2. EXPERIMENTAL PROCEDURE
Both the high density plasma nitriding and the RF-sputtering systems are utilized to make pre-hardening and nano-laminated DLC coating on the SKD11 substrate, respectively.
2.1 High Density Plasma Nitriding
In the present plasma nitriding system, both RF- and DC-plasmas are generated by RF and DC power supplies, independently. As shown in Fig. 1, RF-plasma is ignited by dipole electrode, while DC-plasma is driven by DC-bias on the cathode plate. The generated plasma has symmetric configuration in the electricity field since the vacuum chamber is electrically neutral. There is no mechanical matching box; matching between input and output powers is automatically maintained by frequency control around 2 MHz. In addition, the focused nitrogen plasma flux can be irradiated onto the samples by using the magnetic lens. Nitrogen plasma is first utilized for pre-sputtering the samples; mixed gas of nitrogen and hydrogen is used in the plasma nitriding. The flow rate ratio in the mixed gas affects the species of ions and radicals. In particular, the yield of NH-radicals in the generated species increases with reducing the ratio of the hydrogen flow rate to the nitrogen one. In fact, after Ref. [1], the measured intensity of NH-radicals by the emissive light spectroscopy reaches to 25-30 % of the maximum intensity of N2+ ion as shown in Fig. 2. This measured spectrum proves that high yield of NH-radicals as well as activated nitrogen atoms are available in the present plasma nitriding.