A comparative study on the effect of electrodeposition current density on the grain size of Ni and Ni-W nanocrystalline coatings

The current density of electroplating is one of the vital synthesis variables for control microstructure and properties of electrodeposited nanocrystalline coatings. The mechanism of its effect on the grain size of single element deposits may be different from alloy coatings. In order to evaluate this hypothesis, nickel and nickel-tungsten were electrodeposited on the copper samples by direct current plating at several current densities ranging from 15 mA/cm2 to 90 mA/cm2. The microstructure of the both type prepared coatings were characterized by X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray spectrometry. It is observed by increasing the current density, the weight percentage of W in Ni-W coating decreased. Also the grain size of Ni-W coating was remained about 5 nm, independent of current density up to 60 mA/cm2. But beyond 60 mA/cm2 the coating with coarser grains was obtained as current density increased. In comparison, a different behavior was observed for the nickel coating, so that the average grain size was decreased according to inverse power relation with increasing current density up to 60 mA/cm2 and its change was not noticeable at current density beyond 60 mA/cm2. These behavioral difference was explained by the competition between current density and amount of reduced tungsten ions effects on nucleation and growth processes. Also, by introducing a relative reduction current and its calculating using voltammetry data, the effect of current density on the amount of tungsten in Ni-W nanocrystalline coating was modeled.