The effect of heat treatment on the microstructural changes in electrodeposited Ni-Mo coatings

Nanocrystalline Ni-Mo (16 wt% of Mo) coatings electrodeposited on steel substrates from citrate electrolyte solution were annealed in argon atmosphere, in the temperature range from 300 to 800 ˚C. The effect of the heat treatment on the coatings’ microstructure (surface morphology, phase composition and grain size), as well as their mechanical, tribological and corrosion properties was determined. It was found that, up to 500 °C, the average crystallite size remains almost unchanged, but relaxation of grain boundaries leads to significant increase of the coatings’ microhardness (up to 1190 HV). For Ni-Mo alloys annealed at 500 °C, the wear index and friction coefficient increase significantly from 33·10−6 mm3/Nm and 0.4 (for as-deposited samples) to 89·10−6 mm3/Nm and 0.68. Above 500 °C, thermally-induced gradual fluctuations of chemical composition in the Ni-Mo alloy, and interdiffusion of iron (from the substrate) and nickel (from the coating), were observed. The segregation of elements promotes an increase of grain size (from 10 nm to 33 nm for as-deposited and annealed at 800 °C Ni-Mo coatings, respectively) resulting in decreased microhardness to 487 HV. However, the coatings become less stiff (EIT up to 205 GPa) and are characterised by enhancement of wear (Wv about 2.2–7.8⋅10−6 mm3/Nm) and friction resistance (about 0.4–0.5). No significant influence of the heat treatment temperature on the corrosion parameters of the coatings in the sulphate environment was found. The corrosion resistance of the Ni-Mo coating annealed at 800 °C decreased slightly (Icorr=5.9 μA/cm2, Rp = 1530 Ω) in comparison to the as-deposited alloy (Icorr=4.8 μA/cm2, Rp = 2430 Ω) and that annealed at 500 °C (Icorr = 4.7 μA/cm2, Rp = 2220 Ω).