Determination of the controlling parameters for dislocation nucleation in SrTiO3: An investigation by nanoindentation

We conduct nanoindentation to investigate dislocation nucleation in SrTiO3 (STO) single crystals with surface orientations of (0 0 1), (0 1 1), and (1 1 1) with loading/unloading rates of 25, 250, and 2500 mu N/s. Results reveal that the critical loads (P-c) at which "pop-in" event occurs depend strongly on surface orientations, but slightly related to loading rate. Based on P-c, the critical shear stress that triggers dislocation nucleation was determined by extracting the maximum resolved shear stress (tau(max)) along the slip systems of STO using the Hertzian solution. The dislocation activation shear stress (tau(a)) was determined by averaging tau(max). The determined tau(a) is 9.0-12.0 GPa, close to the shear strength (similar to G/2 pi) of STO, indicating that homogeneous dislocation nucleation dominates the pop-in events. The consistency of the determined tau(a) demonstrates that the frameworks for nanoindentation pop-in analysis established for metals can be extended to ceramics, whereas the influence of the limited slip systems should be taken into consideration. Additionally, we estimated the activation volume and the activation energy via the statistical model proposed by Schuh et al. The small values of the determined activation volume (0.6-9.8 angstrom(3)) and the activation energy (0.13-0.70 eV) indicate that the dislocation nucleation possibly begins from a single-atom migration and local point defects may participate in the dislocation nucleation process. That is, heterogeneous nucleation may exist initially but the homogeneous dislocation nucleation dominates the pop-in events.