Characteristic Comparison of α-Si_3N_4 and γ-Si_3N_4 Sintered under Ultra-high Pressure

Ultra-high pressure sintering of γ-Si3N4 and α-Si3N4 with Y2O3-Al2O3-La2O3 as additives was studied under pressures of 5.4-5.7 GPa and at temperatures of 1 620-1 770 K.The relative density,Vichers hardness and phase composition of sintered compacts were investigated.The results show that both γ-Si3N4 and α-Si3N4 transformed to β-Si3N4 completely after sintering.The relative densities and Vickers hardness of γ-Si3N4 sintered compacts were higher than those of α-Si3N4 sintered compacts prepared under the same conditions.The highest relative density and Vickers hardness in all the as-sintered samples of γ-Si3N4 and α-Si3N4 were 99.20% and 23.42 GPa,98.78% and 21.87 GPa,respectively.The sintered compacts were composed of elongated β-Si3N4 rod crystals with disordered orientation and uniform microstructure.