A comparison of oxidation kinetics of O′-SiAlON and β-SiAlON powders synthesized from bauxite

The oxidation mechanism and kinetics of beta-SiAlON and O'-SiAlON powders synthesized from bauxite have been investigated using thermogravimetric analysis, X-ray diffraction (XRD), and scanning electron microscopy. Oxidation experiments were carried out in both isothermal and nonisothermal modes in oxygen atmosphere. beta-SiAlON begins to oxidize at above 1000 K, and the rate increases significantly after 1200 K, forming SiO2 and Al2O3 at low temperature and mullite at high temperature. By comparison, the oxidation of O'-SiAlON powder starts at about 1250 K and after 1300 K the rate increases significantly. The O'-SiAlON phase remains dominant up to 1623 K, as demonstrated by XRD analysis, and the oxidation products are SiO2 as well as a small amount of mullite. At higher temperature, liquid phase resulting from the fusion of SiO2 and the impurities in O'-SiAlON powder hinders the progress of oxidation, which causes O'-SiAlON to have more oxidation resistance. Diffusion is the controlling step during both the oxidation processes. Based on the experimental data, a new model for predicting the oxidation process is developed. The application of this new model to the two systems demonstrates the validity of this new model. The activation energy of the oxidation of O'-SiAlON and beta-SiAlON has been calculated to be 224.1 and 175.7 kJ/mol, respectively.