Disclosure of Invention
The invention aims to provide an analysis method for the antifriction and antiwear performance of a nanoparticle additive in lubricating oil, which realizes shearing simulation under different temperatures, shearing speeds and pressures, observes the change of a system in the shearing process from an atomic scale, effectively analyzes the influence of the nanoparticle additive on the contact area and the surface abrasion of a friction pair under different conditions, and explores the antiwear behavior of nanoparticles and the low friction mechanism thereof.
In order to achieve the purpose, the technical scheme of the invention is as follows: an analysis method for the friction-reducing and wear-resisting properties of a nanoparticle additive in lubricating oil comprises the following steps:
step S1, establishing a molecular dynamics simulation model: constructing a rough wall model of crystal orientation [100] metallic iron by using LAMMPS software; respectively constructing a lubricating oil model and a nano-particle model through Materials Studio software; combining the wall model, the lubricating oil model and the nano-particle model into a boundary lubricating model containing nano-particles;
s2, selecting a potential function, utilizing LAMMPS software to perform shearing simulation on the boundary lubrication model containing the nano particles, obtaining a simulation result, and storing the result in an output file;
and S3, carrying out quantitative calculation and visual expression on the simulation result, obtaining the change rule of the friction factor and the wall stress at different moments according to the change of the positive pressure and the friction force in the shearing process, and analyzing the friction reduction and wear resistance mechanism of the nanoparticle additive at different temperatures, pressures and shearing speeds.
In an embodiment of the present invention, the step S1 specifically includes:
step S11, constructing a rough wall model of the metallic iron by using LAMMPS software, and selecting alpha-Fe with a lattice constant of
Crystal orientation of [100]](ii) a Selecting nano copper particles as an additive with a lattice constant of
The base oil is n-hexadecane; constructing a lubricating oil model and a nano-particle model by using an Amorphous Cell module in Materials Studio software, and optimizing the space structure of the constructed model by using a Forcite module;
s12, establishing a lubrication model of an upper wall surface, an oil film and a lower wall surface, and combining the wall surface model, the lubrication model and the nanoparticle model into a boundary lubrication model containing nanoparticles by using a Build module in Materials Studio software; the upper wall surface and the lower wall surface are divided into 6 layers in total, wherein the outer layer is a rigid layer for applying boundary conditions, the middle layer is a constant temperature layer for providing environmental influence factors, and the inner layer is a free deformation layer for extracting mechanical properties;
step S13, setting boundary conditions of a boundary lubrication model containing nano particles, setting periodic boundary conditions in the x direction and the y direction, and setting contractive boundary conditions in the z direction;
and S14, selecting a potential function, wherein the lubricating oil molecules adopt a joint atomic model, the interaction between the molecules adopts a joint atomic force field TrAPE-UA, the interaction between iron atoms adopts an Embedded Atom Method potential, and the interaction of a solid-liquid interface comprises the interaction between the iron atoms and copper atoms, the interaction between the iron atoms and the lubricating oil molecules and the interaction between the copper atoms and the lubricating oil molecules adopt a Lennard-Jones potential.
In one embodiment of the invention, the force field parameters of the combined atomic force field TrAPE-UA used are determined by quantum computation, or by using the disclosed and applicable force field parameters.
In one embodiment of the invention, a model constructed by Materials Studio software is converted into a data file which can be identified by LAMMPS through a self-contained msi2lmp tool of the LAMMPS.
In an embodiment of the present invention, the step S2 specifically includes:
s21, relaxing the system under the conditions of regular ensemble and micro-regular ensemble, relaxing the system by using a Nose-Hoover hot bath method to enable an initial model of the system to reach an equilibrium state, simulating the density of the system, and verifying the rationality of a boundary lubrication model containing nano particles;
step S22, in the pressurizing stage, the regular ensemble during relaxation is removed, the temperature of the constant temperature layer is set, the lower wall surface rigid layer is fixed, and the pressure intensity is applied to the upper wall surface rigid layer, so that the system reaches a stable state, and the lubricating conditions under different temperatures and different pressure intensities are simulated;
step S23, in the shearing stage, keeping the pressure unchanged, and simultaneously enabling the two rigid layers to move at the same speed and in opposite directions along the x axis;
and step S24, data processing, namely performing molecular dynamics simulation calculation on the program file by using LAMMPS software, counting the calculation result to obtain related data of the simulation process and the calculation result, and outputting a file, wherein the output file comprises variable parameters and change information of atomic coordinates.
In an embodiment of the invention, the time step of the molecular dynamics simulation has an important influence on both the simulation process and the simulation result, and it should be ensured that the selected step not only can improve the calculation efficiency, but also can obtain effective analysis data.
In one embodiment of the present invention, the shearing movement distance of the shearing stage should be sufficient to ensure the sufficient interaction between the nanoparticles and the two nano-roughness peaks.
In one embodiment of the present invention, to improve computational efficiency, a joint atomic model is applied to the molecules of the lubricant, and the interaction between the molecules uses a joint atomic force field (TraPPE-UA). In the TraPPE-UA force field, the non-bonding interactions between atoms on different molecules and atoms on the same molecule spaced more than three atoms adopt the L-J potential:
in the formula, E
nonbondIs the L-J potential between atoms, i represents the ith atom, J represents the jth atom, ε
ijAs a characteristic value of energy, σ
ijIs the characteristic length of the molecule, q is the atomic charge, r
ijIs the distance between atoms.
In one embodiment of the invention, σ is calculated by Lorentz-Bertholt binding rule
ijAnd ε
ijThe calculation formulas are respectively
And
in an embodiment of the present invention, the step S3 specifically includes: the method comprises the steps of visually expressing an output data file by adopting open source software OVITO, carrying out data processing on the output file by using Origin software, obtaining change rules of friction factors and wall stress at different moments according to changes of positive pressure and friction force in a shearing process, and analyzing the friction-reducing and wear-resisting mechanisms of the nanoparticle additive under different conditions.
Compared with the prior art, the invention has the following beneficial effects: the method overcomes the defect that the existing physical test cannot carry out on-line measurement, and quantitatively predicts the oil film thickness, density, stress and the like of the nano-gap lubricant from the atomic scale; the friction-reducing and wear-resisting mechanism of the nano-particle additive is explored. The method is simple, convenient and flexible, provides reliable basis for the practical application of the nano-particle additive, and has wide application prospect and guidance value.
Detailed Description
The technical scheme of the invention is specifically explained below with reference to the accompanying drawings.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
The invention relates to a method for analyzing the antifriction and antiwear performances of a nanoparticle additive in lubricating oil, which comprises the following steps:
step S1, establishing a molecular dynamics simulation model: constructing a rough wall model of crystal orientation [100] metallic iron by using LAMMPS software; respectively constructing a lubricating oil model and a nano-particle model through Materials Studio software; combining the wall model, the lubricating oil model and the nano-particle model into a boundary lubricating model containing nano-particles;
s2, selecting a potential function, utilizing LAMMPS software to perform shearing simulation on the boundary lubrication model containing the nano particles, obtaining a simulation result, and storing the result in an output file;
and S3, carrying out quantitative calculation and visual expression on the simulation result, obtaining the change rule of the friction factor and the wall stress at different moments according to the change of the positive pressure and the friction force in the shearing process, and analyzing the friction reduction and wear resistance mechanism of the nanoparticle additive at different temperatures, pressures and shearing speeds.
The following are specific embodiments of the present invention.
As shown in fig. 1, in this example, an analysis method for an antifriction and antiwear mechanism of a nanoparticle additive based on molecular dynamics simulated shear is provided, in this example, an external load is set to be 100Mpa, a wall surface temperature is set to be 300K, and a shear rate is set to be 5m/s, so as to simulate an actual working condition, and the specific steps are as follows:
step S1, establishing a molecular dynamics simulation model, and constructing a crystal orientation [100] by using LAMMPS software]A rough wall surface model of metal iron, the size of the model is
The number of iron atoms on the upper wall surface and the lower wall surface is 27260; constructing a lubricating oil model and a nano-particle model by using Materials Studio software, wherein the number of base oil molecules is 730, and the number of nano-particles is 1; the wall model, the lubricant model and the nanoparticle model were combined into a boundary lubrication model containing nanoparticles, as shown in fig. 2.
And step S2, selecting a proper potential function, utilizing LAMMPS software to perform shearing simulation on the boundary lubrication model containing the nano particles, obtaining a simulation result, and storing the result in an output file.
And S3, carrying out quantitative calculation and visual expression on the simulation result, obtaining the change rule of the friction factor and the wall stress at different moments according to the change of the positive pressure and the friction force in the shearing process, and analyzing the antifriction and antiwear mechanism of the nanoparticle additive under different conditions.
In this embodiment, in step S1, a lubricating oil model and a nanoparticle model are constructed using a Materials Studio Cell module, and a spatial structure of the constructed model is optimized by a fortite module.
In this embodiment, in step S1, the wall model, the lubricant model, and the nanoparticle model are combined into a boundary lubrication model containing nanoparticles by using a Build module in Materials Studio software, and the upper and lower walls are divided into 6 layers: the outer layer is a rigid layer for applying boundary conditions, the middle layer is a constant temperature layer for providing environmental influencing factors, and the inner layer is a free deformation layer for extracting mechanical properties.
In this example, in step S1, a suitable potential function is selected, the joint atomic model is used for the lubricant molecules, the joint atomic force field (TraPPE-UA) is used for the interaction between the molecules, the Embedded Atom Method (EAM) potential is used for the interaction between the iron atoms, and the Lennard-Jones (L-J) potential is used for the interaction between the solid-liquid interface including the interaction between the iron atoms and the copper atoms, the iron atoms and the lubricant molecules, and the copper atoms and the lubricant molecules.
In this embodiment, in the LAMMPS simulation process, 2fs is selected for reducing the simulation time, so that the calculation efficiency is improved, and effective analysis data can be obtained.
Furthermore, a model constructed by Materials Studio software is converted into a data file which can be identified by the LAMMPS through a msi2lmp tool carried by the LAMMPS.
In the present embodiment, in step S2, the system is relaxed under the conditions of regular ensemble and micro-regular ensemble, the system is relaxed by 200ps under the condition of temperature 300K by using the Nose-Hoover hot bath method, and the temperature damping coefficient is set to 200 fs.
In this embodiment, in step S2, after the system is sufficiently relaxed, the regular ensemble during relaxation is released, the temperature of the constant temperature layer is set to 300K, the lower wall rigid layer is fixed, the pressure of 100MPa is applied to the upper wall rigid layer, and the pressurizing time is 600ps, so that the system is in a stable state.
In this embodiment, the pressure during the pressing phase is maintained constant while the two rigid layers are moved in the positive and negative directions along the x-axis at a speed of 5m/S, respectively, and the shearing distance is taken in step S2
In the present embodiment, the friction factor is calculated by the formula FL=μ(L0+FN)=F0+μ·FNIn the formula: fLIs friction force, L0As a friction offset, FNIs a positive pressure, FL=μ(L0+FN)=F0+μ·FNIs the friction factor.
In the embodiment, the positive pressure and the friction force of the lubricating system without the added nanoparticles mainly consist of two parts, namely the acting force of a lubricating oil film on a free deformation layer and the acting force between the two free deformation layers when a rough peak is contacted, and the acting force of the nanoparticles on the free deformation layer also exists in the lubricating system with the added nanoparticles.
And (3) performing molecular dynamics simulation calculation on the self-written program file by utilizing LAMMPS software, counting the calculation result, and obtaining an output log file and a dump file of the related data of the simulation process and the calculation result.
And performing data processing on the output log file, and performing visual expression on the output dump file through open source software OVITO software, as shown in FIGS. 3 and 4. It can be seen from fig. 3 that the lubricating system added with nanoparticles has rough peak contact, the lubricating oil film is broken, the iron atoms near the contact surface have lattice distortion, and the friction force is rapidly increased, while for the lubricating system added with nanoparticles, the increase of the friction force is reduced, and the bearing capacity is increased; from fig. 4, it can be obtained that the friction factor of the lubricant to which the nanoparticles are added is 0.137.
The change rule of the Von Mises stress and the surface wear amount of the contact area between the friction pairs is visually expressed by open source software OVITO software, as shown in fig. 5 and 6. As can be seen from FIG. 6, the maximum stress of the wall surface of the lubrication system with the added nanoparticles is 16.89GPa, which is much smaller than the maximum stress of 26.12GPa without the nanoparticles.
In conclusion, the friction-reducing and wear-resisting properties of the nanoparticle additive in the lubricating oil are analyzed through molecular dynamics, and oil film lubrication, oil film fracture, friction force and stress, nanoparticle fracture and nano-roughness peak damage of a lubricating system during shearing under a load condition can be calculated and observed from an atomic scale.
The above are preferred embodiments of the present invention, and all changes made according to the technical scheme of the present invention that produce functional effects do not exceed the scope of the technical scheme of the present invention belong to the protection scope of the present invention.