Effect of initial phase composition on the phase composition, microstructure, and mechanical properties of Si3N4 ceramics

Silicon nitride (Si3N4) ceramics with initial beta-Si3N4 content from 3% to 88% were prepared by spark plasma sintering from 1450 to 1700 degrees C. The effect of initial beta-Si3N4 content on the phase transition, microstructure evolution, and mechanical properties of the Si3N4 ceramics was studied. The existence of initial beta-Si3N4 accelerated the alpha-beta phase transition. When the initial beta-Si3N4 content was 12% and 46%, the aspect ratio of the grains was increased. When the initial beta-Si3N4 content was 88%, the grains were mainly composed of short rod-like grains. As the initial beta-Si3N4 content increased, the densification process was slowed down. The hardness and fracture toughness of samples reached about 18 GPa and 6 MPa<middle dot>m(1/2). It is proposed that the mechanical properties of samples are connected with the densification process and beta-Si3N4 content.