Preparation and properties of CR-C-P hard coatings annealed at high temperature for high temperature applications

Abstract

Chromium plating from the trivalent state is relatively environmentally friendly as compared to a hexavalent chromium bath. Incorporation of non-metallic and metalloid elements into the coating should lead to enhanced properties. The relationship between composition, structure, and properties of annealed Cr—C—P layers electrodeposited from chromium-based trivalent baths is discussed. These coatings are amorphous in the as-deposited state, but upon thermal treatments, chromium nanocrystallization, as well as precipitation of carbides and phosphides occurs. Incorporation of phosphorous strongly influences the structural evolution and mechanical properties. Electroplated Cr—C alloy coatings exhibit significant increases in hardness and strength, when exposed to temperatures up to about 600° C., owing to the evolution of their nanostructure. This evolution can be shifted to higher temperatures (approaching 850° C.), through a ternary addition of phosphorous. The resulting Cr—C—P coatings may be suitable for applications at higher service temperatures, where more conventional Cr-based coatings soften rapidly.

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