Rate Sensitive Indentation Response of a Coarse-Grained Magnesium Aluminate Spinel

The effect of loading rate on indentation hardness, brittleness, and fracture modes during static and dynamic Vickers indentation of a transparent coarse grain (250 mu m average grain size) polycrystalline magnesium aluminate spinel was investigated. Static hardness was measured using a conventional Vickers tester while the dynamic hardness was measured using a custom test fixture based on split Hopkinson pressure bar technique with the capability to produce single indentations at strain rates of roughly 10(3) s(-1). Static and dynamic hardness values as well as characteristic fracture response are compared at a range of indentation loads. It was found that Vickers micro hardness increased by an average of 5% over the entire load range, while the calculated brittleness factor increased when subjected to dynamic indentation loads. Static and dynamic indentations were found to produce radial and lateral cracks with evidence of spall appearing only during dynamic loading. Dynamic indentations also revealed an increased level of transgranular cracking, whereas intergranular cracking was more dominant during static indentations. Indentation cracks generated due to static indentations placed in the vicinity of a grain boundary deflected along a grain boundary, whereas these cracks cut across the grain boundary into the neighboring grain during dynamic indentations. Static and dynamic indentations that were placed directly on grain boundaries generated intergranular (grain boundary) cracks that were on average 18% longer than accompanying intragranular cracks. The implications of these fracture modes on dynamic impact response of polycrystalline spinel are discussed.