Electroplating of Wear‐ and Corrosion‐Resistant CrCoNi Medium‐Entropy Alloys beyond Hard Chromium Coatings

Abstract High‐entropy alloys (HEAs) and medium‐entropy alloys (MEAs) are a new class of alloys that attract attention because of their mechanical properties. The application of such alloys for coating is highly desired; however, the number of coating technologies remains limited. Although electrodeposition is expected to be an environmentally friendly and energy‐saving coating technology, neither the CrMnFeCoNi HEA nor any of its derivatives is successfully electrodeposited because of the difficulty in controlling the composition and the difficulty in alloying Cr, a key component, as a crystal. Here, the successful electrodeposition of a CrCoNi MEA is demonstrated using a mixture of an ionic liquid and an aqueous solution containing metal salts. The resultant CrCoNi MEA layer exhibits high wear and corrosion resistance superior to that of conventional hard Cr coatings prepared using toxic Cr(VI) ions. Mesoscopic phase separation of the mixture is shown to be key to the successful electrodeposition of the MEA. The CrCoNi MEA coating is a strong potential substitute for conventional hard Cr coatings; thus, it is believed that the electrodeposition of the CrCoNi MEA is an important advancement in wear and anticorrosion surface coatings.