Microstructure and Grain‐Boundary Composition of Hot‐Pressed Silicon Nitride With Yttria and Alumina

Abstract

The microstructure of two hot‐pressed silicon nitrides containing Y 2 O 3 and Al 2 O 3 was examined by electron microscopy, electron diffraction, and quantitative, energy‐dispersive X‐ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y 2 O 3 +2 wt% Al 2 O 3 , as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high‐resolution dark‐field and lattice imaging. The second phase in a material with additives of S wt% Y 2 O 3 +5 wt% Al 2 O 3 was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot‐stage experiments in a high‐voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si 3 N 4 grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y 2 O 3 /Al 2 O 3 fluxing.