Nitriding and DLC Coating of Aluminum Alloy Using High-Current Pressure-Gradient-Type Plasma Source

Nitriding of an A5052 aluminum alloy followed by coating with a diamond-like carbon (DLC) film using a pressure-gradient-type plasma source was performed. The plasma source was operated at a low discharge voltage of 60-100 V and a high current of 60-130 A. First, the aluminum alloy was plasma nitrided for 4 h at 520 °C under 0.09-1.1 Pa. The DLC film was then coated using acetylene gas with the same apparatus. The Vickers microhardness of the surface nitrided at 0.51 Pa increased to approximately 305 HV from an initial value of 125 HV for the base material. In addition, glow discharge-optical emission spectrometry (GDOES) revealed that nitrogen was concentrated in the surface region. After the DLC coating, the sample became reddish brown. The GD-OES results indicated that a carbon-rich region was formed at the top surface (DLC film), followed by the formation of a nitrogen-rich region (nitrided layer). Nanoindentation tests revealed that the hardness of the top surface (DLC film) was 10.3 GPa. The DLC coating also exhibited good tribological performance in a ball-on-disk wear test, with friction coefficients of approximately 0.17, which is considered a low value for DLC. In addition, an intermediate AlN layer was deposited on the nitrided layer using the ion-plating method to enhance the adhesion between the DLC film and the substrate. Rockwell indentation measurements revealed good adhesion. Moreover, the addition of another Si-containing DLC intermediate layer was shown to further improve the adhesion between the nitrided layer and DLC film.


Introduction
Aluminum alloys are beneficial for applications that require low weight because their specific gravity is approximately one-third that of steels. How- To improve the hardness of aluminum alloys by plasma nitriding and to determine whether the poor adhesion of DLC films could be overcome by forming DLC films on a nitrided layer using the plas-ma chemical vapor deposition (CVD), the authors previously examined the combination of these sur-face treatments [3]. Improvement of the adhesion properties has also been reported with the addition of a graded AlN film as an intermediate layer on the surface of an aluminum alloys using the unbalanced magnetron sputtering method [4,5].
Nitriding of aluminum alloys is recognized as a difficult technology because the process of nitride formation is inhibited by the oxide layer formed on their surfaces [6][7][8][9][10]. The high-vacuum process for the oxide layer removal of and prevention of re-oxidation is considered key to successful nitriding. The authors previously observed that DLC films can be formed using a pressure-gradient-type plasma source in a high-vacuum area and that hydrogen-less nitriding of an austenitic stainless steel SUS 304 is feasible [11,12]. In this study, using the same technique, the A5052 aluminum alloy was subjected to nitriding and then cooled while maintaining the vacuum, and DLC film formation on the top surface was performed. Attempts were also made to improve the adhesion by introducing an AlN film or a Si-containing DLC intermediate layer as the intermediate layer between the nitride layer and the DLC film.

Experimental material
The sample used in this study was a disk material of ϕ25 × 5 mm of the A5052 Al alloy containing 2.49 mass% of Mg. The surface to be treated was mirror-polished using electrolytic polishing.  are mounted between the cathode and the anode to stabilize the discharge. Plasma generation is possible in the pressure range from the latter half of 10 -2 Pa to 1 Pa, and discharge is possible up to 250 A at 200 V or less. This plasma source is used for thinfilm production using the reactive ion plating (RIP) [15]. Specifically, a high deposition rate of 14 nm/s was achieved for the formation of the MgO films for plasma displays; furthermore, (110), (200), and (220) crystal orientations were controllable [16].

Nitriding and DLC film coating
First, heating was performed with the heater on top while exhausting until a vacuum level of 10 -4 Pa was attained. Thereafter, plasma was generated by the pressure-gradient-type plasma source, a pulse bias voltage was applied, and Ar bombardment was performed.
Subsequently, a pulse bias voltage of -150 V was applied, 500-sccm N 2 gas at a pressure of 0.51 Pa was introduced into the chamber, horizontal plasma was generated with a discharge current of 150 A, and hydrogen-less plasma nitriding was performed at a sample temperature of 520 °C for 360 min. After nitriding and cooling under vacuum, when the temperature was sufficiently lowered, the DLC film with a target thickness of 1.5 μm was formed in 29 min under the following conditions: Pressure of 0.21 Pa, discharge current of 60 A, C 2 H 2 flow rate of 71 sccm, and pulse bias voltage of -100 V.

Nitriding, AlN intermediate layer, and DLC film coating
To improve the adhesion between the nitrided was generated from the plasma gun toward the inside of the chamber by supplying discharge-assisting Ar gas to the plasma source, and preventing the backflow of ions generated in the chamber, in addition, by introducing reactive gases such as N 2 and C 2 H 2 to the cathode side, damage to the cathode was prevented. The shape of the plasma and the site of irradiation could be easily modified using a combined magnetic field generated by the combination of an air core coil and a magnet [13]. Figure  1 shows an example wherein the plasma is bent at a right angle and irradiated the crucible arranged at the bottom.
The vacuum chamber could be evacuated to a level of 10 -5 Pa. An infrared heater was installed as the heating source and used to control the temperature of the processed materials. A pulse power source of 5-350 kHz and 0 to -800 V was connected to the substrate holder, enabling the application of a pulse bias voltage to the substrate. As described above, the plasma shape could be easily modified, and the plasma could be horizontally discharged. Figure 2 presents photographs of plasma bent at a right angle bent plasma and the horizontal plasma.

Pressure-gradient-type plasma source
The pressure-gradient-type plasma source, developed by Uramoto [14], enables large direct current (large-DC) discharge using a LaB 6 plate cathode. The structure of the plasma gun is shown in Figure  3; the cathode part is a composite of a Ta pipe and a LaB 6 plate. Moreover, the hollow cathode discharge is facilitated by supplying discharge-assisting Ar gas from the inside of the cathode. Two grid electrodes flow rate of 500 sccm, pulse bias voltage of -150 V, and sample temperature of 300 °C. After nitriding and cooling under vacuum, the discharge circuit was changed and an AlN film was formed using RIP.
The target film thickness of AlN was 3.4 μm, and processing was performed for 55 min under the following conditions: Pressure of 0.22 Pa, discharge output of 8 kW, and N 2 flow rate of 300 sccm with-layer and the DLC film, an AlN film was formed as an intermediate layer via RIP. Accordingly, for nitriding of the base material, a thin nitrided layer was formed at a low temperature of 300 °C.
After the Ar bombardment treatment described above plasma nitriding was performed for 60 min under the following conditions: Treatment pressure of 0.56 Pa, discharge current of 130 A, N 2 gas

Results and Discussion
Nitriding and DLC film coating  speed film formation at low temperature is possible using RIP. For the Rockwell indentation test, some cracks and peeling were observed around the indentation; however, good adhesion was attained. A plausible reason for this good adhesion is that AlN formed by nitriding and that formed by RIP have the same elemental composition and the bondability to the interface is enhanced. Another factor to consider is that a harder AlN layer could be formed using RIP.
For the sample coated with the DLC film after forming the AlN intermediate layer, a black surface characteristic similar to those for the DLC film were observed. The nanoindentation hardness of the surface was 26.7 GPa. Figure 8 presents the EPMA results for the surface after the Rockwell indentation test. Al and N were detected around the indentation, suggesting a separation between the DLC film and AlN intermediate layer. Therefore, though abrasion marks were observed on the surface of the DLC film, the wear track was extremely small, and its position could not be specified in the cross-sectional profile. Furthermore, μ was 0.18 at the beginning of the friction test and 0.17 after 100 m, indicating that the measurement values were stable. This superior friction resistance is reflective of the characteristics of a DLC film.

Nitriding, AlN intermediate layer, and DLC film coating
To improve the adhesion between the nitrided layer and the DLC film, an AlN film was formed as an intermediate layer using RIP. The thin, golden surface of the AlN film formed after nitriding is shown in Figure 7. The surface hardness increased to 638 HV and the nanoindentation hardness was 11.4 GPa. In addition, the thickness of the AlN film was 3.4 μm, which demonstrates that high- nitrided layer/base material sample, even though slight cracks and separation were observed for part of the indentation edge portion, good adhesion was attained. Figure 10 Figure 9 shows the appearance of the sample after the treatment and presents the Rockwell indentation and nanoindentation surface hardness measurements. A glossy black color characteristic of DLC films was observed for all cases.  surface region. The GD-OES results also demonstrated that after the DLC coating, a carbon-rich region formed at the top surface of the DLC film followed by the formation of a nitrided layer. Nanoindentation tests indicated that the hardness of the DLC film was 10.3 GPa. The DLC coating also exhibited good tribological performance in a ballon-disk wear test, with friction coefficients of approximately 0.17, which is considered a low value

Conclusions
In this study, a DLC film and nitrided layer were deposited onto an aluminum alloy using a pressure-gradient-type plasma source with N 2 and C 2 H 2 gases. The Vickers microhardness of the surface nitrided at 0.51 Pa reached approximately 305 HV compared with an initial hardness of 101 HV for the base material. In addition, GD-OES analysis revealed that nitrogen was concentrated in the     anism of nitrided layers on aluminum substrates by thermal plasma nitriding. Metals 9: 523.