Grain growth of β-silicon nitride in RE-Mg-Si-O-N liquid

Isothermal growth of β-Si₃N₄ crystals dispersed in a series of rare-earth (RE = Y, Gd, Nd and La) oxynitride liquids (RE-Mg-Si-O-N) was studied after heat treatment at 1773K for 1 to 8 h. The microstructural characterization of α- and β-Si₃N ₄ crystals was quantitatively analyzed by electron microscopy. The number of α-Si₃N₄ crystals in oxynitride glasses decreased and that of β-Si₃N₄ crystals gradually increased with heat treatment time. And the mean grain size of β-Si ₃N₄ crystals also increased. It is determined that the rate of α/β transformation in the oxynitride liquid with Y ₂O₃ additive can be faster than that in oxynitride liquids with other additives. Likewise, the radii of rare-earth elements make significant differences in the mean grain size and aspect ratio. In all cases mentioned above, a marked change was observed in the order of ionic radius from La to Y. The mean diameter of the needlelike β-Si₃N₄ crystals heat-treated with La₂O₃ additive was almost constant in the time range of heat treatment in this study. However, the aspect ratio of β-Si₃N₄ crystals with La₂O ₃ additive was found to be much larger than that in oxynitride liquids with other additives, indicating negligible growth of β-prism plane.