Elastic properties of metallic thin films and multilayers

Molecular dynamics computer simulations were performed on metallic thin films and layered systems in order to understand their elastic properties at the atomic level. Atomic interactions were modelled using various forms of the Embedded Atom Method potentials, including a form which had been modified in order to more accurately reproduce known or expected values of surface energy and stress for the transition metals. It was observed that the surface stress and its strain derivative play very important roles in determining the biaxial modulus of these layered structures. Thin unsupported metal films were found to have large values of equilibrium biaxial stress, which varied inversely with film thickness. As a result, the thin film elastic constants were found to deviate substantially from their bulk values. Multilayers, composed of alternating layers of transition metals, were seen to have smaller values of interface stress and did not display any significant modulus anomalies. This result is in contradiction with many early published results concerning the so-called "supermodulus" effect, but is supported by recent experimental work. A thermodynamic model including surface/interface stresses was developed which explains the observed dependence of elastic moduli on film or layer thickness.