Phases, microstructures, and microchemistry of silicon carbide-based ceramics

Pure SiC. Transparent, translucent and opaque regions of high purity bulk SiC grown by chemical vapor deposition (CVD) were characterized for physical properties as well as for microstructure and chemical purity to correlate the degree of optical transparency with other material characteristics. A good correlation was obtained between SiC IR transmission and its microstructure. The transparent material was characterized by pure, essentially defect-free, {dollar}beta{dollar}-SiC (cubic) grains was highly oriented, with a {dollar}langle 111rangle{dollar} direction parallel to the deposition surface. The translucent material of various colors was mostly cubic in structure but contained large amounts of twins. Opaque CVD SiC was randomly oriented, and contained one directional disorder with hexagonal ({dollar}alpha{dollar}-SiC) symmetry in the majority of grains and high density of dislocations elsewhere.; The SiC-AlN system. Formation of solid solutions and their stability have been investigated. Early stages of transformation from the cubic phase were determined. Thin stacking fault lamellae developed first, followed by formation of twin bands, enhanced by the presence of ALN. Hexagonal ({dollar}alpha{dollar}) polytypes with AlN in solid solution formed, with transitional ordered or disordered regions at the transformation interface. The diffusion mechanism in the SiC-AlN system was investigated, finding (1) Strong coupling of Al-N and Si-C pairs in the diffusion process and Diffusion Rate of SiC in AlN {dollar}>{dollar} Diffusion Rate of AlN in SiC; (2) Neutron diffusion study indicated that Si and C atoms are replaced with Al and N atoms receptively; (3) AlN solubility in SiC is proportional to hexagonality; (4) AlN is located only at the hexagonal-type SiC layer in SiC-AlN polytypes.; SiC-matrix composites. The effects of microstructural parameters on toughness of SiC matrix composite with TiC particles have been investigated. Fracture toughness increased considerably by addition of TiC particles to SiC. The principal mechanisms for toughening in the SiC-TiC system were found to be crack deflection and crack bridging, with a possible contribution from microcracking. Microchemical analysis revealed limited mutual solubilities of TiC and SiC: the solubility of SiC in TiC. TiC did not have an effect on SiC phase stability.