Studies on electrochemical deposition and physicochemical properties of nanocrystalline Ni‐Mo alloys

•Plating conditions for obtaining superior nanocrystalline Ni‐Mo coatings were developed.•Effect of electrodeposition parameters on Ni‐Mo composition and properties was studied.•Molybdenum acts as a modifier reducing the grain size of Ni‐Mo alloys from 50 to 2 nm.•The resulting Ni‐Mo coatings are characterised by high hardness and good wear resistance.•For alloys of Mo content exceeding 16 wt% the inverse Hall-Petch relation is observed.

Nanocystalline Ni‐Mo coatings were deposited from citrate-ammonia solution, in a model system with a rotating disc electrode (RDE) using low carbon steel discs as the cathode. The effect of parameters such as electrolyte solution pH (from 4 to 10), temperature (from 20 °C to 60 °C), and hydrodynamic conditions (RDE speed in the range of 0 to 640 rpm) on chemical and phase composition, microstructure, average crystallite size of coatings and current efficiency of electrodeposition was investigated. It was confirmed that, in the induced co-deposition process, molybdenum acts as a grain refinement modifier. This, apart from influencing changes in surface morphology, has a significant effect on the mechanical properties of the layers. For alloys containing less than 16 wt% of Mo (related to the crystallite size of 7 nm) loss of strengthening effect (the inverse Hall-Petch relationship) was observed, which was correlated with the decrease in coatings' resistance to wear. Hence, for electrodeposition nanocrystalline Ni‐Mo alloys characterised by high microhardness, good resistance to wear and adhesion to the steel substrate, the suitable conditions for electrolysis were determined. These conditions include: electrolyte solution pH above 7, plating temperature in the range of 20 °C to 40 °C, and hydrodynamic conditions corresponding to the RDE speed of 260 to 640 rpm.