Transmission electron microscopy of the microstructural changes of a sintered Si3N4 associated with high temperature soaking in air

In this paper a systematic investigation was carried out on the microstructural evolution of a sintered Si3N4 ceramics associated with high temperature soaking in air. The soaking was conducted at 1275 °C for various periods of time, ranging from 16 to 40 hours. High resolution transmission electron microscopy (HRTEM) and conventional TEM were used to study the microstructures of the virgin (as received) and soaked samples. It was found that the Si3N4 ceramics has been severely attacked even inside the material, most probably by oxygen. The originally distinctly shaped Si3N4 grain boundaries and triple junctions became more rounded and the apparent volume fraction of the amorphous phases was increased due to high temperature thermal soaking. Studies on samples prepared with the electron transparent region deliberately located in various specific regions showed that in general the thickness of the amorphous grain boundary phase expanded due to soaking. The clear-cut atomic steps or the faceted Si3N4 grains along the grain boundaries were smeared out. All these changes could be explained on the basis of oxygen attacks through fast path diffusion of oxygen species along the grain boundary phase, surface diffusion in the micropores or even through volume diffusion in the Si3N4 grains along specific orientation. This may contribute to a better understanding of the high temperature behavior of this material in oxidizing atmosphere.