# High Density RF-DC Plasma Nitriding under Optimized Conditions by Plasma-Diagnosis
## 3 Experimental Results and Discussion
### Low Temperature Plasma Nitriding of AISI304 Stainless Steels  
Two plasma nitriding experiments were performed to investigate the hydrogen gas content effect on the nitriding process with the use of the hollow cathode. AISI304 stainless steel disc specimen with a diameter of 20 mm and a thickness of 5 mm was employed as a work. In the first experiment, the hydrogen gas content was only varied to be 10%, 25% and  
Figure 7: Variation in the peak intensity ratio for NH-radicals to N\({}_{2}^{+}\) ions with the increase in hydrogen gas content. 50%. The other plasma parameter was fixed in the pressure of 35 Pa, the DC of \(-\)600 V, and the RF-voltage of +250 V. AISI304 specimen was nitrided at 673 K for 14.4 ks. The surface hardness was measured by micro-Vickers testing with an applied load of 1 N (or 100 gf). Table 2 lists the measured surface hardness at each condition. The highest surface hardness is attained when [H\({}_{2}\)] = 25%. This proves that plasma diagnosis on the N\({}_{2}\) + H\({}_{2}\) plasmas is effective in determining the optimum nitriding conditions, and to make full use of NH radicals, as well as nitrogen ions. In the second experiment, AISI304 specimen was nitrided at 623 K under the RF-voltage of +250 V, the DC-bias of \(-\)500 V, the pressure of 70 Pa and the gas flow rate ratio of 30 mL/min for hydrogen gas to 160 mL/min for nitrogen gas. Very few studies have reported DC-nitriding (or ion-nitriding) and the pulsed-DC-nitriding (or radical-nitriding) at 623 K; the nitrided layer thickness was limited to be negligibly thin [22,23]. Since the nitrogen diffusion governed those inner nitriding processes, a higher holding temperature was necessary to drive the nitriding process into the depth of the substrate. This plasma nitriding at 623 K provides a chance to reconsider the inner nitriding mechanism under the high nitrogen atom flux through the optimization of plasma conditions. Figure 8 depicts the SEM image on the cross-section of the nitrided AISI304 substrate and its nitrogen mapping. The nitrided layer advanced to a depth of 50 \(\upmu\)m, even at 623 K for 14.4 ks. Large grains were seen below the nitriding front end, while no grain boundaries were detected above this front. This suggests that the nitrided layer at 623 K has too fine grains distinguished by SEM with low magnification. This grain refinement process is discussed in later. After [4,8,24], this nitrided layer thickness is slightly proportional to the holding temperature (T\({}_{\rm H}\)) for 623 K \(<\) T\({}_{\rm H}\) \(<\) 693 K, while very few thicknesses were noticed in the normal nitriding at this low temperature [22,23]. In addition, this low temperature nitriding process is characterized by high nitrogen content, as depicted in Figure 8b. No iron and chromium nitrides precipitate in the nitrided layer so that nitrogen atoms are present as an interstitial solute in the iron lattice with high concentration. After pointwise detection by EDX in depth (d), the nitrogen content [N] at the surface reaches to [N] = 7.4 mass % \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**H\({}_{2}\) Percentage (\%)**} & \multicolumn{1}{c}{**Hardness (HV)**} \\ \hline No treatment & 370 \\
10 & 760 \\
25 & 1100 \\
50 & 680 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The hydrogen gas content effect on the surface hardness of nitrided AISI304 disc specimen at 673 K for 14.4 ks. Figure 8: SEM image on the cross-section of nitrided AISI304 substrate at 623 K for 14.4 ks and its nitrogen mapping. **(a)** SEM image on the cross-section, and **(b)** nitrogen solute mapping. or 23.3 at %, while [N] = 4.5 mass % on average for 5 \(\upmu\)m \(<\) d \(<\) 45 \(\upmu\)m. Since the nitrogen solutes supersuraturate the iron cells in the nitrided AISI304 stainless steels, the hardness is much higher than the original hardness of raw AISI304 (400 HV), due to the solid solution hardening mechanism. In fact, the surface hardness reaches 1640 HV; the average hardness is 1500 HV in the nitrided layer.