The effect of thermal cycling on oxide scale structure and morphology on a hot-pressed silicon nitride

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Dense Si 3N 4 hot-pressed with the aid of 9 wt% Y 2O 3 was oxidised in ‘dry’ synthetic air at both 1000 and 1200°C for up to 500 h under thermal cycling conditions. The experiments revealed that thermal cycling has little effect on oxidation kinetics although the morphology of surface oxidation products is affected by the cycling frequency. Cracks formed in the oxide layers on cooling healed immediately on re-exposure to high temperature, and there was no apparent change in the oxidation rate controlling mechanism over the time period investigated. The exposure of the material at 1000°C did not result in catastrophic oxidation as observed for some other Y 2O 3-doped hot-pressed Si 3N 4 compositions. Additionally, it was observed that crystallisation of the oxide layer with time (assisted in part by the outward diffusion of intergranular phase cations from the bulk ceramic to the surface scale) leads to non-parabolic kinetics owing to reduced rates of diffusion through crystalline phases in the surface scale. ©