Oxynitride Glasses Stuart Hampshire* and Michael J. Pomeroy Materials and Surface Science Institute, University of Limerick, Limerick, Ireland 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. Introduction Oxynitride glasses are effectively silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. 1 They are present at grain boundaries of silicon nitride- based materials, either at triple point junctions or as intergranular (IG) films 2 as shown in Fig. 1. Sintering additives in silicon nitride, usually a mixture of a rare- earth (RE) oxide, such as yttria, plus alumina, react with silicon nitride and silica present on the nitride particle surface to form liquid phases that result in densification of the ceramics and transformation of the a-silicon nit- ride to the b form. 3 The liquid phases cool as IG oxy- nitride glass films and pockets 2,4 and their chemistry, particularly the content of the modifying cation (Y or RE), and their volume fractions within the silicon nit- ride ceramic control its mechanical properties. 3–5 For example, as the Y:Al ratio increases, fracture toughness also increases, which is indicative of easier debonding at the silicon nitride grain interfaces. 5 Thus, since the early days of development of silicon nitride and sialon 2,6,7 ceramics, there has been a grow- ing desire to understand the nature of these IG oxynit- ride glass phases. This has resulted in a number of investigations on oxynitride glass formation, structure, properties, and crystallization in various M–Si–O–N, M–Si–Al–O–N, and M–Si–Mg–O–N systems, where M is a modifying cation such as one of the alkaline earths (Ba, 8 Ca, 9–12 Mg 10,11,13,14 ), Li, 15,16 Y, 10,11,17–26 Sc 27,28 or one of the RE lanthanides. 29–36 Some authors have studied mixed modifying cations. 37–41 Studies of properties have shown oxynitride glasses to possess high- er refractoriness, elastic modulus, viscosity, and hardness compared with the corresponding oxide glasses as a re- sult of the presence of nitrogen within the glass struc- ture. 10,11,17–19,21,22,25,26,28,30–34 Preparation of oxynitride glasses 1 typically involves wet ball milling of appropriate powders—silica, al- umina, the modifying oxide(s) plus silicon nitride, or aluminum nitride—in isopropyl alcohol using sialon milling media, followed by evaporation of the alcohol. Int. J. Appl. Ceram. Technol., 5 [2] 155–163 (2008) DOI:10.1111/j.1744-7402.2008.02205.x Ceramic Product Development and Commercialization *stuart.hampshire@ul.ie r 2008 The American Ceramic Society