Studies on electroless Ni–W–P and Ni–W–Cu–P alloy coatings using chloride-based bath

Abstract

In the present investigation ternary electroless Ni–W–P alloy coatings were prepared using alkaline citrate-based bath. Nickel chloride and sodium tungstate were used as nickel and tungsten sources, respectively, and sodium hypophosphite was used as a reducing agent. Deposits were characterized for their structure, morphology, chemical composition and microhardness. A single peak was obtained in XRD for ternary Ni–W–P alloy. Addition of 3 mM of copper chloride in the bath has resulted in a very smooth quaternary Ni–W–Cu–P alloy. Also, codeposition of copper in ternary alloy has increased the peak sharpness and the calculated grain sizes for ternary and quaternary deposits are 2.5 and 8.8 nm, respectively. XPS studies show that in ternary Ni–W–P deposits from the present bath nearly 60 at.% of tungsten is present in the elemental form, W 0, as compared to that from nickel chloride and sulphate additives bath in which W 0 state is only 27 at.%. Addition of copper chloride to the present bath has increased this elemental form by nearly 10 at.% in the quaternary deposit. SEM micrographs show that ternary alloy films are nodular whereas quaternary alloy films are smooth and nodule-free. Microhardness measurements show a marginal difference in the hardness between ternary and quaternary deposits.