Lubricant adsorption and friction regulation and control method based on molecular dynamics simulation

Abstract

The invention discloses a lubricant adsorption and friction regulation and control method based on molecular dynamics simulation, which comprises the following steps: S1, constructing a lubricant molecule and friction pair metal unit cell model, generating topological structure information based on a minimum structure unit, and constructing a lubrication system model with a sandwich structure; s2, giving force field parameters to the lubrication system model, and performing geometric structure optimization and dynamic relaxation; s3, giving surface charges to the upper and lower friction pairs for targeted adsorption simulation of the lubricant; and S4, applying uniformly distributed contact pressure and constant sliding speed to the upper friction pair to perform friction simulation of the lubrication system. According to the lubricant adsorption and friction regulation and control method based on molecular dynamics simulation, a targeted adsorption strategy of a solid-liquid interface is established, the microcosmic adsorption process of the lubricant is reproduced, controllable regulation of friction characteristics is achieved, the human resource cost in actual experiments is reduced, and manual operation errors are reduced.

Description

Technical Field

[0001] The present invention relates to the technical fields of materials science and tribology, and specifically to a method for lubricant adsorption and friction regulation based on molecular dynamics simulation. Background Art

[0002] Friction and wear are one of the main causes of material energy loss and mechanical equipment failures, while lubrication is an efficient strategy to reduce frictional energy consumption, reduce waste gas emissions, and extend service life. The use of liquid lubricants effectively avoids the direct contact of friction pairs and thus improves the friction and wear performance of the lubrication system. However, with the rapid development of high-end technical equipment, equipment units are gradually facing a series of problems that need to be solved urgently, such as harsh operating conditions, complex operating environments, and improved performance requirements, which leads to the lubrication system often being in the boundary lubrication state of severe friction and wear between asperities.

[0003] To improve this problem, lubricant additives have emerged. The polar functional groups rich in the additives will adsorb on the surface of the friction pair to form a physical adsorption film or a tribochemical reaction film to relieve the mechanical contact at the interface, thereby improving the tribological performance of the lubrication system. However, there are still several main problems in the current existing technologies: there are compatibility differences and competitive adsorption relationships between the additives and the base oil, the adsorption binding strength at the solid-liquid interface is very limited, it is difficult to dynamically and quantitatively characterize the adsorption process of lubricant molecules by macroscopic experiments, and the structure-activity relationship between the adsorption configuration and the friction characteristics is still unclear. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for lubricant adsorption and friction regulation based on molecular dynamics simulation to solve the above defects.

[0005] To achieve the above purpose, the present invention provides the following technical solutions:

[0006] A method for lubricant adsorption and friction regulation based on molecular dynamics simulation, comprising the following steps:

[0007] S1. Construct a lubricant molecule and a friction pair metal unit cell model, and generate topological structure information based on the minimum structural unit to construct a lubrication system model with a sandwich structure;

[0008] S2. Assign force field parameters to the lubrication system model and perform geometric structure optimization and dynamic relaxation;

[0009] S3. Assign charges to the upper and lower friction pair surfaces for targeted adsorption simulation of the lubricant;

[0010] S4. Apply a uniform contact pressure and a constant sliding speed to the upper friction pair for friction simulation of the lubrication system.

[0011] Preferably, in step S1, the lubricant molecules are one or more of polar or non-polar base oils or additives containing carbon, hydrogen, oxygen, nitrogen, and phosphorus elements and being in a liquid state under normal temperature and pressure.

[0012] Preferably, in step S1, the friction pair metals are one or more of iron, copper, aluminum, and their alloy materials.

[0013] Preferably, in step S1, the specific steps for constructing the lubrication system model with a sandwich structure include:

[0014] S11. Construct a single lubricant molecule model through the visualization operation interface of AuToFF or MS software, and based on the topological structure information of the single molecule model, construct a lubricant bulk phase structure model through the AC module of MS or the programming algorithm of Moltemplate;

[0015] S12. Obtain the crystal structure information of the friction pair metal atoms through the open-source crystal structure database Materials Project, and use the supercell algorithm of MS software or the replicate algorithm of LAMMPS software to perform cell expansion processing to construct a smooth friction pair surface model, or construct a rough friction pair surface model through the LAMMPS code;

[0016] S13. Combine the lubricant bulk phase structure model with the smooth friction pair surface model or the rough friction pair surface model through the buildlayer algorithm of MS software or the merging script of Packmol software to form a lubrication system model with a sandwich structure.

[0017] Preferably, in step S2, the lubricant is described by force field parameters using CVFF or PCFF or OPLS, the metal friction pair is described by force field parameters using the EAM or EAM / FS or EAM / ALLOY potential function, and the non-bonded interactions between different components of the lubrication system are described by the L-J potential function of the L-B mixing rule for force field parameters.

[0018] Preferably, in step S2, geometric structure optimization and kinetic relaxation are performed. Specifically, geometric structure optimization is achieved based on the conjugate gradient algorithm, 500 ps of kinetic relaxation is completed through the NPT ensemble to make the lubrication system reach a reasonable structural equilibrium state, and the Nosé-Hoover method is used to control the temperature at 300 K and one atmosphere to simulate the normal temperature and pressure environment.

[0019] Preferably, in step S3, the targeted adsorption simulation is specifically as follows:

[0020] S31. Based on the lubrication system model after relaxation equilibrium, equal amounts of positive or negative charges are assigned to all metal atoms of the upper and lower friction pairs to construct solid-liquid electrostatic interactions, and the strength of the solid-liquid interaction is regulated by the magnitude of the charge quantity.

[0021] S32. Simulate the targeted adsorption behavior of the lubricant towards the metal interface through 500 ps of relaxation in the NVT ensemble.

[0022] S33. Quantitatively evaluate the targeted adsorption behavior of the lubricant through the microscopic structure evolution and atomic number density distribution parameters during the simulation process.

[0023] Preferably, in step S4, the friction simulation is specifically as follows:

[0024] S41. Fix the atoms of the lower friction pair to be stationary, apply a contact pressure uniformly distributed in the normal direction to the atoms of the upper friction pair, and relax for 500 ps in the NVT ensemble to achieve loading and compression.

[0025] S42. Apply a constant sliding velocity in the transverse direction to the atoms of the upper friction pair, and relax for 500 ps in the NVT ensemble to achieve shear friction.

[0026] S43. Calculate the normal force Fn and tangential force Ff of the metal atoms of the upper friction pair during the friction process, and calculate the friction coefficient based on Amontons-Coulomb's law CoF = Ff / Fn to characterize the tribological properties of the lubrication system.

[0027] The beneficial effects of the present invention are as follows:

[0028] (1) The method for regulating lubricant adsorption and friction based on molecular dynamics simulation in the present invention establishes a targeted adsorption strategy for the solid-liquid interface through the charge property relationship between the lubricant and the friction pair, reproduces the microscopic adsorption process of the lubricant through a quantitative characterization method, and realizes the controllable adjustment of friction characteristics through a matching mapping relationship, providing theoretical guidance for the design of new lubrication systems under extreme working conditions.

[0029] (2) The method for regulating lubricant adsorption and friction based on molecular dynamics simulation in the present invention can enhance the binding strength of the solid-liquid interface, observe the targeted adsorption process of lubricant molecules, and at the same time can reveal the structure-activity relationship of the lubrication system between the adsorption configuration and friction characteristics, realize the controllable adjustment of friction characteristics, reduce the human resource cost in actual experiments, and reduce the human operation error. Description of the Drawings

[0030] Figure 1 is the process flow chart of the method of the present invention;

[0031] Figure 2 is the model diagram of the lubrication system composed of stearic acid lubricant and metal iron friction pair in Example 1 of the present invention;

[0032] Figure 3 It is a comparative diagram of the steady-state adsorption configurations of Example 1 of the present invention under different surface charge conditions;

[0033] Figure 4 It is a comparative diagram of the adsorbed atom number density of Example 1 of the present invention under different surface charge conditions;

[0034] Figure 5 It is a comparative diagram of the friction coefficients of Example 1 of the present invention under different surface charge conditions;

[0035] Figure 6 It is a comparative diagram of the adsorbed atom number density of Example 2 of the present invention under different surface charge conditions;

[0036] Figure 7 It is a comparative diagram of the friction coefficients of Example 2 of the present invention under different surface charge conditions;

[0037] Figure 8 It is a comparative diagram of the adsorbed atom number density of Example 3 of the present invention under different surface charge conditions;

[0038] Figure 9 It is a comparative diagram of the friction coefficients of Example 3 of the present invention under different surface charge conditions;

[0039] Figure 10 It is a comparative diagram of the adsorbed atom number density of Example 4 of the present invention under different surface charge conditions;

[0040] Figure 11 It is a comparative diagram of the friction coefficients of Example 4 of the present invention under different surface charge conditions. Specific embodiments

[0041] The following further illustrates the present invention in conjunction with embodiments. It should be noted that this is only an example and illustration of the concept of the present invention. Those skilled in the art of the present technology can make various modifications or supplements to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined by this claim book, they should be regarded as falling within the protection scope of the present invention.

[0042] Example 1:

[0043] The present invention provides a method for regulating lubricant adsorption and friction based on molecular dynamics simulation. The specific process is as Figure 1 shown. Specifically, it includes the following steps:

[0044] S1. Construct a lubricant molecule and a friction pair atom unit cell model, and generate topological structure information based on the smallest structural units of the single molecule and single unit cell models to construct a lubrication system model with a sandwich structure. The specific steps include:

[0045] S11. Construct a single-molecule model of the lubricant through the visualization operation interface of AuToFF or MS software, and based on the topological structure information of the single-molecule model, construct a lubricant bulk-phase structure model through the AC module of MS or the programming algorithm of Moltemplate.

[0046] S12. Obtain the crystal structure information of the metal atoms of the friction pair through the open-source crystal structure database Materials Project, and use the supercell algorithm of MS software or the replicate algorithm of LAMMPS software to perform cell expansion processing to construct a smooth friction pair surface model, or construct a rough friction pair surface model through the LAMMPS code.

[0047] S13. Combine the lubricant bulk-phase structure model with the smooth friction pair surface model or the rough friction pair surface model through the buildlayer algorithm of MS software or the merging script of Packmol software to form a lubrication system model with a sandwich structure.

[0048] S2. Assign force field parameters to stearic acid lubricant and metal iron friction pair respectively, and perform geometric structure optimization and long-time kinetic relaxation on the lubrication system model.

[0049] The lubricant is described by force field parameters through CVFF or PCFF or OPLS. The metal friction pair is described by force field parameters using the EAM or EAM / FS or EAM / ALLOY potential function. The non-bonded interaction between different components of the lubrication system is described by the L-J potential function of the L-B mixing rule for force field parameters.

[0050] Perform geometric structure optimization and kinetic relaxation. Specifically: realize geometric structure optimization based on the conjugate gradient algorithm, complete 500 ps kinetic relaxation through the NPT ensemble to make the lubrication system reach a reasonable equilibrium state, and use the Nosé-Hoover method to control the temperature at 300 K and one atmosphere to simulate the normal temperature and pressure environment.

[0051] S3. Assign equal amounts of surface charge to the upper and lower metal iron friction pairs, and perform targeted adsorption simulation of the lubricant through long-time kinetic relaxation; extract the microscopic structural morphology of the lubricant molecules during the simulation process, and quantitatively evaluate the targeted adsorption behavior of the lubricant through the atomic number density distribution of the lubrication system. The specific steps include:

[0052] S31. Based on the lubrication system model after relaxation equilibrium, assign equal amounts of positive or negative charges to all metal atoms of the upper and lower friction pairs to construct solid-liquid electrostatic interaction, and regulate the strength of the solid-liquid interaction through the magnitude of the charge amount.

[0053] S32. Simulate the targeted adsorption behavior of the lubricant towards the metal interface through a 500 - ps NVT ensemble relaxation.

[0054] S33. Quantitatively evaluate the targeted adsorption behavior of the lubricant through the microscopic structure evolution and atomic number density distribution parameters during the simulation process.

[0055] S4. First, apply a uniformly distributed normal contact pressure to the upper metal iron friction pair, and then apply a constant lateral sliding speed to the upper metal iron friction pair to conduct friction simulation of the lubrication system; calculate the normal force Fn and tangential force Ff of the metal atoms on the upper friction pair during the friction process, and calculate the friction coefficient based on Amontons - Coulomb's law CoF = Ff / Fn to characterize the tribological properties of the lubrication system. The specific steps are as follows:

[0056] S41. Fix the atoms of the lower friction pair to be stationary, apply a uniformly distributed normal contact pressure to the atoms of the upper friction pair, and relax for 500 ps in the NVT ensemble to achieve loading and compression.

[0057] S42. Apply a constant lateral sliding speed to the atoms of the upper friction pair, and relax for 500 ps in the NVT ensemble to achieve shear friction.

[0058] S43. Calculate the normal force Fn and tangential force Ff of the metal atoms on the upper friction pair during the friction process, and calculate the friction coefficient based on Amontons - Coulomb's law CoF = Ff / Fn to characterize the tribological properties of the lubrication system.

[0059] In this Example 1, polar stearic acid is used as the lubricant, and metal iron is used as the friction pair. During the adsorption simulation, a smooth friction pair is used to clearly characterize the targeted adsorption behavior, and during the friction simulation, a rough friction pair is used to simulate the boundary lubrication state.

[0060] In step S1, use the drawing tool of the MS software to construct a single - molecule model of stearic acid C 18 H 36 O2 lubricant, and randomly fill 300 stearic acids through the Monte Carlo method of the AC module to construct a bulk lubricant model with a size of Import the unit cell of body - centered cubic lattice metal iron through the open - source crystal structure database Materials Project, and use the supercell function to expand the iron unit cell to construct a smooth surface model with a size of Use the LAMMPS sine function code to construct a model with a size of The rough surface model is constructed by superimposing the lubricating phase model and the friction pair surface model using the build layer function to construct a smooth surface adsorption simulation model and a rough surface friction simulation model. The model file is converted into an input file recognizable by LAMMPS using the msi2lmp tool, and the model is visualized using the OVITO software, such as Figure 2 shown.

[0061] In step S2, the stearic acid lubricant in the model is described by the CVFF potential function, and the metal iron friction pair is described by the EAM potential function. The lubricant and the friction pair are described by the LJ potential function of the LB mixing rule. The model is set to periodic boundary conditions in the X and Y directions and non-periodic boundary conditions in the Z direction. The model is first energy minimized by the gradient descent algorithm, and then the Nosé-Hoover temperature and pressure control method is used to perform 1ns dynamic relaxation on the model under the NPT ensemble.

[0062] In step S3 of this embodiment 1, first, all atoms in the upper and lower metal iron friction pairs are assigned an equal amount of zero charge, and the Nosé-Hoover temperature control method is used to perform an adsorption simulation of the model for 500 ps under the NVT ensemble; secondly, all atoms in the upper and lower metal iron friction pairs are assigned an equal amount of negative charge, and the Nosé-Hoover temperature control method is used to perform an adsorption simulation of the model for 500 ps under the NVT ensemble; finally, all atoms in the upper and lower metal iron friction pairs are assigned an equal amount of positive charge, and the Nosé-Hoover temperature control method is also used to perform an adsorption simulation of the model for 500 ps under the NVT ensemble.

[0063] After adsorption equilibrium of stearic acid on zero / positive / negatively charged surfaces, the stable adsorption conformations of lubricant molecules were extracted, e.g. Figure 3 As shown. In order to clearly show the effect of electrostatic interaction at the solid-liquid interface on targeted adsorption, the stearic acid molecules in the model only highlight the polar carboxyl functional groups and hide the non-polar hydrocarbon tail chains. The results show that a large number of stearic acid molecules in the zero-charge system are entangled with each other in the lubricating phase to form a clustered structure, and only a small number of polar head groups are loosely adsorbed on the metal interface to form a single-layer adsorption film. Compared with the zero-charge system, the agglomeration behavior of stearic acid molecules in the negatively charged system is weakened, and the polar head groups adsorbed at the solid-liquid interface increase to form a multi-layer adsorption film structure. However, when the friction pair surface is positively and negatively charged, more stearic acid molecules are separated from the lubricating phase and aggregated and adsorbed on the charged friction pair interface to form a multi-layer adsorption film with a denser structure. When the surface is negatively charged, the positively charged H atoms in the polar head group are preferentially adsorbed on the interface, while when the surface is positively charged, the negatively charged O atoms in the polar head group are preferentially adsorbed on the interface. This shows that stearic acid and the positively charged surface form a more dense and ordered adsorption film under electrostatic interaction, and the targeted adsorption behavior of the lubricant can be achieved by regulating the charge properties of the friction pair.

[0064] Calculate the atomic number density distribution of the polar head groups of stearic acid molecules along the Z direction to quantitatively characterize the targeted adsorption behavior, as Figure 4 shown. The atomic number density distributions of all systems exhibit the characteristics of "high peak value at the solid-liquid interface and low peak value inside the lubricant", which is attributed to the fact that the polar head groups of stearic acid molecules will preferentially adsorb on the friction pair interface under van der Waals and electrostatic interactions to form a stable physical adsorption film. Among them, there is a main peak at the upper and lower friction pair interfaces in the zero-charge system, indicating that the polar head groups form a monolayer adsorption film. However, in the positive-charge system, the peak value of the atomic number density at the solid-liquid interface increases significantly and there are two main peaks at the upper and lower friction pair interfaces respectively, while in the negative-charge system, the peak value of the atomic number density at the solid-liquid interface decreases significantly and four secondary peaks appear at the upper and lower friction pair interfaces. This shows that the presence of surface positive charges effectively enhances the electrostatic attraction and then induces the targeted adsorption behavior of lubricant molecules at the metal interface, and the presence of surface negative charges effectively enhances the electrostatic repulsion and then induces the loose adsorption behavior of lubricant molecules at the metal interface. The adsorption binding strength at the solid-liquid interface can be regulated by surface charges.

[0065] In step S4, first apply a 1 GPa contact pressure uniformly distributed along the normal direction to all atoms in the rough upper metal iron friction pair, and use the Nosé-Hoover temperature control method to relax the model for 500 ps in the NVT ensemble. Then apply a constant sliding velocity along the transverse direction to all atoms in the rough upper metal iron friction pair for 500 ps of friction simulation; calculate the total normal force Fn and the total tangential force Ff of the upper friction pair metal atoms during the friction process, and calculate the friction coefficient based on Amontons-Coulomb's law CoF = Ff / Fn to characterize the tribological properties of the lubrication system.

[0066] The quantitative characterization results of the friction coefficients of different lubrication systems are as Figure 5 shown. It should be noted that the friction coefficient in molecular dynamics simulation is greater than the experimental value, which is related to factors such as the contact state, contact scale, and chemical composition of the interface. The results show that the friction coefficient of the zero-charge system is 0.323, while the friction coefficient of the positive-charge system is 0.303, and the friction coefficient of the negative-charge system is 0.375. Compared with the zero-charge system, the presence of negative charges on the friction pair surface increases the friction coefficient, and the presence of positive charges on the friction pair surface decreases the friction coefficient. This is attributed to the fact that the polar head group of stearic acid is negatively charged, showing a repulsive effect with the negative-charge surface and an attractive effect with the positive-charge surface. Combining the relevant analysis in step S3 of this embodiment, it fully shows that the targeted adsorption ability of lubricant molecules, the binding strength at the solid-liquid interface, and the tribological properties of the lubrication system can be significantly enhanced through surface charge regulation.

[0067] Example 2:

[0068] The present invention provides a method for regulating lubricant adsorption and friction based on molecular dynamics simulation, and the specific process is as follows Figure 1 as shown.

[0069] In Example 2, the specific steps in S1, S2, S3, and S4 are basically the same as those in Example 1, except that:

[0070] In this Example 2, polar stearic acid is used as the lubricant, and metallic copper is used as the friction pair. During the adsorption simulation, a smooth friction pair is used to clearly characterize the targeted adsorption behavior, and during the friction simulation, a rough friction pair is used to simulate the boundary lubrication state.

[0071] In step S1, the unit cell of metallic copper with a face-centered cubic lattice is imported through the open-source crystal structure database Materials Project, and the supercell function is used to expand the copper unit cell to construct a smooth surface model with a size of . The LAMMPS sine function code is used to construct a rough surface model with a size of .

[0072] The atomic number density distribution of the polar head group of the stearic acid molecule along the Z direction is calculated to quantitatively characterize the targeted adsorption behavior, as Figure 6 shown. The atomic number density distributions of all systems show the characteristics of "high peak value at the solid-liquid interface and low peak value inside the lubricant", which is attributed to the fact that the polar head groups of stearic acid molecules will preferentially adsorb on the friction pair interface under van der Waals and electrostatic interactions to form a stable physical adsorption film. Among them, there is a main peak at the upper and lower friction pair interfaces in the zero-charge system, indicating that the polar head groups form a monolayer adsorption film. However, in the positive-charge system, the peak value of the atomic number density at the solid-liquid interface increases and there are two main peaks at the upper and lower friction pair interfaces respectively, while in the negative-charge system, the peak value of the atomic number density at the solid-liquid interface decreases and three secondary peaks appear at the upper and lower friction pair interfaces. This shows that the presence of surface positive charges effectively enhances the electrostatic attraction and thus induces the targeted adsorption behavior of lubricant molecules at the metal interface, and the presence of surface negative charges effectively enhances the electrostatic repulsion and thus induces the loose adsorption behavior of lubricant molecules at the metal interface. The adsorption binding strength at the solid-liquid interface can be regulated by surface charges.

[0073] The quantitative characterization results of the friction coefficient are as Figure 7As shown in the figure. The friction coefficient of the zero-charge system is 0.322, while that of the positive-charge system is 0.270, and that of the negative-charge system is 0.373. Compared with the zero-charge system, the presence of negative charges on the friction pair surface increases the friction coefficient, and the presence of positive charges on the friction pair surface decreases the friction coefficient. This is attributed to the fact that the polar head group of stearic acid is negatively charged, showing a repulsive interaction with the negative-charge surface and an attractive interaction with the positive-charge surface. The decrease in the attractive interaction weakens the targeted adsorption ability of the lubricant molecules and the binding strength with the metal surface, thereby deteriorating the lubrication performance. While the increase in the attractive interaction enhances the targeted adsorption ability of the lubricant molecules and the binding strength at the solid-liquid interface, thereby improving the lubrication performance. This fully demonstrates that the tribological characteristics of the lubrication system can be regulated by surface charges.

[0074] Example 3:

[0075] The present invention provides a method for regulating lubricant adsorption and friction based on molecular dynamics simulation. The specific process is as Figure 1 shown.

[0076] In this Example 3, the specific steps in S1, S2, S3, and S4 are basically the same as those in Example 1, except that:

[0077] Using non-polar poly-α-olefin as the lubricant and metallic iron as the friction pair. During the adsorption simulation, a smooth friction pair is used to clearly characterize the targeted adsorption behavior, and during the friction simulation, a rough friction pair is used to simulate the boundary lubrication state.

[0078] In step S1, use the drawing tool of the MS software to construct a 1-decene trimer C 30 H 62 lubricant single-molecule model, and randomly fill 200 poly-α-olefins through the Monte Carlo method of the AC module to construct a lubricant bulk-phase model with a size of . The remaining steps in S1 are exactly the same as those in Example 1, and the specific steps in S2, S3, and S4 are also exactly the same as those in Example 1, so they will not be elaborated here.

[0079] Calculate the atomic number density distribution of poly-α-olefin molecules along the Z direction to quantitatively characterize the targeted adsorption behavior, as Figure 8As shown. The atomic number density distributions of all systems exhibit the characteristics of "high peak value at the solid-liquid interface and low peak value inside the lubricant", which is attributed to the fact that poly-α-olefin molecules will preferentially adsorb on the friction pair interface under van der Waals and electrostatic interactions to form a stable physical adsorption film. Among them, there is a main peak and a secondary peak at the upper and lower friction pair interfaces in the zero-charge and negative-charge systems, respectively, and the intensity of the secondary peak in the negative-charge system is higher than that in the zero-charge system. This indicates that non-polar hydrocarbon chains preferentially form a dense and a loose monolayer adsorption film on the surface of metallic iron, and the presence of surface negative charges improves the compactness of the adsorption film, enhances the electrostatic attraction with positively charged hydrogen atoms, and thus induces the targeted adsorption behavior of lubricant molecules at the metal interface. However, due to poly-α-olefin being a non-polar molecule with positively charged hydrogen atoms and negatively charged carbon atoms, the targeted adsorption behavior is not obvious. Nevertheless, there is only one main peak in the atomic number density at the solid-liquid interface in the positive-charge system. This shows that the presence of surface positive charges enhances the electrostatic repulsion with positively charged hydrogen atoms, thereby inducing the loose adsorption behavior of lubricant molecules at the metal interface, and the adsorption binding strength at the solid-liquid interface can be regulated by surface charges.

[0080] The quantitative characterization results of the friction coefficient are as Figure 9 shown. The friction coefficient of the zero-charge system is 0.296, while that of the positive-charge system is 0.271, and that of the negative-charge system is 0.323. Compared with the zero-charge system, the presence of negative charges on the friction pair surface increases the friction coefficient, and the presence of positive charges on the friction pair surface decreases the friction coefficient. This is attributed to the fact that the hydrogen atoms of poly-α-olefin are positively charged and the carbon atoms are negatively charged. At this time, there is an attractive interaction with the negative-charge surface and a repulsive interaction with the positive-charge surface. The decrease in the attractive interaction weakens the targeted adsorption ability of lubricant molecules and the binding strength with the metal surface, while the increase in the attractive interaction enhances the targeted adsorption ability of lubricant molecules and the binding strength at the solid-liquid interface, thus indicating that the tribological properties of the lubrication system can be regulated by surface charges.

[0081] Example 4:

[0082] The present invention provides a method for regulating lubricant adsorption and friction based on molecular dynamics simulation, and the specific process is as Figure 1 shown.

[0083] In this Example 4, the specific steps in S1, S2, S3, and S4 are basically the same as those in Example 1, except that:

[0084] In this Example 4, non-polar poly-α-olefin is used as the lubricant and metallic copper is used as the friction pair. During the adsorption simulation, a smooth friction pair is used to clearly characterize the targeted adsorption behavior, and during the friction simulation, a rough friction pair is used to simulate the boundary lubrication state.

[0085] In step S1, the face-centered cubic lattice metal copper unit cell is imported through the open-source crystal structure database Materials Project, and the supercell function is used to expand the copper unit cell to construct a smooth surface model with a size of . The LAMMPS sine function code is used to construct a rough surface model with a size of . The remaining steps in S1 are exactly the same as those in Example 3, and the specific steps in S2, S3, and S4 are also exactly the same as those in Example 3, so they will not be elaborated here.

[0086] The atomic number density distribution of poly-α-olefin molecules along the Z direction is calculated to quantitatively characterize the targeted adsorption behavior, as shown in Figure 10 . The atomic number density distribution of all systems shows the characteristics of "high peak value at the solid-liquid interface and low peak value inside the lubricant", which is attributed to the fact that poly-α-olefin molecules will preferentially adsorb on the friction pair interface under van der Waals and electrostatic interactions to form a stable physical adsorption film. There is a main peak and a secondary peak at the upper and lower friction pair interfaces in all systems, as well as internal peak characteristics with decreasing intensity. The intensity of the secondary peak in the negatively charged system is the highest, and the intensity of the secondary peak in the positively charged system is the lowest. It shows that the non-polar hydrocarbon chain preferentially forms a dense and multi-layer loose adsorption film on the metal iron surface, and the presence of surface negative charges improves the compactness of the adsorption film, enhances the electrostatic attraction with positively charged hydrogen atoms, and then induces the targeted adsorption behavior of lubricant molecules at the metal interface. However, since poly-α-olefin is a non-polar molecule and hydrogen atoms are positively charged while carbon atoms are negatively charged, the targeted adsorption behavior is not obvious. The presence of surface positive charges enhances the electrostatic repulsion with positively charged hydrogen atoms, thereby inducing the loose adsorption behavior of lubricant molecules at the metal interface, and the adsorption binding strength at the solid-liquid interface can be regulated by surface charges. The quantitative characterization results of the friction coefficient are shown in Figure 11 . The friction coefficient of the zero-charge system is 0.290, while the friction coefficient of the positively charged system is 0.258, and the friction coefficient of the negatively charged system is 0.327. Compared with the zero-charge system, the presence of negative charges on the friction pair surface increases the friction coefficient, and the presence of positive charges on the friction pair surface decreases the friction coefficient. This is attributed to the fact that the hydrogen atoms of poly-α-olefin are positively charged while the carbon atoms are negatively charged. At this time, there is an attractive interaction with the negatively charged surface and a repulsive interaction with the positively charged surface. The decrease in the attractive interaction weakens the targeted adsorption ability of lubricant molecules and the binding strength with the metal surface, while the increase in the attractive interaction enhances the targeted adsorption ability of lubricant molecules and the binding strength at the solid-liquid interface, thus indicating that the tribological properties of the lubrication system can be regulated by surface charges.

[0087] It can be seen from Example 1, Example 2, Example 3, and Example 4 that for a lubricant adsorption and friction regulation method based on molecular dynamics simulation according to the present invention, a targeted adsorption strategy for the solid-liquid interface is established through the charge property relationship between the lubricant and the friction pair. The microscopic adsorption process of the lubricant towards the metal interface is reproduced through a quantitative characterization method. The controllable adjustment of friction characteristics is achieved through a matching mapping relationship, providing theoretical guidance for the design of a new lubrication system under extreme working conditions. The method of the present invention can enhance the bonding strength of the solid-liquid interface, observe the targeted adsorption process of lubricant molecules, and at the same time can reveal the structure-activity relationship of the lubrication system between the adsorption configuration and the friction characteristics, achieve the controllable adjustment of friction characteristics, reduce the human resource cost in actual experiments, and reduce the human operation error.

[0088] This application is carried out under the research and development background of the general project of the National Natural Science Foundation of China that has already been applied for. The specific project name is: Research on the Stable Superlubricity and Regulation Mechanism of the Coupling Friction Interface Characteristics of Solid Nanoparticles, and the project number is: 52475183.

[0089] The above is an exemplary description of the invention. Obviously, the specific implementation of the present invention is not limited by the above methods. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A method for lubricant adsorption and friction control based on molecular dynamics simulation, characterized in that: The following steps are involved: S1. Construct lubricant molecules and friction pair metal unit cell models, generate topological structure information based on the minimum structural unit, and construct a lubrication system model with a sandwich structure; S2, assigning force field parameters to the lubrication system model and performing geometric structure optimization and dynamic relaxation; S3, giving electric charges to the upper and lower friction pairs to simulate the targeted adsorption of lubricants; S4. Apply uniform contact pressure and constant sliding speed to the upper friction pair to simulate the friction of the lubrication system.

2. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S1, the lubricant molecules are one or more of polar or non-polar base oils or additives of carbon, hydrogen, oxygen, nitrogen, and phosphorus elements that are liquid at normal temperature and pressure.

3. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S1, the friction pair metal is one or more of iron, copper, aluminum and their alloy materials.

4. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S1, the lubrication system model of the sandwich structure is constructed by the following specific steps: S11. Construct a lubricant single molecule model through the visual operation interface of AuToFF or MS software, and construct a lubricant phase structure model through the AC module of MS or the programming algorithm of Moltemplate based on the topological structure information of the single molecule model; S12. Obtain the crystal structure information of metal atoms of the friction pair through the open source crystal structure database Materials Project, and use the supercell algorithm of MS software or the replicate algorithm of LAMMPS software to expand the cell to construct a smooth friction pair surface model, or use the LAMMPS code to construct a rough friction pair surface model; S13. By using the buildlayer algorithm of MS software or the merge script of Packmol software, the lubricating body phase structure model and the smooth friction pair surface model or the rough friction pair surface model are combined to form a lubrication system model with a sandwich structure.

5. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S2, the lubricant is described by force field parameters using CVFF or PCFF or OPLS, the metal friction pair is described by force field parameters using EAM or EAM / FS or EAM / ALLOY potential function, and the non-bonded interaction between different components of the lubrication system is described by force field parameters using LJ potential function of LB mixing rule.

6. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S2, geometry optimization and dynamic relaxation are performed, specifically: geometry optimization is achieved based on the conjugate gradient algorithm, 500 ps dynamic relaxation is completed through the NPT ensemble to make the lubrication system reach a structurally reasonable equilibrium state, and the Nosé-Hoover method is used to control the temperature of 300 K and one atmosphere to simulate a normal temperature and pressure environment.

7. The lubricant adsorption and friction control method based on molecular dynamics simulation according to claim 1, characterized in that: In step S3, the targeted adsorption simulation is specifically: S31. Based on the lubrication system model after relaxation equilibrium, all metal atoms in the upper and lower friction pairs are given equal positive or negative charges to construct solid-liquid electrostatic interaction, and the strength of the solid-liquid interaction is regulated by the amount of charge; S32, 500ps NVT ensemble relaxation to simulate the targeted adsorption behavior of lubricants to metal interfaces; S33. The targeted adsorption behavior of lubricants is quantitatively evaluated by the microstructure evolution and atomic number density distribution parameters during the simulation process.

8. The method for lubricant adsorption and friction control based on molecular dynamics simulation according to claim 1, characterized in that: In step S4, the friction simulation is specifically as follows: S41, fix the lower friction pair atom to be stationary, apply a uniformly distributed contact pressure along the normal direction to the upper friction pair atom, and relax for 500ps in the NVT ensemble to achieve loading compression; S42, a constant sliding velocity is applied to the upper friction pair atoms in the lateral direction, and the shear friction is achieved by relaxing for 500 ps in the NVT ensemble; S43. Calculate the normal force Fn and the tangential force Ff of the metal atoms in the upper friction pair during the friction process. Based on the Amonton-Coulomb law CoF=Ff / Fn, calculate the friction coefficient to characterize the tribological properties of the lubrication system.

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