Viscosity of high-nitrogen content Ca–Si–O–N glasses

The viscosity of three high-nitrogen content Ca–Si–O–N glasses, with 30–58 e/o N and 36–39 e/o Ca, was determined by micro-indentation. The measurements were made using an automated set-up, designed and built in-house, capable of measurements up to 1200 °C with applied loads of 0.01–15 N. The viscosity increases significantly with the nitrogen content and reaches viscosity values close to reported values for rare-earth silica oxynitride glasses. The glass transition temperatures range between 878 and 995 °C and are in very good agreement with values measured by differential thermal analysis. The apparent viscosity activation energies are very high, ranging from 855 to 2170 kJ/mol. The glasses can accordingly be classified as being both very refractory and very fragile. Implications of the viscosity values and mechanical properties of the glasses for their structures are discussed.