A Phenomenological Model for High Temperature Oxidation of Si<sub>3</sub>N<sub>4</sub>-TiN Composites

Abstract

Based on microstructural and chemical analyses, a phenomenological model is proposed in order to describe the high temperature oxidation of Si 3 N 4 -TiN ceramics. The model consists of three steps in the 1000-1200°C temperature range. In a first step, the oxidation of the TiN phase is controlled by the diffusion of Ti through TiO 2 formed at the outer surface, leading to the formation of a porous sub-layer. Simultaneously, the Si 3 N 4 phase oxidation is controlled by oxygen diffusion through SiO 2 . In a second step, the oxidation of inner TiN phase is controlled by the diffusion of oxygen through TiO 2 . The Si 3 N 4 transformation into SiO 2 leads to a high molar volume increase, SiO 2 , which is vitreous in the considered temperature range, creeps in the sub-layer porosity. In a third step, SiO 2 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°C and 1100°C. For higher temperatures, fluctuations in the experimental curves seem to indicate a succession of fracture and crack healing in the oxide layer.