Influence of pulsed power supply parameters on active screen plasma nitriding

•The influence of pulsed power supply parameters is studied in an ASPN setup.•The peak discharge current is found to be the most important parameter.•Optical emission spectroscopy is used to clarify results from surface characterization.•At constant time-averaged current, the surface hardness is maximum at lowest duty cycle.•Using lowest duty cycle, hardness can be significantly improved.

It is well known that active screen plasma nitriding (ASPN) is driven using a pulsed D.C. power supply, due to several advantages such as improved control over temperature, nitrided layer phase composition and active species generation. In addition, the pulsed power supply helps to avoid the transition to the arc regime. The ASPN configuration in particular is preferred over conventional plasma nitriding due to complete elimination of edge effect, as plasma is not in direct contact with the samples. In this work, the influence of pulsed power supply parameters such as duty cycle, peak voltage, peak current and time averaged current is studied in an ASPN setup, on nitrided layer phase composition and surface hardness of austenitic stainless steel (ASS). In addition, optical emission spectroscopy (OES) is used to measure the excitation, vibrational and rotational temperatures (Texc, Tvib, Trot), nitrogen dissociation fraction and concentration of active species N2(C3Πu) and N2+(B2Σu+) (in terms of emission intensities) to clarify the results obtained from surface characterization. The results show that keeping time-averaged current constant, the surface hardness decreases with the duty cycle of the pulsed D.C. power supply. The results from OES and surface analysis are correlated, and as a result plasma parameters (Texc, Tvib, Trot) and active species concentration (N, N2, N2+) are found to be important precursors in surface hardening of ASS.