Effects of Y2O3 addition on the sinterability and microstructure of mullite (Part 2) - crystallization of liquid phase and grain growth

The phenomena in the sintering process of Y₂O₃-added mullite ceramics and the crystallization behavior of liquid phase in sintered bodies were studied using differential thermal analysis, X-ray powder diffractometry, scanning and transmission electron microscopies. The mullite powder compacts containing 2 to approximately 10 wt%Y₂O₃ were sintered at 1500 °C for 2 h, and were then cooled in furnace or quenched in order to study the crystallization phenomena. During the firing process, a liquid phase containing Y, Al, Si and O components was formed in the sintered body. When the specimen was cooled in furnace, the liquid phase solidified into glassy state or crystallized depending on the amount of Y₂O₃. The glass phase in the furnace-cooled specimen could also crystallize on annealing. Some crystallization phenomena of the liquid phase were observed. A small α-Al₂O₃ grain, which was formed by the solid state reaction between mullite and Y₂O₃ during the firing process, was observed inside a mullite grain. Larger α-Al₂O₃ grains, which were formed from the solution-reprecipitation process of liquid phase, were observed in the sintering body.