Performance of a borosilica forming Mo-silicide coating in active oxidation environments

Active oxidation is a mechanism in which silica forming materials form a volatile oxide, SiO, causing rapid erosion of the substrate material. Most studies on active oxidation focus on SiC and there is limited information on other silica forming materials. To address this limitation, a borosilica forming Mo-silicide coating was synthesized on pure Mo and tested under the same active oxidation conditions (1500°C and Po ₂=10 ⁻⁴ atm) as preoxidized SiC. While active oxidation was observed for the borosilica Mo-silicide coating, it showed superior performance to the SiC. After 20 h of exposure, the coated sample gained 0.6 mg/cm ³, while SiC lost over 67 mg/cm ³. The borosilica Mo-silicide coating underwent active oxidation until enough Si was lost so that Mo ₅Si ₃ and MoSi ₃ formed as the primary phases, and SiO ₂ became the stable oxide once again. This re-stabilization behavior allows the Mo-silicide coating to have protection from active oxidation at a lower Po ₂ than SiC. A thermodynamic analysis is presented to explain why the Mo-silicides have a larger range of operation than SiC.