Developing plating baths for the production of reflective Ni–Cu films

Electrolytic baths have been developed to obtain bright nickel–rich nickel–copper micrometric films with good mechanical properties and high reflectivity, valid as components of optical mirrors. As metal source, the developed baths contain nickel (II) sulfamate and copper (II) acetate. Citrate was used as buffer and complexing agent to favour the simultaneous electrodeposition of copper and nickel. The influence of citrate concentration, solution pH, [Ni(II)]/[Cu](II) ratio and applied potential on the composition was studied. In order to achieve constant composition throughout the deposit thickness, potentiostatic technique was selected. The optimization process was followed according to a simple factorial design. By adjusting the conditions, compact, uniform and fine grained nickel–copper (Ni–Cu) electrodeposits with high Ni percentages and low roughness were obtained. The deposits show mechanical properties suitable for electroforming. The optical properties (reflectivity and gloss) of the Ni–Cu films make them adequate to be elements of optical mirrors.

► Stable and less aggressive nickel–copper–citrate solutions have been developed to prepare deposits with low copper content.
► Factorial design in optimization process leads to high [Ni(II)]/[Cu(II)] ratios, pH's values between 4 and 4.5 need high copper complexation.
► Low copper incorporation improves mechanical properties maintaining nickel reflectivity.
► Composition is less sensitive to applied potential at the negative values.