The beneficial effect of water vapour on the oxidation at 600 and 700 °C of a MoSi2-based composite

The oxidation characteristics of a MoSi2-based composite in O2 and O2+10% H2O at 600 and 700 °C were investigated. The effects of temperature and water vapour on oxidation were examined. The oxidation kinetics were studied using a thermobalance and furnace exposure, while the morphologies and compositions of the oxides were examined using XRD, ESEM/EDX, and SEM/EDX. We propose that oxidation proceeds by the initial formation of MoO3 crystals and amorphous SiO2 on the surface. The MoO3 is then evaporated; as volatile (MoO3)3 species in O2 and additional MoO2(OH)2 species in O2+10% H2O, which results in a porous, Mo-depleted oxide. However, the pores in the Mo-depleted SiO2 scale heal, and a protective crystalline scale is established eventually. The vapour pressures of the abovementioned volatile species increase with temperature and/or water vapour content in the atmosphere, which leads to accelerated Mo depletion from the oxide scale. A shorter time elapses before the oxide scale is transformed into the relatively Mo-free protective SiO2 scale, which results in less oxide being formed. Thus, the formed scale becomes thinner in O2+10% H2O than in O2. Thereby the Mo removal is beneficial when water vapour is added to the exposure atmosphere.