Abstract
An ultrafine Si3N4 powder was synthesized by the vapor phase reaction of SiCl4 and NH3 in a thermal plasma arc jet. The as-prepared powder was white and amorphous, and had an average particle size of 30 to 40nm. The production rate was 200g/h. The heat treatment above 1380°C decreased the specific surface area and increased the crystallinity, implying that the grain growth and the crystallization occurred simultaneously. The oxygen content was reduced to 1wt% at 1450°C. The powder thus obtained was the crystallized hexagonal grain with the average size 0.2μm. X-ray diffraction indicated the presence of α-Si3N4 phase more than 95% and β phase as remainder. The sintering at 1700°C with the aid of 5wt% Al2O3 and 5wt% Y2O3 resulted in the relative density 98%, Vicker's hardness 16 GN/m2 and the fracture toughness 7 MN/m3/2.
