Oxidation Behavior of Si3N4 at Low Oxygen Partial Pressures.

The oxidation behavior of Si3N4 containing small amounts of impurities in addition to 5 mass% Y2O3 and 5 mass% Al2O3 used as the sintering aids were examined at 1400°C for 15h at low oxygen partial pressures (PO2=0.2-1kPa). The oxidation reaction was followed by measuring the O2 consumption caused by oxidation every minute using a quadrupole mass spectrometer. The weight change of the bodies before and after the oxidation was measured and their oxidized surfaces were analyzed by SEM and EDX.α-cristobalite and Y2O3·2SiO2 were formed as the crystalline oxidation products and an apparently homogeneous protective layer was also formed on the oxidized surface after the oxidation at 1400°C for 15h at low oxygen partial pressures (PO2=0.2-1kPa).In the case of the oxygen partial pressure of 1kPa, the passive oxidation accompanied by weight gain and the active oxidation accompanied by weight loss were considered to be concurrent. In the cases of 0.5 and 0.2kPa, the passive oxidation was observed and any sign of the active oxidation could not be detected.These facts, contradictory to the generally accepted concept that lowering of oxygen pressure favors the active oxidation mechanism, indicate the marked depression of oxygen partial pressure in the flowing gas contacting with the sample surface at the downstream side in the case of 1kPa.