Oxidation Behavior of Boron‐Modified Mo ₅ Si ₃ at 800°–1300°C

Mo 5 Si 3 shows promise as a high‐temperature creep‐resistant material. The high‐temperature oxidation resistance of Mo 5 Si 3 has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo 5 Si 3 exhibits pest oxidation at 800°C. Mass loss occurs in the temperature range 900°–1200°C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°–1300°C. The oxidation rate of Mo 5 Si 3 was decreased by 5 orders of magnitude at 1200°C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800°C. The mechanism for improved oxidation resistance of borondoped Mo 5 Si 3 is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.