Plastic response of the native oxide on Cr and Al thin films from in situ conductive nanoindentation

Thin native oxide layers can dominate the mechanical properties of metallic thin films. However, to date there has been little quantification of how such overlayers affect yield and fracture during indentation in constrained film systems. To gain insight into such processes, electrical contact resistance was measured in situ during nanoindentation on constrained thin films of epitaxial Cr and polycrystalline Al, both possessing a native oxide overlayer. Measurements during loading of the films show both increases and decreases in current, which can then be used to distinguish between various sources of plasticity. Ex situ measurements of the oxide thickness are used to provide a starting point for elasticity simulations of stress in both systems. The results show that dislocation nucleation in the metal film can be differentiated from oxide fracture during indentation.