Influence of zirconia addition on reaction bonded silicon nitride produced from various silicon particle sizes

Reaction bonded silicon nitride ceramics have been produced from Si powders of different particle size and both with and without zirconia as a nitride catalyzing element. The effect of the ZrO2 on the nitridation rate of the specimens was studied as a function of temperature, and the mechanism of enhanced nitridation was discussed. At all temperatures, the degree of nitridation of specimens sintered with ZrO2 was higher than those without additions for all particle sizes. In the case of nitridation of specimens produced without ZrO2, the degree of nitridation was dependent on particle size, with larger sizes resulting in less nitridation. For specimens with ZrO2 the finest powder also showed highest transformation, but there was no difference in the degree of nitridation of medium and coarse powders. It was found that during heating of the specimens the ZrO2 was transformed to ZrN at low temperature and enhanced nitridation is considered to be due to this phase re-converting to ZrO2 at higher temperature and thereby supplying nitrogen to the interior of the specimens. At lower temperatures, the strength of the specimens with ZrO2 were higher than those without, attributed to the enhanced nitridation. However, comparing strengths of fully nitrided specimens, those with ZrO2 were lower than those without, thought to be due to the presence of ZrO2 particles disrupting the connectivity of adjacent silicon nitride grains. (c) 2008 The Ceramic Society of Japan. All rights reserved.