Kinetics of Oxidation of Hot-Pressed Silicon Nitride Containing Magnesia

The oxidation of NC 132 hot‐pressed silicon nitride in dry oxygen was studied at 1521 to 1731K. The rate of O2 uptake and N2 release, as a function of time, was measured volumetrically. The ratio of N2 released to O2 consumed was smaller than expected from the stoichiometry of the oxidation reaction for Si3N4. The low value was attributed to impurities in the material. The oxidation rate followed the parabolic law with an activation energy of 440 kJ/mol. The corrosion scales (examined by microsurface techniques) were porous due to bubbles of released N2. The scales consisted of a mixture of unoxidized silicon nitride grains, crystalline oxides (SiO2 and MgSiO3containing other elements), and a glass phase. Reoxidation showed that the oxide scale was not protective. The magnesium profile in the subsurface layers indicated that the diffusion of magnesium in the unoxidized material was the rate‐limiting step in the oxidation.