Glassy grain-boundary phase crystallization of silicon nitride: Kinetics and phase development

摘要

Pressureless sintering of fully densified silicon nitride can be achieved by adding to the silicon nitride starting powder sintering aids, such as Y_2O_3, Al_2O_3, AlN or Yb_2O_3. At the sintering temperature the additives form a eutectic liquid with silicon nitride which promotes densification by the liquid phase sintering process [1,2]. On cooling, the liquid phase leads generally to the formation of a glassy grain-boundary phase that degrades the high temperature mechanical behaviour. The high temperature mechanical properties of these materials can be improved by decreasing the glass content. This can be achieved by crystallizing the amorphous phase with adequate post-sintering heat treatments. In the case of a Y2O3-Al_2O_3 mixture, crystallized phases such as Y_3Al_5O_(12) (YAG) and Y_2Si_2O_7 are generally obtained [3,4]. In this letter, which is part of a most important work on the effects of crystallization on the creep properties, the crystallization kinetics and the composition of the various phases that crystallize successively from the initial glassy phase are studied by transmission electron microscopy (TEM) in the case of a commercial silicon nitride.