Effect of CeO2, MgO and Y2O3 additions on the sinterability of a milled Si3N3 with 14.5 wt% SiO2

Specimens of milled -Si₃N₄ with 0 to 5.07 equivalent per cent of CeO₂, MgO or Y₂O₃ additions were pressureless sintered at 1650 to 1820° C for 4 h in static nitrogen at 34.5 kPa (5 psi) gauge pressure and while covered with a mixture of Si₃N₄+SiO₂ powders. The density — per cent addition — temperature plots showed maxima which, for all three additives, occurred between 1.2 and 2.5 equivalent per cent. Maximum densities resulted on sintering in the 1765 to 1820° C range and were 99.6 per cent of theoretical with 2.5 equivalent per cent CeO₂, 98.5 per cent of theoretical with 1.24 to 1.87 equivalent per cent MgO, and 99.2 per cent of theoretical with 2.5 equivalent per cent Y₂O₃. Also, densities 94 per cent of theoretical were obtained with as little as 0.62 equivalent per cent additive (1.0 MgO, 2.11 CeO₂ or 1.85 Y₂O₃, in wt%). X-ray diffraction showed that the materials were predominantly -Si₃N₄ with some or no Si₂N₂O. Scanning electron photomicrographs showed microstructures of elongated grains with aspect ratios of about 5, with all additives.