Silicon nitride sintering process parameter optimization method based on numerical simulation

Abstract

The invention discloses a numerical simulation-based silicon nitride sintering process parameter optimization method, which comprises the following steps of sequentially performing degreasing and sintering experiments on photocuring 3D printing silicon nitride ceramic, and performing microscopic characterization on the sintered ceramic to obtain a second characterization result; establishing a viscoelastic constitutive equation of the sintering shrinkage deformation of the silicon nitride ceramic, and constructing a sintering finite element model in Abaqus based on the viscoelastic constitutive equation to obtain a simulation result of a transient temperature field and a simulation result of the sintering shrinkage deformation; correcting the finite element model based on the second representation result to obtain a verified finite element model; the original sintering process curve is optimized according to the verified finite element model, the sintering process curve with the lowest cost and the maximum influence on the reduction degree of the sintering stress is selected as the optimal sintering process, and then optimization of the sintering process is achieved. According to the sintering process parameter optimization method, the cost is reduced, the experimental period is short, and the production efficiency is high.

Description

Technical Field

[0001] The invention relates to the technical field of silicon nitride sintering, and in particular to a method for optimizing silicon nitride sintering process parameters based on numerical simulation. Background Art

[0002] After the photocuring printing ceramic structure is completed, the ceramic often contains 20wt%-30wt% of resin and high molecular organic matter. It is necessary to remove the resin and other high molecular organic matter from the ceramic as much as possible by degreasing. However, degreasing is often carried out at a temperature lower than the temperature at which the grains in the ceramic aggregate and grow. At this time, although the grains in the ceramic after degreasing move, the overall volume occupied by the pores does not change. At this time, the density of the ceramic is low and cannot meet normal use. Therefore, sintering becomes a process step that must be carried out before the ceramic can be used normally. However, during the sintering process, different sintering processes need to be set for different materials because different microstructure properties of ceramics determine different grain growth laws.

[0003] During the sintering process, sintering stress is defined as the force generated by balancing the shrinkage and densification movement inside the ceramic. Therefore, the greater the sintering stress, the higher the shrinkage deformation and densification degree of the corresponding ceramic, and the greater the possibility of ceramic damage. For some ceramics, the increase in sintering stress will cause structural damage to key parts due to excessive shrinkage during the shrinkage deformation process. Therefore, the sintering stress needs to be controlled. The energy of ceramic shrinkage deformation is determined by the internal energy provided by the sintering process. Therefore, by controlling the sintering process parameters, the model sintering stress can be effectively controlled to improve the production efficiency of the sintering process.

[0004] Regarding the optimization of the sintering process of silicon nitride ceramics, reference 1 "Wang Huan, Xuan Weidong, Yang Zhigang, et al. Optimization of pressureless sintering process and performance research of silicon nitride ceramics [J]. Bulletin of the Chinese Silicate Society, 2016, 35(09): 2747-2752. DOI: 10.16552 / j.cnki.issn1001-1625.2016.09.009" uses orthogonal experiments to study the effects of molding pressure, holding time, holding time, sintering temperature, sintering aid content and ratio on the porosity and flexural strength of silicon nitride ceramics.

[0005] Reference 2 "Xiao Fei, Xu Zhuangzhi, Xue Jian, et al. Research on gas pressure sintering process of silicon nitride ceramics [J]. Journal of Ceramics, 2019, 40(03): 382-386. DOI: 10.13957 / j.cnki.tcxb.2019.03.019.", by analyzing the effects of different sintering temperatures and gas pressures on the microstructure, relative density and hardness of silicon nitride, the sintering process of silicon nitride ceramics was optimized.

[0006] Reference 3 "Gao Xiaoju, Jin Ying, Li Jinfu, et al. Effect of sintering process on the properties of Si3N4 foam ceramics [J]. Bulletin of the Chinese Silicate Society, 2010, 29(05):1055-1059. DOI:10.16552 / j.cnki.issn1001-1625.2010.05.025", by analyzing and discussing the effects of heating rate and sintering temperature on the microstructure, phase and mechanical properties of silicon nitride porous ceramics, the sintering process of silicon nitride foam ceramics was optimized.

[0007] Reference 4 "Zhou Changling, Fan Jinglin, Hu Xiaoqing, et al. Effect of sintering process on β-silicon nitride ceramics [J]. Ceramics, 2005, (10): 30-33+43. DOI: 10.19397 / j.cnki.ceramics.2005.10.007" studied the effects of sintering aid mass fraction, sintering temperature and holding time on the densification degree and mechanical properties of β-silicon nitride ceramics.

[0008] Reference 5 "Zou Qiang. Research on the sintering process of silicon nitride ceramic materials for radomes [D]. Tianjin University, 2004", analyzed and studied the effects of changes in sintering atmosphere, temperature, and sintering aids on the sintering properties of silicon nitride ceramics, and established the sintering mechanism of the system of nano silicon nitride powder, micron silicon nitride, cordierite sintering aid, β-spodumene sintering aid, and atmosphere.

[0009] Reference 6 "Zhu Yunrui, He Yunpeng, Yang Jian, et al. Research status of influencing factors of high thermal conductivity silicon nitride ceramic substrates [J]. Bulletin of the Chinese Silicate Society, 2024, 43(07): 2649-2660. DOI: 10.16552 / j.cnki.issn1001-1625.2024.07.029", analyzed the influencing factors of the thermal conductivity of silicon nitride ceramics, and optimized the sintering process by comparing the advantages and disadvantages of its performance under different sintering processes.

[0010] Reference 7 "Gao Zhen, Yin Ruiming, Li Guang, et al. Influence of sintering method on the properties of silicon nitride ceramics [J]. China Ceramic Industry, 2024, 31(03): 47-53. DOI: 10.13958 / j.cnki.ztcg.2024.03.008", compared the effects of different sintering processes on the densification, porosity, mechanical properties, thermal conductivity and chemical composition of silicon nitride ceramics, and analyzed different sintering processes.

[0011] Reference 8 "Li Xiaolei, Liu Yun, Zhou Miao, et al. High thermal conductivity silicon nitride ceramic materials with excellent mechanical properties and preparation methods [P]. Tianjin: CN202211264872.X, 2023-11-03", after the characterization of the microstructure, a two-step sintering scheme was carried out, and a method for preparing high thermal conductivity silicon nitride ceramics with excellent mechanical properties was produced through the collaboration of optimized sintering process and sintering aids.

[0012] Document 9 "Zhao Hongwei. A large-size silicon nitride ceramic with high strength and high thermal conductivity and its preparation method [P]. Jiangsu Province: CN202110940354.4, 2023-10-20", invented a preparation method suitable for the production of large-size silicon nitride ceramics with high strength and high thermal conductivity.

[0013] Reference 10 "Yang Zhou, Li Hongtao, Zhang Chunyan, et al. Effects of gas pressure one-step sintering and two-step sintering on the structure and properties of silicon nitride ceramics [J]. Journal of Ceramics, 2023, 44(04): 712-718. DOI: 10.13957 / j.cnki.tcxb.2023.04.011", by analyzing the effects of different sintering schemes on the density, phase and micromorphology, and mechanical properties of silicon nitride ceramics, a process method for preparing silicon nitride ceramics with better properties was determined.

[0014] However, the optimization of silicon nitride ceramic sintering process recorded in the above literature is mostly based on a large number of experiments to analyze the impact of sintering process optimization on the performance of silicon nitride ceramics. Usually the sintering process includes: a heating stage, a heat preservation stage and a cooling stage. The trial and error method is used to determine the sintering process, which is costly and has a long experimental cycle. Summary of the invention

[0015] Based on the technical problems existing in the background technology, the present invention proposes a method for optimizing silicon nitride sintering process parameters based on numerical simulation, which not only reduces the cost but also shortens the experimental cycle and improves the production efficiency.

[0016] The present invention proposes a method for optimizing silicon nitride sintering process parameters based on numerical simulation, comprising the following steps:

[0017] Step 1: Perform a degreasing experiment on the photocuring 3D printed silicon nitride ceramics, perform microscopic characterization on the degreased ceramics, and obtain a first characterization result;

[0018] Step 2: performing a sintering experiment on the degreased ceramic according to the original sintering process curve, and performing microscopic characterization on the sintered ceramic to obtain a second characterization result;

[0019] Step 3: Establish a viscoelastic constitutive equation for sintering shrinkage deformation of silicon nitride ceramics, and construct a sintering finite element model in Abaqus based on the viscoelastic constitutive equation to obtain simulation results of transient temperature field and sintering shrinkage deformation;

[0020] Step 4: Calibrate and correct the finite element model based on the second characterization result. When the relative error between the simulation result output by the finite element model and the second characterization result obtained by the experiment is less than or equal to 10%, a verified finite element model is obtained.

[0021] Step 5. Optimize the original sintering process curve according to the verified finite element model, and bring the optimized sintering process curve back into the transient heat conduction simulation in the finite element model to simulate the sintering shrinkage deformation, so as to evaluate the influence of the optimized sintering process curve on the sintering stress, and select the sintering process curve with the lowest cost and the greatest influence on the degree of sintering stress reduction as the optimal sintering process, thereby realizing the optimization of the sintering process.

[0022] Furthermore, in step three, the viscoelastic constitutive equation is constructed as follows:

[0023]

[0024] in, is the total strain rate, is the elastic strain rate, is the thermal strain rate, Creep strain rate.

[0025] Further, The formula is as follows:

[0026]

[0027]

[0028]

[0029]

[0030] in, is the stress rate, C is the elastic constant matrix, α is the thermal expansion coefficient of silicon nitride ceramics, Δ is the Hamiltonian vector differential operator, is the temperature change rate, σ′ is the deviatoric stress tensor, is the shear viscosity modulus, ηψ is the bulk viscosity modulus, σ h is the hydrostatic pressure, σ s is the sintering stress, I is the unit matrix when the material is isotropic, η is the viscosity expression, η0 is the pre-exponential coefficient of the viscosity term of the material, Q vis the viscous flow activation energy, R is the universal gas constant, T abs is the absolute temperature at the current temperature.

[0031] Furthermore, in step three, the process of establishing the finite element model is as follows:

[0032] Establish geometric models in the pre-processing of Abaqus, the engineering simulation finite element software;

[0033] Transient temperature field simulation: define the temperature properties of the geometric model, define transient heat conduction in the analysis step, set the total time of the analysis step to the sintering process time, and define the sintering process curve in the amplitude curve in the load as a boundary condition;

[0034] Sintering shrinkage deformation simulation: define the physical properties of the geometric model, select the viscosity analysis step as the analysis step, set the total time of the analysis step to the sintering process time, call the transient temperature field file in the load, and apply boundary conditions that correctly describe the sintering behavior to the geometric model based on the actual boundary conditions of the ceramic in the sintering furnace.

[0035] Furthermore, in step 4, the verified finite element model is obtained as follows:

[0036] The second characterization result is used to compare the shrinkage deformation, grain size and relative density of the finite element model output model;

[0037] If the relative error is less than or equal to 10%, the verified finite element model is obtained;

[0038] If the relative error is greater than 10%, return to step 3 to modify the pre-exponential coefficient η0 of the viscosity term in the viscoelastic constitutive equation until the relative error is less than or equal to 10%.

[0039] Furthermore, in step five, specifically:

[0040] Performing segmented sintering process optimization on the initial sintering process curve in step 2, and using the segmented sintering process curve as the first optimized sintering process curve;

[0041] By changing the influence of sintering temperature on sintering stress, the sintering process curve after the first optimization is optimized to obtain the sintering process curve after the second optimization;

[0042] By changing the influence of the sintering rate in the high temperature stage on the sintering stress, the sintering process curve after the second optimization is optimized, and the sintering process curve after the third optimization is obtained;

[0043] By changing the effect of the high temperature holding time on the sintering stress, the sintering process curve after the third optimization is optimized, and the sintering process curve after the fourth optimization is obtained;

[0044] The above four optimized sintering process curves were respectively brought into the transient heat conduction simulation in the finite element model to simulate the sintering shrinkage deformation and evaluate the influence of each optimized sintering process curve on the sintering stress, so that the sintering process curve after the fourth optimization was taken as the optimal sintering process curve.

[0045] Furthermore, the sintering process of the initial sintering process curve is: a heating rate of 10 K / min from room temperature 20 °C to 1900 °C, and a heat preservation time of 60 min;

[0046] The sintering process of the first optimized sintering process curve is as follows: the heating rate from room temperature 20°C to 1400°C is 10K / min, and the temperature is kept at 1400°C for 30min, and then the heating rate from 1400°C to 1900°C is 5K / min, and the temperature is kept at 1900°C for 60min;

[0047] The sintering process of the second optimized sintering process curve is as follows: the heating rate from room temperature 20°C to 1400°C is 10K / min, and the temperature is kept at 1400°C for 30min, and then the heating rate from 1400°C to 1900°C is 5K / min, and the temperature is kept at 1900°C for 60min;

[0048] The sintering process of the sintering process curve after the third optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, and the temperature is kept at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and the temperature is kept at 1900℃ for 60min;

[0049] The sintering process of the sintering process curve after the fourth optimization is: the heating rate from room temperature 20°C to 1400°C is 10K / min, keeping at 1400°C for 30min, then the heating rate from 1400°C to 1900°C is 5K / min, and keeping at 1900°C for 60min.

[0050] Further, in step 1, the debinding experiment specifically includes: determining the temperature section where the weight loss of silicon nitride ceramic is the most serious through thermogravimetric analysis, and determining the debinding process through segmented sintering;

[0051] The silicon nitride ceramic is placed in a tube furnace for degreasing and decarburization to obtain a degreasing ceramic.

[0052] Furthermore, in steps one and two, the same characterization method is used to perform microscopic characterization on the degreased ceramics and the sintered ceramics. The characterization method is: measuring the grain size and grain distribution inside the ceramics by a scanning electron microscope (SEM), determining the relative density of the ceramics according to the Archimedes drainage method, and measuring the geometric dimensions of the ceramics using a vernier caliper.

[0053] Furthermore, in step 2, the sintering experiment is specifically as follows: the degreased ceramic is placed in a ceramic pressure furnace for sintering, N2 needs to be introduced into the ceramic pressure furnace as a protective atmosphere, and the gas pressure is selected to be 1 MPa.

[0054] The advantage of the silicon nitride sintering process parameter optimization method based on numerical simulation provided by the present invention is that the sintering process parameters of silicon nitride ceramics are optimized by numerical simulation. Only a few sintering experiments are needed to provide data to verify the model, and the sintering process can be optimized by a faster numerical simulation method, thereby achieving the purpose of reducing costs and improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is a schematic diagram of the process of the present invention;

[0056] Figure 2a An optimized process curve diagram for optimizing the initial sintering process curve by segmented sintering process;

[0057] Figure 2b for Figure 2a Sintering stress curve corresponding to the optimized process curve;

[0058] Figure 3a An optimization process curve diagram for selecting four sintering temperatures to further optimize the sintering curve after the first optimization;

[0059] Figure 3b for Figure 3a Sintering stress curve corresponding to the optimized process curve;

[0060] Figure 4a An optimized process curve diagram for selecting four heating rates in the high temperature stage to further optimize the sintering curve after the second optimization;

[0061] Figure 4b for Figure 4a Sintering stress curve corresponding to the optimized process curve;

[0062] Figure 5a An optimization process curve diagram for further optimizing the sintering curve after the third optimization by selecting two high temperature section holding times in the holding stage;

[0063] Figure 5b for Figure 5a Sintering stress curve corresponding to the optimized process curve. DETAILED DESCRIPTION

[0064] Below, the technical solution of the present invention is described in detail through specific embodiments. Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific implementation disclosed below.

[0065] like Figures 1 to 5b As shown, the present invention proposes a method for optimizing silicon nitride sintering process parameters based on numerical simulation, comprising the following steps:

[0066] Step 1: Perform a degreasing experiment on the photocuring 3D printed silicon nitride ceramics, perform microscopic characterization on the degreased ceramics, and obtain a first characterization result;

[0067] The degreasing experiment is specifically as follows: through thermogravimetric analysis, the temperature section where the weight loss of silicon nitride ceramics is the most serious is determined, and the degreasing process is determined through segmented sintering; the silicon nitride ceramics are placed in a tubular furnace for degreasing. Since the organic matter in the silicon nitride ceramics cannot be completely discharged from the interior of the green body during degreasing, some of the organic matter will be deposited on the surface in the form of residual carbon. After the green body is decarburized, a sintering test specimen (i.e., the degreased ceramic) is obtained.

[0068] The same characterization method is used to perform microscopic characterization on the degreased ceramics and the sintered ceramics. The characterization method is: the grain size and grain distribution inside the ceramics at different magnifications are measured by scanning electron microscopy (SEM), the relative density of the ceramics is determined according to the Archimedes drainage method, and the geometric dimensions of the ceramics are measured using a vernier caliper.

[0069] The geometric parameters of the silicon nitride block in this embodiment are: 15 mm in length, 5 mm in width, and 5 mm in height.

[0070] Step 2: performing a sintering experiment on the degreased ceramic according to the original sintering process curve, and performing microscopic characterization on the sintered ceramic to obtain a second characterization result;

[0071] The sintering experiment is as follows: Since the internal grains of silicon nitride ceramics need high energy for cohesion and growth, and protective gas needs to be introduced, silicon nitride ceramics are placed in a high-temperature ceramic gas pressure furnace for sintering. Among them, silicon nitride ceramics are easy to react with air during the sintering process to generate: silicon carbide, silicon oxide and other impurities, that is, if silicon nitride ceramics are directly degreased in the air, silicon nitride ceramics will be oxidized and decomposed by oxygen, resulting in the generation of second-phase impurities. Therefore, N2 needs to be introduced into the gas pressure furnace as a protective atmosphere. According to process experience, the gas pressure is selected to be 1MPa. The unoptimized initial sintering process (i.e., the original sintering process curve) is as follows Figure 2aAs shown in process 1, the temperature is raised from room temperature 20°C to 1900°C at 10K / min and kept at this temperature for 60 minutes. After sintering, the silicon nitride ceramic is cooled in the gas pressure furnace.

[0072] Step 3: Establish a viscoelastic constitutive equation for sintering shrinkage deformation of silicon nitride ceramics, and construct a sintering finite element model in Abaqus based on the viscoelastic constitutive equation to obtain simulation results of transient temperature field and sintering shrinkage deformation;

[0073] (a1) When establishing the viscoelastic constitutive equation, since sintering involves the elastic deformation, creep deformation and thermal expansion deformation of the internal grains of the ceramic, the elastic deformation rate is considered. Thermal strain rate and creep strain rate The elastic constitutive equation is established as follows:

[0074]

[0075] in, is the total strain rate;

[0076] The expression is:

[0077]

[0078]

[0079]

[0080] in, is the stress rate, C is the elastic constant matrix, α is the thermal expansion coefficient of silicon nitride ceramics, Δ is the Hamiltonian vector differential operator, is the temperature change rate, σ ′ is the deviatoric stress tensor, is the shear viscosity modulus, is the bulk viscosity modulus, is the hydrostatic pressure, is the sintering stress, γ s is the surface energy, g is the grain size, I is the unit matrix when the material is isotropic, is the viscosity expression, η0 is the pre-exponential coefficient of the viscosity term of the material, η is a function of temperature T, Q v is the viscous flow activation energy, R is the universal gas constant, T abs is the absolute temperature at the current temperature.

[0081] Among them, the growth dynamics formula of g is: D is the pre-exponential coefficient of the internal grain size growth term of silicon nitride ceramics, Q Gis the activation energy of grain growth inside silicon nitride ceramics.

[0082] (a2) The construction of the finite element model is as follows (a2-1) to (a2-3):

[0083] (a2-1) Establish geometric model in the pre-processing of engineering simulation finite element software Abaqus;

[0084] A 15x5x5mm geometric model of silicon nitride was established. Since the geometric model is relatively simple, it was directly established in the Abaqus pre-processing.

[0085] (a2-2) Transient temperature field simulation: define the temperature properties of the geometric model, such as specific heat capacity, thermal conductivity, and density. Define transient heat conduction in the analysis step. Set the total time of the analysis step to the sintering process time. Define the sintering process curve in the amplitude curve in the load and introduce it as a boundary condition. Discrete the model into a grid, select the unit type as DC3D8, and submit the calculation after creating the job.

[0086] (a2-3) Sintering shrinkage deformation simulation: define the physical properties of the geometric model, such as specific heat capacity, thermal conductivity, density, elastic modulus, Poisson's ratio, independent variables, expansion coefficient, creep expansion strain and creep strain, select the viscosity analysis step as the analysis step, set the total time of the analysis step to the sintering process time, call the transient temperature field file in the load, and apply boundary conditions that correctly describe the sintering behavior to the geometric model according to the actual boundary conditions of silicon nitride ceramics in the sintering furnace; in this embodiment, the boundary condition is selected to constrain the vertical degrees of freedom of the bottom surface of the set model, apply gravity load, divide the discrete model into grids, select the unit type as DC3D8R, create a job, call the Creep subroutine and submit the calculation.

[0087] Step 4: Calibrate and correct the finite element model based on the second characterization result. When the relative error between the simulation result output by the finite element model and the second characterization result obtained by the experiment is less than or equal to 10%, a verified finite element model is obtained.

[0088] In this embodiment, the shrinkage deformation, grain size and relative density of the finite element model output model are compared by the second characterization result; if the relative error is less than or equal to 10%, the verified finite element model is obtained; if the relative error is greater than 10%, return to step 3 to modify the pre-exponential coefficient η0 of the viscosity term in the viscoelastic constitutive equation, and the other parameters such as activation energy are not changed, because η0 fluctuates greatly with temperature, until the relative error is less than or equal to 10%; the comparison results are shown in Table 1:

[0089] Table 1 Comparison of simulation results and measured results of the second characterization

[0090] Characterization parameters Measurements Analog value Relative error xy-direction dimensions 6.40mm 6.62mm 3.36％ Dimension in z direction 13.60mm 14.06mm 3.36％ Grain size 0.99μm 0.93μm 5.28％ Relative density 98.00％ 94.11％ 3.97％

[0091] It can be found from Table 1 that the simulation value (numerical simulation result) is close to the measured value (second characterization result), and the maximum relative error is kept within 5.3%, which is within the relative error requirement limit of 10%, thereby verifying the validity of the finite element model and indicating that the established model can more accurately simulate the actual sintering state of silicon nitride ceramics.

[0092] Step 5. Optimize the original sintering process curve according to the verified finite element model, and bring the optimized sintering process curve back into the transient heat conduction simulation in the finite element model to simulate the sintering shrinkage deformation, so as to evaluate the influence of the optimized sintering process curve on the sintering stress, and select the sintering process curve with the lowest cost and the greatest influence on the degree of sintering stress reduction as the optimal sintering process curve, thereby realizing the optimization of the sintering process.

[0093] When optimizing the sintering process of silicon nitride ceramics, the sintering process parameters such as sintering temperature, heating rate, holding time, and staged sintering process are optimized according to the verified finite element model, and the optimized sintering process curve is brought back into the simulation of the transient temperature field. The specific steps for optimizing the sintering process parameters are (b1) to (b4):

[0094] (b1) performing segmented sintering process optimization on the initial sintering process curve in step 2, and using the segmented sintering process curve as the sintering process curve after the first optimization;

[0095] For the unoptimized initial sintering process (i.e., the corresponding initial sintering process curve) used in step 2, the segmented sintering process is first optimized. Since silicon nitride ceramics will begin to turn from α phase to β phase at around 1400°C, the optimization process is as follows: Figure 2a As shown in the figure, that is, the heating rate from room temperature (20℃) to 1400℃ is 10K / min, and the temperature is kept at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and the temperature is kept at 1900℃ for 60min. By comparing the two sintering processes (before and after optimization), although the overall sintering time of the optimized sintering process is extended by 2h, Figure 2b As shown in the figure, the sintering stress corresponding to the optimized sintering process is less than that of the unoptimized sintering process during the sintering process, and the segmented sintering process curve is selected as the sintering process after the first optimization. That is, the sintering process of the sintering process curve after the first optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, and the temperature is kept at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and the temperature is kept at 1900℃ for 60min.

[0096] (b2) optimizing the sintering process curve after the first optimization by changing the influence of the sintering temperature on the sintering stress, and obtaining the sintering process curve after the second optimization;

[0097] The sintering curve after the first optimization was optimized again by changing the sintering temperature. Four sintering temperatures (1950℃, 2000℃, 2050℃, and 2100℃) were selected to optimize the sintering process after the first optimization. Figure 3a As shown, Figure 3b As shown in the figure, it can be found that the sintering stress of the geometric model at 1900℃ and 2000℃ is lower than that of the other optimized processes. Considering the sintering cost in the actual sintering process, the sintering temperature of 1900℃ is selected as the sintering process after the second optimization. That is, the sintering process of the sintering process curve after the second optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, keeping at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and keeping at 1900℃ for 60min.

[0098] (b3) optimizing the sintering process curve after the second optimization by changing the influence of the sintering rate in the high temperature stage on the sintering stress, and obtaining the sintering process curve after the third optimization;

[0099] The sintering rate in the high temperature stage will seriously affect the sintering quality of silicon nitride ceramics. Therefore, the sintering rate in the 1400℃-1900℃ process is changed as follows: Figure 4a As shown in Figure 2, four heating rates (5K / min, 8K / min, 10K / min, and 15K / min) are selected as the optimization conditions. By analyzing the effect of different heating rates on the sintering stress, the results are as follows: Figure 4b As shown, it can be found that the internal sintering stress of the ceramic is the smallest at 5K / min, and because the high temperature section is short, changing the heating rate will not significantly affect the sintering cost. In order to protect the performance of the sintering furnace in the high temperature section, the high temperature section generally tends to choose a process with a slower heating rate, so the 1400℃-1900℃ section with a heating rate of 5K / min is selected as the sintering process after the third optimization. That is, the sintering process of the sintering process curve after the third optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, and the temperature is kept at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and the temperature is kept at 1900℃ for 60min.

[0100] (b4) optimizing the sintering process curve after the third optimization by changing the influence of the holding time of the high temperature section on the sintering stress, and obtaining the sintering process curve after the fourth optimization;

[0101] The insulation stage is the stage in which the internal structure of the ceramic is balanced and stabilized. Figure 5aBy changing the high temperature holding time (holding for 60 minutes, holding for 180 minutes), the influence on the sintering stress is analyzed and the results are as follows: Figure 5b As shown, it can be found that the change of the holding time in the high temperature section will not significantly affect the sintering stress. Considering the sintering cost, the longer the holding time in the high temperature section, the higher the cost. Therefore, the holding time of 1h at 1900℃ is selected as the final optimized sintering process, that is, the sintering process of the sintering process curve after the fourth optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10℃ / min, keeping at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5℃ / min, and keeping at 1900℃ for 60min.

[0102] In the above (b1) to (b4), each optimized sintering process curve is brought into the simulation of transient heat conduction in the finite element model to simulate the sintering shrinkage deformation, evaluate the influence of each optimized sintering process curve on the sintering stress, and take the sintering process curve after the fourth optimization as the optimal sintering process curve.

[0103] Through steps one to five, the transient heat conduction and sintering shrinkage deformation are jointly simulated in the commercial software Abaqus. The two are sequentially coupled to obtain the temperature field corresponding to the transient heat conduction model and the ceramic shrinkage deformation corresponding to the sintering deformation model. The finite element model is compared with the results of microscopic characterization in the sintering experiment to verify the validity of the finite element model. The sintering process is changed by adjusting the process parameters (viscoelastic constitutive equation). After adjusting the transient heat conduction temperature field of the finite element model, the finite element simulation of silicon nitride shrinkage deformation is performed; since sintering stress is often considered to be the driving force for shrinkage deformation densification during sintering, the degree of reduction of sintering stress is analyzed by changing the process parameters, thereby optimizing the sintering process.

[0104] The method of this embodiment optimizes the ceramic sintering process parameters by means of numerical simulation. After only a few sintering experiments are required to provide data to verify the model, the sintering process can be optimized by a faster numerical simulation method, thereby achieving the purpose of reducing costs and improving production efficiency.

[0105] Therefore, this embodiment can effectively optimize the sintering process parameters of photocuring 3D printed silicon nitride, and in the process of optimizing the sintering process, it takes into account both the reduction of sintering stress and the sintering cost. The calculation method is relatively simple, which provides a low-cost and high-efficiency method for optimizing the sintering process parameters and provides an effective guidance scheme for optimizing the sintering process parameters of molded silicon nitride ceramics.

[0106] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for optimizing silicon nitride sintering process parameters based on numerical simulation, characterized in that: The steps include: Step 1: Perform a degreasing experiment on the photocuring 3D printed silicon nitride ceramics, perform microscopic characterization on the degreased ceramics, and obtain a first characterization result; Step 2: performing a sintering experiment on the degreased ceramic according to the original sintering process curve, and performing microscopic characterization on the sintered ceramic to obtain a second characterization result; Step 3: Establish a viscoelastic constitutive equation for sintering shrinkage deformation of silicon nitride ceramics, and construct a sintering finite element model in Abaqus based on the viscoelastic constitutive equation to obtain simulation results of transient temperature field and sintering shrinkage deformation; Step 4: Calibrate and correct the finite element model based on the second characterization result. When the relative error between the simulation result output by the finite element model and the second characterization result obtained by the experiment is less than or equal to 10%, a verified finite element model is obtained. Step 5. Optimize the original sintering process curve according to the verified finite element model, and bring the optimized sintering process curve back into the transient heat conduction simulation in the finite element model to simulate the sintering shrinkage deformation, so as to evaluate the influence of the optimized sintering process curve on the sintering stress, and select the sintering process curve with the lowest cost and the greatest influence on the degree of sintering stress reduction as the optimal sintering process, thereby realizing the optimization of the sintering process.

2. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 1, characterized in that: In step 3, the viscoelastic constitutive equation is constructed as follows: in, is the total strain rate, is the elastic strain rate, is the thermal strain rate, Creep strain rate.

3. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 1, characterized in that: The formula is as follows: in, is the stress rate, C is the elastic constant matrix, α is the thermal expansion coefficient of silicon nitride ceramics, Δ is the Hamiltonian vector differential operator, is the temperature change rate, σ′ is the deviatoric stress tensor, is the shear viscosity modulus, ηψ is the bulk viscosity modulus, σ h is the hydrostatic pressure, σ s is the sintering stress, I is the unit matrix when the material is isotropic, η is the viscosity expression, η0 is the pre-exponential coefficient of the viscosity term of the material, Q v is the viscous flow activation energy, R is the universal gas constant, T abs is the absolute temperature at the current temperature.

4. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 1, characterized in that: In step three, the finite element model is established as follows: Establish geometric models in the pre-processing of Abaqus, the engineering simulation finite element software; Transient temperature field simulation: define the temperature properties of the geometric model, define transient heat conduction in the analysis step, set the total time of the analysis step to the sintering process time, and define the sintering process curve in the amplitude curve in the load as a boundary condition; Sintering shrinkage deformation simulation: define the physical properties of the geometric model, select the viscosity analysis step as the analysis step, set the total time of the analysis step to the sintering process time, call the transient temperature field file in the load, and apply boundary conditions that correctly describe the sintering behavior to the geometric model based on the actual boundary conditions of silicon nitride ceramics in the sintering furnace.

5. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 3, characterized in that: In step 4, the verified finite element model is obtained as follows: The second characterization result is used to compare the shrinkage deformation, grain size and relative density of the finite element model output model; If the relative error is less than or equal to 10%, the verified finite element model is obtained; If the relative error is greater than 10%, return to step 3 to modify the pre-exponential coefficient η0 of the viscosity term in the viscoelastic constitutive equation until the relative error is less than or equal to 10%.

6. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 1, characterized in that: In step five, specifically: Performing segmented sintering process optimization on the initial sintering process curve in step 2, and using the segmented sintering process curve as the first optimized sintering process curve; By changing the influence of sintering temperature on sintering stress, the sintering process curve after the first optimization is optimized to obtain the sintering process curve after the second optimization; By changing the influence of the sintering rate in the high temperature stage on the sintering stress, the sintering process curve after the second optimization is optimized, and the sintering process curve after the third optimization is obtained; By changing the effect of the high temperature holding time on the sintering stress, the sintering process curve after the third optimization is optimized, and the sintering process curve after the fourth optimization is obtained; The above four optimized sintering process curves were respectively brought into the transient heat conduction simulation in the finite element model to simulate the sintering shrinkage deformation and evaluate the influence of each optimized sintering process curve on the sintering stress, so that the sintering process curve after the fourth optimization was taken as the optimal sintering process curve.

7. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 6, characterized in that: The sintering process of the initial sintering process curve is: the heating rate from room temperature 20°C to 1900°C is 10K / min, and the temperature is kept for 60min; The sintering process of the first optimized sintering process curve is as follows: the heating rate from room temperature 20°C to 1400°C is 10K / min, and the temperature is kept at 1400°C for 30min, and then the heating rate from 1400°C to 1900°C is 5K / min, and the temperature is kept at 1900°C for 60min; The sintering process of the second optimized sintering process curve is as follows: the heating rate from room temperature 20°C to 1400°C is 10K / min, and the temperature is kept at 1400°C for 30min, and then the heating rate from 1400°C to 1900°C is 5K / min, and the temperature is kept at 1900°C for 60min; The sintering process of the sintering process curve after the third optimization is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, and the temperature is kept at 1400℃ for 30min, and then the heating rate from 1400℃ to 1900℃ is 5K / min, and the temperature is kept at 1900℃ for 60min; The sintering process of the sintering process curve after the fourth optimization is: the heating rate from room temperature 20°C to 1400°C is 10K / min, keeping at 1400°C for 30min, then the heating rate from 1400°C to 1900°C is 5K / min, and keeping at 1900°C for 60min.

8. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to any one of claims 1 to 7, characterized in that: In step 1, the debinding experiment specifically includes: determining the temperature section where the weight loss of silicon nitride ceramics is the most serious through thermogravimetric analysis, and determining the debinding process through segmented sintering; The silicon nitride ceramic is placed in a tube furnace for degreasing and decarburization to obtain a degreasing ceramic.

9. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to claim 8, characterized in that: In steps one and two, the same characterization method is used to perform microscopic characterization on the degreased ceramics and the sintered ceramics. The characterization method is: measuring the internal grain size and grain distribution of the ceramics by scanning electron microscopy (SEM), determining the relative density of the ceramics according to the Archimedes drainage method, and measuring the geometric dimensions of the ceramics using a vernier caliper.

10. The method for optimizing silicon nitride sintering process parameters based on numerical simulation according to any one of claims 1 to 7, characterized in that: In step 2, the sintering experiment is specifically as follows: the degreased ceramic is placed in a ceramic gas pressure furnace for sintering, N2 needs to be introduced into the ceramic gas pressure furnace as a protective atmosphere, and the gas pressure is selected to be 1 MPa.

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