Structure, morphology and thermal stability of electrochemically obtained Ni-Co deposits

Nanostructured nickel-cobalt alloy powder deposits were obtained electrochemically on Cu substrates in the current density range 40-400 mA cm-2. The influence of the current density and of the Ni2+/Co2+ ratio in the bath on the microstructure and phase composition of the Ni-Co deposits was studied by SEM and X-ray diffraction methods. Both the bath composition and the current density strongly influence the deposit growth mechanism as well as the deposit composition, microstructure, grain size and surface morphology. If the concentration ratio in the electrolyte is Ni2+/Co2+ = 4, the deposit has a cauliflower structure with mean grain size of 13 nm. In contrast, the particles deposited from the electrolyte with Ni2+/Co2+ = 0.25 show platelet structure with preferred orientations and mean grain size of 20 nm. When electrodeposition was performed at high overpotentials, far from equilibrium conditions, face-centered cubic (FCC) solid solutions of Ni and Co were generated while at low overpotentials, as well as at higher content of cobalt in the electrolyte, hexagonal-close packed (HCP) Co was formed. The structure of nanocrystalline deposits exhibits a strong tendency to structural changes under annealing. DSC of the alloy deposits shows a stepwise process of structural changes in the temperature range from 393 to 823 K. It was found that under annealing, HCP → FCC phase transformation occurs in nanocrystalline deposit obtained from electrolyte with a concentration ratio Ni2+/Co2+ = 0.25.