Preparation method of silicon nitride ceramic based on polysilazane coated modified silicon nitride powder

Abstract

The invention belongs to the technical field of ceramic materials, and particularly discloses a silicon nitride ceramic preparation method based on polysilazane coated modified silicon nitride powder, the method comprises the following steps: step S1, preparing polysilazane coated modified silicon nitride powder; s2, preparing photocuring ceramic slurry for printing; s3, green body printing is carried out; and S4, degreasing and sintering to prepare the silicon nitride ceramic based on the polysilazane coated modified silicon nitride powder. According to the preparation method of the silicon nitride ceramic based on the polysilazane coated modified silicon nitride powder, the surface of the silicon nitride powder is modified, and preparation of silicon nitride slurry with high curing performance and high-toughness silicon nitride ceramic can be realized based on a photocuring additive manufacturing technology.

Description

Preparation method of silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder Technical Field

[0001] This invention belongs to the field of ceramic materials technology, specifically relating to a method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder. Background Technology

[0002] Silicon nitride possesses excellent flexural strength, fracture toughness, hardness, wear resistance, thermal shock resistance, low dielectric constant, and wave transmission properties, leading to its widespread application in various fields, including ceramic cutting tools, bearing balls, artificial joints, and missile infrared antenna radomes. Photopolymerization 3D printing effectively overcomes the bottlenecks inherent in machining or mold manufacturing. While ensuring part forming accuracy, it significantly improves forming efficiency and preform strength, offering greater precision advantages compared to other additive manufacturing technologies.

[0003] However, compared to other commonly used ceramic powders such as alumina, silicon nitride has a greater difference in refractive index to light compared to resin. Silicon nitride has a refractive index of 2.1, while the refractive index of resin to ultraviolet light is typically between 1.4 and 1.6. This significant difference leads to a reduction in curing depth and affects curing performance. Furthermore, silicon nitride powder is grayish, which is beneficial for absorbing ultraviolet light, further impacting the curing performance of ceramic slurries.

[0004] Surface oxidation of silicon nitride powder can improve its optical properties. The resulting silicon oxide has a low refractive index, which can reduce the refractive index difference between ceramics and resins, thus improving the curing performance of the slurry. However, the prepared slurry has a low solid content, and the mechanical properties of silicon oxide are poor. Furthermore, introducing too much silicon oxide can lead to poor mechanical properties of the sintered silicon nitride ceramics. Although coating the surface of silicon nitride powder with organic matter and sintering aids can improve the rheological and curing properties of the slurry, during the debinding process, the organic matter coated on the ceramic powder surface undergoes pyrolysis and volatilization, leaving a large number of pores, which reduces the density and mechanical properties of the silicon nitride ceramics. In other words, the higher the resin content of the coating, the more difficult it is for the silicon nitride ceramics to densify during sintering. Therefore, although the ceramic precursor conversion method can effectively improve the curing and rheological properties of the slurry, a large amount of carbon that was not fully removed during the pyrolysis stage remains after sintering, thus affecting the mechanical properties of the silicon nitride ceramics.

[0005] Therefore, there is a need in the field to develop a method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder, which can effectively solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder. By modifying the surface of the silicon nitride powder and using photocurable additive manufacturing technology, it is possible to prepare silicon nitride slurry with high curing performance and high-toughness silicon nitride ceramics.

[0007] To achieve the above objectives, this invention provides a method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder, comprising the following steps: Step S1, preparing polysilazane-coated modified silicon nitride powder; Step S2, preparing a photocurable ceramic slurry for printing; Step S21, weighing 85-95 parts by mass of polysilazane-coated modified silicon nitride powder, 3-9 parts by mass of yttrium oxide powder, and 2-6 parts by mass of alumina powder, adding 0.8-1 times the total mass of the powder to anhydrous ethanol, ball milling, drying in an oven at 70°C for 8 hours, and passing through a 150-mesh sieve to obtain a mixed powder; wherein, anhydrous ethanol... The concentration is 99.7%; Step S22: Weigh 42-45 parts of mixed powder and 55-58 parts of photosensitive resin by volume, and mix them in a planetary degassing mixer to obtain a photocurable ceramic slurry for printing; Step S3: Print green blanks; Step S4: Degrease and sinter to prepare silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder; Step S41: Set the heating rate and holding time to perform thermal degreasing on the green blanks to obtain semi-degreased green blanks; Step S42: Transfer the semi-degreased green blanks to an atmosphere sintering furnace for sintering to obtain silicon nitride ceramics based on polysilazane-coated modified silicon nitride powders.

[0008] Preferably, step S1 specifically includes: Step S10, pretreating the silicon nitride powder; A. Preparing a 5% (w / w) dilute nitric acid solution, using a polytetrafluoroethylene beaker, weighing 85-90 parts by weight of dilute nitric acid and 10-15 parts by weight of silicon nitride powder, placing them in an ultrasonic cleaner for ultrasonic dispersion for 15 minutes, and then magnetically stirring at 500 rpm for 30 minutes to remove impurities from the powder surface, obtaining a slurry forming silanol groups, thus avoiding uneven crosslinking reaction in the subsequent polysilazane coating process; B. Transferring the slurry to a centrifuge tube, rotating at a speed of... C) Centrifuge at 4000 rpm for 5 min, discard the supernatant waste acid liquid; D) Add the same mass of deionized water as dilute nitric acid, place in an ultrasonic cleaner and ultrasonically disperse for 10 min, centrifuge at 4000 rpm for 5 min, discard the supernatant, repeat this step 5 times until the pH of the supernatant is 6.5-7.0; E) After discarding the supernatant, add the same mass of anhydrous ethanol as dilute nitric acid, repeat the ultrasonic-centrifugation cycle in C twice; after removing the supernatant, place in a vacuum oven and dry at 70℃ for 8 h to obtain the pretreated product. Step S11: Weigh 10-15 parts by mass of polysilazane and solvent and mix them. Stir magnetically at 500 rpm for 15 minutes. Add 80-90 parts of pretreated silicon nitride powder and place in an ultrasonic cleaner for ultrasonic dispersion treatment for 30 minutes to prevent powder agglomeration. Stir magnetically at 500 rpm for 10 hours while heating simultaneously to obtain a pre-cured polysilazane mixed powder slurry. Step S12: Place the pre-cured polysilazane mixed powder slurry in an oven for heating. After curing and removing the solvent, add 0.8-1 times the total mass of anhydrous ethanol to the powder, ball mill to mix evenly, and finally dry in an oven at 70℃ for 8 hours. Pass through a 150-mesh sieve to obtain polysilazane-coated modified silicon nitride powder with a polysilazane coating thickness of 30-50nm; wherein the concentration of anhydrous ethanol is 99.7%; the ball milling is specifically as follows: add 3 times the total mass of alumina balls with diameters of 4mm, 7mm and 10mm, in a mass ratio of 6:3:1, and ball mill at 150rpm for 12 hours using a flat roller mill.

[0009] Preferably, in step S11, the polysilazane is a vinyl polysilazane that can be cured by UV light and heat, and it is coated onto the surface of silicon nitride powder using a heat curing method; the solvent is n-butyl ether, and the amount of solvent added is 9-11 times the mass of polysilazane; by controlling the mass ratio of solvent to polysilazane, the viscosity of the polysilazane solution is significantly reduced by using a high dilution ratio, causing the polymer chains entangled in the polysilazane to extend. When the solvent evaporates, the silicon-hydrogen bonds in the polysilazane will combine with the silanol bonds on the surface of the acid-washed silicon nitride, inducing the polysilazane to spread directionally on the surface of the silicon nitride particles, controlling the thickness of the polysilazane coating layer to be 30-50 nm, forming a core-shell structure with uniform thickness; the silicon nitride powder is α-phase silicon nitride, and its particle size D50 The thickness is 0.7 μm; the heating temperature is 95 °C, and the heating time is 10 h.

[0010] Preferably, in step S12, the thermosetting specifically involves: pre-curing the oven at 95°C for 6 hours to fix the coating layer morphology; then adjusting the oven temperature to 180°C and 240°C and holding for 2 hours respectively to initiate a high-temperature cross-linking reaction, forming a polysilazane coating layer on the silicon nitride surface.

[0011] Preferably, in step S21, the particle size D of the yttrium oxide powder is... 50 The alumina powder is α-phase alumina with a particle size of 50 nm and a particle size D. 50 The particle size is 30nm; the ball milling process is as follows: add alumina balls with a total mass of 3 times the powder, the diameter of the alumina balls is 4mm, 7mm and 10mm, the mass ratio is 6:3:1, and ball mill at 150rpm for 12h using a flat roller mill.

[0012] Preferably, in step S22, the photosensitive resin is composed of monomers, dispersants, and photoinitiators; the monomers are composed of 40 parts by volume of trimethylolpropane triacrylate and 60 parts by volume of 1,6-hexanediol diacrylate, which enables the ceramic slurry to have high curing depth and low over-curing width during the printing process, resulting in ceramic green bodies with high molding accuracy; the dispersant is Lubrizol Solsperse 41000, and the amount of dispersant added is 2-4% of the mass of the mixed powder; the photoinitiator is phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide, and the amount of photoinitiator added is 1% of the mass of the photosensitive resin.

[0013] Preferably, in step S22, the stirring speed is 400 r / min and the stirring time is 4 h; wherein, the solid content of the photocurable ceramic slurry used for printing is 42-45 vol.%.

[0014] Preferably, step S3 specifically comprises: Step S31, designing a model using 3D modeling software, and setting the printing layer thickness, exposure intensity, and exposure time, and printing a green body using surface exposure projection; wherein the printing layer thickness is 20μm, the wavelength of the ultraviolet light used is 405nm, and the exposure intensity is 19mW / cm². 2 The exposure time was 3 seconds, and the exposure energy was 57 mJ / cm². 2 Step S32: Use a cleaning agent and simultaneously operate an ultrasonic cleaner for 20 minutes, then wipe the surface of the green body clean with a non-woven cloth to remove any remaining slurry. The cleaning agent is 99.9% 1,6-hexanediol diacrylate.

[0015] Preferably, step S41 specifically involves: placing the green blank in a degreasing furnace, evacuating it to below 10 Pa, introducing nitrogen gas to atmospheric pressure and maintaining a nitrogen flow rate of 50 mL / min to start heating, heating to 145°C at a rate of 0.2°C / min and holding for 180 min; heating to 215°C at a rate of 0.1°C / min and holding for 210 min; heating to 285°C at a rate of 0.1°C / min and holding for 300 min; heating to 475°C at a rate of 0.1°C / min and holding for 180 min; heating to 525°C at a rate of 0.1°C / min and holding for 180 min; heating to 595°C at a rate of 0.1°C / min and holding for 180 min; heating to 615°C at a rate of 0.1°C / min and holding for 180 min; and cooling to room temperature at a rate of 1°C / min to obtain a semi-degreased green blank.

[0016] Preferably, step S42 specifically involves: placing the semi-degreased green blank into a boron nitride crucible, placing it into an atmosphere sintering furnace, using nitrogen as the protective atmosphere, heating it to 1400°C at a rate of 10°C / min, then heating it to 1800°C at a rate of 5°C / min and holding it at that temperature for 120 min, and then cooling it to room temperature at a rate of 5°C / min to obtain silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder.

[0017] The present invention adopts the above-mentioned method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder, and the beneficial effects are as follows: (1) The present invention adopts a simple thermosetting process, and the coating method used successfully coats polysilazane uniformly onto silicon nitride powder. Using a polytetrafluoroethylene beaker can successfully demold the silicon nitride powder after coating, avoiding adhesion to the beaker wall. By controlling the mass ratio of solvent to polysilazane, the thickness of the polysilazane coating layer on the surface of silicon nitride is precisely controlled between 30-50nm, thereby effectively reducing the refractive index difference between silicon nitride powder and resin after coating with polysilazane, while avoiding excessive coating layer leading to increased slurry viscosity, thus improving the curing performance and rheological properties of slurry.

[0018] (2) In this invention, the polysilazane on the surface of silicon nitride powder will decompose to generate silicon nitride during sintering, which improves the density and mechanical properties of silicon nitride ceramics and makes up for the shortcomings of powder surface modification methods such as thermosetting resin coating and surface oxidation; the carbon component generated by the decomposition of polysilazane will undergo a carbothermic reduction reaction with the natural oxide layer on the surface of silicon nitride powder during sintering, which effectively reduces the oxygen content at the grain boundaries and reduces the residue of amorphous glass phase; the prepared silicon nitride ceramics form a self-toughened microstructure, and the density is 98.4% as measured by Archimedes method and the fracture toughness is 8.9 MPa·m as measured by single-sided notched beam method. 1 / 2 Compared to silicon nitride ceramics prepared from powder with surface oxidation modification, both fracture toughness and density are improved.

[0019] (3) The curing performance of the polysilazane-coated modified silicon nitride slurry for photopolymerization 3D printing prepared in this invention is improved, and the curing performance is improved when the exposure intensity is 57 mJ / cm. 2 At that time, the curing depth of a single layer is 48μm; during the photocuring process, the polysilazane coated on the powder surface can undergo a copolymerization reaction with the resin monomer, which can significantly improve the strength of the green body and avoid defects such as cracks and warping in the post-processing stage.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 shows the EDS distribution of C and N elements in the polysilazane-coated modified silicon nitride powder prepared according to the embodiment of the method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder of the present invention; Figure 2 shows the surface SEM image of the silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder after alkaline etching according to the embodiment of the method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder of the present invention; Figure 3 shows the surface SEM image of the surface-oxidized silicon nitride ceramic prepared by Comparative Example 1 of the method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder of the present invention after alkaline etching; Figure 4 shows the surface SEM image of the unmodified silicon nitride ceramic prepared by Comparative Example 2 of the method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder of the present invention after alkaline etching; Figure 5 shows the performance comparison diagram of various ceramics in the experimental examples of the method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0024] The embodiment describes a method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder, comprising the following steps: Step S1: Preparing polysilazane-coated modified silicon nitride powder; Step S10: Pre-treating the silicon nitride powder; A. Preparing a 5% (w / w) dilute nitric acid solution. Using a polytetrafluoroethylene (PTFE) beaker, weigh 85 parts by weight of dilute nitric acid and 15 parts by weight of silicon nitride powder, place them in an ultrasonic cleaner, ultrasonically disperse for 15 minutes, and magnetically stir at 500 rpm for 30 minutes to remove impurities from the powder surface, obtaining a slurry with silanol groups; B. Transferring the slurry to a centrifuge tube, centrifuging at 4000 rpm for 5 minutes, and discarding the upper waste acid liquid; C. Adding the same mass of deionized water as the dilute nitric acid, and placing it in an ultrasonic cleaner... The mixture is ultrasonically dispersed in a wave cleaner for 10 minutes, centrifuged at 4000 rpm for 5 minutes, and the supernatant is discarded. This cycle is repeated 5 times until the pH of the supernatant is 6.5-7.0. D. After discarding the supernatant, anhydrous ethanol of the same mass as dilute nitric acid is added, and the ultrasonic-centrifugation cycle in C is repeated twice. After removing the supernatant, the mixture is placed in a vacuum oven and dried at 70℃ for 8 hours to obtain pretreated silicon nitride powder. Step S11: 10 parts by mass of vinyl polysilazane and 9 times the mass of vinyl polysilazane n-butyl ether are weighed and mixed. The mixture is magnetically stirred at 500 rpm for 15 minutes. After stirring evenly, 90 parts of pretreated silicon nitride powder are added. The silicon nitride powder is α-phase silicon nitride with a particle size D. 50 The particle size was 0.7 μm. The mixture was magnetically stirred at 500 rpm for 10 hours while simultaneously heated to 95°C for 10 hours to obtain a pre-cured polysilazane mixed powder slurry. In step S12, the pre-cured polysilazane mixed powder slurry was placed in an oven for thermal curing to remove n-butyl ether. The oven temperature was 95°C, and the holding time was 6 hours. The oven temperature was then adjusted to 180°C and 240°C, and held for 2 hours each to thermally cure the polysilazane on the surface of the silicon nitride powder. After adding 0.8 times the total mass of the powder to 99.7% anhydrous ethanol, the mixture was ball-milled until homogeneous. Specifically, alumina balls with diameters of 4 mm, 7 mm, and 10 mm were added at a mass ratio of 6:3:1. The mixture was ball-milled at 150 rpm for 12 hours using a flat roller mill.

[0025] Finally, the powder was dried in an oven at 70℃ for 8 hours and passed through a 150-mesh sieve to obtain polysilazane-coated modified silicon nitride powder with a polysilazane coating thickness of 30-50 nm.

[0026] Step S2: Prepare photocurable ceramic slurry for printing; Step S21: Weigh 90 parts of polysilazane-coated modified silicon nitride powder, 6 parts of yttrium oxide powder, and 4 parts of alumina powder according to the mass ratio, add 0.8 times the total mass of the powder with 99.7% anhydrous ethanol and ball mill. Specifically, add 3 times the total mass of the powder with alumina balls of diameters of 4mm, 7mm, and 10mm, in a mass ratio of 6:3:1, and ball mill at 150rpm for 12h using a flat roller mill.

[0027] Finally, the mixture was dried in an oven at 70℃ for 8 hours and then passed through a 150-mesh sieve to obtain the mixed powder; the particle size D of the yttrium oxide powder was... 50 The alumina powder is α-phase alumina with a particle size of 50 nm and a particle size D. 50 It is 30nm.

[0028] Step S22: Weigh 42 parts of the mixed powder and 58 parts of the photosensitive resin by volume, and mix them in a planetary degassing mixer at a speed of 400 r / min for 4 hours to obtain a photocurable ceramic slurry for printing with a solid content of 42-45 vol.%. The photosensitive resin is composed of monomers, dispersants, and photoinitiators. The monomers are composed of 40 parts of trimethylolpropane triacrylate and 60 parts of 1,6-hexanediol diacrylate by volume. The dispersant is Lubrizol Solsperse 41000, and the amount of dispersant added is 3% of the mass of the mixed powder. The photoinitiator is phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and the amount of photoinitiator added is 1% of the mass of the photosensitive resin.

[0029] Step S3: Perform green body printing; Step S31: Design the model using 3D modeling software, designing a cuboid with printing model dimensions of 4mm×6mm×35mm, and setting the printing layer thickness to 20μm, the wavelength of the ultraviolet light used to be 405nm, and the exposure intensity to 19mW / cm². 2 The exposure time was 3 seconds, and the exposure energy was 57 mJ / cm². 2 The single-layer curing depth of the slurry is controlled to be 50μm, and the green body is printed by surface exposure projection. In step S32, 1,6-hexanediol diacrylate with a concentration of 99.9% is used, and an ultrasonic cleaner is used for 20 minutes. Then, the slurry residue on the surface of the green body is wiped clean with a non-woven cloth.

[0030] Step S4: Debinding and sintering to prepare silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder; Step S41: Setting the heating rate and holding time to perform thermal debinding on the green body to obtain a semi-debinded green body; placing the green body in a debinding furnace, evacuating to below 10 Pa, introducing nitrogen to atmospheric pressure and maintaining a nitrogen flow rate of 50 mL / min to start heating, heating to 145 °C at a rate of 0.2 °C / min and holding for 180 min; heating to 215 °C at a rate of 0.1 °C / min and holding for 210 min. The mixture is heated to 285°C at a rate of 0.1°C / min and held for 300 min; then heated to 475°C at a rate of 0.1°C / min and held for 180 min; then heated to 525°C at a rate of 0.1°C / min and held for 180 min; then heated to 595°C at a rate of 0.1°C / min and held for 180 min; then heated to 615°C at a rate of 0.1°C / min and held for 180 min; and finally cooled to room temperature at a rate of 1°C / min to obtain a semi-degreased green body.

[0031] Step S42: Transfer the semi-degreased green body to an atmosphere sintering furnace for sintering to obtain silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder.

[0032] The semi-degreased green body is placed in a boron nitride crucible and then placed in an atmosphere sintering furnace. The protective atmosphere is nitrogen. The furnace is heated to 1400°C at a rate of 10°C / min, then heated to 1800°C at a rate of 5°C / min and held for 120 min. Finally, the furnace is cooled to room temperature at a rate of 5°C / min to obtain silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder.

[0033] As shown in Figure 1, the polysilazane-coated modified silicon nitride powder prepared in this embodiment has a carbon enrichment layer on its surface, proving that this embodiment successfully coated the silicon nitride powder surface with polysilazane. The high dilution ratio of the solvent to the polysilazane causes the entangled polymer chains in the polysilazane to extend. When the solvent evaporates, the silicon-hydrogen bonds in the polysilazane combine with the silanol bonds on the acid-washed silicon nitride surface, inducing the polysilazane to spread directionally on the silicon nitride particle surface. The thickness of the polysilazane coating layer is controlled to be 30-50 nm, forming a uniform core-shell structure. The prepared slurry is exposed at an energy of 57 mJ / cm². 2 At that time, the curing depth of a single layer was 48μm.

[0034] As shown in Figure 2, the silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder prepared in this embodiment exhibits a high degree of densification, forming long columnar β-phase silicon nitride. The density of the silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder was measured to be 98.4% using the Archimedes method, and the fracture toughness was measured to be 8.9 MPa·m using the single-sided notched beam method. 1 / 2 .

[0035] Comparative Example 1 differs from the Example in that: Step S1, silicon nitride powder is placed in an alumina crucible, placed in a sintering furnace, heated to 1200°C at 5°C / min in air atmosphere and held for 60 min, then cooled to 800°C at 5°C / min and cooled with the furnace. The oxidized silicon nitride powder is ground and pulverized. 55 parts by weight of the oxidized silicon nitride powder and 45 parts by weight of anhydrous ethanol are ball-milled at 150 r / min for 12 h using a flat roller mill. After ball milling, the powder is placed in an oven and dried at 70°C for 6 h. The powder is then sieved through a 150-mesh sieve to obtain surface-oxidized silicon nitride powder.

[0036] Step S2: Weigh 90 parts of surface-oxidized silicon nitride powder, 6 parts of yttrium oxide powder, and 4 parts of alumina powder according to the mass ratio. Add 0.8 times the total mass of the powder to anhydrous ethanol with a concentration of 99.7% and ball mill. Specifically, add 3 times the total mass of the powder to alumina balls with diameters of 4mm, 7mm, and 10mm, in a mass ratio of 6:3:1. Ball mill at 150rpm for 12 hours using a flat roller mill.

[0037] Finally, the mixture was dried in an oven at 70℃ for 8 hours and then passed through a 150-mesh sieve to obtain the mixed powder; the particle size D of the yttrium oxide powder was... 50 The alumina powder is α-phase alumina with a particle size of 50 nm and a particle size D. 50 It is 30nm.

[0038] The remaining steps are the same as in the example, and the silicon nitride ceramic with surface oxidation modification is finally prepared.

[0039] As shown in Figure 3, the silicon nitride ceramic prepared after surface oxidation in Comparative Example 1 has a low degree of densification, contains many pores, and has a small aspect ratio. The excessive silica glass phase restricts grain growth, resulting in short, thick rod-shaped crystals. The slurry prepared after surface oxidation modification, at an exposure energy of 57 mJ / cm², showed poor densification. 2 At that time, the single-layer curing depth was 42 μm. The prepared surface-oxidized silicon nitride ceramic had a density of 96.2% as measured by the Archimedes method and a fracture toughness of 4.94 MPa·m as measured by the single-sided notched beam method. 1 / 2 .

[0040] Comparative Example 2 differs from the Example in that: step S1 is omitted; in step S2, 90 parts of unmodified silicon nitride powder, 6 parts of yttrium oxide powder, and 4 parts of alumina powder are weighed according to their mass ratios, and 0.8 times the total mass of the powder is added with 99.7% anhydrous ethanol and ball milling is performed. Specifically, alumina balls with diameters of 4 mm, 7 mm, and 10 mm are added to the total mass of the powder, and the mass ratio of the alumina balls is 6:3:1. The mixture is ball milled at 150 rpm for 12 hours using a flat roller mill.

[0041] Finally, the mixture was dried in an oven at 70℃ for 8 hours and then passed through a 150-mesh sieve to obtain the mixed powder; the particle size D of the yttrium oxide powder was... 50 The alumina powder is α-phase alumina with a particle size of 50 nm and a particle size D. 50 It is 30nm.

[0042] The remaining steps are the same as in the example, and the final product is silicon nitride ceramic without surface modification.

[0043] As shown in Figure 4, the unmodified silicon nitride ceramic in Comparative Example 2 exhibits numerous pores, and its grain aspect ratio is smaller compared to the examples. The slurry prepared in Comparative Example 2, at an exposure energy of 57 mJ / cm², showed... 2 At that time, the single-layer curing depth was 28 μm, and the curing performance was poor. The unmodified silicon nitride ceramic had a density of 97.1% as measured by the Archimedes method, and a fracture toughness of 5.27 MPa·m as measured by the single-sided notched beam method. 1 / 2 .

[0044] The experimental examples tested the performance of silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder prepared in the examples, silicon nitride ceramics with surface oxidation modification prepared in Comparative Example 1, and silicon nitride ceramics without surface modification prepared in Comparative Example 2.

[0045] As shown in Figure 5, polysilazane coating modification is the most effective modification method among the three methods, which can significantly improve the fracture toughness and density of silicon nitride ceramics at the same time; surface oxidation modification has the worst effect on performance improvement, and is even weaker than the unmodified sample in fracture toughness; the performance of unmodified silicon nitride ceramics is in between the two.

[0046] Therefore, the present invention adopts the above-mentioned method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder. By modifying the surface of silicon nitride powder and based on photocuring additive manufacturing technology, it is possible to prepare silicon nitride slurry with high curing performance and high-toughness silicon nitride ceramics.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder, characterized in that, Includes the following steps: Step S1: Prepare polysilazane-coated modified silicon nitride powder; Step S2: Prepare photocurable ceramic slurry for printing; Step S21: Weigh 85-95 parts by mass of polysilazane-coated modified silicon nitride powder, 3-9 parts by mass of yttrium oxide powder, and 2-6 parts by mass of alumina powder, add 0.8-1 times the total mass of anhydrous ethanol, ball mill, dry in an oven at 70℃ for 8 hours, and pass through a 150-mesh sieve to obtain a mixed powder; wherein, the concentration of anhydrous ethanol is 99.7%; Step S22: Weigh 4 parts by volume of... 2-45 parts of mixed powder and 55-58 parts of photosensitive resin are mixed in a planetary degassing mixer to obtain a photocurable ceramic slurry for printing; Step S3: Print green body; Step S4: Debind and sinter to prepare silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder; Step S41: Set the heating rate and holding time to perform thermal debinding on the green body to obtain a semi-debinded green body; Step S42: Transfer the semi-debinded green body to an atmosphere sintering furnace for sintering to obtain silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder.

2. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 1, characterized in that, Step S1 specifically involves: Step S10, pretreating the silicon nitride powder; A. Preparing a 5% (w / w) dilute nitric acid solution. Using a polytetrafluoroethylene (PTFE) beaker, weigh 85-90 parts by weight of dilute nitric acid and 10-15 parts by weight of silicon nitride powder, place them in an ultrasonic cleaner, ultrasonically disperse for 15 minutes, and magnetically stir at 500 rpm for 30 minutes to remove impurities from the powder surface, obtaining a slurry forming silanol groups; B. Transferring the slurry to a centrifuge tube, centrifuging at 4000 rpm for 5 minutes, and discarding the supernatant. C. Add the same mass of deionized water as dilute nitric acid, place in an ultrasonic cleaner and ultrasonically disperse for 10 min, centrifuge at 4000 rpm for 5 min, discard the supernatant, and repeat this cycle 5 times until the pH of the supernatant is 6.5-7.0; D. After discarding the supernatant, add the same mass of anhydrous ethanol as dilute nitric acid, and repeat the ultrasonic-centrifugation cycle in C twice; after removing the supernatant, place in a vacuum oven and dry at 70℃ for 8 h to obtain pretreated silicon nitride powder; Step S1 1. Weigh 10-20 parts by weight of polysilazane and solvent and mix them. Stir magnetically at 500 rpm for 15 min. Add 80-90 parts of pretreated silicon nitride powder and place in an ultrasonic cleaner for ultrasonic dispersion treatment for 30 min to avoid powder agglomeration. Stir magnetically at 500 rpm for 10 h while heating simultaneously to obtain a pre-cured polysilazane mixed powder slurry. Step S12: Place the pre-cured polysilazane mixed powder slurry in an oven for heat curing and remove the solvent. Add anhydrous ethanol at 0.8-1 times the total mass of the powder, ball mill and mix evenly, and finally dry in an oven at 70℃ for 8 hours. Pass through a 150-mesh sieve to obtain polysilazane-coated modified silicon nitride powder with a polysilazane coating thickness of 30-50nm; wherein the concentration of anhydrous ethanol is 99.7%; the ball milling is specifically as follows: add alumina balls with a diameter of 4mm, 7mm and 10mm, and a mass ratio of 6:3:1, and ball mill at 150rpm for 12 hours using a flat roller mill.

3. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 2, characterized in that: In step S11, the polysilazane is vinyl polysilazane; the solvent is n-butyl ether, and the amount of solvent added is 9-11 times the mass of the polysilazane; the high dilution ratio of the solvent to the polysilazane is used to extend the polymer chains entangled in the polysilazane; the chemical bonding between the silicon-hydrogen bonds and the silanol bonds on the powder surface is used to induce the directional spreading of the polysilazane, controlling the thickness of the polysilazane coating layer to a uniform core-shell structure of 30-50 nm; the silicon nitride powder is α-phase silicon nitride, and its particle size D 50 The thickness is 0.7 μm; the heating temperature is 95 °C, and the heating time is 10 h.

4. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 2, characterized in that, In step S12, the thermosetting process specifically involves: when the oven temperature is 95°C, the holding time is 6 hours to pre-cure and fix the coating layer morphology; then the oven temperature is adjusted to 180°C and 240°C and held for 2 hours respectively to initiate a high-temperature cross-linking reaction, forming a polysilazane coating layer structure on the silicon nitride surface.

5. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 1, characterized in that: In step S21, the particle size D of the yttrium oxide powder 50 The alumina powder is α-phase alumina with a particle size of 50 nm and a particle size D. 50 The particle size is 30nm; the ball milling process is as follows: add alumina balls with a total mass of 3 times the powder, the diameter of the alumina balls is 4mm, 7mm and 10mm, the mass ratio is 6:3:1, and ball mill at 150rpm for 12h using a flat roller mill.

6. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 1, characterized in that: In step S22, the photosensitive resin is composed of monomers, dispersants, and photoinitiators; the monomers are composed of 40 parts by volume of trimethylolpropane triacrylate and 60 parts by volume of 1,6-hexanediol diacrylate; the dispersant is Lubrizol Solsperse 41000, and the amount of dispersant added is 2-4% of the mass of the mixed powder; the photoinitiator is phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and the amount of photoinitiator added is 1% of the mass of the photosensitive resin.

7. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 6, characterized in that: In step S22, the stirring speed is 400 r / min and the stirring time is 4 h; wherein, the solid content of the photocurable ceramic slurry used for printing is 42-45 vol.%.

8. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 1, characterized in that, Step S3 specifically involves: Step S31, designing a model using 3D modeling software, setting the printing layer thickness, exposure intensity, and exposure time, and printing a green body using surface exposure projection; wherein the printing layer thickness is 20μm, the wavelength of the ultraviolet light used is 405nm, and the exposure intensity is 19mW / cm². 2 The exposure time was 3 seconds, and the exposure energy was 57 mJ / cm². 2 Step S32: Use a cleaning agent and simultaneously operate an ultrasonic cleaner for 20 minutes, then wipe the surface of the green body with a non-woven cloth to remove any remaining slurry; the cleaning agent is 99.9% 1,6-hexanediol diacrylate.

9. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 1, characterized in that, Step S41 is as follows: Place the green body in a degreasing furnace, evacuate to below 10 Pa, introduce nitrogen to atmospheric pressure and maintain a nitrogen flow rate of 50 mL / min to start heating, heat to 145℃ at a rate of 0.2℃ / min and hold for 180 min; heat to 215℃ at a rate of 0.1℃ / min and hold for 210 min; heat to 285℃ at a rate of 0.1℃ / min and hold for 300 min; heat to 475℃ at a rate of 0.1℃ / min and hold for 180 min; heat to 525℃ at a rate of 0.1℃ / min and hold for 180 min; heat to 595℃ at a rate of 0.1℃ / min and hold for 180 min; heat to 615℃ at a rate of 0.1℃ / min and hold for 180 min; cool to room temperature at a rate of 1℃ / min to obtain a semi-degreased green body.

10. The method for preparing silicon nitride ceramics based on polysilazane-coated modified silicon nitride powder according to claim 9, characterized in that, Step S42 specifically involves placing the semi-degreased green body into a boron nitride crucible, placing it in an atmosphere sintering furnace, using nitrogen as the protective atmosphere, heating it to 1400°C at a rate of 10°C / min, then heating it to 1800°C at a rate of 5°C / min and holding it at that temperature for 120 min, and finally cooling it to room temperature at a rate of 5°C / min to obtain silicon nitride ceramic based on polysilazane-coated modified silicon nitride powder.

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