Initial stages of Ni-P electrodeposition: growth morphology and composition of deposits

The evolution of growth morphology and composition of deposits during the initial stages of Ni-P electrodeposition is studied using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Combined electrochemical and surface analytical measurements show that the deposition process starts at relatively low cathodic potentials by instantaneous formation and growth of hemispherical centres. The phosphorus content of deposits in the initial deposition stages is found to increase gradually with the deposition time. Additional electrochemical and XPS measurements, carried out on Ni substrates under same polarisation conditions in a Ni2+ ion free electrolyte solution, show the occurrence of a time dependent Ni-P surface alloy formation indicating a strong Ni-P interaction. It is suggested that the very early stages of Ni-P electrodeposition involve a primary instantaneous nucleation of Ni followed by a Ni-P alloy formation induced by the strong Ni-P interaction. AFM images show that in advanced deposition stages the coalescence of growing Ni-P centres leads to formation of larger growth mounds. The evolution of the resulting surface roughness is analysed on the basis of the so-called dynamic scaling concept. The estimated values for the roughness exponent and the growth exponent (alpha = 1.07 +/- 0.05 and beta = 0.28 +/- 0.05) correspond to a model involving a smoothing of the growing surface driven by surface diffusion. (C) 2002 Elsevier Science B.V. All rights reserved.