Deposition of High-Entropy Alloy (HEA) coatings by HVOF and cold gas spray

High-entropy alloy (HEA) coatings were deposited by high-velocity oxygen fuel (HVOF) and cold gas spraying (CGS) using mechanically alloyed feedstock materials, as an alternative to conventional coatings such as Stellite. Cr-Mn-Fe-Co-Ni and Alx(Cr20Mn25Fe40Ni15)100-x (x = 0, 10, 14) alloys were considered. Due to the high reactivity of the mechanically alloyed feedstock, the HVOF-sprayed HEAs contained 20 - 40 vol% oxide inclusions, with greater oxidation in the Al-free compositions. Conversely, CGS coatings retained the microstructure of the feedstock powders, which contained finely dispersed oxides from the mechanical alloying process. Nanoindentation mapping revealed that both the FCC and BCC phases in the coatings were much harder than those in bulk HEAs, because of the small grain size and finely dispersed oxides. On the macroscale, the brittle oxide inclusions in the HVOF coatings resulted in a lower hardness and indentation modulus than those of the CGS coatings. However, oxide inclusions were beneficial to sliding wear resistance because they limited the size of the metal areas that could be sheared off. In fact, the metallic phases suffered adhesive wear because they did not exhibit strengthening mechanisms such as stress-induced phase changes. The sliding wear rates of the coatings were therefore higher than those of HVOF-sprayed Stellite, which undergoes stress-induced martensitic transformation. In electrochemical polarization tests, most of the HVOF HEA coatings had lower corrosion current densities than HVOF-sprayed Stellite, despite narrower passivity ranges that were further worsened by adding Al. The CGS coatings had poor corrosion resistance because their plastically deformed microstructure increased their reactivity.