Effect of current density on deposition process and properties of nanocrystalline Ni–Co–W alloy coatings

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Abstract

The nanostructure Ni–Co–W alloy coatings were electrodeposited onto a copper substrate using different applied current densities, in a modified Watts-type bath. The coatings were single-phase solid solutions with average grain sizes about 6–11 nm, calculated from X-ray diffraction patterns using the Scherrer equation. EIS results showed that the adsorption and reduction of W-containing ion complexes dominated at all applied current densities. However, the diffusion of the ion complexes reached to a limitation at higher current densities. The W and Co contents of the coatings decreased with an increase in the applied current density. A homogeneous nodular surface morphology was obtained at all current densities. The coatings produced at low current densities, containing higher amount of alloying elements, showed lower corrosion resistance.

Introduction

The electrodeposited Ni–Co coating is an important engineering material widely used in various applications of magnetic devices, especially in micro-system technologies to manufacture sensors, actuators, micro-relays and inductors [1]. The electrodeposited Ni–W coating exhibits higher hardness and scratch resistance as compared with the finest pure nanocrystalline Ni alloys, although the contribution of solid-solution strengthening from W is expected to be essentially negligible [2]. In fact, interest in the electroplating of W-containing alloys stems from the tendency to enhance the hardness and also the corrosion resistance of the coating [3]. Regarding the characteristics of Co–W alloy that possesses interesting catalytic properties, but shows lower corrosion resistance, nickel is added into the bath to improve the corrosion resistance of coatings produced [4]. On the other hand, it has been reported that introducing W into Ni–Co coatings improves durability, hardness and resistance to high temperatures [3].

For the first time, the ternary Ni–Co–W coating was utilized in the production of magnetic-film memories, just because of its desirable soft magnetic properties. This coating also found wide applications in surface micromachining as well as magnetic or magneto resistive [5], heat conductive [6], good wear resistance and electrocatalytic activity [7]. No any reference could be traced out in the literature for electrodeposition of this ternary alloy except the results presented by Singh et al. [8].

The aim of this work is to study the electrodeposition mechanism of Ni–Co–W alloys by electrochemical impedance spectroscopy (EIS) technique. Factors influencing amounts of W and Co content, morphology and corrosion resistance of the coatings are also studied.

Section snippets

Materials and experimental procedures

Copper substrate was used in disk shape with 0.85 cm2 surface area. The substrate was mechanically polished up to 1200 grade abrasive paper and then electropolished in a solution containing 65 wt% phosphoric acid and 35 wt% distilled water for 15 min. Ni–Co–W alloy was electrodeposited from a modified Watts-type bath at 25 °C. The composition of the bath is shown in Table 1. The number of coulombs passed for all coatings was kept constant at 36 C. According to Faraday's law, this should lead to a

Deposition current densities

Suitable current densities for electrodeposition were selected based on the cathodic scan plot (Fig. 1). As it can be seen, by decreasing the cathodic potential, within the range of −500 to −800 mV (region I), the current density remains almost constant. This potential range is most probably related to the limitation arises due to predominant hydrogen evolution. In this way, hydroxide species may be formed by hydrolysis due to the local pH increase near the cathode surface [11]. For coating

Conclusions

  • 1.

    Increasing the deposition current density would decrease the W and Co contents of Ni–Co–W coatings.

  • 2.

    In spite of increasing the deposition current density, grain size of the Ni–Co–W coatings remained constant at 6–11 nm.

  • 3.

    The highest corrosion resistance was obtained for the coatings produced at high current densities (region III on cathodic polarization reading). It seems that the contrary effects of Co and W elements determine the corrosion resistance of Ni–Co–W coatings.

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