Characterization of Cu/Ni multilayer coatings by nanoindentation and atomic force microscopy

Cu/Ni multilayer coatings prepared by RF/DC magnetron sputtering process were characterized using x-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) techniques. Films deposited under certain deposition conditions showed the appearance of satellite reflections around the principal reflections in the XRD data, indicating the formation of superlattice structure. Nanohardness measurements were performed on the films prepared under different deposition conditions, e.g. modulation wavelength (Λ), copper to nickel thickness ratio (tCu/tNi) and substrate temperature (Ts). Nanohardness data revealed that the hardness was enhanced by a factor of ∼2.5 times that of the rule-of-mixtures value. This enhancement in hardness occurred over a limited range of Λ. The results indicated that the hardness also depends on the copper to nickel thickness ratio and for tCu/tNi≃0.30 the films exhibited a maximum hardness. The hardness increased almost linearly with substrate temperature up to 200 °C and got saturated at 250 °C. No considerable change in the hardness was observed after vacuum annealing of the coatings. Imaging of the residual imprints by AFM revealed pile-up formation after indentation. The extent of pile-up, a measure of plastic flow of the material, was dependent on the preparation conditions of the multilayer coatings.