Effect of polyethylene glycol molecular weight on the electrodeposition of nanostructured nickel

Electrodeposition is an attractive approach to synthesize nanostructured metals and their microstructure can be effectively controlled by tuning the process parameters. Here, two molecular weight (MW) types of polyethylene glycol (PEG) were used as additives for nickel electrodeposition in Watts bath. The surface morphology, crystal orientation, microstructure and microhardness of the Ni film were investigated. Experimental results revealed that the MW of PEG played a decisive role in controlling the microstructure of the deposits. As the current density increased from 25 to 100 mA/cm2, the microstructure of the Ni film deposited from the PEG600-containing bath evolved from ultrafine grains with nanotwin lamella perpendicular to the substrate surface to randomly oriented nanotwins, where the average grain size decreased from 118 nm to 78 nm. Within the same current density range, the Ni film obtained from the PEG10000-containing bath presented fine nanograins with multifold nano-twins (e.g., fivefold nanotwins) in random orientations with the average grain sizes of around 10 nm, little influenced by the current density. Such microstructure transition could be attributed to that the addition of PEG facilitated the formation of twins, and higher MW PEG induced a greater inhibition effect, promoting significant grain refinement.