Investigation on the pulsed DC plasma nitriding with optical emission spectroscopy

We have investigated nitriding behavior by a pulsed DC plasma nitriding process. For effective control, it is necessary to monitor the density of the active nitrogen species in the plasma, which is responsible for the enhanced plasma–ferro-metal reaction. An optical emission spectroscopy (OES) method was employed for diagnostic analysis of the plasma during the processes. In this study, AISI H13 steels were nitrided at 550 °C for 4 h with various discharge times and nitrogen concentrations. Active species such as N2+, N2* were evaluated in terms of the first negative system [N2+(B2Σu+, ν′)→N2+(X2Σg+, ν″+hν], the second positive system [N2(C3Πu, ν′)→N2 (B3Πg, ν″)+hν] of nitrogen and Balmer series of hydrogen by OES. In addition, the structures of the samples were analyzed by XRD. It was found that the compound layer consists of the dominant γ′-Fe4N phase.