Silicon nitride composite material for grading wheel and preparation method of silicon nitride composite material

Abstract

The invention discloses a silicon nitride composite material for a grading wheel and a preparation method of the silicon nitride composite material in the field of silicon nitride composites.A polysilazane structure with a main chain containing a benzene ring is formed through silane ammonolysis and subjected to coordination doping with metal ions and a crosslinking reaction of diisocyanate, the metal ions and an organic carbon chain structure are introduced, and the silicon nitride composite material for the grading wheel is obtained; the preparation method comprises the following steps: forming a three-dimensional cross-linked polysilazane precursor containing metal and organic carbon, carrying out molding and sintering treatment to form a compact biscuit, converting the organic carbon into SiCN in situ at a high temperature, carrying out synergistic growth on Si3N4-SiCN-metal nitride, and finally forming the high-density Si3N4-SiCN-metal nitride composite material, and fine grains and a low-defect structure are combined, the high-load requirement of components such as the grading wheel is met, the high hardness and self-lubricating property of SiCN and the hard spot effect of metal nitride are superposed, and the service life is remarkably prolonged.

Description

Technical Field

[0001] This invention belongs to the field of silicon nitride composite material technology, specifically referring to a silicon nitride composite material for grading wheels and its preparation method. Background Technology

[0002] Powder classification is a key unit operation in fields such as new energy materials, building materials, biomedicine, and electronic information. The classifying wheel, as the core moving component of the classification equipment, directly determines the particle size distribution accuracy, production efficiency, and purity of the powder product. In actual operation, the classifying wheel needs to generate a centrifugal force field through high-speed rotation (typically 3000-15000 r / min), utilizing the force difference between centrifugal force and airflow drag to achieve separation—fine particles enter the collection system with the airflow, while coarse particles are thrown against the machine wall and discharged under centrifugal force. During this process, the classifying wheel operates in an extremely complex environment: it must withstand continuous centrifugal stress (up to hundreds of MPa) generated by high-speed rotation, face high-frequency impacts and frictional wear from powder particles, and in some scenarios, resist chemical corrosion from acidic / alkaline powders and temperature fluctuations of 200-600℃. This places extremely stringent comprehensive performance requirements on the prepared materials.

[0003] From an industrial application perspective, materials for grading wheels must meet four core indicators. First, excellent mechanical properties are fundamental: high hardness resists frictional wear of powder particles, preventing a decrease in wheel surface shape accuracy due to wear and ensuring long-term stability of grading particle size; high bending strength and fatigue strength can withstand centrifugal stress from high-speed rotation, preventing plastic deformation or fracture of the wheel under long-term dynamic loads; simultaneously, good fracture toughness is required to buffer the impact of hard particles such as silicon carbide and corundum, preventing brittle fracture. Second, chemical corrosion resistance is crucial for ensuring powder purity: the material must maintain chemical stability over a wide range of environments, not reacting with the powder to generate impurities, nor exhibiting surface corrosion or component dissolution. This is especially important in new energy lithium battery materials and pharmaceutical powder grading, where metal ion (such as Cr and Ni) contamination directly affects product performance. Third, thermal and dimensional stability are indispensable: the material must have a low coefficient of linear expansion to prevent temperature fluctuations from causing a decrease in wheel dimensional accuracy or internal stress concentration; and it must maintain stable mechanical properties within the operating temperature range, without softening or phase transformation. Fourth, high density and uniformity determine the stability of equipment operation: the inside of the wheel body should be free of obvious pores and have a small density gradient to ensure dynamic balance accuracy during high-speed rotation, reduce the impact of equipment vibration on grading accuracy and bearing wear.

[0004] Current grading wheel materials on the market cannot fully meet the above requirements. Metallic materials, such as 304 stainless steel and titanium alloys, while possessing good toughness and machinability, suffer from low hardness and poor wear resistance. Their service life in hard powder grading is typically less than 1000 hours, and they are prone to electrochemical corrosion with acidic powders, leading to powder contamination, making them unsuitable for high-end applications. Resin-based composite materials have low density and excellent corrosion resistance, but poor high-temperature performance and low surface hardness, making them only suitable for low-temperature soft powder grading (such as plastic powders). Among traditional ceramic materials, alumina ceramics have high hardness but low fracture toughness, making them susceptible to impact cracking; silicon carbide ceramics have slightly better toughness, but require high-temperature hot-pressing sintering above 1900℃, resulting in high manufacturing costs and poor thermal shock resistance, making them prone to cracking under temperature fluctuations.

[0005] Silicon nitride ceramics have become a research hotspot for grader materials due to their combination of high specific strength, excellent toughness, and corrosion resistance. However, existing silicon nitride materials still suffer from four key defects that restrict their industrial application:

[0006] Firstly, there are performance limitations caused by sintering aids. Silicon nitride has an extremely low self-diffusion coefficient, requiring the addition of sintering aids such as Y₂O₃ and Al₂O₃ to lower the sintering temperature (1600-1800℃). However, these aids can form low-melting-point glassy phases (such as Y-Si-Al-ON glassy phases) at grain boundaries. At high temperatures, the softening of the glassy phase leads to a sharp drop in high-temperature flexural strength and a deterioration in creep resistance. Furthermore, the Y and Al elements in the glassy phase readily react with acidic powders, initiating grain boundary corrosion and forming surface pores and internal microcracks. This not only reduces mechanical properties but may also contaminate the powder through corrosion products.

[0007] Secondly, its impact resistance and toughness remain insufficient. Although silicon nitride has better toughness than alumina ceramics, it is prone to edge chipping or microcrack propagation when handling hard particles such as diamond micro powder and silicon carbide coarse powder with a particle size ≥50μm. After long-term use, microcracks penetrate the wheel body, leading to fracture failure.

[0008] Third, the preparation process presents challenges related to uniformity and defects. The dry pressing method commonly used for silicon nitride grading wheels can easily lead to uneven density of the blank, resulting in local porosity ≥5% after sintering. During high-speed rotation, vibrations are generated due to uneven distribution of centrifugal force. If grain growth becomes uncontrolled during sintering, it will reduce the grain boundary bonding force, forming stress concentration points, which will further weaken the fracture toughness and impact resistance. Summary of the Invention

[0009] To address the above issues and overcome the shortcomings of existing technologies, this invention provides a silicon nitride composite material for grading wheels and its preparation method. This invention involves the ammonolysis of silane to form a polysilazane structure with a benzene ring in the main chain. This is further followed by coordination doping with metal ions and crosslinking with diisocyanate, introducing metal ions and organic carbon chain structures. This ultimately forms a three-dimensional crosslinked polysilazane precursor containing metal and organic carbon. After molding and sintering to form a dense preform, the organic carbon is converted in situ to SiCN at high temperature. Simultaneously, Si3N4-SiCN-metal nitride synergistically grows, ultimately forming a highly dense Si3N4-SiCN-metal nitride composite material. The high toughness of Si3N4 and the grain boundary strengthening of SiCN work synergistically, combined with a fine grain and low-defect structure, meeting the high load requirements of components such as grading wheels. The high hardness of SiCN, its self-lubricating properties, and the hard particles of the metal nitride significantly extend its service life.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention proposes a method for preparing a silicon nitride composite material for a grading wheel, specifically including the following steps:

[0011] A1. Dissolve 1,4-di(dimethylsilyl)benzene in anhydrous toluene, add triethylamine, mix well, then pass in a flowing inert gas and add thionyl chloride dropwise while maintaining room temperature. After the addition is complete, raise the reaction temperature to carry out the chlorination reaction. After the reaction is complete, cool, add anhydrous diethyl ether to the reaction system for dilution, add sodium carbonate to neutralize the reaction system, filter, collect the filtrate, and distill under reduced pressure to obtain 1,4-di(dimethylchlorosilyl)benzene.

[0012] A2. Add 1,4-bis(dimethylchlorosilyl)benzene, methyldichlorosilane, and methylvinyldichlorosilane prepared in step A1 to a flask in sequence, add anhydrous toluene, mix until the reactants are completely dissolved, then introduce flowing nitrogen gas, transfer to an ice bath to cool the reaction system thoroughly, introduce dry ammonia gas into the reaction system, remove the ice bath, raise the temperature to carry out the ammonolysis reaction, after the reaction is completed, let stand at room temperature, filter, collect the filtrate, and distill under reduced pressure to obtain the polysilazane copolymer;

[0013] A3. Dissolve the polysilazane copolymer prepared in step A2 in anhydrous toluene, introduce flowing nitrogen gas, dissolve the organometallic compound in anhydrous toluene, and sonicate to obtain a metal doping solution. Add the metal doping solution dropwise using a constant pressure funnel. After the addition is complete, adjust the reaction temperature to carry out the doping reaction. After the reaction is completed, the polysilazane doping solution is obtained.

[0014] A4. Add a catalyst to the polysilazane doped solution prepared in step A3, dissolve the diisocyanate compound in anhydrous toluene, add it dropwise to the reaction system, raise the temperature to carry out the crosslinking reaction, after the reaction is completed, cool, and distill under reduced pressure to obtain the precursor solution.

[0015] A5. Place the precursor solution prepared in step A4 into a vacuum degassing tank and let it stand under a vacuum of -0.095MPa to degas. Take the steel mold and perform surface treatment with silane coupling agent. Inject the degassed precursor solution into the mold, centrifuge, and simultaneously solidify. After centrifugation, continue to solidify to obtain the green blank.

[0016] A6. The green blank prepared in step A5 is sintered under a nitrogen atmosphere according to the heating program, and then cooled to obtain silicon nitride composite material.

[0017] Preferably, in step A1, the mass-to-volume ratio of 1,4-bis(dimethylsilyl)benzene to triethylamine is 2-3 g / 3-4.8 mL;

[0018] Preferably, in step A1, the mass-to-volume ratio of 1,4-bis(dimethylsilyl)benzene to thionyl chloride is 2-3 g / 1.7-3 mL;

[0019] Preferably, in step A1, the chlorination reaction temperature is 50-60°C and the chlorination reaction time is 3-4 hours;

[0020] Preferably, in step A2, the mass-to-volume ratio of 1,4-bis(dimethylchlorosilyl)benzene, methyldichlorosilane, and methylvinyldichlorosilane is 2-4 g: 2.0-3.2 mL: 1-2 mL;

[0021] Preferably, in step A2, the flow rate of the ammonia gas is 50-80 mL / min, and the ammonia gas introduction time is 20-40 min;

[0022] Preferably, in step A2, the reaction temperature of the ammonolysis reaction is 40-50°C, and the reaction time is 4-6 hours.

[0023] Preferably, in step A3, the organometallic compound includes at least one of ferrocene, nickel dicene, cobalt dicene, and aluminum isopropoxide;

[0024] Preferably, in step A3, the mass of the organometallic compound added is 3%-5% of the mass of the polysilazane copolymer;

[0025] Preferably, in step A3, the reaction temperature of the doping reaction is 0-30°C, and the reaction time of the doping reaction is 2-4 hours;

[0026] Preferably, in step A4, the catalyst comprises at least one of dibutyltin dilaurate, stannous octoate, and dibutyltin diacetylacetonate;

[0027] Preferably, in step A4, the mass of the catalyst added is 0.3%-0.5% of the mass of the polysilazane copolymer described in step A3;

[0028] Preferably, in step A4, the diisocyanate compound includes at least one of toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, toluene-2,5-diisocyanate, terephthalic diisocyanate, and isophthalic diisocyanate;

[0029] Preferably, in step A4, the mass of the diisocyanate compound added is 5%-15% of the mass of the polysilazane copolymer described in step A3;

[0030] Preferably, in step A4, the crosslinking reaction temperature is 50-60°C, and the crosslinking reaction time is 4-6 hours.

[0031] Preferably, in step A5, the centrifugation speed is 800-1200 rpm and the centrifugation time is 45-60 min;

[0032] Preferably, in step A5, the curing temperature of the curing treatment is 50-60℃, and the curing time is 2.5-4.5h;

[0033] Preferably, in step A6, the heating procedure for the sintering process is as follows:

[0034] Phase 1: Increase the temperature from room temperature to 120-150℃ at a rate of 1℃ / min, and hold for 1.5-2 hours.

[0035] Second stage: Increase the temperature to 300-320℃ at a rate of 0.5℃ / min, and hold for 3-4 hours;

[0036] Three stages: Heat to 600-650℃ at a rate of 1℃ / min, and hold for 5-6 hours;

[0037] Fourth stage: Increase the temperature to 1000-1100℃ at a rate of 2℃ / min, and hold for 2-3 hours;

[0038] Five stages: Increase the temperature to 1400-1500℃ at a rate of 1℃ / min, and hold for 3-4 hours.

[0039] The present invention also provides a silicon nitride composite material for grading wheels prepared according to the above preparation method.

[0040] The beneficial effects achieved by this invention are as follows:

[0041] A silicon nitride composite material for grading wheels and its preparation method are disclosed. This invention involves the ammonolysis of silane to form a polysilazane structure with a benzene ring in the main chain. Further coordination doping with metal ions and cross-linking reactions with diisocyanate are then carried out to introduce metal ions and organic carbon chain structures, ultimately forming a three-dimensional cross-linked polysilazane precursor containing metal and organic carbon. After molding and sintering to form a dense preform, the organic carbon is converted in situ to SiCN at high temperature. Simultaneously, Si3N4-SiCN-metal nitride synergistically grows, ultimately forming a highly dense Si3N4-SiCN-metal nitride composite material. The high toughness of Si3N4 and the grain boundary strengthening of SiCN work synergistically, combined with a fine grain and low-defect structure, to meet the high load requirements of components such as grading wheels. The high hardness of SiCN, its self-lubricating properties, and the hard particles of the metal nitride significantly extend its service life. In this application, 1,4-bis(dimethylchlorosilyl)benzene, methyldichlorosilane, and methylvinyldichlorosilane are used as raw materials for ammonolytic polycondensation. The nitrogen atom in ammonia gas contains a lone pair of electrons, which act as a nucleophile to attack the electron-deficient Si atom, replacing the Cl⁻ in the Si-Cl bond to generate Si-NH₂ or Si-NH-Si groups. Simultaneously, NH₄Cl precipitate is released. The bifunctional monomers are linked through Si-N bonds, undergoing a polycondensation reaction to form a polysilazane copolymer. The benzene ring backbone has a rigid structure, enhancing the deformation resistance of the polysilazane backbone. The metal ions (Fe) in the organometallic compound... 2+ Ni 2+ Al 3+Metals (such as toluene-2,4-diisocyanates) possess empty orbitals that form coordinate bonds with the N atoms (containing lone pairs of electrons) of the Si-NH- or Si-NH2 groups in polysilazane, allowing the metal elements to be uniformly dispersed in the polysilazane molecular chain rather than agglomerated, forming "metal coordination crosslinking points." These points serve as precursors for metal nitrides, laying the foundation for the subsequent formation of hard point phases during sintering. Under catalytic action, the aromatic ring carbon (benzene ring), alkyl carbon (methyl), and carbonyl carbon (-NCO) of diisocyanates (such as toluene-2,4-diisocyanate) covalently bind to the polysilazane network through reaction. In the final crosslinked structure, carbon exists as uniformly dispersed organic carbon (non-free carbon), laying the foundation for "atomic-level dispersion" for subsequent high-temperature conversion to SiCN. During the high-temperature sintering stage, the SiCN phase... Further growth and fusion with Si3N4 grains form a continuous Si3N4-SiCN phase structure. After molding, a low-defect, high-density green blank is formed. After multi-stage sintering treatment, residual solvent (toluene) and low-molecular-weight volatiles are removed to avoid cracking of the green blank. After preliminary pyrolysis, organic groups (methyl, vinyl) are slowly removed, and the Si-N framework rearranges. Deep pyrolysis can completely remove organic groups, and the Si-N framework polymerizes to form Si3N4 crystal nuclei. Metal elements are converted into metal nitrides. During crystallization, Si3N4 crystal nuclei grow, and metal nitrides adhere to the surface of the crystal nuclei, inhibiting excessive grain growth. Finally, through densification treatment, Si3N4 grains grow further, and metal nitrides fill the grain boundary voids to form a high-density composite structure. Attached Figure Description

[0042] Figure 1 The diagram shows the Vickers hardness results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention.

[0043] Figure 2 The graph shows the bending strength results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention at room temperature / high temperature.

[0044] Figure 3 The graph shows the thermal shock resistance results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention.

[0045] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0048] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the experimental materials and test strains used in the following examples were purchased from commercial channels.

[0049] Example 1

[0050] This embodiment provides a method for preparing a silicon nitride composite material for a grading wheel, specifically including the following steps:

[0051] A1. Add 20g of 1,4-di(dimethylsilyl)benzene to a dry flask, add 1000mL of anhydrous toluene and stir until the 1,4-di(dimethylsilyl)benzene is completely dissolved. After passing flowing nitrogen gas into the reaction system, add 30mL of triethylamine and stir at 300rpm. After mixing evenly, keep at room temperature and add 17mL of thionyl chloride dropwise at a rate of 1 drop / s. After the addition is complete, continue stirring and raise the reaction temperature to 50℃ to carry out the chlorination reaction. The reaction is carried out for 4 hours. After the reaction is completed, let the reaction system cool naturally to room temperature, add 500mL of anhydrous diethyl ether to dilute the reaction system, add sodium carbonate to neutralize the reaction system, filter, collect the filtrate, and distill under reduced pressure to obtain 1,4-di(dimethylchlorosilyl)benzene.

[0052] A2. Add 20g of 1,4-di(dimethylchlorosilyl)benzene, 20mL of methyldichlorosilane, and 10mL of methylvinyldichlorosilane prepared in step A1 to a flask in sequence. Add 500mL of anhydrous toluene and stir at 300rpm until the reaction system is uniformly mixed. Then, introduce flowing nitrogen gas. After 30min, transfer the reaction system to an ice-water bath and continue stirring to allow the reaction system to cool down sufficiently. Dry the reaction system with anhydrous CaCl2 gas through an anhydrous CaCl2 drying tower and introduce it into the reaction system at a flow rate of 50mL / min. After 20min, remove the ice-water bath and raise the reaction temperature to 50℃ to carry out the ammonolysis reaction. React for 4h. After the reaction is completed, let it stand at room temperature for 2h, filter and collect the filtrate, wash with anhydrous diethyl ether, filter, collect the filtrate, and distill under reduced pressure to obtain the polysilazane copolymer.

[0053] A3. Dissolve 20g of the polysilazane copolymer prepared in step A2 in 80mL of anhydrous toluene, purge with flowing nitrogen gas, and after 30min, dissolve 1.0g of ferrocene in 20mL of anhydrous toluene, sonicate at 300W for 40min to obtain a metal doping solution. Add the metal doping solution dropwise to the reaction system at a rate of 1 drop / s using a constant pressure funnel. After the addition is complete, adjust the reaction temperature to 30℃ and carry out the doping reaction for 3h. After the reaction is completed, the polysilazane doped solution is obtained.

[0054] A4. Add 0.1 g of dibutyltin dilaurate to the polysilazane doped solution prepared in step A3. Dissolve 1 g of toluene-2,5-diisocyanate in 5 mL of anhydrous toluene and add it dropwise to the reaction system at a rate of 1 drop / s. After mixing evenly, raise the reaction temperature to 50°C and react for 6 h. After the reaction is completed, allow the reaction system to cool naturally to room temperature and then distill under reduced pressure to obtain the precursor solution. The solid content of the precursor solution is 72.7%.

[0055] A5. Place the precursor solution prepared in step A4 in a vacuum degassing tank and let it stand under a vacuum of -0.095MPa for 30 minutes to degas. Take a 45# steel mold and perform surface treatment with KH-550. Inject the degassed precursor solution into the mold and centrifuge at 1200rpm for 15 minutes. At the same time, heat and cure it in a 60℃ oven. After 30 minutes, stop centrifugation and continue to cure at 60℃ for 2 hours to obtain the green blank.

[0056] A6. Place the green blank prepared in step A5 into a heating furnace, and introduce flowing nitrogen gas at a rate of 50 mL / min. Under the nitrogen atmosphere, raise the temperature from room temperature to 120°C at a rate of 1°C / min and hold for 2 hours. Raise the temperature to 300°C at a rate of 0.5°C / min and hold for 4 hours. Raise the temperature to 600°C at a rate of 1°C / min and hold for 6 hours. Raise the temperature to 1000°C at a rate of 2°C / min and hold for 3 hours. Raise the temperature to 1400°C at a rate of 1°C / min and hold for 4 hours. Stop heating and allow the reaction system to cool naturally to room temperature to obtain the silicon nitride composite material.

[0057] This embodiment also provides a silicon nitride composite material for grading wheels prepared according to the above preparation method.

[0058] Example 2

[0059] This embodiment provides a method for preparing a silicon nitride composite material for a grading wheel, specifically including the following steps:

[0060] A1. Add 25g of 1,4-di(dimethylsilyl)benzene to a dry flask, add 1000mL of anhydrous toluene and stir until the 1,4-di(dimethylsilyl)benzene is completely dissolved. After passing flowing nitrogen gas into the reaction system, add 40mL of triethylamine and stir at 300rpm. After mixing evenly, keep at room temperature and add 24mL of thionyl chloride dropwise at a rate of 1 drop / s. After the addition is complete, continue stirring and raise the reaction temperature to 55℃ to carry out the chlorination reaction for 3.5h. After the reaction is completed, let the reaction system cool naturally to room temperature, add 500mL of anhydrous diethyl ether to dilute the reaction system, add sodium carbonate to neutralize the reaction system, filter, collect the filtrate, and distill under reduced pressure to obtain 1,4-di(dimethylchlorosilyl)benzene.

[0061] A2. Add 30g of 1,4-di(dimethylchlorosilyl)benzene, 27mL of methyldichlorosilane, and 15mL of methylvinyldichlorosilane prepared in step A1 to a flask in sequence. Add 500mL of anhydrous toluene and stir at 300rpm until the reaction system is uniformly mixed. Then, introduce flowing nitrogen gas. After 30min, transfer the reaction system to an ice-water bath and continue stirring to allow the reaction system to cool down sufficiently. Dry the system with anhydrous CaCl2 gas through an anhydrous CaCl2 drying tower and introduce it into the reaction system at a flow rate of 65mL / min. After 40min, remove the ice-water bath and raise the reaction temperature to 45℃ for ammonolysis reaction. React for 5h. After the reaction is completed, let it stand at room temperature for 2h, filter and collect the filtrate, wash with anhydrous diethyl ether, filter, collect the filtrate, and distill under reduced pressure to obtain the polysilazane copolymer.

[0062] A3. Dissolve 30g of the polysilazane copolymer prepared in step A2 in 120mL of anhydrous toluene, introduce flowing nitrogen gas, and after 30min, dissolve 0.9g of cobalt dicene in 30mL of anhydrous toluene, sonicate at 300W for 40min to obtain a metal doping solution. Add the metal doping solution dropwise to the reaction system at a rate of 1 drop / s using a constant pressure funnel. After the addition is complete, adjust the reaction temperature to 20℃ and carry out the doping reaction for 4h. After the reaction is completed, the polysilazane doped solution is obtained.

[0063] A4. Add 0.12 g of dibutyltin dilaurate to the polysilazane doped solution prepared in step A3. Dissolve 3 g of toluene-2,5-diisocyanate in 5 mL of anhydrous toluene and add it dropwise to the reaction system at a rate of 1 drop / s. After mixing evenly, raise the reaction temperature to 55°C and react for 5 h. After the reaction is completed, allow the reaction system to cool naturally to room temperature and then distill under reduced pressure to obtain the precursor solution. The solid content of the precursor solution is 64.8%.

[0064] A5. Place the precursor solution prepared in step A4 in a vacuum degassing tank and let it stand under a vacuum of -0.095MPa for 30 minutes to degas. Take a 45# steel mold and perform surface treatment with KH-550. Inject the degassed precursor solution into the mold and centrifuge at 1000rpm for 20 minutes. At the same time, heat and cure in an oven at 55℃. After 30 minutes, stop centrifugation and continue to cure at 55℃ for 3 hours to obtain the green blank.

[0065] A6. Place the green blank prepared in step A5 into a heating furnace, and introduce flowing nitrogen gas at a rate of 50 mL / min. Under the nitrogen atmosphere, raise the temperature from room temperature to 150°C at a rate of 1°C / min and hold for 1.5 h. Raise the temperature to 320°C at a rate of 0.5°C / min and hold for 3 h. Raise the temperature to 650°C at a rate of 1°C / min and hold for 5 h. Raise the temperature to 1100°C at a rate of 2°C / min and hold for 2 h. Raise the temperature to 1500°C at a rate of 1°C / min and hold for 3 h. Allow the reaction system to cool naturally to room temperature to obtain the silicon nitride composite material.

[0066] This embodiment also provides a silicon nitride composite material for grading wheels prepared according to the above preparation method.

[0067] Example 3

[0068] This embodiment provides a method for preparing a silicon nitride composite material for a grading wheel, specifically including the following steps:

[0069] A1. Add 30g of 1,4-di(dimethylsilyl)benzene to a dry flask, add 1000mL of anhydrous toluene and stir until the 1,4-di(dimethylsilyl)benzene is completely dissolved. After passing flowing nitrogen gas into the reaction system, add 48mL of triethylamine and stir at 300rpm. After mixing evenly, keep at room temperature and add 30mL of thionyl chloride dropwise at a rate of 1 drop / s. After the addition is complete, continue stirring and raise the reaction temperature to 60℃ to carry out the chlorination reaction. The reaction is carried out for 3h. After the reaction is completed, let the reaction system cool naturally to room temperature, add 500mL of anhydrous diethyl ether to dilute the reaction system, add sodium carbonate to neutralize the reaction system, filter, collect the filtrate, and distill under reduced pressure to obtain 1,4-di(dimethylchlorosilyl)benzene.

[0070] A2. Add 40g of 1,4-di(dimethylchlorosilyl)benzene, 32mL of methyldichlorosilane, and 20mL of methylvinyldichlorosilane prepared in step A1 to a flask in sequence. Add 500mL of anhydrous toluene and stir at 300rpm until the reaction system is uniformly mixed. Then, introduce flowing nitrogen gas. After 30min, transfer the reaction system to an ice-water bath and continue stirring to allow the reaction system to cool down sufficiently. Dry the system with anhydrous CaCl2 gas through an anhydrous CaCl2 drying tower and introduce it into the reaction system at a flow rate of 80mL / min. After 40min, remove the ice-water bath and raise the reaction temperature to 40℃ to carry out the ammonolysis reaction. After the reaction is completed, let it stand at room temperature for 2h, filter and collect the filtrate, wash it with anhydrous diethyl ether, filter it, collect the filtrate, and distill it under reduced pressure to obtain the polysilazane copolymer.

[0071] A3. Dissolve 40g of the polysilazane copolymer prepared in step A2 in 160mL of anhydrous toluene, purge with flowing nitrogen gas, and after 30min, dissolve 1.6g of aluminum isopropoxide in 40mL of anhydrous toluene, sonicate at 300W for 40min to obtain a metal doping solution. Add the metal doping solution dropwise to the reaction system at a rate of 1 drop / s using a constant pressure funnel. After the addition is complete, adjust the reaction temperature to 0℃ and carry out the doping reaction for 2h. After the reaction is completed, the polysilazane doped solution is obtained.

[0072] A4. Add 0.16 g of dibutyltin dilaurate to the polysilazane doped solution prepared in step A3. Dissolve 6 g of toluene-2,5-diisocyanate in 5 mL of anhydrous toluene and add it dropwise to the reaction system at a rate of 1 drop / s. After mixing evenly, raise the reaction temperature to 60°C and react for 4 h. After the reaction is completed, allow the reaction system to cool naturally to room temperature and then distill under reduced pressure to obtain the precursor solution. The solid content of the precursor solution is 60.3%.

[0073] A5. Place the precursor solution prepared in step A4 in a vacuum degassing tank and let it stand under a vacuum of -0.095MPa for 30 minutes to degas. Take a 45# steel mold and perform surface treatment with KH-550. Inject the degassed precursor solution into the mold and centrifuge at 800rpm for 30 minutes. At the same time, heat and cure it in a 50℃ oven. After 30 minutes, stop centrifugation and continue to cure at 50℃ for 4 hours to obtain the green blank.

[0074] A6. Place the green blank prepared in step A5 into a heating furnace, and introduce flowing nitrogen gas at a rate of 50 mL / min. Under the nitrogen atmosphere, raise the temperature from room temperature to 140°C at a rate of 1°C / min and hold for 1.5 h. Raise the temperature to 310°C at a rate of 0.5°C / min and hold for 4 h. Raise the temperature to 630°C at a rate of 1°C / min and hold for 5.5 h. Raise the temperature to 1100°C at a rate of 2°C / min and hold for 2 h. Raise the temperature to 1400°C at a rate of 1°C / min and hold for 4 h. Allow the reaction system to cool naturally to room temperature to obtain the silicon nitride composite material.

[0075] This embodiment also provides a silicon nitride composite material for grading wheels prepared according to the above preparation method.

[0076] Comparative Example 1

[0077] This comparative example provides a silicon nitride composite material and its preparation method. The only difference between this example and Example 1 is that the preparation method of the silicon nitride composite material does not include step A1, and step A2 does not include 1,4-bis(dimethylchlorosilyl)benzene.

[0078] Comparative Example 2

[0079] This comparative example provides a silicon nitride composite material and its preparation method. The only difference between this example and Example 1 is that the preparation method of the silicon nitride composite material does not include step A3.

[0080] Comparative Example 3

[0081] This comparative example provides a silicon nitride composite material and its preparation method. The only difference between this example and Example 1 is that the preparation method of the silicon nitride composite material does not include step A4.

[0082] Experimental Example

[0083] Performance tests were conducted on Examples 1-3 and Comparative Examples 1-3, with the main test indicators including Vickers hardness, flexural strength, resistance to deformation, and thermal shock stability.

[0084] 1. Vickers hardness test, according to GB / T 4340.1-2009, uses a square pyramidal diamond indenter (136° apex) to press into the surface of a silicon nitride composite material under a specified test force. After holding for a certain time, the test force is removed, and the lengths of the two perpendicular diagonals of the indentation are measured. The Vickers hardness value (HV) is calculated using the following formula. The hardness value reflects the material's ability to resist localized plastic deformation; the higher the value, the stronger the material's surface hardness and basic properties such as scratch resistance and wear resistance. A digital Vickers hardness tester (model: MH-6, Shanghai Everone Precision Instruments Co., Ltd.) is used. Before testing, the test surface of the sample needs to be polished to a surface roughness Ra≤0.8μm. The parameters are set as follows: applied load of 98N, loading time of 15s. Then, the diagonal lengths of the indentation are measured. The final result can be calculated using the following formula to determine the Vickers hardness (HV).

[0085] ;

[0086] Where F is the applied load (N); d is the diagonal length of the indentation (mm).

[0087] Figure 1The figures show the Vickers hardness results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention. As shown, the core influencing factors of Vickers hardness include material density, microstructure, reinforcing phase, and chemical bond rigidity. In Example 1, the 1,4-bis(dimethylchlorosilyl)benzene synthesized in step A1 contains a rigid benzene ring structure. Introducing this into the polysilazane backbone can improve the rigidity of the molecular chain, resulting in a more stable silicon nitride skeleton after sintering and enhanced resistance to plastic deformation. In step A3, after ferrocene doping, iron elements form a second phase such as Fe4N or Fe-Si alloy during sintering, which is uniformly dispersed in the silicon nitride matrix. This "dispersion strengthening" hinders dislocation movement and significantly improves hardness. A4 The isocyanate modification in the steps increases the crosslinking degree of the precursor, increases the density of the green blank (reducing porosity), and results in fewer defects in the sintered material with a higher basic hardness. In Example 2, the metal doping solution was replaced with a cobalt diacene doping solution. Cobalt and iron are both transition metals. After cobalt diacene doping, a second phase such as Co4N or Co-Si alloy can be formed, which can also improve hardness through dispersion strengthening. However, the atomic radius of cobalt is slightly larger than that of iron, and its compatibility with the silicon nitride matrix is ​​slightly weaker than that of iron. The dispersion uniformity of the second phase is slightly worse, so the hardness is slightly lower than that of Example 1. In Example 3, the metal doping solution was replaced with an aluminum isopropoxide doping solution. After aluminum doping, AlN or Al2O3 second phase is mainly formed, and its hardness is lower than that of Fe4N or Co4N. In Comparative Example 1, the monomer containing a rigid benzene ring in step A1 is the core component for enhancing the strength of the silicon nitride framework. Without it, the polysilazane backbone consists only of methyldichlorosilane (a flexible chain) and methylvinyldichlorosilane, resulting in insufficient molecular chain rigidity. After sintering, the silicon nitride structure is loose and prone to forming coarse grains, leading to the lowest hardness. In Comparative Example 2, metal doping is the core of "second-phase strengthening." Without it, the silicon nitride matrix lacks a dispersed hard phase and relies solely on its own framework strength, significantly reducing its resistance to plastic deformation. Simultaneously, the absence of metal elements may lead to coarse grain growth, resulting in a decrease in Vickers hardness. In Comparative Example 3, isocyanate modification in step A4 can react with active groups (such as Si-OH) in the polysilazane through the -NCO group, increasing the precursor viscosity and crosslinking degree, and reducing the porosity of the green body. Without it, the precursor crosslinking is insufficient, the green body density decreases, and after sintering, internal defects increase, leading to a decrease in hardness.

[0088] 2. Bending strength is a core indicator of the load-bearing fracture resistance of silicon nitride composites. The three-point bending method was used, referring to the national standard GB / T 6569-2006 "Test Method for Bending Strength of Fine Ceramics". The silicon nitride composites from Examples 1-3 and Comparative Examples 1-3 were prepared into uniformly sized specimens: 40mm × 5mm × 3mm. The upper and lower surfaces and sides were sequentially polished with 400#, 800#, and 1200# sandpaper to remove cutting defects, ensuring a surface roughness Ra ≤ 0.8μm. Five parallel specimens were prepared for each group. An Instron 5969 universal testing machine was used, equipped with a three-point bending fixture, and the load sensor was calibrated. The parameters were set as follows: span (S) = 16 × H, loading speed = 5mm / min. The specimen was placed stably on the lower support, ensuring the load application point was located at the specimen's centerline to avoid displacement, until the specimen fractured. The maximum fracture load was read. The three-point bending strength test was conducted in a high-temperature air atmosphere of 1200℃, and the bending strength (σ) was calculated according to the following formula:

[0089] ;

[0090] Among them, F max S is the maximum breaking load (N); S is the span (mm); W is the specimen width (mm); H is the specimen height (mm).

[0091] Figure 2 The figures show the bending strength results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention at room temperature / high temperature. In the examples, the benzene ring in 1,4-di(dimethylchlorosilyl)benzene provides rigid support for the polysilazane backbone. After sintering, the silicon nitride (Si3N4) grains grow directionally along the rigid skeleton to form a uniform fibrous whisker structure. The whiskers can consume fracture energy through the "bridging" and "pull-out" effects, significantly improving the bending resistance. The metal doping can form nanoparticles during sintering, which are dispersed at the Si3N4 grain boundaries, enabling grain growth. At the same time, the nanoparticles have a high interfacial bonding energy with Si3N4, which can effectively prevent crack propagation along the grain boundaries. The isocyanate modification increases the crosslinking degree of the precursor, reduces the "stress concentration points" under bending load, and improves the bending strength of the examples. In Comparative Example 1, the polysilazane backbone is mainly composed of flexible methyl chains. After sintering, the Si3N4 grains grow randomly and have low density. The combined effect of numerous pores and coarse grains leads to the material fractures under low loads. In Comparative Example 2, the Si3N4 grains grow into coarse columnar crystals without metal ion inhibition during sintering. Furthermore, there are no nanoparticle pinning at the grain boundaries, making it easy for cracks to propagate rapidly along the grain boundaries. Without the "bridging" effect of the metal phase, the material consumes less energy when fractured. In Comparative Example 3, the lack of cross-linking reaction reduces the density of the green body. Numerous pores become "crack initiation points" under bending loads. The unmodified precursor easily forms coarse grains after sintering, and the intergranular bonding force is weak.

[0092] Under high temperature conditions, in the examples, 1,4-bis(dimethylchlorosilyl)benzene contains a rigid benzene ring structure, which can significantly increase the crosslinking density of polysilazane when used as a comonomer in copolymerization with methyldichlorosilane and methylvinyldichlorosilane. During the subsequent sintering process at 1400℃, the highly cross-linked precursor more easily forms a low-porosity silicon nitride matrix. At 1200℃, porosity is the core source of stress concentration. A dense matrix can prevent porosity from expanding into cracks under load, directly improving fracture resistance. After metal doping, a high-temperature stable second-phase reinforcement system can be constructed, inhibiting excessive growth of silicon nitride grains. Metal nitrides, acting as grain boundary pinning points, can control the size of silicon nitride grains. The grain boundary bonding of the fine-grained structure is stronger, making it less prone to intergranular fracture at 1200℃. At the same time, it can also enhance the grain boundary softening resistance. At 1200℃ in air, a small amount of grain boundary glass phase may exist in the silicon nitride matrix. Metal nitrides can react with the glass phase to generate high-temperature resistant silicates, avoiding crystal failure caused by glass phase softening, and significantly improving high-temperature load-bearing capacity. Example 3 shows the best reinforcement effect. The lattice matching degree between AlN and silicon nitride is higher than that of FeN / CoN, allowing for more uniform dispersion in the matrix. The second-phase pinning and grain boundary modification effects are more balanced, thus strengthening the... The highest strength was achieved with Fe and Co nitrides, which, while resistant to high temperatures, exhibited slightly inferior dispersibility. Therefore, the strengths of Examples 1 and 2 were slightly lower but similar. In Example 1, without 1,4-di(dimethylchlorosilyl)benzene, the comonomers were only methyldichlorosilane and methylvinyldichlorosilane. Their low crosslinking density meant that at 1200°C, the pores amplified stress, leading to rapid formation of through-cracks under load and consequently lower flexural strength. In Comparative Example 2, without metal doping, the matrix was only pure silicon nitride, without AlN / F. In the eN / CoN second phase, silicon nitride grains tend to grow into coarse grains with weak grain boundary bonding. At 1200℃, the grain boundary glass phase softens directly and cannot bear the load, making the material prone to intergranular fracture, which is the lowest among all samples. In Comparative Example 3, without toluene-2,5-diisocyanate modification, the precursor crosslinking is uneven, and a large number of micron-sized microcracks exist in the matrix after sintering. At 1200℃, the microcracks tend to propagate rapidly under load, although the metal doping from step A3 is retained (the second phase still plays a role).

[0093] 3. Thermal shock stability is a key indicator of the resistance of ceramic materials to thermal deformation. Simulating the sudden cooling and heating scenarios that the grading wheel may face, such as "high temperature operation - normal temperature shutdown", the core evaluation is whether the material will crack, peel off or weaken after drastic temperature changes. The silicon nitride composite materials from Examples 1-3 and Comparative Examples 1-3 were prepared into uniform-sized specimens: 40mm × 5mm × 3mm. The upper and lower surfaces and sides were sequentially sanded with 400#, 800#, and 1200# sandpaper to remove cutting defects, ensuring a surface roughness Ra ≤ 0.8μm. Five parallel specimens were prepared for each group. The specimens were placed in a muffle furnace and heated to the set thermal shock temperature at a rate of 5℃ / min. Three temperature gradients were selected: 800℃, 1000℃, and 1200℃. The temperature was held for 30 minutes, and the specimens were immediately removed from the muffle furnace and immersed in a room-temperature cold water bath for 10 minutes to simulate rapid cooling. The "heating-holding-cooling" cycle was repeated for 10 cycles. After 10 cycles, the bending strength of the specimens was tested using a three-point bending tester with a loading rate of 0.5mm / min and a span of 30mm. The thermal shock strength retention rate (%) of each group of specimens was calculated using the following formula:

[0094] Strength retention rate (%) = (Average flexural strength after thermal shock / Average flexural strength of un-thermal-shocked sample) × 100;

[0095] Figure 3 The figures show the thermal shock resistance results of the silicon nitride composite materials prepared in Examples 1-3 and Comparative Examples 1-3 of this invention. As shown in the figures, the thermal shock resistance of Examples 1-3 is significantly higher than that of the comparative examples. In these examples, the benzene ring in 1,4-bis(dimethylchlorosilyl)benzene provides a rigid framework for the silicon nitride matrix, forming a directional fibrous grain structure after sintering. This structure has a lower coefficient of thermal expansion (CTE) and less anisotropy. When the temperature changes abruptly, the overall thermal stress distribution is uniform, and local stress concentration is less likely to occur. The metal source is transformed into nanomaterials during the sintering process. The second phase (AlN, Fe4N, Co4N) has a CTE closer to that of silicon nitride, while Fe4N and Co4N have larger CTE differences compared to silicon nitride. Therefore, Example 3 exhibits stronger thermal stress buffering capacity and the highest strength retention rate after thermal shock at 1200℃. Metal nitride particles can enhance the fracture toughness of the material through the "crack deflection" and "crack bridging" effects, making the material less prone to through-cracks under thermal stress. Isocyanate modification makes the precursor cross-linking more complete, resulting in a density of 94%~96% after sintering (porosity <4%). During thermal shock, porosity acts as a "stress concentration amplifier" (the stress at the pore tip can reach 5~10 times the average stress). Low porosity reduces crack initiation points, thus the strength retention rate of Example 3 is significantly higher than that of Comparative Example 3, which lacks step A4. The thermal shock resistance of the Comparative Example is significantly reduced; high porosity, high CTE difference, and low toughness lead to thermal stress accumulation and rapid crack propagation.

[0096] Application examples

[0097] The silicon nitride composite material for grading wheels described in this invention is used to prepare grading wheels, and the specific method is as follows:

[0098] B1. Design and Modeling of the Grading Wheel Structure: Design the three-dimensional structure of the grading wheel using CA software (such as SolidWorks) and determine key parameters: wheel diameter, number of blades, blade inclination angle, hub bore diameter, and wheel thickness.

[0099] B2. Pretreatment of composite material blanks: Take silicon nitride composite material, detect internal defects by ultrasonic flaw detection, screen blanks without obvious defects, cut the blanks into rough blanks slightly larger than the design size using a grinding wheel cutter, remove the surface oxide layer and burrs, ultrasonically clean with anhydrous ethanol, and dry in an 80℃ oven for 2 hours.

[0100] B3. Rough Machining: Fix the pre-treated blank in the ceramic-specific fixture of the CNC machining center, and perform rough machining using a diamond-coated end mill.

[0101] B4. Finishing and Precision Correction: Replace with a high-precision diamond grinding wheel (800-1200 mesh) for finishing;

[0102] B5. Stress Relief and Surface Strengthening: The finely machined graded wheel is placed in a vacuum furnace and subjected to low-temperature annealing under a nitrogen protective atmosphere (flow rate 30 mL / min): the temperature is increased to 200℃ at 2℃ / min, held for 2 hours, and then cooled to room temperature at 1℃ / min to eliminate residual processing stress. The surface processing altered layer is removed by plasma etching, and then a coating is deposited by magnetron sputtering to improve surface hardness and wear resistance.

[0103] B6. Performance testing and assembly: Sampling is performed for three-point bending strength test, impact toughness test and dynamic balance test. Qualified graded wheels and drive shafts are assembled by heat fitting method, and locating pins and anti-loosening nuts are installed. Finally, the whole system is put into trial operation.

[0104] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

[0105] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention. The actual application is not limited to this. In conclusion, if those skilled in the art are inspired by this description and design similar methods and embodiments without departing from the spirit of the present invention, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a silicon nitride composite material for a grading wheel, characterized in that: Specifically, the following steps are included: A1. Dissolve 1,4-di(dimethylsilyl)benzene in anhydrous toluene, add triethylamine, mix well, then pass in a flowing inert gas and add thionyl chloride dropwise while maintaining room temperature. After the addition is complete, raise the reaction temperature to carry out the chlorination reaction. After the reaction is complete, cool, add anhydrous diethyl ether to the reaction system for dilution, add sodium carbonate to neutralize the reaction system, filter, collect the filtrate, and distill under reduced pressure to obtain 1,4-di(dimethylchlorosilyl)benzene. A2. Add 1,4-bis(dimethylchlorosilyl)benzene, methyldichlorosilane, and methylvinyldichlorosilane prepared in step A1 to a flask in sequence, add anhydrous toluene, mix until the reactants are completely dissolved, then introduce flowing nitrogen gas, transfer to an ice bath to cool the reaction system thoroughly, introduce dry ammonia gas into the reaction system, remove the ice bath, raise the temperature to carry out the ammonolysis reaction, after the reaction is completed, let stand at room temperature, filter, collect the filtrate, and distill under reduced pressure to obtain the polysilazane copolymer; A3. Dissolve the polysilazane copolymer prepared in step A2 in anhydrous toluene, introduce flowing nitrogen gas, dissolve the organometallic compound in anhydrous toluene, and sonicate to obtain a metal doping solution. Add the metal doping solution dropwise using a constant pressure funnel. After the addition is complete, adjust the reaction temperature to carry out the doping reaction. After the reaction is completed, the polysilazane doping solution is obtained. A4. Add a catalyst to the polysilazane doped solution prepared in step A3, dissolve the diisocyanate compound in anhydrous toluene, add it dropwise to the reaction system, raise the temperature to carry out the crosslinking reaction, after the reaction is completed, cool, and distill under reduced pressure to obtain the precursor solution. A5. Place the precursor solution prepared in step A4 into a vacuum degassing tank and let it stand under a vacuum of -0.095MPa to degas. Take the steel mold and perform surface treatment with silane coupling agent. Inject the degassed precursor solution into the mold, centrifuge, and simultaneously solidify. After centrifugation, continue to solidify to obtain the green blank. A6. The green blank prepared in step A5 is sintered under a nitrogen atmosphere according to a heating program, and then cooled to obtain silicon nitride composite material.

2. The method for preparing a silicon nitride composite material for a grading wheel according to claim 1, characterized in that: In step A1, the mass-to-volume ratio of 1,4-di(dimethylsilyl)benzene to triethylamine is 2-3 g / 3-4.8 mL; the mass-to-volume ratio of 1,4-di(dimethylsilyl)benzene to thionyl chloride is 2-3 g / 1.7-3 mL; the chlorination reaction temperature is 50-60 °C, and the chlorination reaction time is 3-4 h.

3. The method for preparing a silicon nitride composite material for a grading wheel according to claim 2, characterized in that: In step A2, the mass-to-volume ratio of 1,4-bis(dimethylchlorosilyl)benzene, methyldichlorosilane, and methylvinyldichlorosilane is 2-4 g: 2.0-3.2 mL: 1-2 mL; the flow rate of ammonia is 50-80 mL / min, and the ammonia introduction time is 20-40 min; the reaction temperature of the ammonolysis reaction is 40-50 °C, and the reaction time of the ammonolysis reaction is 4-6 h.

4. The method for preparing a silicon nitride composite material for a grading wheel according to claim 3, characterized in that: In step A3, the organometallic compound includes at least one of ferrocene, nickel dicene, cobalt dicene, and aluminum isopropoxide; the added mass of the organometallic compound is 3%-5% of the mass of the polysilazane copolymer.

5. The method for preparing a silicon nitride composite material for a grading wheel according to claim 4, characterized in that: In step A3, the reaction temperature of the doping reaction is 0-30℃, and the reaction time is 2-4h.

6. The method for preparing a silicon nitride composite material for a grading wheel according to claim 5, characterized in that: In step A4, the catalyst includes at least one of dibutyltin dilaurate, stannous octoate, and dibutyltin diacetylacetonate; the mass of the catalyst added is 0.3%-0.5% of the mass of the polysilazane copolymer described in step A3.

7. The method for preparing a silicon nitride composite material for a grading wheel according to claim 6, characterized in that: In step A4, the diisocyanate compound includes at least one of toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, toluene-2,5-diisocyanate, terephthalic diisocyanate, and isophthalic diisocyanate; in step A4, the added mass of the diisocyanate compound is 5%-15% of the mass of the polysilazane copolymer described in step A3; the crosslinking reaction temperature is 50-60℃, and the crosslinking reaction time is 4-6h.

8. A method for preparing a silicon nitride composite material for a grading wheel according to claim 7, characterized in that: In step A5, the centrifugation speed is 800-1200 rpm and the centrifugation time is 45-60 min; the curing temperature of the curing treatment is 50-60℃ and the curing time is 2.5-4.5 h.

9. A method for preparing a silicon nitride composite material for a grading wheel according to claim 8, characterized in that: In step A6, the specific temperature rise procedure for the sintering process is as follows: Phase 1: Increase the temperature from room temperature to 120-150℃ at a rate of 1℃ / min, and hold for 1.5-2 hours. Second stage: Increase the temperature to 300-320℃ at a rate of 0.5℃ / min, and hold for 3-4 hours; Three stages: Heat to 600-650℃ at a rate of 1℃ / min, and hold for 5-6 hours; Fourth stage: Increase the temperature to 1000-1100℃ at a rate of 2℃ / min, and hold for 2-3 hours; Five stages: Increase the temperature to 1400-1500℃ at a rate of 1℃ / min, and hold for 3-4 hours.

10. A silicon nitride composite material for a grading wheel, characterized in that: The silicon nitride composite material is prepared by the preparation method according to any one of claims 1-9.

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