Molecular Dynamics Study Of Nanosacle Friction Of Graphene

The problem of friction, especially occuring on the booming developments of nano-robots, nanoelectromechanical systems (NEMSs), is becoming a crucial limitation owing to serious interfacial friction and wear in nanoscale. How to control and realize the low frictional state and understanding the mechanism of nanoscale friction and energy dissipation of solid-solid contact is a ubiquitous interest. Recent discoveries of structural superlubricity in layered crystalline incommensurate contact interfaces (such as graphite or carbon nanotube systems) provide an ideal solution.Adhesion between interfaces contributes to the high friction in nanoscale. Hence, we can reduce the frictional state by modulating the interfacial interaction. Since graphite/graphite interface is a typically physical interaction, the atomic structures, stacking modes and dynamic behavior are main parameters to be considered. Despite numerous experimental and theoretical studies having been performed to illustrate different factors such as velocity, temperature and normal load on nanoscale friction, some factors remain being ignored and elusive. For example, the inhomogeneity of charge distribution on graphene surface caused by adsorptions, which is beyond the normal consideration during nanotribological experiment, is largely unexplored. Moreover, the main experimental technique based on atomic force microscope can only evaluate friction with velocity range from μm/s to mm/s. Hence, the dynamic behavior such as diffusion of adsorption on layered material is another intriguing problem. Additionally, how to predict the breakdown of superlubricity and extend service time of structural superlubricity is another interesting question to address to provide some guidance for industrial applications.Computational techniques are powerful tools to address these challenging issues discussed above with the atomic scale resolution. In this thesis, we employ molecular dynamics (MD) simulation and density functional theory (DFT) to study these factors (charge distribution, commensurability and diffusion behavior) on frictional behavior of a graphene flake on graphene substrate, which is considered as a prototypical system. The main conclusions are as follows:(1) We evaluate the coupling effect of charge distribution and commensurability on frictional behavior with the help of graphene/graphene and boron nitride/boron nitride system. And it is proved that modulating charge distribution of interface can tune interfacial friction. The existence of charge distribution will obviously increase interfacial friction, even more serious compared with commensurability effect.(2) The diffusion mechanism of graphene flake is developed and we demonstrate the importance of rotational degree of freedom of adsorption on its surface diffusion. The diffusion mechanism of graphene flake is related with energy barrier of commensurate/ incommensurate transition with the introduction of thermal activation. It shows a "stick-slip" motion at low temperature, whereas diffuses with the rotation of graphene flake at high temperature. Oscillation caused by rotation of flake provides driving force to motivate the jump of atoms across energy minima at low temperature, whereas it exhibits a "Levy-flight" diffusion behavior to help transportation with freezing the rotation at high temperature. Besides, we propose the strategy to persist the superlubric state of graphene/graphene system with the existence of rotation during sliding process:higher velocity, larger graphene size, high symmetry of graphene flake and at lower temperature.(3) We also develop a statistical indicator to predict the breakdown of superlubric state in one-dimensional Frenkel-Kontorova model and graphitic system. The autocorrelation coefficient of center-of-mass velocity can be applied to avoid the catastrophic breakdown of structural superlubricity. A "phonostat" method is proposed and demonstrated that it is efficient to extend the lifetime of superlubricity with the help of critical indicator in simulations.Our findings provide insights into the understanding of the mechanism of nanoscale friction in single crystalline physical interaction interface, especially the mechanism of structural superlubricity, and may help to develop future NEMS.