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Many crystalline materials exhibit an indentation size effect (ISE), i.e., an intrinsic increase in hardness with decreasing indentation depths during indentation with geometrically self-similar indenters such as pyramids and cone. During indentation testing, the material underneath the indenter is heavily deformed, introducing strain gradients in the materials, causing high local dislocation densities. For better understanding the small-scale mechanical properties, in the present work, the 3D dislocation structure evolution around and underneath the spherical and Berkovich indentations have been resolved for the first time in (001) oriented strontium titanate (STO) single crystal at room temperature via a Sequential Polishing and Etching Technique (SPET). The Scanning Electron Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD) were used to analyze the dislocation microstructures at various depths below the surface. The indentation data combined with dislocation etch-pit technique revealed that the incipient plasticity (manifested as sudden indenter displacement burst) was strongly influenced by pre-existing dislocations. Etching revealed a well-defined asterisk-shaped etch-pit symmetry around the residual impressions, aligned along the and directions, which evolved step-by-step by increasing indentation load. SPET obtained cross-sections confirmed the presence of a high dislocation density region below the indentations. At larger polishing depths, a dislocation free region surrounded by box-shaped dislocation etch-pits pattern was observed. From dislocation etch-pits shape and tracking of dislocation etch-pit pile-ups, it was found that the slip along {110} planes is more favorable underneath the indentations. The dislocation etch-pits were digitized for calculating the dislocation densities at multiple depths with subsequent high-resolution electron backscatter diffraction (HR–EBSD) measurements at each polishing depth. The dislocation density quantified from etch-pit analysis includes both Statistically Stored Dislocations (SSDs) and (Geometrically Necessary Dislocations) GNDs, whereas HR-EBSD provides only the minimum GND density necessary to generate the measured orientation distribution. Both HR-EBSD and etch pit analysis show for each normalized radius a higher dislocation density at smaller loads. This result qualitatively validates the assumption in the Nix-Gao model that lower indentation depths result in higher GND densities. Furthermore, elevated temperature (350 °C) Berkovich nanoindentation experiments were performed on (001) oriented STO to analyze the influence of temperature on the ISE and the dislocation structure around the residual impression. It was found that STO exhibits an ISE, which was strongly reduced at 350 °C compared to 25 °C. At 25 °C, dislocation pile-ups were found shorter as compared to 350 °C. This also correlates with the smaller size effects at 350 °C. Peach-Koehler forces and the elastic-plastic indentation stress field were used to model the influence of the lattice frictional stress on the dislocation pile-ups. Based on an equilibrium position of the outermost dislocations, an average lattice frictional stresses were calculated to be 89 MPa and 46 MPa at 25 °C and 350 °C, respectively. |
