2018 Volume 126 Issue 12 Pages 968-976
This study reviews the development of fabrication processes for silicon nitride ceramics with excellent mechanical properties, such as high fracture strength and fracture toughness. Silicon nitride ceramics with high wear resistance and thermal conductivity have been produced by tuning their composition and utilizing highly efficient processing techniques. The nitridation behavior of silicon powder in the presence of Zr compounds has been studied in order to assess the nitridation enhancement effect of zirconium during formation of reaction bonded silicon nitride. Thermogravimetric analysis has revealed that the addition of ZrO2 to Si powder reduces the temperature of the main nitridation reaction. It is possible to produce reaction-bonded silicon nitride using a rapid nitridation technique with a heating rate of over 5 °C/min, which is 80 times higher than that for the conventional reaction bonding process. A nitridation enhancement effect has also been observed with the addition of some rare-earth oxides. The reaction bonding process has been used to fabricate high-thermal-conductivity silicon nitride ceramics with a relatively short processing time. Si3N4/carbon fiber composites have been developed using randomly dispersed high-tensile-modulus carbon fibers. The aligned silicon nitride grains and short carbon fibers in the composites result in both high fracture strength and toughness and a low friction coefficient under dry and water sliding conditions. The mechanical properties of the composites are anisotropic with respect to the grain alignment, with both strength and toughness being highest parallel to the extrusion direction. In this direction, the seeded specimen, with both silicon nitride grains and carbon fibers aligned, has higher fracture toughness and higher fracture strength than those of the non-seeded specimen, with only fibers aligned.