Pulse and DC Electrodeposition of Ni-W-TiO2 Thin Film Composites
Authors: Zhang Yujia , Podlaha Elizabeth J 
Published: February 2020

Tungsten alloys with nickel are well known for their superior properties in the following areas: corrosion resistance, hardness, wear resistance, and catalytic activity towards hydrogen evolution, as comprehensively reviewed.1 In aqueous electrolytes, tungsten cannot be electrodeposited alone, but requires iron-group ions, such as Ni(II), in the electrolyte to induce its reduction.2 Composite electrodeposition of Ni-W alloys allows for further tailoring of properties. For example, Kumar et al.
3
showed that the hardness was further increased with the addition of titania in the deposit, and when pulse deposited there was an enhancement in corrosion resistance. Similarly, Goldasteh and Rastegari4 reported an increase in hardness due to refining of the crystallite size in the deposit with titania. Corrosion resistance was found to be better in pulse deposited Ni-W-TiO2 composites, compared to their DC counterparts. Both of these studies target low tungsten deposit composition and use ammonium-containing electrolytes. In the presented work, composites with high tungsten content are examined from an ammonium-free electrolyte. The influence of the TiO2particle on the metal ion reduction rate and on the deposit morphology is investigated under DC and pulsed current density (PC) deposition.
A boric acid, sodium citrate, ammonium-free electrolyte with micro- or nano-scaled titania particles, at a pH of 8, and 25 °C, is used to electrodeposit the alloys onto a rotating cylinder Hull (RCH) cell. The RCH was employed to evaluate the deposit composition over a range of current densities, and an assessment of primary or secondary current distribution was made. Frequency was varied, from 0.2 Hz to 200 Hz, to examine the effect of PC deposition on the deposit composition. The composition and thickness of the deposits were measured by X-ray fluorescence (XRF). The partial current densities of Ni and W were calculated by Faraday’s law. Scanning electron microscopy (SEM) was used to examine the morphology of the deposits at both low and high magnifications.
Under DC deposition, the trend in particle incorporation with the amount of particles in the deposit followed a limit of the Guglielmi model,5where a Langmuir adsorption behavior dominates. The amount of particle incorporation for nano-particles compared to micro-particles was lower. Under PC deposition, the composition was slightly altered compared to DC. For example, with micro-particles, the tungsten deposit content decreased with PC compared to DC more at low current densities (PC 45 wt % W vs. DC 55 wt % W) and had little effect once the side reaction was high. SEM images show that there was a significant change in the surface morphology with the inclusion of particles; adding particles produced a rougher surface. PC deposition with different frequencies did not significantly alter the roughness at the micro-scale, but it did effect the presence of small nano-pits observed at low current densities; the higher the frequency the smaller the pit diameter and density found on the deposit surface. A discussion on the influence of frequency with both nano- and micro-sized particles will be presented.
The authors acknowledge support from NASF and NSF.

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