Effect of annealing on structure and properties of pulse electroplated Ni-W alloy coating

Abstract

The structure and properties of Ni-W alloy coatings, deposited through pulse-plating route from an aqueous sulphate-citrate bath, have been evaluated and compared with those of coatings annealed under vacuum for durations of 2 h, 4 h or 6 h. The effect of annealing on microstructural evolution of the alloy coatings has been examined with emphasis on identification of constituent phases by X-ray diffraction, along with studies of their morphology by scanning and transmission electron microscopy with selected area electron diffraction. Annealing of Ni-W alloy coatings has resulted in the replacement of globular cluster-type appearance by a faceted morphology with the increase in crystallite size and reduction of the micro-strain in the alloy matrix. Both Nanoindentation hardness and elastic modulus of the coatings appear to reach their peak values on annealing for 2 h owing to the formation of NiW and Ni4W intermetallic phases. However, on annealing for durations ≥4 h, the hardness is decreased owing to grain coarsening, whereas the Young’s modulus is lowered due to diffusion of Cu from the substrate. The corrosion resistance of the annealed Ni-W alloy coatings, evaluated through potentiodynamic polarization technique, is found to be inferior as compared to that of the as-deposited coatings.