The role of water vapour in the oxidation of glass in the YSiAlON system

The effect of water vapour on the oxidation of an oxynitride glass in the system YSiAlON has been studied in the temperature range 1050–1200°C. Three categories of oxidising atmospheres were used; dry oxygen/nitrogen, nitrogen plus water vapour and oxygen plus water vapour. The water vapour pressure was varied within the range 360–2690 Pa. The oxidation kinetics, derived from thermogravimetric data, and the reaction products, examined by XRD and SEM, were analysed in order to understand the reaction mechanisms. The reaction kinetics and product morphology indicate that the rate of oxidation is controlled by the chemical reaction at the internal interface (oxide scale/substrate). The oxidation of the glass is found to be much faster in an atmosphere of nitrogen plus water vapour than in pure oxygen, showing that water vapour is an effective independent oxidant. The oxidation rate for oxygen plus water vapour is greater than the sum of the individual oxidation rates, indicating a synergy. The relationships between the reaction rate and the water vapour pressure and oxidation temperature are determined.