Functional Plasma-Synthesized Tungsten–Boron Coating on an Al2O3 Substrate

Abstract—An experimental coating, which is formed during plasma synthesis of tungsten borides and the reduction of metallic tungsten from a mixture based on a scheelite concentrate and boric acid, is studied. The coating is formed on an Al₂O₃ substrate. The step-by-step formation of borides on the substrate surface and the reduction of metallic tungsten using an electric arc plasma generator built in an experimental high-temperature synthesis facility is described. The coating on the substrate consists of reduced metallic tungsten and its borides synthesized in one technological stage during condensation from a dispersed vapor–drop state. To conduct a series of experiments, we designed a prototype of an indirect-acting plasma generator to generate an electric arc plasma flow with a specific power g > 10⁴–10⁵ W/cm². The mixture is destructurized and then sublimated in the form of a vapor–drop phase during the action of a high-temperature plasma flow on the complex structures of the mineral concentrate and its constituent tungsten oxide. Tungsten borides are synthesized during chemical transformations when a dispersed material is removed from a heated plasma flow, nucleation phases form, and the vapor–drop phase condenses on the substrate surface. The synthesis is also accompanied by significant boron sublimation from compounds, which leads to the reduction of metallic tungsten. The material formed during plasma synthesis forms a W–B system and structures, the physicochemical properties of which depend on the composition of the mixture, the flow density, and the plasma pressure and temperature. The results of chemical analysis of the particles forming the W–B coating as a solid solution of dendrites on the Al₂O₃ substrate surface are presented. Using electron microprobe analysis, we determined the phase composition of the coating and revealed the presence of tungsten borides W₂B₅, WB₂, W₂B, and WB and metallic tungsten. The results of producing W–B coatings or films using mineral multicomponent raw materials can be useful in various high-tech industries, including the hydrometallurgical or chemical industry.