High Strain Rate Deformation and Damage in Ceramic Materials

The objective of this investigation is to use a plate impact recovery experiment to identify the dominant failure mechanisms in conventional α-Al₂O₃ ceramics and thereby gain insight into the most promising, failure-resistant microstructures. A “soft-recovery” configuration is used wherein a star-shaped flyer impacts a square specimen. The impedances, shapes, thicknesses and orientation of all plates are designed to ensure a known history of longitudinal, planar stress waves throughout a central octagonal region of the specimen. The plane waves generated from this experiment are monitored by a laser interferometer system that allows data to be collected at four separate locations. The validity of the approach is demonstrated by a shot in which all plates were stressed within their elastic range. Subsequently, several experiments were conducted at nearly the same stress level with commercially sintered aluminas having different grain size and different glass content. These experiments, taken as a whole, demonstrate that improvement in alumina’s dynamic compressive properties is obtained by reducing the grain size. In compression, a reduction in grain size lowers average residual stresses at triple junctions and grain boundaries and makes the material less susceptible to inelastic deformation and sliding at triple junctions and grain boundaries. A reduction in the weight percent of pre-processing impurities (and therefore the amount of intergranular glassy phase) yields strong improvements in the dynamic tensile strength of the ceramic. A decrease in the amount of glassy phase tends to make tensile damage less likely by improving grain boundary strength. These trends were tested by conducting recovery experiments on a high-purity, small-grain alumina, processed in-house through hot pressing. Both the compressive resistance and, especially, the tensile resistance were superior to those found for all other tested specimens. The overall results suggest that the best failure resistance will be obtained for new, high-purity, ultrafine-grain ceramics that are prepared by hot pressing of nanometer scale powders.