Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si₃N₄ material which was produced by the nitridation pressureless sintering (NPS) method using Y₂O₃ and Al₂O₃ as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.