Molecular modeling as enabler for exploring of tribological systems

摘要

There is no doubt that tribological interactions have a profound impact on design more efficient machine parts and results in the better maintenance of those elements. This project is aimed toward a general understanding what cause underlies this behavior at the molecular level. The model tested in this work does not necessarily capture the friction mechanism for all boundary lubrication systems, but the framework can be a good flexible approach to test different mechanisms. The model tested shows good agreement with experiments, indicating that, once calibrated, it has predictive capabilities that provide a beneficial tool for further exploration of boundary-lubricated systems.The investigated friction modifiers were found to form self-assembled monolayers (SAMs) on either of the considered surfaces (steel/hematite, silica, cellulose,epoxy and aramid). Key differences are packing density and the tilting of the alkyl chains. This is most pronounced for C18 (no unsaturation) which exhibits the largest tilt angle and C18' (two double bonds) which shows almost untitled SAMs. Upon shearing, C18 SAMs show elastic deformation which lowers friction forces until flipping of the tilt angles. Moreover, molecules may be ripped off the surface during asperity contacts. Both phenomena depend on the velocity of shearing. The coarse-grained model of the C18-modulated asperity contact of rough steel-silica/aramid/cellulose surfaces demonstrates the desired positive friction behavior.With atomistic simulations, a molecular scale mechanistic picture of the behavior of different SAM types upon direct contact could be established. Using such models, friction coefficients are still qualitative in nature. However, the overall response to shear could be investigated and a mechanistic insight of the involved processes were elaborated for the different SAM types. The study of tribofilm removal and its chemical and physical effects on friction of the boundary-lubricated contacts is still the subject of the ongoing research.