An Investigation Into The Mechanism Of Formation Of Electrodeposited Composite Coatings

A mechanism for the formation of electrodeposited composite coatings has been proposed which was based on a study of the acid copper sulphate-alumina system. Apparatus was constructed to provide a controlled cathodic vibration, and this was used to examine the effect of bath parameters on the adsorption of particles onto the cathode and on the formation of a solid particle-cathode bond by the deposition of metal. The strength of the bond established between the particle and the cathode without the influence of deposited metal was found to be independent of pH, temperature and the chemical nature of the particle and substrate. However, the ability of this bond to hold the particle onto the cathode, long enough for it to be permanently fixed as a result of metal deposition, would be influenced by the prevailing agitation conditions and by the particle size and density. Electrostatic attraction was found to have an insignificant influence on the strength of this bond. The adhesion of: the particle was found to rapidly increase as a result of metal deposition, the rate of formation of this bond increasing as a function of metal ion adsorption on the particle. Although metal ion adsorption increased with temperature, it was deduced from particle content - current density curves that the rate of formation of the bond, at low current densities, decreased with increasing temperature. This was attributed to the relative ease of reduction of metal ions from the bulk solution compared to those adsorbed on the particle. At low current density the rate of formation of the particle-cathode bond controls code position. As the current density is increased the efficiency of bond formation increases such that at high current density the rate of arrival of particles to the cathode surface becomes the controlling factor. The particle arrival rate was shown to increase firstly with temperature, due possibly to a reduction in the thickness of the boundary layer at the cathode surface and secondly with reducing surface roughness, The rate of formation of the solid/solid bond was found to be highly sensitive to chloride in the case of the copper-alumina system; its removal by the addition of silver or thallium promoted code position.