FORMATION OF IRON SULFIDE BY PLASMA-NITRIDING USING SUBSIDIARY CATHODE

Chromium-Molybdenum steel was plasma-nitrided at 823 K for 10.8 ks in an atmosphere of 30% $N_2$-70% $H_2$ gas under 665 Pa without and with a subsidiary cathode of $MoS_2$ to compare ion-nitriding and plasma-sulfnitriding using subsidiary cathode. When the steel was ion-nitrided without $MoS_2$, iron nitride layer of 4$\mu\textrm{m}$ and nitrogen diffusion layer of 400mm were formed on the steel. A compound layer of 15$\mu\textrm{m}$ and nitrogen diffusion layer of 400$\mu\textrm{m}$ were formed on the surface of the steel plasma-sulfnitrided with subsidiary cathode of $MoS_2$. The compound layer consisted of FeS containing Mo and iron nitrides. The nitrides of $\varepsilon$-$Fe_2$, $_3N$ and $\gamma$-$Fe_4N$ formed under the FeS. The thicker compound layer was formed by plasma-sulfnitriding than ion-nitriding. In plasma-sulfnitriding, the surface hardness was about 730 Hv. The surface hardness of the steel plasma-sulfnitrided with $MoS_2$ was lower than that of ion-nitrided without $MoS_2$. This may be due to the soft FeS layer formed on the surface of the plasma-sulfnitrided steel.