Plasma nitriding combined with a hollow cathode discharge sputtering at high pressures

Abstract

The article describes a new plasma nitriding system with an auxiliary cathode the surface of which is furnished with holes of diameter ∅ and depth d. The auxiliary cathode fulfills two functions: (i) it intensifies the nitriding discharge by hollow cathode discharges generated in the holes and (ii) strongly sputters its material. The sputtered material is transferred onto the surface of nitrided steel sample where it forms a surface layer of nitrides from deposited material. As the material of the auxiliary cathode can be different from that of samples to be nitrided, the surfaces of nitrided substrates can be improved by an addition of selected materials, such as Mo, Cr, Ti, V, etc. The main attention is devoted to the investigation of transfer of the material from the auxiliary cathode onto the surface of nitrided steel samples. It was found that conditions under which a maximum quantity of this material is transferred depend on dimensions of holes in the auxiliary cathode, potential of treated substrates, and on argon content in N2+H2+Ar mixture. The plasma nitriding was carried out at a high pressure of 1067 Pa and a substrate temperature T of 550 °C for a time of 2.5 h. Detailed measurements of elemental depth profiles and structure of nitrided steel samples, carried out by means of glow discharge optical emission spectroscopy and x-ray diffraction, respectively, are given.