Strain-dependent transition of time-dependent deformation mechanism in single-crystal ZnO evaluated by spherical nanoindentation

We have systematically explored the time-dependent plasticity of (0001)-oriented ZnO single crystal through spherical indentation tests. Constant load indentation creep tests were performed in elastic strain and elastic-plastic strain regimes. In both regimes, creep indeed occurred at ambient temperature. With the observed quasi-steady-static strain rate, the stress exponent was found as similar to 1.361 for elastic contact regime and similar to 3.077 for elastic-plastic regime, suggesting predominant creep mechanism of diffusion and dislocation, respectively. This strain-dependent transition of creep mechanism is discussed in terms of possible factors affecting the creep in each regime.