Environmental Resistance of Mo–Si–B Alloys and Coatings

For high temperature application beyond the range of Ni-base superalloys, multiphase Mo–Si–B alloys with compositions, that yield the ternary intermetallic Mo5SiB2 (T2) phase as a key microstructure constituent together with the Mo and Mo3Si phases, offer an attractive balance of high melting temperature, oxidation resistance and mechanical properties. The investigation of reaction kinetics involving the T2 phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis kinetic biasing is used together with pack cementation to develop multilayered coatings and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate attack of silica based coatings, the current pack cementation coatings provide oxidation resistance in water vapor up to at least 1500 °C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments.