Length-scale-based hardening model for ultra-small volumes

Understanding the hardening response of small volumes is necessary to completely explain the mechanical properties of thin films and nanostructures. This experimental study deals with the deformation and hardening response in gold and copper films ranging in thickness from 10 to 400 nm and silicon nanoparticles with particle diameters less than 100 nm. For very thin films of both gold and copper, it was found that hardness initially decreases from about 2.5 to 1.5 GPa with increasing penetration depth. Thereafter, an increase occurs with depths beyond about 5-10% of the film thickness. It is proposed that the observed minima are produced by two competing mechanisms. It is shown that for relatively deep penetrations, a dislocation back stress argument reasonably explains the material hardening behavior unrelated to any substrate composite effect. Then, for shallow contacts, a volume-to-surface length scale argument relating to an indentation size effect is hypothesized. A simple model based on the superposition of these two mechanisms provides a reasonable fit to the experimental nanoindentation data.