Preparation, characterisation and electrochemical study of crack-free nanocrystalline electrodeposited Co-W alloy coating of high hardness

Abstract

In this work, a lustrous metallic grey nanocrystalline Co-W alloy thin coating was successfully fabricated on copper substrates via galvanostatic deposition from an acidic glycine complexing bath. The impacts of various operating conditions e.g. glycine concentrations, applied current, pH and temperature on the cathodic polarisation behaviour, anodic stripping voltammetry, cathodic current efficiency (CCE%), and alloy composition were examined. This gave the opportunity to establish the operating conditions for producing various Co-W alloys. It was found that the W content in the alloy was in the range of 7.7–13.8 wt.% depending on the operating conditions. Moreover, the CCE% of the deposited alloy was characterised by relatively high values (up to 97.0%) in comparison with other baths. The XRD analysis indicates that the Co-W alloy consists of a hexagonal close-packed (hcp) Co lattice crystalline structure of nanosized grains, of average size 15.44 nm, using 25 g L−1 glycine, at 25°C and the highest current density tested.. The microhardness of Co-W alloy coatings is comparable to or even higher than that of hard chromium coatings (∼ 963–1119 kgf mm−2). The Co-W alloy coatings are crack-free as shown from SEM investigations. The glycine bath used for Co-W alloy codeposition is environmentally friendly.