Research On The Mechanism Of Friction And Energy Dissipation On Graphene

The phenomenon of friction exists in all aspects of life,and the research on it can be traced back to the Leonardo da Vinci period.Although the Amontons-Coulomb friction law,which is widely recognized by the public,had been proposed successively,the mechanism behind the friction has not been fundamentally resolved.This also makes it impossible for people to make further breakthroughs in the understanding and application of this phenomenon.With the advent of the nano era,the emergence of micro-nano devices has promoted the rapid development of various research and application fields.How to design and manufacture micro-nano devices reasonably have become an objective criterion for measuring the level of science and technology of a country.Micro-nano devices have extremely high requirements for accuracy and stability,and their high surface-to-volume characteristics make the impact of friction phenomena particularly prominent.Obviously,the empirical theory obtained at the macro level cannot meet the current needs,which requires researchers to give a new explanation.Therefore,the research on friction mechanism has become a hot topic at the moment.Based on molecular dynamics(MD),lattice dynamics,heat transfer and other theories,combined with experimental instruments such as optical microscopes and atomic force microscopes,this paper uses graphene as the main object to study the friction behavior at the nano-scale,revealing that it is different from the macro-scale friction mechanism.Novel findings are summarized as follows:1.A molecular dynamics model of a graphene flake sliding on a double-layer graphene substrate is established to show the non-monotonic of friction variations on sliding velocity,which contradicts with the macroscopic theory predictions such as the Amontons-Coulomb friction law and the Newton law of viscosity.The friction variation trend is divided into regions:the rising phase,the steady phase and the velocity decay phase with the sliding velocities.A modified Prandtl–Tomlinson model was proposed to explain the variation of the instantaneous friction force and instantaneous displacements.The modified model is also extended to a two freedom degree systems in order to discusses the effects of mass,damping and stiffness on friction force.2.The change of the friction force of the graphene flake under the action of the normal vibration excitation is studied.The study shows that the normal vibration excitation can reduce the friction force depending on the frequency and amplitude of the excitation.When the frequency is within a certain range,the friction will decrease with larger amplitude.Once the frequency falls outside a certain range,the excitation will not cause changes in friction.In addition to studying the normal excitation,we established a model of the hemispherical silicon tip sliding on the silicon substrate,considering the influence of the lateral excitation(perpendicular to the sliding direction)on the frictional force.The results show that dynamic lubrication can still be achieved by lateral excitation.By observing the change of potential energy surface under lateral excitation,the study in this paper listed the reasons why lateral excitation can reduce friction,and the influence of velocity on this reduced friction effect.3.A one-dimensional single-atom chain model has been established and the size effect of friction dissipation been studied.By exciting the end atoms to vibrate at different frequencies,it is found that not all the vibration energy can be absorbed by the system.For small-scale systems,there is a minimum cut-off frequency that can absorb energy.If the frequency of the vibration is less than the cut-off frequency,energy dissipation will not arise.This is because vibrations at the frequencies less than the cut-off frequency cannot be scattered into vibrations of other frequencies,which becomes standing waves afterwards.At the same time,the cut-off frequency shifts to high frequency as the system size increases;however,when the excitation frequency is too large and exceeds the intrinsic power spectrum of the system,the system still cannot receive the energy generated by vibration.Only when the vibration in the intrinsic power spectrum is excited and the frequency of the vibration is close to the spectrum line,can the energy be effectively absorbed.4.Based on the influence of vibration at different frequencies on friction dissipation,we first demonstrated the change of the phonon mode due to the flake sliding at the contact interface,and found that as the sliding velocity increased,the phonon mode would shift to higher frequencies.The phonon wavepacket MD simulations are used to study the transmission of the in-plane acoustic mode(longitudinal wave LA and transverse wave TA)and the normal bending acoustic ZA mode at the interface,respectively,to observe the relationship between the change in friction and transmissivity.In addition to studying the phonon transmission,we also studied the changes in phonon lifetime and frequency by decomposing phonon modes.It is found that the lifetime of some phonon modes existed in the first Brillouin zone will decrease with the increase of velocity.The phonon lifetime is affected by both velocity and temperature.The velocity influence part is dominant at low velocity,and the temperature influence part is dominant at high velocity.The change trend of friction with velocity can be explained from the perspective of the change of phonon properties.5.By establishing a molecular dynamics friction model and observing the density of states of phonons directly excited by friction at the substrate contact interface,we found that the directly excited non-equilibrium phonons not only appear at the washboard frequency,but also appear at its harmonics.When the frequency of the excited phonon vibration is consistent with the resonance frequency of the tip,it will cause a resonance effect,thereby opening the dissipation channel and significantly increasing the friction dissipation.At the same time,by analyzing the fast Fourier transform diagram of the instantaneous friction force,we can see that the role of high-frequency components gradually fades as the velocity increases.This finding has been demonstrated experimentally for the first time.Obvious friction peaks can be observed at the predicted velocities when we use atomic force microscopy to test the tip sliding on graphite and molybdenum disulfide substrates.This phenomenon is very stable and will not be affected by temperature and humidity.