Molecular dynamics study on tribological behaviors of DLC films
Date of Issue2016-12-13
Interdisciplinary Graduate School (IGS)
Diamond-like carbon (DLC) films that are a type of amorphous solids exhibit excellent mechanical properties and superior tribological performances and thus can be used as the coating of workpieces to improve their wear resistance and reduce their surface friction. The tribological behaviors of the DLC films are sensitive to many factors such as the film compositions, operational parameters and environment. These sensitivities highly complicate the tribological mechanisms of the DLC films and largely degrade their reliability and performances. Due to the difficulty to observe the evolution of the contact interfaces, particularly their detailed atomic structures at the nanoscale, investigation of the tribological mechanisms of the DLC films is challenging. Molecular dynamics (MD) simulation is a powerful technique used to investigate the nanoscale physical and chemical phenomena which can hardly be observed in experiments. This PhD dissertation adopts MD simulation as the main approach to investigate the nanoscale tribological mechanisms of DLC films under different operational conditions. The effects of the load, velocity and the surface roughness of DLC films on their tribological behaviors are studied for two-body contact cases in which a diamond tip slides against a DLC film. It is found that the increase of the load can induce transition of wear from adhesive to abrasive and highly increase wear rate of the film. Its friction force follows the macroscale Bowden-Tabor model at a small load, but diverges from such model at a large load due to the formation of transfer layers. This keeps consistent with experimental observations in literature and thus demonstrates that the macroscale tribological mechanisms are still valid at the nanoscale. The friction force and wear rate of the film decrease with the velocity due to the reduction of the sliding depth of the diamond tip and number of bonds at the contact interface. The increase of the surface roughness causes that the friction force of the film increases while its wear rate shows a nonmonotonic dependence on the roughness due to the competitions between the adhesive and abrasive wear. This nonmonotonic dependence indicates the existence of a minimum wear of the DLC films and thus shows the difference of their nanoscale tribology from that at the macroscale. The effects of third particles at the interface between DLC films on their tribological behaviors are studied by relative sliding two films with a rigid particle located between them. It is found that friction and wear of the films are determined by adhesion at a small load but dominated by both adhesion and ploughing at a large load. A high velocity can increase the friction of the film but decrease its wear, due to the response of its networks to a high strain rate indicated by such velocity. This indicates that both the surface adhesion and the mechanical response of the DLC films play significant roles in their tribological behaviors at the nanoscale. The friction and wear of the film are also highly influenced by the shape of the particle and its size which can influence its movement mode and wear mechanisms. The effects of testing atmospheres are studied by simulating the friction behaviors of DLC films with the presence of environmental H atoms. It is found that the friction mechanisms depend on the friction temperature. At low friction temperatures, H atoms are concentrated near contact interfaces, and their passivations highly reduce the interfacial adhesion and friction force. However, at high friction temperatures, the diffusions of H atoms and graphitization of the DLC film as well as its thermal expansions cause a wide region with easy-shear properties, thus resulting in a low friction force. This indicates that besides the H-passivations the H diffusions can also reduce the friction force of the DLC films by tailoring structures and properties of their sliding interface, providing a new explanation for the low friction of these films in the H2 environment. Graphene is a single layer of carbon atoms arranged into a honeycomb lattice structures. Since its discovery, graphene has found itself wide applications in nanotechnologies due to its superior mechanical properties including the ultrahigh mechanical strength and superior lubrication performances. One of these applications is to use graphene as a lubrication material to isolate the contact between surface asperities. Simulation of the lubrication of graphene for DLC films shows that its lubrication performance can be improved by the increase of its layer number but degraded by its defects. Under a small normal force, though the puckering effect has been induced, the superlubrication of the graphene can be preserved as indicated by the ultralow friction force. Under a large normal force, the friction force increases due to the tribochemical reactions of graphene and the tribopairs. The tribochemical reactions highly influence the structural stability of the graphene and thus terminate its superlubrication. The simulation also demonstrates that the size increase of the graphene can largely increase its friction force by promoting the puckering effect. This shows that the small-sized graphene may exhibit a better lubrication performance than that with a large size. This PhD dissertation has investigated the tribological behaviors of DLC films under the influence of operational conditions. The results help to understand the tribological mechanisms of the DLC films and promote their wide applications in future.