Oxidation behavior of boron-modified Mo5Si3 at 800 degrees-1300 degrees 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 degrees C. Mass loss occurs in the temperature range 900 degrees-1200 degrees 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 degrees-1300 degrees C. The oxidation rate of Mo5Si3 was decreased by 5 orders of magnitude at 1200 degrees C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800 degrees C. The mechanism for improved oxidation resistance of boron-doped Mo5Si3 is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.