Three Tribolayers Self-Generated from SiC Individually Work for Reducing Friction in Different Contact Pressures

The low friction of silicon carbide (SiC)/water systems is understood to be the result of a self-forming silica-based tribolayer that is produced by tribochemical reactions. Although several experimental studies have revealed that this silica-based tribolayer contains a considerable amount of carbon, the detailed structure of the tribolayer and its role in providing low friction remain unclear. Here, we conducted a reactive molecular dynamics sliding simulation of an amorphous SiC (a-SiC)/water system to elucidate the atomic-scale structure of the self-forming tribolayer and the mechanism underlying its formation. We found that the water selectively oxidized Si atoms at the surface of the a-SiC, resulting in their removal as SiO₂ wear particles. Some of these wear particles dissolved in the water, resulting in the formation of colloidal silica, whereas others were deposited on the a-SiC surface, where they formed a layer of silica hydrate. Meanwhile, carbon atoms remained at the a-SiC surface and formed a C-rich layer, which corresponds to the initial process of an amorphous carbon layer formation. Based on our findings, we propose that colloidal silica, silica hydrate, and amorphous carbon act as individual tribolayers to reduce friction in the low-, intermediate-, and high-contact-pressure areas, respectively. In the low-contact-pressure area, water mainly separates the surfaces, and the silica hydrate holds the water at the sliding interface because of its high hydrophilicity, increasing the load-carrying capacity of the water. In the intermediate-contact-pressure area, the water cannot separate the surfaces, but the colloidal silica layer can because of its high viscosity. In the high-contact-pressure area, where the water and silica-based layers are broken, the amorphous carbon layer works as a solid lubricant. Thus, the three self-forming tribolayers together produce the low friction that characterizes SiC/water systems.