Analytical TEM study of microstructure -property relations in liquid-phase-sintered SiC with AIN- Y₂O₃ additives

The microstructures of liquid-phase sintered SiC with A1N-Y ₂O₃ 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 A1N dissolution into the re-precipitated part of SiC grains. This changes the liquid composition during sintering and induces crystallization of Y ₁₀Al₂Si₃O₁₈N₄ and Y ₂O₃ in the triple-pockets. Amorphous films were observed to wet both grain boundaries and two-phase interfaces. A low ratio of A1N 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.