2019 年 18 巻 2 号 p. 103-104
Diamond-like carbon (DLC) is a class of amorphous carbon materials with excellent friction properties. Graphitization of DLC caused by friction induces wear of DLC. Further, the existence of water molecules decrease the graphitization rate of DLC. In order to reduce the wear caused by graphitization, it is essential to reveal the mechanism by which water molecules decrease graphitization of DLC. Therefore, the friction simulations of DLC in both water and vacuum environment were carried out by reactive molecular dynamics method. We find that terminal-H atoms are removed by friction and dangling bonds are generated in both environments. In water environment, the generated dangling bonds connect with H and OH generated by the dissociation of water molecule, inhibiting the transformation from sp3 to sp2 carbon atoms and the formation of double bonds.We find that these are reasons for the suppression of DLC graphitization and the decrement of DLC wear.
Diamond-like carbon (DLC) coating films possess excellent friction properties. Especially since DLC shows super-low friction coefficient in water environment. However, the graphitization of DLC caused by friction, induces the wear of DLC. To decrease the wear, it is essential to inhibit the graphitization of DLC. Moreover, the experiments show that the existence of water molecules decrease the graphitization rate . A clarification of the mechanism that water molecules use to decrease the graphitization of DLC is required. However, the graphitization is complicatedly connected with tribochemical reactions, and thus it is difficult to clarify the graphitization process of DLC during the friction process by experiments. Therefore, we investigated the reason why water molecules decrease the graphitization of DLC using reactive molecular dynamics (MD) simulation.
The friction simulation model is shown in Figure 1. We use H-terminated DLC surfaces. Friction simulations are carried out by fixing the bottom part of the lower DLC substrate and sliding the topmost part of the upper DLC substrate with a normal load of 10 GPa. Since 1 GPa − 10 GPa was usually used in the previous simulations, here we use 10 GPa to accelerate reactions at friction interfaces . Both friction simulations in water and vacuum environment are carried out. We used a reactive force field (ReaxFF) to consider chemical reactions. The simulations were performed by our developed MD program, LASKYO .
The friction simulation model of DLC.
To evaluate the influence of water environment on the graphitization of DLC, we carried out the friction simulation and investigated the number of sp2 carbon atoms (Csp2) under water and vacuum environments. From Figure 2, in both environments, the numbers of Csp2 increase, which means that friction causes the graphitization of DLC. The increased amount of Csp2 in water environment is much smaller than that in vacuum, indicating water environment suppresses the graphitization of DLC. To expound the reason why the water environment suppresses the graphitization of DLC, we investigated the change in the number of H2O molecules and dangling bonds in DLC substrates, respectively. Figures 3a and 3b show the number of dangling bonds on carbon atoms and H2O molecules, respectively. Figure 3a shows that in both water and vacuum environments the dangling bonds on carbon atoms increase. This is because the contact between two DLC surfaces induces the removal of terminal-H atoms from the DLC surfaces. Thus, dangling bonds are generated on DLC surfaces. The generation of the dangling bonds in water environment is less than that in vacuum. In Figure 3b, H2O molecules in water environment decrease, meaning the chemical reaction between water and DLC. We revealed that terminal-H atoms on the DLC substrates detach from the DLC surfaces and the dangling bonds are generated on the carbon atoms, in friction progresses under both vacuum and water environments (Figure 4). In vacuum environment, these carbon atoms with dangling bonds connect with neighbor carbon atoms. Then, single bonds change to double bonds, leading to the transformation from Csp3 to Csp2 and the graphitization of DLC. Correspondingly, in water environment, these dangling bonds on the carbon atoms connect with the dissociated H and OH from H2O molecules, and then H- and OH-terminations are generated. Therefore, in water environment, the change from single bonds to double bonds is suppressed and the graphitization of DLC is inhibited.
Time variation of Csp2 number during friction.
Time variation of dangling bonds in the DLC substrates (a) and H2O molecule (b) during friction.
Mechanism of graphitization and its inhibit by water.
In this study, we clarify the reason why water molecules suppress the graphitization of DLC during the friction process. In vacuum, the graphitization is induced by the removal of terminal-H atoms during friction. Here we demonstrate that, in water environment, terminal-H and -OH are generated by the dissociative adsorption of water onto DLC surfaces, finally suppressing the graphitization of DLC.