In situ analysis of electrochemical deposition kinetics for nanocrystalline nickel-cobalt alloys with varying bath hydrodynamics

The promising mechanical properties of electrochemically deposited nickel-cobalt alloys (eNiCo) significantly depend on the resultant grain size. This study describes the influence of bath hydrodynamics (BHD) on grain size and thus material properties of the electrodeposited layers. To study the role of BHD in grain growth, in situ analysis using hydrodynamic galvanostatic electrochemical impedance spectroscopy (HGEIS) and in situ hydrodynamic linear sweep voltammetry (HLSV) techniques are used to correlate the influences of BHD on grain growth kinetics. The HGEIS studies revealed that with increasing BHD velocity from 0 cm/s to 42 cm/s, the charge transfer resistance decreased, while the double layer capacitance increased. The HLSV studies revealed that the limiting current density increased with BHD velocity, whereas the corresponding computed diffusion layer thickness decreased from ca. 115 mu m to 96 mu m (16.4 % reduction) for eNiCo coatings and from 120 mu m to 106 mu m (11.6 % reduction) for eNi coatings, respectively. Thorough analysis using ion channelling images from focused ion beam (FIB) cross sections of the deposited films in combination with a qualitative model revealed a strong correlation of BHD with diffusion layer thickness and grain size. Whereas without BHD a predominant columnar growth and only a thin layer formed directly after nucleation with fine grain distribution are observed, at large BHD velocities a fine equiaxed grain growth over the full layer thickness was achieved. The microstructure transformation as function of BHD, consequently resulted in improved micromechanical properties which is exemplified by the microhardness up to 692 HV0.01 of the eNiCo coatings. For eNiCo a strong relation between mean grain size value derived from focused ion beam ion channelling microscopy and Vickers microhardness of the layer was proved.