Glasses: Oxynitride Glass Properties and Crystallization

Abstract

This article provides an overview of the effects of glass composition on properties and crystallization of oxynitride glasses. Oxynitride glass formation occurs in a number of M–Si–O–N, M–Si–Al–O–N and M–Si–Mg–O–N systems and was reviewed in the previous article. The effect of nitrogen substitution for oxygen in silicate and aluminosilicate glasses is to increase properties such as glass transition temperature, elastic modulus, viscosity, hardness and slow crack growth resistance as a result of increased cross-linking within the glass network provided by tri-coordinated nitrogen atoms. Most reports show good linear relationships between glass property values and nitrogen content with correlation coefficients generally of the order of 0.99. The linearity of the relationships can be explained by the fact that each eq% nitrogen substitution introduces the same number of additional crosslinks into the glass network with the consequent increases in rigidity and degree of network compaction (decrease in molar volume). There is a clear linear dependence of Young’s modulus, and also microhardness, on fractional glass compactness. For a constant cation composition, viscosity increases by more than 2 orders of magnitude as ~17 eq% oxygen is replaced by nitrogen. For rare earth oxynitride glasses with constant nitrogen content, viscosity, Young’s modulus and Tg and other properties increase linearly with increasing cation field strength (decreasing ionic radius). An overview of studies on crystallization of oxynitride glasses outlining nucleation and growth of crystal phases, including different yttrium disilicate phases and B-phase (M2SiAlO5N; M=Y, Er, Y+Yb), to form glass ceramics with significant increases in elastic modulus over the parent glasses. Addition of fluorine extends glass formation in oxynitride systems and allows dissolution of higher levels of nitrogen into glasses. Fluorine lowers Tg but does not have any effect on elastic modulus or microhardness.