| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Monday Sessions |
| Session TF-MoE |
| Session: | Nanostructured Surfaces & Thin Films I |
| Presenter: | Jun Chida, Tohoku University, Japan |
| Authors: | J. Chida, Tohoku University, Japan N. Takahashi, Tohoku University, Japan Y. Wang, Tohoku University, Japan Y. Ootani, Tohoku University, Japan T. Nishimatsu, Tohoku University, Japan Y. Higuchi, Tohoku University, Japan N. Ozawa, Tohoku University, Japan K. Adachi, Tohoku University, Japan M. Kubo, Tohoku University, Japan |
| Correspondent: | Click to Email |
Silicon nitride (Si3N4) is used as a low friction material under water lubrication. It is suggested that the low friction is caused by the SiO2･n(H2O) (silica gel) layer generated by chemical reactions between the Si3N4 and the water at a friction interface [1]. Thus, the understanding of formation mechanisms of the silica gel is required to improve the friction properties. However, it is difficult to directly observe the chemical reaction dynamics at atomic scale by experiments. In this work, we investigated the formation mechanism of the silica gel by our tight-binding quantum chemical molecule dynamics simulator [2].
We performed the sliding simulation of Si3N4 under water lubrication. The simulation model consists of two amorphous Si3N4 substrates and H2O molecules. The top of the upper substrate was slid by 100 m/s with a load of 3 GPa. The Si atoms and the N atoms on the surfaces were terminated by OH groups and H atoms, respectively.
First, we employed the flat substrate model. During the friction simulation, an H atom of a Si-OH bond on the Si3N4 surface transferred to a nearby N atom. Then, the O atom of the Si-O bond approached another Si atom in the surface, and a Si-O-Si bond was generated. The Si-OH groups changed to the Si-O-Si groups at the friction interface. This process leads to the formation of Si-O-Si bond network which is the main component of the silica gel.
Next, in order to reveal the influence of surface roughness on the formation mechanism of the silica gel, we employed the Si3N4 substrates with a rough surface. During the friction simulation, the Si-O-Si bonds were generated on the surface of the convex parts, as observed in the simulation with the flat substrates. When the two convex parts collided, the chemical bonds were generated between the two substrates. Then, the convex part of the lower substrate combined with the upper substrate was scraped off. H2O molecules dissociated and adsorbed on a newly generated surface to saturate the dangling bonds. Finally, the flat Si-O-Si chain was formed on the surface. This result indicates that the collision of the convex parts leads to the formation of the flat Si-O-Si bond network.
In this work, we found that the Si-O-Si network, which is the main component of silica gel, is generated at the friction interface of Si3N4 by the chemical reaction of the Si-OH groups and the collision of the convex parts. Remarkably, the flat Si-O-Si network was formed after the scraping the convex parts.
[1] M. Chen et al., Tribol. Lett. 11 (2001) 23.
[2] K. Hayashi, M. Kubo et al., J. Phys. Chem. C, 115, 22981-22986 (2011).