Nanocomposites and nanomaterials

Electrochemical deposition of tungsten from tungstate-borate melts
Malyshev V.V.1, Gab A.I.1, Shakhnin D.B.1, Franko M.M.1
1 Institute of Engineering & Technology, Open International University of Human Development “Ukraine”, 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: viktor.malyshev.igic@gmail.com

The system for the tungsten coatings electrochemical deposition based on Na2WO4–B2O3 melt was studies. The influence of boron oxide concentration, temperature, cathode current density, and duration of electrolysis on cathode deposits composition and structure was investigated; optimal parameters of reversive tungsten deposition from Na2WO4–B2O3 melts were selected. Tungsten is deposited if the boron oxide concentration does not exceed 10 mol %; with higher boron oxide concentrations, tungsten oxides and bronzes are showing at XRD patterns. Continuous coatings were obtained at 1073-1323 K and current density up to 50 A/dm2. The higher is the electrolysis temperature, the coarser are the deposit grains.
Well adhered, uniform, continuous, non-porous coatings were obtained at 1173 K with current density 1-10 A/dm2. In the initial deposition period, the deposit grains size was decreased with the current density increase. With current density higher than 10 A/dm2, grains size was increased significantly, the roughness amplitude increased from 1 up to 2-3 µm, and the progressive growth of individual protrusions turned into dendritic formation preventing the further coating deposition. Below 1 A/dm2, substrate corrosion rate exceeds tungsten deposition rate, causing poor coatings adhesion. Tungsten deposition rate within studied current density range are 15-40 µm/h, and tungsten current yield – about 85%. 
With coating thickening, its surface structure changes from fine-crystalline with uniform grains to coarse-crystalline. To obtain fine-crystalline coatings, a reversive electrolysis mode was used. The cathodic-to-anodic-phase ratio was within 20-40 range; the anodic phase duration was 0.5-2.0 s, and the anode current density was 15-50 A/dm2. To obtain a good coating, the following electrolysis parameters are optimal: cathodic phase duration – 20 s, anodic phase duration – 0.5-1.0 s, cathode current density – 8-10 A/dm2, anode current density – 15-30 A/dm2. Electrolysis carrying out in this mode makes it possible to halve the coating roughness with an increase of its thickness up to 200 microns. 
Continuous, well-adhered to the substrate, tungsten coatings were deposited onto graphite, nickel, copper, molybdenum, tungsten, machine-building structural steels, heat-resistant and tool hard alloys, nitrided and nickel-plated titanium.