Six kinds of pressureless sintered Si
3N
4 with three different additives (CeO
2-Al
2O
3-MgO, Y
2O
3-Al
2O
3, Y
2O
3-Al
2O
3-MgO) were rapidly heated up to 1400°C and then held at this temperature for 15h in pure oxygen. During the oxidation, N
2, NO and CO
2 concentrations in the exhaust gas were continuously measured by a mass spectrometer. In every sample, a good correlation was observed between the amount of N
2 formed and weight gain. The amount of N
2 formed and weight gain after oxidation of Si
3N
4 increased with decreasing sintering temperature. The rates of N
2 and NO formation increased rapidly for 5min after the heating temperature reached 1400°C and then decreased rapidly. After rapid decrease, the N
2 formation curve showed different behavior with three additives. The maximum N
2 formation rate was in the order Y
2O
3-Al
2O
3-MgO>CeO
2-Al
2O
3-MgO>Y
2O
3-Al
2O
3. The total amount of N
2 formed was in the order Y
2O
3-Al
2O
3-MgO>CeO
2-Al
2O
3-MgO≈Y
2O
3-Al
2O
3. From the Si
3N
4 with CeO
2-Al
2O
3-MgO, a large amount of CO
2 was released and the surface of oxide scale was rough.
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