Cathodic deposition of refractory intermetallic compounds from FLINAK melts Part II: Preparative cathodic deposition of TiB2 and ZrB2 and coatings thereof

At 700°C the production of TiB₂ was performed by cathodic deposition of this refractory intermetallic compound on carbon cathodes from FLINAK melts containing Ti(IV) and B(III) in concentration ratios ranging from 1:3 to 1:2 and total concentrations from 2 to 10 mol% at current densities from 0.2 to 3 A cm^–2. Current efficiencies were always lower than 100% but the purity of the deposited TiB₂ was in almost all cases better than 98%. Generating augmented mass transfer by the anode effect, which develops at high current densities, improved cathodic current efficiencies and enhanced the purity of the coarse deposit. Current efficiencies decreased with decreasing temperature. Smooth and dense coatings on carbon substrate were only obtained if the thickness of the coating was kept below a critical value of 0.5 mm. It was impossible to produce dense, well adhering coatings on copper rod electrodes since, due to the mismatch of the thermal expansion coefficients of copper and TiB₂ the coatings detached from the substrate on cooling. To produce pure ZrB₂ material or ZrB₂-coatings necessitates low current densities (i<0.04 A cm^–2). From the impure cathodically deposited mixtures of dendritic ZrB₂, which were obtained at higher current densities, solidified electrolyte and lower valent zirconium compounds could not be separated.