Oxidation and corrosion mechanisms for silicon nitride-based ceramics in air and fused salts

Oxidation and corrosion experiments have been conducted using yttria densified beta-sialons (Si(6-z)AlzOzN(8-z)) with a range of z-values. Oxidation studies were conducted in laboratory air at a temperature of 1350 degrees C for times of up to 256 hours. Complementary corrosion studies were conducted using coupons of the materials immersed in a eutectic (Na:K)(2)SO4 mixture at 1150 degrees C for time periods of up to 96 hours. Using results obtained from detailed energy dispersive X-ray analyses of degraded surfaces and polished sections and X-ray diffraction of surface corrosion products it is concluded that the mechanism by which oxidation at 1350 degrees C and molten sulphate corrosion at 1150 degrees C occurs is due to the solution of beta-sialon grains by a liquid phase. For oxidation, the liquid phase is a Y-Si-Al-O liquid possibly containing nitrogen which forms because of the diffusion of yttrium and aluminium from the ceramic into the oxide scale. For corrosion by the molten sulphate, the liquid phase is complex and comprises an aluminosilicate liquid containing Na,K and Y which is extremely corrosive to all of the materials except the beta-sialon with z=3.0.