Instrumented nano-indentation experiments, using spherical tips with radii of 13.5 and 1µm, were used to explore the deformation behavior of single crystals having basal or prism orientations. By converting the nano-indentation load versus displacement data into stress–strain curves, it was shown that the main reason why the hardening rates were higher for the basal- than for the prism-plane was the activation of dislocations having widely different flow stresses on different sets of slip planes. For the former, glide occurred on basal planes as well as pyramidal planes. For the latter, glide occurred predominantly on basal planes. The basal plane was roughly twice as hard as the prism plane; due probably to the orientation of the basal planes with respect to the indentation axis. Weibull statistical analyses of the pop-in stresses indicated that the inherent defect concentration at, or near to, the surface was the main factor in the initiation of plastic deformation. The strain energy released when the pop-ins occurred determined their extent. The elastic moduli values, as determined by Berkovich nano-indentation, were equal to 135 and 144GPa for the basal and prism planes, respectively. In the basal orientation, repeated indentations to the same stress resulted in fully reversible hysteretic loops that were attributed to the formation of incipient kink bands.

Deformation Micromechanisms of ZnO Single Crystals as Determined from Spherical Nanoindentation Stress–Strain Curves. S.Basu, M.W.Barsoum: Journal of Materials Research, 2007, 22[9], 2470-7