AISI 304 stainless steel was nitrided at different temperature in the plasma equipment using a new auxiliary device. The proposed hollow cathode auxiliary device improves the plasma density, thereby accelerating the nitriding process. The modified surface was characterized by x-ray diffraction, scanning and transmission electron microscopies, atomic force microscopy, surface microhardness testing. Electrochemical corrosion test was used to measure the localized corrosion resistance of the samples. Single S-phase layer without the CrN precipitation was obtained by hollow cathode plasma nitriding at 450 °C, which improved the microhardness, wear and corrosion resistance of the steel surface. Nitriding at higher temperature of 550 °C resulted in substitution of the S-phase towards layer composed from CrN and α-Fe phases, improving the microhardness but decreasing the corrosion resistance.

Plasma nitriding is a plasma-activated thermochemical method widely used to increase the fatigue strength, hardness and wear resistance of low-alloy steels. In this work, a new structure consisting of a hollow cathode metal plate can produce a hollow cathode discharge and heat the workpiece by radiation. This principle is used to perform hollow cathode source plasma nitriding (HCSPN). Herein, 42CrMo steels were treated by conventional plasma nitriding (CPN) and HCSPN treatments. The 42CrMo steels were nitrided at 525 °C for 6 h in NH 3 atmosphere. The mechanical and dry wear properties were measured by nano-indentation, Vickers micro-indentation and ball-on-disk tribometer, respectively. The H 3 /E ⁎2 ratio was 0.0274 GPa (H=11.51 GPa and E ⁎ =235.84 GPa) for the CPN sample and 0.0276 GPa (H=10.87 GPa and E ⁎ =215.54 GPa) for the HCSPN sample. Compared with the untreated 42CrMn steel, all the nitrided samples possess increased fracture toughness. Because the workpiece was not used as a discharge cathode, this HCSPN treatment overcame the disadvantages of the conventional CPN treatment. Additionally, results showed that the surface of the HCSPN sample was smoother than that of the CPN sample, and its tribological performance was better.

The influence of electric potentials on the characteristics of modified layers with plasma nitriding of 2Cr13 stainless steel at a high temperature was investigated. Three active screens were placed on the cathodic plate. Three groups of 2Cr13 stainless steel samples were set with cathodic, anodic, and floating potentials. Samples were nitrided at 500 °C for 5 h in an ammonia atmosphere. The treated samples were analyzed using x‐ray diffraction, scanning electron microscopy with energy dispersive x‐ray spectroscopy, surface roughness tester, and atomic force microscopy. It is proved that nitriding treatments can be suitably carried out at three electric potentials, but the corresponding modified layers take on different microstructure, morphology, and hardness behaviors. It found that the three nitrided samples are mainly composed of the ϵ−Fe2‐3N and γ’−Fe4N phases. The order of thicknesses of the nitrided layers is: floating < anodic < cathodic potential. The results also demonstrated that electric potentials play essential roles in the corrosion resistance and tribological properties in plasma nitriding treatment. Because there is no visible precipitation of chromium nitride in the modified layer, the corrosion resistance of the floating nitrided sample is better than that of the other two samples.