SHI Zhuotao, ZHI Qiang, BAI Yusong, LI Feng, WANG Bo, YANG Jianfeng
(State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University,Xi’an 710049, Shaanxi, China)
Abstract: 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-Si3N4whiskers 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-Si3N4after 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.
Key words: β-Si3N4 whiskers; porous silicon nitride; effective overlap