Application of Electrolyte Jet to Rapid Composite Electroplating

This paper presents a rapid electroplating process which uses an electrolyte jet containing ceramic particles for the formation of Ni-P matrix composite coatings on an aluminium alloy substrate. The electrolyte jet not only makes the deposition rate of Ni-P matrix extremely fast, but also controls the volume fraction of the particles and phosphorus contents in the deposits by changing the jet velocity. The high deposition rate is caused by a thinned diffusion layer adjacent to the cathode surface as a result of nickel ions forcibly supplied by the electrolyte jet. The limiting current density increases linearly with the square root of jet velocity. As the jet velocity increases, however, it reaches saturation at the current density determined by the hydrogen evolution voltage. The volume fraction of SiC particles in deposits decreases to zero with increasing jet velocity. This is because the particles adsorbed on the cathode surface are subjected to a higher shearing force on desorption when the jet velocity accelerates. In contrast, the adsorbed particles are subjected to a lower shearing force when the jet slows, although the particle adsorbing force increases owing to the cathodic potential. The phosphorus content in matrices increases with the increase in jet velocity and with decrease in the current density. The matrix grain size becomes finer owing to a restraint on the growth rate of nickel crystals by adsorbed phosphorus atoms. Therefore, through a change in both the jet velocity and current density, the jet electroplating process achieves compositionally graded codeposits of the particle volume fraction and/or the phosphorus content with a high deposition rate.