Free silicon and crystallization in silicon nitride based ceramics and in oxynitride glasses

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Abstract
Changes in microstructure induced by heat treatments between 1000 and 1500 °C in Si3N4 based glasses and ceramics were characterized using x‐ray diffraction and Raman scattering. The presence of free silicon appears to be a common feature of these families of materials. Si precipitates were observed by transmission electron microscopy in a bulk glass and in a polycrystalline ceramic. It is shown that heat treatments affect the free silicon content and that its decrease upon annealing is closely related to the crystallization of secondary phases. The incidence of the amount of free silicon on the material color strongly suggests that silicon microcrystals are responsible for most of the color fluctuations reported in silicon nitride glasses and ceramics. The higher the free Si content, the darker they are.

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