Analytical TEM study of microstructure-property relations in liquid-phase-sintered SiC with AlN-Y₂O₃additives

The microstructures of liquid-phase sintered SiC with AlN-Y2O3 additives are systematically investigated by using transmission electron microscopy and analytical electron microscopy. Pure α-SiC as starting powder leads to fine, equiaxied microstructure. Introduction of α-SiC seed crystals into β-SiC powder accelerates the β-to-α-SiC phase transformation through a solution-precipitation process and promotes anisotropic grain growth, which results in a plate-like microstructure. Core/rim structures were found in both cases as a result of AlN dissolution into the re-precipitated part of SiC grains. This changes the liquid composition during sintering and induces crystallization of Y10Al2Si3O18N4 and Y2O3 in the triple-pockets. Amorphous films were observed to wet both grain boundaries and two-phase interfaces. A low ratio of AlN to Y2O3 in the sintering additive accelerates the devitrification of triple-pockets. Additional annealing can further devitrify the triple-pockets as well as the amorphous GB films, leading to a microstructure with potentially higher creep resistance.