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 s ystem YSiAlON has been studied in the temperature range 1050-1200 degreesC. Three categories of oxidising atmospheres were used; dry oxygen/nitrogen, nitrogen plus water vapour and oxygen plus water vapour. The water vapour p ressure was varied within the range 360-2690 Pa. The oxidation kinetics, de rived from thermogravimetric data, and the reaction products, examined by X RD and SEM. were analysed in order to understand the reaction mechanisms. T he reaction kinetics and product morphology indicate that the rate of oxida tion is controlled by the chemical reaction at the internal interface (oxid e scale/substrate). The oxidation of the glass is found to be much faster i n an atmosphere of nitrogen plus water vapour than in pure oxygen, showing that water vapour is an effective independent oxidant. The oxidation rate f or oxygen plus water vapour is greater than the sum of the individual oxida tion rates, indicating a synergy. The relationships between the reaction ra te and the water vapour pressure and oxidation temperature are determined. (C) 2001 Elsevier Science Ltd. All rights reserved.