A preparation process for modified graphite ceramic products

Abstract

This invention discloses a preparation process for modified graphite ceramic products, relating to the ceramics field. The preparation process includes the following steps: loading composite modified graphite into a mold, sealing it, and then performing cold isostatic pressing; followed by vacuum hot pressing sintering under a nitrogen atmosphere; and finally, post-treatment to obtain the modified graphite ceramic product. Specifically, the composite modified graphite is prepared by first coating flake graphite powder with aluminum nitrate nonahydrate and silica sol using a sol-gel process, drying, and calcining under an argon atmosphere to form an amorphous silica-alumina composite coating on the graphite surface, thus obtaining pretreated graphite. This pretreated graphite is then calcined at high temperature under an argon atmosphere to generate needle-like or columnar mullite crystals tightly bonded to the graphite surface in situ, resulting in modified and toughened graphite. Finally, it is mixed with silicon nitride nanoparticles, a dispersant, and a binder, and then vacuum dried and granulated to obtain the final product. The introduction of composite modified graphite in this invention simultaneously improves mechanical strength, creep resistance, impermeability, and wear resistance.

Description

Technical Field

[0001] This invention relates to the field of ceramic technology, and specifically to a preparation process for modified graphite ceramic products. Background Technology

[0002] Graphite ceramics are a novel structural and functional composite material that combines the excellent thermal conductivity, corrosion resistance, and self-lubricating properties of graphite with the high strength and wear resistance of ceramic materials. They have been widely used in industrial fields such as high-temperature mechanical seals, metallurgical crucibles, and heat exchangers. Modified graphite ceramic products, through techniques such as graphite surface coating, ceramic phase composites, and optimized sintering processes, have to some extent improved the inherent defects of pure graphite, such as low mechanical strength and susceptibility to wear.

[0003] However, in practical applications, modified graphite ceramic products still have the following performance shortcomings: First, the interfacial bonding strength between graphite and ceramic phases is insufficient. Due to the large difference in wettability between the two, microcracks and defects are easily formed at the interface. Under load, the cracks easily propagate along the interface, making it difficult for the material's impact resistance and flexural strength to meet the requirements of high-load conditions. Second, high-temperature oxidation and thermal stability are poor. The graphite phase will undergo oxidative weight loss in an oxidizing environment above 600℃. At the same time, the mismatch in the thermal expansion coefficients of the ceramic phase and graphite will generate large thermal stress during temperature cycling, causing coating cracking and interlayer delamination. Third, the material density and impermeability are insufficient. Existing sintering processes cannot completely eliminate internal pores. In corrosive media and molten metal environments, the media can easily penetrate into the material through the pores, accelerating matrix corrosion and failure. Fourth, it is difficult to synergistically improve multiple properties. Increasing the graphite content can improve self-lubrication but will significantly reduce mechanical strength. Increasing the proportion of ceramic phase can enhance strength but will sacrifice self-lubrication performance, making it difficult to achieve simultaneous optimization of compressive strength, creep resistance, and wear resistance. Therefore, the compressive strength, impact resistance, flexural strength, creep resistance, impermeability and wear resistance of existing modified graphite ceramic products still need to be improved simultaneously. Summary of the Invention

[0004] The purpose of this invention is to provide a preparation process for modified graphite ceramic products, thereby solving the following technical problems: Existing modified graphite ceramic products still suffer from poor compressive strength, impact resistance, flexural strength, creep resistance, impermeability, and wear resistance.

[0005] The objective of this invention can be achieved through the following technical solutions: A process for preparing modified graphite ceramic products includes the following steps: S1: The composite modified graphite is loaded into a rubber mold, then sealed and subjected to cold isostatic pressing to obtain a green blank; S2: Under a nitrogen atmosphere, the green blank is subjected to vacuum hot pressing sintering treatment, and after demolding, a sintered green body is obtained; S3: After coarse and fine grinding of the sintered green body, it is first rinsed with deionized water, then ultrasonically cleaned with anhydrous ethanol, and finally dried and cooled to obtain modified graphite ceramic products. The composite modified graphite is prepared by first coating flake graphite powder with aluminum nitrate nonahydrate and silica sol through a sol-gel process, drying, and calcining at 400°C under an argon atmosphere to form an amorphous silica-alumina composite coating on the graphite surface, thus obtaining pretreated graphite. The pretreated graphite is then calcined at a high temperature of 1350-1370°C under an argon atmosphere to generate needle-like or columnar mullite crystals that are tightly bonded to the graphite surface in situ, thus obtaining modified and toughened graphite. Finally, the modified and toughened graphite is mixed with silicon nitride nanoparticles, dispersant, and binder, and then vacuum dried and granulated to obtain the final product.

[0006] Preferably, the method for preparing the silica sol is as follows: Tetraethyl orthosilicate, anhydrous ethanol, and deionized water were mixed and stirred for 20 minutes. The pH was then adjusted to 2.7-2.9, and the mixture was stirred at 50°C for 60 minutes to obtain silica sol.

[0007] Preferably, the mass ratio of tetraethyl orthosilicate, anhydrous ethanol, and deionized water is 110:200:80-90.

[0008] Preferably, the preparation method of the composite modified graphite is as follows: A1: Add aluminum nitrate nonahydrate to anhydrous ethanol and stir for 40-50 min. Then add flake graphite powder and stir for 30 min. Then add silica sol and adjust the pH to 4.0-4.4. Stir for 10 min, spread it out and dry it at 120℃ for 15-16 h. Grind it and sieve it. Then heat it to 400℃ at 5℃ / min under argon atmosphere and hold it for 2 h. After cooling, the pretreated graphite is obtained. A2: Modified and toughened graphite is obtained by breaking up agglomerated graphite after high-temperature calcination in an argon atmosphere. A3: Add silicon nitride nanopowder and dispersant to anhydrous ethanol and ultrasonically disperse for 60 min. Then add binder and stir for 50 min. Then add modified and toughened graphite and stir at 60℃ and -0.08MPa vacuum until the ethanol is completely evaporated. After drying, sieve and then roll granulate through a 40-mesh sieve to obtain composite modified graphite.

[0009] Preferably, the mass ratio of anhydrous ethanol, aluminum nitrate nonahydrate, flake graphite powder, and silica sol in A1 is 500:560-570:680:390-400.

[0010] Preferably, the high-temperature calcination treatment described in A2 is as follows: first, the temperature is increased to 500°C at 10°C / min and held for 1 hour, then the temperature is increased to 1350-1370°C at 5°C / min and held for 3 hours, and then cooled with the furnace.

[0011] Preferably, the mass ratio of anhydrous ethanol, silicon nitride nanopowder, dispersant, binder, and modified toughened graphite in A3 is 100:45:1:28:650-700.

[0012] Preferably, the dispersant is either polyethylene glycol 2000 or polyethylene glycol 6000.

[0013] Preferably, the adhesive is any one of polyvinyl butyral, polymethyl methacrylate, and ethyl cellulose.

[0014] Preferably, the vacuum hot pressing sintering process described in S2 is as follows: first, the temperature is increased to 500°C at 2°C / min and held for 1 hour; then, the temperature is increased to 1400°C at 5°C / min and a constant axial pressure of 30 MPa is applied; then, the temperature is increased to 1680-1700°C at 5°C / min and held for 1 hour; finally, the pressure and nitrogen atmosphere are maintained and the furnace is cooled to room temperature.

[0015] The beneficial effects of this invention are: This invention provides a preparation process for modified graphite ceramic products. The invention simultaneously improves the compressive strength, impact resistance, flexural strength, creep resistance, impermeability resistance and wear resistance of modified graphite ceramic products through the following methods.

[0016] (1) In this invention, a highly active amorphous silica-alumina oxide precursor is uniformly coated onto the surface of flake graphite using a sol-gel method. After low-temperature heat treatment at 400℃ in an argon inert atmosphere, the organic components and bound water in the gel are removed, forming a continuous, dense, and highly reactive amorphous silica-alumina composite coating. Pure graphite has extremely low surface energy and very poor wettability with ceramic phases. Direct mixing would result in a large number of gaps and defects at the interface, making it impossible to effectively transfer loads. However, the silica-alumina oxide coating transforms the graphite surface from hydrophobic and inert to hydrophilic and active, providing an interfacial basis for the subsequent in-situ formation of mullite and the bonding of silicon nitride. The uniform oxide coating formed at the specific reaction temperature of this invention transforms the simple mechanical bond between graphite and the subsequent ceramic phase into a stronger chemical / diffusion bond, significantly enhancing the interfacial bonding force. This lays the foundation for the effective transfer of load from the ceramic phase to the graphite matrix, greatly improving the basic strength of the composite material. If the pretreatment temperature is too high, the precursor will over-crystallize and sinter, losing its high reactivity. Simultaneously, the gases generated by rapid decomposition can cause the coating to crack and peel off, further deteriorating the interfacial bonding. The oxide coating can, to some extent, hinder the high-temperature slippage of graphite sheets. A continuous oxide coating can seal some of the open pores on the graphite surface, reducing the diffusion channels of gas within the graphite and significantly decreasing gas permeability. The oxide coating has a higher hardness than graphite, reducing direct wear on the graphite surface.

[0017] (2) In the modification and toughening treatment of pretreated graphite in this invention, the pretreated graphite is subjected to high-temperature heat treatment at 1350-1370℃ in an argon inert atmosphere, causing amorphous silica and alumina on the graphite surface to undergo a solid-phase reaction, generating in-situ needle-like or columnar mullite crystals that are firmly bonded to the graphite surface. The in-situ generated mullite crystals do not have interface contamination problems, and their needle-like / columnar structures will interpenetrate and entwine between graphite sheets, forming an interwoven three-dimensional network structure; at the same time, the thermal expansion coefficient of mullite is between that of graphite and silicon nitride, which can effectively alleviate the thermal mismatch stress between different phases and reduce the generation of microcracks during sintering. When cracks propagate inside the material, they will encounter needle-like mullite crystals, resulting in crack deflection, crack bridging, and crystal pull-out, consuming a large amount of fracture energy and significantly improving the toughness and bending strength of the material; at the same time, the three-dimensional network structure of mullite can effectively constrain the deformation of graphite sheets, further improving the compressive strength. Mullite crystals interspersed between graphite sheets act as physical pinning agents, strongly inhibiting the slippage and rearrangement of graphite sheets at high temperatures, improving creep resistance at 1000℃, and significantly reducing the steady-state creep rate. Mullite crystals fill the tiny gaps between graphite sheets, cutting off some interconnected gas diffusion channels and further reducing permeability. Mullite's high hardness and strong bond with graphite effectively withstand wear loads, reducing graphite shedding and wear, and significantly improving wear resistance.

[0018] (3) In the composite modification process of flake graphite of the present invention, the modified and toughened graphite is uniformly mixed with nano-silicon nitride powder, and a composite powder with good flowability and high density is prepared by granulation process. This step introduces the nano-silicon nitride dispersion reinforcing phase and provides a good forming basis for subsequent cold isostatic pressing and hot pressing sintering. The nano-silicon nitride particles are mainly uniformly distributed at the grain boundaries of graphite and mullite, playing the role of pinning grain boundaries, refining the structure and filling micropores. During the high-temperature hot pressing process, the addition of nanoparticles can also increase the densification rate and promote the material to approach full density. The dispersion strengthening and grain boundary pinning effect of nano-silicon nitride can hinder dislocation movement and grain boundary slip, so that the strength of the material can be further improved. The nano-silicon nitride particles distributed at the grain boundaries can strongly pin the grain boundaries, inhibit grain boundary slip and diffusion creep at high temperature, and improve the creep resistance at high temperature. The nano-silicon nitride particles can effectively fill the residual micropores at the grain boundaries, improve the material density, reduce the gas permeability, and give the material excellent sealing performance. Uniformly distributed silicon nitride particles can form a hard, wear-resistant skeleton, significantly resisting abrasive and adhesive wear, thus greatly improving wear resistance. Therefore, in the three-step modification of this invention, pretreatment is fundamental, solving the interfacial wetting and bonding problems between graphite and ceramic phases, and providing the necessary interfacial conditions for subsequent modification; modification and toughening are the core, achieving toughening through in-situ generation of mullite whiskers, fundamentally improving the inherent defect of high brittleness in pure graphite; composite modification is the enhancement, using nano-silicon nitride to achieve dispersion strengthening and further densification, comprehensively improving the material's strength, wear resistance, and high-temperature stability. The synergistic effect of these three steps results in a significant improvement in key indicators such as mechanical properties, wear resistance, impermeability, and high-temperature creep resistance of the final modified graphite ceramic product compared to pure graphite or single-step modified materials, effectively solving the application limitations of pure graphite under harsh conditions such as high temperature, high pressure, and wear.

[0019] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] Figure 1 This is a SEM image of the needle-like mullite crystals on the surface of the modified and toughened graphite prepared in Example 1 of this application. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. The embodiments described below are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: The preparation process of a modified graphite ceramic product is as follows: S1: Mix 110g tetraethyl orthosilicate, 200g anhydrous ethanol and 80g deionized water and stir for 20min. Then adjust the pH to 2.7 with 1mol / L nitric acid aqueous solution. Then stir at 50℃ for 60min to obtain silica sol. S2: Add 560g of aluminum nitrate nonahydrate to 500g of anhydrous ethanol and stir for 40min. Then add 680g of flake graphite powder (200 mesh, purity ≥99.9%) and stir for 30min. Then add 390g of silica sol at 10mL / min. At the same time, adjust the pH to 4.0 with 1mol / L ammonia water. After stirring for 10min, spread it into a 1cm thick layer and dry it at 120℃ for 15h. Grind it and pass it through a 100-mesh standard sieve. Then put it into a corundum crucible and place it in a tube furnace. Finally, pass 99.99% pure argon gas at 100mL / min and exhaust it for 30min. Then raise the temperature to 400℃ at 5℃ / min and hold it for 2h. After cooling to room temperature in the furnace, the pretreated graphite is obtained. S3: 700g of pretreated graphite was placed in a corundum crucible and placed in a high-temperature tube furnace. Argon gas with a purity of 99.99% was introduced at a rate of 100mL / min. After 30min of gas introduction, the temperature was increased to 500℃ at a rate of 10℃ / min and held for 1h. Then the temperature was increased to 1350℃ at a rate of 5℃ / min and held for 3h. The furnace was then allowed to cool naturally to room temperature and removed. Finally, the agglomerates were gently broken up to obtain modified and toughened graphite. Figure 1 This refers to the SEM image of needle-like mullite crystals that are in situ formed on the surface of modified and toughened graphite and are tightly bonded to the graphite surface; from Figure 1 As can be seen, this application uses tetraethyl orthosilicate as the silicon source and aluminum nitrate nonahydrate as the aluminum source. Through the sol-gel method, an amorphous silicon-aluminum oxide precursor is coated on the surface of flake graphite, and then subjected to high-temperature heat treatment in an inert gas atmosphere to generate needle-like mullite crystals in situ on the graphite surface. The mullite crystals grown in situ on the graphite sheets in this application play a physical pinning role, improving the material's creep resistance, wear resistance, and reducing permeability. S4: Add 45g of silicon nitride nanopowder (50nm, α phase ≥95%, purity ≥99.5%) and 1g of polyethylene glycol 2000 to 100g of anhydrous ethanol and ultrasonically disperse in an ice-water bath for 60min. Then add 28g of polyvinyl butyral in three portions. Stir for 10min after the first two additions and stir for 30min after the third addition. Then add 650g of modified toughened graphite and stir at 60℃ and -0.08MPa vacuum until the ethanol is completely evaporated. After drying, pass through an 80-mesh standard sieve and then roll-granulate through a 40-mesh sieve to obtain composite modified graphite. S5: The composite modified graphite is loaded into a rubber mold, then sealed and cold isostatically pressed at 200MPa for 5 minutes to obtain the green blank. S6: Place the green blank into a graphite mold with boron nitride release agent on the inner wall, place it in a vacuum hot press sintering furnace, evacuate to below 5 Pa and maintain for 30 min, then introduce nitrogen gas with a purity of 99.999% to a positive pressure of 0.15 MPa in the furnace (maintain positive pressure protection throughout the process), then raise the temperature to 500℃ at 2℃ / min and hold for 1 h, then raise the temperature to 1400℃ at 5℃ / min and apply a constant axial pressure of 30 MPa, then raise the temperature to 1680℃ at 5℃ / min and hold for 1 h, finally maintain the pressure and nitrogen atmosphere and cool with the furnace to room temperature, and obtain the sintered green body after demolding; S7: The sintered blank is coarsely ground (180-mesh grinding wheel) and finely ground (1000-mesh grinding wheel) using diamond grinding wheels to achieve the required dimensional accuracy (tolerance ±0.02mm) and surface finish (Ra≤1.6μm). Then, the surface is rinsed with 10MPa high-pressure deionized water for 3 minutes to remove residual abrasives, followed by ultrasonic cleaning with anhydrous ethanol for 15 minutes. Finally, it is dried at 110℃ for 2 hours and cooled to obtain the modified graphite ceramic product.

[0024] Example 2: The preparation process of a modified graphite ceramic product is as follows: S1: Mix 110g tetraethyl orthosilicate, 200g anhydrous ethanol and 85g deionized water and stir for 20min. Then adjust the pH to 2.8 with 1mol / L nitric acid aqueous solution. Then stir at 50℃ for 60min to obtain silica sol. S2: Add 565g of aluminum nitrate nonahydrate to 500g of anhydrous ethanol and stir for 45min. Then add 680g of flake graphite powder (200 mesh, purity ≥99.9%) and stir for 30min. Then add 395g of silica sol at 10mL / min. At the same time, adjust the pH to 4.2 with 1mol / L ammonia water. After stirring for 10min, spread it into a 1.3cm thick layer and dry it at 120℃ for 15.5h. Grind it and pass it through a 100-mesh standard sieve. Then put it into a corundum crucible and place it in a tube furnace. Finally, pass 99.99% pure argon gas at 100mL / min and exhaust it for 30min. Then raise the temperature to 400℃ at 5℃ / min and hold it for 2h. After cooling to room temperature in the furnace, the pretreated graphite is obtained. S3: 700g of pretreated graphite was placed in a corundum crucible and placed in a high-temperature tube furnace. Argon gas with a purity of 99.99% was introduced at a rate of 100mL / min. After 30min of gas introduction, the temperature was increased to 500℃ at a rate of 10℃ / min and held for 1h. Then the temperature was increased to 1360℃ at a rate of 5℃ / min and held for 3h. The furnace was then allowed to cool naturally to room temperature and removed. Finally, the agglomerates were gently broken up to obtain modified and toughened graphite. S4: Add 45g of silicon nitride nanopowder (50nm, α phase ≥95%, purity ≥99.5%) and 1g of polyethylene glycol 6000 to 100g of anhydrous ethanol and ultrasonically disperse in an ice-water bath for 60min. Then add 28g of polymethyl methacrylate in three portions. Stir for 10min after the first two additions and stir for 30min after the third addition. Then add 675g of modified toughened graphite and stir at 60℃ and -0.08MPa vacuum until the ethanol is completely evaporated. After drying, pass through an 80-mesh standard sieve and then roll-granulate through a 40-mesh sieve to obtain composite modified graphite. S5: The composite modified graphite is loaded into a rubber mold, then sealed and cold isostatically pressed at 200MPa for 6 minutes to obtain the green blank. S6: Place the green blank into a graphite mold with boron nitride release agent on the inner wall, place it in a vacuum hot press sintering furnace, evacuate to below 5 Pa and maintain for 30 min, then introduce nitrogen gas with a purity of 99.999% to a positive pressure of 0.15 MPa in the furnace (maintain positive pressure protection throughout the process), then raise the temperature to 500℃ at 2℃ / min and hold for 1 h, then raise the temperature to 1400℃ at 5℃ / min and apply a constant axial pressure of 30 MPa, then raise the temperature to 1690℃ at 5℃ / min and hold for 1 h, finally maintain the pressure and nitrogen atmosphere and cool with the furnace to room temperature, and obtain the sintered green body after demolding; S7: The sintered blank is coarsely ground (180-mesh grinding wheel) and finely ground (1000-mesh grinding wheel) using diamond grinding wheels to achieve the required dimensional accuracy (tolerance ±0.02mm) and surface finish (Ra≤1.6μm). Then, the surface is rinsed with 10MPa high-pressure deionized water for 3 minutes to remove residual abrasives, followed by ultrasonic cleaning with anhydrous ethanol for 15 minutes. Finally, it is dried at 110℃ for 2 hours and cooled to obtain the modified graphite ceramic product.

[0025] Example 3: The preparation process of a modified graphite ceramic product is as follows: S1: Mix 110g tetraethyl orthosilicate, 200g anhydrous ethanol and 90g deionized water and stir for 20min. Then adjust the pH to 2.9 with 1mol / L nitric acid aqueous solution. Then stir at 50℃ for 60min to obtain silica sol. S2: Add 570g of aluminum nitrate nonahydrate to 500g of anhydrous ethanol and stir for 50min. Then add 680g of flake graphite powder (200 mesh, purity ≥99.9%) and stir for 30min. Then add 400g of silica sol at 10mL / min. At the same time, adjust the pH to 4.4 with 1mol / L ammonia water. After stirring for 10min, spread it into a 1.5cm thick layer and dry it at 120℃ for 16h. Grind it and pass it through a 100-mesh standard sieve. Then put it into a corundum crucible and place it in a tube furnace. Finally, pass 99.99% pure argon gas at 100mL / min and exhaust it for 30min. Then raise the temperature to 400℃ at 5℃ / min and hold it for 2h. After cooling to room temperature in the furnace, the pretreated graphite is obtained. S3: 700g of pretreated graphite was placed in a corundum crucible and placed in a high-temperature tube furnace. Argon gas with a purity of 99.99% was introduced at a rate of 100mL / min. After 30min of gas introduction, the temperature was increased to 500℃ at a rate of 10℃ / min and held for 1h. Then the temperature was increased to 1370℃ at a rate of 5℃ / min and held for 3h. The furnace was then allowed to cool naturally to room temperature and removed. Finally, the agglomerates were gently broken up to obtain modified toughened graphite. S4: Add 45g of silicon nitride nanopowder (50nm, α phase ≥95%, purity ≥99.5%) and 1g of polyethylene glycol 2000 to 100g of anhydrous ethanol and ultrasonically disperse in an ice-water bath for 60min. Then add 28g of ethyl cellulose in three portions. Stir for 10min after the first two additions and stir for 30min after the third addition. Then add 700g of modified toughened graphite and stir at 60℃ and -0.08MPa vacuum until the ethanol is completely evaporated. After drying, pass through an 80-mesh standard sieve and then roll-granulate through a 40-mesh sieve to obtain composite modified graphite. S5: The composite modified graphite is loaded into a rubber mold, then sealed and cold isostatically pressed at 200MPa for 7 minutes to obtain the green blank. S6: Place the green blank into a graphite mold with boron nitride release agent on the inner wall, place it in a vacuum hot press sintering furnace, evacuate to below 5 Pa and maintain for 30 min, then introduce nitrogen gas with a purity of 99.999% to a positive pressure of 0.15 MPa in the furnace (maintain positive pressure protection throughout the process), then raise the temperature to 500℃ at 2℃ / min and hold for 1 h, then raise the temperature to 1400℃ at 5℃ / min and apply a constant axial pressure of 30 MPa, then raise the temperature to 1700℃ at 5℃ / min and hold for 1 h, finally maintain the pressure and nitrogen atmosphere and cool with the furnace to room temperature, and obtain the sintered green body after demolding; S7: The sintered blank is coarsely ground (180-mesh grinding wheel) and finely ground (1000-mesh grinding wheel) using diamond grinding wheels to achieve the required dimensional accuracy (tolerance ±0.02mm) and surface finish (Ra≤1.6μm). Then, the surface is rinsed with 10MPa high-pressure deionized water for 3 minutes to remove residual abrasives, followed by ultrasonic cleaning with anhydrous ethanol for 15 minutes. Finally, it is dried at 110℃ for 2 hours and cooled to obtain the modified graphite ceramic product.

[0026] Comparative Example 1: Compared with Example 1, this comparative example only replaces "heating to 400°C at 5°C / min and holding for 2 hours" in the preparation process of S2 with "heating to 600°C at 5°C / min and holding for 2 hours". All other steps and parameters are the same, and will not be repeated in this comparative example. Finally, modified graphite ceramic products are obtained.

[0027] Comparative Example 2: Compared with Example 1, this comparative example only replaces the "pretreated graphite" added in the preparation process of S3 with "flake graphite powder (200 mesh, purity ≥99.9%)". All other steps and parameters are the same, and will not be repeated in this comparative example. Finally, modified graphite ceramic products are obtained.

[0028] Comparative Example 3: Compared with Example 1, this comparative example only replaces the "modified toughened graphite" added in the preparation process of S4 with the "pretreated graphite" prepared in S2. All other steps and parameters are the same, and will not be repeated in this comparative example. Finally, modified graphite ceramic products are obtained.

[0029] Comparative Example 4: Compared with Example 1, this comparative example only did not add "silicon nitride nanopowder" in the preparation process of S4. All other steps and parameters were the same, and will not be repeated here. The modified graphite ceramic product was finally obtained.

[0030] Performance testing: Determination of compressive strength: Referring to GB / T 13465.3-2014 standard, the compressive strength (MPa) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was determined at a loading rate of 1 kN / s. The test results are shown in Table 1.

[0031] Impact resistance testing: Referring to GB / T 13465.4-2014 standard, the impact strength (kJ / m²) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was determined at a pendulum energy of 15J. 2 The test results are shown in Table 1.

[0032] Determination of flexural strength: Referring to GB / T 13465.2-2014 standard, the flexural strength (MPa) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was determined at a loading rate of 0.5 mm / min. The test results are shown in Table 1.

[0033] Determination of creep resistance: Referring to GB / T 34218-2017 standard, the steady-state creep rate (h⁻¹) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was determined under a nitrogen atmosphere at 1000℃ and a stress of 80 MPa. -1 The test results are shown in Table 1.

[0034] Determination of permeability: Referring to GB / T 13465.11-2025 standard, the gas permeability (cm³) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was determined at 23°C, with dry nitrogen as the test gas and a pressure difference of 0.1 MPa across the sample. 2 The test results ( / s) are shown in Table 2.

[0035] Abrasion resistance testing: The volumetric wear rate (mm) of modified graphite ceramic product specimens with dimensions of 19mm × 12mm × 12mm prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was measured under the following conditions: the grinding ring was made of No. 45 steel, the sliding speed was 2m / s, the normal load was 100N, and the test time was 1h. 3 ·N -1 ·m -1 The test results are shown in Table 2.

[0036] Determination of thermal conductivity: Referring to GB / T 13465.12-2023 standard, the thermal conductivity (W·m) of the modified graphite ceramic products prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was measured. -1 ·K -1 The test results are shown in Table 2.

[0037] Table 1: Performance test results of Examples 1-3 and Comparative Examples 1-4

[0038] Table 2: Performance test results of Examples 1-3 and Comparative Examples 1-4

[0039] Data Analysis: As can be seen from Tables 1 and 2, the modified graphite ceramic products prepared in the embodiments of the present invention simultaneously possess excellent compressive strength, impact resistance, flexural strength, creep resistance, impermeability, and wear resistance.

[0040] In Comparative Example 1, the only difference was that the calcination temperature of the pretreated graphite was increased from 400℃ to 600℃; all other process parameters remained identical. The compressive strength of the sample prepared in Comparative Example 1 decreased to 218.6 MPa, and the impact strength decreased to 3.5 kJ / m². 2 The flexural strength decreased to 75.5 MPa; the steady-state creep rate at 1000℃ increased to 1.5 × 10⁻⁶. -5 h -1 Gas permeability increased by an order of magnitude, and volumetric wear rate increased to 6.0 × 10⁻⁶. -5 mm 3 ·N -1 ·m -1 Thermal conductivity decreased to 96.2 W·m -1 ·K -1 As can be seen from the above, the 400℃ low-temperature calcination used in the technical solution of this application only removes the organic components of the sol-gel, retaining the highly active amorphous silicon-aluminum composite coating; while raising the temperature to 600℃ will cause the coating to crystallize prematurely, release internal gas to form microcracks, significantly reduce the interfacial bonding force between graphite and subsequent mullite and silicon nitride, hinder load transfer, and simultaneously deteriorate mechanical strength, high-temperature creep resistance, and sealing wear resistance. This proves that the 400℃ low-temperature pretreatment of this invention is a necessary condition to ensure interfacial activity.

[0041] Comparative Example 2 directly used raw flake graphite instead of pretreated graphite, without the sol-gel coating process. The compressive strength of the sample prepared in Comparative Example 2 was only 119.4 MPa, less than 40% of that in Example 1; the impact and flexural strengths also decreased significantly; the creep rate increased by nearly 40 times; and the gas permeability reached 2.0 × 10⁻⁶. -12 cm 2 / s, the wear resistance deteriorates by an order of magnitude. As can be seen from the above, pure graphite has an inert surface and extremely poor wettability with silicon aluminum oxide and silicon nitride. When there is no continuous coating transition, there are a large number of pore defects at the interface, which cannot generate mullite toughening phase in situ. The interface pinning and crack deflection strengthening mechanism are lost, which directly proves that sol-gel coating pretreatment is a basic prerequisite for achieving high performance.

[0042] Comparative Example 3 only prepared pretreated graphite without undergoing high-temperature calcination at 1350-1370℃ to generate needle-like / columnar mullite crystals. The compressive, impact, and flexural strengths of the samples prepared in Comparative Example 3 were significantly reduced, while the creep rate increased by more than three times, and the permeability and wear rate increased simultaneously. Comparative Example 3 lacked the three-dimensional network pinning structure of mullite crystals, resulting in easy slippage of graphite sheets, no deflection or bridging toughening mechanism for crack propagation, and no buffering of thermal expansion mismatch stress. This significantly reduced the material's toughness, high-temperature stability, and density, confirming that in-situ mullite generation is the core strengthening and toughening process of this invention.

[0043] Comparative Example 4 did not add silicon nitride nanoparticles; it relied solely on modified and toughened graphite for molding and sintering. The mechanical properties of the sample prepared in Comparative Example 4 were significantly reduced, with a marked deterioration in high-temperature creep rate, permeability, and wear resistance. This indicates that silicon nitride nanoparticles fill grain boundary micropores, pin grain boundaries, and refine the sintered structure. Without them, the material density decreases, and the lack of a hard, wear-resistant framework prevents dispersion strengthening, confirming that silicon nitride composite granulation is a key supplementary process for comprehensive performance improvement.

[0044] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A preparation process for modified graphite ceramic products, characterized in that, Includes the following steps: S1: The composite modified graphite is loaded into the mold, then sealed and subjected to cold isostatic pressing to obtain the green blank; S2: Under a nitrogen atmosphere, the green blank is subjected to vacuum hot pressing sintering treatment, and after demolding, a sintered green body is obtained; S3: After coarse and fine grinding of the sintered green body, it is first rinsed with deionized water, then ultrasonically cleaned with anhydrous ethanol, and finally dried and cooled to obtain modified graphite ceramic products. The composite modified graphite is prepared by first coating flake graphite powder with aluminum nitrate nonahydrate and silica sol through a sol-gel process, drying, and calcining at 400°C under an argon atmosphere to form an amorphous silica-alumina composite coating on the graphite surface, thus obtaining pretreated graphite. The pretreated graphite is then calcined at a high temperature of 1350-1370°C under an argon atmosphere to generate needle-like or columnar mullite crystals that are tightly bonded to the graphite surface in situ, thus obtaining modified and toughened graphite. Finally, the modified and toughened graphite is mixed with silicon nitride nanoparticles, dispersant, and binder, and then vacuum dried and granulated to obtain the final product.

2. The preparation process of the modified graphite ceramic product according to claim 1, characterized in that, The method for preparing the silica sol is as follows: Tetraethyl orthosilicate, anhydrous ethanol, and deionized water were mixed and stirred for 20 minutes. The pH was then adjusted to 2.7-2.9, and the mixture was stirred at 50°C for 60 minutes to obtain silica sol.

3. The preparation process of the modified graphite ceramic product according to claim 2, characterized in that, The mass ratio of tetraethyl orthosilicate, anhydrous ethanol, and deionized water is 110:200:80-90.

4. The preparation process of the modified graphite ceramic product according to claim 1, characterized in that, The preparation method of the composite modified graphite is as follows: A1: Add aluminum nitrate nonahydrate to anhydrous ethanol and stir for 40-50 min. Then add flake graphite powder and stir for 30 min. Then add silica sol and adjust the pH to 4.0-4.

4. Stir for 10 min, spread it out and dry it at 120℃ for 15-16 h. Grind it and sieve it. Then heat it to 400℃ at 5℃ / min under argon atmosphere and hold it for 2 h. After cooling, the pretreated graphite is obtained. A2: Modified and toughened graphite is obtained by breaking up agglomerated graphite after high-temperature calcination in an argon atmosphere. A3: Add silicon nitride nanopowder and dispersant to anhydrous ethanol and ultrasonically disperse for 60 min. Then add binder and stir for 50 min. Then add modified and toughened graphite and stir at 60℃ and -0.08MPa vacuum until the ethanol is completely evaporated. After drying, sieve and then roll granulate through a 40-mesh sieve to obtain composite modified graphite.

5. The preparation process of the modified graphite ceramic product according to claim 4, characterized in that, The mass ratio of anhydrous ethanol, aluminum nitrate nonahydrate, flake graphite powder, and silica sol described in A1 is 500:560-570:680:390-400.

6. The preparation process of the modified graphite ceramic product according to claim 4, characterized in that, The high-temperature calcination treatment described in A2 is as follows: first, the temperature is increased to 500℃ at 10℃ / min and held for 1 hour, then the temperature is increased to 1350-1370℃ at 5℃ / min and held for 3 hours, and then cooled with the furnace.

7. The preparation process of the modified graphite ceramic product according to claim 4, characterized in that, The mass ratio of anhydrous ethanol, silicon nitride nanopowder, dispersant, binder, and modified toughened graphite in A3 is 100:45:1:28:650-700.

8. The preparation process of the modified graphite ceramic product according to claim 1, characterized in that, The dispersant is either polyethylene glycol 2000 or polyethylene glycol 6000.

9. The preparation process of the modified graphite ceramic product according to claim 1, characterized in that, The adhesive is any one of polyvinyl butyral, polymethyl methacrylate, and ethyl cellulose.

10. The preparation process of the modified graphite ceramic product according to claim 1, characterized in that, The vacuum hot pressing sintering process described in S2 is as follows: first, the temperature is increased to 500℃ at 2℃ / min and held for 1 hour; then, the temperature is increased to 1400℃ at 5℃ / min and a constant axial pressure of 30MPa is applied; then, the temperature is increased to 1680-1700℃ at 5℃ / min and held for 1 hour; finally, the pressure and nitrogen atmosphere are maintained and the furnace is cooled to room temperature.

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