Simulation method for damage of stray light absorbing functional thin films caused by nanosecond laser

Abstract

The present invention discloses a simulation method for damage to a functional film absorbing stray light caused by nanosecond laser action, the method comprising: establishing a three-dimensional model of a functional film absorbing stray light; setting a laser light source and progressively loading; setting temperature-dependent parameters and parameter boundary conditions of each material; performing gradient meshing on the three-dimensional model; performing dynamic thermal coupling simulation based on the mesh until the set simulation time is reached, and obtaining the functional film damage morphology and multi-physical field evolution law according to the set material temperature-dependent parameters and parameter boundary conditions. The scheme of the present invention can effectively reveal the damage mechanism of the functional film and predict the damage spot morphology and multi-physical field evolution law.

Description

Technical Field

[0001] The invention relates to the field of laser-acted oxide ceramic functional films, and in particular to a simulation method for damage to a stray light absorbing functional film caused by nanosecond laser action. Background Art

[0002] Functional films based on aluminum oxide / silicon oxide can greatly improve the absorption rate of stray light, effectively preventing stray light from acting on aluminum alloys with low laser absorption rate and then being reflected multiple times in high-energy laser systems to form high-order ghost light that is difficult to calculate. However, since the average power of stray light can reach 10 8 W will induce thermal stress in the functional film in a very short time, causing the material to splash and irreversibly contaminate the high-energy laser system. At present, it is difficult to accurately and comprehensively capture this process dynamically through experimental means. Therefore, it is important and challenging to use numerical methods to accurately analyze the multi-physical fields and damage evolution involved in the stray light action process while exploring the response relationship of the action effect to the laser parameters. The current research has fully discussed and estimated the temperature / material distribution and material injection rate in the metal target caused by the laser action. However, for brittle materials, the non-negligible thermal stress caused by the sudden increase in temperature in the extremely short pulse time is not taken into account, so the conclusion will have a very large deviation. Therefore, the current simulation method is not suitable for ceramic oxide functional film materials, and a more effective and complete simulation method is needed to simulate the laser action process. Summary of the invention

[0003] The present invention provides a simulation method for damage to a functional film absorbing stray light caused by nanosecond laser action, which can effectively reveal the damage mechanism of the functional film and predict the morphology of the damage spot and the evolution law of multi-physical fields.

[0004] To this end, the present invention provides the following technical solutions:

[0005] A method for simulating damage to a stray light absorbing functional film caused by nanosecond laser action, the method comprising:

[0006] Establish a three-dimensional model of stray light absorption functional film;

[0007] Set up the laser light source and progressively load it;

[0008] Set the temperature dependence parameters and parameter boundary conditions of each material;

[0009] Performing gradient meshing on the three-dimensional model;

[0010] Based on the grid, dynamic and thermal coupling simulation is performed until the set simulation time is reached, and the functional film damage morphology and multi-physical field evolution law are obtained according to the set material temperature dependence parameters and parameter boundary conditions.

[0011] Optionally, the three-dimensional simulation model includes: an upper protective layer SiO2, a middle light absorption layer Al2O3, and a bottom substrate AA6061 layer.

[0012] Optionally, the method further comprises:

[0013] The nanoporous structure of the middle absorption layer Al2O3 and the CdSe filler with full-band spectral absorption capability are simplified, the nano-scale CdSe particles are not modeled, and the laser absorption rate of aluminum oxide is directly converted to the absorption rate value of the film system after adding CdSe.

[0014] Optionally, the step of setting a laser light source and progressively loading includes:

[0015] Using Gaussian light source:

[0016]

[0017] Among them, Q is the laser power density distribution function, A is the laser absorptivity, P is the laser power, and r is the spot radius

[0018] Set the laser pulse width and laser power;

[0019] After the damage deletion condition of the current layer unit is met, the current layer unit is deleted;

[0020] A new laser light source is applied on the lower unit surface.

[0021] Optionally, the method further comprises: setting a material thermal-mechanical composite damage judgment criterion (temperature reaches a material vaporization value or stress state parameter reaches a material damage value);

[0022] Determine whether the current layer unit damage deletion condition is met in the following way:

[0023] Get the temperature and stress state of the current layer unit;

[0024] Determining whether the temperature and stress state of the current layer unit meet the material thermal-mechanical composite damage judgment criterion;

[0025] If so, it is determined that the current layer unit damage deletion condition is met.

[0026] Optionally, the judgment criterion is: the temperature of the current layer unit reaches a material vaporization value or the stress state parameter reaches a material damage value.

[0027] Optionally, the temperature-dependent parameters of each material include any one or more of the following: elastic modulus, Poisson's ratio, flexible damage parameter, JC constitutive and damage parameter, JH-2 constitutive and damage parameter, bilinear constitutive parameter, thermal expansion coefficient, thermal conductivity, specific heat capacity, inelastic heat share, latent heat, melting and boiling points, and absorptivity;

[0028] The boundary conditions include any one or more of the following boundary conditions: initial temperature, heat source boundary on the model surface, fixed constraint on the model ground, heat outlet on the model side wall, air convection heat transfer, and surface thermal radiation.

[0029] Optionally, the performing gradient meshing on the three-dimensional model comprises:

[0030] The three-dimensional model is divided by a gradient transition grid method, so that the radial inner grid is uniformly dense at 2 μm, the radial outer grid is sparse at 20 μm, and the axial grid is 33 nm.

[0031] Optionally, the performing dynamic thermal coupling simulation based on the grid includes:

[0032] Simulate the evolution of material removal damage caused by thermal-mechanical combined effects in the domain;

[0033] Material splashing process;

[0034] The transient values ​​and evolution process of stress, strain, displacement, temperature, phase state, velocity and acceleration on the surface and inside of the material.

[0035] Optionally, the transient values ​​and evolution processes of the material surface and internal stress strain, displacement, temperature, phase state, velocity, acceleration include:

[0036] Perform post-processing data extraction and analysis, including the evolution of extreme values ​​over time, the distribution laws of physical fields, and the current phase of the material.

[0037] The simulation method for damage to the functional film caused by stray light absorption by nanosecond laser provided in the embodiment of the present invention establishes a three-dimensional laser-induced thermal-mechanical coupling model for the functional film, introduces the temperature dependence of the thermal parameters and introduces the progressive damage mechanism, and obtains the laser damage morphology and multi-physical field evolution law of the functional film. It provides an important reference for improving the cleanliness of high-energy laser systems and promoting the development of related industries, and also provides upstream guidance for the design of the terminal optical system structure and the KDP (potassium dihydrogen phosphate) crystal frame wind knife. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a flow chart of a simulation method of damage to a stray light absorbing functional film caused by nanosecond laser action according to an embodiment of the present invention;

[0039] Figure 2The structure diagram of the stray light absorption functional film and the micro-macro characterization diagram in the embodiment of the present invention;

[0040] Figure 3 This is a flow chart of unit damage determination in an embodiment of the present invention;

[0041] Figure 4 This is a diagram showing the effect of gradient mesh division in an embodiment of the present invention;

[0042] Figure 5 An exemplary diagram showing damage to a stray light absorbing functional film caused by nanosecond laser action at different laser power densities in an embodiment of the present invention;

[0043] Figure 6 An example diagram of the collapse and splashing of a stray light absorbing functional film caused by nanosecond laser action at different laser power densities in an embodiment of the present invention;

[0044] Figure 7 This is a graph showing the evolution of the damage spot depth / width of the stray light absorption functional film caused by nanosecond laser action under different laser power densities in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the above-mentioned objects, features and beneficial effects of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0046] like Figure 1 As shown, the flowchart of the simulation method of the damage of the stray light absorption functional film caused by nanosecond laser action in an embodiment of the present invention comprises the following steps:

[0047] Step 101, establishing a three-dimensional model of a stray light absorbing functional film.

[0048] Specifically, Figure 2 The schematic diagram of the stray light absorption functional film structure and the micro- and macroscopic characterization diagrams shown are the simulated object, and its substrate is AA6061-T6.

[0049] The preparation method of the stray light absorption functional thin film specimen is as follows:

[0050] First, the substrate is mechanically polished and then the specimen is degreased and alkali-etched to remove the surface organic matter and atmospheric oxidation layer. Due to the presence of elements such as Si and Cu, the alkali solution dissolves the surface of the specimen unevenly, and it needs to be soaked in a neutralizing solution to restore the metallic luster.

[0051] Anodization is then performed, the bath solution is composed of sulfuric acid with a mass fraction of more than 10% and a small amount of nano-dispersed particles, and then immersed in an electrolytic deposition bath solution to add CdSe with spectral absorption ability to the pores of alumina.

[0052] Finally, by soaking in a silicon-containing solution at high temperature, a silicon dioxide film is formed on its surface, which achieves the purpose of surface modification and completes the pore sealing at the same time.

[0053] In order to simplify the calculation, the simulation method simplifies the nanoporous structure of the middle absorption layer Al2O3 and the CdSe filler with full-band spectrum absorption capacity. The nano-scale CdSe particles are not modeled, and the laser absorption rate of aluminum oxide is directly converted to the absorption rate value of the film system after adding CdSe. Because CdSe has full-band spectrum absorption capacity, it can improve the laser absorption rate of aluminum oxide film. The so-called "conversion" means that the nano-scale CdSe particles are not modeled, and the laser absorption rate of aluminum oxide is directly set to the absorption rate value of the film system after adding CdSe.

[0054] In order to characterize the particle splashing phenomenon generated during laser irradiation, a three-dimensional model was used for calculation. The simulation domain established was 2mm×2mm×30μm, of which the SiO2 layer was 100nm, the Al2O3 layer was 24μm, and the rest was AA6061. Ideally, the different material layers were in perfect contact, and their contact strength was equal to the material strength. This three-dimensional model is aimed at the damage and multi-physical field evolution process caused by the thermal-mechanical coupling of the material after being acted on by the laser. The calculation time scale of the model is a quasi-steady-state process after the laser acts for a certain time (such as 6ns) and the material cools naturally for a certain time (such as 1μs).

[0055] Step 102, setting the laser light source and progressively loading.

[0056] Specifically, VDFLUX can be used to set the laser light source. VDFLUX is a secondary development subroutine for dynamics simulation, which can customize non-uniform heat sources.

[0057] In an embodiment of the present invention, the laser light source may adopt a Gaussian distribution heat source:

[0058]

[0059] Where Q is the laser power density distribution function, A is the laser absorption rate, P is the laser power, and r is the spot radius. For example, the spot radius can be 480μm, and the laser energy density can be set to 0.5~2J / cm 2 .

[0060] In specific applications, it is necessary to set the laser pulse width and laser power.

[0061] The scanning is performed unit by unit through double loop traversal of the analysis time step and the number of units. After the light source distribution function is input by programming, the temperature interface is used to traverse the current analysis time step unit. When the unit temperature is greater than the material boiling point or the stress state is greater than the damage judgment, the current unit is deleted, and the first surface of the lower unit in the laser direction of the unit is identified at the same time, and then the heat source loading of the current unit is stopped and a new laser light source is loaded on the surface of the lower unit, so as to cooperate with the unit thermal-mechanical composite damage deletion in step 103 to perform progressive loading of the heat source.

[0062] Specifically, the material thermal-mechanical composite damage judgment criteria can be preset. Accordingly, whether the current layer unit damage deletion condition is met is determined in the following manner: specifically, the temperature and stress state of the current layer unit are obtained; whether the temperature and stress state of the current layer unit meet the material thermal-mechanical composite damage judgment criteria; if yes, it is determined that the current layer unit damage deletion condition is met. The judgment criteria are specifically: the temperature reaches the material vaporization value or the stress state parameter reaches the material damage value.

[0063] like Figure 3 As shown, it is a flowchart of unit damage determination in an embodiment of the present invention, which includes the following steps:

[0064] First, in step 301, a heat source scan is performed on the simulation domain to determine the current phase-related laser absorptivity and loaded laser power density of each unit one by one.

[0065] Then, in step 302, temperature damage determination is performed on each current unit in turn. The specific determination method is as follows:

[0066] Step 321, obtaining the current unit temperature and stress-strain state.

[0067] Step 322, determine whether the current unit temperature is greater than the material boiling point; if so, execute step 324; otherwise, execute step 323.

[0068] Step 323, determine whether the current unit stress meets the damage condition; if yes, execute step 324; otherwise, execute step 321 to determine the next unit.

[0069] Step 324, delete the current unit.

[0070] It should be noted that each layer has multiple units. If the current unit is judged to meet the damage condition, the current unit is deleted and the laser loading of the next unit layer of the current unit is performed. In other words, if any damage in thermal-mechanical damage occurs, while deleting the current unit, the next unit layer in the laser irradiation direction of the unit is identified, and the laser loading is performed on the surface of the next unit layer, so as to realize the progressive loading of the laser heat source.

[0071] It should be noted that there are many ways to use the constitutive damage criterion for mechanical damage, such as: flexible / tough damage criterion and JC (Johnson-Cook) criterion for flexible materials, brittle damage criterion and JH-1 (Johnson-Holrdquist) / JH-2 (Johnson-HolrdquistⅡ) criterion for hard and brittle materials, bilinear constitutive damage criterion, etc. In practical applications, one or more of the criterion criteria can be arbitrarily selected, and the embodiments of the present invention are not limited to this.

[0072] In step 103, the temperature dependency parameters and parameter boundary conditions of each material are set.

[0073] The thermo-mechanical-optical parameters of materials are not fixed and are very sensitive to temperature changes. In the scheme of the present invention, the temperature-dependent material parameters used include any one or more of the following: elastic modulus, Poisson's ratio, flexible damage parameter, JC constitutive and damage parameter, JH-2 constitutive and damage parameter, bilinear constitutive parameter, thermal expansion coefficient, thermal conductivity, specific heat capacity, inelastic heat share, latent heat, melting and boiling point, and absorptivity.

[0074] The boundary conditions include any one or more of the following: initial temperature, heat source boundary on the model surface, fixed constraint on the model ground, heat outlet on the model side wall, air convection heat transfer, and surface thermal radiation.

[0075] Continue to refer to Figure 1 In step 104, the three-dimensional model is gradient meshed.

[0076] Specifically, the three-dimensional model is divided into gradient grids so that the radial inner grid is uniformly dense at 2 μm, the radial outer grid is sparse at 20 μm, and the axial grid is 33 nm. Figure 4 Of course, other granularity sizes may also be used, which is not limited in the present embodiment. The purpose is to reduce the calculation cost as much as possible while ensuring the calculation accuracy.

[0077] Step 105, performing a dynamic-thermal coupling simulation based on the grid until a set simulation time is reached, and obtaining the functional film damage morphology and multi-physical field evolution law according to the set material temperature dependence parameters and boundary conditions.

[0078] The purpose of simulation is to describe the dynamic evolution of the material during the laser action on the film system and the cooling period, including heat and force fields, specifically involving the material's temperature, heat flow evolution, stress and strain evolution, model phase explosion state (damage and particle splashes) and other microscopic multi-physical states. The simulation process is started and ended by time control, which is a transient simulation.

[0079] Specifically, the simulation process mainly includes the following:

[0080] The simulation domain unit temperature reaches the material boiling point, resulting in the evolution of material removal damage caused by thermal action, and the unit stress reaches the material strength limit, resulting in the evolution of material removal damage;

[0081] Due to the removal of the unit, the insufficient unit constraint leads to the material splashing process;

[0082] The simulation domain is loaded by the heat source, which produces changes in the thermal-force field, thereby triggering the transient values ​​and evolution process of stress, strain, displacement, temperature, phase state, velocity, and acceleration on the surface and inside of the material.

[0083] The present invention proposes a simulation method for thermal-mechanical composite damage caused by laser action on functional thin films. The present invention can reveal the damage morphology and multi-physical field evolution law, including: the evolution process of material removal damage caused by thermal-mechanical composite action in the simulation domain; the material splashing process; the transient values ​​and evolution process of stress strain, displacement, temperature, phase state, velocity, and acceleration on the surface and inside of the material. The simulation results are described in detail using damage morphology and evolution process; material splashing as an example:

[0084] Figure 5 The damage spot morphology and temperature distribution under different laser energy densities and different ceramic material constitutive conditions at different times are shown. The SiO2 layer is hidden in the 2nd and 4th rows for easy comparison. For ease of expression, the sub-graph positions are described in matrix form.

[0085] like Figure 5 As shown in the figure, with the increase of laser energy density, the diameter of the damage spot increases accordingly. At 6ns, the surface of the SiO2 layer has no obvious damage, and there are residual particles in the damage spot of the Al2O3 layer. The damage spot is stepped, and there is heat concentration at the edge, which leads to stress concentration, making the boundary position more susceptible to damage. This effect is particularly Figure 5 -[2,6](i.e. Figure 5 This is consistent with the layer-by-layer effect of the laser (adaptive progressive loading of surface heat source), and the use of different constitutive models has little effect on the effect.

[0086] After cooling for 1μs, the effects under different constitutive conditions are essentially different. Using the bilinear constitutive model, the surface of the functional film is damaged and splashed, the damage of the Al2O3 layer extends radially outward, and the gradient damage becomes the spalling damage of the vertical boundary. In comparison, when the JH-2 constitutive model is used to describe the thermal-mechanical properties of the material, no further damage extension is observed after cooling for 1μs, only heat transfer occurs, resulting in a decrease in surface temperature, and the gradient damage structure is retained.

[0087] The constitutive model, also known as the mechanical constitutive equation of the material, or the stress-strain model of the material, is a mathematical expression that describes the mechanical properties of the material (stress-strain-strength-time relationship).

[0088] From the perspective of the reliability of simulation results: the use of bilinear constitutive model can obtain more realistic similar Figure 5 -[3,4](i.e. Figure 5 The “rough” surface of the model (row 3, column 4) simulates the material debris produced by brittle splashing, which is not available in the model using the JH-2 constitutive model.

[0089] From the perspective of material splashing effect of simulation results: Figure 6 As shown in the figure, when the bilinear constitutive laser was applied for 6ns, no splashing was observed. At this time, due to the continuous action of the laser, the temperature difference was relatively small compared to the cooling stage, the thermal stress remained within a controllable range, and only damage occurred. At this time, the thermal stress generated by the laser action material was not enough to cause instability in the simulation domain, the kinetic energy was always maintained at a low level, and the material speed / acceleration was low. During the cooling stage, the material suddenly lost the effect of the heat source, the thermal gradient at the boundary position increased rapidly, and the instantaneous thermal stress of the material caused the unit to fall off and splash at high speed. The maximum flight speed could reach 153.9m / s when cooling for 1μs. However, when the JH-2 constitutive model was used, no splashing occurred in the material. Generally speaking, material (unit) damage caused by stress is more likely to cause a sudden increase in kinetic energy and unit splashing. The kinetic energy of unit failure caused by temperature is small, and it is difficult to produce splashing.

[0090] In summary, the damage of the unit under the bilinear constitutive model is the combined effect of heat and force, while the damage form of the JH-2 constitutive unit is mainly thermal damage. The damage caused by stress is not well reflected due to the poor matching between the constitutive model and the model.

[0091] Figure 7 The figure shows the evolution of the damage spot depth / width of the stray light absorption functional film caused by nanosecond laser under different laser power densities. The horizontal axis represents the laser energy density and the vertical axis represents the damage spot width.

[0092] from Figure 7 It can be seen that the maximum width of the damage spot has little relationship with the stage, and no matter which constitutive model is used, the width of the damage spot increases with the increase of laser energy density, such as Figure 7 As shown in the figure on the left. 2 Except for the laser energy density of 2J / cm, the central damage depth at different stages is basically the same, but the maximum damage depth increases significantly. 2 When the maximum damage depth in the cooling stage is consistent with the central damage, it is 7 times the maximum depth in the laser action stage. Figure 7 As shown in the figure on the right.

[0093] The central damage is: when the material is acted upon by laser, the central position of the damage spot is the position with the highest energy. The maximum damage depth is: when the brittle material is damaged after being irradiated by laser, its maximum damage depth is not necessarily located at the central position of the damage.

[0094] The above simulation results can fully illustrate that the simulation method for nanosecond laser-induced stray light absorption functional film damage provided by the embodiment of the present invention can effectively predict the laser damage process of ceramic oxide functional films, provide accurate guidance for later experiments, and save a lot of manpower and material resources. Furthermore, the present invention provides important guidance for improving the cleanliness of high-energy laser systems and promoting related industries, including the design of terminal optical system structures, especially KDP crystal frame wind knives, and the development and application of lithography equipment.

[0095] The embodiments of the present invention are described in detail above. The present invention is described in detail using specific implementation methods herein. The description of the above embodiments is only used to help understand the method and device of the present invention. It is only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should belong to the scope of protection of the present invention, and the content of this specification should not be understood as limiting the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A simulation method for damage to a stray light absorbing functional film caused by nanosecond laser action, characterized in that: The method comprises: Establish a three-dimensional simulation model of stray light absorption functional film; Set up the laser light source and progressively load it; Set the temperature-dependent parameters and parameter boundary conditions of each material; Performing gradient meshing on the three-dimensional simulation model; Based on the grid, dynamic thermal-mechanical coupling simulation is performed until the set simulation time is reached, and the functional film damage morphology and multi-physical field evolution law are obtained according to the set material temperature dependence parameters and parameter boundary conditions; The three-dimensional simulation model includes: an upper protective layer SiO2, a middle light absorption layer Al2O3, and a bottom substrate AA6061 layer; The step of setting the laser light source and gradually loading the laser light source comprises: Using Gaussian light source: Wherein, Q is the laser power density distribution function, A is the laser absorptivity, P is the laser power, and r is the spot radius; Set the laser pulse width and laser power; After the damage deletion condition of the current layer unit is met, the current layer unit is deleted; Applying a new laser light source to the lower unit surface; The dynamic thermal coupling simulation based on the grid includes: Simulate the evolution of material removal damage caused by thermal-mechanical combined effects in the domain; Material splashing process; The transient values ​​and evolution process of stress, strain, displacement, temperature, phase state, velocity and acceleration on the surface and inside of the material.

2. The method according to claim 1, characterized in that The method further comprises: The nanoporous structure of the middle light absorption layer Al2O3 and the CdSe filler with full-band spectral absorption capability are simplified, the nano-scale CdSe particles are not modeled, and the laser absorption rate of aluminum oxide is directly converted to the absorption rate value of the film system after adding CdSe.

3. The method according to claim 1, characterized in that The method further comprises: setting a material thermal-mechanical composite damage determination criterion; Determine whether the damage deletion condition of the current layer unit is met in the following way: Get the temperature and stress state of the current layer unit; Determining whether the temperature and stress state of the current layer unit meet the material thermal-mechanical composite damage judgment criterion; If so, it is determined that the current layer unit damage deletion condition is met.

4. The method according to claim 3, characterized in that The judgment criterion is: the temperature of the current layer unit reaches the material vaporization value or the stress state parameter reaches the material damage value.

5. The method according to claim 1, characterized in that The temperature-dependent parameters of each material include any one or more of the following: elastic modulus, Poisson's ratio, flexible damage parameter, JC constitutive and damage parameter, JH-2 constitutive and damage parameter, bilinear constitutive parameter, thermal expansion coefficient, thermal conductivity, specific heat capacity, inelastic heat share, latent heat, melting and boiling points, and absorptivity; The boundary conditions include any one or more of the following boundary conditions: initial temperature, heat source boundary on the model surface, fixed constraint on the model ground, heat outlet on the model side wall, air convection heat transfer, and surface thermal radiation.

6. The method according to claim 1, characterized in that The gradient meshing of the three-dimensional model comprises: The three-dimensional model is divided by a gradient transition grid method, so that the radial inner grid is uniformly dense at 2 μm, the radial outer grid is sparse at 20 μm, and the axial grid is 33 nm.

7. The method according to claim 6, characterized in that The transient values ​​and evolution processes of the material surface and internal stress strain, displacement, temperature, phase state, velocity, acceleration include: Perform post-processing data extraction and analysis, including the evolution of extreme values ​​over time, the distribution laws of physical fields, and the current phase of the material.

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