Monostructural and spectral investigations of the mechanism of surface layer formation during the oxidation of hot-pressed Si3N4 in air at high temperatures

Hot-pressed silicon nitride specimens containing magnesia additives were prepared and oxidized in air at high temperatures. The oxidized specimens were examined using a scanning electron microscope. The electronic and IR absorption spectra were measured. The scanning electron microscopy results indicated a decrease in the grain size with increasing intergranular area. The grain boundaries become sharper with increasing oxidation temperature and magnesia content. The sharpness of the grain boundaries is due to high temperature thermal etching effects. Electronic and IR absorption spectra revealed band broadening, a slight shift towards longer wavelength and the appearance of new bands as a function of MgO content and firing temperature. In conformity with all the present data, this could also be correlated with the decreased degree of crystallinity, the enlargement of the intergranular areas and the formation and propagation of oxidation products. In the oxidation mechanism additive and impurity cations diffuse outwards from the glassy phase in the bulk Si₃N₄ through the oxide film. The driving force is the formation of a reaction couple between the surface silica and the phase containing bulk magnesia and impurities.