Nanoindentation of single-crystalline gold thin films: Correlating hardness and the onset of plasticity

Understanding the mechanical properties of materials with external dimensions on the nanometer scale is crucial for the design and fabrication of nanoelectronics and nanosystems. Metal thin films exhibit a size-dependent hardening effect that scales inversely with the film thickness down to 200 nm. The thickness range below 200 nm is mostly unexplored and initial experiments indicate a change in the scaling law. Here, the mechanical properties of single-crystalline Au films are investigated in the thickness range from 31 to 858 nm by nanoindentation. Maximum shear stresses at the onset of plasticity are determined by the finite element method. While the hardness increases with decreasing film thickness, as expected from macroscopic experiments, the onset of plasticity shifts to lower shear stresses for thinner films. These observations are interpreted with respect to detailed observations of the microstructures of the films investigated.