A method for optimizing sintering process parameters of silicon nitride based on numerical simulation

Abstract

The application discloses a kind of based on numerical simulation's silicon nitride sintering process parameter optimization method, including the following steps: to photo-curing 3D printing silicon nitride ceramic is successively implemented and sintered experiment is debound, micro characterization is carried out to the ceramic after sintering, and second characterization result is obtained;Establish the viscoelastic constitutive equation of silicon nitride ceramic sintering shrinkage deformation, based on the viscoelastic constitutive equation in Abaqus Sintering finite element model is constructed, to obtain the simulation result of transient temperature field and the simulation result of sintering shrinkage deformation;Second characterization result is corrected to finite element model, and verified finite element model is obtained;According to verified finite element model, the original sintering process curve is optimized, and the sintering process curve with the lowest cost and the greatest influence on sintering stress reduction is selected as the optimal sintering process, to realize the optimization of sintering process.The sintering process parameter optimization method not only reduces the cost, but also shortens the experimental period, and the production efficiency is high.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of silicon nitride sintering, and in particular to a silicon nitride sintering process parameter optimization method based on numerical simulation. BACKGROUND

[0002] After light-cured printing of a ceramic structure, the ceramic often contains 20wt%-30wt% of resin and high organic matter, and the resin and the high organic matter need to be removed from the ceramic as much as possible through a degreasing method. However, degreasing is usually performed at a temperature lower than that at which the ceramic internal grains aggregate and grow. At this time, after degreasing, the ceramic internal grains move, but the total volume of the pores does not change. At this time, the ceramic internal densification degree is low, and cannot meet the normal use. Therefore, sintering becomes a process step that must be performed before the ceramic is used normally. However, in the sintering process, different sintering processes need to be set for different materials because different microstructure properties of the ceramic determine different grain growth rules.

[0003] In the sintering process, sintering stress is defined as the force generated by balancing the shrinkage densification movement of the ceramic. Therefore, the greater the sintering stress, the higher the shrinkage deformation and the densification degree of the ceramic, and the greater the possibility of damage to the ceramic. For some ceramics, the increase of the sintering stress will lead to the destruction of the structure due to excessive shrinkage in the shrinkage deformation process, and therefore the sintering stress needs to be controlled. The energy of the ceramic shrinkage deformation is determined by the internal energy provided by the sintering process, and therefore the sintering process parameters can be controlled to effectively control the sintering stress of the model and improve the production efficiency of the sintering process.

[0004] For the optimization of the silicon nitride ceramic sintering process, document 1 “Wang H, Xuan WD, Yang ZG, et al. Pressureless sintering process optimization and performance of silicon nitride ceramics [J]. Bulletin of the Chinese Ceramic Society, 2016, 35(09): 2747-2752. DOI:10.16552 / j.cnki.issn1001-1625.2016.09.009” studies the effects of forming pressure, pressure holding time, holding time, sintering temperature, sintering additive content and ratio on the porosity and bending strength of silicon nitride ceramics through orthogonal experiment.

[0005] Document 2 “Xiao F, Xu Z, Xue J, et al. Sintering process of silicon nitride ceramics under gas pressure [J]. Journal of the Chinese Ceramic Society, 2019, 40(03): 382-386. DOI:10.13957 / j.cnki.tcxb.2019.03.019.” optimizes the sintering process of silicon nitride ceramics by analyzing the effects of different sintering temperatures and gas pressures on the microstructure, relative density and hardness of silicon nitride.

[0006] Document 3 "Gao X Q, Jin Y, Li J F, et al. Effect of sintering process on properties of Si3N4 foam ceramics [J]. Bulletin of the Chinese Ceramic Society, 2010, 29(05): 1055-1059. DOI: 10.16552 / j.cnki.issn1001-1625.2010.05.025", by analyzing the effects of heating rate and sintering temperature on the microstructure, phase and mechanical properties of silicon nitride mesh foam ceramics, the sintering process of silicon nitride foam ceramics is optimized.

[0007] Document 4 "Zhou C L, Fan J L, Hu X Q, 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", the effects of sintering additive mass fraction, sintering temperature and holding time on the densification degree and mechanical properties of β-silicon nitride ceramics are studied.

[0008] Document 5 "Zou Q. Sintering process of silicon nitride ceramic materials for radome [D]. Tianjin University, 2004", the effects of sintering atmosphere, temperature and sintering additives on the sintering properties of silicon nitride ceramics are analyzed, and the sintering mechanism of the system of nano-silicon nitride powder, micron silicon nitride, cordierite sintering aid, β-lithium feldspar sintering aid and atmosphere is established.

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

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

[0011] Document 8 "Li Xiaolei, Liu Yun, Zhou Miao, et al. High thermal conductivity silicon nitride ceramic material with excellent mechanical properties and preparation method [P]. Tianjin: CN202211264872. X, 2023-11-03", the microstructure is characterized after two-step sintering scheme, and the sintering process and sintering aid are produced by optimizing the sintering process and sintering aid.

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

[0013] Document 10 "Yangzhou, Li Hongtao, Zhang Chunyan, et al. Effect of one-step sintering and two-step sintering on structure and performance of silicon nitride ceramic [J]. Journal of ceramics, 2023, 44(04): 712-718. DOI: 10.13957 / j.cnki.tcxb.2023.04.011", by analyzing the influence of different sintering schemes on the density, phase and micro-morphology, mechanical properties of silicon nitride ceramic, a process method for preparing silicon nitride ceramic with good performance is determined.

[0014] However, the sintering process optimization of the above-mentioned documents is mostly based on the analysis of the influence of sintering process optimization on the performance of silicon nitride ceramic through a large number of experiments. The sintering process usually includes heating stage, holding stage and cooling stage. The cost of sintering process determined by trial and error method is high, and the experimental period is long. SUMMARY

[0015] Based on the technical problems existing in the background art, the present application proposes a silicon nitride sintering process parameter optimization method based on numerical simulation, which not only reduces the cost, but also shortens the experimental period and improves the production efficiency.

[0016] The silicon nitride sintering process parameter optimization method based on numerical simulation proposed by the present application comprises the following steps:

[0017] Step 1, carry out debinding experiment on photocured 3D printed silicon nitride ceramic, and carry out micro characterization on the debound ceramic to obtain first characterization result;

[0018] Step 2, according to the original sintering process curve, carry out sintering experiment on the debound ceramic, and carry out micro characterization on the sintered ceramic to obtain second characterization result;

[0019] Step three, establish the viscoelastic constitutive equation of silicon nitride ceramic sintering shrinkage deformation, and based on the viscoelastic constitutive equation, build a sintering finite element model in Abaqus to obtain the simulation results of transient temperature field and sintering shrinkage deformation;

[0020] Step four, calibrate and correct the finite element model based on the second characterization results, and when the relative error between the simulation results output by the finite element model and the second characterization results obtained by the experiment is less than or equal to 10%, the verified finite element model is obtained;

[0021] Step five, according to the verified finite element model, optimize the original sintering process curve, and bring the optimized sintering process curve into the simulation of transient heat conduction in the finite element model again 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 reduction of sintering stress as the optimal sintering process, thereby realizing the optimization of sintering process.

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

[0023]

[0024] Wherein, is the total strain rate, represents the elastic strain rate, represents the thermal strain rate, is the creep strain rate.

[0025] Further, The formula of is as follows:

[0026]

[0027]

[0028]

[0029]

[0030] Wherein, is the stress rate, C is the elastic constant matrix, α is the thermal expansion coefficient of silicon nitride ceramic, Δ is the Hamilton 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 viscosity term of the material, Q vis the activation energy for viscous flow, R is the universal gas constant, and T is the absolute temperature at the current temperature. abs is the absolute temperature at the current temperature.

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

[0032] A geometric model is established in the Abaqus pre-processing of the engineering simulation finite element software;

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

[0034] Sintering shrinkage deformation simulation: define the physical properties of the geometric model, select the viscous analysis step for the analysis step, set the total time of the analysis step as the sintering process time, call the transient temperature field file in the load, and apply the boundary condition correctly describing the sintering behavior according to the actual boundary condition of the ceramic in the sintering furnace to the geometric model.

[0035] Further, in step four, the process of obtaining the verified finite element model is as follows:

[0036] Compare the shrinkage deformation, grain size, and relative density output by the finite element model with the second characterization results;

[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 three to modify the viscous term coefficient η0 in the viscoelastic constitutive equation, until the relative error is less than or equal to 10%.

[0039] Further, in step five, specifically:

[0040] Subsection sintering process optimization is performed on the initial sintering process curve in step two, and the subsection sintering process curve is used as the first optimized sintering process curve;

[0041] The first optimized sintering process curve is optimized by changing the sintering temperature to affect the sintering stress, and the second optimized sintering process curve is obtained;

[0042] The second optimized sintering process curve is optimized by changing the sintering rate at high temperature to affect the sintering stress, and the third optimized sintering process curve is obtained;

[0043] The third optimized sintering process curve is optimized by changing the holding time at high temperature to affect the sintering stress, and the fourth optimized sintering process curve is obtained;

[0044] The four optimized sintering process curves are respectively introduced into the simulation of transient heat conduction in the finite element model to simulate the sintering shrinkage deformation, evaluate the influence of the sintering process curve after each optimization on the sintering stress, and take the sintering process curve after the fourth optimization as the optimal sintering process curve.

[0045] Further, the sintering process of the initial sintering process curve is: the heating rate from room temperature 20℃ to 1900℃ is 10K / min, and the holding time is 60min;

[0046] 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, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min;

[0047] 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, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 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, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min;

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

[0050] Further, in step one, the debinding experiment is specifically: through thermogravimetric analysis, the temperature section with the most serious weight loss of silicon nitride ceramic is determined, and the debinding process is determined through staged sintering;

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

[0052] Further, in steps one and two, the same characterization method is used to characterize the debound ceramic and the sintered ceramic, and the characterization method is: the internal grain size and grain distribution of the ceramic are measured by scanning electron microscopy SEM, the relative density of the ceramic is determined according to Archimedes drainage method, and the geometric size of the ceramic is measured by vernier caliper.

[0053] Further, in step two, the sintering experiment is specifically as follows: after debinding, the ceramic is placed in a ceramic gas pressure furnace for sintering, N2 is introduced into the ceramic gas pressure furnace as a protective atmosphere, and the gas pressure is 1 MPa.

[0054] The method has the advantages that: the sintering process parameters of the silicon nitride ceramic are optimized through numerical simulation, after a small amount of sintering experiment data is provided to verify the model, the sintering process is optimized through a fast numerical simulation method, and the purposes of reducing cost and improving production efficiency are achieved. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 The figure is a flowchart of the present application;

[0056] Figure 2a The figure is an optimization process curve diagram for segmenting the initial sintering process curve to optimize the sintering process;

[0057] Figure 2b The figure is a sintering stress curve corresponding to the optimization process curve of Figure 2a The figure is a sintering stress curve corresponding to the optimization process curve of

[0058] Figure 3a The figure is an optimization process curve diagram for selecting four sintering temperatures to continue optimizing the sintering curve after the first optimization;

[0059] Figure 3b The figure is a sintering stress curve corresponding to the optimization process curve of Figure 3a The figure is a sintering stress curve corresponding to the optimization process curve of

[0060] Figure 4a The figure is an optimization process curve diagram for selecting four sintering temperatures to continue optimizing the sintering curve after the first optimization;

[0061] Figure 4b The figure is a sintering stress curve corresponding to the optimization process curve of Figure 4a The figure is a sintering stress curve corresponding to the optimization process curve of

[0062] Figure 5a The figure is an optimization process curve diagram for selecting four sintering temperatures to continue optimizing the sintering curve after the first optimization;

[0063] Figure 5b The figure is a sintering stress curve corresponding to the optimization process curve of Figure 5a The figure is a sintering stress curve corresponding to the optimization process curve of DETAILED DESCRIPTION

[0064] The technical solutions of the present application will be described in detail below through specific embodiments. In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application. However, the present application can be implemented in many different ways other than those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application. Therefore, the present application is not limited to the specific implementations disclosed below.

[0065] As shown in Figures 1 to 5b A silicon nitride sintering process parameter optimization method based on numerical simulation is provided, which comprises the following steps:

[0066] Step one, carry out debinding experiment on the photocured 3D printed silicon nitride ceramic, and perform micro characterization on the debound ceramic to obtain first characterization results;

[0067] The debinding experiment is specifically: determine the temperature section with the most serious weight loss of the silicon nitride ceramic through thermal gravimetric analysis, and determine the debinding process through segmented sintering; place the silicon nitride ceramic in a tube furnace for debinding. Since the organic matter cannot be completely discharged from the inside of the green body during the debinding of the silicon nitride ceramic, part of the organic matter will be deposited on the surface in the form of residual carbon. After decarburization treatment of the green body, the sintering test specimen (i.e. the debound ceramic) is obtained.

[0068] The debound ceramic and the sintered ceramic are micro characterized in the same way, which is: measuring the grain size and grain distribution inside the ceramic under different magnifications through scanning electron microscopy (SEM), determining the relative density of the ceramic according to the Archimedes drainage method, and measuring the geometric size of the ceramic using a vernier caliper.

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

[0070] Step two, sinter the debound ceramic according to the original sintering process curve, and micro characterize the sintered ceramic to obtain second characterization results;

[0071] The sintering experiment is specifically: the silicon nitride ceramic needs high energy for the mutual adhesion and growth of its internal grains, and needs to be purged with a protective gas, so the silicon nitride ceramic is placed in a high-temperature ceramic gas pressure furnace for sintering. Since the silicon nitride ceramic is prone to react with air during sintering to generate impurities such as silicon carbide and silicon oxide, if the silicon nitride ceramic is directly debound in air, the silicon nitride ceramic will be oxidized and decomposed by oxygen, resulting in the generation of secondary phases and other impurities. Therefore, N2 needs to be purged as a protective atmosphere in the gas pressure furnace. According to the process experience, the gas pressure is selected as 1MPa. The initial sintering process (i.e. the original sintering process curve) that has not been optimized is as shown in Figure 2aThe sintering of the silicon nitride ceramic is completed from room temperature 20℃ to 1900℃ at a rate of 10K / min, and the silicon nitride ceramic is cooled with the gas pressure furnace after sintering for 60min.

[0072] Step three, establish the viscoelastic constitutive equation of the sintering shrinkage deformation of the silicon nitride ceramic, and build the sintering finite element model in Abaqus based on the viscoelastic constitutive equation to obtain the simulation results of the transient temperature field and the sintering shrinkage deformation;

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

[0074]

[0075] wherein, is the total strain rate;

[0076] The expression of is as follows:

[0077]

[0078]

[0079]

[0080] wherein, is the stress rate, C is the elastic constant matrix, α is the thermal expansion coefficient of the silicon nitride ceramic, Δ is the Hamilton 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 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] wherein, the growth kinetics formula of g is: D is the pre-exponential factor of the grain size growth term of the silicon nitride ceramic, Q GThe activation energy for the grain growth of the silicon nitride ceramic.

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

[0083] (a2-1) A geometric model is established in the Abaqus pre-processing finite element software engineering simulation.

[0084] A geometric model of silicon nitride 15x5x5mm is established, because the geometric model is relatively simple, the geometric model is 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, density, define transient heat conduction in analysis step, set the total time of analysis step as the sintering process time, define the sintering process curve in the amplitude curve as the boundary condition in the load, divide the grid of the discrete model, select the element type as DC3D8, and submit the calculation after establishing 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 variable, expansion coefficient, creep expansion strain and creep strain, select viscous analysis step in analysis step, set the total time of analysis step as the sintering process time, call the transient temperature field file in the load, and apply the boundary condition correctly describing the sintering behavior to the geometric model according to the actual boundary condition of the silicon nitride ceramic in the sintering furnace; In this embodiment, the boundary condition is selected as the constraint set model bottom vertical direction freedom degree, the gravity load is applied, the grid of the discrete model is divided, the element type is selected as DC3D8R, the job is established, the Creep subroutine is called, and the calculation is submitted.

[0087] Step four, based on the second characterization result, the finite element model is calibrated and corrected, when the relative error between the simulation result of the finite element model output and the second characterization result obtained by experiment is less than or equal to 10%, the 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 with 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%, the modification of the viscous term coefficient η0 in the viscoelastic constitutive equation is returned, and the rest of the activation energy and other parameters are not changed, because η0 has a large fluctuation 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 table of numerical simulation results and measured values of second characterization results

[0090] Characterization parameter Measured value Simulated value Relative error x-y direction dimension 6.40 mm 6.62 mm 3.36％ z direction dimension 13.60 mm 14.06 mm 3.36％ Grain size 0.99 μm 0.93 μm 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), the maximum relative error is kept within 5.3%, which is within the relative error requirement limit of 10%, thereby verifying the effectiveness of the finite element model, and indicating that the established model can accurately simulate the actual sintering state of silicon nitride ceramic.

[0092] Step five, according to the verified finite element model, the original sintering process curve is optimized, the optimized sintering process curve is brought into the simulation of transient heat conduction of the finite element model again to simulate the sintering shrinkage deformation, so as to evaluate the influence of the optimized sintering process curve on the sintering stress, select the sintering process curve with the lowest cost and the greatest influence on the reduction of sintering stress as the optimal sintering process curve, and then realize the optimization of the sintering process.

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

[0094] (b1) segmenting the initial sintering process curve in step two to optimize the sintering process, and taking the segmented sintering process curve as the first optimized sintering process curve;

[0095] The unoptimized initial sintering process (i.e. corresponding to the initial sintering process curve) in step two is first optimized by segmenting the sintering process, and since silicon nitride ceramic will start to transform from α phase to β phase near 1400℃, the optimized process is as shown in Figure 2a , that is, from room temperature (20℃) to 1400℃ at a heating rate of 10K / min, and holding at 1400℃ for 30min, then from 1400℃ to 1900℃ at a heating rate of 5K / min, and holding at 1900℃ for 60min. By comparing the two sintering processes (before and after optimization), although the overall time of the optimized sintering process is extended by 2h, as Figure 2b shown, the sintering stress corresponding to the optimized sintering process is smaller than that of the unoptimized sintering process during the sintering process, and the segmented sintering process curve is selected as the first optimized sintering process. That is, the sintering process of the first optimized sintering process curve is: from room temperature 20℃ to 1400℃ at a heating rate of 10K / min, holding at 1400℃ for 30min, then from 1400℃ to 1900℃ at a heating rate of 5K / min, and holding at 1900℃ for 60min.

[0096] (b2) By changing the effect of sintering temperature on sintering stress, the sintering process curve after the first optimization was optimized, and the sintering process curve after the second optimization was obtained.

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

[0098] (b3) By changing the effect of the sintering rate at the high temperature stage on the sintering stress, the sintering process curve after the second optimization was optimized, and the sintering process curve after the third optimization was obtained.

[0099] The sintering rate during the high-temperature stage significantly affects the sintering quality of silicon nitride ceramics. Therefore, for the 1400℃–1900℃ process, the sintering rate is adjusted as follows: Figure 4a As shown, four heating rates (5 K / min, 8 K / min, 10 K / min, and 15 K / min) were selected as optimization conditions. The results obtained by analyzing the effect of different heating rates on sintering stress are as follows. Figure 4b As shown, the sintering stress inside the ceramic is lowest at a rate of 5 K / min. Furthermore, due to the shorter high-temperature range, changing the heating rate does not significantly affect the sintering cost. To protect the performance of the sintering furnace in the high-temperature range, a slower heating rate is generally preferred. Therefore, a heating rate of 5 K / min was selected for the 1400℃–1900℃ range as the sintering process after the third optimization. Specifically, the sintering process curve after the third optimization is as follows: a heating rate of 10 K / min from room temperature (20℃) to 1400℃, holding at 1400℃ for 30 min, then a heating rate of 5 K / min from 1400℃ to 1900℃, and holding at 1900℃ for 60 min.

[0100] (b4) By changing the effect of the high-temperature section holding time on the sintering stress, the sintering process curve after the third optimization was optimized, and the sintering process curve after the fourth optimization was obtained.

[0101] The heat preservation stage is the stage in which the internal structure of the ceramic reaches equilibrium and stability, such as Figure 5aThe effect of changing the holding time in the high-temperature section (holding for 60 min, holding for 180 min) on the sintering stress was analyzed, and the results were as follows Figure 5b As shown in the figure, it can be found that the change in the holding time in the high-temperature section does not significantly affect the sintering stress. Considering the sintering cost, the longer the holding time in the high-temperature section, the higher the cost required. Therefore, the holding time of 1 h at 1900 ℃ is selected as the final optimized sintering process, that is, the sintering process curve of the fourth optimization is as follows: the heating rate from room temperature 20 ℃ to 1400 ℃ is 10 ℃ / min, the holding time at 1400 ℃ is 30 min, the heating rate from 1400 ℃ to 1900 ℃ is 5 ℃ / min, and the holding time at 1900 ℃ is 60 min.

[0102] In the above (b1) to (b4), the sintering process curve after each optimization is brought into the simulation of transient heat conduction in the finite element model to simulate the sintering shrinkage deformation, evaluate the effect of the sintering process curve after each optimization 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 simulated in the commercial software Abaqus, and 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 in sequence. The effectiveness of the finite element model is verified by comparing the results of the microcharacterization in the sintering experiment. The sintering process is changed by adjusting the process parameters (viscoelastic constitutive equation), and the finite element simulation of the shrinkage deformation of silicon nitride is performed after adjusting the transient heat conduction temperature field of the finite element model. Since the sintering stress is often considered as the driving force for the densification of shrinkage deformation in the sintering process, the degree of reduction of the sintering stress is analyzed by changing the process parameters, so as to realize the optimization of the sintering process.

[0104] The method of the embodiment optimizes the ceramic sintering process parameters by numerical simulation. After a small amount of sintering experiment data is provided to verify the model, the sintering process can be optimized by a faster numerical simulation method, so as to reduce the cost and improve the production efficiency.

[0105] Therefore, the embodiment can effectively optimize the sintering process parameters of the light-cured 3D printing silicon nitride, and the sintering stress is reduced during the optimization of the sintering process, and the sintering cost is also considered. The calculation method is relatively simple, which provides a low-cost and high-efficiency method for the optimization of the sintering process parameters, and provides an effective guidance scheme for the optimization of the sintering process parameters of the silicon nitride ceramic.

[0106] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for optimizing sintering process parameters of silicon nitride based on numerical simulation, characterized in that, Comprising the following steps: Step one, carry out debinding experiment on photocured 3D printed silicon nitride ceramic, and carry out micro characterization on the debound ceramic to obtain first characterization result; Step two, sinter the debound ceramic according to the original sintering process curve, and carry out micro characterization on the sintered ceramic to obtain second characterization result; Step three, establish a viscoelastic constitutive equation of sintering shrinkage deformation of silicon nitride ceramic, and build a sintering finite element model based on the viscoelastic constitutive equation in Abaqus to obtain simulation results of transient temperature field and sintering shrinkage deformation; Step four, calibrate and correct the finite element model based on the second characterization result, and when the relative error between the simulation results output by the finite element model and the second characterization results obtained by experiment is less than or equal to 10%, the verified finite element model is obtained; Step five, according to the verified finite element model, optimize the original sintering process curve, and bring the optimized sintering process curve into the simulation of transient heat conduction in the finite element model again to simulate the sintering shrinkage deformation, so as to evaluate the influence of the optimized sintering process curve on the sintering stress, select the sintering process curve with the lowest cost and the greatest influence on the reduction of sintering stress as the optimal sintering process, and then realize the optimization of sintering process; In step five, specifically: Carry out segmented sintering process optimization on the initial sintering process curve in step two, and take the segmented sintering process curve as the first optimized sintering process curve; Optimize the first optimized sintering process curve by changing the influence of sintering temperature on sintering stress to obtain the second optimized sintering process curve; Optimize the second optimized sintering process curve by changing the influence of sintering rate at high temperature stage on sintering stress to obtain the third optimized sintering process curve; Optimize the third optimized sintering process curve by changing the influence of holding time at high temperature stage on sintering stress to obtain the fourth optimized sintering process curve; Bring the above four optimized sintering process curves into the simulation of transient heat conduction in the finite element model respectively to simulate the sintering shrinkage deformation, and evaluate the influence of each optimized sintering process curve on the sintering stress, so as to take the fourth optimized sintering process curve as the optimal sintering process curve.

2. The method for optimization of sintering process parameters of silicon nitride based on numerical simulation according to claim 1, characterized in that, In step three, the construction of the viscoelastic constitutive equation is as follows: ； wherein, is the total strain rate, represents the elastic strain rate, represents the thermal strain rate, represents the creep strain rate.

3. The method for optimization of sintering process parameters of silicon nitride based on numerical simulation according to claim 2, characterized in that, , , The formula is as follows: ； ； ； ； wherein is the stress rate, is the elastic constant matrix, is the thermal expansion coefficient of silicon nitride ceramic, is the Hamiltonian vector differential operator, is the temperature change rate, is the deviatoric stress tensor, is the shear viscosity modulus, is the bulk viscosity modulus, is the hydrostatic pressure, is the sintering stress, is the unit matrix when the material is isotropic, is the viscosity expression, is the viscosity term coefficient of the material, is the viscous flow activation energy, is the universal gas constant, is the absolute temperature at the current temperature.

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

5. The method for optimization of sintering process parameters of silicon nitride based on numerical simulation according to claim 3, wherein, In step four, the process of obtaining the verified finite element model is as follows: The second characterization result is compared with the finite element model output to obtain the shrinkage deformation, grain size and relative density of the 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 three to modify the coefficient of the viscous term in the viscoelastic constitutive equation Modify until the relative error is less than or equal to 10%.

6. The method of optimizing the sintering process parameters of silicon nitride based on numerical simulation according to claim 1, wherein, The sintering process of the initial sintering process curve is: the heating rate from room temperature 20℃ to 1900℃ is 10K / min, and the holding time is 60min; The sintering process of the first optimized sintering process curve is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min; The sintering process of the second optimized sintering process curve is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min; The sintering process of the third optimized sintering process curve is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min; The sintering process of the fourth optimized sintering process curve is: the heating rate from room temperature 20℃ to 1400℃ is 10K / min, the holding time at 1400℃ is 30min, the heating rate from 1400℃ to 1900℃ is 5K / min, and the holding time at 1900℃ is 60min.

7. The method of optimizing sintering process parameters for silicon nitride based on numerical simulation according to any one of claims 1 to 6, characterized in that, In step one, the debinding experiment is as follows: through thermogravimetric analysis, the temperature section with the most serious weight loss of silicon nitride ceramic is determined, and the debinding process is determined by staged sintering; The silicon nitride ceramic is placed in a tube furnace for debinding and decarburization to obtain the debound ceramic.

8. The method of claim 7, wherein, In steps one and two, the same characterization method is used to characterize the debound ceramic and the sintered ceramic, and the characterization method is as follows: the grain size and grain distribution of the ceramic are measured by scanning electron microscopy SEM, the relative density of the ceramic is measured by Archimedes drainage method, and the geometric size of the ceramic is measured by vernier caliper.

9. The method of optimizing the sintering process parameters of silicon nitride based on numerical simulation according to any one of claims 1 to 6, characterized in that, In step two, the sintering experiment is as follows: the debound ceramic is placed in a ceramic gas pressure furnace for sintering, N2 is introduced as protective atmosphere in the ceramic gas pressure furnace, and the gas pressure is 1MPa.

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