Determining the Mechanical Properties of Oxide-Coated Copper Nano-films using Reactive Molecular Dynamics

Metal-oxide layers are likely to be present on metallic nano-structures due to either environmental exposure during use, or high temperatures processing techniques, such as annealing. It is well known that nano-structured metals have vastly different mechanical properties from bulk metals; however, difficulties in modeling the transition between metallic and ionic bonding have prevented the computational investigation of the effects of oxide surface layers. Newly developed COMB potentials are used to perform fully reactive molecular dynamics simulations, which elucidate the effects that metal-oxide layers have on a copper nano-film's mechanical properties. Simulated tensile tests are performed on thin-films while using different strain-rates, temperatures, and oxide thicknesses to evaluate changes in yield stress, modulus, and failure mechanisms. Findings indicate that copper-thin film mechanical properties are strongly affected by native oxide layers. The oxide structures formed are quite amorphous, with per-volume bond-densities much lower than bulk CuO. It is found that oxidation will cause a reduction in elastic modulus, due to these softened (lower net binding energy) CuO layers. While under strain, structural reorganization within the oxide layers facilitates yielding at lower strain values through nucleation of partial dislocations across the oxide/metal interface. These results are supported by comparison to experimental studies. The mechanical properties are fit to a thermodynamic model based on classical nucleation theory. The fit implies that the oxidation of the films reduces the activation volume for yielding,