic added as low and high temperature lubricants respectively. Potential applications for the PS200 coating are cylinder wall/piston ring lubrication for Stirling engines and foil bearing journal lubrication. In this preliminary program, the silver film overlay thickness was optimized based upon tests using a pin-on-disk tribometer. The friciton and wear studies were performed in a helium atmosphere at temperatures from 25 to 760°C with a sliding velocity of 2.7 m/s under a 4.9 N load. Films between 100 and 150 nm thick provide the best lubrication of the counterface material. Though the thin films were quickly worn through to the substrate PS200 coating, they do provide a beneficial effect by enriching the sliding surface with lubricants. This reduced the initial abrasiveness of the as ground, plasma-sprayed coating surface, thus reducing counterface wear.