Achieving maximum hardness in semi-coherent multilayer thin films with unequal layer thickness

Sources of plastic strengthening in [0 0 1] epitaxial Cu/Ni multilayer thin films are examined using measurements of in-plane lattice parameter and hardness (H) for films of different bilayer period (Λ) and Ni volume fraction (% Ni). Similar to other investigations, H for 50% Ni-50% Cu films increases with decreasing bilayer period down to Λ = 20 nm, where interfaces are coherent. A new finding is that H for semi-coherent films increases with % Ni. This strategy yields the largest reported H for this system (5.2 GPa for 60% Ni/40% Cu, Λ = 60 nm), showing that smaller is not always stronger. The rationale for the increased H is the development of a large interfacial dislocation density during the elasto-plastic transition to fully plastic yield. This strengthens Cu/Ni interfaces to slip propagation. The results are interpreted with a dislocation-based model that furnishes estimates of interfacial dislocation line energies, pinning strengths to confined layer slip, and interface barrier strengths to slip transmission.