Development of nickel-free barrier coating for the electroplating industry

Abstract

The European Union has been an important market for our watches, spectacle frames and jewellery. Nickel was used as the electroplated barrier coating to control the migration of the basis metal to the gold and gold alloy over-plated decorative. Since 1994, the Council of European Union has controlled the used of nickel in objects intended to be in contact with skin. The search for a barrier coating to replace Nickel has been a popular research topic over the years. Palladium and copper tin alloys have been explored but the price of the former and the temperature instability of the latter have caused concerned to the industry. Cobalt is less toxic in terms of skin toxicity and has low solubility for copper. Cobalt and its alloys have been proved to be good barriers for copper diffusion. However, the corrosion resistance of cobalt is worse than nickel. The main aim of this project is to develop binary and ternary electroplated cobalt alloys as barrier coatings which possess both good barrier and corrosion resistant properties. The plating solution compositions and the process conditions were investigated. The barrier and corrosion resistant properties for a range of coatings were compared. By using citrate as complexing agent, the cobalt-molybdenum(Co-Mo), cobalt-phosphorus(Co-P) and cobalt-molybdenum-phosphorus(Co-Mo-p) alloy coatings were successfully developed and electrodeposited. The Co-Mo with 31.6% of molybdenum and Co-P with 22.1% P were optimized. By combining the Co-Mo and Co-P plating baths, the Co-Mo-P bath was obtained. The coating contains 8% Mo and 20% phosphorus. To study the diffusion property, the interdiffusion coefficients of copper/barrier were determinated. The micro-profiling and Boltzmann-Matano Method were used to obtain the interdiffusion coefficients. It was found that the interdiffusion coefficients for nickel are higher than that of the cobalt base alloys. The results of this project have indicated the potentials of these alloys as effective and economical substitutes for nickel.