Effect of native oxide layers on copper thin-film tensile properties: A reactive molecular dynamics study

MD Skarlinski and DJ Quesnel, JOURNAL OF APPLIED PHYSICS, 118, 235306 (2015).

DOI: 10.1063/1.4938384

Metal-oxide layers are likely to be present on metallic nano-structures due to either environmental exposure during use, or high temperature 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 charge-optimized many body Liang et al., Mater. Sci. Eng., R 74, 255 (2013) potentials are used to perform fully reactive molecular dynamics simulations which elucidate the effects that metal-oxide layers have on the mechanical properties of a copper thin-film. 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 formed oxide layers have an amorphous structure with lower Cu-O bond- densities than bulk CuO, and a mixture of Cu2O and CuO charge character. It is found that oxidation will cause modifications to the strain response of the elastic modulii, producing a stiffened modulii at low temperatures (<75 K) and low strain values (<5%), and a softened modulii at higher temperatures. While under strain, structural reorganization within the oxide layers facilitates brittle yielding through nucleation of defects across the oxide/metal interface. The oxide-free copper thin- film yielding mechanism is found to be a tensile-axis reorientation and grain creation. The oxide layers change the observed yielding mechanism, allowing for the inner copper thin-film to sustain an FCC-to-BCC transition during yielding. 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. (C) 2015 AIP Publishing LLC.

Return to Publications page