Effect of Plasma Treatment on Friction Coefficient of Diamond-like Carbon Films

Abstract

1Introduction Diamond-like carbon （DLC） films have been intensively studied in the past due to their excellent characteristics, such as chemical inertness, high hardness, low friction coefficients, wear resistance , and biocompatibility 1） , 2）. Recent studies have reported that surface-modified DLC films improve biocompatibility, lubricity, stability, and cell adhesion 3） ～ 6）. These characteristics are related to surface roughness 7） , 8） , structural bonds 9） , 10） , and whether the film is hydrophilic or hydrophobic 11） , 12）. Surface modification of DLC has been performed by doping with suitable elements 3） ～ 5） , 13） and plasma treatment 14） , 15）. Owing to the chemically active species in plasma and the ease of processing, plasma treatment is the most widely used method for modifying DLC surfaces. Recently, O2 plasma treatment has become widely used on both experimental and industrial scales; for instance, Santos et al. found that O2 plasma treatment enhances the surface energy of DLC films, which may further improve their surface hardness 16）. In addition, Choi et al. also devoted their enthusiastic attention towards oxygen plasma in DLC films to improve their tribological properties 17）. The improvement of the surface energy and friction coefficient of DLC is a direct consequence of the surface activation processes required for many technological applications, including use as a biomaterial 15） , 18） , 19）. In this context, O2 and CF4 plasma treatment of DLC film surfaces may provide a new and efficient approach to improve the wetting and tribological properties of the films without the need for complex hybrid system geometry. Thus, further work on this topic should exhibit great promise. In this study, we have focused on the effect of O2 and CF4 plasma treatment on friction coefficient, and have evaluated the bonding structure and wettability properties of the DLC films. 2Experimental details A schematic of the PBII system used for the deposition and plasma treatment of the DLC films has been previously shown in the literature 20）. Si （100） wafers, 0.7 mm thick, were used as sub-strates. The wafers were sputter-cleaned with Ar ＋ for 20 min to remove residual surface contaminants and surface oxides using a negative-pulsed bias voltage of 10 kV. A pulse width of 5 μs, a pulse delay of 25 μs, and a pulse frequency of 1 kHz were utilized during the sputter-cleaned process. Using a negative-pulsed bias voltage of 20 kV, the DLC film interlayer was first deposited with CH4 for 60 min to improve adhesion between the film and the substrate. A pulse width of 5 μs, a pulse delay of 60 μs, and a pulse frequency of 1 kHz were utilized during the creation of interlayer process. The deposition of the DLC films was performed at a negative-pulsed bias voltage of 5 kV with C2H2 for 150 min. A pulse width of 5 μs, a pulse delay of 25 μs, and a pulse frequency of 1 kHz were utilized during the coating process. The deposition pressure was set to 2 Pa, and the total deposited thickness of the films was approximately 450 nm. After deposition, O2 or CF4 plasma was applied in the chamber with various RF powers of 100, 300, and 500 W for 30 min. The plasma treatment pressure was also set to 2 Pa. The film properties were studied using several characterization techniques. The surface information from a depth close to the film surfaces was carried out with sufficient accuracy by means of fully gas-purging for IR measuring device （JASCO FT/ IR-4200） , not only a sample room but also a beam splitter room using dry N2, and by using the highly sensitive technique for the film surface called Attenuated total reflectance （ATR）. Therefore , the spectrum which does not have influence of the noise by In this study, diamond-like carbon （DLC） films are modified using O2 and CF4 plasma generates by radio frequency （RF） , taking into account surface wetting and tribological properties. Surface properties of O2 and CF4 plasma-treated DLC are characterized using atomic force microscopy （AFM） , Attenuated total reflectance-Fourier transform infrared （ATR-FTIR） spectroscopy, ball-on-disk friction testing, and contact angle measurements. The results indicate that hydroxyl group generates on the surface of the O2 plasma-treated DLC with surface roughness of approximately 0.20-0.25 nm, while fluorinated group also generates on the surface of the CF4 plasma-treated DLC with surface roughness of approximately 0.16-0.21 nm. The O2 plasma-treated DLC films present hydrophilic surfaces due to their low contact angles and high surface energies. In contrast, CF4 plasma-treated DLC show hydrophobic surfaces, as evidence by their high contact angles and low surface energies. Further, CF4 plasma-treated DLC film surfaces exhibit friction coefficients as low as untreated DLC, but O2 plasma-treated DLC exhibit higher friction coefficients than that of the untreated DLC. It is concluded that CF4 plasma treatment can be used to produce hydrophobic DLC, making DLC a favorable non-wetting surface and improving its friction coefficient.