Mechanical properties of oxidized silicon nitride ceramics

The effect of oxidation on the room-temperature mechanical properties of silicon nitride ceramics with four different compositions is studied. Room temperature values of bending strength and elastic modulus of the whole oxidized material and of hardness, the fracture toughness and elastic modulus of the internal region (without the strongly oxidized surface layer) are determined for the materials oxidized at five different temperatures up to 1400°C. The strength decreases with increasing oxidation temperature for all the materials investigated. The fracture toughness of the internal region decreases upon oxidation at 1000°C. A reduction of the elastic modulus of the materials oxidized at 1400°C is also observed. The oxidation-induced changes in the mechanical properties are partially explained by the changes in the phase composition.