Research On The Technology Of Plasma Surface Modification With Hollow Cathode Discharge

DC plasma nitriding is one of the widely used surface engineering technologies to improve the surface hardness, wear and corrosion resistance of various engineering materials, such as low alloy steels. The plasma nitriding process is accomplished in a vacuum chamber where the specimen is connected to a cathode. A high voltage (about 400-1000V) is applied between the cathode and the anode (vessel). By the nitriding process, the a-Fe solid solution becomes enriched with nitrogen and, if the chemical potential of nitrogen in the nitriding atmosphere is sufficiently high, different iron nitrides may form. However, the conventional plasma nitriding has some inherent shortcomings, such as damage caused to parts by arcing, the'edging effect','hollow cathode effect' and difficulty in maintaining a uniform chamber temperature, particularly in fullworkloads of components with varied dimensions.Several models have been proposed in the past to explain the mass transfer mechanism in dc plasma nitriding. These include the models of sputtering and deposition, nitrogen adsorption, and neutral and ion adsorption. However, any of these models can easily be explained the phenomenon of plasma nitriding process.In order to overcome above shortcomings, a double active screen was designed in our research. Using hollow cathode discharge effect exiting between inner and outer layers the large screen is easily to be heated to higher temperature. Radiation provides the heat that brings components to the required temperature for treatment.The samples used in this work are 42CrMo low alloy steel and AISI 316L austenitic stainless steel.The samples are placed in a cathodic, anodic, floating potential or subject to a relative lower bias voltage. Nitrided layers were investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Confocal laser scanning microscope(CLSM), X-ray photoelectron spectroscopy (XPS) and so on. The published results showed that it is possible to achieve a similar hardening effect as conventional plasma nitriding, without the common problems such as edge effect and arcing on surface.After nitriding of the 42CrMo low alloy steel, the compound layer(ε-Fe2₃ and y'-Fe4N) is produced on the component surface and a nitrogen diffusion layer is formed beneath it. AISI 316L austenitic stainless steels were effectively plasma nitrided at the anodic potential. The nitrided layer mainly composed of nitrogen expanded austenite phase was formed at the low temperatures (below 480℃) for 4h. While plasma nitriding was carried out at high temperature (above 510℃) for 4h, a 3-layered structure consisting of a top compound (γ'-Fe4N+γN+CrN) layer, an intermediate (γ'-Fe4N+γN+CrN+α) layer and a lower layer was formed. In particular, it has been found that when nitriding time is sufficiently short, theγN phase can be produced on the surface of an austenitic stainless steel at high temperature. The results showed that rapid plasma nitriding of austenitic stainless steel is a suitable process for improving the surface hardness and wear resistance properties without deteriorating corrosion resistance.Based on the theories of plasma discharge and the principles of the thermochemical surface treatment, flexible change potential of samples in the plasma environment, this includes cathodic, anodic (zero) and floating potential, to investigate the influence of the nitrogen ions and neutral nitrogen particle on the plasma nitriding, providing experimental data for the establishment of the plasma nitriding model.