The Influence of Water Vapor on the Oxidation of MoSi<sub>2</sub> AT 450°C

Abstract

The oxidation of a MoSi2 composite was studied in dry air, oxygen and oxygen saturated with 10% water vapour at 450°C. The kinetics were investigated using TGA as well as oxide thickness measurements. Detailed analyses were performed on the morphology and composition of the oxide using XRD, ESEM, SEM, and EDX. It is shown that the oxidation rate increases drastically in the presence of water vapour, and the growth of Mo03 crystals on the oxide surface increases considerably. The different regions in the oxide cross-section are Mo-depleted compared with the corresponding regions in the bulk when oxidised in oxygen saturated with 10% water vapour. However, the samples oxidised in dry oxygen only shows Mo-depletion in some outer parts of the oxide. Accelerated growth of the MoSi2-oxide layer during exposure in 02+10%H20 compared to that in 02 can be related to the fact that more volatile Mo-species form in the presence of water vapour, resulting in a substantial loss of Mo03 from the inner part of the oxide. The voids left behind are not healed by the silica at this low temperature, which leaves the oxide with an open structure. As a result, the oxidation rate increases.