Abstract
Oxidation mechanisms in single- and two-phase Si-Al-O-N ceramics have been studied using scanning and transmission electron microscopy together with energy-dispersive X-ray microanalysis. Silicate layers formed on single-phase (β′) ceramics are non-crystalline, with viscosity and resulting oxidation kinetics controlled by outward diffusion of grainboundary segregated impurities. Aluminium substitution in β′ is important in compensating for the viscosity reduction imposed by the divalent ion impurities and inhibiting crystallization. Crystallization, induced only on slow furnace cooling, produces mullite and cristobalite phases. Two-phase (β′and matrix) ceramics exhibit comparatively poor oxidation kinetics with formation of a porous crystalline silicate layer due to the continued availability of a high concentration of metallic ions in the matrix phase.
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