Effect of Strain Rate on Nano-Scale Mechanical Behavior of A-Plane (11(2)over-bar0) ZnO Single Crystal by Nanoindentation

In this study, nanoindentation tests at three different strain rates within 100 nm indentation depth were conducted on an a-plane (11 (2) over bar0) ZnO single crystal to investigate the effect of strain rate on its nano-scale mechanical behavior. The load-indentation-depth curves, pop-in events, hardness and Young's moduli of an a-plane (11 (2) over bar0) ZnO single crystal at different strain rates were investigated at the nano-scale level. The results indicated that, with the indentation depth increasing, the load increased gradually at each maximum indentation depth, hma, during the loading process. A distinct pop-in event occurred on each loading curve except that corresponding to the h(max) of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane (11 (2) over bar0) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young's modulus of the a-plane (11 (2) over bar0) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity.