Mechanical deformation of single-crystal ZnO

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 (greater than or similar to4-13 mN with an similar to4.2 mum 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 5.0+/-0.1 and 111.2+/-4.7 GPa, respectively, for a plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO are discussed. (C) 2002 American Institute of Physics.