Moseler, MichaelSchilling, TanjaReichenbach, ThomasThomasReichenbach2023-02-282023-02-282022urn:nbn:de:bsz:25-freidok-2321895https://publica.fraunhofer.de/handle/publica/43710410.6094/UNIFR/232189This work addresses tribologically loaded silicon- and carbon-based materials using atomistic simulations. The main focus is on nonequilibrium processes such as shear-induced structural transformations of the two solids in contact or of a solid lubricant between them. Additionally, dry, passivated tribological interfaces that undergo purely elastic deformations are studied with the goal of understanding the fundamental energy dissipation mechanisms. First, nonequilibrium phase transitions at cold-welded diamond-cubic silicon and diamond sliding interfaces are investigated. Reactive molecular dynamics simulations show the mechanically induced formation of amorphous solids with a liquidlike structure at the sliding interface. Analogous to the solid-to-liquid transitions, the crystalline-to-amorphous transitions lead to a densification in silicon and to a volume expansion in diamond. As a result, pressure enhances amorphization in silicon and suppresses it in diamond. In silicon, the formation of the amorphous shear bands between the two crystals is limited by a shear-induced recrystallization process resulting in a constant thickness of the amorphous interface layer. The interplay of amorphization and recrystallization can cause this layer to migrate in the direction normal to the interface plane as a result of different shear elastic responses of the two crystals, i.e., one crystal grows at the expense of the other. This ‘‘triboepitaxy’’ concept could form the basis for a novel mechanical nanolithography technology for the selective growth of crystalline nanostructures. Next, the lubrication of steel contacts by a solid polytetrafluoroethylene (PTFE) nanolayer is studied. Motivated by accompanying model experiments of PTFE-lubricated rolling bearings, a force field based on density-functional theory is developed for this system. It allows for large-scale molecular dynamics simulations which show that shear induces a chain alignment in PTFE leading to a pseudo-crystalline arrangement of the molecules that reduces friction. Moreover, friction is shown to be strongly affected by the local shear rates in PTFE, which in turn can be controlled by the thickness of the film. The last part of the thesis explores dry sliding without any material transformation. In this case, friction originates from elastic instabilities. However, the impact of the surface structure on friction is rather elusive. Hydrogen- and fluorine-terminated diamond and diamond-like carbon interfaces are compared to understand the relationship between the chemical surface passivation and friction. The simulations (based on a force field developed in this thesis) show that the optimization of the ratio and arrangement of the terminations for minimal friction is essentially a geometric problem to minimize the interface energy corrugation. The presented approach provides a general framework for computer-aided design of surface chemistry for minimal friction.enTribologieMolekulardynamikdoctoral thesis