Electrodeposition of Ni-Fe-Mo-W Alloys - Part 10
Tenth Quarterly Report - AESF Research Project #R-117. This NASF-AESF Foundation research project report covers the tenth quarter of project work (April-June 2015). In this period, Graduate student Avinash Kola has continued work on the influence of deposition conditions on the properties of the Ni-W alloys.
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Prof. E.J. Podlaha-Murphy* and A. Kola
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the tenth quarter of project work (April-June 2015). Progress on the previous quarters has been published in summary in the NASF Report in Products Finishing and in full at www.pfonline.com.** A printable PDF version is available by clicking HERE.
The project, initiated in January 2013, addresses the induced codeposition of molybdenum and tungsten alloys with nickel and iron having a focus on developing a toolbox of plating conditions to deposit different combinations of Ni, Fe, Mo and W. This paper covers progress made during the tenth quarter.
In previous reports, the effect of 2 butyne-1,4-diol (BD) on Ni-W plating from an ammonia-free electrolyte at pH 2 and 8 were examined. In both cases the addition of BD concentration in the electrolyte caused a lower current efficiency, but an expected smoother deposit surface. In this report, the addition of boric acid was investigated to examine, in particular, if the current efficiency can be improved in the presence of BD, and if, in turn, the addition of boric acid affects the deposit composition.
Boric acid is known to be a good buffering agent,1 has been shown to enhance the deposition rate of nickel by lowering its overvoltage2,3 and can help to suppress the hydrogen evolution side reaction.4-6 Thus, it would be expected to improve current efficiency. Electrolytes used to electrodeposit Ni-W thin films typically contain ammonia. Electrolytes without ammonia have been developed, with a large quantity of tungsten in the deposit, but at a loss of current efficiency.7-11 In previous reports from our lab, Ni-W thin films and nanowires were successfully deposited in an ammonia-free, boric acid-citrate electrolyte.12 The motivation to use boric acid was adopted from previous literature reports of using boric acid in Co-W electrodeposition.13,14
In Ni-W electrodeposition from ammonium containing electrolytes, Donten, et al.15,16 included boric acid in the electrolyte to obtain amorphous tungsten-alloys with nickel, iron and cobalt, though the effect of boric acid was not explicitly studied. Wu, et al.17 comprehensively examined the effect of boric acid addition on the deposit composition, structure and hardness of electrodeposited Ni-W with a plating bath containing ammonium salts. They showed that the tungsten content increased gradually with increasing boric acid electrolyte additions, with a parabolic change in current efficiency. The Ni-W current efficiency was improved between with an addition of 0 to 0.5 M boric acid, and then decreased as the boric acid concentration was further increased. Grain refinement was observed with the increase in the boric acid electrolyte, with an accompanying composition increase in wt% tungsten. Improved hardness of the deposit was correlated with a decrease in the grain size up to a point. It then fell with higher wt% tungsten. Thus, it is possible, based on these studies, that there may also be an influence of boric acid on the deposit composition with an ammonia-free electrolyte. To this end, the electrodeposition of Ni-W is examined in an ammonia-free electrolyte, pH 8, with the addition of the following amounts of boric acid: 0, 0.5 and 1.0 M.
A traditional Hull cell with air agitation was used to deposit Ni-W alloys from an ammonia-free electrolyte. The electrolyte contained 0.1 M nickel sulfate, 0.15 M sodium tungstate, 0.285 M trisodium citrate, 5 mM BD and variable amounts of boric acid, at a pH of 8 and at room temperature (RT), 22-23°C. Further, the influence of temperature was considered by inspecting one condition at 60°C, with the electrolyte containing 0.5 M boric acid. The average applied current density was -50 mA/cm2 on a copper plate, and with an agitation rate of 3 L/min created from the use of air jets near the cathode surface. The deposition time was 30 min. Polarization data was obtained by placing the electrodes parallel to each other using a three-electrode cell configuration with a Ag/AgCl reference electrode. The scan rate of potential was 10 mV/sec. The curves were corrected for ohmic drop with impedance spectroscopy (not shown). Composition and thickness were measured using x-ray fluorescence (XRF).
Results and discussion
The polarization curves in Fig. 1 show that there is an influence on the total current density with boric acid concentration at room temperature. With increasing concentrations of boric acid, there is an inhibitory effect on the total current in the potential range of -0.6 to -1.2 VAg/AgCl (see dashed region in box) at low current densities. Above a cathodic current density of 50 mA/cm2 the total current density doesn’t follow a trend with the addition of boric acid. There is a sharp increase in current density for the 0.5 M boric acid case at a potential of about -1.1 V, indicative of the exponential increase in water reduction, while for the 1.0 M boric acid case the rise in current density occurs at more negative potential values. A closer look at the partial current densities helps to understand whether the metal or side reaction partial current density, or both, is affected by the boric acid addition. In order to determine the partial current densities the deposition composition and mass were evaluated.