Effect of HIP Sintering on the Crystal Structure and Fracture Behavior of α-SiC Platelets Embedded in Si3N4 Matrix

The effect of severe HIP sintering conditions (2050°C, 180MPa) on the crystal structure and fracture behavior of SiC platelets embedded in an Si3N4 matrix has been examined. The starting SiC platelets were single crystals of hexagonal structure (α-SiC) mainly constituted of the 6H and 4H polytypes with a minor fraction of 15R type. Etch-pit analysis revealed that this latter structure was a host in 6H or 4H crystallites and did not exist as a single crystallite. The SiC crystallites were separated from the nitride matrix by a chemical procedure and characterized by XRD, SEM and AFM. Then, the results were compared with that by TEM and SEM on the bulk composite specimen. No phase transformations were detected and the polytype fractions were unchanged after HIP sintering. However, locally unbalanced shear forces by neighboring crystals deformed the SiC platelets plastically during sintering causing the slip of close-packed (0001) planes and resulting in the introduction of high density dislocations into the crystals by glide mechanism. The dislocation density in the SiC platelets after HIP sintering was typically in the order 1015m-2. Dislocations had little or no effect on the cleavage fracture of the SiC platelets but produced some kinking and local splitting of the crack path for propagation along other crystallographic planes.