New trends on nitriding in low pressure are discharges studied by optical emission spectroscopy

The purpose of this work is to present the first results obtained by optical emission spectroscopy (OES) in a high current, low voltage and low pressure are discharge used for steel nitriding. The apparatus is mainly constituted of an ionisation chamber mounted on the top of the nitriding reactor itself. In this process, workpieces are nitrided at floating potential in Ar-N-2 gas mixtures. OES measurements are done in pure argon for different are currents and different gas pressures in the reactor. This plasma diagnostic technique has been used to characterize the different excitation processes of both neutral and ionic argon species. In the case of Ar-N-2 gas mixtures a strong quenching effect of nitrogen on the electron energy distribution function is observed. OES measurements obtained for various nitrogen percentages in Ar-N-2 gas mixtures are compared to the results obtained by metallurgical analysis of nitrided Armco iron substrates in the same conditions.