# High Density RF-DC Plasma Nitriding under Optimized Conditions by Plasma-Diagnosis
## 4 Conclusions  
Plasma diagnosis with the use of OES is utilized to describe the effect of RF-DC plasma processing parameters and hydrogen gas content on the yield of N\({}_{2}^{+}\) ions and NH radicals. In particular, the yield ratio of NH-radicals to N\({}_{2}^{+}\) ions is employed as a parameter to investigate the effect of hydrogen gas content on the nitriding process. This ratio maximizes at a hydrogen content of 20 to 25%. When the hydrogen content is less than 20%, the reaction cross-section to yield the NH-radical increases with the hydrogen content, while this reaction cross-section significantly decreases with the increase in hydrogen content by the quenching effect. In particular, the yield of NH-radicals and N-ions significantly reduces with the increase in hydrogen content of up to 50% and results in low hardness. This optimization of hydrogen content in the nitriding process is demonstrated by the actual nitriding of AISI304 stainless steels at 673 K for 14.4 ks. The highest surface hardness is attained at this optimum hydrogen content. This proves that a sufficient amount of nitrogen atom flux at the substrate surface drives the inner nitriding process. This enriched nitrogen atom flux from the substrate surface drives the nitrogen supersaturation and diffusion co-process. Even at 623 K, when conventional nitriding only forms a very thin nitrided layer due to slow nitrogen diffusion, the AISI304 substrate can be nitrided to produce a thick nitrogen supersaturated layer with high hardness by the present plasma processing. The nitrogen supersaturation and diffusion co-process terminates by itself when the nitrogen diffusion network in the granular microstructure of AISI304 becomes too sparse to sustain the co-process.  
In conventional plasma nitriding industries, the holding temperature is a main processing parameter since the inner nitriding process strongly depends on the nitrogen body-diffusion. In the presence of a rich nitrogen atom flux from the substrate surface to the depth, the inner nitriding takes place even at a much lower holding temperature. Since the nitrogen supersaturation and diffusion co-process is sustained by self-formation of the nitrogen diffusion network, the holding temperature can be lowered to broaden the application of plasma nitriding. In particular, the stainless-steel wires and tools for medical applications requires their hardening and strengthening without any additional alloyingmetallic elements. The present low temperature plasma nitriding provides a solution by high hardening and strengthening them only with the enrichment of nitrogen solute contents in their depth.  
Conceptualization, T.A.; methodology, T.A.; validation, T.A., I.R.; formal analysis, E.E.Y.; investigation, T.A., I.R. and E.E.Y.; resources, T.A.; data curation, I.R.; writing--original draft preparation, T.A.; writing--review and editing, T.A.; visualization, I.R.; supervision, T.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors would like to express their gratitude to [PERSON] and [PERSON] (University of Brawijaya, Indonesia) and [PERSON] (Graduate School of Engineering, University of Tokyo) for the help in experiments and discussion.  
The authors declare no conflict of interest.