| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Thin Films | Monday Sessions |
| Session TF-MoM |
| Session: | Self-Organized and Nanostructured Thin Films |
| Presenter: | Shandan Bai, Tohoku University, Japan |
| Authors: | S. Bai, Tohoku University, Japan Y. Niiyama, Tohoku University, Japan Y. Kobayashi, Tohoku University, Japan Y. Higuchi, Tohoku University, Japan N. Ozawa, Tohoku University, Japan K. Adachi, Tohoku University, Japan S. Mori, Tohoku University, Japan K. Kurihara, Tohoku University, Japan M. Kubo, Tohoku University, Japan |
| Correspondent: | Click to Email |
[Introduction] Diamond-Like Carbon (DLC) coatings have low friction and anti-wear tribological performances. Furthermore, water lubrication improves the friction properties of DLC films and reduces the emission of CO2. The friction coefficient of the DLC films drastically changes under water lubrication, since some tribo-chemical reactions occur during the sliding. However, tribo-chemical reactions are difficult to be revealed only by experimental analyses in details. The computational technique is efficient method to investigate the low friction mechanism [1]. In this study, we reveal the tribo-chemical reaction between DLC film and water using the computational method on an atomic scale.
[Method] To clarify the tribo-chemical reactions of DLC films under water lubrication, we use our tight-binding quantum chemical molecular dynamics (TB-QCMD) method [2]. We construct the sliding simulation model consisting of 80 water molecules and two DLC substrates. The thickness of water is approximately 1.0 nm. The friction simulation is performed for 100,000 steps with the time step of 0.1 fs. We apply contact pressures of 0.5 and 5 GPa on the top layer of upper substrate of DLC films, while it is forcibly slid with a horizontal velocity of 10 m/s. The simulation temperature is set at 300 K, achieved by velocity scaling method.
[Results and Discussion] We perform our TB-QCMD calculations to investigate the low friction properties of DLC in water lubrication. Under a contact pressure of 0.5 GPa, one C-OH bond is generated on the DLC surface at 0.045 ps during the sliding, because of the dissociation of a water molecule. Furthermore, at 3.655 ps, we observe another C-OH bond generation on the surface. The result indicates that OH terminates the DLC surface under a contact pressure of 0.5 GPa. Under a contact pressure of 5 GPa, generation of a C-OH bond is observed on the surface at 1.380 ps. Furthermore, at 3.880 ps, it is very interesting to see the generation of C-O-C on the DLC surface, which is a different chemical reaction with that under pressure of 0.5 GPa. The friction coefficients are 0.81 and 0.05 under contact pressures of 0.5 and 5 GPa, respectively. Those results indicate that the friction coefficient decreases with increasing a contact pressure. We think that the chemical reaction leads to the structure change on the DLC surface and the low friction properties of DLC in water lubrication under high contact pressures.
[1] S. Bai, M. Kubo et al., J. Phys. Chem. C, 116, 12559, (2012).
[2] S. Bai, M. Kubo et al., RSC Adv., DIO: 10.1039/c4ra04065a.