Effect of Surface Deposited Layer on Active Screen Plasma Nitriding

Abstract

The nitriding process is widely used to improve the tribological properties and wear resistance of metal surfaces. An “edge effect” occurs due to distortions of the electric field around the corners and edges of the components, although the components are well heated. This results in nonuniformity in properties such as the hardness and thickness of the surface layer. Recently, there has been considerable interest in active screen plasma nitriding (ASPN). In this process, the edge effect is completely eliminated because the plasma is produced on the cage and not directly on the samples, and sputtering of material from screen is important for ASPN. In this study, austenitic stainless steel AISI 316L and silicon (Si) were nitrided by ASPN to investigate the effect of surface deposits from the screen. ASPN experiments were carried out using a DC plasma-nitriding unit. The sample was placed on the sample stage in a floating potential and isolated from the cathodic screen and the anode. The screen, which was AISI 316L expanded metal with 38 % open area ratio, was mounted on the cathodic stage around the sample stage. ASPN was performed in 25 % N2 + 75 % H2 atmosphere for 7.2–90 ks at 673 K under 200 Pa. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction, Vickers microhardness, and glow discharge optical emission spectroscopy. The nitrogen-expanded austenite (S phase) was formed on the surface of both samples. Additionally, γ′-Fe4N and CrN were also formed on the Si surface. The thickness of the surface deposited layer increased linearly with increasing nitriding time. That of the nitriding layer of S phase also increased, whereas the deviation from parabolic law was observed. This result suggested that the surface deposited layer suppressed the formation of a nitriding layer.