Electrodeposition of Ni-Al2O3 composite coatings with combined addition of SDS and HPB surfactants

•Role of HPB in electrodeposition of Ni-Al2O3 coatings was studied systematically.•Zeta potentials of the particles were measured in both Watt's bath and water.•Dispersibility of particles is improved dynamically in Watt's bath with agitation.•Co-deposition of Al2O3 particles is enhanced by cathode capture with HPB addition.•Surfactants facilitate a smooth and dense coating by improving wetting of bath.

The effect of hexadecylpyridinium bromide (HPB) combined with sodium dodecyl sulfate (SDS) on electrodeposition of Ni-Al2O3 composite coatings using fine Al2O3 particles of average size of 150 nm and a typical watt's bath was investigated by evaluating the dispersibility, migration of alumina particles in the plating bath and their capture and embedding in the resulted coatings with varying HPB concentrations. The dispersibility and stability of the Al2O3 particles suspended in the electroplating bath were evaluated through measuring the particle size distribution and the particle sedimentation. The depositing dynamics of Al2O3 particles was discussed based on analysis of particle surface chemistry, zeta potential with simultaneous particle motion observation, and contact angle of bath on the substrate and coatings, respectively. The dispersibility and stability of the alumina particles in Watt's bath seemed not to have been improved by the surfactants according to the static measurement results of particle size distribution and sedimentation. However, the dynamics of Al2O3 particles co-deposition were apparently modified by the surfactant addition with analysis of Al2O3 particle motion in the electroplating bath as increasing the HPB addition at a fixed SDS content, where a random movement of particles in the bath is observed correlating to a fluctuation of zeta potential, in contrast to continuous increasing of zeta potential in water-diluted case as also reported in previous studies for co-electrodeposition of both oxide and carbide particles reinforced composite coatings. It is concluded that, directional electrophoresis migration of ceramic particles across the bath is limited due to the high ionic concentration in the bath environment. The combined addition of HPB with SDS led to the highest concentration of co-deposition of Al2O3 particles into composite coatings, over both the cases of SDS addition and without surfactants. Moreover, HPB addition alone deteriorated the coating quality with notable coating exfoliation. The adsorption of cationic HPB on the surface of Al2O3 particles promoted the particle capture onto cathodes and favored particle dispersion especially with agitation of the bath by forming “soft” agglomerates of the particles, while the anionic surfactant of SDS may facilitate a smooth and dense coating without pore defect by significantly improving wetting of bath on both copper substrate and nickel coating.