Synthesis of nanocrystalline Ni-B alloy by electroless deposition and effect of heat treatments on their structural state.

Thick films (up to 35 µm) of Ni-B alloys were synthesized by the electroless process. Coatings were then subjected to structural and morphological investigation by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The SEM study, carried out at various stages of the deposition process, showed that the deposit morphology is very much influenced by the synthesis route in aqueous solution. A columnar morphology with branching of the columns inside the coating is thus observed, as well as a cauliflower-like surface texture.
The XRD and SEM studies revealed that the deposits are constituted of a non-equilibrium nanocrystalline supersaturated solid solution of boron in nickel. The nano-grains are arranged in a wavy pattern that follows the shape of the previous deposit-liquid interface.

The effects of heat treatment were also investigated. The usual industrial heat treatment for electroless-nickel coatings (400 °C during 1 h under a controlled atmosphere containing 95% Ar and 5% H2) was applied to some of the samples.
Observation of the treated samples confirmed that the structure of the coating tends towards the equilibrium structure by the formation of nickel borides crystallites. The columnar morphology and cauliflower-like surface texture are conserved and traces of the wave-like organisation of the grains in the coatings are observables.