Deformation micromechanisms of ZnO single crystals as determined from spherical nanoindentation stress-strain curves

In this work, instrumented nanoindentation experiments with two spherical tips with radii of 13.5 and 1 mu m were used to explore the deformation behavior of ZnO single crystals with two orientations, C (basal) and A (prism). By converting the nanoindentation load-displacement data to stress-strain curves, we show that the main reason the hardening rates are higher for the C plane than they are for the A plane is the activation of dislocations-with widely different flow stresses-on different sets of slip planes. For the former, glide occurs on basal planes as well as pyramidal planes; for the latter, glide occurs predominantly on basal planes. The C plane is roughly twice as hard as the A plane, probably due to the orientation of basal planes with respect to the indentation axis. A Weibull statistical analysis of the pop-in stresses indicates that the inherent defect concentration at or near the surface is the main factor for the initiation of plastic deformation. The strain energy released when the pop-ins occur determines their extent. The elastic moduli values, determined by Berkovich nanoindentation, are 135 +/- 3 GPa and 144 +/- 4 GPa for the C and A planes, respectively. In the C orientation repeated indentations to the same stress result in fully reversible hysteretic loops that are attributed to the formation of incipient kink bands.