Microstructure evolution of Si3N4 ceramics with high thermal conductivity by using Y2O3 and MgSiN2 as sintering additives

Silicon nitride (Si3N4) ceramics were prepared by gas-pressure sintering using Y2O3-MgSiN2 as a sintering additive. The densification behavior, phase transition, and microstructure evolution were investigated in detail, and the relevance between the microstructure and the performance (including thermal conductivity and mechanical properties) was further discussed. A significant change from a bimodal to a homogeneous microstructure and a decreased grain size occurred with increasing Y2O3-MgSiN2 content. When the small quantity of preformed & beta;-Si3N4 nuclei grew preferentially and rapidly in a short time, an obvious bimodal microstructure was obtained in the sample with 4 mol% and 6 mol% Y2O3-MgSiN2. When more & beta;-Si3N4 nuclei grew at a relatively rapid rate, the sample with 8 mol% Y2O3-MgSiN2 showed a microstructure consisting of numerous abnormally grown & beta;-Si3N4 grains and small grains. When more & beta;-Si3N4 nuclei grew simultaneously and slowly, there was a homogeneous microstructure and smaller grains in the sample containing 10 mol% Y2O3-MgSiN2. Benefitting from the completely dense, significant bimodal microstructure, low grain boundary phase, and excellent Si3N4-Si3N4 contiguity, the sample containing 6 mol% Y2O3-MgSiN2 exhibited great comprehensive performance, with a maximum thermal conductivity and fracture toughness of 84.1 W/(m & BULL;K) and 8.97 MPa m1/2, as well as a flexural strength of 880.2 MPa.