The New Examples of Tungsten Induced Codeposition Cu-W and Cu-Ni-W Alloys
Authors: Bacal Pawel , Donten Mikolaj , Stojek Zbigniew Jan 
Published: February 2020

Electrodeposition of new binary copper-tungsten alloys and ternary tungsten-copper-nickel alloys contradict the commonly assumed fact that tungsten cannot be electrocodeposited from aqueous solutions with other than iron-group (Co, Ni, Fe) metals [i]. This unusual electrodeposition behavior was named ‘induced codeposition’ by Brenner. The process of electrodeposition the new Cu alloys also can be also classified to this phenomenon [ii]. The obtained alloys, besides of their great potential of industrial applications, bring a new look into the induced codeposition phenomenon.

It is commonly known that the binary Ni/Fe/Co-W alloys obtained under particular conditions of galvanic process form amorphous/nanocrystalline phases [iii]. This internal organization strongly influences their mechanical and corrosion behavior. Excellent values of the parameters, which are important in the industrial usage, such as hardness and thermal-, wear- and corrosion resistance make them advanced materials with a wide range of possible applications. The W-Ni/Fe/Co alloys can be used as a replacement for hard chrome coatings and as protective layers [iv]. The new obtained materials (Cu-W and Cu-Ni-W) exhibit similar properties, thus they may also find the applications analogous to the previously mentioned Ni/Fe/Co-W alloys.

Both materials, Cu-W and Cu-Ni-W, were obtained via direct current electrodeposition from the citrate plating bath. Authors were able to obtain the materials with a wide range of elemental composition ratio. The required parameters to obtain the desired combination of composition and morphology were established and carefully investigated. The deposits were prepared on silver substrate, to avoid contamination of the deposited alloy with the substrate material; however; authors were also able to deposit the new alloys on other substrate materials such as stainless steel, copper, zinc and many others. All depositions were performed at elevated temperature (70 ◦C) in the presence of amorphous structure promoters such as boric acid and phosphoric acid in the bath.

The Faradaic Efficiency of the deposition and its correlation with both: bath composition and current density was investigated in details. The process was optimized to obtain the material containing the high amount of tungsten with as low as possible number of surface defects. In comparison to the Ni/Fe/Co-W alloys, the best achieved CuW and CuNiW alloys exhibited less cracks and defects, what was probably caused by a decrease in the internal structure stress. Hardness of the layers was correlated with alloys composition. In the obtained alloys the content of W could reach up to 14 at.%. (Cu-W) or even 28 at.%. (Cu-Ni-W). The preliminary corrosion experiments revealed that the materials hardness and corrosion resistance changes significantly with W content. The internal structure of those materials was also investigated by using TEM microscopy of thin lamellas cut out from the layers and by SEM micrographs of the cross brakes and the cross-cuts. These investigations revealed peculiar and unique internal structure of the Cu-W alloy. Powder XRD and SEM with EDS measurements supported the existence of the nanocrystalline/amorphous tungsten rich phases which were decorated by polycrystalline, mesoscale-size excretions of metallic copper.

The presented data enable new possibilities in electrodeposition of tungsten heavy alloys (WHAs). The same time they are significant contribution in investigations of phenomenon of tungsten (and molybdenum) codeposition.

This work was financed by Polish NCN Grant 2015/17/N/ST5/03352

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