Composition Optimization of Self-Lubricating Chromium-Carbide-Based Composite Coatings for Use to 760°C

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Abstract
This paper describes new compositions of self-lubricating coatings that contain chromium carbide. A bonded chromium carbide was used as the “base stock” because of the known excellent wear resistance and the chemical stability of chromium carbide. “Additives” were silver and barium fluoride/calcium fluoride eutectic. The coating constituents were treated as a ternary system consisting of: (1) the bonded carbide base material, (2) silver, and (3) the eutectic. A study to determine the optimum amounts of each constituent was performed. The various compositions were prepared by powder blending. The blended powders were then plasma-sprayed onto superalloy substrates and diamond ground to the desired coating thickness. Friction and wear studies were performed at temperatures from 25 to 760°C in helium and hydrogen. A variety of counterface materials was evaluated with the objective of discovering a satisfactory metal/coating sliding combination for potential applications, such as piston ring/cylinder liner couples for Stirling engines. In general, silver and fluoride additions to chromium carbide reduced the friction coefficient and increased the wear resistance relative to the unmodified carbide coating. Silver and the fluoride eutectic acted synergistically in reducing friction and counterface wear. Counterface materials included cobalt, nickel, and ferrous alloys. Several coating compositions gave good results in hydrogen and in helium to 760°C.

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