The electrodeposition and characterisation of compositionally modulated tin-cobalt alloy coatings as lead-free plain bearing material

Traditionally, lead-based bearing overlays dominate the commercial automotive market and it has been proven that an excellent combination of properties can be attained through their use. However, lead is a toxic metal and a cumulative poison in humans. According to the European Union End-of-Life Vehicle (ELV) Directive proposed in 1997, vehicles that registered in'all the member states after 1st July 2003 should contain no lead, mercury, cadmium and hexavalent chromium. In this study, a new sulphate-gluconate electrolyte was used to produce multilayer SnCo coatings, aimed at a lead-free overlay for future market use. Tin-cobalt compositionally modulated alloy (CMA) coatings produced from sulphategluconate electrolytes have been previously examined as a potential replacement for lead-free bearing overlays [1]. However, some obstacles may exist which limit their potential use on an industrial scale. For example, long electroplating times are required to produce a thick coating which is very undesirable from an industrial viewpoint, and also the possible elemental interdiffusion occurring in the coating system under engine operating temperatures could rapidly deteriorate the coating properties. In addition, there is an increasing demand from automotive industry to further improve bearing overlay properties, for example for high performance and high compression ratio engines... cont'd.