High-temperature oxidation of silicon nitride-based ceramics by water vapour

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.