DEVELOPMENT, CHARACTERIZATION AND OXIDATION BEHAVIOR OF SI3N4-AL2O3 CERAMICS

Five Si3N4-Al2O3 ceramic grades were prepared by hot pressing at 1650- degrees-C. Backscattered electron micrographs revealed four different phases. Quantitative electron probe microanalysis allowed the identifi cation of these phases as X-sialon, alpha-Al2O3, O'-sialon and beta-si alon. The maximum solubility of Al2O3 in Si3N4 and in Si2N2O at 1650-d egrees-C was determined as well as the chemical composition of X-sialo n. Based on these results, a slightly revised phase diagram is propose d. The general features describing the microstructure of the various p hases have been investigated by transmission electron microscopy. The influence of the various phases on the mechanical properties was inves tigated; hardness, fracture toughness and elastic modulus were measure d. The oxidation behaviour has been studied in air at 1300-degrees-C a nd 1450-degrees-C The metastable phase diagram of Al2O3-SiO2 in the ab sence of mullite can be used to predict the oxidation products and rel ative amounts formed in the oxide layers.