REACH regulation challenge: Development of alternative coatings to hexavalent chromium for minting applications

Abstract

Aiming to improve the protection of human health and the environment, REACH regulation was enforced across European Union, accelerating the need to find alternative solutions to the electrodeposition processes of hexavalent chromium employed in several industrial applications. Electroplated chromium coatings are widely used due to their physical and mechanical properties that allow a longer service life of products that usually fail due to wear. Minting is one of those applications where good mechanical and tribological properties are required and where hard chromium has an important usage without being outmatched by other coatings. This study presents the development of an alternative coating to hard chromium with outstanding mechanical properties, in particular high resistance to cyclic loads . Cr( Al , Si )N coatings have been produced by reactive DC magnetron sputtering . Focusing on mechanical properties, an increase in the hardness was observed with the consecutive addition of elements to the base matrix (CrN), ranging from ≈10 GPa in CrN coatings up to ≈17 GPa for the CrAlSiN coatings. In nano-impact tests, CrAlN coatings had the best performance and no crack events by fatigue were observed by SEM after 50 impacts, with load maximum 20mN, with an estimated maximum contact pressure of 1.5 GPa much higher than the one requested for coinage. The fatigue wear was governed by the H/E ratio and plasticity index showing that Cr( Al , Si )N coatings deposited by reactive magnetron sputtering can be considered excellent candidates for cyclic loading, as in minting applications. • In nano-impact tests, CrAlN coatings withstood 1.5 GPa without failure. • Depth of initial indentation after 50 impacts was lower in CrAlN than in steel. • Cr( Al , Si )N coatings are viable alternatives to hard chromium under cyclic loads.