Diagnostic of arc discharges for plasma nitriding by optical emission spectroscopy

Abstract

A new high current (100–300 A), low voltage (25–45 V) and low pressure (0.4–1 Pa) arc discharge plasma used for steel nitriding is presented and characterized by optical emission spectroscopy (OES). In this process, the nitriding rate is high even if the workpieces are nitrided at floating potential without hydrogen in Ar−N2 gas mixtures. With H2, there is no appreciable gain in nitrogen content or in growth rate of the diffusion layer. In this paper, the OES diagnostic technique has been used to characterize the different excitation processes of both neutral and ionic argon species in Ar−N2, Ar−H2 and Ar−N2−H2 gas mixtures. The reactivity of this process is evaluated by using AISI 316 L austenitic stainless steel substrates nitrided for different treatment times at 690 K.