Discharge conditions and emission spectroscopy of N2 and N2 active species in a variable power dc pulsed plasma used for steel nitriding

The variations of stainless steel cathode temperature (Tc) as a function of nominal plasma power (driven by discharge frequency) and duty cycle at different pressures (0.6-3.2 torr) in nitrogen plasma are reported in conditions favourable for plasma nitriding. Above power at 200 kHz, it is found that increase in power does not have any significant effect on Tc. Corresponding optical emission spectroscopy (OES) study near the cathode surface shows that the presence of vibrational bands of second positive system of N2 (∆v = 0, -1, -2, -3) and first negative system of N2 (∆v = 0, -1, -2) are dominant. The variation of intensity of those nitrogen molecular bands as a function of discharge power at various pressures has been studied. At 2 torr, the vibrational temperature of N2 molecule is calculated from N2 vibrational spectra at different discharge power regime as frequency is varied from 100 to 500 kHz. The variation of power in this frequency range 50-500 kHz has a similar effect on Tc and optical emission intensity. Floating potential in variable power conditions has been measured by Langmuir probe which shows a plateau after 3.2 Watt/cm (at 200 kHz) plasma power density.