CORROSION KINETICS OF SILICON-NITRIDE IN DRY AIR CONTAINING SODIUM-NITRATE VAPORS

The gaseous alkali corrosion kinetics of Si3N4 were systematically inv estigated from 950 degrees to 1100 degrees C in dry air containing 0.9 8 vol% sodium nitrate vapors. The linear reaction rates observed at al l temperatures indicate that the alkali corrosion of Si3N4 is interfac e-controlled with an activation energy of 199 kJ/mol. The overall reac tion involves a complex absorption-dissolution-oxidation process and t he rate-controlling step appears to be the interfacial oxidation of Si 3N4 to SiO2. A comparision of the reaction kinetics and mechanism betw een the oxidation of Si3N4 in pure oxygen and in the alkali-containing atmosphere is presented.