GRAIN-BOUNDARY MICROSTRUCTURE AND CHEMISTRY OF A HOT ISOSTATICALLY PRESSED HIGH-PURITY SILICON-NITRIDE

Two high-purity Si3N4 materials were fabricated by hot isostatic press ing without the presence of sintering additives, using an amorphous la ser-derived Si3N4 powder with different oxygen contents. High-resoluti on transmission electron microscopy and electron energy-loss spectrosc opy (EELS) analysis of the Si3N4 materials showed the presence of an a morphous SiO2 grain-boundary phase in the three-grain junctions. Spati ally resolved EELS analysis indicated the presence of a chemistry simi lar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The ch emical composition of the grain-boundary film was SiNxOy (x approximat e to 0.53 and y approximate to 1.23), and the triple pocket correspond ed to the amorphous SiO2 containing similar to 2 wt% nitrogen. The equ ilibrium grain-boundary-film thickness was measured and found to be sm aller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larg er calcium concentration in the material with the lower amount of SiO2 grain-boundary phase, because the concentration of foreign ions has b een shown to affect the grain-boundary thickness.