Tribological behaviors of DLC films with hierarchical surface textures under water lubrication: A molecular dynamic simulation

Tribological behaviors of diamond-like carbon (DLC) films with different levels of hierarchical surface textures with lubricant water molecules are investigated through molecular dynamics simulation. The friction forces stabilize at a small value for small normal loads, due to the complete separation between DLC films by water molecules, while friction forces with large normal loads show complicated changes under the cooperation of interfacial evolution and water behaviors. Under large normal loads, friction force increases firstly due to the direct contact of surface textures which are subsequently worn and graphitized, resulting in the temporary stabilization of friction force at a large value. With their further wearing, the amount of interfacial carbon clusters decreases and water molecules distribute evenly at interface, which leads to the gradual decrease and final stabilization of friction force. During the sliding, the water molecules show a restoration in the structure and amount of hydrogen bonds, thus making these molecules play different roles in various stages, i.e., these molecules demonstrate a better diffusion during the friction rise and an enhanced separating effect for DLC films during the friction stabilization. Furthermore, the same amount of water molecules in the one-level hierarchical (L1) model has a larger bearing capacity than that in the two-level hierarchical (L2) model. When the normal load exceeds the bearing capacity of water, the friction force for model L2 is more stable and smaller than that for model L1 after running-in periods due to flattened interfaces and evenly distributed water molecules.