[SY-O7] Symposium O-7

[SY-O7] Molecular Dynamics Simulation Study on the Structure, Role, and Formation Mechanism of Tribofilms of Silicon-Based Materials in Water

^○Yusuke Ootani¹, Jingxiang Xu¹, Naoki Takahashi¹, Koshi Adachi², Momoji Kubo¹ (1.Institute for Materials Research, Tohoku University, Japan, 2.Department of Mechanical Systems Engineering, Graduate School of Engineering, Tohoku University, Japan)

It is known that silicon-based materials such as silicon carbide have low friction coefficients in water lubrication system. A tribofilm formed at a sliding interface is considered to reduce the friction coefficient, however detailed mechanism is still unclear because in-situ observation of the atomic scale sliding interface is difficult. Therefore, in this work, we investigated the structure, role and formation mechanism of the tribofilm of silicon-based materials by using molecular dynamics method. We performed sliding simulations of a SiO₂ substrate, which is a model of a native oxide layer of silicon-based materials, in water environment. We found that hydrolysis reaction of Si-O bond in SiO₂ surfaces occurred at contact areas of the surfaces as, Si-O + H₂O → Si-OH + OH. The hydrolysis reaction was mediated by proton transfer process. Since the hydrolysis reaction dissociated Si-O bonds, Si-O bond network of the SiO₂ surfaces became sparse, and then, several SiO₂ clusters were removed from the surfaces as wear debris. The wear debris were dissolved in the water layer between two surfaces, forming a colloidal silica layer. On the other hand, the water molecules penetrated into the sparse SiO₂ surfaces and hydrate the surfaces, forming a hydrophilic silica gel layers on the SiO₂ surfaces. Therefore, at the sliding interface, the colloidal silica layer was sandwiched by two silica gel layers. The colloidal silica layer prevents the contact of the surfaces and reduces a friction force, whereas the hydrophilic silica gel layer holds the colloidal silica layer at the sliding interface. Thus, the colloidal silica and silica gel layer that formed by hydrolysis reaction reduces the friction coefficient of silicon-based materials in water lubrication systems.