A First-principles Investigation Of The Friction Properties Of H-BN/Gr Heterogeneous Interface

Friction is ubiquitous in our lives.According to statistics,more than one-third of the world’s disposable energy is lost due to friction,and about eighty percent of mechanical parts and components fail due to wear and tear.Therefore,exploring methods to reduce friction in order to minimize energy waste and extend the service life of mechanical components is an urgent issue under the concept of sustainable development.Materials play a very important role in promoting the development of science and technology.The weak interlayer interactions of two-dimensional materials have led to excellent lubrication performance,which has aroused intense research interest.Usually,both experimental and computer simulation methods are used to study the friction properties of materials.As people’s exploration of friction gradually deepens to nanoscale or even smaller scales,the first-principles methods of understanding friction from the perspective of electronic interaction are increasingly being valued.There are two main models,namely the constant interface distance model(CDM)and the constant load model(CLM),which are used for the first-principles calculation of friction performance.The results obtained using the CLM can be compared to the friction experimental data,but require an extremely complex data processing process.The simplified CDM is commonly used to calculate friction,but the rationality of this simplification has not been proven.In addition,heterojunctions,as a special presence in two-dimensional materials,exhibit excellent lubrication performance during sliding due to their naturally formed lattice mismatch.However,there are still limited researches on the relevant mechanisms of heterojunction friction.In view of this,this thesis uses the first-principles method based on density functional theory to establish the transformation bridge of friction properties calculated by the CDM and CLM,and summarizes the law of friction properties of typical two-dimensional materials changing with load,and explores the origin of friction from the perspective of electronic interactions.The main findings of the study contain the following three aspects:1.Equivalence verification of the friction properties calculated under the constant load model and constant interface distance model.Based on the first-principles of density functional theory,this thesis focuses on four typical two-dimensional materials:graphene/graphene,hexagonal boron nitride/hexagonal boron nitride,molybdenum disulfide/molybdenum disulfide,and molybdenum disulfide/molybdenum selenide,and calculates the potential energy surfaces(PES)and friction properties of these interface systems using the CLM and CDM respectively.By comparing the PES calculated under the two models,this thesis verifies the equivalence between the friction properties obtained under the two models.In addition,by the strategy of defining the load under the constant interface distance model as the average load along the sliding path,a transformation bridge is established between the friction properties obtained under the constant load model and the constant interface distance model.Moreover,this thesis further verifies the universality of this conversion method in the graphene/hexagonal boron nitride heterojunction interface system.2.Study of the friction properties and mechanism analysis of Gr/h-BN heterogeneous interface.Based on the first-principles of density functional theory,this thesis takes the heterogeneous interface Gr/h-BN which is composed of a conductive graphene(Gr)monolayer and an insulating hexagonal boron nitride(h-BN)monolayer as the research object.The PESs are calculated to characterize the friction properties of the heterogeneous interface system as well as the electronic structure of the heterogeneous interface system such as charge density distribution and charge redistribution.The results of the study showed that: similar to the Gr/Gr and h-BN/h-BN interfaces,the Gr/h-BN heterogeneous interface exhibits pressure-induced friction collapse under high load conditions,but the critical pressure for the occurrence of superlubricity is different.The Gr/Gr interface corresponds to 261 GPa,while the Gr/h-BN heterogeneous interface corresponds to 305 GPa;The interface friction properties are determined by the redistribution of interface charge density.The more charge redistribution,the greater the friction force;The negative work done by the load during sliding at the Gr/h-BN heterogeneous interface leads to a negative friction coefficient.3.Effect of layer number on the friction properties of Gr/h-BN heterogeneous interfaces.Based on the first-principles of density functional theory,this thesis calculates the PESs of six types of Gr/h-BN heterostructures constructed by using Gr or h-BN monolayers as the substrate and increasing the number of substrate layers from one to three.The results indicated that: for the systems with Gr as the substrate,the potential energy difference of the optimal slip path at the critical interlayer distance is 0.0045 e V for the system with one layer of Gr as the substrate,which indicates a pressure-induced friction collapse phenomenon.As the number of added Gr substrate layers increases from one to three,the interlayer distance values corresponding to the pressure-induced low friction phenomenon show a continuous increase from 1.875 (?) to 2.016 (?) and then to 2.075 (?);for the systems with h-BN substrate,as the number of h-BN substrate layers increases from one to three,the interlayer distance values corresponding to the pressure-induced low friction phenomenon show a transition from1.505 (?) to 1.603 (?) and then to 1.545 (?).