The Anti‐Friction Mechanism of Si‐ <scp>DLC</scp> Films by Water Molecules: A <scp>ReaxFF</scp> Molecular Dynamics Study

ABSTRACT Identifying appropriate application conditions for various doped‐DLC films is essential for maximizing their performance and extending service life, which also highlights the need to understand how environmental factors influence their tribological behaviour. Given that the tribological properties of Si‐DLC films are strongly affected by the presence of water molecules, this study employs ReaxFF molecular dynamics simulations to explore how water modifies their frictional behaviour and to uncover the underlying mechanisms. This study demonstrates that when only a limited number of water molecules are present, two separate mechanisms are responsible for reducing friction in Si‐DLC films. Under high‐friction conditions, water molecules undergo decomposition and the released H and O atoms interact with the Si‐DLC surface, passivating it and consequently lowering friction. Meanwhile, the migration of Si atoms to the contact surface results in the generation of a silicon‐rich tribolayer with low shear strength, further reducing resistance. By contrast, in water‐rich systems, Si‐OH groups generated on the contact surface promote water adsorption and lead to the generation of a continuous hydration layer, which markedly enhances lubrication and reduces friction. These results confirm that the improvement of tribological properties in Si‐DLC films by water molecules is closely linked to tribochemical reactions, offering new insights into the behaviour of Si‐DLC in humid environments.