Relationship between film composition and microhardness of electrodeposited Ni–W–B films prepared using a citrate–glycinate bath

In the present study, the authors fabricated the electrodeposited Ni–W–B films with various W and B contents and evaluated their composition, surface morphology, crystal structure and microhardness. The Ni–W–B films were deposited using a Ni–W plating bath containing dimethylamine borane as the boron source and citrate and glycine as complexing agents. It was possible to prepare Ni–W–B films with a wide range of W (0–19 at.%) and B (0–18 at.%) content by controlling the plating condition (tungstate concentration, glycine concentration and current density). The hardness of the Ni–W–B films with a nanocrystalline phase was proportional to the square root of the grain size, in accordance with the Hall–Petch relation. However, in the region of amorphous or nanocrystalline phase of 2–3 nm or less in grain size, hardness decreased. The maximum hardness of the Ni–W–B films was about 850 Hv, comparable to that of hard chromium plated films. It is very important to optimize the film composition in order to fabricate Ni–W–B films with high hardness.