Hot-pressed Si3N4, sintered Si3N4 and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO2, Y2Si2O7 and Y4A1209. The oxidation rate was in the order sialon > sintered Si3N4 > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y2O3 content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol-1.
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