Elastic-plastic deformation behavior of sapphire M-plane under static loading using nano-indentation

Elastic-plastic response of M-plane single crystal sapphire was explored via a nano-indenter with a Berkovich tip. Elastic-plastic transition was observed with eight different points (ranging from 0.30 to 0.55 mN) subject to respective peak load. Mechanical properties i.e., Oliver-Pharr hardness and elastic modulus were also determined in the onset elastic and elastic-plastic regions. Stable value of elastic modulus estimated from Oliver-Pharr nanoindentation experiments was around 430 +/- 15.0 GPa. However, Oliver-Pharr hardness in purely elastic and elastic-plastic (ISE) regions was approximately 2.19 and 2.0 times greater than the non-ISE hardness values respectively. Values of hardness in the non-ISE region were also in compliance with the depth independent hardness calculated through Nix-Gao and proportional specimen resistance models. Additionally, principal stresses and maximum shear stress were estimated on pop-in burst using Hertzian contact theory. Values of the critical resolved shear stress (CRSS) and maximum possible shear strength were also calculated at the first pop-in burst. Moreover, plastic zone size enhanced 1.36 times by shifting of critical load from 0.30 to 0.55 mN. Multiplication of Schmid factor and interplanar spacing indicated two slip systems i.e., prism {01 (11) over bar} <10<(11)over bar>> and pyramidal {11 (2) over bar0} <<(1)over bar>100>, which are verified in the Transmission electron microscopy (TEM) images. Estimated values of maximum contact pressures at respective critical loads were much lower in comparison to phase transformation pressure of sapphire. Maximum tensile stress was also calculated using Hertzian contact theory relations. Obtained value of maximum tensile strength was 3.58 times lower than cleavage fracture stress at the first pop-in.