High-entropy Fe-Cr-Ni-Co-(Cu) coatings produced by vacuum electro-spark deposition for marine and coastal applications

High-entropy coatings (HECs) are of a great interest for the protection of structural steels and alloys used in the costal and offshore areas. Here, thick, dense, and uniform Fe-Cr-Ni-Co-(Cu) coatings with a crack-free surface have been successfully deposited on AISI 420S steel by vacuum electro-spark deposition using CrNiCo and CrNiCoCu electrodes. The coatings consist of columnar grains (approximately 300 nm in diameter) and subgrains (10-50 nm thick) of an fcc phase and spherical inclusions of mixed SiO2 + (Cr,Ti)2O3 oxide, 30-50 nm in size. Although Cu is an element prone to segregation, the experimental results show that Cu does not form its own phase and is in the metal solid solution. Molecular dynamics simulation shows that Cu has a slight tendency to self-clustering and form Cu-rich clusters in FeCrNiCo-Cu HECs. However, several regions enriched in Cu are observed in the FeCrNiCo-Cu samples. FeCrNiCo coatings tested in artificial seawater and the Black Sea exhibited enhanced corrosion resistance. In tribocorrosion tests, FeCrNiCo-(Cu) coatings performed better than steel substrate due to faster recovery of a passive film. The addition of Cu has a positive effect on the antibacterial activity of FeCrNiCo coatings against Gram-positive B. cereus Arc30 and B. cereus F strains.