Optimization Study of N2-H2 Mixture Pulsed DC Plasma Ionitriding Reactor

Abstract

In modern science and technology the glow discharges have a wide varity of applications. In microelectronics industry the glow discharges are used for etching of surfaces to from topographical surface features, as well as for deposition of thin films. In material processing industry the glow discharges are used extensively for deposition of various thin films, coatings and surface layers. In the present work the diagnostics of pulsed DC generated nitrogen-hydrogen mixture plasma, using an Active Screen Cage is performed so that the optimum working conditions for the purpose of material processing are obtained. Trace rare gas optical emission spectroscopy is used to investigate the effect of current density, filling pressure and hydrogen concentration for the measurement of excitation temperature, vibrational temperature, dissociation fraction and nitrogen atom density. The nitrogen plasma is generated by using 50 Hz pulsed-DC power source. The excitation temperature is determined from Ar-I line intensities, using Boltzmann’s plot method. It has been observed that the excitation temperature increases with both current density and hydrogen concentration, where as it decreases with filling pressure. In order to find out the vibrational temperature of the second positive 𝑁ʋ(𝐶3𝛱𝑢,𝒱́→𝐵3𝛱𝑔,𝒱̋) system, the Δ𝒱 = -2 sequence is used due to comparatively longer lifetime (τ ~ 36 ns) using Boltzmann’s plot method. The behavior of the vibrational temperature remains similar like in the case of excitation temperature. The nitrogen dissociation fraction is calculated using actinometery and line ratio methods. It is observed that the dissociation fraction increases by adding 40% of hydrogen in the nitrogen plasma and then upon further increase of hydrogen concentration it decreases sharply. The atomic density of nitrogen is also calculated using actinometery method, which also increase with hydrogen concentration up to 40% hydrogen in the mixture. Using the optimum condition of current density, filling pressure and hydrogen concentration, different types of steels including AISI 316, AISI 304, mild steel and high xviii carbon steel are nitrided in the presence of the active screen cage. The treated samples are analysed by X-ray diffractrometery (XRD) to investigate the changes in the crystallographic structure. Scanning electron microscope (SEM) is used to investigate the surface morphology of the plasma irradiated samples, where the changes in the surface hardness are measured by Vickers microhardness tester. The XRD pattern of all the samples confirms the presence of nitrides with iron, carbon and chorimum. Microhardness results reveal a 3-7 times harder surface for different samples. The nitrogen mass transfer mechanism in active screen cage plasma nitriding process is also investigated using optical emission spectroscopy. The dominant species including NH, Fe-I, 𝑁2+, N-I and N2 along with 𝐻𝛼and 𝐻𝛽 lines are observed using optical emission spectroscopy (OES). The factor of sample treatment time for both of Active screen cage and DC plasma nitriding of AISI 316 stainless steel are investigated. Increasing trend in microhardness is observed in both cases but three-fold more hardness is achieved using Active Screen Cage in comparision to direct current plasma nitriding. On the basis of metallurgical and OES observations a new phenomelogical scheme for a nitrogen mass transfer mechanism in active screen cage plasma nitriding process is proposed