FeMnNiCoCr-based high entropy alloy coatings : Effect of nitrogen additions on microstructural development, mechanical properties and tribological performance

(FeMnNiCoCr)N_x high entropy alloy (HEA) coatings were deposited onto M2 steel substrates via direct current (DC) magnetron sputtering at several nitrogen flow rates (4, 8, 15, 25 sccm). The microstructures exhibit a fine columnar pattern. Higher flow rates promoted higher nitrogen contents that led to a transformation from a fcc to bcc structure. Self-organising behaviours of metallic atoms, enhanced by nitrogen gas flow, led to the formation of the bcc structure, wherein metal atoms randomly occupy equilibrium lattice sites and nitrogen atoms segregate at grain boundaries. Coatings with low nitrogen content (~6 at. %) exhibit a fcc structure, offering the best scratch toughness and adhesion strength, but inferior hardness (~11 GPa) and wear resistance; coatings with medium nitrogen contents (~15–22 at. %) exhibit a bcc structure, which display improved hardness (~13–15 GPa) and wear resistance, but inferior scratch responses, that was attributed to the presence of the harder, but more brittle bcc phase. At high nitrogen content (~26 at. %) an impressive combination of superior hardness (~17 GPa), wear resistance and good scratch response were identified with a well-defined bcc phase. It is these bcc grains, together with nitrogen-reinforced grain boundaries that overcame the incompatibility between the strength and ductility.