Preparation and Properties of Silicon Nitride Porous Ceramics through Molding with High Aspect Ratio β-Si₃N₄ Whiskers

In order to improve the flexural strength and high temperature resistance of porous silicon nitride, porous ceramics composed of β-Si₃N₄ whisker bonded with α-Si₃N₄ were prepared by using press molding, phenolic resin impregnation and carbonization and carbothermic reduction reaction between SiO vapor and residual carbon, through pressureless sintering. The effects of whisker size, forming pressure and impregnation time of the β-Si₃N₄ whisker on properties of the porous ceramics were studied. It was found that the addition of sintering aid Y₂O₃ and the use of loose sintering are effective to increase the aspect ratio of the β-Si₃N₄ whiskers. Average aspect ratio of the β-Si₃N₄ whiskers reached 13.8, while the maximum is 25.7. Increasing the pressure during molding improved the densification of the materials and hence the orientation degree of the whiskers, increased overlap point number of the whiskers and reduced the porosity, thus leading to improvement in strength of the materials. Increasing the number of impregnations raised the content of α-Si₃N₄ after carbothermic reduction. Resin impregnation for 1–3 times improved the connecting of the whiskers, so that the strength could be greatly increased. Only a slight increase in strength was observed above three impregnations, whereas the porosity was reduced, due to the enhanced effective bonding among the whiskers. The porosity and flexural strength of the sample molded at 120 MPa after five times of dipping are 41.0% and 213.4 MPa.