抄録
Hot-pressed silicon nitride specimens were oxidized in air flow at 1200°C for up to 3000h. Tridymite was detected by X-ray diffraction after 750h oxidation, and the crystals of 3-4×200-300μm were observed by composition image on the 3000h oxidized layer. The composition change in the phases by oxidation were examined with an EDS. The relative amount of SiO2 in the Y2S phase decreased during oxidation leading to the stoichiometric composition (Y2O3⋅2SiO2). The glassy phase formed after 3000h oxidation contained several metallic oxides such as Y2O3, CaO, MgO, Na2O, K2O and 10wt% Al2O3. Incorporation of alkali and other metallic oxides into the glassy phase lowered the melting temperature of the silica (cristo balite)-rich phase. The glassy phase acted as a flux of silica to promote the crystallization of tridymite.
