A research method for controlling friction by coupling strain and velocity

Abstract

The present invention relates to a research method for regulating friction by coupling strain and speed, comprising the following steps: (1) establishing a molecular dynamics friction model, and realizing biaxial strain applied to a substrate by directly changing the coordinates of MoS2 atoms; (2) performing simulation and processing data: (1) calculating the friction force under different strains and temperatures, and obtaining the relationship between the change curve of friction force and strain; (2) calculating the potential energy surface, state density and friction interface thermal resistance under different strain and speed coupling, and determining the contribution of state density to friction force; (3) comparing the potential barrier height and friction interface temperature amplitude under different strains and temperatures, and determining the dependence of friction force on strain and temperature; (4) analyzing the friction force and friction interface thermal resistance under different strain and speed coupling, and determining the influence of strain and speed on the friction force of MoS2 / MoS2 interface. The present invention can provide a new perspective and theoretical guidance for the field of nanomaterial tribology by studying the friction energy dissipation between MoS2 / MoS2 interfaces based on the coupling of biaxial strain and sliding speed.

Description

Technical Field

[0001] The invention relates to the technical field of molecular dynamics, and in particular to a research method for regulating friction through coupling of strain and speed. Background Art

[0002] Friction is a ubiquitous form of energy dissipation that converts kinetic energy into heat energy and exists in many mechanical systems. According to statistics, more than one-third of the world's energy is consumed through various forms of friction, which has a huge negative impact on global economic benefits. With the development of high-precision mechanical systems, nanomaterials are considered to have extremely outstanding development prospects due to their excellent performance. Today, the gradual rise of two-dimensional materials (2D) belonging to the category of nanomaterials has led researchers to explore methods and solid lubricants for reducing friction and wear. Studies have found that the friction behavior of 2D materials depends not only on the normal load, sliding velocity and substrate stiffness, but also on the interface commensurability and substrate thickness. Recent 2D material research has reduced friction to superlubrication levels.

[0003] Molybdenum disulfide (MoS2) is a semiconductor with a honeycomb structure and is also recognized as one of the smoothest solid lubricants. It has made great progress in energy conversion and storage. Due to its weak interlayer van der Waals potential, MoS2 has excellent tribological properties and lubrication properties. Kalyan et al. combined graphene and MoS2 as solid lubricants and found that friction in steel hydrogenated DLC (diamond-like carbon) contacts can be reduced even at high contact pressures and high sliding speeds. In addition, Quereda et al. studied the surface roughness and lateral friction resistance of single-layer MoS2 on different substrates by atomic force microscopy (AFM) and found that the surface roughness strongly depends on the type of substrate material, and the lateral friction decreases with the decrease in roughness. Shi et al. revealed the effect of the interlayer distance of MoS2 on the sliding friction. The results showed that as the distance between MoS2 flakes decreases, the increase in interlayer Coulomb repulsion will cause the Mo-S bonds to deform, thereby reducing interlayer friction sliding. Mohammad et al. demonstrated the atomic friction angle relationship when the nanoprobe slides on monolayer MoS2 and bulk MoS2, and found that the contact quality formed at the friction interface will affect the interface energy morphology, and thus affect the friction energy dissipation.

[0004] But nanoscale friction properties can also be regulated by applying strain. Kitt et al. studied the friction process of multilayer graphene sliding on SiO2 samples and found that graphene can be smoothed by applying in-plane strain, thereby significantly reducing friction. Yang et al. studied the friction coefficient of strained graphene at different scratch depths and found that the change in friction coefficient caused by strain was attributed to the change in contact area at the atomic scale. Xin et al. studied the effect of strain on the friction between graphene layers. Therefore, applying strain to the substrate will reduce the commensurability of the substrate surface, resulting in a transition from commensurate to incommensurate contact. Wang et al. used density functional theory to study the atomic-scale friction behavior of MoS2 and found that the in-plane compressive strain system has lower friction dissipation than the tensile strain. Ding et al. revealed the effect of strain on the thermal conductivity of single-layer MoS2 and found that the thermal conductivity can be reduced by more than 60% under a tensile strain of 12%.

[0005] The above studies show that the applied strain has a decisive influence on the friction between MoS2 layers, but it does not provide the internal mechanism and method of regulating friction through the coupling of strain and velocity. At the same time, the internal mechanism of the effect of temperature on friction in different strain ranges also needs further exploration and discussion. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a research method for regulating friction through the coupling of strain and speed.

[0007] In order to solve the above problems, the present invention provides a research method for controlling friction by coupling strain and velocity, comprising the following steps:

[0008] (1) A molecular dynamics friction model consisting of a probe and a substrate of molybdenum disulfide / molybdenum disulfide (MoS2 / MoS2) was established, and the biaxial strain applied to the substrate during the simulation was achieved by directly changing the coordinates of the MoS2 atoms;

[0009] The probe comprises a free motion layer I; each atom in the molybdenum disulfide probe is provided with a spring in the x, y and z directions; wherein: the spring in the x direction pulls the probe atom to perform friction sliding at a constant speed of 10 m / s, and the springs in the y and z directions are used to eliminate the translation and rotation of the probe in the y direction, as well as the movement in the z direction during sliding; a periodic boundary condition is set in the x direction, and a free boundary condition is set in the y and z directions;

[0010] The substrate comprises a bottom fixed layer, a side fixed layer, a side temperature regulating layer and a free movement layer II;

[0011] A friction interface is formed between the free movement layer I and the free movement layer II;

[0012] (2) Execute simulation and process data:

[0013] ① Taking the strain range of 0-0.2 and the speed of 10m / s, the instantaneous friction force and average friction force of the MoS2 / MoS2 interface at temperatures of 0K and 300K were calculated respectively, and then the relationship between the change curve of friction force and strain was obtained;

[0014] ② Take the strain as 0, 0.002 and 0.1, and the speed as 10m / s, calculate the potential energy surface, state density and friction interface thermal resistance under different strain and speed coupling to determine the contribution of state density to friction under different strains;

[0015] ③ Compare the potential barrier height and friction interface temperature amplitude under different strains and temperatures to determine the dependence of friction force on strain and temperature;

[0016] ④ Analyze the friction force and friction interface thermal resistance under different strain and speed coupling, and determine the influence of strain and speed on the friction force of the MoS2 / MoS2 interface.

[0017] The lateral temperature adjustment layer in step (1) refers to stably controlling the system temperature at 0K and 300K.

[0018] The step (2) of executing simulation and processing data also includes: taking the strain range as 0 to 0.2, the speed as 10 m / s, setting the temperature as 0 K and 300 K, calculating and analyzing the instantaneous friction force and average friction force under different strain and speed coupling, and determining the influence of strain and temperature on the friction force of the MoS2 / MoS2 interface during the friction process.

[0019] The step (2) of executing simulation and processing data also includes: taking strains of 0, 0.002 and 0.1, and speed of 10m / s, and statistically analyzing the potential energy surface and potential energy difference under different strain and speed couplings; calculating the phonon state density of the friction interface under strains of 0.002 and 0.1; taking the strain range of 0-0.2, speed of 10m / s, temperature of 0K and 300K, and calculating the friction interface thermal resistance of MoS2 / MoS2 under different strain and speed couplings; comparing the potential energy surface, state density of the probe and the substrate, and the friction interface thermal resistance under different strains to determine the dependence of the friction force and the interface thermal resistance on the state density under different strains.

[0020] The step (2) of executing simulation and processing data also includes: taking the strain as 0.002, 0.01, 0.16 and 0.19, the speed as 10m / s, and calculating the barrier height at the temperature of 0K and 300K respectively; taking the strain range as 0-0.2, and calculating the temperature amplitude of the MoS2 / MoS2 interface probe and the substrate at the temperature of 0K and 300K respectively; analyzing the barrier height and temperature amplitude results to determine the influence of the barrier height and temperature amplitude at different strains and temperatures on the friction force of the MoS2 / MoS2 interface.

[0021] The step (2) of executing simulation and processing data also includes: taking strains of 0, 0.05 and 0.1 and temperature of 0 K, calculating the MoS2 / MoS2 interface friction force and friction interface thermal resistance at different speeds, analyzing the friction force distribution under different strain and speed couplings, and determining the relationship between strain and speed on friction force and friction interface thermal resistance.

[0022] The step (2) of executing simulation and processing data also includes: taking the strain as 0 and 0.1, the temperature as 0K, performing fast Fourier transform (FFT) on the instantaneous friction force at different speeds, analyzing the FFT spectrum under different strain and speed coupling, and revealing the intrinsic reason for the friction peak in the friction force-speed change curve.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The present invention takes MoS2 / MoS2 as the research object, first establishes a molecular dynamics model, then runs the model under different strains and speeds, calculates the instantaneous friction force in the friction process, and processes and analyzes it, and determines the influence of strain and speed coupling on the friction characteristics between MoS2 / MoS2 interfaces: with the increase of strain, the stick-slip phenomenon in the instantaneous friction force change curve gradually disappears, and the average friction force also decreases with the increase of strain; in the case of slight deviation from no strain, the contact interface will have greater friction energy dissipation; and temperature has different effects on the interface friction force under different strain ranges. Therefore, the present invention provides an effective method for clarifying the influence of different biaxial strains and sliding speed coupling on the MoS2 / MoS2 interface friction force, and further provides a new perspective and help for the nanotribology research between MoS2 layers.

[0025] 2. The present invention statistically analyzes the potential energy surface and potential energy difference of the friction interface under different strain and velocity coupling, calculates the interface phonon state density, and calculates the friction interface thermal resistance of the MoS2 / MoS2 system under different strain and velocity coupling, determines the relationship between the state density and friction force under different strains, and the significant contribution of the state density to the friction interface thermal resistance.

[0026] 3. The present invention calculates the barrier height and the temperature amplitude of the friction interface at temperatures of 0K and 300K; analyzes the results of the barrier height and temperature amplitude, and determines the relationship between the barrier height and temperature amplitude and the interface friction force under different strains and temperatures.

[0027] 4. The present invention calculates the friction force and friction interface thermal resistance under different sliding speeds, analyzes the friction force distribution under different strain and speed couplings, and determines the contribution of strain and speed to the friction force and interface thermal resistance: the friction force will increase with the increase of sliding speed and decrease with the increase of strain; the friction interface thermal resistance will increase with the increase of strain and speed.

[0028] 5. In order to explore the reason for the friction peak in the friction force-speed change curve during the friction process, the present invention performs fast Fourier transform (FFT) on the instantaneous friction force at different speeds, analyzes the FFT spectrum under different strain and speed coupling, and reveals the reason for the friction peak in the friction force-speed change curve: when the sliding speed reaches v f =120.50m / s (i.e. the washboard frequency is equal to the natural frequency), friction resonance will occur on the contact surface, resulting in a larger amplitude and energy dissipation, thus forming a friction resonance peak on the friction force-velocity curve. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings.

[0030] Figure 1 This is the molecular dynamics friction model of the molybdenum disulfide system in the embodiment of the present invention.

[0031] Figure 2 1 is a curve showing the variation of instantaneous friction force with sliding time under different strain and velocity coupling in an embodiment of the present invention.

[0032] Figure 3 : is the functional relationship between the average friction force and the strain under different temperature and speed coupling in the embodiment of the present invention.

[0033] Figure 4 1 is the potential energy morphology of the friction interface under different strain and velocity coupling in the embodiment of the present invention.

[0034] Figure 5 : is the state density of the probe and the substrate under different strain and velocity coupling in the embodiment of the present invention.

[0035] Figure 6 : is the functional relationship between the friction interface thermal resistance and the strain and velocity coupling in the embodiment of the present invention.

[0036] Figure 7 1 is a curve showing the change of barrier height with sliding displacement under different strains and temperatures in an embodiment of the present invention.

[0037] Figure 8 Graph 1 is the dependence of the friction interface temperature amplitude on different temperatures and strains in the embodiment of the present invention.

[0038] Fig. 9 : is the functional relationship between the average friction force and the sliding speed under different strains in the embodiment of the present invention.

[0039] Fig.10 1 is a function curve of the friction interface thermal resistance changing with speed under different strains in the embodiment of the present invention.

[0040] Fig.11 Spectral distribution of amplitude changing with different strains and velocity coupling in the embodiment of the present invention. DETAILED DESCRIPTION

[0041] A research method for controlling friction by coupling strain and velocity, comprising the following steps:

[0042] ⑴ A molecular dynamics friction model consisting of a probe and a substrate of molybdenum disulfide / molybdenum disulfide (MoS2 / MoS2) was established, and the biaxial strain applied to the substrate during the simulation was achieved by directly changing the coordinates of the MoS2 atoms.

[0043] The probe comprises a free motion layer I; each atom in the molybdenum disulfide probe is provided with a spring in the x, y and z directions; wherein: the spring in the x direction pulls the probe atom to slide frictionally at a constant speed of 10 m / s, and the springs in the y and z directions are used to eliminate the translation and rotation of the probe along the y direction, as well as the movement along the z direction during sliding; a periodic boundary condition is set in the x direction, and a free boundary condition is set in the y and z directions.

[0044] The substrate comprises a bottom fixed layer, a side fixed layer, a lateral temperature adjustment layer and a free motion layer II, wherein the lateral temperature adjustment layer is to stably control the system temperature at 0K and 300K.

[0045] A friction interface is formed between the free movement layer I and the free movement layer II.

[0046] (2) Execute simulation and process data:

[0047] ① Taking the strain range of 0-0.2 and the speed of 10m / s, the instantaneous friction force and average friction force of the MoS2 / MoS2 interface at temperatures of 0K and 300K were calculated respectively, and then the relationship between the change curve of friction force and strain was obtained;

[0048] ② Take the strain as 0, 0.002 and 0.1, and the speed as 10m / s, calculate the potential energy surface, state density and friction interface thermal resistance under different strain and speed coupling to determine the contribution of state density to friction under different strains;

[0049] ③ Compare the potential barrier height and friction interface temperature amplitude under different strains and temperatures to determine the dependence of friction force on strain and temperature;

[0050] ④ Analyze the friction force and friction interface thermal resistance under different strain and speed coupling, determine the influence of strain and speed on the friction force of MoS2 / MoS2 interface, and thus provide a new perspective and help for the nanotribology research between MoS2 layers.

[0051] The method of performing simulation and processing data in the present invention also includes one of the following:

[0052] The strain range is 0-0.2, the speed is 10 m / s, the temperature is set at 0 K and 300 K, and the instantaneous friction force and average friction force under different strain and speed coupling are calculated and analyzed to determine the influence of strain and temperature on the friction force of the MoS2 / MoS2 interface during the friction process.

[0053] The strains were taken as 0, 0.002 and 0.1, and the speed was 10m / s, and the potential energy surface and potential energy difference under different strain and speed coupling were statistically analyzed; the phonon state density of the friction interface under strains of 0.002 and 0.1 was calculated; the strain range was taken as 0-0.2, the speed was 10m / s, the temperature was 0K and 300K, and the friction interface thermal resistance of MoS2 / MoS2 under different strain and speed coupling was calculated; the potential energy surface, the state density of the probe and the substrate, and the friction interface thermal resistance under different strains were compared to determine the dependence of the friction force and the interface thermal resistance on the state density under different strains.

[0054] The strains were taken as 0.002, 0.01, 0.16 and 0.19, and the speed was 10m / s, and the barrier heights at temperatures of 0K and 300K were calculated respectively; the strain range was taken as 0-0.2, and the temperature amplitudes of the MoS2 / MoS2 interface probe and substrate at temperatures of 0K and 300K were calculated respectively; the barrier height and temperature amplitude results were analyzed to determine the effects of the barrier height and temperature amplitude on the friction force of the MoS2 / MoS2 interface at different strains and temperatures.

[0055] Taking strains of 0, 0.05 and 0.1 and temperature of 0 K, the friction force and friction interface thermal resistance of MoS2 / MoS2 interface at different speeds are calculated. The friction force distribution under different strain and speed coupling is analyzed to determine the relationship between strain and speed on friction force and friction interface thermal resistance.

[0056] Taking strains of 0 and 0.1 and temperature of 0 K, the instantaneous friction force at different speeds is subjected to fast Fourier transform (FFT). The FFT spectra at different strains and coupling speeds are analyzed to reveal the intrinsic reasons for the friction peak in the friction force-speed variation curve.

[0057] Embodiment The present invention takes MoS2 / MoS2 system as the research object and explores the research method of the mechanism of the influence of strain and speed on the interface friction.

[0058] The present invention reveals two abnormal phenomena. One is that greater friction energy dissipation will occur in a state slightly deviated from the state of no strain. The other is that within a small strain range, friction decreases with increasing temperature. When the strain exceeds a critical value, the increased temperature will lead to an increase in friction. In addition, the significant contribution of sliding velocity to friction under different strains is studied. Finally, the fast Fourier transform spectrum of instantaneous friction is used to further explain the intrinsic reason for the friction peak in the function curve of strain and velocity coupling.

[0059] The specific process is as follows:

[0060] (1) Establishment of molecular dynamics model

[0061] like Figure 1 Figure 2 shows a typical system for studying the friction between MoS2 layers, where a square MoS2 probe slides on a MoS2 substrate supported by rigid atoms.

[0062] The lengths of the substrate along the x and y directions are 13.44 nm and 28.82 nm, respectively, and the size of the probe is 13.44 × 13.30 nm. 2 . The atoms in the bottom area of ​​the substrate and on both sides of the upper layer are restricted to fixed walls, and the six degrees of freedom of all atoms in the fixed walls are frozen to prevent them from translating and rotating. The atomic region adjacent to the fixed wall of the upper layer of the substrate along the y direction is set as a temperature regulation domain, and the atoms in other areas are moving layers and are all affected by Newtonian mechanics. In addition, in order to eliminate the boundary effect in the microscopic world, periodic boundary conditions are adopted in the x direction of the probe and the substrate, thereby simulating an infinite plane size to maintain the dynamic balance between the probe and the substrate. Free boundary conditions are set in the y and z directions.

[0063] In the present invention, a NVE ensemble with constant atomic number, volume and energy is selected for all friction calculations; the Berendsen thermostat stably controls the system temperature at 0K and 300K. x A spring is set on each atom of the MoS2 probe and slides at a speed v s Connect it to the slider; apply springs to each atom of the probe in the y and z directions, with spring stiffness coefficients k y and k z , the purpose is to eliminate the translation and rotation of the probe in the y direction, as well as the movement in the z direction when sliding.

[0064] In the present invention, the interaction between the fixed wall at the bottom of the substrate and MoS2, as well as the interaction between the probe and the substrate can be described by Lennard-Jone (LJ) potential. The LJ potential strength between the probe and the substrate remains constant and is not affected by the sliding speed. The Stillinger-Weber potential is used to characterize the Mo-S covalent bonds between atoms in the MoS2 layer.

[0065] In the present invention, 24 atoms are selected from the probe and the substrate to monitor the phonon state density of the MoS2 / MoS2 friction interface, and finally the biaxial strain applied to the substrate during the simulation is achieved by directly changing the coordinates of the MoS2 atoms.

[0066] (2) Determine the effect of strain and temperature on the friction force at the MoS2 / MoS2 interface

[0067] An experiment was conducted on the above-mentioned MoS2 / MoS2 friction system. At the beginning of operation, the friction system reached the relaxation temperature and maintained a constant temperature under the action of the temperature regulating layer.

[0068] Taking strains of 0, 0.002, 0.05 and 0.1 and a speed of 10 m / s, the instantaneous friction of the MoS2 / MoS2 system under different strain and speed couplings is calculated to determine the effect of strain on the instantaneous friction.

[0069] The friction system of the present invention is used to study the nanofriction behavior between the probe and the substrate under different strains, temperatures and speeds. The instantaneous spring force during the sliding process can be regarded as the instantaneous friction force, and the spring force can be expressed as: F f =N·k x (v s tx t ), where N is the number of probe atoms, t is the sliding time, and x t Represents the displacement of the probe mass center. The average friction force can be obtained by averaging the instantaneous friction force (ie, the friction force in the present invention).

[0070] Figure 2 The instantaneous friction force curves with time under different strains were statistically analyzed. The results show that under no strain or small strain, stick-slip phenomenon will occur during the friction sliding process; as the strain increases, the instantaneous friction force (i.e., the maximum static friction force) gradually decreases, and the symmetry of the stick-slip curve gradually weakens, indicating that the friction energy dissipation decreases. The friction state gradually transitions from the stick-slip stage to the continuous sliding stage.

[0071] Figure 3 The friction changes caused by applying in-plane strain to the MoS2 substrate are shown. Overall, the strain-friction curves at different temperatures have similar trends; the average friction force decreases with increasing strain, but when ε xx =εyy =0.002, compared with the unstrained case, the maximum instantaneous friction force decreases, while the average friction force increases. In other words, the maximum average friction force is obtained under small strain, which may be due to the small strain applied to the substrate causing the probe to be properly pulled apart synchronously and increasing the deformation. The present invention predicts that under small strain, the atomic lattice between the friction interface is not mismatched and still maintains a perfectly matched lattice structure. At the same time, it is confirmed by extracting the phonon state density of the probe and the substrate under small strain and calculating the thermal resistance of the MoS2 / MoS2 friction interface.

[0072] (3) Determine the dependence of friction and interfacial thermal resistance on state density under different strains

[0073] In order to verify that the atoms on the friction contact surface still maintain a matching lattice structure under small strain, the present invention extracts the phonon state density of the probe and the substrate. By analyzing the state density under different strains, the dependence of friction force and interface thermal resistance on the state density is determined.

[0074] Applying in-plane strain will change the lattice constant between adjacent atoms, thereby affecting the period length of the interaction between atoms during the sliding process. In the present invention, the sliding period of the probe under different strains is firstly revealed. Figure 4 The potential energy surface under different strains is shown. As the strain increases from 0 to 0.1, the potential energy difference decreases from 440eV to 6.3eV; when the probe slides equidistantly on the substrate, the potential energy difference decreases with the increase of strain. Figure 4 The distance between two adjacent maximum energy values ​​in (a) and (b) represents the period length. Figure 4 (a) is the potential energy morphology of the probe and substrate under no strain. Since the lattice constant between atoms on the contact surface does not change, the period length remains (a is the Mo-S bond length). Although the strain is only applied to the substrate, the potential energy morphology of the probe and the substrate remains the same, and the period length is (where a0 = (1 + 0.002)a). Figure 4 (c) and (d) show the potential energy morphologies of the probe and substrate under strain 0.1, respectively. Figure 4 The difference between (a) and (b) is that after applying strain to the substrate, the potential energy cycle length of the probe does not change synchronously and remains (like Figure 4 (c)), and the period length of the substrate is a1=(1+0.1)a(e.g. Figure 4 (d)).

[0075] Friction energy dissipation is related to the coupling strength of the acoustic modes excited between the sliding surfaces. Each atom in contact is affected by the relative potential energy, and the vibration frequency of the atoms on the contact surface generated by periodic excitation is concentrated on the washboard frequency and its harmonics. Therefore, under small strain, the washboard frequency of the probe and the substrate is: At the maximum strain of 0.1, the washboard frequency of the substrate is: But the probe's washboard frequency is: where v s Represents the sliding speed.

[0076] The phonon state density calculation process in the present invention is as follows:

[0077] In molecular dynamics simulations, the velocity autocorrelation function VAF can be calculated as:

[0078]

[0079] Where: M represents the number of atoms in the selected area; <> is the time average operator; Δt is the time step; t' is the delay time number; N cor is the number of autocorrelation speeds; v j,i is the thermal velocity of atom j corresponding to the sliding time i.

[0080] The state density is calculated by VAF through fast Fourier transform:

[0081]

[0082] Where: ω is the angular frequency, τ is the total sliding time.

[0083] Figure 5 The phonon state density of the friction interface under different strains is given. Figure 5 It can be found that under small strains, the acoustic modes excited between the probe and the substrate are completely matched and are both located at the washboard frequency and its harmonics, which indicates that there is no mismatch in the lattice structure of the atoms on the contact surface; under larger strains, although the excited acoustic modes are also at the washboard frequency and its harmonics, due to the difference in washboard frequencies, the acoustic modes excited on the contact surface no longer match (the dotted lines in the figure indicate the positions of the frequencies and harmonics).

[0084] Therefore, the present invention shows that after applying a small strain to the substrate, the atoms on the contact surface still maintain a perfectly matched lattice structure, and applying a larger strain increases the deformation of the probe. In other words, additional work is required during the friction process, resulting in greater friction energy dissipation.

[0085] The present invention further studies the friction interface thermal resistance under different strains and temperatures. In the present invention, the friction force calculated by the spring is equal to the friction force calculated by the heat absorbed by the thermostatic layer, that is, the friction dissipation rate is equal to the heat extraction rate of the thermostatic layer, so

[0086] The friction force can also be expressed as Where J represents the heat flow, which can be expressed based on Fourier transform calculation as: Q represents the total friction energy.

[0087] When heat flows through the interface, it usually manifests as a sudden drop in the interface temperature difference. Assuming that the heat between the contact surfaces is evenly distributed, the heat passing through the friction interface is equal to half of the total heat absorbed by the thermostatic layer. Based on Fourier's law, the friction interface thermal resistance ITR can be expressed as:

[0088] Where: R is ITR; A represents the contact area of ​​the friction interface; ΔT is the interface temperature difference.

[0089] The present invention Figure 6 It shows that with the increase of strain, ITR generally shows an upward trend. Under no strain or micro-strain, ITR is almost zero. This is because the acoustic modes excited between the probe and the substrate are perfectly matched. The degree of matching reflects the coupling strength of the vibration modes of the contact interface. The strong coupling between the acoustic modes forms an effective energy dissipation channel at the friction interface, thereby improving the transmission efficiency of friction energy, so ITR is minimal. These research results provide a theoretical basis and guidance for regulating the influence of strain on interface friction.

[0090] (4) Determine the influence of barrier height and temperature amplitude on interface friction

[0091] In order to reveal the reason why temperature has different effects on interface friction under different strains, the present invention further determines the dependence of interface friction under different strains on barrier height and temperature amplitude during the friction process by calculating and analyzing the barrier height and temperature amplitude under different conditions.

[0092] In the present invention, Figure 3 Another anomaly shown is that when ε xx =ε yy When ε is less than 0.1, the friction force at 300K is less than that at 0K; however, when ε xx =ε yy When >0.1, the friction force at high temperature is greater than that at low temperature. The magnitude of the friction force can be attributed to the difficulty of crossing the potential barrier. Figure 7a shows that the barrier height decreases with increasing strain. At the same strain, high temperature reduces the energy barrier, which means that only a smaller force is needed to overcome the barrier, thereby reducing the interface friction. It should be noted that thermal lubrication plays a decisive role in the friction in this process. However, Figure 7 The results of b show that the potential barrier height at high temperature is higher than that at 0K, which means that when the strain exceeds the critical value of 0.1, the probe needs a larger driving force to pass through the potential barrier at high temperature. Therefore, the friction energy dissipation at high temperature will be greater. To further reveal this reason, the present invention extracts the temperature amplitude of the friction interface.

[0093] In the present invention, Figure 8 As shown in the figure, the amplitude peak caused by temperature is formed near the microstrain, and the interface temperature amplitude at 300K is much larger than 0K. In fact, the increase in temperature will make the thermal vibration of atoms more intense, and the sulfur atoms on the friction interface may further deviate from their equilibrium position, thereby increasing the collision probability between atoms. The friction will be affected by the inelastic collision between atoms, which also means that when the strain exceeds the critical value, the thermal collision motion of atoms begins to dominate the friction.

[0094] The present invention shows that: in the friction process within the strain range, the thermal lubrication and thermal collision of the interface atoms are always in a state of mutual competition.

[0095] (5) Determine the effect of strain and velocity coupling on friction and interfacial thermal resistance

[0096] The present invention further explores the correlation between friction, friction interface thermal resistance, speed and strain. By analyzing the friction under different strain and speed coupling of MoS2 / MoS2 system, the dependence of friction and interface thermal resistance on strain and speed is determined. The specific contents are as follows:

[0097] Taking strains of 0, 0.05 and 0.1 and temperature of 0 K, the friction force and interfacial thermal resistance of the MoS2 / MoS2 system at different speeds are calculated respectively. The relevant calculation results are analyzed to determine the influence of strain and speed coupling on the friction force and interfacial thermal resistance during the friction process.

[0098] The results show that: Fig. 9 As shown in , the friction force shows a non-monotonic upward trend with increasing speed, and this trend is not affected by strain. The faster the probe slides, the faster the atomic layer wrinkles caused by the applied strain move. The wrinkle motion is actually the result of the motion of the substrate atoms, which means that the substrate atoms have a greater sliding speed during the separation and adhesion process, and therefore consume more energy. Fig.10It can be seen that the friction interface thermal resistance shows a gradual increase trend with the increase of strain and speed. This is because, in the absence of strain, the interface atomic lattice is perfectly matched, and there are more energy dissipation channels at the friction interface, so that the energy generated by friction is efficiently transferred from the probe to the substrate. Therefore, the friction interface thermal resistance is the weakest. However, as the strain increases, the interface atomic lattice structure mismatches, the energy dissipation channel weakens, and the interface friction energy cannot be transferred. Therefore, the interface thermal resistance increases with the increase of strain. In the present invention, Fig. 9 It was also found that when the sliding speed reached about 120.50 m / s, a friction peak was formed (e.g. Fig. 9 dotted line).

[0099] The present invention shows that the friction force in the absence of strain is greater than that after the strain is applied, and the friction force decreases with the increase of strain; the thermal resistance of the friction interface increases with the increase of strain and speed; when the sliding speed reaches about 120.50m / s, a friction peak will be generated.

[0100] The present invention predicts that the formation of the friction peak may be caused by friction resonance. In order to reveal the cause of the friction peak, the present invention analyzes the Fourier transform spectrum of the instantaneous friction force.

[0101] (6) Determine the influence of Fourier transform spectrum on interface friction peak during friction process

[0102] In order to further explore the influence of friction resonance on the convex peak, the present invention performs fast Fourier transform on the instantaneous friction force. By calculating and analyzing the spectrum under different strain and velocity coupling, the influence of the FFT spectrum on the friction peak in the friction force-velocity change curve during the friction process is determined.

[0103] The Fourier transform spectrum represents the vibration frequency and amplitude of the probe center of mass. The natural frequency is denoted as f n ( m is the mass of the probe). The present invention further calculates the sliding velocity corresponding to the natural frequency And found that v f It is exactly equal to 120.50 m / s, thus confirming the prediction of friction resonance of the present invention. Fig.11 (a), (b) and (c) show that in the absence of strain, the FFT spectrum distribution is located at the washboard frequency f t When the speed increases to v f =120.50m / s, f t With f nKeep consistent, at this moment, friction resonance occurs at the contact interface; at the same time, the amplitude reaches the maximum value and forms a friction resonance peak. When the washboard frequency is greater than or less than the natural frequency, the friction resonance peak will disappear, accompanied by a decrease in amplitude, resulting in reduced energy dissipation. When the strain reaches 0.1, the atomic lattice of the friction interface is mismatched, and the lower energy barrier makes the interface potential energy constraint weaker than in the case of no strain. Fig.11 As shown in (d), (e) and (f), as the sliding speed increases, the interface potential energy constraint weakens, and the FFT spectrum distribution is mainly concentrated on the natural frequency of the probe; similar to the unstrained case, when the speed reaches 120.50m / s, the vibration amplitude reaches the maximum and a friction resonance peak is generated. In addition, no matter how the sliding speed changes, the amplitude in the unstrained state is much larger than the amplitude under 0.1 strain; the higher the vibration amplitude of the probe, the greater the energy dissipated during the friction process, and the greater the corresponding friction force.

[0104] The present invention shows that the friction peak in the friction force-speed variation curve is caused by the friction resonance of the interface. These conclusions reveal the contribution of strain and speed to interface friction and the internal mechanism of their influence.

[0105] In summary, the study on friction energy dissipation at the MoS2 / MoS2 interface based on the coupling of biaxial strain and sliding velocity in the present invention can provide a new perspective and theoretical guidance for the field of nanomaterial tribology.

[0106] The technical solution provided by the present invention is described in detail above. The principle and implementation mode of the present invention are described in detail using specific embodiments herein, and the explanation of the above embodiments is only used to help to deeply understand the method and core idea of ​​the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A research method for regulating friction by coupling strain and velocity, comprising the following steps: (1) Establish a molecular dynamics friction model consisting of a MoS2 / MoS2 probe and a substrate, and realize the biaxial strain applied to the substrate during the simulation by directly changing the MoS2 atomic coordinates; The probe comprises a free motion layer I; each atom in the MoS2 probe is x , y , z Springs are set in both directions; in: x The spring in the direction will pull the probe atom to slide at a constant speed of 10m / s. y and z The spring in the direction is used to eliminate the probe along the y direction, and along the z movement in the direction of x Set periodic boundary conditions in the direction. y and z Direction setting free boundary conditions; The substrate comprises a bottom fixing layer, a side fixing layer, a side temperature regulating layer and a free movement layer II; A friction interface is formed between the free movement layer I and the free movement layer II; ⑵Execute simulation and process data: ① Taking the strain range of 0~0.2 and the speed of 10m / s, the instantaneous friction force and average friction force of the MoS2 / MoS2 interface at the temperature of 0K and 300K were calculated respectively, and then the relationship between the change curve of friction force and strain was obtained; ② Take the strain as 0, 0.002 and 0.1, and the speed as 10m / s, calculate the potential energy surface, state density and friction interface thermal resistance under different strain and speed coupling to determine the contribution of state density to friction under different strains; ③ Compare the potential barrier height and friction interface temperature amplitude under different strains and temperatures to determine the dependence of friction force on strain and temperature; ④ Analyze the friction force and friction interface thermal resistance under different strain and speed coupling, and determine the influence of strain and speed on the friction force of the MoS2 / MoS2 interface.

2. A method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The lateral temperature adjustment layer in step (1) refers to stably controlling the system temperature at 0K and 300K.

3. A method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The step (2) of executing simulation and processing data also includes: taking the strain range as 0~0.2, the speed as 10m / s, setting the temperature as 0K and 300K, calculating and analyzing the instantaneous friction force and average friction force under different strain and speed coupling, and determining the influence of strain and temperature on the friction force of the MoS2 / MoS2 interface during the friction process.

4. A method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The step (2) of executing simulation and processing data also includes: taking strains of 0, 0.002 and 0.1, and speed of 10 m / s, and statistically analyzing the potential energy surface and potential energy difference under different strain and speed couplings; calculating the phonon state density of the friction interface under strains of 0.002 and 0.1; taking the strain range of 0~0.2, speed of 10 m / s, temperature of 0K and 300K, and calculating the friction interface thermal resistance of MoS2 / MoS2 under different strain and speed couplings; comparing the potential energy surface, state density of the probe and the substrate, and the friction interface thermal resistance under different strains to determine the dependence of the friction force and the interface thermal resistance on the state density under different strains.

5. A research method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The step (2) of executing simulation and processing data also includes: taking strains of 0.002, 0.01, 0.16 and 0.19 and a speed of 10 m / s, respectively calculating the barrier heights at temperatures of 0 K and 300 K; taking the strain range of 0 to 0.2, respectively calculating the temperature amplitudes of the MoS2 / MoS2 interface probe and the substrate at temperatures of 0 K and 300 K; analyzing the barrier height and temperature amplitude results to determine the effects of the barrier height and temperature amplitude on the MoS2 / MoS2 interface friction at different strains and temperatures.

6. A method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The step (2) of executing simulation and processing data also includes: taking strains of 0, 0.05 and 0.1 and temperature of 0K, calculating the MoS2 / MoS2 interface friction force and friction interface thermal resistance at different speeds, analyzing the friction force distribution under different strain and speed couplings, and determining the relationship between strain and speed on friction force and friction interface thermal resistance.

7. A method for controlling friction by coupling strain and velocity as claimed in claim 1, characterized in that: The step (2) of executing simulation and processing data also includes: taking strain as 0 and 0.1, temperature 0K, performing fast Fourier transform on the instantaneous friction force at different speeds, analyzing the FFT spectrum under different strain and speed coupling, and revealing the intrinsic reason for the friction peak in the friction force-speed change curve.

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