Generation of Plasma in Non-Self-Sustained Glow Discharge With Hollow Cathode for Nitriding Inner Surfaces of Elongated and Complex Shaped Cavities

Abstract

This article describes a plasma-generating system based on a non-self-sustained glow discharge with external electron injection and a hollow cathode in the shape of a tube with an internal diameter of 25 mm and a length of 300 mm made of stainless steel AISI 321. It is proved that the parameters of the non-self-sustained glow discharge under (0.2–1) Pa, (100–280) V, and (2–6) A depend on the auxiliary discharge current to a great extent, which is due to the determining influence of electrons injected from the auxiliary discharge plasma on the physical processes in the main discharge. The study of uniformity of temperature distribution on the hollow cathode walls has shown that it can reach ±10% of the average value and greatly depends not only on electric characteristics of the discharge but also on its working pressure. The study further demonstrates that such discharge can burn not only in elongated hollow rectilinear cathodes but also in hollow cathodes of a complex shape produced by welding two tubes at a 90% angle. The efficiency of using emitted electrons to sustain discharge and uniformity of temperature distribution on the walls is improved in the latter configuration. The discharge can be used for ion plasma nitriding of inner surfaces with treatment uniformity of ±8.5% of the average value for complex-shaped hollow cathodes. The mean thickness of the nitrided layer after 1 h of processing steel AISI 321 at 630 °C was $\approx 50~\mu \text{m}$ and the hardness of the nitrided layer exceeded the original value by $\approx 3$ times.