High temperature oxidation behavior of Si3N4-based ceramics was investigated systematically. The test has been made at 13000C in dry air environment up to 2000 hours. The Si3N4 specimens tested were as follows. a) S-1: Si3N4 added 8mass% Y2O3. b) S-2: Si3N4/SiC added 8mass% Y2O3. c) S-3: Si3N4 added 5mass% Y2O3 and 3mass% Al2O3. d) S-4: Si3N4/SiC added 5mass% Y2O3 and 3mass% Al2O3. The main conclusions are as follows. 1) Oxidized layer thickness of S-3 and S-4 were much thicker than those of S-1 and S-2. 2) Relationship between the oxidation time and oxidized layer thickness obeyed the parabolic law in all specimens. 3) The Y concentration under the oxidized layer was reduced considerably. The zone was defined as diffused layer. The thickness of diffused layer was very large in S-3 and S-4 samples. 4) Crystalline phases in the oxidized layer were mainly SiO2 and Y2Si2O7. 5) Effect of SiC composition on the oxidation behavior was slight.
