Electroless deposition of group eight metals
Electroless plating is the controlled chemical reduction of metals without the use of an applied external potential. The use of electroless plating has a number of advantages over electroplating including the ability to deposit coatings onto complex-shaped objects which can be metallic or non-metallic, such as plastics. The most widely used electroless coating is electroless nickel prepared using sodium hypophosphite as reductant. This results in coatings which contain phosphorus. The properties of metal coatings containing non-metals are frequently superior compared to those of the pure metal.The first part of present work was concerned with the development of an electroless plating solution capable of depositing palladium-boron coatings. The plating solution is based on EDTA-stabilised palladium (H) ions in the presence of dimethylamine borane as the reducing agent. At a pH of 10.5 the solution deposits bright adherent coatings at 1 μm per 10 h. The plating solution operates at room temperature within narrow limits of composition and concentration: small deviations in composition from the optimum results in solution decomposition.Characterisation of electroless palladium-boron coatings indicates that the inclusion of boron decreases the corrosion resistance of the coating. Examination of palladium-boron coatings by X-ray diffractometry show that the lattice parameter has been increased by 2.86%.The second part of the present work was concerned with the development of pure ruthenium coatings. The plating solution developed uses hydrazine hydrate as the reducing agent with methylamine as stabiliser. The plating solution operates at a pH of 13 and at 60°C, and deposits 65% of the available ruthenium in 5 h. This corresponds to a 3 gm coating over a substrate surface area of 25 cm2.The coatings produced are bright and adherent. The roughness of the substrate surface was found to be important to obtaining adherent coatings over 2 gm thick. A roughness in the order of 1100 A was found to be satisfactory. Scanning electron microscopy shows the coatings to be dense with no obvious pores. The porosity of the coatings was found to be low, decreasing with increasing coating thickness. However, the electrochemical properties of the coatings indicate that ruthenium coatings have poor corrosion resistance.The chemistry of the plating solution, particularly for ruthenium, is considered in some detail.