Characterization and oxidation behavior of silicide coating on multiphase Mo–Si–B alloy

Moss-based Mo–Si–B multiphase alloys possess improved fracture toughness compared to its single-phase silicide counterparts (MoSi2, Mo5Si3 or Mo5SiB2) without sacrifying high temperature strength, but they degrade rapidly in an oxidizing environment at elevated temperature. In the present study, an MoSi2 coating was applied to protect the Moss-based alloy from high temperature oxidation via pack cementation. A boron-modified MoSi2 coating was also produced in an attempt to enhance the oxidation resistance of MoSi2 coating via co-deposition of B and Si, or using a two-step process. Isothermal and cyclic oxidation tests on coated Mo–12Si–8B (at.%) alloy were performed at 1100 °C and 1600 °C, respectively up to 200 h. The MoSi2 coating provides enough protection from oxidation in both static and cyclic conditions at 1100 °C, while at 1600 °C it degraded after 10 h by forming less protective T1 (Mo5Si3Bx) phase via interdiffusion between coating and substrate. The coating lifetime of MoSi2 was limited by the Si depletion due to diffusion into substrate.