Effect of Surface Deposits on Nitriding Layer Formation of Active Screen Plasma Nitriding

Austenitic stainless steel SUS 316L was nitrided by active-screen plasma nitriding (ASPN) to investigate the effect of surface deposits from the screen on the nitriding layer formation. ASPN experiments were carried out using a DC plasma-nitriding unit. The sample was placed on the sample stage in a floating potential (bias-off) and a cathodic potential (bias-on). The screen, which was SUS 316L expanded metal with 38% open area ratio, was mounted on the cathodic stage around the sample stage. Nitriding was performed in a nitrogen-hydrogen atmosphere with 25% N2 + 75% H2 for 18–180 ks at 673 K under 200 Pa by the ASPN process. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction, Vickers microhardness and glow discharge optical emission spectroscopy. From the surface observation of the nitrided sample, deposits were observed on the top surface of the sample nitrided with bias-off whereas deposits were not on that nitrided with bias-on. The nitrogen-expanded austenite (S phase) was formed on the surface of both samples. Layer thickness of the S phase increased with increasing the nitriding time. Additionally, the degree of an increase of the layer thickness of the S phase nitrided with bias-on was approximately 2.5 times greater than that nitrided with bias-off. This result suggests that the ASPN treatment with bias-on is effective for the increase of the nitriding layer thickness