Effect of direct current density on performance of tungsten coating electroplated from Na2WO4-WO3-B2O3 system

• Tungsten coatings were electroplated from the Na2WO4-WO3-B2O3 melt.
• Tungsten coating comprised a tooth-like layer and a columnar growth layer.
• The surface roughness increases as increasing current density.
• At high current densities the coating process favors the growth of the (211) orientation.
• The growth rate of tungsten crystal nucleus is higher than the nucleation rate.

Tungsten coatings with body-centered cubic (bbc) structure were successfully electroplated from Na2WO4-WO3-B2O3 molten salt at 1153 K. Compact and void-free coatings were obtained from the molten salt when the current density was below 80 mA cm−2. As the increasing of current density, the trend of crystal growth was promoted and the grain size and surface roughness of tungsten coatings increased. The preferred orientation of the coatings changed from (200) to (211). All coatings comprised an inner layer of tooth-like grains and an outer layer of columnar grains. It could be concluded that the nucleation easily occurred on Cu alloy substrate for tungsten atom. When the current density was up to 100 mA cm−2, the show increase of the thickness was responsible for the rapid decreases of current efficiency. All the tungsten coatings and copper alloy substrates exhibited excellent interface bonding with an adhesive strength which is over 32 MPa.