Mechanical and thermal properties of Si3N4 ceramics prepared by gelcasting using high-solid-loading slurries

In this study, according to the Dinger model, aqueous Si3N4 slurries with a solid loading of 63 vol% and a viscosity lower than 1.0 Pa s were successfully prepared using two-grade Si3N4 powders with D50 = 7.2 mu m and 9.4 mu m as starting materials. These resulting slurries exhibited good rheological properties and suspension sta-bility. Notably, the presence of beta-Si3N4 seeds existed in the starting materials effectively reduced the nucleation barrier by transforming homogeneous nucleation into heterogeneous nucleation. Consequently, it facilitated the reconstructive phase transformation of alpha-Si3N4 to beta-Si3N4, promoted the anisotropic growth of beta-Si3N4 nucle-ation, and resulted in the final formation of the bimodal microstructure. This unique microstructure significantly enhanced the mechanical properties of Si3N4 ceramics, resulting in a high flexural strength of 1011.4 MPa and a fracture toughness of 14.8 MPa m1/2. These excellent properties were achieved through the combination of gelcasting and gas pressure sintering. Simultaneously, the Si3N4 ceramics exhibited a low thermal conductivity of 47.7 W m- 1 K-1, mainly owing to the low density of 95.3 %, the presence of low-thermal-conductivity silicate-based noncrystalline substances, and the possible existence of lattice oxygen in Si3N4 crystals.