Nanoindentation Study of Pop‐in Phenomenon Characteristics and Mechanical Properties of Sapphire (102) Crystal

Abstract

The pop‐in behavior and mechanical properties of sapphire crystal vertically indented to its rhombohedral R (102) plane were investigated by nanoindentation using a Berkovich indenter. Effect of loading rate on pop‐in load and pop‐in extension width was observed within the indentation depth of h < 120 nm. The indentation size effect (ISE) of hardness within an indentation depth of 60 nm was systematically analyzed using Nix‐Gao and Al‐Rub models. Our experiments provided the consistent evaluations of hardness (H = 27.5 GPa), true hardness (Htrue = 68.9 GPa) at the non‐ISE region and effective indentation modulus (M = 423 GPa) for the contact depth of hc > 20 nm. Using the Hertzian contact theory, Schmid's law, and energy principle of indentation, the possible dominant slip system, which mainly contributed to the first pop‐in event when indented normal to the (102) plane, was estimated as {101} <20>. The distributions of corresponding resolved shear stress and principal stresses at the slip plane were also estimated.