Static and dynamic indentation response of basal and prism plane sapphire

Static and dynamic indentation experiments are conducted on sapphire along the c[0 0 0 1] and a [ 1   1   2 ¯   0 ] crystallographic axes to determine the loading rate dependence on its mechanical response. Static hardness is measured using a conventional Vickers tester while the dynamic hardness is 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 103 s−1. Static and dynamic hardness values as well as characteristic fracture response for both axes are compared at a range of indentation loads. The results indicate that dynamic hardness increases by 10% and 12% over static hardness for basal and prism plane sapphire, respectively. Observed crack orientations due to indentation of the a- and c-planes are consistent with the expected location of twinning or slip on the r ( 0   1   1 ¯   2 ) , ( 1   1 ¯   0   2 ) , ( 1 ¯   0   1   2 )     and     c ( 0   0   0   1 ) planes for both static and dynamic indentations. Prism slip is also suspected to interact with rhombohedral twinning systems to create a weakened zone oriented along the c-axis. Static indentations yielded well defined crystallographically oriented cracks with repeated directional switchbacks indicating a zone of competition between the two crack systems. Dynamic indentations, on the other hand, resulted in shorter and less jagged cracks accompanied by increased localized spalling beneath each indentation site as compared to the static indentations. This mode of fracture would suggest a shift in energy dissipation mechanism from crystal axis oriented long crack propagation under static loading to micro cracking that is independent of crystallographic orientation under higher strain rate events.