Calcia partition in phase-separated intergranular glass and interfaces in doped silicon nitride produced by hot isostatic pressing

It was previously found that CaO additives in Si3N4 containing SiO2 segregate to the ∼1 nm thin amorphous grain boundaries in preference to dissolving in the interior of the SiO2 rich glass pockets. Moreover, Ca was detected at some of the β-Si3N4/silica interfaces. Using sensitive imaging and analytical methods available from modern transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), the Ca distribution was accurately mapped with a resolution approaching 1 nm. Liquid–liquid phase separation occurs at the liquid (now glass) triple pockets at higher Ca levels. Electron energy-loss spectrometry provides compositions of the separated phases, one being nearly pure SiO2 and the other containing significant Ca and N, and also insights about their bonding nature. The Ca and N containing liquid tends to reside in the tips of the liquid pockets with re-entrant geometries. The two β-Si3N4/liquid interfaces and the actual triple line regions were also assessed. Evidence emerged that these entities are diffuse spanning well more than a nanometer in width and having high N levels; Ca also often adsorbs to these. The β-Si3N4/liquid interfaces for the two liquids have different compositions, and that of the grain boundary film is intermediate between these values. Thus, this chemical information yields a comprehensive picture of this phase separation and of the various interfaces in this system.