| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuE |
| Session: | Next-generation Protective Coatings and Tribological Applications |
| Presenter: | Fumiya Nakamura, Tohoku University, Japan |
| Authors: | F. Nakamura, Tohoku University, Japan Y. Wang, Tohoku University, Japan N. Miyazaki, Tohoku University, Japan Y. Ootani, Tohoku University, Japan N. Ozawa, Tohoku University, Japan K. Adachi, Tohoku University, Japan M. Kubo, Tohoku University, Japan |
| Correspondent: | Click to Email |
Water lubrication has the characteristic of low environmental burden. It is known that silicon carbide (SiC) shows low friction coefficient due to the formation of a tribofilm by chemical reaction at sliding interfaces under water lubrication. Thus, understanding of the chemical reaction mechanism is essential to improve friction characteristic for practical use and application. However, it is difficult to observe directly such a complicated phenomenon including friction and chemical reaction by experiments. Therefore, in this study, molecular dynamics (MD) simulation using reactive force field, which can simulate chemical reaction, was conducted to analyze the structure and formation mechanism of tribofilm in the friction process of amorphous SiC under water environment.
In the simulation, we used a model in which amorphous SiC ball and disk were rubbed in water (See supplementary document (SD) Fig.1). The SiC ball was slid on the SiC surface with nominal pressure of 0.5 GPa and sliding speed of 100 m/s.
During the friction, the surface wears as the SiC ball and the SiC disk come into contact with each other. Silica (SiO2) particle and hydrocarbon were confirmed as wear debris. It was also observed that the SiO2 particles dissolved in water and forms colloidal silica. Firstly, we investigated the changes in number of water molecules and Si-O-Si and C-H bonds during the friction (See SD Fig. 2, Fig.3). It was found that the number of water molecules decrease continuously while the number of Si-O-Si and C-H bonds increase correspondingly. This result indicates that hydrolysis reaction of SiC (SiC + H2O → Si-OH + C-H) occurs at the sliding interface. Next, we investigated the change in the number of Si and C contained in the wear debris (See SD Fig. 4). We found that the number of Si in the debris was always larger than that of C in the debris. This result indicates that the Si atoms are easily dissolved from the SiC surface as SiO2 particle, whereas the C atoms tend to remain on the SiC surface. The distribution of C, Si, H, and O atoms except H2O molecules shows that the C and H atoms were concentrated on the surface of the SiC disk and SiC ball (See SD Fig. 5, Fig. 6). On the other hand, the Si and O atoms were dissolved in water, forming the colloidal silica. Meanwhile, the Si and O atoms also located between the SiC disk and SiC ball. This fact indicates that the colloidal silica prevent the contact of the surfaces, leading to low friction. Thus, we concluded that colloidal silica, which is produced by the hydrolysis reaction of SiC surface, lowers the friction of SiC in water lubrication.