Oxidation modelling of a Si3N4-TiN ceramic : Microstructure and kinetic laws

The oxidation of a silicon nitride-titanium nitride ceramic was studied and an oxidation kinetic model was proposed. For temperatures < 1000 C, only the TiN phase was oxidised. The oxidation process was then controlled by oxygen diffusion through TiO2, described by a parabolic oxidation kinetic law. The process was more complex > 1000 C, because of the simultaneous oxidation of both Si3N4 and TiN phases. Three oxidation modes, controlled by distinct diffusion mechanisms, took place successively. In the first step, Si3N4 and TiN phases were independently oxidised, respectively into SiO2 and TiO2 phases. Si3N4 oxidation was controlled by oxygen diffusion through SiO2, while TiN oxidation was controlled by titanium diffusion through TiO2. In the second step, the TiN oxidation was controlled by oxygen diffusion through TiO2 and through SiO2 formed by Si3N4 oxidation. In the third step, oxidation of the TiN and Si3N4 phases was controlled by oxygen diffusion through the silica layer. Kinetic laws were proposed for each of these three oxidation modes. 34 refs.