Nanoindentation of lithium niobate: hardness anisotropy and pop-in phenomenon

Lithium niobate is a non-centrosymmetric crystal. Due to its non-centrosymmetric nature, the properties such as hardness and fracture toughness are likely to be different on opposite faces of the same wafer. In many wafer production processes such as wiresawing, lapping, and polishing, opposite faces of the wafer are processed simultaneously. Thus, it is important to understand the nature of anisotropy on the opposite faces of the same wafer. As a first step, hardness anisotropy of three common orientations of lithium niobate wafers is studied using nanoindentation technique. The hardness values on opposite faces for the same direction are found to be different for most cases. Lithium niobate exhibits a significant indentation size effect. Lithium niobate also exhibits pop-in phenomenon similar to the pop-in behavior of other materials. The pop-in phenomenon in lithium niobate is dictated by crystallographic orientation.