Time-resolved plasma diagnostics of cathodic cage plasma nitriding system with variable pulsed duty cycle and surface modification of plain carbon steel

Abstract

Cathodic cage plasma nitriding (CCPN) is an efficient technique to improve the surface properties of metallic and non-metallic materials, including steels. Usually, CCPN is powered by pulsed voltage sources due to additional control of the nitrided layer's thickness and composition as well as the discharge transition to the arc region can be controlled by varying the pulsed duty cycle. As the behavior of pulsed plasma varies with time, it is important to monitor variation in plasma parameters by using time-resolved plasma diagnostics. Unfortunately, the fundamental time-resolved diagnostics of such discharges are still very limited. Here, we report time-resolved measurements of current, voltage, plasma emission, space, and time-resolved emission from a single hole in the cathode as a function of pulsed duty cycle (15–75 %) at constant total current. The results show that the electron temperature is higher if a higher peak current and the shortest 15 % pulsed duty cycle are used. The results obtained from plasma diagnostics are confirmed by treatment of plain carbon steel samples in identical conditions and found that the best hardness, wear, and corrosion resistance is obtained at the lowest pulsed duty cycle. The study suggests that plasma generated at a low pulsed duty cycle and a higher peak discharge current exhibits higher electron temperature and less fluctuations in plasma emission which are more efficient in attaining high surface hardness, better wear, and corrosion resistance to nitride plain carbon steel.