Effects of Oxidation on Surface Stresses and Mechanical Properties of Liquid Phase Pressureless-Sintered SiC-AlN-Gamma2O3 Ceramics

The understanding of the oxidation mechanism of 50 wt% SiC-50 wt% AlN composites obtained by means of pressureless sintering without the protective powder bed and with Y2O3 as sintering-aid were significantly improved by means of Raman spectroscopy. These analyses put in evidence that amorphous carbon started to be formed at 1300 °C as main effect of active oxidation of SiC. At higher temperature the crystallization process began and it was completed at 1500 °C when only graphite could be recognized. On the basis of these new evidences, oxidation effects on the mechanical properties of SiC-AlN-Y2O3 composites were defined. First of all, heat treatment in air was able to induce a compressive surface stress due to the volume gain associated to the oxidation of the intergranular phase. As a consequence apparent fracture toughness showed a value of 6.6 MPa m1/2 after a heat treatment at 1300 °C, while at higher temperature effects of active oxidation caused a decreasing up to 4.7 MPa m1/2. This toughening mechanism was also used to improve the resistance to thermal shock, which was evaluated by performing quenching tests. Furthermore, passive oxidation induced the healing of superficial flaws by means of the formation of α-cristobalite. This phenomenon was assumed to be responsible for the increasing of the flexural strength.