Optical Emission Spectroscopy of N2-H2-Natural CH4 Mixture Plasmas Generated By Pulsed-dc for Surface Hardening of Austenitic Stainless Steel

Abstract

Nitrogen glow discharge plasma is generated by number of power sources and has many applications in material processing such as Nitriding of different metal alloys. The material processing efficiency of the plasma may be enhanced by optimizing the related parameters of the discharge. The concentration of the active nitrogen species in the plasma may depend on various operating conditions such as input power of the discharge, pressure and gas flow rate etc. In order to improve the various excitation and ionization processes occurring in the plasma for the generation of active species, one way is to mix some inert gas in the nitrogen plasma. In the present study the diagnostics of Pulsed-dc generated nitrogen-methane-hydrogen mixture plasma is carried out to analyze the optimum working conditions for its application. Spectroscopic measurements of hydrogen Balmer-β and Balmer-γ line profiles are performed in an abnormal glow region of Pulsed-dc sustained nitrogen-hydrogen- methane plasma for investigating dependencies of their line shapes and intensities on discharge parameters. The excitation temperature Te is determined from Ar-I emission line intensities by using Boltzmann’s plot method of Balmer lines is found to be increased with methane mixing in nitrogen plasma. The electron density is extracted from Stark broadening (FWHM) of the Hβ emission profile. It is found that both the emission intensity and the broadening of the Balmer-β and Balmer-γ lines show significant dependence on the filling pressure in the same manner. However, both the emission intensity and broadening of Hβ line exhibit weak dependence on input power in contrast to the Hγ emission line. The concentration of active species N 2 (C 3  u )   and N 2 ( B 2  u ) are monitored in terms of the emission intensities of nitrogen bands of the second positive and the first negative systems respectively. The concentration of N 2 (C 3  u ) active species is appreciably enhanced by methane mixing signifying the role of argon meta-stables in the excitation and dissociation processes. The effect of mixing of methane-hydrogen is studied on the surface nitriding os AISI-304 stainless steel substrate at temperature of 500 0C generated by 50 Hz pulsing source at powers of 300 and 500 watt and filling pressure of 1, 3, 5 mbar at different treated times 1, 2, 3, 4 and 5 hours. The samples are then analyzed for plasma induced changes in their surface properties by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Vicker’s micro hardness testing. The XRD pattern confirms the formation of expanded austenite phase (γN) resulted by the incorporation of nitrogen as interstitial solid solution in the iron lattice. The EDX spectra show peaks corresponding to elements that are present in the surface of the substrate. It can be noticed that no additional peak corresponding to any impurity element, other than nitrogen, which is supposed to be present owing to nitrogen incorporation in the iron lattice is found in EDX spectra. The effect of ion etching is apparent from the SEM micrograph, which is normally expected during the nitriding process. All the nitrided samples show an increase in surface hardness as compared to that of untreated one. It is revealed that the surface hardness increase significantly with increasing nitriding time. Based upon optical measurement reported here, plasma discharge parameters can be optimized for the production of active species, excitation temperature Te and number density.