Oxidation kinetics of silicon nitride ceramics: influences of the crystallisation and additive enrichment in the oxide scale

This article contributes to the description of the effects of the crystallisation and additive enrichment in the oxide scale on the passive oxidation kinetics. It shows that the apparent activation energies as usually deduced from the parabolic growth law must be larger than the activation energy for the rate-limiting diffusion process in the amorphous scale, provided that the additive ions diffuse into the scale. It also explains under what conditions the scale growth would turn from parabolic to non-parabolic regime and when the scale grows more slowly at higher temperatures. A direct method is proposed and demonstrated to calculate, from the experimental scale growth curves, the diffusivity of the diffusion species in the scale and the degree of the scale crystallinity. (orig.)