| Reduction of friction is one of the most important and significant problems in physics,which may play an instrumental role in diverse systems that covers from macroscopic equipment,through the nanometer contacts in nanomachines to biological molecular motors.In macroscopic scale,friction force(f)was found to increase with increasing normal load(N)according to Da Vinci-Amontons law,f=μN,where the friction coefficient μ is a positive value(μ>0).In modern tribology,structural superlubricity has emerged as a new promising remedy to reduce friction and wear,which generally originates from the effective cancelation of lateral forces due to the lattice mismatch,thus achieving a very low friction state.That is,the realization of superlubricity depends on the incommensurate contact conditions of the structure.Here,even under the superlubricity state,the friction coefficient μ is still a very small positive value.However,in this thesis,using the first-principles calculation,negative friction coefficient(μ<0),where sliding barrier decreases upon increasing normal load,is predicted for two-dimensional(2D)ferroelectric In2Se3 homogeneous commensurate contact,which simultaneously breaking both the prevailing incommensurate contact condition invoked to result in superlubricity and the classic Da Vinci-Amontons law.The physical origin of this counterintuitive phenomenon lies in that the increase of the sliding energy barrier contributed by vander Waals(vdW)interactions can be thoroughly compensated by the enhanced electrostatic energy reductions which is a spontaneous dipole-alignment-dependent response towards the interfacial charge transfer under load.Furthermore,the sliding friction coefficient can be further tuned from negative to positive,through quasi-zero,by applying an appropriate external electric field or controlling the specific alignments of both the out-of-plane and in-plane electric polarization directions of the individual In2Se3 component layers.The present findings are expected to play an instrumental role in the future design of novel nanostructures with desirable functionalities for potential applications in tribology,nanomachines,nanosensors,nanocatalysis,and many other related physical and chemical phenomena.The specific chapter content of this thesis is arranged as follows:Chapter 1,Introduction.The development process of friction,including liquid lubricant,solid lubricant and their defects,is briefly described,and the concept of structural superlubricity is introduced.Next,the development of friction and related experimental progress in recent years are introduced from the perspectives of nanometer,micron and centimeter sizes.At the same time,the bottleneck problem encountered in superlubricity and the research motivation of this thesis are proposed.Chapter 2,Theory and methods.We briefly introduced the theoretical basis of first-principles calculation,the simulation software used in the thesis,and the parameters set in our calculationChapter 3,The properties of 2D ferroelectric material In2Se3,that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations,are introduced.According to the arrangement of out-of-plane polarization directions,the three most stable sliding models are constructed.Then the appropriate sliding pathway,the fixing method of the atoms during sliding,and the calculation method related to friction are chosen for the next calculation.Chapter 4,we studied the friction coefficient of the three most stable sliding models with the normal load(N).We found that the three models exhibit different friction behavior under normal load,and the friction coefficients are μ<0,μ≈0,andμ>0,respectively.Our further research shows that the above results are achieved by a significant reduction in electrostatic energy to compensate for the increase in sliding energy barrier caused by vdW interaction.In addition,we explain the deeper mechanisms that affect the above three friction properties from the perspective of charge transfer.The results show that the three different frictional responses under load are due to the process of redistribution of spontaneous polarized charges.Chapter 5,based on the above friction behavior,we further studied the effect of in-plane polarization on friction.First,while maintaining the same direction as the previous out-of-plane polarization,we changed the in-plane polarization directions of the three configurations.We found that with the change of the in-plane polarization direction,the response of the friction force to the load also showed different trends.In addition,we also found that by applying an appropriate external electric field,the frictional force showed a decreasing or constant trend in different configurations.These interesting findings provide new and feasible solutions for the regulation of nanoscale friction coefficients.Chapter 6,The summary and prospect of the thesis.Different from the previous incommensurate contact method,this study uses homogenous commensurate contact of In2Se3 and achieves a negative coefficient of friction by selecting a suitable dipole-dipole arrangement.It is also important that the friction coefficient μ can be adjusted by further changing the in-plane polarization direction and applying an appropriate external electric field.This discovery is expected to play an important role in the future design of new nanostructures and nanomachines and has potential application value in various nanodevices,nanosensors and related fields. |
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