Effect of agarose content on microstructures and mechanical properties of porous silicon nitride ceramics produced by gelcasting

Porous Si₃N₄ self-reinforce ceramics were prepared by gelcasting using agarose solutions. By changing the agarose content in the slurries, the porous silicon nitride ceramics with different porosities, α→β-Si₃N₄ phase transformation, and mechanical properties were obtained. When the agarose content changed from 0.2% to 0.8% (w/w, based on powder), the porosities increased from 10.3% to 21.4%, while the fracture strength decreased from 455 to 316 MPa and the fracture toughness decreased from 6.6 to 5.5 MPa•m^1/2. Many fibrous β-Si₃N₄ grains grown from the internal wall of the round pores is the typical microstructure of the gelcasting porous silicon nitride ceramic. Both elongated β-Si₃N₄ grains and suitable interfacial bonding strength contributes to high fracture toughness by favoring crack deflection and bridging. The growth mechanisms of fibrous grains resulted from the synergy of solution-diffusion-reprecipitation and vapor-liquid-solid (VLS).