Nano-structure of Co–W alloy electrodeposited from gluconate bath

A series of ten Co–W coatings have been electrodeposited from the same gluconate bath. The W content of the samples is found to decrease from 22.7 at.% in the first sample to 11.0 at.% in the final sample as the consequent W content of the bath is reduced. The third coating in the series with 18.0 at.% W has been investigated by SEM, TEM and XRD and microhardness testing. This is shown to have a unique, well-ordered, self-organised two-phase nanostructure of platelets parallel to the growth direction separated by a relatively thick grain boundary phase. The platelets have a very well-defined width of 4–5 nm and the interfacial phase is roughly 1 nm thick. The Gibbs free energy for a solid-solution of W in HCP Co is shown to possess spinodal chemistry between the ranges of 8 and 23 at.% W. All the coatings deposited have compositions within this range and this is proposed as a possible mechanism for the origin of the nano-platelet formation. This provides a very well-defined length scale for the lamellar nanostructure. It is shown that the resulting nanostructure is not in compositional equilibrium, but that a final equilibrium state for the self-organised nanocrystalline structure does exist and can be achieved through further W segregation, propagated by annealing at 600 °C. The hardness of the coating is found to increase with the nominal W content, and this is predicted to arise from solid-solution strengthening of the platelets rather than an increase in the area of dislocation-pinning grain boundaries. The uniformity of the underlying nanostructure is expected to significantly enhance the mechanical properties of the coating.