Modeling method for generating rough surface and coating film for molecular dynamics simulation

Abstract

This invention discloses a molecular dynamics simulation method for generating and coating rough surfaces. The model is a cuboid structure consisting of a substrate and a lubricating film. The substrate surface is a three-dimensional random rough surface, and the lubricating film is attached to the substrate surface. The main steps include: Step 1: Outputting a model file with a three-dimensional random rough surface; Step 2: Establishing a molecular dynamics model containing the random rough surface and the lubricating film; Step 3: Pressing down with an indenter to attach the lubricating film to the substrate surface; Step 4: Outputting the atomic information of the substrate and the lubricating film to establish the target model. Compared with traditional molecular dynamics models, this invention is more realistic, simpler to operate, and lays the foundation for studying the friction and wear mechanisms of rough surfaces at the nanoscale.

Description

Technical Field

[0001] This invention belongs to the field of molecular dynamics technology, specifically relating to a modeling method for generating rough surfaces and coating them with films. Background Technology

[0002] Molecular dynamics simulation is a commonly used numerical simulation method in fields such as tribology. LAMMPS software is a frequently used tool for molecular dynamics simulations, effectively simulating the mechanical properties of relative motion between atoms in a model. In tribological research, common molecular dynamics models mainly consist of an indenter and a matrix. The matrix is ​​typically a cuboid structure with an ideally smooth surface. A few models have regularly rough or two-dimensionally randomly rough surfaces. However, in actual production, it's impossible to produce such ideal surfaces; instead, three-dimensional randomly rough surfaces with a certain degree of roughness are required. Therefore, it's necessary to study the various mechanical properties of a matrix surface with three-dimensional randomly roughness. Applying a lubricating film to the surface of a part is a common method to reduce friction and wear in relatively moving mechanisms. In actual production, lubricating films are mainly applied to the matrix surface through sputtering, spraying, etc. However, existing molecular dynamics models rarely apply lubricating films to three-dimensional randomly rough surfaces. Therefore, it's necessary to study the friction and wear problem when a lubricating film is applied to a three-dimensional randomly rough surface; that is, it's necessary to establish a molecular dynamics model based on the above practical problems for subsequent research.

[0003] Existing tribological studies based on molecular dynamics primarily simplify the substrate surface as a smooth surface. A few models further simplify it to a three-dimensional, regularly rough surface or a two-dimensional, randomly rough surface. For example, the article "Lubricity of graphene onrough Au surfaces," published in the *Journal of Physics D: Applied Physics*, Volume 51, Issue 43, 2018, established a model consisting of graphene, a substrate, and an indenter. The substrate surface was a two-dimensional, randomly rough surface with graphene attached to it. The study investigated the lubrication performance of graphene attached to this surface. However, under actual working conditions, the substrate surface should be a three-dimensional, randomly rough surface. To overcome these shortcomings, this invention establishes a cuboid model consisting of a substrate and a lubricating film. The substrate surface is a three-dimensional, randomly rough surface with the lubricating film attached to it, laying the foundation for studying the friction and wear mechanisms of rough surfaces at the nanoscale. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a molecular dynamics simulation method for generating and coating rough surfaces. The model is a cuboid structure composed of a substrate and a lubricating film. The substrate surface is a three-dimensional random rough surface, and the lubricating film is attached to the substrate surface. The main steps include: Step 1: Outputting a model file with a three-dimensional random rough surface; Step 2: Establishing a molecular dynamics model containing the random rough surface and the lubricating film; Step 3: Pressing down the indenter to attach the lubricating film to the substrate surface; Step 4: Outputting the atomic information of the substrate and the lubricating film to establish the target model. Compared with traditional molecular dynamics models, this invention is more realistic, simpler to operate, and lays the foundation for studying the friction and wear mechanisms of rough surfaces at the nanoscale.

[0005] The technical solution adopted by this invention to solve its technical problem includes the following steps:

[0006] Step 1: Output a model file with a 3D randomly rough surface;

[0007] Step 1-1: Use the Gaussian random surface generation program to generate a set of random points containing coordinate information. Let A be the set of random points, where the Z coordinate of the points in set A is a complex number.

[0008] Step 1-2: Take the real part of the Z-coordinate of each point in set A to obtain the set of pure real parts B1. Calculate set B according to the following formula (1):

[0009] z b (x i ,y j )=h0+k(z b1 (x i ,y j )-z b0 (1)

[0010] In the formula, h0 is the initial reference for establishing the rough surface model; k is the magnification factor; z b1 (x i ,y j ) is the midpoint (x) of set B1 i ,y j The corresponding Z-coordinate value; z b0 The average of the Z-coordinates of all points in set B1; z b (x i ,y j (x) is the midpoint of set B. i ,y j The corresponding Z-coordinate value; x i and y jLet i = 1, 2, ..., m, j = 1, 2, ..., n, where m and n represent the number of atoms in set B1 in the X and Y directions, respectively.

[0011] Steps 1-3: Calculate the three-dimensional coordinates of each atom in the rough surface model based on the atomic arrangement rules; let the Z-coordinate of any atom in the rough surface model be z(x). i ,y j ); Compare the Z-coordinate value of each atom with the Z-coordinate value of the corresponding point in set B. If z(x) i ,y j )≤z b (x i ,y j If z(x) is true, then the atom number, atom type, and three-dimensional coordinates of that atom will be output to the file "initialSubstrate.txt". Conversely, if z(x) is false, then the atom number, atom type, and three-dimensional coordinates of that atom will be output to the file "initialSubstrate.txt". i ,y j )>z b (x i ,y j If the atom is selected, its atomic number, atomic type, and three-dimensional coordinates will be output to the file "initialIndenter.txt".

[0012] Steps 1-4: Using the least squares method, calculate the datum surface equation of the model corresponding to the file "initialSubstrate.txt", and then calculate the surface roughness value of the model. If the roughness value does not meet the set modeling requirements, return to step 1-1 and start modeling again.

[0013] Step 2: Establish a molecular dynamics model containing a random rough surface and a lubricating film;

[0014] Write the infile required to run the LAMMPS software. Read the atomic information from the file "initialIndenter.txt" into the infile, set the corresponding molecular dynamics model as the indenter, and move these atoms upwards by a distance d. Read the atomic information from the file "initialSubstrate.txt" into the infile, set the corresponding molecular dynamics model as the matrix, and establish a lubricating film between the indenter and the matrix. Both the matrix and the indenter consist of a fixed layer, a thermostatic layer, and a Newtonian layer. The lubricating film is set as a separate group.

[0015] Step 3: Press down the pressure head to allow the lubricating film to adhere to the substrate surface;

[0016] Step 3-1: Set the boundary conditions of the simulation region, the interatomic interaction potential function, the initial velocity of the atoms, the system temperature, and the integration step size; after setting, the model first relaxes under the canonical ensemble NVT, and then switches to relaxation under the microcanonical ensemble NVE.

[0017] Step 3-2: Press down the pressure head, with a pressing distance of d1.

[0018] d1 = dh f -c1, d1>0 (2)

[0019] In the formula, d represents the distance between the indenter and the substrate; h f This indicates the thickness of the lubricating film; c1 is a constant introduced to ensure that the substrate is not affected during the pressing process of the indenter.

[0020] Step 3-3: After the pressure head is pressed down, relax the model under the microcanonical ensemble NVE for the preset time length;

[0021] Step 4: Output the atomic information of the matrix and lubricating film to establish the target model;

[0022] Step 4-1: Output the atomic information of the matrix and the lubricating film respectively. The atomic information includes the atomic number, atomic type and three-dimensional coordinates of the atom. Let the file containing only the atomic information of the matrix be "finalSubstrate.txt" and the file containing only the atomic information of the lubricating film be "finalFilm.txt". Reorder the atomic numbers in the output information files to ensure that the atomic numbers are continuous. The structure of the output file is the same as that of "initialSubstrate.txt".

[0023] Step 4-2: Write the final in file required to run the LAMMPS software. Read the atomic information contained in the file "finalSubstrate.txt" into the final required in file, and set the corresponding model as the matrix. Read the atomic information contained in the file "finalFilm.txt" into the final required in file, and set the corresponding model as the lubricating film. This completes the model establishment of the lubricating film attached to the three-dimensional random rough surface for molecular dynamics simulation.

[0024] Preferably, the output file in steps 1-3 consists of a part recording model information and a part recording information of each atom: The first part, which records model information, has a blank line in the first line or content starting with "#", the second line records the total number of atoms in the model, the third line records the total number of atom types in the model, and the fourth to sixth lines record the dimensions of the model in the X, Y, and Z directions, respectively; The second part, which records information of each atom, has "Atoms" in the first line, and after a blank line, each line records the atom information of one atom. The information in each line is in the following order: atom number, atom type, atom X coordinate, atom Y coordinate, and atom Z coordinate. Each data is separated by a space. No blank line is required between the first part and the second part.

[0025] Preferably, in steps 1-4, the least squares method is used to calculate the reference surface equation of the model corresponding to the file "initialSubstrate.txt", and then the surface roughness value of the established model is calculated as follows:

[0026] Let set C be the set of each (x) in the file "initialSubstrate.txt". i ,y j The set of points whose Z-coordinate values ​​are the largest, and let the equation of the datum plane be:

[0027] f(x,y)=a+bc+cy (3)

[0028] The formula for calculating the three unknowns a, b, and c in the equation of the datum surface is derived as follows:

[0029]

[0030] In the formula z c (x i ,y j ) represents the midpoint (x) of set C. i ,y j The corresponding Z-coordinate value, Let X, Y, and Z represent the average values ​​of the coordinates of all points in set C in the X, Y, and Z directions, respectively.

[0031] Calculate the roughness parameter of a three-dimensional random rough surface—the surface arithmetic mean deviation S. a :

[0032]

[0033] Preferably, the value of c1 is in the range of 0.2-0.3 nm.

[0034] The beneficial effects of this invention are as follows:

[0035] Compared with traditional molecular dynamics modeling methods, this invention better reflects actual working conditions and improves the accuracy of simulation results. Under actual working conditions, the surfaces of relatively moving parts cannot be smooth, ideally rough, or two-dimensional randomly rough. However, most existing simulation methods focus on simplifying the model surface to these forms, rarely establishing it as a three-dimensional randomly rough surface. Therefore, this modeling method is more realistic. Considering that relatively moving parts often undergo lubrication measures during operation (such as applying a solid lubricant to the surface), this invention, taking these factors into account, establishes a final model where the lubricating film is attached to a substrate with a three-dimensional randomly rough surface, which matches the actual situation and lays the foundation for subsequent research. Attached Figure Description

[0036] Figure 1 This is a flowchart of the present invention.

[0037] Figure 2 This is a flowchart of step 1 of the present invention.

[0038] Figure 3 This is the file structure of "initialSubstrate.txt" in an embodiment of the present invention.

[0039] Figure 4 This is the molecular dynamics model established in the embodiments of the present invention.

[0040] Figure 5 This is a flowchart of the model motion process in step 3 of the present invention.

[0041] Figure 6 This is the matrix model extracted in an embodiment of the present invention.

[0042] Figure 7 This is a lubricating film model extracted from an embodiment of the present invention.

[0043] Figure 8 This is the final molecular dynamics model established for the embodiments of the present invention. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0045] Existing tribological studies based on molecular dynamics mainly simplify the substrate surface as a smooth surface. A few models simplify the substrate surface as a three-dimensional regular rough surface or a two-dimensional random rough surface. However, in actual production, the substrate surface should be a three-dimensional random rough surface. At the same time, considering that attaching a lubricating film to the substrate surface is a common way to reduce friction and wear of relatively moving parts, and that existing molecular dynamics models rarely attach a lubricating film to a three-dimensional random rough surface, this invention proposes a modeling method for generating a rough surface and coating it with a film for molecular dynamics simulation. This method can establish a molecular dynamics model with a three-dimensional random rough surface as the substrate surface and a lubricating film attached to the substrate surface for subsequent calculations.

[0046] A molecular dynamics simulation method for generating rough surfaces and coating them includes the following steps:

[0047] Step 1: Output a model file with a 3D randomly rough surface;

[0048] Step 1-1: Use the Gaussian random surface generation program to generate a set of random points containing coordinate information. Let A be the set of random points, where the Z coordinate of the points in set A is a complex number.

[0049] Step 1-2: Take the real part of the Z-coordinate of each point in set A to obtain the set of pure real parts B1. Calculate set B according to the following formula (1):

[0050] z b (x i ,y j )=h0+k(z b1 (x i ,y j )-z b0 (1)

[0051] In the formula, h0 is the initial reference for establishing the rough surface model; k is the magnification factor; z b1 (x i ,y j ) is the midpoint (x) of set B1 i ,y j The corresponding Z-coordinate value; z b0 The average of the Z-coordinates of all points in set B1; z b (x i ,y j (x) is the midpoint of set B. i ,y j The corresponding Z-coordinate value; x i and y j Let i = 1, 2, ..., m, j = 1, 2, ..., n, where m and n represent the number of atoms in set B1 in the X and Y directions, respectively.

[0052] Steps 1-3: Calculate the three-dimensional coordinates of each atom in the rough surface model based on the atomic arrangement rules; let the Z-coordinate of any atom in the rough surface model be z(x). i ,y j ); Compare the Z-coordinate value of each atom with the Z-coordinate value of the corresponding point in set B. If z(x) i ,y j )≤z b (x i ,y j If z(x) is true, then the atom number, atom type, and three-dimensional coordinates of that atom will be output to the file "initialSubstrate.txt". Conversely, if z(x) is false, then the atom number, atom type, and three-dimensional coordinates of that atom will be output to the file "initialSubstrate.txt". i ,y j )>z b (x i ,y j If the atom is selected, its atomic number, atomic type, and three-dimensional coordinates will be output to the file "initialIndenter.txt".

[0053] The output file consists of a section recording model information and a section recording information about each atom: The first part, which records model information, begins with a blank line or content starting with "#", the second line records the total number of atoms in the model, the third line records the total number of atom types in the model, and the fourth to sixth lines record the dimensions of the model in the X, Y, and Z directions, respectively; The second part, which records information about each atom, begins with "Atoms", followed by a blank line, and each subsequent line records the information for one atom. The information in each line is in the following order: atom number, atom type, atom X coordinate, atom Y coordinate, and atom Z coordinate. Each data point is separated by a space. No blank line is needed between the first and second parts.

[0054] Steps 1-4: Using the least squares method, calculate the datum surface equation of the model corresponding to the file "initialSubstrate.txt", and then calculate the surface roughness value of the established model.

[0055] Let set C be the set of each (x) in the file "initialSubstrate.txt". i ,y j The set of points whose Z-coordinate values ​​are the largest, and let the equation of the datum plane be:

[0056] f(x,y)=a+bx+cy (3)

[0057] The formula for calculating the three unknowns a, b, and c in the equation of the datum surface is derived as follows:

[0058]

[0059] In the formula z c (x i ,y j ) represents the midpoint (x) of set C. i ,y j The corresponding Z-coordinate value, Let X, Y, and Z represent the average values ​​of the coordinates of all points in set C in the X, Y, and Z directions, respectively.

[0060] Calculate the roughness parameter of a three-dimensional random rough surface—the surface arithmetic mean deviation S. a :

[0061]

[0062] If the roughness value does not meet the set modeling requirements, return to step 1-1 and start modeling again;

[0063] Step 2: Establish a molecular dynamics model containing a random rough surface and a lubricating film;

[0064] Write the infile required to run the LAMMPS software. Read the atomic information from the file "initialIndenter.txt" into the infile, set the corresponding molecular dynamics model as the indenter, and move these atoms upwards by a distance d. Read the atomic information from the file "initialSubstrate.txt" into the infile, set the corresponding molecular dynamics model as the matrix, and establish a lubricating film between the indenter and the matrix. Both the matrix and the indenter consist of a fixed layer, a thermostatic layer, and a Newtonian layer. The lubricating film is set as a separate group.

[0065] Step 3: Press down the pressure head to allow the lubricating film to adhere to the substrate surface;

[0066] Step 3-1: Set periodic boundary conditions in the X and Y directions of the simulation region to reduce size effects. Obtain the interactions between atoms through the interaction potential functions between different element atoms in the system. Set the initial velocities of all atoms in the system according to the Maxwell-Boltzmann energy distribution function, which corresponds to the temperature of their respective equilibrium states. Set the system temperature to 300K and the integration step size to 2fs (1fs = 1e(-15)s). After completing the above settings, under the canonical ensemble (NVT), calculate the potential energy, position, and velocity of each atom in the system according to the initial position and initial velocity of each atom in the system based on the selected potential function. Apply equilibrium constraints to the pressure head and matrix atoms to make the initial model of the system reach an equilibrium state. Then, under the microcanonical ensemble (NVE), perform internal equilibrium evolution of the system to make the system reach an equilibrium state.

[0067] Step 3-2: Press down the pressure head, with a pressing distance of d1.

[0068] d1 = dh f -c1, d1>0 (2)

[0069] In the formula, d represents the distance between the indenter and the substrate; h f This indicates the thickness of the lubricating film; c1 is a constant introduced to ensure that the substrate is not affected during the pressing process of the indenter; the value of c1 ranges from 0.2 to 0.3 nm.

[0070] Step 3-3: After the indenter is pressed down, relax the model under the microcanonical ensemble NVE for a period of time;

[0071] Step 4: Output the atomic information of the matrix and lubricating film to establish the target model;

[0072] Step 4-1: Output the atomic information of the matrix and the lubricating film respectively. The atomic information includes the atomic number, atomic type and three-dimensional coordinates of the atom. Let the file containing only the atomic information of the matrix be "finalSubstrate.txt" and the file containing only the atomic information of the lubricating film be "finalFilm.txt". Reorder the atomic numbers in the output information files to ensure that the atomic numbers are continuous. The structure of the output file is the same as that of "initialSubstrate.txt".

[0073] Step 4-2: Write the final in file required to run the LAMMPS software. Read the atomic information contained in the file "finalSubstrate.txt" into the final required in file, and set the corresponding model as the matrix. Read the atomic information contained in the file "finalFilm.txt" into the final required in file, and set the corresponding model as the lubricating film. This completes the model establishment of the lubricating film attached to the three-dimensional random rough surface for molecular dynamics simulation. Specific implementation examples:

[0075] See Figure 1 As shown in the figure, this embodiment provides a modeling method for generating rough surfaces and coating them for molecular dynamics simulations. The specific steps are as follows:

[0076] Step 1: Output a model file with a 3D randomly rough surface;

[0077] See Figure 2 As shown, this step includes the following sub-steps:

[0078] Step 1: Use the Gaussian random surface generation program to generate a set of random points containing coordinate information. Let A be the set of random points, where the Z coordinates of the points in set A are complex numbers.

[0079] Step 2: Take the real part of the Z coordinate of each point in set A, and let the set of pure real parts be B1. Calculate set B according to the following formula.

[0080] z b (x i ,y j )=h0+k(z b1 (x i ,y j )-z b0 )

[0081] In the formula, h0 is the initial reference for establishing the rough surface model, which depends on the size of the rough surface model in the Z direction; k is the magnification factor; z b1 (x i ,y j ) is the midpoint (x) of set B1 i ,y j The corresponding Z-coordinate value; z b0 The average of the Z-coordinates of all points in set B1; z b (x i ,y j (x) is the midpoint of set B. i ,y j The corresponding Z-coordinate value; x i and y j Let i = 1, 2, ..., m, j = 1, 2, ..., n, where m and n represent the number of atoms in set B1 in the X and Y directions, respectively, depending on the size of the rough surface model in the X and Y directions.

[0082] Step 3: Calculate the three-dimensional coordinates of each atom in the complete model based on the atomic arrangement rules. Let the Z-coordinate of any atom in the complete model be z(x). i ,y j ), where x i and y j Let i and j represent the X and Y coordinates of the atom, respectively. The values ​​of i and j have the same signs as the corresponding points in set B. Compare the Z coordinate value of each atom with the Z coordinate value of the corresponding point in set B. If z(x) i ,y j )≤z b (x i ,y jIf the atomic number, atomic type, and three-dimensional coordinates of the atom are output to the file "initialSubstrate.txt", otherwise the corresponding information of the atom will be output to the file "initialIndenter.txt". The output file consists of a part that records model information and a part that records information of each atom.

[0083] Step 4: Using the least squares method, calculate the datum surface equation of the model corresponding to the file "initialSubstrate.txt", and then calculate the surface roughness value of the constructed model. Let set C be the set of each (x) in the file "initialSubstrate.txt". i ,y j The set of points whose Z-coordinate values ​​are the largest, and let the equation of the datum plane be:

[0084] f(x,y)=a+bx+cy

[0085] The formula for calculating the three unknowns a, b, and c in the equation of the datum surface is derived as follows:

[0086]

[0087] In the formula z c (x i ,y j ) represents the midpoint (x) of set C. i ,y j The corresponding Z-coordinate value, Let i, j, m, and n represent the average values ​​of the coordinates of all points in set C in the X, Y, and Z directions, respectively. The values ​​of i, j, m, and n are exactly the same as those in set B.

[0088] Substituting the coordinates of each point in set C into the derived formula, the specific expression for the datum surface equation is obtained. The datum surface equation of the three-dimensional random rough surface generated from the file "initialSubstrate.txt" is calculated as follows:

[0089] f(x,y)=103.1111-0.0194x+0y

[0090] Calculate the roughness parameter of a three-dimensional random rough surface—the surface arithmetic mean deviation S. a .

[0091]

[0092] In this embodiment, the roughness value obtained through calculation is If the roughness value does not meet the modeling requirements, return to step 1 to remodel.

[0093] See Figure 3 As shown, taking the file "initialSubstrate.txt" as an example, the output file of step 3 in step one consists of a part recording model information and a part recording information of each atom. The first part, which records model information, has a blank line in the first line or content starting with "#", a line recording the total number of atoms in the model, a line recording the total number of atom types in the model, and lines four to six recording the dimensions of the model in the X, Y, and Z directions, respectively. The second part, which records information of each atom, has "Atoms" in the first line, followed by a blank line, and each subsequent line records the information of one atom. The information in each line is in the following order: atom number, atom type, atom X coordinate, atom Y coordinate, and atom Z coordinate. Each data is separated by a space. There is no blank line between the second part and the first part.

[0094] Step two involves establishing a molecular dynamics model containing a randomly rough surface and a lubricating film. The method for this step is as follows:

[0095] Write the .in file required to run the LAMMPS software. Read the atomic information from the file "initialIndenter.txt" into the .in file, set the corresponding molecular dynamics model as the pressure head, and move these atoms upwards by a distance 'd'. Read the atomic information from the file "initialSubstrate.txt" into the .in file, set the corresponding molecular dynamics model as the matrix, and establish a lubricating film between the pressure head and the matrix. For example... Figure 4 As shown, the model is grouped. Both the substrate and the indenter consist of a fixed layer, a thermostatic layer, and a Newtonian layer. Atoms within one unit cell of the bottom of the substrate and the top of the indenter are designated as the fixed layer. Atoms within one unit cell of the fixed layer above the substrate and below the fixed layer of the indenter are designated as thermostatic layer atoms to absorb heat generated during movement. The remaining atoms of the substrate and the indenter are designated as Newtonian layers, where the atoms in both the thermostatic layer and the Newtonian layer obey Newton's second law. The lubricating film is designated as a separate group. In this embodiment, the substrate contains 161,618 atoms, the indenter contains 97,582 atoms, and the lubricating film contains 9,282 atoms.

[0096] See Figure 5 Step three involves pressing down the pressure head to allow the lubricating film to adhere to the substrate surface. This step includes the following sub-steps:

[0097] Step 1: Set periodic boundary conditions in the X and Y directions of the simulation region to reduce size effects. Obtain the interactions between atoms through the interaction potential functions between different element atoms in the system. In this embodiment, three types of atoms are included: gold (Au), sulfur (S), and molybdenum (Mo). The matrix and indenter are both composed of gold atoms, and the lubricating film is molybdenum disulfide, composed of sulfur and molybdenum atoms. The EAM potential is used between gold atoms in the matrix and gold atoms in the indenter. The Morse potential is used between gold atoms in the matrix and gold atoms in the indenter. The REBO potential is used between sulfur atoms and molybdenum atoms in the lubricating film. The LJ potential is used between gold atoms in the indenter and matrix and sulfur and molybdenum atoms in molybdenum disulfide. The initial velocities of all atoms in the system are set according to the Maxwell-Boltzmann energy distribution function, which corresponds to the temperature of their respective equilibrium states. The system temperature is set to 300K, and the integration step size is selected as 2fs. Under the canonical ensemble (NVT), based on the initial position and initial velocity of each atom in the system, the potential energy, position and velocity of each atom in the system are calculated according to the selected potential function. The pressure head and matrix atoms are balanced and constrained so that the initial model of the system reaches the equilibrium state. After completion, the internal equilibrium state evolution of the system is carried out under the microcanonical ensemble (NVE) so that the system reaches the equilibrium state.

[0098] Step 2: Press down the pressure head, with a pressing distance of d1.

[0099] d1 = dh f -c1,d1>0

[0100] In the formula, d represents the distance between the indenter and the substrate, and h f The thickness of the lubricating film is indicated by c1, which is an amount introduced to ensure that the substrate is not affected during the pressing process of the pressure head. It is advisable to take 0.2-0.3nm.

[0101] Step 3: After the indenter is pressed down, the model is relaxed in the (NVE) ensemble for a period of time. In this embodiment, the relaxation step size is 10,000 steps, or 20,000 fs.

[0102] Step four outputs the atomic information of the matrix and lubricating film to establish the target model. This step includes the following sub-steps:

[0103] Step 1: Output the atomic information of the matrix and lubricating film separately. The atomic information includes the atom number, atom type, and three-dimensional coordinates of the atom. Let the file containing only the matrix atomic information be "finalSubstrate.txt" and the file containing only the lubricating film atomic information be "finalFilm.txt". Reorder the atom numbers in the output files to ensure that the atom numbers are consecutive. The structure of the output files should be the same as "initialSubstrate.txt". Figure 6 and Figure 7 The images show the models corresponding to the files "finalSubstrate.txt" and "finalFilm.txt", respectively.

[0104] Step 2: Compile the final in-file required to run the LAMMPS software. Read the atomic information from the file "finalSubstrate.txt" into the final in-file, setting the corresponding model as the matrix. Similarly, read the atomic information from the file "finalFilm.txt" into the final in-file, setting the corresponding model as the lubricating film. This completes the model creation for molecular dynamics simulations, showing the lubricating film attached to a three-dimensional random rough surface. Figure 8 As shown.

Claims

1. A modeling method for generating rough surfaces and coating them using molecular dynamics simulations, characterized in that, Includes the following steps: Step 1: Output a model file with a 3D randomly rough surface; Step 1-1: Use the Gaussian random surface generation program to generate a set of random points containing coordinate information. Let A be the set of random points, where the Z coordinate of the points in set A is a complex number. Step 1-2: Take the real part of the Z-coordinate of each point in set A to obtain the set of pure real parts B1. Calculate set B according to the following formula (1): With b (x i ,y j )=h0+k(z b1 (x i ,y j )-With b0 ) (1) In the formula, h0 is the initial reference for establishing the rough surface model; k is the magnification factor; z b1 (x i ,y j ) is the midpoint (x) of set B1 i ,y j The corresponding Z-coordinate value; z b0 The average of the Z-coordinates of all points in set B1; z b (x i ,y j (x) is the midpoint of set B. i ,y j The corresponding Z-coordinate value; x i and y j Let i = 1, 2, ..., m, j = 1, 2, ..., n, where m and n represent the number of atoms in set B1 in the X and Y directions, respectively. Steps 1-3: Calculate the three-dimensional coordinates of each atom in the rough surface model based on the atomic arrangement rules; let the Z-coordinate of any atom in the rough surface model be z(x). i ,y j ); Compare the Z-coordinate value of each atom with the Z-coordinate value of the corresponding point in set B. If z(x) i ,y j )≤z b (x i ,y j If z(x) is true, then the atom number, atom type, and atom three-dimensional coordinate information of that atom will be output to the file "initialSubstrate.txt". Otherwise, if z(x) is false, then the atom number, atom type, and atom three-dimensional coordinate information will be output to the file "initialSubstrate.txt". i ,y j )>z b (x i ,y j If the atom is selected, its atomic number, atomic type, and three-dimensional coordinates will be output to the file "initialIndenter.txt". Steps 1-4: Using the least squares method, calculate the datum surface equation of the model corresponding to the file "initialSubstrate.txt", and then calculate the surface roughness value of the model. If the roughness value does not meet the set modeling requirements, return to step 1-1 and start modeling again. Step 2: Establish a molecular dynamics model containing a random rough surface and a lubricating film; Write the infile required to run the LAMMPS software. Read the atomic information from the file "initialIndenter.txt" into the infile, set the corresponding molecular dynamics model as the indenter, and move these atoms upwards by a distance d. Read the atomic information from the file "initialSubstrate.txt" into the infile, set the corresponding molecular dynamics model as the matrix, and establish a lubricating film between the indenter and the matrix. Both the matrix and the indenter consist of a fixed layer, a thermostatic layer, and a Newtonian layer. The lubricating film is set as a separate group. Step 3: Press down the pressure head to allow the lubricating film to adhere to the substrate surface; Step 3-1: Set the boundary conditions of the simulation region, the interatomic interaction potential function, the initial velocity of the atoms, the system temperature, and the integration step size; after setting, the model first relaxes under the canonical ensemble NVT, and then switches to relaxation under the microcanonical ensemble NVE. Step 3-2: Press down the pressure head, with a pressing distance of d1. d1=d-h f -c1,d1>0 (2) In the formula, d represents the distance between the indenter and the substrate; h f This indicates the thickness of the lubricating film; c1 is a constant introduced to ensure that the substrate is not affected during the pressing process of the indenter. Step 3-3: After the pressure head is pressed down, relax the model under the microcanonical ensemble NVE for the preset time length; Step 4: Output the atomic information of the matrix and lubricating film to establish the target model; Step 4-1: Output the atomic information of the matrix and the lubricating film respectively. The atomic information includes the atomic number, atomic type and three-dimensional coordinates of the atom. Let the file containing only the atomic information of the matrix be "finalSubstrate.txt" and the file containing only the atomic information of the lubricating film be "finalFilm.txt". Reorder the atomic numbers in the output information files to ensure that the atomic numbers are continuous. The structure of the output file is the same as that of "initialSubstrate.txt". Step 4-2: Write the final in file required to run the LAMMPS software. Read the atomic information contained in the file "finalSubstrate.txt" into the final in file and set the corresponding model as the matrix. Read the atomic information contained in the file "finalFilm.txt" into the final in file and set the corresponding model as the lubricating film. This completes the model establishment of the lubricating film attached to the three-dimensional random rough surface for molecular dynamics simulation.

2. The modeling method for generating rough surfaces and coating them using molecular dynamics simulation according to claim 1, characterized in that, The output file in steps 1-3 consists of a section recording model information and a section recording information about each atom: The first part, which records model information, begins with a blank line or content starting with "#", the second line records the total number of atoms in the model, the third line records the total number of atom types in the model, and the fourth to sixth lines record the dimensions of the model in the X, Y, and Z directions, respectively; The second part, which records information about each atom, begins with "Atoms", followed by a blank line, and each subsequent line records the information for one atom. The information in each line is in the following order: atom number, atom type, atom X coordinate, atom Y coordinate, and atom Z coordinate. Each data point is separated by a space. No blank line is needed between the first and second parts.

3. The modeling method for generating rough surfaces and coating them using molecular dynamics simulation according to claim 1, characterized in that, In steps 1-4, the least squares method is used to calculate the datum surface equation of the model corresponding to the file "initialSubstrate.txt", and then the surface roughness value of the established model is calculated as follows: Let set C be the set of each (x) in the file "initialSubstrate.txt". i ,y j The set of points whose Z-coordinate values ​​are the largest, and let the equation of the datum plane be: f(x,y)=a+bx+cy (3) The formula for calculating the three unknowns a, b, and c in the equation of the datum surface is derived as follows: In the formula z c (x i ,y j ) represents the midpoint (x) of set C. i ,y j The corresponding Z-coordinate value, Let X, Y, and Z represent the average values ​​of the coordinates of all points in set C in the X, Y, and Z directions, respectively. Calculate the roughness parameter of a three-dimensional randomly rough surface—the surface arithmetic mean deviation S. a :

4. The modeling method for generating rough surfaces and coating them using molecular dynamics simulation according to claim 1, characterized in that, The value of c1 ranges from 0.2 to 0.3 nm.

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