Active screen plasma nitriding of FeCrNiCoMo_0.2 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 μm) top γ′-M₄N 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′-M₄N, α-Fe, and ε-M₃N. 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.