Abstract
The high temperature oxidation behaviors of gas-pressure-sintered (GPS) Si3N4 containing small amounts of impurities in addition to 5wt% Y2O3 and 0, 2 or 5wt% Al2O3 (A0, A2, A5) as the sintering aids were examined at 1400°C for 15h in pure oxygen. For comparison, the GPS Si3N4 obtained from highly pure Si3N4 powder (B5) was examined. The oxidation reaction was followed every minute by measuring the evolved N2 and NO using a quadrupole mass spectrometer. The weight changes of the bodies before and after the oxidation were measured and their oxidized surfaces were analyzed by SEM and EDX.The oxidation behavior of GPS Si3N4 with Y2O3 and Al2O3 additives obeyed an asymptotic kinetics possibly due to the formation of protective scales. The N2 evolution rate, the parabolic rate constant of the initial oxidation stage, the amount of N2 evolved, and the weight gain increased with an increase in the amount of Al2O3. The SEM observation of the oxidized surface revealed that the proportion of a crystalline phase identified as Y2O3·2SiO2 decreased and a glassy phase increased with increasing Al2O3 content.The oxidation behavior of B5 was fairly fitted with two stage parabolic kinetics. B5 exhibited a larger amount of N2 evolved and larger weight gain than A5.
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