Electodeposition of compositionally modulated tin-cobalt alloy multilayer coatings as plain bearing overlays

Lead-based alloy electrodeposits have been used as overlays for plain bearings for many years. The toxicity of lead has not been a problem for the industry until recently with the initiation of the End-of-Life Vehicle (ELV) directive in the European Union. This legislation requires the member states to phase out lead in the future, thus promoting a search for lead-free alternatives. The aim of this research was to investigate the electrodeposition and the fatigue properties of compositionally modulated Sn-Co alloy multilayer coatings as possible overlays for plain bearings. Following a comprehensive review on the electrodeposition of Sn-Co alloys, a novel sulfate/gluconate electrolyte was established, from which compositionally modulated multilayer coatings can be deposited by varying the applied current density only, i. e. pure tin coatings were deposited at current densities lower than the limiting current density for the deposition of tin, whilst Sn-Co alloy coatings were deposited at much higher current densities. It was found that the pH value of the solution was crucial to maintain the desired deposit composition/current density relationship, with pH 3.5-4 being the optimum range. The main function of sodium gluconate was to inhibit the hydrolysis of tin(II) ions. The total concentration of tin(II) ions in the electrolyte and the agitation strength were the two factors which were employed to markedly increase the limiting current density for the deposition of tin, leading to an increased overall deposition rate for coatings. It was also found that the addition of boric acid and a non-ionic surfactant, Tween 20, to the solution was essential for the deposition of satisfactory Sn-Co alloy coatings. The electrodeposition of Sn-Co alloys was also carried out using pulsed current, in order to enhance the electrocrystallisation process and thus prevent the formation of dendritic coatings. Compositional analysis indicated that Sn-Co alloy coatings obtained under pulsed current conditions had more uniform cobalt content within a wide current density range of 1-4 A/dm 2 than under direct current conditions. After ruling out the selective dissolution of cobalt during the off-time by AES depth profiling, the effect of pulsed current parameters on the deposit composition was explained on the basis of the mass transport theory derived from Ibl's duplex diffusion layer model for pulse electroplating, leading to the conclusion that pulsed current was able to slow down the deposition rate of the more noble component of an alloy by controlling the mass transport in the non-stationary diffusion layer. (Continues...)