Multilayer thin films exhibit strong size effects in their mechanical behavior such as increasing yield strength or hardness with decreasing layer thickness. Different deformation mechanisms have been suggested for different layer thicknesses and types of interfaces such as dislocation pile-up, confined layer slip, or dislocation transmission across the interfaces. For layer thicknesses in the sub-micrometer and nanometer regimes, the interface between the layers becomes increasingly important and its role in the deformation process needs to be studied in detail in order to tailor the mechanical properties by selecting certain material combinations.
In this study, Cu-based metallic multilayers, i.e. Cu-Ni, Cu-Au, and Cu-Cr, were investigated by nanoindentation and microcompression. All films with total thicknesses between 1 and 2 µm were deposited on (111) Si substrates by magnetron sputtering. The layer thicknesses were varied between 25 and 100 nm. Focused ion beam prepared pillars with interfaces perpendicular and at different tilt angles to the loading direction were tested in order to investigate the shear resistance of incoherent (Cu-Cr) and semi-coherent interfaces (Cu-Ni, Cu-Au). The interfacial shear stresses were determined from in-situ microcompression in a scanning electron microscope using digital image correlation.
For all material systems, the hardness or yield strength increases when the the individual layer thickness decreases from 100 to 25 nm. Comparing all three systems with a fixed layer thickness of 25 nm, a higher yield strength for the Cu-Cr system can be observed, whereas the yield strength of the Cu-Ni and Cu-Au system are in the same range. For the Cu-Ni system, it was observed that the addition of 10at% Ag to the Cu layers resulted in a 20% increase of the yield strength. It is assumed that the addition of Ag increases the misfit dislocation density at the interface. Differences in the shear resistance were observed for the different types of interfaces.