Influence of α-Si3N4 coarse powder on densification, microstructure, mechanical properties, and thermal behavior of silicon nitride ceramics

Silicon nitride (Si3N4) ceramics, with different ratios of fine and coarse α-Si3N4 powders, were prepared by spark plasma sintering (SPS) and heat treatment. Further, the influence of coarse α-Si3N4 powder on densification, microstructure, mechanical properties, and thermal behavior of Si3N4 ceramics was systematically investigated. Compared with fine particles, coarse particles exhibit a slower phase transition rate and remain intact until the end of SPS. The remaining large-sized grains of coarse α-Si3N4 induce extensive growth of neighboring β-Si3N4 grains and promote the development of large elongated grains. Noteworthy, an appropriate number of large elongated grains distributed among fine-grained matrix forms bimodal microstructural distribution, which is conducive to superior flexural strength. Herein, Si3N4 ceramics with flexural strength of 861.34 MPa and thermal conductivity of 65.76 W m−1 K−1 were obtained after the addition of 40 wt% coarse α-Si3N4 powder.