A phenomenological model for high temperature oxidation of Si3N4-TiN composites

Based on microstructural and chemical analyses, a phenomenological model is proposed in order to describe the high temperature oxidation of Si3N4-TiN ceramics. The model consists of three steps in the 1000-1200 degreesC temperature range. In a first step, the oxidation of the TiN phase is controlled by the diffusion of Ti through TiO2 formed at the outer surface, leading to the formation of a porous sub-layer. Simultaneously, the Si3N4 phase oxidation is controlled by oxygen diffusion through SiO2. In a second step, the oxidation of inner TiN phase is controlled by the diffusion of oxygen through TiO2. The Si3N4 transformation into SiO2 leads to a high molar volume increase, SiO2, which is vitreous in the considered temperature range, creeps in the sub-layer porosity. In a third step, SiO2 forms a continuous sub-layer, and oxidation is controlled by the diffusion of O through this layer. The kinetic laws proposed from this phenomenological model are in good agreement with thermogravimetric analyses between 1000 degreesC and 1100 degreesC. For higher temperatures, fluctuations in the experimental curves seem to indicate a succession of fracture and crack healing in the oxide layer.