**2. High-density plasma nitriding system**

The high-density RF (Radio Frequency) - DC (Direct Current) plasma nitriding system is depicted in **Figure 2a**. It consists of the vacuum chamber, the evacuation system, the gas supply, the RF/DC-power generator, and the control panel. On the basis of the hollow cathode device [13, 18], the experimental setup was redesigned with the use of plasma diagnosis [19]. **Figure 2b** depicts a typical hollow-cathode setup for the present nitriding processes. High brightness only in the inside of the hollow proves how much the ion and radical populations are present in the ignited plasmas.

### **Figure 2.**

*High-density plasma nitriding system. a) Overview of RF-DC plasma nitriding system, and b) hollow cathode set up for high-density nitriding process.*

**Figure 3.** *A typical optical emission spectrum from the nitrogen and hydrogen plasmas is measured by the plasma diagnosis.*

*Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K… DOI: http://dx.doi.org/10.5772/intechopen.102387*

**Figure 4.**

*Variation of the intensity ratio (RNH) of NH-radical peak to N2 <sup>+</sup> peak by OES with increasing the hydrogen gas content in N2 + H2 mixture gas.*

Among various plasma diagnosis methods, OES (Optical Emissive-light Spectroscopy) was employed to describe the activated species in the nitrogen – hydrogen plasmas. **Figure 3** depicts a typical spectrum. Both N2 <sup>+</sup> and NH are main species with high intensity. After [20], N2 <sup>+</sup> is a mother species to generate a series of nitrogen ions by electron attachment and detachment reactions in plasmas. The N2 + peak with the highest intensity and the NH peak at Λ = 340 nm was employed for diagnosis.

After optimizing the relationship among the RF-voltage, the DC-bias, and the pressure, the nitrogen–hydrogen gas ratio was varied to search for optimization of the intensity ratio (RNH) of NH-radical peak to N2 <sup>+</sup> peak. **Figure 4** shows the variation of RNH with increasing the hydrogen gas content [H2] to the nitrogen– hydrogen mixture gas. When [H2] < 5%, less NH-peaks were detected by OES; RNH increases with increasing [H2]. After RNH becomes maximum at [H2] = 20-25%, this ratio significantly decreases with increasing [H2] due to the hydrogen quenching effect in the plasma synthesis of NH radicals. In the following nitriding experiments, the nitrogen and hydrogen gas flow rate were respectively fixed by 160 mL/ min and 30 mL/min after this plasma diagnosis.
