Effect of composition on viscosities of rare earth oxynitride glasses

M-Si-Al-O-N glasses (where M = Mg, Y or rare earth cation) are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of such oxynitride glass phases determine the properties of the material, in particular, high temperature creep phenomena. A number of investigations on oxynitride glass formation and properties have shown that nitrogen increases glass transition and softening temperatures, viscosity, elastic modulus and hardness. By changing the cation ratios or the type of rare earth cation, properties such as viscosity can be increased further. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity. These effects have important implications for creep in silicon nitride based ceramics where amorphous intergranular films control the creep resistance.