Grain boundaries in hot-pressed SiC with a sintering additive, Al2O3, were investigated using both high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) with a view to explaining the correlation between high-temperature strength at 1773 K and the grain boundary segregation of both Al and O atoms. High-resolution transmission electron micrographs showed no thin secondary phase at the grain boundary interface. EDS line scans using a 3 nm probe showed the segregation of both Al and O atoms to grain boundaries. This grain boundary segregation lowers grain boundary strength and causes a transition from transgranular cleavage to intergranular fracture. We concluded that the high-temperature strength was degraded by the grain boundary weakening caused by the grain boundary segregation. Strengthening has been tried by controlling grain boundary composition according to equilibrium segregation theory. The high-temperature strength is improved from 200 MPa to 700 MPa, which is equivalent to room-temperature strength, by decreasing the grain boundary concentration of both Al and O.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys