Surface-Modified Layer Formed by Plasma Nitriding Using Chromium Screen

Low carbon steel sample S15C was nitrided by active screen plasma nitriding (ASPN) and direct current plasma nitriding with screen (S-DCPN) using a chromium screen to form simultaneously chromium nitride coating/nitrogen-diffusion layer on the sample surface. The sample was placed on the sample stage in a floating potential and a cathodic potential. A chromium screen was mounted on the cathodic stage parallel to the top of the sample. Plasma nitriding treatments in a floating potential (ASPN) and a cathodic potential (S-DCPN) were performed in a nitrogen-hydrogen atmosphere with 75% N-2-25% H-2 for 120 min at 773K and 873K under 200 Pa. After nitriding, the nitrided microstructure was examined with a scanning electron microscope, glow discharge optical emission spectroscopy and X-ray diffraction studies. In addition, the hardness and corrosion properties were also measured. The nitrided layer formed by S-DCPN consisted of a Cr-concentrated layer (similar to 750 HV) followed by a nitrogen-diffusion layer whereas that formed by ASPN consisted of deposited layer of chromium nitrides and iron nitrides without nitrogen-diffusion layer. Corrosion resistance of the samples treated by S-DCPN was better than that of untreated sample and sample treated by ASPN.