Oxidation Behavior of Boron-Modified Mo5Si3 at 800°–1300°C

Mo₅Si₃ shows promise as a high-temperature creep-resistant material. The high-temperature oxidation resistance of Mo₅Si₃ has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo₅Si₃ 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 Mo₅Si₃ 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 Mo₅Si₃ is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.