A multilayer composite ceramic coating with high-temperature stability and corrosion resistance on a graphite surface and a preparation method thereof

Abstract

The application discloses a kind of multilayer composite ceramic coating with high-temperature stability and corrosion resistance on graphite surface and a preparation method thereof, ultra-high temperature ceramic oxide slurry is brushed on the surface of graphite substrate, first heat treatment is carried out, porous transition layer is obtained, then ultra-high temperature carbide slurry is brushed on the surface of porous transition layer, ultra-high temperature carbide layer is obtained after drying, then roughening treatment is carried out on the surface of ultra-high temperature carbide layer using ultra-high temperature carbide slurry, then second heat treatment is carried out, rough carbide layer is obtained, then dense projection is obtained by point pressure on the surface of rough carbide layer using metal carbide ceramic mixed slurry, and protective layer material is sprayed, and the method of the application can significantly improve the firmness of each layer, effectively avoid the reaction between graphite substrate and surface protective layer at high temperature, leading to coating failure and falling off, further enhance the use limit of protective layer, and have wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of corrosion-resistant multilayer composite ceramic coatings on graphite surfaces, specifically relating to a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on graphite surfaces and its preparation method. Background Technology

[0002] Graphite materials, due to their excellent high-temperature stability (melting point up to 3850℃±50℃), chemical corrosion resistance (able to withstand most strong acids and alkalis), and good machinability, have been widely used in key fields such as metallurgy, semiconductors, and nuclear energy. However, their intrinsic properties cause them to oxidize at 400℃ in oxygen-containing environments, and their use as crucible materials presents carbon contamination problems, severely limiting their application in high-end equipment manufacturing.

[0003] In existing technologies, preparing ceramic coatings on graphite substrates using chemical vapor deposition (CVD), plasma spraying (APS), or sol-gel methods is the mainstream solution. Specific implementation methods include: 1. Carbide system: SiC coating (thermal expansion coefficient 4.5 × 10⁻⁶). -6 / ℃ with the best matching degree with graphite), TaC, TiC, etc.; 2. Oxide system: Y2O3, Al2O3, ZrO2; 3. Boron / nitride system: ZrB2 (excellent thermal shock resistance), TiN, Si3N4; 4. Composite coating: gradient structure SiC / Si3N4, multilayer design ZrB2-SiC-ZrC, etc.

[0004] However, experimental verification shows that under high-temperature service conditions (>1200℃), significant interfacial reactions occur between the non-carbide coating and the graphite substrate. For example, in the nitride system, TiN undergoes a disproportionation reaction at 1350℃ (TiN(s) + C(s) → TiC(s) + 1 / 2N2↑), resulting in an increase in coating porosity of 15-20%; in the boride system, ZrB2 reacts with C at 1600℃ to generate ZrC+ free boron (ΔG=-210 kJ / mol), causing a volume shrinkage of 3.8%; in the oxide system, Y2O3 reacts with C in a vacuum environment at 1550℃ to generate Y2O2C (Y2O3 + 3C → Y2O2C + 2CO↑), with a reaction rate of 0.15 mm / h.

[0005] The main defects of the existing technology are: 1. Mismatch between the thermal expansion coefficients of the coating and the substrate (e.g., the thermal expansion coefficient of Al2O3 is 8.4×10). -61. The interfacial stress accumulation is caused by the diffusion of C into SiC coatings (e.g., the diffusion depth of C into SiC coatings reaches 50 μm-1500℃ / 100h); 2. The insufficiency of the grain boundary activation energy of the coating (e.g., the grain boundary diffusion activation energy of ZrB2 is only 280 kJ / mol) leads to rapid oxidation failure; 3. The interfacial bonding strength of the multilayer structure is insufficient (usually <15 MPa), which easily leads to interlayer peeling. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the first objective of this invention is to provide a method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface. This method not only significantly improves the bonding strength between the layers but also effectively prevents the graphite substrate and the surface protective layer from reacting at high temperatures, thus avoiding coating failure and detachment. This further enhances the service life of the protective layer and has broad application prospects.

[0007] The second objective of this invention is to provide a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on the graphite surface prepared by the above-described method. The multilayer composite ceramic coating provided by this invention exhibits strong interfacial bonding with the graphite matrix, excellent shock resistance, high-temperature stability, superior corrosion resistance, and a long service life, providing a solid guarantee for the application of graphite in high-temperature fields.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] This invention discloses a method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface. The method involves brushing an ultra-high temperature ceramic oxide slurry onto the surface of a graphite substrate, performing a first heat treatment to obtain a porous transition layer, then brushing an ultra-high temperature carbide slurry onto the surface of the porous transition layer, drying to obtain an ultra-high temperature carbide layer, then roughening the surface of the ultra-high temperature carbide layer with the ultra-high temperature carbide slurry, followed by a second heat treatment to obtain a rough carbide layer, then applying a metal carbide-ceramic mixed slurry to the surface of the rough carbide layer to create dense protrusions, and finally spraying a protective layer material to obtain the final product.

[0010] The ultra-high temperature ceramic oxide slurry includes ultra-high temperature ceramic oxide powder and silicon carbide powder;

[0011] The ultra-high temperature carbide slurry contains ultra-high temperature carbide powder A;

[0012] The metal carbide ceramic mixture slurry comprises metal powder B and ultra-high temperature carbide powder B;

[0013] The protective layer material comprises ceramic powder, which is selected from at least one of oxide powder, silicide powder, boride powder, and nitride powder.

[0014] The preparation method of this invention involves first coating a slurry containing ultra-high temperature ceramic oxide powder and silicon carbide powder onto the surface of a graphite substrate. During heat treatment, the ultra-high temperature oxide reacts with the graphite substrate to form porous carbides. The silicon carbide particles in the slurry effectively prevent the sintering and growth of the reacted carbide grains, maintaining the porous structure. The porous transition layer obtained through the above reaction has uniform micron-sized pores and is metallurgically bonded to the graphite substrate. This effectively alleviates stress cracking in the coating caused by the large difference in thermal expansion coefficients between the substrate and the rough coating, and between the outermost protective layer, and ensures that graphite with different thermal expansion coefficients and different types can be used as substrates. Then, a rough carbide layer is deposited on the surface of the porous transition layer. The carbides are stable and do not react with graphite or the protective layer material at high temperatures. Simultaneously, the surface has a rough structure, constructing a coating with excellent corrosion resistance on the surface of the rough carbide layer. This layered system significantly enhances the overall corrosion resistance of graphite substrates. While ensuring stable bonding with the substrate, this structure fully releases the protective efficacy of the outermost coating. The ceramic roughness of the rough carbide layer forms a robust physical-mechanical interlocking structure with the outermost coating. Furthermore, based on the rough carbide layer, a metal carbide-ceramic mixture is first applied to the surface of the rough carbide layer to create dense protrusions, followed by the spraying of a protective layer material. The protrusions formed by the metal carbides can form a point-to-point metallurgical bond with the surface-prepared protective layer after high-temperature sintering. This further improves the bonding strength between the rough carbide layer and the protective layer while mitigating stress cracking or even detachment caused by differences in thermal expansion coefficients between different materials. Under high-temperature conditions, the metallic phase distributed on the rough sites diffuses into and out of the coating, forming metallurgical bonding points with the outer protective layer. These metallurgical bonding points not only have high bonding strength but also do not generate significant thermal stress during formation. In complex environments involving alternating high and low temperature cycles, the mechanical interlocking structure and metallurgical bonding points work synergistically to effectively enhance the adhesion between the coating and the substrate. Furthermore, the low diffusion rate of carbon in the carbide coating prevents the carbon in the substrate from adversely affecting the protective layer's performance. Under high-temperature corrosive conditions, the metal points melt to form a liquid phase. This liquid phase effectively seals any cracks and pores that may appear on the coating surface, further strengthening the coating's protective properties. Therefore, the multilayer composite ceramic coating provided by this invention exhibits high interfacial bonding strength and excellent oxidation resistance.

[0015] In a preferred embodiment, the graphite substrate is first ultrasonically cleaned and then dried.

[0016] In a preferred embodiment, the graphite in the graphite substrate is selected from any one of fine-grained graphite, high-purity graphite, and isostatically pressed graphite, and the coefficient of thermal expansion of the graphite substrate is 2-8×10⁻⁶. -6 / ℃.

[0017] In a preferred embodiment, the ultra-high temperature ceramic oxide slurry comprises, by mass percentage, the following: 50-80 wt% ultra-high temperature ceramic oxide powder, 3-5 wt% silicon carbide powder, 1-5 wt% first binder, 0.1-1 wt% first dispersant, and the balance being the first solvent.

[0018] Silicon carbide powder in ultra-high temperature ceramic oxide slurry can effectively avoid the problems of ceramic particle growth and sintering, maintain the porous state of the transition layer, and silicon carbide powder is inexpensive and has good high-temperature performance.

[0019] In a further preferred embodiment, the ultra-high temperature ceramic oxide powder is selected from at least one of titanium oxide, zirconium oxide, hafnium oxide, tantalum oxide, niobium oxide, vanadium oxide, and chromium oxide.

[0020] In a further preferred embodiment, the first adhesive is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0021] In a further preferred embodiment, the first dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0022] In a further preferred embodiment, the first solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water.

[0023] In actual operation, the ultra-high temperature ceramic oxide slurry is obtained by ball milling and mixing each raw material according to the design ratio of the ultra-high temperature ceramic oxide slurry.

[0024] In a preferred embodiment, the ultra-high temperature ceramic oxide slurry is brushed onto the surface of the graphite substrate 4-16 times, with a total coating thickness of 20-80 μm. After each brushing is completed, the substrate is dried. After complete drying and cooling, the next brushing cycle is performed. The drying temperature is ≤60℃.

[0025] In this invention, ultra-high temperature ceramic oxide slurry is brushed onto the graphite surface layer by layer and dried at a temperature of ≤60℃, which ensures that macroscopic cracks will not occur due to excessive coating thickness during the drying process.

[0026] In a preferred embodiment, the first heat treatment is performed in a vacuum environment, the temperature of the first heat treatment is 1500-1900℃, the duration of the first heat treatment is 2-4 hours, and the heating rate is ≤10℃ / min.

[0027] In a further preferred embodiment, during the first heat treatment, the temperature is first raised to 900-1000℃ at a heating rate of 9-10℃ / min, then raised to 1200-1300℃ at a heating rate of 5-8℃ / min, and then raised to 1700-1900℃ at a heating rate of 1-4℃ / min.

[0028] By performing the first heat treatment through the above-mentioned staged heating process, it is possible to avoid excessively violent reactions between the coating and the substrate, which could lead to a large number of defects in the coating.

[0029] In a preferred embodiment, the ultra-high temperature carbide slurry, by mass percentage, comprises: 50-80 wt% ultra-high temperature carbide powder A, 1-2 wt% metal powder A, 1-5 wt% second binder, 0.1-1 wt% second dispersant, and the balance being a second solvent.

[0030] In a further preferred embodiment, the ultra-high temperature carbide powder A is selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide.

[0031] In the aforementioned carbides, carbon has a very low bulk diffusion coefficient within its grains and does not react significantly with most materials, such as silicon oxide, aluminum oxide, molybdenum silicide, ytterbium silicate, and titanium nitride, effectively ensuring the stable coexistence of the ultra-high temperature carbide layer and the protective layer at high temperatures.

[0032] In a further preferred embodiment, the ultra-high temperature carbide powder A is composed of ultra-high temperature carbide powder A1 with a particle size of 0.5-1μm and ultra-high temperature carbide powder A2 with a particle size of 15-20μm, and the mass ratio of ultra-high temperature carbide powder A1: ultra-high temperature carbide powder A2 is 60-80: 20-40.

[0033] In ultra-high temperature carbide slurry, ultra-high temperature carbide powder A is preferably ultra-high temperature carbide powder with different particle sizes. Coarse particles can form ceramic rough spots in the rough carbide layer. During the sintering process, these spots will engulf nearby smaller ceramic particles through the Oswald ripening mechanism. The energy of large particles is lower than that of small particles, while the energy of small particles is higher and their solubility is higher than that of large particles. Small particles that dissolve at high temperature will redeposit on the large particles to form larger ceramic particles, thereby increasing the roughness of the coating.

[0034] In a further preferred embodiment, the metal in the metal powder A is selected from at least one of nickel, chromium, cobalt, iron, and copper.

[0035] In a further preferred embodiment, the second adhesive is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0036] In a further preferred embodiment, the second dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0037] In a further preferred embodiment, the second solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water.

[0038] In a preferred embodiment, the thickness of the ultra-high temperature carbide slurry brushed onto the surface of the porous transition layer is 20-50 μm.

[0039] In this invention, a carbide slurry with a certain thickness and uniformity is first brushed onto the surface of the porous transition layer. After being slowly dried in an oven, it is brushed again before spot pressing is performed to obtain a rough carbide coating. This is because, firstly, one of the functions of the rough carbide layer is to prevent the graphite from reacting with the material of the protective layer. Therefore, it needs to have a certain thickness and cannot have large defects such as cracks or holes in order to play a protective role. The purpose of slow drying is to avoid generating large stress during the drying process of the coating, which would cause the coating to crack during sintering. Secondly, the second function of the rough carbide layer is to obtain rough spots, which are obtained by spot pressing. Spot pressing after drying can form rougher spots. Otherwise, spot pressing directly on an undried surface can easily cause the rough spots to gradually level out, reducing the roughness.

[0040] The preferred method involves using an ultra-high temperature carbide slurry for roughening treatment as follows: First, apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer using a sponge brush, with the coating thickness controlled to be 0.2 mm or less. Then, use a texture roller to apply the ultra-high temperature carbide slurry back and forth to the surface 3-6 times. Finally, use the tip of a stiff-bristled brush to apply the ultra-high temperature carbide slurry evenly to create dense, raised dots on the surface; the density of these raised dots is 50-80 dots / cm². 3 .

[0041] In a further preferred embodiment, the height difference between the protrusions and depressions on the surface of the texture roller is less than 0.3 mm.

[0042] Further optimization involves ensuring that the tip of the stiff-bristled brush head picks up no more than 2g of the rough layer slurry each time, and that the brush is pressed 5-10 times at each point before being re-picked up with slurry.

[0043] In the preferred embodiment, the temperature of the second heat treatment is 2200-2500℃, and the time of the second heat treatment is 2-6 hours.

[0044] In a preferred embodiment, the metal carbide ceramic slurry comprises, by mass percentage, the following: 10-20 wt% ultra-high temperature carbide powder B, 45-60 wt% metal powder B, 0-10% silicon powder, 1-5 wt% third binder, 0.1-1 wt% third dispersant, and the balance being a third solvent.

[0045] In a further preferred embodiment, the ultra-high temperature carbide powder B is selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide.

[0046] In a further preferred embodiment, the metal in the metal powder B is selected from at least one of nickel, chromium, and aluminum.

[0047] In a further preferred embodiment, the third binder is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0048] In a further preferred embodiment, the third dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0049] In a further preferred embodiment, the third solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water.

[0050] In a preferred embodiment, the process of applying a metal carbide-ceramic mixture slurry to the surface of a rough carbide layer involves using the tip of a stiff-bristled brush to apply the metal carbide-ceramic mixture slurry evenly to the surface of the rough carbide layer, creating dense raised areas. Each time, the tip of the stiff-bristled brush applies no more than 2g of the metal carbide-ceramic mixture slurry, and the brush is reapplied after each 5-10 application.

[0051] The preferred method involves applying a metal carbide-ceramic mixture slurry to the surface of a rough carbide layer using point pressing to obtain dense protrusions with a density of 10-20 per cm². 3 .

[0052] In this invention, a rough carbide layer is first obtained through sintering. The ceramic sintering densifies the material, providing protection. Secondly, it forms rough ceramic spots, which can act as mechanical interlocking points. Simultaneously, these spots can fix the molten protective layer material and promote metallurgical bonding between the metal-ceramic spots and the molten protective layer material during subsequent plasma spraying. Then, through the point pressing of the metal carbide-ceramic mixed slurry, the metal-ceramic rough spots are formed, along with the rough spots that are metallurgically bonded during the subsequent protective layer preparation process. The combination of these two types of rough spots combines the effects of metallurgical bonding and mechanical bonding, improving the bonding strength between coatings while avoiding stress cracking caused by differences in material properties between coatings. By controlling the density of metal carbide protrusions within the range of this invention, the final performance is optimal. If the metal carbide protrusions are too dense, the stress between coatings will be very high; if they are too small, the bonding performance of the coatings will be reduced.

[0053] In a preferred embodiment, the process of obtaining the protective layer material is as follows: ball milling and mixing ceramic powder, fourth binder and fourth dispersant for 2-4 hours to obtain mixed powder, then mixing the mixed powder with water to obtain protective layer slurry, and then granulating the protective layer slurry to obtain spherical particles of 20-100μm, which is the protective layer material.

[0054] The solid content of the protective layer slurry is 40-60 wt%. In the mixed powder, the mass fraction of the fourth binder is 0.5-2%, and the mass fraction of the fourth dispersant is 0.1-1%. The fourth binder is selected from at least one of PVA and CMC, and the fourth dispersant is selected from ammonium polyacrylate.

[0055] In a preferred embodiment, the oxide powder is selected from at least one of alumina, silicon oxide, yttrium oxide, and mullite; the silicide powder is selected from one of molybdenum silicide, tungsten silicide, and ytterbium silicate; and the nitride powder is selected from at least one of titanium nitride, tantalum nitride, and silicon nitride.

[0056] In a preferred embodiment, the spraying is plasma spraying, and the process parameters for plasma spraying are as follows: main gas Ar: 35-50 L / min; auxiliary gas H2: 5-10 L / min; arc current: 500-800 A; arc voltage: 40-80 V; power: 30-60 kW; powder feeding rate: 20-50 g / min; spraying distance: 80-150 mm; spraying angle: perpendicular to the graphite substrate surface; spray gun moving speed: 50-200 mm / s.

[0057] The present invention also provides a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on the graphite surface prepared by the above preparation method, wherein the multilayer composite ceramic coating consists of a porous transition layer, a rough carbide layer, and a surface protective layer from bottom to top.

[0058] In a preferred embodiment, the porous transition layer is composed of ultra-high temperature carbide C and silicon carbide, the rough carbide layer has a rough structure and is made of ultra-high temperature carbide D, and the surface protective layer is made of ceramic. The ultra-high temperature carbide C and ultra-high temperature carbide D are selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide, and the ceramic is selected from at least one of alumina, silicon oxide, yttrium oxide, mullite, molybdenum silicide, tungsten silicide, ytterbium silicate, titanium nitride, tantalum nitride, and silicon nitride.

[0059] Principles and advantages

[0060] The preparation method of this invention involves first coating a slurry containing ultra-high temperature ceramic oxide powder and silicon carbide powder onto the surface of a graphite substrate. During heat treatment, the ultra-high temperature oxide reacts with the graphite substrate to form porous carbides. The silicon carbide particles in the slurry effectively prevent the sintering and growth of the reacted carbide grains, maintaining the porous structure. A porous transition layer is obtained through the above reaction. Then, a dense, rough carbide layer of a certain thickness is formed on the basis of the porous transition layer. The porous transition layer has uniform micron-sized pores and is metallurgically bonded to the graphite substrate, which can effectively alleviate the thermal expansion between the substrate and the rough carbide layer and the outermost protective layer. Excessive differences in coefficients lead to stress cracking in the coating. To address this, a dense protrusion is created on the surface of the rough carbide layer by applying a metal carbide-ceramic mixture. A protective layer is then sprayed on. The protrusions formed by the metal carbide allow for point-to-point metallurgical bonding with the surface-prepared protective layer after high-temperature sintering. This further enhances the bond strength between the rough carbide layer and the protective layer while mitigating stress cracking and even detachment caused by differences in thermal expansion coefficients between different materials. Under high-temperature conditions, the metallic phase distributed at the rough sites diffuses into and out of the coating, forming metallurgical bonding points with the outer protective layer. These metallurgical bonding points not only have high bonding strength but also do not generate significant thermal stress during formation. In complex environments involving alternating high and low temperature cycles, the mechanical interlocking structure and the metallurgical bonding points work synergistically to effectively enhance the adhesion between the coating and the substrate. Furthermore, the low diffusion rate of carbon in the carbide coating prevents the carbon in the substrate from adversely affecting the performance of the protective layer. When exposed to high-temperature corrosive environments, the metal sites melt to form a liquid phase. This liquid phase can promptly seal any cracks and pores that may appear on the coating surface, further enhancing the protective performance of the coating. As a result, the multilayer composite ceramic coating provided by this invention has high interfacial bonding strength and excellent oxidation resistance.

[0061] Compared with the prior art, the present invention has at least the following advantages:

[0062] 1. The present invention alleviates the thermal stress between graphite and rough layer through a porous transition layer, thereby improving the thermal shock resistance of the coating.

[0063] 2. In this invention, the rough carbide layer improves the bonding force between the coating and the protective layer with different physicochemical properties through the mechanical interlocking structure and metallurgical bonding points, and avoids the reaction between the graphite substrate and the protective layer, so as to give full play to the performance of the protective layer.

[0064] 3. This invention can prepare high-purity coatings by adjusting the raw material ratio and sintering process, and can be applied to fields such as semiconductors where high purity is required. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of the rough coating prepared by the method of the present invention. From bottom to top, the layers are: substrate 100, porous transition layer 200, rough carbide layer 300, and surface protective layer 400.

[0066] Figure 2 The images shown are secondary electron images of the cross-section and surface of the transition layer in Example 1, wherein... Figure 2 Image (a) is a secondary electron image of a cross-section of the transition layer provided in Example 1, obtained by scanning electron microscopy. Figure 2 (b) is a secondary electron image of the surface of the transition layer provided in Example 1, obtained by scanning electron microscopy.

[0067] Figure 3 This is a secondary electron image of the coating cross-section from a scanning electron microscope in Example 1.

[0068] Figure 4 This is a macroscopic view of the coating sample from Example 1.

[0069] Figure 5 This is a macroscopic view of the coating sample in Comparative Example 1.

[0070] Figure 6 The coating failure diagram provided in Comparative Example 2 shows that... Figure 6 (a) in the image is a macroscopic view of the substrate after the coating has peeled off. Figure 6 (b) in the image represents the peeled-off coating. Detailed Implementation

[0071] Example 1

[0072] This embodiment provides a method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface, comprising the following steps:

[0073] (1) Cut the graphite substrate into blocks of 50×50×10mm, polish the graphite surface with 600-grit diamond sandpaper, and then clean the graphite with acetone and ethanol in sequence using ultrasonic cleaning.

[0074] (2) Weigh 70 wt% tantalum oxide (with a particle size of about 500 nm), 20 wt% ethanol, 5 wt% toluene, 1 wt% ethyl cellulose, 0.5 wt% polyethyleneimine, and 3.5 wt% silicon carbide (1-5 μm) in proportion, and then use a planetary ball mill to ball mill for 4 hours at a ball-to-material ratio of 5:1. After ball milling, an ultra-high temperature ceramic oxide slurry is obtained. Use a fine brush to dip the ultra-high temperature ceramic oxide slurry and then brush it evenly on the graphite surface 10 times. Each time, the slurry needs to be dried in a 50℃ oven before the next brushing. The brushed tantalum oxide coating is about 40 μm thick.

[0075] (3) The graphite substrate coated with ultra-high temperature ceramic oxide slurry is placed in a high temperature carbon tube furnace and heated to 1000℃ at a heating rate of 10℃ / min, then heated to 1300℃ at a heating rate of 5℃ / min, then heated to 1800℃ at a heating rate of 2℃ / min and held for 1h to obtain a porous transition layer on the graphite surface.

[0076] (4) Weigh 70 wt% tantalum carbide (composed of 80 wt% tantalum carbide with a particle size of 500 nm-1 μm and 20 wt% tantalum carbide with a particle size of 15-20 μm), 22.5 wt% ethanol, 5 wt% toluene, 1 wt% ethyl cellulose, 0.5 wt% polyethyleneimine, and 1 wt% nickel according to the specified proportions, and then ball mill them for 4 hours using a planetary ball mill at a ball-to-material ratio of 5:1 to obtain an ultra-high temperature carbide slurry. First, use a fine brush to dip the ultra-high temperature carbide slurry and then evenly brush it onto the graphite surface 8 times. Each time, the slurry needs to be dried in a 50°C oven before the next brushing step. After brushing and drying, an ultra-high temperature carbide layer of about 35 μm is obtained. Next, use a sponge brush to apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer, controlling the coating thickness to 0.2mm. Then, use a texture roller (with a surface unevenness of less than 0.3mm and an average height difference of about 0.25mm) to apply roughening slurry back and forth to the surface 4 times. Finally, use the tip of a stiff-bristled brush to apply ultra-high temperature carbide slurry evenly to the surface, creating dense raised areas. Each time, the tip of the stiff-bristled brush should not pick up more than 2g of coarse carbide slurry. After each 5-10 applications, re-pick the slurry and repeat the application until the entire surface forms a uniform, dense raised area (with an average height of about 0.3mm), and the density of the raised areas is 50-80 per cm². 3 .

[0077] (5) The sample coated with ultra-high temperature carbide slurry was placed in a high temperature carbon tube furnace and heated to 2300℃ at a heating rate of 5℃ / min and kept at that temperature for 2h to obtain a graphite-porous transition layer-rough carbide layer.

[0078] (6) Weigh 20wt% tantalum carbide (composed of 80wt% tantalum carbide with a particle size of 500nm-1μm and 20wt% tantalum carbide with a particle size of 15-20μm), 45wt% Cr, 5wt% Ni, 22.5wt% ethanol, 6wt% toluene, 1wt% ethyl cellulose, and 0.5wt% polyethyleneimine according to the specified ratio, and then ball mill them for 4 hours using a planetary ball mill at a ball-to-material ratio of 5:1 to obtain a metal carbide slurry. Use the tip of a stiff-bristled brush to apply the metal carbide slurry evenly to the rough surface to form dense protrusions. Each time, the tip of the stiff-bristled brush should not take more than 2g of coarse carbide slurry. After each 5-10 application, reapply the slurry and repeat the application to the surface until the entire surface forms a uniform dense protrusion (the average height of the protrusion is about 0.3mm), and the density of the protrusions is 10-20 per cm. 3 .

[0079] (7) 60wt% alumina (1-5μm), 38wt% ytterbium silicate (1-5μm) powder, 1.5wt% binder (90wt% PVA, 10wt% CMC), and 0.5wt% dispersant ammonium polyacrylate were ball-milled for 3 hours. The mixed powder was then mixed with water to prepare a slurry with a solid content of 50wt%. The slurry was then placed in a spray dryer for granulation to form spherical particles of about 45μm. The granulated powder was then loaded into a powder feeding device. Using atmospheric plasma spraying (the spraying process is as follows: main gas Ar: 50 L / min; auxiliary gas H2: 8 L / min; arc current: 600A; arc voltage: 70 V; power: 42kW; powder feed rate: 35g / min; spraying distance: 100 mm; spraying angle: perpendicular to the substrate surface; spray gun moving speed: 150mm / s), a protective layer of about 200μm is sprayed onto the rough layer surface to obtain a multi-layer composite ceramic coating consisting of a porous transition layer, a rough carbide layer, and a protective layer on the graphite surface.

[0080] The prepared transition layer scanning electron microscope secondary electron image is as follows: Figure 2 As shown, where Figure 2 Image (a) is a secondary electron image of a cross-section of the transition layer provided in Example 1, obtained by scanning electron microscopy. Figure 2 (b) is a secondary electron image of the surface of the transition layer provided in Example 1, from which uniform micron-sized pores can be clearly seen.

[0081] Figure 3 The image shown is a secondary electron image of the coating cross-section obtained by scanning electron microscopy in Example 1. The outermost layer in the image is the protective layer. As can be seen from the image, the protective layer and the intermediate rough layer form a mechanically interlocked structure. There is a porous transition layer between the rough layer and the bottom substrate.

[0082] Figure 4This is a macroscopic view of the multilayer composite ceramic coating prepared in Example 1. As can be seen from the figure, the coating is dense, flat, and free of defects.

[0083] Example 2

[0084] This embodiment provides a method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface, comprising the following steps:

[0085] (1) Cut the graphite substrate into blocks of 50×50×10mm, polish the graphite surface with 600-grit diamond sandpaper, and then clean the graphite with acetone and ethanol in sequence using ultrasonic cleaning.

[0086] (2) Weigh 56wt% tantalum oxide, 14wt% hafnium oxide (with a particle size of about 500 nm), 20wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, 0.5wt% polyethyleneimine, and 3.5wt% silicon carbide (1-5μm) in proportion, and then use a planetary ball mill to ball mill for 4 hours at a ball-to-material ratio of 5:1. After ball milling, use a fine brush to dip the tantalum oxide slurry and then brush it evenly on the graphite surface 10 times. Each time, the slurry needs to be dried in a 50℃ oven before the next brushing. The tantalum oxide coating is about 40μm thick.

[0087] (3) The graphite substrate coated with tantalum oxide slurry is placed in a high-temperature carbon tube furnace and heated to 1000°C at a heating rate of 10°C / min, then heated to 1300°C at a heating rate of 5°C / min, and then heated to 1900°C at a heating rate of 2°C / min and held for 1 hour to obtain a porous transition layer on the graphite surface.

[0088] (4) Weigh 56wt% tantalum carbide and 14wt% hafnium carbide (composed of 80wt% tantalum carbide with a particle size of 500nm-1μm and 20wt% tantalum carbide with a particle size of 15-20μm), 22.5wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, 0.5wt% polyethyleneimine, and 1wt% nickel in proportion, and then ball mill them for 4 hours using a planetary ball mill at a ball-to-material ratio of 5:1 to obtain an ultra-high temperature carbide slurry. First, use a fine brush to dip the ultra-high temperature carbide slurry and then brush it evenly on the graphite surface 8 times. Each time, the slurry needs to be dried in a 50℃ oven before the next brushing. After brushing and drying, an ultra-high temperature carbide layer of about 35μm is obtained. First, use a sponge brush to apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer, controlling the coating thickness to 0.2 mm or less. Then, use a texture roller (with a surface elevation difference of less than 0.3 mm and an average height difference of about 0.25 mm) to apply the roughening layer slurry back and forth to the surface 5 times. Finally, use the tip of a stiff-bristled brush to apply the roughening layer slurry evenly to the surface, creating dense raised areas. Each time, the tip of the stiff-bristled brush should not pick up more than 2 g of rough carbide slurry. After each 5-10 applications, re-pick the slurry and repeat the application process until the entire surface forms a uniform, dense raised area (with an average height of about 0.3 mm) and a density of 50-80 raised areas per cm². 3 .

[0089] (5) The sample coated with ultra-high temperature carbide slurry was placed in a high temperature carbon tube furnace and heated to 2500℃ at a heating rate of 5℃ / min and kept at the temperature for 2h to obtain a graphite-porous transition layer-rough carbide layer coating.

[0090] (6) Weigh 8% tantalum carbide and 2% hafnium carbide (composed of 80wt% tantalum carbide with a particle size of 500nm-1μm and 20wt% tantalum carbide with a particle size of 15-20μm), 45wt% Cr, 8wt% Ni, 8wt% Al, 22.5wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, and 0.5wt% polyethyleneimine according to the specified proportions. Then, use a planetary ball mill to ball mill the mixture for 4 hours at a ball-to-material ratio of 5:1 to obtain a metal carbide slurry. Use the tip of a stiff-bristled brush to apply the metal carbide slurry to the rough surface and press it evenly to create dense protrusions. Each time, the tip of the stiff-bristled brush should not take more than 2g of the coarse carbide slurry. After each 5-10 presses, re-apply the slurry and repeat the pressing process on the surface until the entire surface forms a uniform dense protrusion (the average height of the protrusion is about 0.3mm), with a protrusion density of 10-20 per cm. 3 .

[0091] (7) Mix 50wt% hafnium carbide (1-5μm), 40wt% molybdenum silicide (1-5μm), 8wt% hafnium boride (1-5μm) powder, 1.5wt% binder (90wt% PVA, 10wt% CMC), and 0.5wt% dispersant ammonium polyacrylate by ball milling for 2-4 hours. The mixed powder is then mixed with water to prepare a slurry with a solid content of 50wt%. The slurry is then placed in a spray dryer for granulation to form spherical particles of about 45μm. The granulated powder is then loaded into a powder feeding device. Using atmospheric plasma spraying (the spraying process is as follows: main gas Ar: 50 L / min; auxiliary gas H2: 8 L / min; arc current: 600A; arc voltage: 70 V; power: 42kW; powder feed rate: 35g / min; spraying distance: 100 mm; spraying angle: perpendicular to the substrate surface; spray gun moving speed: 150mm / s), a protective layer of about 200μm is sprayed onto the rough layer surface to obtain a multi-layer composite ceramic coating consisting of a porous transition layer, a rough carbide layer, and a protective layer on the graphite surface.

[0092] Example 3

[0093] This embodiment provides a method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface, comprising the following steps:

[0094] (1) Cut the graphite substrate into blocks of 50×50×10mm, polish the graphite surface with 600-grit diamond sandpaper, and then clean the graphite with acetone and ethanol in sequence using ultrasonic cleaning.

[0095] (2) Weigh 63wt% zirconium oxide and 7wt% niobium oxide (with a particle size of about 500 nm), 20wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, 0.5wt% polyethyleneimine, and 3.5wt% silicon carbide (1-5μm) in proportion, and then use a planetary ball mill to ball mill for 4 hours at a ball-to-material ratio of 5:1. After ball milling, use a fine brush to dip into tantalum oxide slurry and then brush it evenly on the graphite surface 10 times. Each time, the slurry needs to be dried in a 50℃ oven before the next brushing. The tantalum oxide coating is about 40μm thick.

[0096] (3) The graphite substrate coated with tantalum oxide slurry is placed in a high-temperature carbon tube furnace and heated to 1000°C at a heating rate of 10°C / min, then heated to 1300°C at a heating rate of 5°C / min, and then heated to 1700°C at a heating rate of 2°C / min and kept at 1h to obtain a porous transition layer on the graphite surface.

[0097] (4) Weigh 63wt% zirconium carbide and 7wt% niobium carbide (composed of 80wt% tantalum carbide with a particle size of 500nm-1μm and 20wt% tantalum carbide with a particle size of 15-20μm), 22.5wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, 0.5wt% polyethyleneimine, and 1wt% nickel in proportion, and then ball mill them for 4 hours using a planetary ball mill at a ball-to-material ratio of 5:1 to obtain an ultra-high temperature carbide slurry. First, use a fine brush to dip the ultra-high temperature carbide slurry and then brush it evenly on the graphite surface 8 times. Each time, the slurry needs to be dried in a 50℃ oven before the next brushing. After brushing and drying, an ultra-high temperature carbide layer of about 35μm is obtained. First, use a sponge brush to apply the carbide slurry to the transition layer surface, controlling the coating thickness to 0.2mm or less. Then, use a texture roller (with a surface elevation difference of less than 0.3mm and an average height difference of approximately 0.25mm) to apply the roughening layer slurry back and forth to the surface 6 times. Finally, use the tip of a stiff-bristled brush to apply the roughening layer slurry evenly and densely to the surface, creating dense raised areas. Each time, the tip of the stiff-bristled brush should not pick up more than 2g of the coarse carbide slurry. After each 5-10 applications, re-pick the slurry and repeat the application process until the entire surface forms a uniform, densely tossed surface (with an average height of approximately 0.3mm), and the density of the raised areas is 50-80 per cm². 3 .

[0098] (5) The sample coated with ultra-high temperature carbide slurry was placed in a high temperature carbon tube furnace and heated to 2200℃ at a heating rate of 5℃ / min and kept at that temperature for 2h to obtain a graphite-porous transition layer-rough carbide layer coating.

[0099] (6) Weigh 8% zirconium carbide and 2% niobium carbide (composed of 80wt% tantalum carbide with a particle size of 500nm-1μm and 20wt% tantalum carbide with a particle size of 15-20μm), 40wt% Cr, 5wt% Si, 5wt% Al, 22.5wt% ethanol, 5wt% toluene, 1wt% ethyl cellulose, and 0.5wt% polyethyleneimine in the specified proportions. Then, use a planetary ball mill to ball mill the materials at a ball-to-material ratio of 5:1 for 4 hours to obtain a metal carbide slurry. Use the tip of a stiff-bristled brush to apply the metal carbide slurry to the rough surface and press it evenly to create dense protrusions. Each time, the tip of the stiff-bristled brush should not take more than 2g of the coarse carbide slurry. After each 5-10 presses, re-apply the slurry and repeat the pressing process on the surface until the entire surface forms a uniform dense protrusion (the average height of the protrusion is about 0.3mm), with a protrusion density of 10-20 per cm. 3 .

[0100] (7) Mix 50wt% zirconium carbide (1-5μm), 20wt% ytterbium silicate (1-5μm), 10% yttrium oxide (1-5μm), 10% molybdenum silicide (1-5μm), 8wt% zirconium boride (1-5μm) powder, 1.5wt% binder (90wt% PVA, 10wt% CMC), and 0.5wt% dispersant ammonium polyacrylate by ball milling for 2-4 hours. The mixed powder is then mixed with water to prepare a slurry with a solid content of 50wt%. The slurry is then placed in a spray dryer for granulation to form spherical particles of about 45μm. The granulated powder is then loaded into a powder feeding device. Using atmospheric plasma spraying (the spraying process is as follows: main gas Ar: 50 L / min; auxiliary gas H2: 8 L / min; arc current: 600A; arc voltage: 70 V; power: 42kW; powder feed rate: 35g / min; spraying distance: 100 mm; spraying angle: perpendicular to the substrate surface; spray gun moving speed: 150mm / s), a protective layer of about 200μm is sprayed onto the rough layer surface to obtain a multi-layer composite ceramic coating consisting of a porous transition layer, a rough carbide layer, and a protective layer on the graphite surface.

[0101] The samples with multilayer composite ceramic coatings obtained in Examples 1-3 and Comparative Examples 2-3 were subjected to oxidation experiments at 1500℃. The time when the weight loss rate reached 3wt% or more or the coating detached was identified as the oxidation failure time. The time when the weight loss rate reached 3wt% or more in Examples 1-3 and the time when the coating failed and detached in the comparative examples are shown in Table 1.

[0102]

[0103] Comparative Example 1

[0104] Compared to Example 1, the difference lies in omitting steps (2) and (3), i.e., not preparing a porous transition layer on the graphite surface. This results in the coatings detaching from the graphite substrate surface after other coatings are prepared due to significant differences in their coefficients of thermal expansion, such as... Figure 5 As shown.

[0105] Comparative Example 2

[0106] Compared with Example 1, the difference lies in the omission of steps (4) and (5), i.e., the absence of a rough carbide layer on the graphite surface. Due to the lack of a dense rough carbide layer and the high porosity of the porous transition layer, the carbon source in the graphite substrate easily diffuses at high temperatures and reacts with the oxides in the protective layer on the surface. This results in the coating failing and peeling off after 5 hours of use at 1500°C. Figure 6 As shown, where Figure 6 (a) in the image is a macroscopic view of the substrate after the coating has peeled off. Figure 6 (b) in the image represents the peeled-off coating.

[0107] Comparative Example 3

[0108] Compared to Example 1, the difference lies in the omission of step (6), i.e., the absence of a metal carbide coating on the rough carbide layer before spraying the oxide coating. Because there is no metal to promote the bonding between the rough carbide and the oxide and to relieve the stress therein, the coating begins to peel off after 30 hours of use at 1500°C.

Claims

1. A method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface, characterized in that: Ultra-high temperature ceramic oxide slurry is brushed onto the surface of a graphite substrate, followed by a first heat treatment to obtain a porous transition layer. Then, ultra-high temperature carbide slurry is brushed onto the surface of the porous transition layer and dried to obtain an ultra-high temperature carbide layer. Next, ultra-high temperature carbide slurry is used to roughen the surface of the ultra-high temperature carbide layer, followed by a second heat treatment to obtain a rough carbide layer. Then, a metal carbide ceramic mixed slurry is used to poke the surface of the rough carbide layer to obtain dense protrusions. Finally, a protective layer material is sprayed on to obtain the final product. The ultra-high temperature ceramic oxide slurry includes ultra-high temperature ceramic oxide powder and silicon carbide powder; The ultra-high temperature carbide slurry contains ultra-high temperature carbide powder A; The metal carbide ceramic mixture slurry comprises metal powder B and ultra-high temperature carbide powder B; The protective layer material comprises ceramic powder, which is selected from at least one of oxide powder, silicide powder, boride powder, and nitride powder.

2. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The graphite substrate is first ultrasonically cleaned and then dried; The coefficient of thermal expansion of the graphite substrate is 2-8×10⁻⁶. -6 / ℃.

3. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The ultra-high temperature ceramic oxide slurry, by mass percentage, comprises the following: 50-80 wt% ultra-high temperature ceramic oxide powder, 3-5 wt% silicon carbide powder, 1-5 wt% first binder, 0.1-1 wt% first dispersant, and the balance being the first solvent; The ultra-high temperature ceramic oxide powder is selected from at least one of titanium oxide, zirconium oxide, hafnium oxide, tantalum oxide, niobium oxide, vanadium oxide, and chromium oxide; The first adhesive is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral; The first dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate; The first solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water; The ultra-high temperature ceramic oxide slurry is brushed onto the surface of the graphite substrate 4-16 times, with a total coating thickness of 20-80μm. After each brushing, the substrate is dried. After complete drying and cooling, the next brushing cycle is performed. The drying temperature is ≤60℃. The first heat treatment is carried out in a vacuum environment, the temperature of the first heat treatment is 1500-1900℃, the time of the first heat treatment is 2-4h, and the heating rate is ≤10℃ / min.

4. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 3, characterized in that: During the first heat treatment, the temperature is first raised to 900-1000℃ at a heating rate of 9-10℃ / min, then raised to 1200-1300℃ at a heating rate of 5-8℃ / min, and then raised to 1700-1900℃ at a heating rate of 1-4℃ / min.

5. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The ultra-high temperature carbide slurry, by mass percentage, is composed of the following: ultra-high temperature carbide powder A 50-80wt%, metal powder A 1-2wt%, second binder 1-5wt%, second dispersant 0.1-1wt%, with the balance being the second solvent; The ultra-high temperature carbide powder A is selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide; The ultra-high temperature carbide powder A is composed of ultra-high temperature carbide powder A1 with a particle size of 0.5-1μm and ultra-high temperature carbide powder A2 with a particle size of 15-20μm. By mass ratio, ultra-high temperature carbide powder A1: ultra-high temperature carbide powder A2 = 60-80: 20-40. The metal in the metal powder A is selected from at least one of nickel, chromium, cobalt, iron, and copper; The second adhesive is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral; The second dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate; The second solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water; The thickness of the ultra-high temperature carbide slurry brushed onto the surface of the porous transition layer is 20-50 μm.

6. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The roughening process using ultra-high temperature carbide slurry is as follows: First, use a sponge brush to apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer, with the coating thickness controlled to be 0.2 mm or less. Then, use a texture roller to apply ultra-high temperature carbide slurry back and forth to the surface 3-6 times. Finally, use the tip of a stiff-bristled brush to apply ultra-high temperature carbide slurry evenly to the surface to create dense raised dots; the density of the dense raised dots is 50-80 dots / cm. 3 ; The height difference between the protrusions and depressions on the surface of the texture roller is less than 0.3 mm; The tip of the stiff-bristled brush should not pick up more than 2g of the rough layer of slurry each time, and should be pressed 5-10 times at each point before re-dipping in the slurry. The temperature of the second heat treatment is 2200-2500℃, and the time of the second heat treatment is 2-6 hours.

7. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The metal carbide ceramic slurry, by mass percentage, is composed of the following: 10-20 wt% ultra-high temperature carbide powder B, 45-60 wt% metal powder B, 0-10% silicon powder, 1-5 wt% third binder, 0.1-1 wt% third dispersant, and the balance being a third solvent. The ultra-high temperature carbide powder B is selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide; The metal in the metal powder B is selected from at least one of nickel, chromium, and aluminum; The third adhesive is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral; The third dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate; The third solvent is selected from at least one of ethanol, toluene, benzyl alcohol, and pure water.

8. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The process of applying a metal carbide ceramic mixture slurry to the surface of a rough carbide layer is as follows: use the tip of a stiff-bristled brush to apply the metal carbide ceramic mixture slurry evenly to the surface of the rough carbide layer to create dense raised areas. Each time, the tip of the stiff-bristled brush should apply no more than 2g of the metal carbide ceramic mixture slurry. After each application of the slurry 5-10 times, the brush should be reapplied. A dense protrusion density of 10-20 per cm³ was obtained by tapping a metal carbide-ceramic mixture onto the surface of a rough carbide layer. 3 .

9. The method for preparing a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The process of obtaining the protective layer material is as follows: ceramic powder, fourth binder and fourth dispersant are ball-milled and mixed for 2-4 hours to obtain mixed powder, then the mixed powder is mixed with water to obtain protective layer slurry, and then the protective layer slurry is granulated to obtain spherical particles of 20-100μm, which is the protective layer material. The solid content of the protective layer slurry is 40-60 wt%. In the mixed powder, the mass fraction of the fourth binder is 0.5-2%, and the mass fraction of the fourth dispersant is 0.1-1%. The fourth binder is selected from at least one of PVA and CMC, and the fourth dispersant is selected from ammonium polyacrylate. The oxide powder is selected from at least one of alumina, silicon oxide, yttrium oxide, and mullite; the silicide powder is selected from one of molybdenum silicide, tungsten silicide, and ytterbium silicate; and the nitride powder is selected from at least one of titanium nitride, tantalum nitride, and silicon nitride. The spraying is plasma spraying, and the process parameters of plasma spraying are as follows: main gas Ar: 35-50 L / min; auxiliary gas H2: 5-10 L / min; arc current: 500-800 A; arc voltage: 40-80 V; power: 30-60 kW; powder feeding rate: 20-50 g / min; spraying distance: 80-150 mm; spraying angle: perpendicular to the graphite substrate surface; spray gun moving speed: 50-200 mm / s.

10. A multilayer composite ceramic coating on a graphite surface that is high-temperature stable and corrosion-resistant, prepared by the preparation method according to any one of claims 1-9, characterized in that: The multilayer composite ceramic coating consists of a porous transition layer, a rough carbide layer, and a surface protective layer from bottom to top. The porous transition layer is composed of ultra-high temperature carbide C and silicon carbide. The rough carbide layer has a rough structure and is made of ultra-high temperature carbide D. The surface protective layer is made of ceramic. The ultra-high temperature carbide C and ultra-high temperature carbide D are selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide. The ceramic is selected from at least one of alumina, silicon oxide, yttrium oxide, mullite, molybdenum silicide, tungsten silicide, ytterbium silicate, titanium nitride, tantalum nitride, and silicon nitride.

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