Local Ion Nitriding of Martensitic Structural Steel in Plasma of Glow Discharge with Hollow Cathode

Abstract

A probe study of glow discharge plasma is carried out according to the technique of a single cylindrical Langmuir probe at a pressure of 60 Pa in a gas mixture of 70% Ar + 30% N2. The distribution of density of charged particles in a cavity formed by a technological screen to create a hollow cathode effect and by the treated surface of the material is obtained. It is established that the hollow cathode effect leads to increasing ion concentration near the cathode surface during ion nitriding in a glow discharge. The maximum ion concentration in the center of a cathode cavity is ni = 3.5 × 1016 m–3. Effects of the local ion nitriding in the plasma of glow discharge with a hollow cathode as well as the duration of the plasma treatment process on the microhardness and structure of diffusion layer of martensitic structural steel at the temperature of 550°C are studied. The process of diffusion saturation for martensitic steel local ion nitriding is found to be described by an inverse parabolic law. It is proved that smoothing of microhardness distribution through the hardened case rises with the rise of duration of nitriding in plasma of glow discharge with hollow cathode. Three zones are formed in the steel surface as a result of the local ion nitriding: plasma modified zone, transition zone, and unmodified zone. The thickness of the diffusion layer in the zone modified with plasma of glow discharge with hollow cathode effect is two times greater than that in the zone modified without hollow cathode effect.