Mechanisms of Superplastic Deformation of Nanocrystalline Silicon Carbide Ceramics.

摘要

This project was undertaken to obtain preliminary data on the effect of nanograin size SiC materials on its properties. Using starting SiC powders with an average particle size of about 30 nm and small amounts of carbon and oxygen impurities, several processing techniques were used to fabricate bulk samples. These included the following: standard hot isostatic pressing (HIP), spark plasma sintering, ultra-high pressure HIP, and a multianvil pressure apparatus. The ultra-high pressure HIP technique achieved a final average grain size less than 100 nm and a relative percent of theoretical density of 96.8%; the hardness of this material was 22.7 GPa. A theoretical analysis of the effect of grain size on critical resolved shear stress to nucleate dislocations suggested a critical grain size of about 400 nm, below where it was easier to move partial dislocations. However, above this size, it was easier to move perfect dislocations. In addition, using more conventional hot-pressing techniques, the effect of different silicon and carbon contents was also investigated. It was found that increases in the free carbon and silicon content decreased the resulting grain size and influenced their strain rate sensitivity and flow stress.