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

Silicon nitride (Si 3 N 4 ) ceramics were prepared by gas-pressure sintering using Y 2 O 3 –MgSiN 2 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 Y 2 O 3 –MgSiN 2 content. When the small quantity of preformed β-Si 3 N 4 nuclei grew preferentially and rapidly in a short time, an obvious bimodal microstructure was obtained in the sample with 4 mol% and 6 mol% Y 2 O 3 –MgSiN 2 . When more β-Si 3 N 4 nuclei grew at a relatively rapid rate, the sample with 8 mol% Y 2 O 3 –MgSiN 2 showed a microstructure consisting of numerous abnormally grown β-Si 3 N 4 grains and small grains. When more β-Si 3 N 4 nuclei grew simultaneously and slowly, there was a homogeneous microstructure and smaller grains in the sample containing 10 mol% Y 2 O 3 –MgSiN 2 . Benefitting from the completely dense, significant bimodal microstructure, low grain boundary phase, and excellent Si 3 N 4 –Si 3 N 4 contiguity, the sample containing 6 mol% Y 2 O 3 –MgSiN 2 exhibited great comprehensive performance, with a maximum thermal conductivity and fracture toughness of 84.1 W/(m⋅K) and 8.97 MPa m 1/2 , as well as a flexural strength of 880.2 MPa.