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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2006-04, Vol.422 (1), p.51-65
• Main Authors: Gu, Hui, Cannon, Rowland M., Tanaka, Isao, Rühle, Manfred
• Format: Article
• Language: English
• Subjects: Analytical electron microscopy; Electron energy-loss spectroscopy; Interface; Intergranular glass; Phase separation; Silicon nitride
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2006.01.012

It was previously found that CaO additives in Si 3N 4 containing SiO 2 segregate to the ∼1 nm thin amorphous grain boundaries in preference to dissolving in the interior of the SiO 2 rich glass pockets. Moreover, Ca was detected at some of the β-Si 3N 4/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 SiO 2 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-Si 3N 4/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-Si 3N 4/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.