Studies on optical emission spectroscopy of nitriding plasma and characterization of nitrided steel

Abstract

Plasma in nature is abundant and appears quite beautiful in colour as can be seen in glow of sun and auroras. Plasma produced in laboratories is also quite interesting as a correspondence of the naturally occurring plasmas from a glow discharge to a tokamak. However, plasmas tuned to special conditions have been successfully utilized for material processing, of which, plasma nitriding is one and foremost. The work of the thesis focuses on the setting up of a plasma nitriding system with various diagnostics applied to plasma and plasma treated stainless steels. Emission spectroscopy and probe diagnostics are applied to study the optical and plasma properties whereas austenitic stainless steels are characterized by x-ray diffraction (XRD) and hardness test in order to demonstrate the application of the process. The optical and plasma properties during nitriding are motivating, as these properties will lead to conditions for the formation of surface layer. The formation of iron and chromium nitrides is another important asset of the plasma nitriding process, which is tested by XRD technique, while the surface hardness is tested by hardness test. Optical properties are found from optical emission spectroscopy (OES) in terms of emission intensities and wavelengths. Langmuir probe (LP) is used for finding variations of plasma parameters with respect to process parameters and to make a comparison of plasma parameters with those of optical and discharge parameters. A correlation between OES and material characterization properties is sought in order to make a clear understanding of the process. In Chapter 1, introduction to plasma, the possible applications with relevance to plasma nitriding and spectroscopy are described. Plasma nitriding (termed in the title as nitriding plasma for convenience) has emerged as a powerful tool in modifying surface properties of a material without affecting the bulk properties. The various advantages over a conventional gas nitriding process have made this plasma-assisted method a...