Spectroscopic study for pulsed DC plasma nitriding of narrow deep holes

The nitriding behavior of the inner surfaces of narrow blind holes with various diameters was investigated by the pulsed DC plasma nitriding process. Optical emission spectroscopy with an optical probe was employed for diagnostic analysis of the plasma during the processes. Active species were analyzed using an optical probe in front of narrow blind holes with diameters of 3–5 mm and a penetration depth of 30 mm. AISI H13 steel was nitrided at 550 °C for 3 h at various pulsed DC duty cycles, nitrogen and hydrogen partial pressures. The hollow-cathode discharge was effectively stabilized by controlling the pulse duty cycle and working pressure. The plasma stability in a narrow hole appeared to be enhanced by increasing working pressure and decreasing pulse duty cycle.