Dependence of Vickers Hardness on Layer Thickness in Electrodeposited Ni–Co–Cu/Cu Multilayered Films

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The layer thickness dependence of Vickers hardness was investigated in Ni–Co–Cu/Cu multilayered films having layer thicknesses h ranging from 10 nm to 300 nm. The Ni–Co–Cu/Cu multilayered films were fabricated by electrodeposition. In the layer thickness range of h ≥ 75 nm, the hardness increased with decreasing layer thickness. Conversely, the hardness decreased with decreasing layer thickness at h ≤ 75 nm. At h = 10 nm, the hardness decreased to 97 HV, although the local maximum at h = 75 nm was 210 HV. In the X-ray diffraction (XRD) profiles around the fcc (111) peak, the 10 nm multilayered film revealed a single peak, while two peaks corresponding to the Ni–Co–Cu and Cu layers were detected in the other multilayered films. The low hardness of the 10 nm film can be understood from the absence of interface strengthening, which was deduced from the single XRD peak. For 20 nm ≤ h ≤ 75 nm, the two (111) peaks approached each other. The decreased hardness in this region could be related to sparse misfit dislocations, which can be estimated from the XRD peak angles.