Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti-24Al-11Nb, Ti-22Al-27Nb, and Ti-20Al-22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO2 scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO2. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi2. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500-1000 degrees C for the Ti-Al-Nb alloys, or at 500-875 degrees C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.