Properties of Sintered Cantor High-Entropy Alloy Pure-Nickel-Screen-Plasma-Nitrided at Varying Gas Pressures

The excellent tensile strength and ductility of the CoCrFeMnNi high-entropy alloys (HEAs) at low temperatures have been recently reported. Regardless of the fabrication method, the existing CoCrFeMnNi alloys tend to exhibit lower hardness than conventional steel materials. To mitigate this issue, surface-modification treatments have been deployed as effective methods for improving the hardness of CoCeFrMnNi HEAs. Studies have demonstrated that the maximum thickness of the nitrided layer of the alloy was obtained by the treatment of its surface with a pure Ni screen at 873 K. In furtherance, this study was aimed at evaluating the properties of the nitrided layer by subjecting the pure-Ni-screen-sintered CoCrFeMnNi HEA to screen-assisted direct-current (DC) plasma nitriding (PN) treatments (S-DCPN) at varying gas pressures. The gas-atomized CoCrFeMnNi HEA powder was processed by ball-milling for 10 h to prepare a sintered body, which was subsequently subjected to S-DCPN treatments for 15 h at 873 K and varying gas pressures (200, 400, 600 Pa; gas compositions: 75% N2 and 25% H2). For the PN procedure, a pure Ni screen was installed as an auxiliary cathode to ensure uniform heating and enhanced nitrogen supply. Furthermore, the nitrided samples were characterized by X-ray diffraction analysis, cross-sectional microstructure observations, surface-morphology observations, glow discharge optical emission spectroscopy, and surface hardness, corrosion, and wear tests. A black modified layer was observed on the sample after nitriding, becoming darker as the gas pressure increased. Notably, the surface hardness of the HEA samples increased significantly after nitriding.