Mechanical deformation of single-crystal ZnO

Abstract

The deformation behavior of bulk ZnO single crystals is studied by a combination of spherical nanoindentation and atomic force microscopy. Results show that ZnO exhibits plastic deformation for relatively low loads (≳4–13 mN with an ∼4.2 μm radius spherical indenter). Interestingly, the elastic–plastic deformation transition threshold depends on the loading rate, with faster loading resulting, on average, in larger threshold values. Multiple discontinuities (so called “pop-in” events) in force–displacement curves are observed during indentation loading. No discontinuities are observed on unloading. Slip is identified as the major mode of plastic deformation in ZnO, and pop-in events are attributed to the initiation of slip. An analysis of partial load–unload data reveals values of the hardness and Young’s modulus of $\text{5.0±0.1}$ and $\text{111.2±4.7\hspace{0.17em}}\mathrm{GPa},$ respectively, for a plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO are discussed.