## 3 Results and Discussion  
The surface of the DLC thin films was evaluated to identify their tribological properties. The FESEM images of the RF-PECVD DLC thin films showed uniform and smooth surfaces in all specimens; the only significant difference was that the number of small grains tended to slightly increase as thickness increased, from which can be inferred that the density in the DLC thin films increased with growth. Figure 1 shows the root mean square (rms) surface roughness of the DLC thin films with a change in thicknesses. The three-dimensional (3D) AFM images identified a smooth and uniform surface similar to the FESEM images [3,4]. The rms surface roughness increased from 0.70 to 0.94 nm with an increase in thickness, which is believed to have been contributed by the increase in the number of small grains in the surfaces due to the increased kinetic energy of carbon and hydrogen ions in the plasma [17,18], increased bonding for thin-film growth, and increased ion collisions near the surface [19,20] with an increase in the deposition time for increasing the thickness of the DLC thin films with the fixed RF power. Figure 2 shows the hardness, elastic modulus, and friction coefficient with changes in the thickness of the RF-PECVD DLC thin films. As the thickness of the DLC thin films increased, the hardness and elastic modulus increased, and the maximum values of the hardness and elastic modulus were approximately 14 and 210 GPa, respectively, at thickness of 100 nm. These results indicate that the increased hardness and elastic modulus of the DLC thin films make the thin films denser with increasing thickness, and the ratio of diamond-like (\(sp^{3}\)) content to amorphous bonding (\(sp^{2}\)) in the DLC thin films increases [21,22]. Increasing the deposition time to increase the thickness of the thin film improves the bonding ratio of carbon and hydrogen, and the increased collisions of ions in the plasma near the surface increase the density of the DLC thin films and contribute to the increase in hardness and elastic modulus [6,7]. The friction coefficient of the DLC thin films showed a tendency to decrease from 0.017 to 0.013 with an increase in thickness. Although the slightly rougher surface in the thicker thin films, as shown in the FESEM and AFM results (Figure 1), had some effect on the increase in the friction coefficient [23], the  
Figure 1: Field effect scanning electron microscope (FESEM) surface images of the radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) diamond-like carbon (DLC) thin films with various film thicknesses: (**a**) 20, (**b**) 40, (**c**) 60, and (**d**) 80 nm. (**e**) Three-dimensional (3D) images and (**f**) rms surface roughness of the RF-PECVD DLC thin films for the same conditions obtained by atomic force microscopy (AFM). increased film density of the DLC thin film as the thickness increased reduced the friction coefficient [24], which was affected by the modification in the ratio of \(sp^{3}\) representing the binding of carbon and hydrogen in the thin films [6,7]. Figure 3 shows the contact angle of the DLC thin films with a change in thickness. The value of the contact angle on the surface exhibits a decreasing tendency from \(86^{\circ}\) to \(84^{\circ}\), but it was within the scope of a small change at varying DLC thin films thicknesses. The slight decrease in contact angle might be caused by the increase in the ratio of \(sp^{3}\) showing hydrophilic properties on the surface. Generally, the surface shows the hydrophilic/hydrophobic properties according to the ratio of \(sp^{3}/sp^{2}\) in the thin films [25]. The DLC thin films with higher \(sp^{3}\) bonding fraction had a more hydrophobic surface than those with a higher \(sp^{2}\) bonding fraction because the surface energy of the \(sp^{3}\)-terminated surface is very high due to its strong covalent character [26]. Owing to the hydrophobic properties of the DLC thin films, they are used as surface protectable and self-cleanable thin films.