New Reduced Models For Structural Superlubricity

Friction is the main reason for energy dissipation,wear and aging of devices in the industrial field.It has become one of the major technical challenges faced by many fields and limited their further development.Since the concept “Superlubricity” was proposed by Hirano in 1990 s,much work has been done and the structural superlubricity in the layered materials are considered to be one of the most ideal techniques for solving the above challenges,especially when structural superlubricity was realized in micrometer graphite flakes and centimeters-long carbon nanotubes in air conditions.The breakthroughs of large scale superlubricity in experiments bring more challenges for the theoretical research of superlubricity.The present theoretical methods studying superlubricity are mainly divided by two categories: molecular dynamics(MD)and reduced models,such as Prandtl-Tomlinson(PT)model and Frenkel-Kontorova(FK)model.Recent studies show that the out-of-plane deformation of the 2D crystal materials and the complex deformation of the edge chemical groups may have important influences on the behavior of superlubricity in layered materials.How to study these factors theoretically is very improtant for the further developmement of structural superlubricity,especially for large scale structural superlubricity.However,the present theoretical methods are not qualified to study the effect of elastic deformation on large scale superlubricity: the MD method can describe complex deformation of materials but can only simulate quite small system due to the large amount of calculation,on the other hand,although the PT model and FK model can simulate large systems,they can only describe stretching and compressing deformation.Based on this background,this dissertation developed two simple but quite effective theoretical models which can consider the effect of out-of-plane deformation of 2D crystal materials and complex deformaiton of edge chemical groups,respectively.We apply the two models to study large scale superlubricity.The main achievements of this dissertation are as follows:Firstly,we established a 2D reduced model which can mimic the out-of-plane deformation of 2D atomically thin sheets and deduced an analytic potential describing the Van der Waals interaction between two crystal surfaces.We proved the effectiveness and validity of the 2D reduced model and the analytic potential through theoreticalanalysis(self-consistent screening approximation,SCSA)and MD simulations.The amount of calculation of the 2D reduced model and analytic potential is quite small.Thus,they can simulate rather complex systems with much larger size than MD,which lays a solid foundation for the futher theorectical study of large scale superlubricity.Then,based on the 2D reduced model and the analytic potential,we developed a theoretical model to study the frictional properties of nanojunctions including atomically thin sheets.The results show that the frictional properties are determined by the interplay between the lattice mismatch of the contacting surfaces and out-of-plane displacements of the sheet.We designed a heterostructure to reduce the out-of-plane deformation of the 2D sheet to achieve superlow friction based on the interplay of the two factors.Our results provide new insights and avenues for realizing large scale superlubricity.Finally,we proposed a generalized Prandtl-Tomlinson model which including the inner degrees of freedom of the edge chemical groups.Based on this model,we found that the structure,the deformation of the edge chemical group and its relative orientation respect to the surface play very important roles on the frictional behavior of the system.The results of generalized PT model agree well with the experimental results for single molecule.According to these properties,we suggest an avenue for controlling the friction behavior at nanoscale.