MICROSTRUCTURE OF SILICON-NITRIDE CERAMICS SINTERED WITH RARE-EARTH-OXIDES

Microstructures of Si3N4 with crystallized grain boundaries of rare-earth silicates were investigated. beta-Si3N4 grains in samples sintered with Yb2O3 were more elongated than those sintered with Dy2O3. Amorphous thin layers between crystallized grain boundaries of silicate and Si3N4 grains were recognized for both specimens sintered with Dy2O3 and with Yb2O3. In the sample sintered with Dy2O3, very clean grain boundaries between Si3N4 grains which did not contain the heavy elements were frequently recognized, while samples with Yb2O3 had an amorphous grain boundary phase containing Yb. Unusual structures with distorted lattice images in regions rich in Si and O were discovered in crystalline Dy-silicate phases.