First principles multiscale modeling of physico-chemical aspects of tribology

We have been developing methods to obtain a fundamental understanding of the physico-chemistry of sliding systems in contact that underlies the materials science and mechanics issues essential to tribology. Advances in theory and methods are now making it practical to consider fully first principles (de novo) predictions of surface and interface structures and their role in determining tribological properties of materials. Despite the progress, there remains an enormous gap between the distances and time scales of quantum mechanics (QM) simulations and the quantitative macroscopic continuum models essential in engineering design of tribosystems. In order to bridge this gap, we use a hierarchy of simulations, beginning with QM, continuing through Molecular Dynamics (MD), then to mesoscale dynamics, to continuum mechanics, and finally to engineering design. A recent advance here is the development of first principles Force Fields (FF) based on QM that allow MD to describe bond breaking, plasticity, and phase transitions. Such FF are being used to extract mesoscale parameters for describing the dislocations for metals and their role in plasticity. Based on these FF, we used steady state nonequilibrium MD (NEMD) to study Crack Initiation and Spallation failure in such systems. We illustrate the atomistic approach to tribology of metal surfaces by determining the shear and friction for Ni(001)/Ni(001) as a function of misorientation. These results suggest that some degree of plasticity occurs even for careful experiments on clean samples. We illustrate the use of NEMD to study the rheology of confined lubricants, considering a system of two oxidized iron surfaces, covered with DTP wear inhibitors, and lubricated with hexadecane. These results illustrate the dramatic effects of nanoconfinement. We illustrate the use of bond dissociation consistent FF, by examining the use of nanotubes as nanotribological probes.