Grain-boundary microstructure and chemistry of a hot isostatically pressed high-purity silicon nitride

Two high-purity Si3N4 materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser-derived Si3N4 powder with different oxygen contents. High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) analysis of the Si3N4 materials showed the presence of an amorphous SiO2 grain-boundary phase in the three-grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The chemical composition of the grain-boundary film was SiNxOy (x approximate to 0.53 and y approximate to 1.23), and the triple pocket corresponded to the amorphous SiO2 containing similar to 2 wt% nitrogen. The equilibrium grain-boundary-film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO2 grain-boundary phase, because the concentration of foreign ions has been shown to affect the grain-boundary thickness.