SiC offers a promise for high strength applications at high temperature; however, poor fracture resistance has inhibited its utility. Recent developments to control microstructure during hot pressing have improved fracture toughness >3 fold, while also improving strength 50% above that of a commercial SiC, Hexoloy. This ABC-SiC (designated for the Al, B, and C additives) utilizes liquid phase sintering to obtain full densification at 1650°C, and to induce the β-3C to α-4H phase transformation below 1900°C. Interlocking, plate-like, α grains, coupled with a thin (˜1 nm) amorphous layer, provide for tortuous intergranular fracture and high toughness.
This study focuses on the developing microstructure; how the α-4H polytype grows as a stacking modification of the β-3C grains, and how amorphous grain boundaries and crystalline triple point phases develop and interact with the crack geometry. HR-TEM and Image-Filtered EELS characterize the amorphous grain boundaries. Field Emission - SEM, EDS and Auger Electron Spectroscopy characterize the fracture morphology and the chemistry of grain boundaries and triple points. Electron Diffraction and HR-TEM depict an epitaxial relationship between triple point phases (Al8B4C7 and A14O4C) and matrix α-SiC grains, the development of which affects the mechanical toughening. The transformation to toughen SiC is compared to the well-studied transformation processing in Si3N4. A distinct advantage is the interlocked nature of the plate-like grains which causes strong elastic bridging behind the crack tip.