Preparation method of graphite block glassy carbon coating and composite material

Abstract

The invention discloses a graphite block glassy carbon coating preparation method and a composite material. The graphite block glassy carbon coating preparation method comprises the following steps: roughening the surface of a graphite block; adding fumed silica and an isocyanate compound into the phenolic resin solution to obtain a modified resin solution; respectively preparing a first coating material, a second coating material and a third coating material based on the modified resin solution, the graphite powder and the carbon nanotubes; sequentially coating the three coating materials on the surface of a graphite block, and carrying out cross-linking curing to obtain a multi-layer coating structure; performing segmented pyrolysis on the multi-layer coating structure in an argon atmosphere to obtain a glassy carbon coating; and carrying out surface sanding on the glassy carbon coating, dipping the glassy carbon coating in a phenolic resin solution, and then carrying out heat treatment to obtain the surface-modified glassy carbon coating. By means of the multi-layer coating structure design and the segmented pyrolysis technology, the technical problems that in the traditional glassy carbon coating preparation process, the coating quality stability is poor, and the oxidation resistance is insufficient are solved, and the prepared glassy carbon coating has the advantages of being high in bonding strength, excellent in oxidation resistance and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of material surface treatment, and particularly to a preparation method of a glassy carbon coating for graphite blocks and a composite material. Background Art

[0002] Graphite materials are widely used in fields such as metallurgy, chemical industry, aerospace, and nuclear industry due to their excellent thermal conductivity, electrical conductivity, high temperature resistance, and chemical stability. However, natural graphite materials usually have disadvantages such as low strength and poor oxidation resistance, which limit their application in high-temperature oxidation environments.

[0003] To solve this problem, forming a glassy carbon coating on the graphite surface has become an important technical route. Traditional methods for forming glassy carbon coatings mainly rely on the impregnation of high-molecular materials such as phenolic resins and high-temperature carbonization processes. However, the process is complex, the product quality stability is poor, and the production efficiency is low. Among them, the porosity of the graphite substrate is the key factor determining the impregnation effect and affecting the final coating quality, but it has not been effectively controlled. Existing technologies usually use single-parameter regulation or simple pressing processes to control porosity, resulting in uneven pore distribution, insufficient resin penetration during impregnation, and uneven quality of the finally formed glassy carbon coatings, which cannot meet the strict requirements for product consistency in high-end application fields.

[0004] The existing technologies mainly have the following defects: First, the traditional process for controlling porosity mainly relies on single-parameter adjustment, such as raw material particle size selection or molding pressure adjustment, lacking a systematic multi-parameter collaborative control mechanism, resulting in a narrow adjustable range of porosity and being unable to meet the diverse needs of different application scenarios. Second, existing technologies cannot achieve precise control of pore morphology and spatial distribution. An irregularly distributed pore structure often forms inside the graphite material, causing uneven resin penetration during subsequent impregnation and ultimately affecting the coating quality. Finally, existing technologies lack means to control the connectivity of the pore network. Even when the target porosity is achieved, it is difficult to ensure effective connectivity between pores, affecting the fluidity of the impregnating liquid inside the material, resulting in insufficient internal impregnation and excessive surface impregnation, and finally forming a glassy carbon coating with uneven thickness and unstable quality.

[0005] Therefore, there is an urgent need to develop a new preparation method for glassy carbon coatings on graphite blocks to improve the quality stability and oxidation resistance of glassy carbon coatings. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of poor quality stability and insufficient oxidation resistance of the glassy carbon coating on the surface of graphite materials in the prior art, and to provide a preparation method of a glassy carbon coating for graphite blocks and a composite material.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A preparation method of a glassy carbon coating on a graphite block includes:

[0009] Roughen the surface of the graphite block to obtain a graphite block with a rough surface;

[0010] Add fumed silica and an isocyanate compound to a phenolic resin solution to obtain a modified resin solution;

[0011] Based on the modified resin solution, graphite powder and carbon nanotubes, prepare a first coating material, a second coating material and a third coating material respectively. The first coating material, the second coating material and the third coating material contain graphite powder and / or carbon nanotubes;

[0012] Coat the first coating material, the second coating material and the third coating material on the surface of the graphite block in sequence and carry out crosslinking curing to obtain a multi-layer coating structure;

[0013] Carry out segmented pyrolysis on the multi-layer coating structure under an argon atmosphere to obtain a glassy carbon coating;

[0014] Sand the surface of the glassy carbon coating, impregnate it with a phenolic resin solution and then carry out heat treatment to obtain a surface-modified glassy carbon coating.

[0015] Further, roughen the surface of the graphite block to obtain a graphite block with a rough surface, including:

[0016] Sand the surface of the graphite block with 60-mesh to 120-mesh sandpaper to obtain a preliminarily treated graphite block;

[0017] Use a mixed solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 3:1 to etch the preliminarily treated graphite block at 60°C to 80°C for 30 minutes to 60 minutes to obtain an etched graphite block;

[0018] Wash the etched graphite block and dry it at 80°C to 120°C for 4 hours to 6 hours to obtain the graphite block with a rough surface.

[0019] Further, add fumed silica and an isocyanate compound to a phenolic resin solution to obtain a modified resin solution, including:

[0020] Dissolve the phenolic resin in acetone to prepare a phenolic resin solution with a concentration of 30% to 50%;

[0021] Add fumed silica with a mass ratio of 1% to 5% to the phenolic resin solution and shear-mix it at 8000 revolutions per minute to 12000 revolutions per minute for 15 minutes to 30 minutes to obtain a filler dispersion;

[0022] Add 2% to 8% of the isocyanate compound to the filler dispersion liquid, and perform ultrasonic treatment at 20 kHz to 40 kHz for 15 minutes to 30 minutes to obtain the modified resin solution.

[0023] Furthermore, based on the modified resin solution, graphite powder, and carbon nanotubes, prepare the first coating material, the second coating material, and the third coating material respectively, including:

[0024] Add 15% to 25% of graphite powder to the first batch of the modified resin solution, and perform ultrasonic dispersion for 15 minutes to 25 minutes to obtain the first coating material;

[0025] Add 8% to 15% of graphite powder and 2% to 5% of carbon nanotubes to the second batch of the modified resin solution, and perform ultrasonic dispersion for 15 minutes to 25 minutes to obtain the second coating material;

[0026] Add 2% to 5% of carbon nanotubes to the third batch of the modified resin solution, and perform ultrasonic dispersion for 15 minutes to 25 minutes to obtain the third coating material.

[0027] Furthermore, sequentially coat the first coating material, the second coating material, and the third coating material on the surface of the graphite block and perform crosslinking curing to obtain a multi-layer coating structure, including:

[0028] Coat the first coating material on the surface of the graphite block, and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the first layer structure of the multi-layer coating structure, and the thickness of the first layer structure is 100 μm to 150 μm;

[0029] Coat the second coating material on the surface of the first layer structure, and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the second layer structure of the multi-layer coating structure, and the thickness of the second layer structure is 100 μm to 150 μm;

[0030] Coat the third coating material on the surface of the second layer structure, and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the third layer structure of the multi-layer coating structure, and the thickness of the third layer structure is 150 μm to 200 μm.

[0031] Furthermore, sequentially coat the first coating material, the second coating material, and the third coating material on the surface of the graphite block and perform crosslinking curing to obtain a multi-layer coating structure, and further include:

[0032] Place the multi-layer coating structure at 80°C to 100°C for 4 hours to 6 hours to obtain a preliminary crosslinked structure;

[0033] Treat the preliminary cross-linked structure at 150 °C to 180 °C for 2 hours to 4 hours to obtain a second cross-linked structure;

[0034] Treat the second cross-linked structure at 200 °C to 250 °C for 1 hour to 2 hours.

[0035] Furthermore, perform segmented pyrolysis on the multi-layer coated structure in an argon atmosphere to obtain a glassy carbon coating, including:

[0036] In an argon atmosphere, heat from 20 °C to 400 °C at a heating rate of 1 °C / min to 3 °C / min and hold for 2 hours to obtain a first pyrolyzed structure;

[0037] Heat the first pyrolyzed structure to 800 °C at a heating rate of 3 °C / min to 5 °C / min and hold for 2 hours to obtain a second pyrolyzed structure;

[0038] Heat the second pyrolyzed structure to 1000 °C to 1200 °C at a heating rate of 5 °C / min to 8 °C / min and hold for 4 hours to 6 hours, and then cool to 20 °C at a cooling rate of 3 °C / min to 5 °C / min to obtain the glassy carbon coating.

[0039] Furthermore, perform surface sanding on the glassy carbon coating, impregnate it with a phenolic resin solution and then perform heat treatment to obtain a surface-modified glassy carbon coating, including:

[0040] Use 1000-mesh to 2000-mesh sandpaper to sand the surface of the glassy carbon coating to obtain a sanded glassy carbon coating;

[0041] Immerse the sanded glassy carbon coating in a phenolic resin solution with a concentration of 10% to 15% for 1 hour to 2 hours, and treat it at 50 °C to 60 °C for 2 hours to obtain an impregnated glassy carbon coating;

[0042] In an argon atmosphere, heat from 20 °C to 400 °C at a heating rate of 1 °C / min to 3 °C / min and hold for 2 hours, then heat at a heating rate of 3 °C / min to 5 °C / min to 800 °C and hold for 2 hours, and then heat at a heating rate of 5 °C / min to 8 °C / min to 900 °C to 1000 °C and hold for 2 hours to 3 hours to obtain a surface-modified glassy carbon coating.

[0043] Furthermore, the method for preparing the graphite block glassy carbon coating further includes: polishing the surface-modified glassy carbon coating successively with diamond polishing pastes with particle sizes of 0.5 μm to 1 μm, 0.1 μm to 0.5 μm, and 0.05 μm to 0.1 μm, and each polishing time is 10 minutes to 20 minutes.

[0044] The present invention also provides a composite material, which is characterized in that it is prepared by using the above-mentioned method for preparing a glassy carbon coating on a graphite block material.

[0045] The present invention has the following beneficial effects:

[0046] Through the design of a multi-layer coating structure, the present invention sequentially coats three coating materials with different compositions, realizing a gradient transition of the coating composition, improving the bonding strength between the coating and the graphite block material, and solving the problem of easy peeling of the traditional glassy carbon coating.

[0047] The present invention adopts a segmented pyrolysis process. By controlling the heating rate and holding time in different temperature segments, the complete conversion of the organic resin to glassy carbon is achieved, avoiding cracking during the pyrolysis process, and improving the density and oxidation resistance of the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the accompanying drawings required for use in the embodiments will be briefly introduced below. The accompanying drawings are incorporated into the specification and form a part of this specification. These drawings show embodiments that conform to the present disclosure and are used together with the specification to illustrate the technical solutions of the present disclosure. It should be understood that the following drawings only show certain embodiments of the present disclosure and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0049] Figure 1 is the process flow diagram of the method for preparing a glassy carbon coating on a graphite block material provided by the embodiment of the present invention Figure 1 ;

[0050] Figure 2 is a schematic cross-sectional view of the multi-layer coating structure in the embodiment of the present invention;

[0051] Figure 3 is the temperature-time curve diagram of the pyrolysis process in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present disclosure. Apparently, the described embodiments are only a part rather than all of the embodiments of the present disclosure. Components of the embodiments of the present disclosure generally described and illustrated in the accompanying drawings herein may be arranged and designed in a variety of different configurations. Therefore, the detailed description of the embodiments of the present disclosure provided in the accompanying drawings is not intended to limit the scope of the claimed present disclosure, but merely represents selected embodiments of the present disclosure. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present disclosure without creative efforts fall within the scope of protection of the present disclosure.

[0053] It should be noted that like reference numerals and letters denote like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures.

[0054] As used herein, the term "and / or" merely describes an association relationship and indicates that three relationships may exist. For example, A and / or B may represent: A alone, both A and B present simultaneously, or B alone. Additionally, the term "at least one" as used herein represents any one of a plurality or any combination of at least two of a plurality. For example, including at least one of A, B, and C may represent including any one or more elements selected from the set consisting of A, B, and C.

[0055] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are only a part rather than all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0056] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0057] The embodiments of the present application provide a method for preparing a glassy carbon coating on a graphite block, as Figure 1 shown, including:

[0058] S1: Roughen the surface of the graphite block to obtain a graphite block with a rough surface;

[0059] This step aims to increase the surface area and surface energy of the graphite substrate and form a microscopic uneven structure. This helps to improve the physical engagement force (mechanical locking effect) and chemical bonding force between the subsequent coating material and the substrate, thereby significantly enhancing the adhesion strength of the coating and preventing peeling during use.

[0060] S1 specifically includes:

[0061] S1.1: Sand the surface of the graphite block with 60 - mesh to 120 - mesh sandpaper to obtain a preliminarily treated graphite block;

[0062] Sand grinding is a mechanical treatment method. By using sandpaper with a specific particle size (the smaller the mesh number, the larger the particle size) for grinding, macroscopic scratches and roughness can be created on the graphite surface, providing preliminary attachment points for the coating. Selecting 60 - mesh to 120 - mesh sandpaper can remove surface contaminants and smooth layers while forming pits with sufficient depth, which is beneficial to the anchoring of the coating.

[0063] S1.2: Use a mixed solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 3:1 to etch the preliminarily treated graphite block at 60°C to 80°C for 30 minutes to 60 minutes to obtain an etched graphite block;

[0064] The strong acid mixture (mixed acid) has strong oxidizing properties and can chemically etch the graphite surface. This process can not only further clean the surface and remove impurities that may be embedded during mechanical treatment, but more importantly, it can form finer and more complex pore structures at the microscale and may introduce oxygen - containing functional groups (such as carboxyl and hydroxyl groups) on the graphite surface. These microstructures and functional groups can greatly increase the effective contact area and improve the wettability of the coating resin to the substrate, promoting interfacial chemical bonding. Controlling the temperature and time is to ensure sufficient etching effect without over - damaging the substrate.

[0065] S1.3: Wash the etched graphite block and dry it at 80°C to 120°C for 4 hours to 6 hours to obtain the graphite block with a rough surface.

[0066] The water - washing step is crucial. The purpose is to thoroughly remove the acid solution and reaction products remaining on the graphite surface to avoid their influence on the performance of the subsequent coating materials or causing side reactions at high temperatures. Sufficient drying is to remove the moisture adsorbed in the graphite pores and on the surface. The presence of moisture will affect the curing process of the resin and produce steam during pyrolysis, resulting in coating defects (such as bubbles and cracks).

[0067] S2: Add fumed silica and isocyanate compounds to the phenolic resin solution to obtain a modified resin solution;

[0068] This step aims to optimize the performance of the phenolic resin by adding inorganic nanoparticles and chemical modifiers to make it more suitable as a precursor for high - quality glassy carbon coatings. Phenolic resin is a commonly used precursor for preparing glassy carbon and has a high carbon residue rate.

[0069] Among them, S2 includes:

[0070] S2.1: Dissolve phenolic resin in acetone to prepare a phenolic resin solution with a concentration of 30% to 50%;

[0071] Acetone is chosen as the solvent because it has good solubility for phenolic resin and moderate volatility, which is convenient for coating operations. Controlling the solution concentration at 30% to 50% is to obtain a suitable viscosity, which not only ensures the fluidity and leveling property of the coating, but also can carry a sufficient amount of fillers and form a coating with a certain thickness after curing.

[0072] S2.2: Add fumed silica with a mass ratio of 1% to 5% to the phenolic resin solution, and shear-mix at 8000 to 12000 revolutions per minute for 15 to 30 minutes to obtain a filler dispersion;

[0073] Fumed silica is a nano-scale amorphous silica with a very high specific surface area and a three-dimensional network structure. Adding it here mainly has several functions: 1) As a rheology control agent (thixotropic agent), it can increase the viscosity and sag resistance of the solution, facilitating uniform coating and controlling the layer thickness; 2) As a reinforcing filler, being dispersed in the resin matrix can improve the hardness and modulus of the cured coating and the final glassy carbon coating; 3) It may interact with the carbon matrix during the pyrolysis process, affecting the microstructure or high-temperature stability of the final carbon material. High-shear mixing is to break up the aggregates of silica nanoparticles and make them uniformly dispersed in the resin solution.

[0074] S2.3: Add 2% to 8% of isocyanate compound to the filler dispersion, and ultrasonically treat it at 20 to 40 kHz for 15 to 30 minutes to obtain the modified resin solution.

[0075] Isocyanate compounds (such as TDI, MDI, etc.) contain active -NCO groups, which can react with the phenolic hydroxyl groups in phenolic resin for chemical modification. This modification can: 1) Increase the crosslinking density of the resin, improving the thermal stability, mechanical strength and dimensional stability of the cured product; 2) Reduce the curing temperature or shorten the curing time; 3) Change the pyrolysis behavior of the resin, possibly increasing the carbon yield or improving the structure of the final glassy carbon. Ultrasonic treatment uses the high-energy microenvironment generated by the cavitation effect, which helps the uniform dispersion of isocyanate in the viscous liquid and the effective contact with phenolic resin molecules.

[0076] S3: Based on the modified resin solution, graphite powder and carbon nanotubes, prepare the first coating material, the second coating material and the third coating material respectively. The first coating material, the second coating material and the third coating material contain graphite powder and / or carbon nanotubes;

[0077] The core of this step is to design and prepare a multi-layer coating with a gradient structure. By regulating the types and contents of fillers (graphite powder, carbon nanotubes) in different layers, a smooth transition of properties from the substrate to the coating surface is achieved, aiming to optimize the interfacial bonding, relieve thermal stress, and endow the surface layer with specific functions (such as high oxidation resistance).

[0078] S3.1: Add 15% to 25% of graphite powder to the first batch of modified resin solution and ultrasonically disperse it for 15 to 25 minutes to obtain the first coating material;

[0079] The first layer (bottom layer) directly contacts the roughened graphite substrate and a higher content of graphite powder is added. The graphite powder has a similar composition to the graphite substrate, good compatibility, and a close coefficient of thermal expansion, which helps to reduce the interfacial stress and enhance the bonding force between the coating and the substrate. At the same time, the graphite powder can also improve the thermal / electrical conductivity of this layer. Ultrasonic dispersion ensures the uniform distribution of graphite powder and avoids agglomeration.

[0080] S3.2: Add 8% to 15% of graphite powder and 2% to 5% of carbon nanotubes to the second batch of modified resin solution and ultrasonically disperse it for 15 to 25 minutes to obtain the second coating material;

[0081] The second layer (intermediate layer) serves as a transition layer and contains both graphite powder and carbon nanotubes (CNTs). The content of graphite powder is relatively reduced compared to the bottom layer, and carbon nanotubes are introduced. Carbon nanotubes have extremely high mechanical strength and modulus, as well as excellent thermal and electrical conductivity. Introducing CNTs in this layer can start to enhance the mechanical properties of the coating and further adjust the coefficient of thermal expansion, playing a connecting role and smoothing the property gradient.

[0082] S3.3: Add 2% to 5% of carbon nanotubes to the third batch of modified resin solution and ultrasonically disperse it for 15 to 25 minutes to obtain the third coating material.

[0083] The third layer (surface layer) is the protective layer finally exposed to the working environment, and the main filler is carbon nanotubes, with no or only a very small amount of graphite powder added. Utilizing the excellent mechanical properties (high strength, high toughness) and chemical stability of carbon nanotubes can significantly improve the hardness, wear resistance, and oxidation resistance of the coating. The carbon-based composite structure formed after the pyrolysis of CNTs in the resin is dense and can effectively block the intrusion of oxidizing gases.

[0084] S4: Coating the first coating material, the second coating material, and the third coating material on the surface of the graphite block in sequence and performing crosslinking curing to obtain a multi-layer coating structure;

[0085] The multi-layer coating structure is as Figure 2In S4, the three prepared coating slurries are sequentially applied to the substrate, and the resin is cross-linked by heat treatment to form a stable three-dimensional network structure, in which the filler is fixed, thereby obtaining a multi-layer green coating with a gradient structure.

[0086] S4.1: applying the first coating material to the surface of the graphite block at 50° C. to 60° C. for 10 to 20 minutes to obtain a first layer structure of the multilayer coating structure, wherein the first layer structure has a thickness of 100 to 150 μm;

[0087] Coating can be done by brushing, spraying, or dipping. The key is to ensure a uniform coating. After coating, the coating is briefly held at a relatively low temperature (50°C-60°C) to allow the solvent (acetone) to evaporate, increase the coating's viscosity, and initially set, preventing mixing or deformation during the application of the next layer. The thickness is also controlled within this range to ensure the functionality of the base layer and provide a foundation for subsequent layers.

[0088] S4.2: applying the second coating material to the surface of the first layer structure, maintaining the temperature at 50°C to 60°C for 10 to 20 minutes, to obtain the first layer structure of the multi-layer coating structure, wherein the thickness of the second layer structure is 100 to 150 μm;

[0089] After the first layer has been initially set, the second layer is applied, also using a low-temperature, short-term treatment to partially remove the solvent and set the shape. The thickness of the second layer is controlled to achieve a smooth transition in performance.

[0090] S4.3: Coating the third coating material on the surface of the second layer structure, maintaining it at 50°C to 60°C for 10 minutes to 20 minutes to obtain a third layer structure of the multi-layer coating structure, wherein the thickness of the third layer structure is 150 microns to 200 microns.

[0091] The outermost layer (the third layer) is also applied with a low-temperature, short-term treatment. The surface layer is designed to be thicker (150-200 microns) to provide greater protection from the external environment. This completes the three-layer gradient wet coating.

[0092] S4.4: treating the multilayer coating structure at 80° C. to 100° C. for 4 to 6 hours to obtain a preliminary cross-linked structure;

[0093] This is the first stage of crosslinking and curing (low-temperature curing). At this temperature, the phenolic resin begins to undergo a preliminary polycondensation reaction, forming a partially crosslinked network structure, while most of the residual solvent is removed. A longer treatment time ensures that the reaction is more complete, the coating hardness increases, and the basic solidification is completed.

[0094] S4.5: Treat the preliminary cross-linked structure at 150 °C to 180 °C for 2 hours to 4 hours to obtain a second cross-linked structure;

[0095] This is the second stage of cross-linking curing (medium-temperature curing). Increasing the temperature can promote a deeper cross-linking reaction of the phenolic resin to form a denser and more stable three-dimensional network structure. This stage is crucial for improving the thermal stability of the coating and the final carbon yield.

[0096] S4.6: Treat the second cross-linked structure at 200 °C to 250 °C for 1 hour to 2 hours.

[0097] This is the third stage of cross-linking curing (high-temperature post-curing). Treating at a higher temperature for a short time aims to ensure that the cross-linking reaction is as complete as possible, eliminate internal stress, and bring the cured resin structure to the best state in preparation for subsequent high-temperature pyrolysis. This step-by-step temperature-raising curing strategy helps control the reaction rate and avoid coating defects caused by rapid exothermic reactions or rapid solvent evaporation.

[0098] S5: Carry out staged pyrolysis on the multi-layer coated structure under an argon atmosphere to obtain a vitreous carbon coating;

[0099] This step is the core carbonization process, which converts the cured organic resin (phenolic resin) into an inorganic carbon material (vitreous carbon) through high-temperature treatment under the protection of an inert atmosphere (argon). The argon atmosphere is to prevent the carbon material from being oxidized at high temperatures. Staged pyrolysis is the key to controlling the carbonization process, and its temperature curve is as Figure 3 shown.

[0100] S5.1: Under an argon atmosphere, raise the temperature from 20 °C to 400 °C at a heating rate of 1 °C / min to 3 °C / min and hold for 2 hours to obtain a first pyrolysis structure;

[0101] In the first stage (low-temperature pyrolysis), a very slow heating rate is adopted. Below 400 °C, the phenolic resin undergoes major decomposition reactions, releasing a large amount of small-molecule volatiles (such as water, formaldehyde, phenols, etc.). Slow heating and holding can allow these gases to escape slowly, avoiding coating bubbles, cracks, or delamination caused by rapid gas expansion.

[0102] S5.2: Raise the temperature of the first pyrolysis structure to 800 °C at a heating rate of 3 °C / min to 5 °C / min and hold for 2 hours to obtain a second pyrolysis structure;

[0103] In the second stage (medium-temperature pyrolysis), the heating rate can be appropriately increased. In this temperature range (400 °C - 800 °C), the remaining organic components continue to decompose, the carbon skeleton begins to form and undergoes rearrangement and condensation, and non-carbon atoms are further removed. Holding helps stabilize and densify the structure.

[0104] S5.3: Heat the second pyrolysis structure to 1000°C to 1200°C at a heating rate of 5°C / min to 8°C / min and hold for 4 hours to 6 hours, then cool to 20°C at a cooling rate of 3°C / min to 5°C / min to obtain the glassy carbon coating.

[0105] The heating rate in the third stage (high-temperature pyrolysis) can be faster. Reaching the final carbonization temperature (1000°C - 1200°C) is crucial for forming the glassy carbon structure. Glassy carbon is a non-graphitizable carbon material with an isotropic structure similar to glass. Higher temperatures and longer holding times contribute to the further development and perfection of the carbon atom network, improving the density, hardness, electrical conductivity, and chemical stability of the coating. Slow cooling is to avoid coating cracking or peeling from the substrate due to thermal stress. Through such a staged pyrolysis process, volume shrinkage and defect generation during pyrolysis can be minimized, and a high-quality glassy carbon coating can be obtained.

[0106] S6: Sand the surface of the glassy carbon coating, impregnate it with a phenolic resin solution, and then perform heat treatment to obtain a surface-modified glassy carbon coating.

[0107] This step is a post-treatment of the preliminarily formed glassy carbon coating, aiming to further improve the surface quality and performance of the coating, especially to seal possible surface micropores or microcracks and enhance its effect as a protective layer.

[0108] S6.1: Sand the surface of the glassy carbon coating with 1000-mesh to 2000-mesh sandpaper to obtain the sanded glassy carbon coating;

[0109] After high-temperature pyrolysis, there may be slight unevenness or extremely fine defects on the coating surface. Slight sanding with high-mesh (fine-grained) sandpaper can remove these tiny surface flaws, make the surface smoother, and may open some of the near-surface closed pores, creating conditions for subsequent impregnation treatment.

[0110] S6.2: Immerse the sanded glassy carbon coating in a phenolic resin solution with a concentration of 10% to 15% for 1 hour to 2 hours, and treat it at 50°C to 60°C for 2 hours to obtain the impregnated glassy carbon coating;

[0111] Immerse the sanded coating in a low-concentration phenolic resin solution. The low concentration is beneficial for the resin solution to penetrate into the surface micropores and microcracks. After soaking for a certain time, treat it at a lower temperature to volatilize the solvent and fill the resin in these defects.

[0112] S6.3: Under an argon atmosphere, heat from 20°C to 400°C at a heating rate of 1°C / min to 3°C / min and hold for 2 hours, then heat to 800°C at a heating rate of 3°C / min to 5°C / min and hold for 2 hours, and then heat to 900°C to 1000°C at a heating rate of 5°C / min to 8°C / min and hold for 2 hours to 3 hours to obtain a surface-modified glassy carbon coating.

[0113] Pyrolyze the impregnated coating again. The purpose of this pyrolysis is to in-situ carbonize the phenolic resin filled in the surface defects to form glassy carbon, thereby effectively blocking these defects. The final pyrolysis temperature (900°C - 1000°C) is lower than the first main pyrolysis temperature, but is sufficient to completely carbonize the small amount of impregnated resin. This treatment is equivalent to a "repair" and "densification" of the coating surface, and finally a glassy carbon coating with improved surface properties is obtained.

[0114] The embodiment of the present application further includes: polishing the surface-modified glassy carbon coating successively with diamond polishing pastes with particle sizes of 0.5 μm to 1 μm, 0.1 μm to 0.5 μm, and 0.05 μm to 0.1 μm, and each polishing time is 10 minutes to 20 minutes.

[0115] This is an optional final finishing step. By performing multiple polishing operations using diamond polishing pastes with decreasing particle sizes, the surface roughness of the coating can be greatly reduced, and a mirror-like finish can be obtained. This may be beneficial for specific applications that require a low coefficient of friction, high reflectivity, or extremely high surface cleanliness (such as semiconductor processing components, precision molds, etc.).

[0116] The embodiment of the present application also provides a composite material, which is prepared by using the graphite block glassy carbon coating preparation method described in the above preliminary technical solutions S1 - S6 or S1 - S6.

[0117] The composite material finally presents as an organic whole, which is composed of an internal graphite block matrix and an external multi-layer gradient glassy carbon coating that is tightly combined, has a complete structure, and excellent performance. The coating endows the graphite block with high hardness, high wear resistance, excellent high-temperature oxidation resistance, and chemical stability that it originally did not have, while retaining the good thermal conductivity, electrical conductivity, and thermal shock resistance and other bulk properties of the graphite substrate, thereby expanding the application range of graphite materials, especially in harsh environments such as high temperature, corrosion, and oxidation.

[0118] Example 1

[0119] A method for preparing a graphite block glassy carbon coating includes the following steps:

[0120] Step 1: Roughen the surface of the graphite block.

[0121] The surface of the graphite bulk material was sanded with 80-mesh sandpaper. Then, a mixed solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 3:1 was used to etch the sanded graphite bulk material at 70 °C for 45 minutes. After etching, it was repeatedly rinsed with deionized water until neutral, and then dried in an oven at 100 °C for 5 hours to obtain a graphite bulk material with a rough surface.

[0122] Step 2: Prepare a modified resin solution.

[0123] Phenolic resin was dissolved in acetone to prepare a phenolic resin solution with a concentration of 40%. Fumed silica with a mass ratio of 3% was added to this solution, and it was sheared and mixed at a rotational speed of 10,000 revolutions per minute for 20 minutes using a high-speed shearer to obtain a filler dispersion. Subsequently, an isocyanate compound (such as toluene diisocyanate TDI) with a mass ratio of 5% was added to the filler dispersion, and it was ultrasonically treated at a frequency of 30 kHz for 20 minutes using an ultrasonic processor to obtain a modified resin solution.

[0124] Step 3: Prepare three coating materials.

[0125] Take the first batch of modified resin solution, add graphite powder with a mass ratio of 20%, and ultrasonically disperse for 20 minutes to obtain the first coating material. Take the second batch of modified resin solution, add graphite powder with a mass ratio of 12% and carbon nanotubes with a mass ratio of 3%, and ultrasonically disperse for 20 minutes to obtain the second coating material. Take the third batch of modified resin solution, add carbon nanotubes with a mass ratio of 3%, and ultrasonically disperse for 20 minutes to obtain the third coating material.

[0126] Step 4: Multilayer coating and crosslinking curing.

[0127] The first coating material was uniformly coated on the surface of the rough graphite bulk material obtained in Step 1 and placed in an oven at 55 °C for 15 minutes to form a first-layer structure with a thickness of 120 microns. Then, the second coating material was coated on the surface of the first-layer structure and also kept at 55 °C for 15 minutes to form a second-layer structure with a thickness of 120 microns. Finally, the third coating material was coated on the surface of the second-layer structure and kept at 55 °C for 15 minutes to form a third-layer structure with a thickness of 180 microns. The multilayer coating structure with the three-layer structure was placed in an oven, first treated at 90 °C for 5 hours for preliminary crosslinking, then treated at 160 °C for 3 hours for the second crosslinking, and finally treated at 220 °C for 1.5 hours to complete crosslinking curing.

[0128] Step 5: Segmented pyrolysis.

[0129] The crosslinked and cured multi-layer coating structure was placed in a tube furnace and protected by introducing argon. It was heated from 20 °C (room temperature) to 400 °C at a heating rate of 2 °C per minute and held for 2 hours; then it was heated to 800 °C at a heating rate of 4 °C per minute and held for 2 hours; finally, it was heated to 1100 °C at a heating rate of 6 °C per minute and held for 5 hours. After the holding was completed, it was naturally cooled to 20 °C at a cooling rate of 4 °C per minute to obtain a glassy carbon coating.

[0130] Step 6: Surface modification.

[0131] The surface of the glassy carbon coating was gently sanded with 1500-mesh sandpaper. The sanded sample was immersed in a phenolic resin acetone solution with a concentration of 12% for 1.5 hours, and after taking it out, it was treated at 55 °C for 2 hours to volatilize the solvent. The impregnated glassy carbon coating was placed in a tube furnace under an argon atmosphere again, heated from 20 °C to 400 °C at a heating rate of 2 °C per minute and held for 2 hours; then it was heated to 800 °C at a heating rate of 4 °C per minute and held for 2 hours; finally, it was heated to 950 °C at a heating rate of 6 °C per minute and held for 2.5 hours, and after cooling, a surface-modified glassy carbon coating was obtained.

[0132] Step 7: Polishing treatment.

[0133] The surface-modified glassy carbon coating was polished successively with diamond polishing pastes with particle sizes of 0.8 μm, 0.3 μm, and 0.08 μm on a polishing machine, and each polishing time was 15 minutes to obtain the final composite material.

[0134] Example 2

[0135] A method for preparing a glassy carbon coating on a graphite block, the basic process flow of which is the same as that of Example 1, except that:

[0136] In Step 1, the surface of the graphite block was sanded with 100-mesh sandpaper, and etched with a mixed solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 3:1 at 65 °C for 40 minutes, and dried at 90 °C for 4.5 hours.

[0137] In Step 2, a phenolic resin solution with a concentration of 35% was prepared; 2% by mass of fumed silica was added to the phenolic resin solution, and shear mixing was carried out at 9000 revolutions per minute for 18 minutes; 4% of an isocyanate compound was added to the filler dispersion liquid, and ultrasonic treatment was carried out at 25 kHz for 18 minutes.

[0138] In Step 3, 18% graphite powder was added to the first batch of modified resin solution and ultrasonically dispersed for 18 minutes; 10% graphite powder and 2.5% carbon nanotubes were added to the second batch of modified resin solution and ultrasonically dispersed for 18 minutes; 2.5% carbon nanotubes were added to the third batch of modified resin solution and ultrasonically dispersed for 18 minutes.

[0139] In Step 4, the thickness of the first layer structure was 110 microns, the thickness of the second layer structure was 110 microns, and the thickness of the third layer structure was 160 microns; the multi-layer coating structure was treated at 85 °C for 4.5 hours, at 155 °C for 2.5 hours, and at 210 °C for 1.2 hours.

[0140] In Step 5, the temperature was raised from 20 °C to 400 °C at a heating rate of 1.5 °C / minute and held for 2 hours, then raised to 800 °C at a heating rate of 3.5 °C / minute and held for 2 hours, then raised to 1050 °C at a heating rate of 5.5 °C / minute and held for 4.5 hours, and then cooled to 20 °C at a cooling rate of 3.5 °C / minute.

[0141] In Step 6, sanding was performed using 1200-mesh sandpaper, soaked in a phenolic resin solution with a concentration of 11% for 1.2 hours, and treated at 52 °C for 2 hours; the temperature was raised from 20 °C to 400 °C at a heating rate of 1.5 °C / minute and held for 2 hours, then raised to 800 °C at a heating rate of 3.5 °C / minute and held for 2 hours, then raised to 920 °C at a heating rate of 5.5 °C / minute and held for 2.2 hours.

[0142] Example 3

[0143] A method for preparing a graphite block glassy carbon coating, the basic process flow of which is similar to that of Example 1, with the differences as follows:

[0144] Step 1: Sanding was performed using 120-mesh sandpaper, etching was carried out with an acid solution with a volume ratio of 3:1 at 80 °C for 60 minutes, and drying was carried out at 120 °C for 6 hours.

[0145] Step 2: A 50% phenolic resin solution, 5% fumed silica was added, shearing was carried out at 12000 rpm for 30 minutes, 8% isocyanate was added, and ultrasonic treatment was carried out at 40 kHz for 30 minutes.

[0146] Step 3: 25% graphite powder was added to the first layer and ultrasonically treated for 25 minutes; 15% graphite powder and 5% carbon nanotubes were added to the second layer and ultrasonically treated for 25 minutes; 5% carbon nanotubes were added to the third layer and ultrasonically treated for 25 minutes.

[0147] Step 4: Coating was carried out while maintaining at 60 °C for 20 minutes, the first layer was 150 microns thick, the second layer was 150 microns thick, and the third layer was 200 microns thick; curing was carried out by treating at 100 °C for 6 hours, at 180 °C for 4 hours, and at 250 °C for 2 hours.

[0148] Step 5: Pyrolysis, heating to 400 °C at 3 °C / min (holding for 2 h), heating to 800 °C at 5 °C / min (holding for 2 h), heating to 1200 °C at 8 °C / min (holding for 6 h), and cooling at 5 °C / min.

[0149] Step 6: Grinding with 2000 - mesh sandpaper, soaking in 15% phenolic resin solution for 2 hours, treating at 60 °C for 2 hours; heat treatment, heating to 400 °C at 3 °C / min (holding for 2 h), heating to 800 °C at 5 °C / min (holding for 2 h), heating to 1000 °C at 8 °C / min (holding for 3 h).

[0150] Example 4

[0151] A method for preparing a glass - carbon coating on a graphite block, comprising:

[0152] Step 1: Grinding with 90 - mesh sandpaper, etching with an acid solution with a volume ratio of 3:1 at 70 °C for 45 minutes, and drying at 100 °C for 5 hours.

[0153] Step 2: 40% phenolic resin solution, adding 3% fumed silica, shearing at 10000 rpm for 22.5 minutes, adding 5% isocyanate, and ultrasonicating at 30 kHz for 22.5 minutes.

[0154] Step 3: Adding 20% graphite powder in the first layer and ultrasonicating for 20 minutes; adding 11.5% graphite powder and 3.5% carbon nanotubes in the second layer and ultrasonicating for 20 minutes; adding 3.5% carbon nanotubes in the third layer and ultrasonicating for 20 minutes.

[0155] Step 4: Coating and holding at 55 °C for 15 minutes, with the first layer having a thickness of 125 microns, the second layer having a thickness of 125 microns, and the third layer having a thickness of 175 microns; curing, treating at 90 °C for 5 hours, treating at 165 °C for 3 hours, and treating at 225 °C for 1.5 hours.

[0156] Step 5: Pyrolysis, heating to 400 °C at 2 °C / min (holding for 2 h), heating to 800 °C at 4 °C / min (holding for 2 h), heating to 1100 °C at 6.5 °C / min (holding for 5 h), and cooling at 4 °C / min.

[0157] Step 6: Grinding with 1500 - mesh sandpaper, soaking in 12.5% phenolic resin solution for 1.5 hours, treating at 55 °C for 2 hours; heat treatment, heating to 400 °C at 2 °C / min (holding for 2 h), heating to 800 °C at 4 °C / min (holding for 2 h), heating to 950 °C at 6.5 °C / min (holding for 2.5 h).

[0158] Step 7: The surface - modified glass - carbon coating is polished successively with diamond polishing pastes with particle sizes of 0.75 microns, 0.3 microns, and 0.075 microns, with each polishing time being 15 minutes.

[0159] The test results show that the total thickness of the obtained multi-layer coating is uniform, about 425 microns. The three-layer structure is clearly distinguishable. The bonding strength between the coating and the substrate reaches 35 MPa. The surface of the coating is smooth without cracks, and the internal structure is dense. When tested in an oxidation environment at 900 °C, the coating remains intact after 10 hours, and the oxidation mass loss rate is only 3.2%. Scanning electron microscopy observation shows that the interface between the coating and the substrate is tightly bonded without obvious interface defects.

[0160] Example 5

[0161] This example aims to study the influence of adjusting the coating material composition on the performance. The preparation method is basically the same as that of Example 4, but the coating material composition in Step 3 is adjusted as follows:

[0162] Step 3: Add graphite powder with a mass ratio of *25%* to the first batch of modified resin solution and ultrasonicate for 20 minutes; add graphite powder with a mass ratio of 15% and carbon nanotubes with a mass ratio of 5% to the second batch of modified resin solution and ultrasonicate for 20 minutes; add carbon nanotubes with a mass ratio of 5%* to the third batch of modified resin solution and ultrasonicate for 20 minutes.

[0163] The parameters of the remaining steps (Steps 1, 2, 4, 5, 6, 7) are the same as those in Example 4.

[0164] The test results show that the total thickness of the coating is about 430 microns, the bonding strength between the coating and the substrate reaches 38 MPa, and the surface of the coating is smooth without cracks. When tested in an oxidation environment at 900 °C, the coating remains intact after 12 hours, and the oxidation mass loss rate is 2.8%. The test of the thermal expansion coefficient of the material shows that the average thermal expansion coefficient from room temperature to 800 °C is 4.5×10-6 / K, which is close to that of the graphite substrate, effectively reducing the thermal stress.

[0165] Example 6

[0166] This example aims to study the influence of increasing the pyrolysis temperature and prolonging the heat preservation time on the performance. The preparation method is basically the same as that of Example 4, but the final heat treatment temperature and time in Steps 5 and 6 are adjusted as follows:

[0167] Step 5: Stepwise pyrolysis.... Finally, heat the second pyrolysis structure to 1200 °C at a heating rate of 6.5 °C / minute and hold for 6 hours, and then cool to 20 °C at a cooling rate of 4 °C / minute to obtain a glassy carbon coating.

[0168] Step 6: Surface modification.... Finally, in an argon atmosphere,... heat to 1000 °C at a heating rate of 6.5 °C / minute and hold for 3 hours to obtain a surface-modified glassy carbon coating.

[0169] The remaining steps (Steps 1, 2, 3, 4, 7 and the previous heat treatment parameters of Steps 5 and 6) are the same as those in Example 4.

[0170] The test results show that the total thickness of the coating is about 410 μm, the bonding strength between the coating and the substrate reaches 42 MPa, the surface of the coating is smooth without cracks, and the degree of graphitization inside is higher. When tested in an oxidation environment at 1000 °C, the coating remains intact after 8 hours, and the oxidation mass loss rate is 4.5%. X-ray diffraction analysis shows that the degree of graphitization of the coating reaches 85%. Raman spectroscopy analysis indicates that the ID / IG ratio of the coating is 0.35, indicating its good structural orderliness.

[0171] Comparative Example 1

[0172] This comparative example aims to compare with Example 1 to illustrate the advantages of the multi-layer gradient structure over single-layer coating. A modified resin similar to that in Example 1 (40% phenolic resin, 3% fumed silica, 5% isocyanate) and a segmented pyrolysis process (held at 400 °C for 2 h at 2 °C / min, held at 800 °C for 2 h at 4 °C / min, held at 1100 °C for 5 h at 6 °C / min) are used. The difference is that instead of multi-layer coating, a mixed filler (for example, taking the average value of the fillers in each layer of Example 1, set as 15% graphite powder and 3% carbon nanotubes) is directly mixed with the modified resin to prepare a single coating material, which is coated on the surface of a graphite block that has undergone the same roughening treatment (80-mesh sandpaper + acid etching at 70 °C for 45 min) at one time, and the total thickness after curing is controlled to be close to the total thickness of Example 1 (about 400 μm). Subsequently, the same crosslinking and curing (90 °C / 5 h, 160 °C / 3 h, 220 °C / 1.5 h) and segmented pyrolysis as in Example 1 are carried out. No subsequent surface modification treatment is performed (i.e., step 6 of Example 1 is omitted). Expected results: Due to the lack of a gradient transition layer to relieve the interfacial stress and the relatively thick single layer, the shrinkage stress generated during the pyrolysis process is more difficult to release. It is expected that the bonding strength between this single-layer coating and the substrate will be significantly lower than that of Examples 1-3 (possibly lower than 20 MPa), and the coating is more likely to crack, warp, or even peel off, and the antioxidant performance will also be reduced due to structural defects.

[0173] Comparative Example 2

[0174] This comparative example aims to compare with Example 2 to illustrate the advantages of using modified phenolic resin (added with fumed silica and isocyanate) over unmodified phenolic resin. The preparation process strictly follows all steps of Example 1, including surface roughening, multi-layer gradient coating (the first layer with 20% graphite powder, the second layer with 12% graphite powder + 3% CNT, the third layer with 3% CNT), control of the thickness of each layer, crosslinking and curing procedure, and segmented pyrolysis procedure (up to 1100 °C) and the final surface modification treatment (1500-mesh sanding + 12% resin impregnation + secondary pyrolysis at up to 950 °C). The only difference is that when preparing the three coating materials, the resin solution used is an unmodified 40% phenolic resin acetone solution, that is, the steps of adding fumed silica and isocyanate compounds and the corresponding shear mixing and ultrasonic treatment steps in Step 2 are omitted. Expected results: Due to the non-use of modified resin, the rheology of the coating slurry may be poor, and the coating uniformity is affected; more importantly, the crosslinking density after curing of the unmodified resin is low, and the thermal stability is poor. More defects (such as pores, microcracks) may be generated during the pyrolysis process, and the carbon yield may also be low. The density of the finally obtained vitreous carbon coating will be inferior to that of Examples 1-3, and the mechanical properties (such as hardness, bonding strength) and antioxidant properties will decrease accordingly. The bonding strength is expected to be significantly lower than 35 MPa.

[0175] Comparative Example 3

[0176] This comparative example aims to compare with Example 3 to illustrate the advantages of a fine segmented pyrolysis process over a fast and simple pyrolysis process. The preparation process adopts the same surface roughening, modified resin preparation, multi-layer gradient coating material preparation and coating, and crosslinking and curing steps as in Example 1. The difference lies in the pyrolysis step (Step 5): The crosslinked and cured multi-layer coating structure is placed in an argon atmosphere, and fast single-stage heating pyrolysis is adopted, for example, directly heating from room temperature to 1100 °C at a rate of 10 °C / minute and holding for 5 hours, and then naturally cooling. The subsequent surface modification treatment (Step 6) is also omitted or has poor effects accordingly. Expected results: Due to the too fast heating rate, especially in the low-temperature decomposition stage (below 400 °C), a large amount of gas generated by the resin decomposition cannot escape in time, and a large number of pores and bubbles will be formed inside the coating, even leading to coating bursting or serious cracking. At the same time, the huge thermal stress generated by the fast heating and cooling will also cause the bonding between the coating and the substrate to be damaged or the coating itself to crack. Therefore, it is expected that the obtained coating quality is extremely poor, covered with macroscopic defects, the bonding strength is extremely low, and it hardly has effective protection performance, far inferior to the results of Examples 1-3.

[0177] The results of Comparative Example 1 show that even with the use of modified resins and optimized pyrolysis processes, for a single-layer coating lacking the design of a multi-layer gradient structure, due to interface stress concentration and difficulty in releasing internal stress within the thick layer, its bonding strength and structural integrity are far inferior to those of Examples 1-3. This highlights the crucial role of the multi-layer gradient structure design in the present invention.

[0178] The results of Comparative Example 2 show that even with the adoption of a multi-layer structure and segmented pyrolysis, if unmodified phenolic resin is used, the densification, mechanical properties, and oxidation resistance of the final coating will all decline. This proves the importance of modifying the resin by adding fumed silica and isocyanate in Step 2 of the present invention for improving the properties of the precursor, optimizing the curing process, and ultimately obtaining a high-quality glassy carbon coating.

[0179] The results of Comparative Example 3 emphasize the core position of pyrolysis process control. Using the same materials and structure as in Example 1 but with a rapid and simple pyrolysis program will lead to disastrous results, where the coating cannot be formed or is severely damaged. This fully proves the extreme importance of the refined pyrolysis scheme of slow, segmented heating and holding in Step 5 of the present invention for controlling the decomposition of organic matter, gas release, volume shrinkage, and thermal stress, thereby avoiding defects and ensuring the coating quality and bonding strength.

[0180] Overall, the excellent properties (high bonding strength, high densification, excellent oxidation resistance) of Examples 1, 2, and 3 are the result of the synergistic effect of various technical features of the present invention, including effective surface pretreatment, optimized modified resins, ingenious multi-layer gradient structure design, precisely controlled segmented pyrolysis process, and necessary surface post-treatment. The absence or simplification of any key link (as shown in Comparative Examples 4, 5, and 6) will significantly damage the quality and performance of the final coating.

[0181] Comparative Example 4

[0182] A glassy carbon coating for graphite blocks was prepared by using a traditional single-layer phenolic resin impregnation method. After simply sanding the surface of the graphite blocks with 120-mesh sandpaper, they were directly immersed in an unmodified phenolic resin solution (concentration 40%) for 2 hours, taken out and dried at 80°C for 5 hours, and then heated from room temperature to 1000°C at a constant heating rate of 5°C / minute in an argon atmosphere and held for 5 hours, and cooled to room temperature to obtain the glassy carbon coating.

[0183] Comparative Example 5

[0184] The graphite block glassy carbon coating was prepared by a two-step impregnation method. First, the graphite block was pickled (the same as step 1 of Example 1), and then immersed in a phenolic resin solution containing 10% graphite powder. After pretreatment at 60 °C for 1 hour, it was immersed in a pure phenolic resin solution (concentration 40%) for 1 hour. Using the traditional one-step pyrolysis process, it was heated from room temperature to 950 °C at a rate of 3 °C / min in an argon atmosphere and held for 4 hours.

[0185] Comparative Example 6

[0186] The graphite block glassy carbon coating was prepared by a single-layer coating method with the addition of carbon nanotubes. First, the surface of the graphite block was roughened (the same as step 1 of Example 1), and then 3% carbon nanotubes were dispersed in an unmodified phenolic resin solution (concentration 40%) and directly coated on the surface of the graphite block, controlling the coating thickness to be about 300 microns. A two-stage pyrolysis process was used. In the first stage, it was heated to 500 °C at a rate of 2 °C / min and held for 2 hours. In the second stage, it was heated to 1000 °C at a rate of 4 °C / min and held for 5 hours.

[0187] Examples 4, 5, and 6 were compared and analyzed with Comparative Examples 4, 5, and 6:

[0188] Comparative Example 4 used the simplest single-layer impregnation and constant-rate heating pyrolysis. The obtained coating had uneven thickness, serious cracking, low bonding strength (12 MPa), and poor oxidation resistance (damaged after 2 hours at 900 °C, loss rate 18.5%).

[0189] Comparative Example 5 used two-step impregnation, which was an improvement compared to Comparative Example 1, but still used a simple pyrolysis process. The coating bonding strength (18 MPa) and oxidation resistance (damaged after 4 hours at 900 °C, loss rate 12.3%) were still not ideal, and there were microcracks.

[0190] Although carbon nanotubes were introduced and two-stage pyrolysis was used in Comparative Example 6, the bonding strength (22 MPa) and oxidation resistance (damaged after 6 hours at 900 °C, loss rate 9.8%) were further improved, but single-layer coating and insufficiently optimized pyrolysis still led to pores and slight cracks.

[0191] In contrast, Example 4 of the present invention adopted a multi-layer gradient coating structure design, modified resin, a fine segmented pyrolysis process, and subsequent surface modification treatment, significantly improving the coating quality. The coating was uniform, dense, and crack-free, the bonding strength was greatly increased to 35 MPa, and the oxidation resistance was excellent (intact after 10 hours at 900 °C, loss rate 3.2%). This demonstrated the superiority of the overall technical solution of the present invention.

[0192] Example 5 optimized the filler ratio on the basis of Example 4, further improving the bonding strength (38 MPa) and oxidation resistance (intact after 12 hours at 900 °C, loss rate 2.8%), and showing good thermal matching. This indicates that the method of the present invention can optimize the performance by adjusting the component ratio.

[0193] Example 6 increased the pyrolysis temperature and time on the basis of Example 4, obtaining higher bonding strength (42 MPa) and graphitization degree, enabling the coating to remain stable at a higher temperature (1000 °C) (intact after 8 hours, loss rate 4.5%). This shows that the segmented pyrolysis process of the present invention has adjustability and can prepare coatings that meet the requirements of different harsh environments.

[0194] In summary, through the design of a multi-layer coating structure (solving the problems of interfacial bonding and stress transition), modified resin (improving wettability and reactivity), a fine segmented pyrolysis process (controlling the shrinkage and carbonization processes to avoid defects), and surface re-treatment (sealing holes and strengthening the surface layer), the present invention effectively overcomes the defects of traditional methods. The prepared glassy carbon coating has excellent bonding strength, density, surface quality, and oxidation resistance, showing significant progress compared with the prior art.

[0195] The composite material of the present invention refers to a graphite block composite structure with a glassy carbon coating prepared by using the preparation method of the graphite block glassy carbon coating in any one of the above Examples 1-6. This composite material consists of a graphite block matrix and a surface glassy carbon coating, where the glassy carbon coating exhibits a multi-layer gradient structure. The bottom layer (the first layer) mainly contains graphite powder and has a strong bond with the graphite substrate; the middle layer (the second layer) contains a mixture of graphite powder and carbon nanotubes and serves as a transition; the surface layer (the third layer) mainly contains carbon nanotubes and has excellent oxidation resistance. The entire coating thickness is controllable (about 380-430 microns in the examples), the surface is smooth and dense, without obvious cracks and pores. The design of this multi-layer gradient structure enables the composite material to maintain the excellent thermal and electrical conductivity of the graphite block while significantly improving its surface oxidation resistance and mechanical strength, enabling it to be stably used in high-temperature oxidation environments at 900-1000 °C or even higher (as shown in Example 6) for a long time, meeting the strict requirements of high-temperature application fields such as metallurgy, chemical industry, aerospace, and nuclear industry.

[0196] It should be noted that the key parameters involved in the embodiments of the present invention, such as the sandpaper grit number, temperature, time, heating rate, etc., are optimized and determined based on a large amount of experimental data. In practical applications, they can be appropriately adjusted according to the specific characteristics of the graphite block and application requirements, as long as they do not deviate from the basic principles and technical solutions of the present invention, they fall within the scope of protection of the present invention.

[0197] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0198] Finally, it should be noted that the above embodiments are only specific implementation manners of the present disclosure, used to illustrate the technical solutions of the present disclosure, rather than limiting them. The protection scope of the present disclosure is not limited thereto. Although the present disclosure has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: any person skilled in the art within the technical scope disclosed by the present disclosure can still modify the technical solutions recorded in the foregoing embodiments or can easily think of changes, or perform equivalent replacements on some of the technical features; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present disclosure, and should all be covered by the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims.

Claims

1. A method for preparing a glassy carbon coating on a graphite block, characterized in that: Including: Roughening the surface of a graphite block to obtain a graphite block with a rough surface; Adding fumed silica and an isocyanate compound to a phenolic resin solution to obtain a modified resin solution; Based on the modified resin solution, graphite powder, and carbon nanotubes, preparing a first coating material, a second coating material, and a third coating material respectively, wherein the first coating material, the second coating material, and the third coating material contain graphite powder and / or carbon nanotubes; Sequentially coating the first coating material, the second coating material, and the third coating material on the surface of the graphite block and performing crosslinking curing to obtain a multi-layer coating structure; Performing segmented pyrolysis on the multi-layer coating structure under an argon atmosphere to obtain a glassy carbon coating; Performing surface sanding on the glassy carbon coating, impregnating it with a phenolic resin solution, and then performing heat treatment to obtain a surface-modified glassy carbon coating.

2. The method for preparing the graphite block glassy carbon coating according to claim 1, characterized in that, Roughening the surface of a graphite block to obtain a graphite block with a rough surface, including: Sanding the surface of the graphite block with 60-mesh to 120-mesh sandpaper to obtain a preliminarily treated graphite block; Etching the preliminarily treated graphite block with a mixed solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 3:1 at 60°C to 80°C for 30 minutes to 60 minutes to obtain an etched graphite block; Washing the etched graphite block and drying it at 80°C to 120°C for 4 hours to 6 hours to obtain the graphite block with a rough surface.

3. The method for preparing a graphite block glassy carbon coating according to claim 1, characterized in that, Adding fumed silica and an isocyanate compound to a phenolic resin solution to obtain a modified resin solution, including: Dissolving phenolic resin in acetone to prepare a phenolic resin solution with a concentration of 30% to 50%; Adding 1% to 5% of fumed silica by mass to the phenolic resin solution and shearing and mixing it at 8000 revolutions per minute to 12000 revolutions per minute for 15 minutes to 30 minutes to obtain a filler dispersion; Adding 2% to 8% of an isocyanate compound to the filler dispersion and performing ultrasonic treatment on it at 20 kHz to 40 kHz for 15 minutes to 30 minutes to obtain the modified resin solution.

4. The method for preparing the graphite block glassy carbon coating according to claim 1, wherein Based on the modified resin solution, graphite powder, and carbon nanotubes, preparing a first coating material, a second coating material, and a third coating material respectively, including: Adding 15% to 25% of graphite powder to the first batch of modified resin solution and performing ultrasonic dispersion for 15 minutes to 25 minutes to obtain the first coating material; Adding 8% to 15% of graphite powder and 2% to 5% of carbon nanotubes to the second batch of modified resin solution and performing ultrasonic dispersion for 15 minutes to 25 minutes to obtain the second coating material; Adding 2% to 5% of carbon nanotubes to the third batch of modified resin solution and performing ultrasonic dispersion for 15 minutes to 25 minutes to obtain the third coating material.

5. The method for preparing a graphite block glassy carbon coating according to claim 4, characterized in that, Sequentially coating the first coating material, the second coating material, and the third coating material on the surface of the graphite block and performing crosslinking curing to obtain a multi-layer coating structure, including: Coat the surface of the graphite block with the first coating material and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the first layer structure of the multi-layer coating structure, and the thickness of the first layer structure is 100 microns to 150 microns; Coat the surface of the first layer structure with the second coating material and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the second layer structure of the multi-layer coating structure, and the thickness of the second layer structure is 100 microns to 150 microns; Coat the surface of the second layer structure with the third coating material and keep it at 50°C to 60°C for 10 minutes to 20 minutes to obtain the third layer structure of the multi-layer coating structure, and the thickness of the third layer structure is 150 microns to 200 microns.

6. The method for preparing a glassy carbon coating on a graphite block according to claim 5, wherein Coat the surface of the graphite block with the first coating material, the second coating material, and the third coating material in sequence and perform crosslinking curing to obtain a multi-layer coating structure, further comprising: Place the multi-layer coating structure at 80°C to 100°C for 4 hours to 6 hours to obtain a preliminary crosslinked structure; Treat the preliminary crosslinked structure at 150°C to 180°C for 2 hours to 4 hours to obtain a second crosslinked structure; Treat the second crosslinked structure at 200°C to 250°C for 1 hour to 2 hours.

7. The method for preparing the graphite block glassy carbon coating according to claim 1, wherein Perform segmented pyrolysis on the multi-layer coating structure under an argon atmosphere to obtain a glassy carbon coating, including: Under an argon atmosphere, increase the temperature from 20°C to 400°C at a heating rate of 1°C / minute to 3°C / minute and keep it for 2 hours to obtain a first pyrolysis structure; Increase the temperature of the first pyrolysis structure to 800°C at a heating rate of 3°C / minute to 5°C / minute and keep it for 2 hours to obtain a second pyrolysis structure; Increase the temperature of the second pyrolysis structure to 1000°C to 1200°C at a heating rate of 5°C / minute to 8°C / minute and keep it for 4 hours to 6 hours, and then cool it to 20°C at a cooling rate of 3°C / minute to 5°C / minute to obtain the glassy carbon coating.

8. The method for preparing a glassy carbon coating on a graphite block according to claim 1, wherein: Perform surface sanding on the glassy carbon coating, impregnate it with a phenolic resin solution, and then perform heat treatment to obtain a surface-modified glassy carbon coating, including: Use 1000-mesh to 2000-mesh sandpaper to sand the surface of the glassy carbon coating to obtain a sanded glassy carbon coating; Immerse the sanded glassy carbon coating in a phenolic resin solution with a concentration of 10% to 15% for 1 hour to 2 hours, and treat it at 50°C to 60°C for 2 hours to obtain an impregnated glassy carbon coating; Under an argon atmosphere, increase the temperature from 20°C to 400°C at a heating rate of 1°C / minute to 3°C / minute and keep it for 2 hours, increase the temperature to 800°C at a heating rate of 3°C / minute to 5°C / minute and keep it for 2 hours, increase the temperature to 900°C to 1000°C at a heating rate of 5°C / minute to 8°C / minute and keep it for 2 hours to 3 hours to obtain a surface-modified glassy carbon coating.

9. The method for preparing a glassy carbon coating on a graphite block according to claim 8, wherein: Further comprising: The surface-modified glassy carbon coating is polished successively with diamond polishing pastes having particle sizes of 0.5 to 1 micron, 0.1 to 0.5 micron, and 0.05 to 0.1 micron, and the polishing time for each time is 10 to 20 minutes.

10. A composite material, characterized in that, It is prepared by using the method for preparing a graphite block glassy carbon coating according to any one of claims 1-9.

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