Theoretical and practical aspects of electrodeposition of metal coatings with improved properties. (Part 2)

This part presents experimental evidence for the existing correlation between leveling performance and some properties (tensile strength, ductility, internal stresses, porosity) of bright metal electrodeposits. Copper, nickel, palladium, and gold coatings, deposited from bright solution in a d-c regime and at various pulse frequencies, corresponding to the extreme values (minimum and maximum) of leveling power, have been examined. The observed correlation is based on the phenomena of diffusion and adsorption of brightener species, confirming the action of a leveling mechanism, which determines the degree of homogeneous distribution of dislocations and other defects within the deposit. Lower values of leveling power correspond to lower tensile strength, elongation and great porosity. Conversely, bright pulse-electrodeposited metal coatings with maximum leveling power are the strongest and most ductile and are characterized by less porosity. The electrochemical porosity measurements performed for these deposits have also demonstrated the existence of such a correlation between leveling performance and coating properties. Given the fact that the minimum porosity of coatings corresponds to the leveling power maximum and vice versa, it can be stated that the leveling performance could be an objective criterion for property estimation of d-c or pulse-electrodeposited metal coatings. Those pulse parameters that enable a maximum value of leveling power represent the plating conditions for electrodeposition of metal coatings with the best properties : electric conductivity, resistance to corrosion, wear and/or abrasion.