Thin Films Critical Friction And Superlubricity Design

In this paper,using molecular dynamics simulation,the following two aspects are explored for systems using few layers of graphene as lubricants:(1)First,we have explored the thickness-dependent friction in the few-layer graphene system.Using molecular dynamics to simulate the sliding process of silicon tips on different layers of suspended graphene,we found that with the number of graphene layers increasing,the friction force experienced by the sliding tip has a tendency to change in even-odd oscillations,which is that the friction force of the odd-numbered layer system is always greater than the frictional force of the evennumbered layer adjacent system.In order to further analyze this phenomenon,we did another three sets of control simulations by changing the loadings,the size of the tip,and the state of grapheme,whose results all show that the friction force has an evenodd oscillation with grapheme thickness.Then we analyzed the overall configuration of graphene,by expanding the existing contact theory,that is,compared to only studying the contact of the tip with the top graphene on the friction force,We have found that indirect contact in the suspended system dominates the odd-even oscillation behavior of the friction,while in the substrate system it exhibits the oscillation behavior of direct contact.Our work enriches the micro-theories of nanofriction and provides new ideas for the design of micro-nano devices and the study of solid-state lubricants.(2)Through the work of the first part,we found that the overall configuration of the layered lubricant has a great influence on the thickness-dependent friction during the sliding process.In addition,based on the previously reported that tip size can affect the thickness-dependent friction and are inspired by molecular robots.A twotip model was designed,in which two different sized tips were connected to control the surface configuration of the lubricant to further explore the microscopic mechanism of the configuration affecting the nano-friction.Through research,we found that in the substrate system,the friction force of sliding forward tip and the friction of backward sliding tip are substantially equal in one layer and two layers of graphene system,and when the number of graphene layers is increased to three and four layers,the mean friction force of the two directions can be different by two times;in the suspension system,the friction force sliding in two directions is greatly different from one layer to four layers.Through analysis,we found that in the substrate system,the folds of the large and small tips in the one-layer and two-layer systems are independent,while the folds in the three-and four-layer systems are connected together,and the root cause of the difference in sliding friction is caused by the connection area of the folds;in the suspended system,from one layer to four layers,there are obvious groove marks on the large tip side,and the front of the small tip is flat due to surface strain.The presence of the grooves causes a large difference in the sliding friction between the two directions.This part of our work further deepens the perception of nano-friction and provides new ideas for the design of nextgeneration nano-devices.