Active screen plasma nitriding of FeCrNiCoMo0.2 high entropy alloy for combating wear

High entropy alloys (HEAs) have been widely studied due to their excellent mechanical and chemical properties; however, most HEAs are characterised by low hardness and poor tribological properties. In this study, advanced active screen plasma nitriding (ASPN) technology has been applied to the surface modification of a single-phase FCC structured FeCrNiCoMo 0.2 high entropy alloy. The microstructure, wear, and corrosion behaviour of the HEA were fully evaluated before and after the ASPN treatments under 400°C, 450°C and 500°C for 10h. A very thin (around 1 micron) top γ'-M 4 N layer was formed due to the deposition of the sputtered material from the AISI316 mesh screen. Below the top deposition layer, nitrogen supersaturated expanded FCC solid solution phase (or S-phase) was generated after low-temperature treatment at 400°C and 450°C, and CrN began to precipitate in the 450°C treated sample. At 500°C, the nitrogen supersaturated S-phase was decomposed into CrN, γ'-M 4 N, α-Fe, and ε-M 3 N. Following the ASPN treatments at 400, 450 and 500°C, the surface hardness of the HEA increased from 197 HV 0.01 for the untreated material to 840, 1174 and 1315 HV 0.01 and the wear rate of the HEA was reduced by 55, 50 and 5.3 times, respectively. The 400°C treated sample remained the overall good performance as for the untreated sample. However, impaired corrosion properties were observed for the 500°C treated sample.