| Friction exists in almost all mechanical equipment.The wear of machine parts caused by friction is the main form of mechanical equipment failure.Globally,it has been estimated that nearly 20%of the energy consumptions are lost due to friction and related phenomena,and about 80%of the mechanical failures are caused by friction and abrasion.However,the mechanism of spatiotemporal response dynamics caused by friction process is still not fully understood to date.Therefore,for the development of industrial economy,it has become an urgent problem to reveal the internal mechanism of friction and controlling the friction energy consumption.In this thesis,based on molecular dynamics（MD）,lattice dynamics,and quantum mechanics theory,combined with experimental instruments such as atomic force microscope and Raman spectrometer,the friction behaviors on contact interfaces between two-dimensional materials and between semiconductors are explored.The two-dimensional materials refer to graphene,Mo S2,and WS2;while the semiconductors include silicon,germanium,and diamond.The details are as follows:A system for investigating the contributions of elastic deformation energy and thermal activation effects to friction has been constructed.In this system,a graphene flake slides on a suspended graphene layer anchored on a bed of springs.The graphene–spring system provides a useful ideal approach to model different layers of graphene through changing the stiffness of the springs.The results first indicate that both the friction force and the elastic deformation energy have an exponential dependence on the support stiffness.Second,the observed non-monotonic variation in friction manifested by peaks and plateaus with increasing temperature results from the changing rate of energy dissipation due to the transition of slip regimes.Therefore,the friction force emanates from the competition between the interfacial energy barrier and out-of-plane elastic deformation energy,as well as the competition between the thermal activation effects and transition of slip regimes.The observation can extend the validity of the Prandtl–Tomlinson model on friction phenomena.Based on the mechanism of energy dissipation of nanofriction,a model system with a graphene flake sliding on a monolayer graphene supported by gradient stiffness is constructed.This system is to analyze the contributions of different regions of the graphene flake to friction force,with the substrate characterized by different stiffness gradients.The results indicate,first,that the soft region of contacts always contributes to the driving force,whereas the hard region leads to the biggest friction force in all column atoms of the flake.Moreover,as the increase of the support stiffness with the stiffness gradient and the midpoint stiffness,the contribution ratio of hard region to the total friction is greater than those of each column atoms in soft regions.Second,the energy barrier decreases with the increasing support stiffness along the stiffness gradient direction of the substrate,which induces a reduction of the resistance forces to the relative motion.Meanwhile,the amplitude of the thermal atomic fluctuation is higher in the softer region while lower in the harder one.Given these points,results reported here suggest that the friction force in each contact region is caused by the coupling of the energy barrier and the elastic deformation between the graphene surfaces.The former contribution,the energy barrier,includes the interfacial potential barrier in commensurate state which is against the sliding of the surfaces with respect to each other,and the potential gradient caused by the different vibration magnitudes of the substrate atoms against the different spring stiffness in the direction of stiffness gradient.The latter contribution,the elastic deformation,is the unbalanced edge energy barrier resulting from the asymmetrical deformation and the different degrees of freedom between the edge atoms of the slider and atoms inside and outside the contact area of the substrate.The interfacial atomic forces of two contacting graphene flakes are calculated to quantitatively illustrate the friction evolution from commensurate to incommensurate contacts.It is found that the atomic force distributions display moirépatterns.The moirépatterns for the contact stress indicate the contact quality,while the patterns for the shear stress are related to the friction force.In both the commensurate and incommensurate contacts,part of the interface atoms experiences positive friction force while the other atoms negative friction force.However,the interfacial friction distributions for the incommensurate state clearly demonstrate that the positive and negative atomic friction forces distribute more symmetry than that in the commensurate state,which produces an ultra-low effective friction force.Frictional phonon dissipation in mono/bilayer graphene is modeled based on the phonon spectrum by means of molecular dynamics simulations.The results demonstrate that the blue shift of the density of states can be attributed to various factors,such as thicker layers,faster sliding velocities,and higher normal loads;these factors also lead to an increase in friction.Combined with the polarized density of state method,it is revealed that the frequencies of the flexural acoustic（ZA）modes shift to higher levels as the thickness increases,resulting in enhanced friction;the higher friction in the faster sliding velocities is caused by an increase in the in-plane acoustic（LA and TA）modes;the relative sliding under larger normal loads could increase LA,TA,and ZA modes,leading to increased friction.It is further indicated that both higher normal loads and thicker layers will create greater deformation of the graphene lattice,which will,in turn,lead to an increased thermal conductivity and,ultimately,a higher friction dissipation efficiency.The increased thermal conductivity is the reason that higher friction leads to lower contact temperatures.To examine phonon transport during the friction process of commensurate–incommensurate transition,the vibrational density of states of contact surfaces is calculated.The results indicate that,compared with the static state,the relative sliding of the contact surfaces causes a blue shift in the interfacial phonon spectrum in or close to commensurate contact,whereas the contrast of the phonon spectrum in incommensurate contact is almost indiscernible.Further findings suggest that the cause of friction in commensurate contact can be attributed to the excitation of new LA and TA modes,which provide the most efficient energy dissipation channels in the friction process.However,when the contact interface is incommensurate,the new excited phonon modes are very few.In addition,when the tip and the substrate are subjected to a same biaxial compressive/tensile strain,fewer new acoustic modes are excited than in the no strain case.Thus,the friction can be controlled by applying in-plane strain even in commensurate contact.Finally,the contribution of the excited acoustic modes to friction at various frequency bands is also calculated,which provides theoretical guidance for controlling friction by adjusting excitation phonon modes.A method to control the matching degree of phonon spectra at the interface through modifying the atomic mass of contact materials is proposed,thereby regulating the interfacial friction force and thermal conductance.Results of Debye theory and MD simulations show that the cutoff frequency of phonon spectrum decreases with increasing atomic mass.In these regards,the coupling strength of phonon modes on contact surfaces makes it possible to gain insight into the nonmonotonic variation of friction force and thermal conductance.It is suggested that when the atomic masses of the contact surfaces are close,the overlap of phonon modes increases energy transport channels and therefore phonon transmission at the interface,and finally,an enhanced energy dissipation in friction and heat transfer ability at interface.Experimental samples of Mo S2-WS2 heterostructures on silica substrate are synthesized by chemical vapor deposition（CVD）,the relationship between friction and wear characteristics of the samples with normal load,sliding velocity,and loading mode is studied.The results show that the friction coefficients on both tip/Mo S2 and tip/WS2 interfaces are lower than that of tip/silica interface,and the friction coefficient of tip/WS2 interface is the lowest.It is further found that the adhesive forces exist between the tip and the surfaces of Mo S2,WS2,and substrate,but the adhesion strength of these interfaces does not change obviously with the increase of normal load.Moreover,the friction on the surfaces of the Mo S2 and WS2 basically increases exponentially with the increase of sliding velocity.However,several friction peaks and plateaus appeared on the friction curve in the whole velocity range,which indicates that the exponential relationship between friction and velocity is not strict.At last,when the normal load exceeds the critical value,the surface of Mo S2 has undergone considerable wear and tear,causing it to peel off from the substrate.In conclusion,the research of this thesis is intended to provide a new technical route for effective control of friction energy consumption. |
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