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The silicon oxynitride glasses take advantage of nitrogen bonding to attain high elastic modulus, increased softening temperatures and viscosities, greater slow crack growth resistance, and modest gains in fracture resistance. Of the oxynitride glasses, the Si–Y–Al‐based oxynitride glasses have been most extensively studied and a degree of success has been achieved in understanding how changes in glass composition affect structural parameters and their relationship with properties. More recent studies have focused on the Si–RE–Me oxynitride glasses, where Me is primarily Al or Mg and rare earth (RE) includes most of the lanthanide series elements. These glasses possess a range of elastic, thermal, mechanical, and optical properties, which can be correlated with the strength of the RE bond in terms of the cationic field strength. However, such correlations require knowledge of not only the RE valence state but also its coordination with the anions. Herein, the current state‐of‐the‐art understanding of the properties and structural parameters of oxynitride glasses and their interrelationships are reviewed.

Glasses of composition (in equivalent percent) (28 −x)Mg:xY:56Si:16Al:(100 −y)O:yN, with x= 0, 14, or 28 for y= 0 and 15 and with x= 0, 7, 14, 21, or 28 for y= 10, were prepared by melting and casting. For glasses where the nitrogen content was varied for a constant cationic ratio, the glass molar volume (MV), compactness (C), Young's modulus (E), glass‐transition temperature (Tg), and dilatometric‐softening temperature (Tds) varied linearly as the nitrogen content increased, with MV decreasing and the other properties increasing. From the incremental changes in these properties with nitrogen content, for glasses with x= 0, 14, and 28, good linear fits (R2 > 0.99) were obtained, and best‐fit slopes are reported here. The property changes and their linearity were consistent with the increased cross‐linking of the glass network by tricoordinated nitrogen. The replacement of magnesium by yttrium led to a nonlinear decrease in glass compactness and to nonlinear increases in MV, Tg, and Tds. However, linear correlations were found for MV and ionic volume and for Tg, Tds, and the coordination of (Si,Al)(O,N) tetrahedra of the glass structural units to the modifier cations not involved in charge compensating aluminum ions in fourfold coordination. The replacement of magnesium by yttrium had little effect on Young's modulus, and this result was related to similar changes in the compactness, C. The present results showed that the effects of substituting nitrogen for oxygen and yttrium for magnesium are independent and additive; thus, no synergistic effects of anion and cation substitutions were observed.

Oxynitride glasses are grain boundary phases within silicon nitride ceramics. The desire to understand their nature led to various investigations on oxynitride glass formation, structure, properties, and crystallization. This paper provides a review of oxynitride glasses and outlines the effect of glass chemistry, including nitrogen contents and cation ratios, on properties such as glass transition temperature, Young's modulus, and viscosity and relates this to structural features within the glass. A short outline of crystallization of oxynitride glasses to form glass-ceramics is presented.

Thirteen glasses of the general formula (M1, M2)9.33Si14Al5.33O41.5N5.67 where M1=La or Nd and M2=Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness (C), Young's modulus (E), microhardness (H), glass transition temperatures (Tg), and dilatometric softening temperatures (Td) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300°C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La–Y and La–Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the α polymorph of yttrium disilicate because of combined La–Y ionic radius effects.

The effects of fluorine and nitrogen substitution for oxygen in a calcium aluminosilicate glass have been examined. Twelve glasses in the Ca–Si–Al–O–N–F system, with constant cation ratio, have been prepared and characterized with respect to density, molar volume (MV), fractional glass compactness (C), Young's modulus (E), microhardness (μHv), and glass transition temperature (Tg). Linear regression analysis of the property data with respect to nitrogen and fluorine substitution for oxygen strongly indicates that each affects property values independent of each other. Nitrogen substitution decreases MV and increases C, E, μHv, and Tg values due to the greater network connectivity associated with tri‐coordinated nitrogen. Fluorine substitution, which reduces network connectivity, decreases Tg values as might be expected. However, MV, C, E, and μHv are little affected by increases in fluorine content. The vastly different effects of nitrogen and fluorine on E and μHv are due to the manner in which their substitution for oxygen affects glass free volume and thus fractional glass compactness (i.e. atomic packing density). Differences in the effects of N and F substitution on Tg reflect their different effects on glass crosslink density. A method for calculating crosslink density is presented here, and the need for this method to take into account the ratio of network forming to network modifying aluminum is discussed.

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