Effect of the distance between screen and sample on active screen plasma nitriding properties

Recently there has been considerable interest in active screen plasma nitriding (ASPN), through-cage plasma nitriding, and cathodic-cage plasma nitriding. In these processes, the edge effect is completely eliminated because plasma is produced on the cage and not directly on the samples. However, little information has been reported on the effect of the distance between the screen and the sample on the ASPN properties. The objective of this study is to investigate this effect by nitriding low-carbon steel S15C and austenitic stainless steel SUS 304 by ASPN using a 304 steel screen.The sample was treated for 18 ks at 723 K under 600 Pa in 25% N2 + 75% H2. The distance between the screen and the sample was changed to 10, 20, 30, 40, and 50 mm. The nitrided samples were characterized by observing their appearance and surface roughness by optical microscopy, scanning electron microscopy, x-ray diffraction, and microhardness testing. After nitriding, polygonal particles with a normal distribution were observed at the entire of all the ASPN-treated sample surfaces. The particles on the sample surface became coarser with a decrease in the distance between the screen and the sample. Furthermore, the thickness of the surface-hardened layer increased with a decrease in the distance between the screen and the sample.