Extra-large grains in the silicon nitride ceramics doped with yttria and hafnia

Microstructural development of silicon nitride doped with Y2O3 and HfO2 was investigated to determine how extra-large grains develop during gas pressure sintering. Grains as long as 200 mu m and a few tens of micrometers wide developed at high temperatures (>2173 K) in a fine-grained matrix containing a limited amount of liquid which had a poor propensity to spread. A small number of grains could grow quite large before their growth was halted by neighboring grains of comparable size.