Study on the indentation size effect in CaF2: Dislocation structure and hardness

In this study nanoindentations have been performed on a cleaved surface of a CaF2 single crystal and the dislocation structure has been investigated by the etch pit technique using atomic force microscopy. The deformation during indentation is first purely elastic until dislocations are created observable in a pop-in in the load displacement data, as well as in a dislocation rosette around the indentation. After pop-in a relatively high hardness is observed, which gradually decreases, until at 3 μm a nearly constant hardness is found. By using sequential polishing, etching and imaging, the dislocation structure underneath indentations with indentation depths of 300 nm and 110 nm (load: 5 mN, 1 mN) is quantified. The dislocation density and radial distribution of dislocation density depend on the indentation depth, where a smaller indentation depth leads to a higher dislocation density, which is in qualitative agreement with the observed increase in hardness.