LOW-FREQUENCY DC PULSED PLASMA FOR IRON NITRIDING

Diagnostics are performed in a low frequency (below 1000 Hz) N2-H-2 d.c. pulsed plasma used for iron nitriding. The experimental conditions are chosen according to previous results, some of them being briefly recalled (especially those concerning the discharge frequency). Measurements of the electron-ion recombination coefficient show that this process is mainly responsible for the electron density decay during the temporal post-discharge (when the discharge is cut off). By taking into account the presence of two different species in the plasma, a global recombination coefficient is measured and its variations as a function of H-2 concentration are reported. The metallurgical analysis of the nitrided samples is correlated with the spectroscopic study of the plasma, which reveals a decrease in the substrate sputtering by introducing a small amount of H-2 in the gas mixture. It is shown that gamma' (Fe4N) nitride layers of better quality are obtained by using a 90%N2-10%H-2 treatment atmosphere.