Growth and oxidation resistance of boron-modified and germanium-doped silicide diffusion coatings formed by the halide-activated pack cementation method

Abstract

Niobium-base and titanium-base alloys and intermetallic compounds possess many attractive properties, but practical application at high temperature is limited by their rapid scaling and embrittlement by oxygen dissolution. Since alloying schemes have not provided an acceptable balance of oxidation resistance and mechanical properties, protective coatings are required. Procedures for codepositing either silicon and boron, or else silicon and germanium, in a single reaction-processing step by a fluoride-activated, pack cementation method have been developed. These B-modified and Ge-doped silicide coatings were grown on commercially pure titanium, Ti−20A1−22Nb, Ti−22Al−27Nb and Nb−40Ti−15A1 alloys by direct conversion of the base metal into a TiSi 2-base coating. For Nb-base alloys, a protective Ge-doped MoSi 2 coating was grown by the diffusive conversion of a Mo layer deposited by physical vapor deposition. These diffusion coatings have been characterized, and their performance was evaluated by cyclic oxidation in air. Microhardness profiles were used to demonstrate the absence of oxygen contamination for the coated substrates. Preliminary fatigue testing of coated substrates is reported.