Spectroscopic investigation of a temporal post-discharge plasma for iron nitriding

A pure nitrogen plasma operating in a low-frequency (<1 kHz) DC pulsed mode is used for iron nitriding and is characterized by means of optical emission spectroscopy. Line intensities related to N 2 or N 2 + states involved in the surface treatment, as well as an iron line linked to the cathode sputtering, are maintained during the discharge and the temporal post-discharge, and are compared with the electron density evolution. The processes governing the population of excited states are quite different when the discharge is on (electron collisions) and when the discharge voltage is cut off (importance of long lifetime metastable N 2 states)-justifying the interest of the post-discharge phase during which active species are created in a surface treatment process.