Electrodeposition of black chromium spectrally selective coatings from a Cr(III)-ionic liquid solution

In the present study, black chromium coatings were electrodeposited from a 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) solution containing Cr(III) under potentiostatic control on copper substrates. The electrochemical behavior of the electrolyte was studied by cyclic voltammetry. The reduction of Cr(III) species in the solution is a two-step process, from Cr(III) to Cr(II) and from Cr(II) to Cr(0). The potential of each reaction shifts positively with the increase of the electrolyte temperature. The parameters for the electrodeposition of black chromium films were optimized. Homogeneous black chromium films with spectrally selective optical properties were produced by applying a potential of −1.5 V for 1800 s, at an electrolyte temperature of 85 °C. The coatings consist of a mixture of chromium oxide/hydroxide and metallic chromium. They are amorphous and present a sub-micrometric granular structure.