Kinetic studies of the oxidation of γ-aluminum oxynitride

The present article deals with the investigations of the oxidation kinetics of gamma-aluminum oxynitride (AION) in the temperature range of 1173 to 1773K by thermogravimetry. Oxidation experiments with AlON powder and plates have been carried out in air, both in isothermal as well as nonisothermal modes. Oxidation of MON resulted in the formation of Al2O3. The results showed that the rate of oxidation was negligible below 1273 K and, at higher temperatures, showed an increase with increasing temperature, Both isothermal studies as well as experiments with ramped temperature clearly indicated that the mechanism of the reaction changes around 1600 K. In the nonisothermal mode, the oxidation curve showed a plateau region in this temperature range. The difference between the two reaction steps was explained on the basis of the formation of a metastable alumina phase in the lower temperature region and stable corundum phase at higher temperatures, The buildup of the product layer would lead to a diffusion-controlled reaction kinetics. In the nonisothermal experiments, the phase transformations in alumina product layer at higher temperatures would lead to crack formation, thereby leading to even direct chemical reaction. From the experiments for the isothermal oxidation of AION powder, the overall activation energy for the reaction rate with chemical control was determined to be 218 kJ/mole in the temperature range of 1273 to 1573 K and 78 kJ/mole in the range of 1573 to 1773 K. The overall activation energy for the diffusion step was found from the isothermal oxidation of AlON plates to be 227 kJ/mole.