Microstructure and Mechanical Properties of SiC-AlN Ceramics after Phase Separation Treatment

Abstract

The microstructure and mechanical properties of SiC-AlN solid solution ceramics and the ceramics after phase separation treatment were evaluated in relation to the temperature and time of annealing. The SiC-AlN (SiC/AlN=70/30) solid solution ceramics with the addition of Y2O3 and Al2O3 were prepared by hot-pressing at 2200°C under 22MPa for 8h in Ar, which were followed by a subsequent heat-treatment in the miscibility gap at a temperature of 1600°C or 1800°C up to 120h in Ar. The modulated structure characteristic of the spinodal decomposition was developed within crystalline grains by the phase separation treatment. The fracture toughness was enhanced by the phase separation at both 1600°C and 1800°C for 8h. The hardness also increased with the phase separation treatment. The changes of these properties are considered to be dependent on the introduction of a modulated structure in the crystalline grains. Both the solid solution ceramics and the ceramics with phase separation exhibited an increase in crack resistance with crack extension. This phenomenon suggests that the crystalline structure of solid solution or the modulated structure contributes effectively to the shielding of crack extension. Although the modulated structures which consist of phases with different Young's modulus and linear thermal expansion coefficient were introduced to the crystalline grains by the phase separation treatment, these two properties of the ceramics were independent of the phase separation.