Plasma diagnostics of pulsed d.c. glow discharge combined with ICP for deep nitriding process

A deep nitriding system was designed to increase the plasma density using dual plasma source and consisted of a pulsed d.c. and r.f. (13.56 MHz). A pulsed d.c. glow discharge combined with inductively coupled plasma (ICP) has been characterized by optical emission spectroscopy (OES) in the r.f. power range of 400–1000 W and pressure between 400 and 800 Pa. For the nitrided AISI 4140 steel at a r.f. power of 900 W, pressure of 700 Pa and cathode voltage of –550 V, the case hardening depth is approximately 1.6 times deeper than that obtained without ICP and the compound layer is slightly thickened as the r.f. power is increased. The increasing trends of normalized intensities of N2 (337.1 nm) and N2+ (393.4 nm) are similar as the increasing trend of the nitriding case depth with r.f. power. The roles of ICP to enlarge the nitriding depth will be discussed.