Effects of α-silicon nitride powder processing on surface oxidation kinetics

The oxidation of two α–Si3N4 powders has been studied using x-ray photoelectron spectroscopy (XPS) and Bremsstrahlung-excited Auger electron spectroscopy (AES). These two powders had comparable specific surface areas but were produced by different methods. The powders were oxidized by heating in air at temperatures between 850° and 1000 °C. The oxide thickness for each heating time and temperature was determined both from the relative O 1s and N 1s peak intensities and from the relative SiO2 and Si3N4 Si KLL peak intensities. In each case, the oxide thickness calculated from the XPS data was systematically higher than those determined from the AES data. This difference may be due to uncertainties in the data analysis procedures. The oxidation of each powder followed a linear rate law for these temperatures. On one of the powders, the XPS data showed evidence of oxynitride formation. Analysis with thermal decomposition mass spectroscopy also showed that much more hydrogen was evolved from this particular powder, particularly at temperatures >700 °C. These differences in bulk impurity levels are believed to be responsible for the differences in the activation energies for oxidation which were measured to be 104 ± 22 kJ/mol and 62 ± 4 kJ/mol for these two powders.