Multi-layer composite ceramic coating with high-temperature stability and corrosion resistance on graphite surface and preparation method of multi-layer composite ceramic coating

Abstract

The preparation method comprises the following steps: brushing ultrahigh-temperature ceramic oxide slurry on the surface of a graphite base material, carrying out first heat treatment to obtain a porous transition layer, then brushing ultrahigh-temperature carbide slurry on the surface of the porous transition layer, and carrying out second heat treatment to obtain the high-temperature-stable and corrosion-resistant multi-layer composite ceramic coating on the surface of the graphite base material. The preparation method comprises the following steps: performing primary heat treatment on the surface of an ultra-high-temperature carbide layer, drying to obtain the ultra-high-temperature carbide layer, performing roughening treatment on the surface of the ultra-high-temperature carbide layer by adopting ultra-high-temperature carbide slurry, performing secondary heat treatment to obtain a rough carbide layer, and performing point pressure on the surface of the rough carbide layer by adopting metal carbide ceramic mixed slurry to obtain dense bulges; according to the method disclosed by the invention, not only can the bonding firmness among the layers be remarkably improved, but also the situation that the graphite base material and the surface protection layer react at high temperature, so that the coating fails and falls off is effectively avoided, the use limit of the protection layer is further enhanced, and the method has a wide application prospect.

Description

Technical Field

[0001] The invention belongs to the field of corrosion-resistant multilayer composite ceramic coatings on graphite surfaces, and particularly relates to a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface and a preparation method thereof. Background Art

[0002] Graphite has been widely used in key sectors such as metallurgy, semiconductors, and nuclear energy due to its excellent high-temperature stability (melting point of 3850°C ± 50°C), chemical resistance (can withstand most strong acids and bases), and good machinability. However, its inherent properties lead to oxidation at 400°C in oxygen-containing environments, and carbon contamination when used as a crucible material seriously restrict its application in high-end equipment manufacturing.

[0003] In the existing technology, the mainstream solution is to prepare ceramic coating on the surface of graphite substrate by chemical vapor deposition (CVD), plasma spraying (APS) or sol-gel method. Specific implementation methods include: 1. Carbide system: SiC coating (thermal expansion coefficient 4.5×10 -6 / ℃ with the best match to graphite), TaC, TiC, etc.; 2. Oxide systems: Y2O3, Al2O3, ZrO2; 3. Boron / nitride systems: ZrB2 (excellent thermal shock resistance), TiN, Si3N4; 4. Composite coatings: gradient structured SiC / Si3N4, multi-layered ZrB2-SiC-ZrC, etc.

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

[0005] The main defects of the existing technology are: 1. The thermal expansion coefficient of the coating and the substrate is mismatched (for example, the thermal expansion coefficient of Al2O3 is 8.4×10 -6 / ℃ while that of graphite is 4.8×10-6 / ℃), which leads to interfacial stress accumulation; 2. High-temperature diffusion leads to element mutual dissolution (for example, the diffusion depth of C into SiC coating reaches 50μm-1500℃ / 100h); 3. Insufficient activation energy of coating grain boundaries (for example, the activation energy of ZrB2 grain boundary diffusion is only 280 kJ / mol), which causes rapid oxidation failure; 4. Insufficient interfacial bonding strength of multilayer structure (usually <15 MPa), which makes interlayer delamination prone to occur. Summary of the Invention

[0006] To address the shortcomings of the existing technology, the first objective of the present invention is to provide a method for preparing a multi-layer composite ceramic coating on a graphite surface that is stable at high temperatures and corrosion-resistant. This method not only significantly improves the bonding strength between the layers but also effectively prevents the graphite substrate from reacting with the surface protective layer at high temperatures, which could lead to coating failure and shedding. This further enhances the protective layer's useful life and has broad application prospects.

[0007] A second object of the present invention is to provide a high-temperature stable and corrosion-resistant multilayer composite ceramic coating on a graphite surface, prepared by the above-mentioned preparation method. The multilayer composite ceramic coating provided by the present invention exhibits strong interfacial bonding with the graphite substrate, excellent seismic performance, high-temperature stability, excellent corrosion resistance, and a long service life, providing a solid foundation for the application of graphite in high-temperature applications.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for preparing a multi-layer composite ceramic coating that is stable at high temperatures and corrosion-resistant on a graphite surface. The method comprises the following steps: applying an ultra-high temperature ceramic oxide slurry to the surface of a graphite substrate, performing a first heat treatment to obtain a porous transition layer, then applying an ultra-high temperature carbide slurry to the surface of the porous transition layer, drying the layer, and then roughening the surface of the ultra-high temperature carbide layer with the ultra-high temperature carbide slurry. The layer is then subjected to a second heat treatment to obtain a rough carbide layer. A metal carbide-ceramic mixed slurry is then applied to the surface of the rough carbide layer for point pressing to obtain dense protrusions, and then a protective layer material is sprayed on the surface to obtain the coating.

[0010] The ultrahigh temperature ceramic oxide slurry comprises ultrahigh temperature ceramic oxide powder and silicon carbide powder;

[0011] The ultrahigh temperature carbide slurry comprises ultrahigh temperature carbide powder A;

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

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

[0014] The preparation method of the present invention is to first apply an ultra-high temperature ceramic oxide slurry containing ultra-high temperature ceramic oxide powder and silicon carbide powder to the surface of a graphite substrate. During the heat treatment process, the ultra-high temperature oxide reacts with the graphite substrate to form a porous carbide, wherein the silicon carbide particles in the slurry can effectively prevent the carbide grains generated by the reaction from sintering and growing, and maintain a porous structure. The porous transition layer obtained by the above reaction has uniform micron pores and is metallurgically bonded to the graphite substrate. It can effectively alleviate the stress cracking of the coating caused by the large difference in thermal expansion coefficient between the substrate and the rough coating and the outermost protective layer, and ensure that graphites with different thermal expansion coefficients and different types can be used as substrates; and then a rough carbide layer is arranged on the surface of the porous transition layer. The carbide has stable performance and does not react with graphite and protective layer materials at high temperatures. At the same time, the surface has a rough structure, and a coating with excellent corrosion resistance is constructed on the surface of the rough carbide layer. The layer system can significantly enhance the overall corrosion resistance of the graphite substrate. This structure can fully unleash the protective effectiveness of the outermost coating while ensuring a stable bond with the substrate. The ceramic roughness of the rough carbide layer can form a stable physical and mechanical interlocking structure with the outermost coating. In addition, based on the rough carbide layer, a metal carbide-ceramic mixed slurry is first used to point-press the surface of the rough carbide layer to obtain dense protrusions, and then the protective layer material is sprayed. The protrusions formed by the metal carbide can form a point-to-point metallurgical bond with the protective layer prepared on the surface after high-temperature sintering. This further improves the bonding strength between the rough carbide layer and the protective layer while also alleviating the problem of stress cracking or even shedding caused by the different thermal expansion coefficients between the different materials. Under high temperature conditions, the metal phase distributed on the rough sites will diffuse into the interior and exterior of the coating, forming metallurgical connection 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 the formation process. In a complex environment characterized by alternating high and low temperature cycles, the mechanical interlocking structure and metallurgical bonding points work synergistically to effectively enhance the bonding between the coating and the substrate. Furthermore, the diffusion of carbon in the carbide coating is low, thereby preventing the carbon in the substrate from adversely affecting the performance of the protective layer. When exposed to high-temperature corrosive environments, the metal points melt to form liquid phases, which can promptly seal any cracks and pores that may appear on the coating surface, further enhancing the protective properties of the coating. This results in the multilayer composite ceramic coating provided by the present invention having high interfacial bonding strength and excellent antioxidant properties.

[0015] In a preferred embodiment, the graphite substrate is first ultrasonically cleaned and 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 isostatic graphite, and the thermal expansion coefficient of the graphite substrate is 2-8×10 -6 / ℃.

[0017] In a preferred embodiment, the ultrahigh temperature ceramic oxide slurry is composed of the following components by mass percentage: 50-80 wt% ultrahigh 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 cheap and has good high-temperature performance.

[0019] Further preferably, 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] Further preferably, the first binder is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0021] Further preferably, the first dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0022] Further preferably, the first solvent is selected from at least one of ethanol, toluene, benzyl alcohol and pure water.

[0023] In actual operation, the ultrahigh temperature ceramic oxide slurry is obtained by mixing the raw materials according to the designed proportion of the ultrahigh temperature ceramic oxide slurry through ball milling.

[0024] In a preferred embodiment, the ultra-high temperature ceramic oxide slurry is brushed on the surface of the graphite substrate 4-16 times, with a total brushing thickness of 20-80 μm. After each brushing, it is dried and completely dried and cooled before the next brushing cycle is carried out. The drying temperature is ≤60°C.

[0025] In the present invention, the ultrahigh temperature ceramic oxide slurry is brushed onto the graphite surface layer by layer and dried at a temperature of ≤60°C, which can ensure that macroscopic cracks are not generated during the drying process of the coating due to the coating being too thick.

[0026] Preferably, the first heat treatment is performed in a vacuum environment, the temperature of the first heat treatment is 1500-1900° C., the time of the first heat treatment is 2-4 hours, and the heating rate is ≤10° C. / min.

[0027] Further preferably, during the first heat treatment, the temperature is first increased to 900-1000°C at a heating rate of 9-10°C / min, then increased to 1200-1300°C at a heating rate of 5-8°C / min, and then increased to 1700-1900°C at a heating rate of 1-4°C / min.

[0028] By performing the first heat treatment through the above-mentioned staged temperature increase procedure, it is possible to avoid excessive reaction between the coating and the substrate, which would cause a large number of defects in the coating.

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

[0030] Further preferably, the ultrahigh temperature carbide powder A is selected from at least one of zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide.

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

[0032] Further preferably, the ultrahigh temperature carbide powder A consists of ultrahigh temperature carbide powder A1 with a particle size of 0.5-1 μm and ultrahigh temperature carbide powder A2 with a particle size of 15-20 μm, and the mass ratio of ultrahigh temperature carbide powder A1:ultrahigh temperature carbide powder A2 is 60-80:20-40.

[0033] In the ultra-high temperature carbide slurry, the ultra-high temperature carbide powder A is preferably an ultra-high temperature carbide powder composed of different coarse and fine particle sizes. The coarse particles can form ceramic rough points in the rough carbide layer. During the sintering process, such points swallow up the smaller ceramic particles nearby through the Oswald ripening mechanism. The energy of large particles is lower than that of small particles, and the energy of small particles is high and the solubility is higher than that of large particles. The small particles dissolved at high temperature will be redeposited on the large particles to form larger ceramic particles, thereby increasing the roughness of the coating.

[0034] Further preferably, the metal in the metal powder A is selected from at least one of nickel, chromium, cobalt, iron and copper.

[0035] Further preferably, the second binder is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0036] Further preferably, the second dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0037] More preferably, the second solvent is selected from at least one of ethanol, toluene, benzyl alcohol and pure water.

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

[0039] In the present invention, a carbide slurry with a certain uniform thickness is first brushed on the surface of the porous transition layer, and then slowly dried in an oven, and then brushed again before point 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 graphite from reacting with the material of the protective layer, so it needs to have a certain thickness and cannot have large defects such as cracks and holes to play a protective role. The purpose of slow drying is to avoid large stress during the drying process of the coating, which causes the coating to crack during sintering. Secondly, the second function of the rough carbide layer is to obtain rough points, and the rough points are obtained by point pressing. Point pressing after drying can form rougher points. Otherwise, direct point pressing on the undried surface will easily cause the rough points to gradually level and reduce the roughness.

[0040] The preferred solution is to use ultra-high temperature carbide slurry for roughening treatment. The process 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, and the brushing thickness is controlled to be 0.2mm or less. Then, use a texture roller to dip the ultra-high temperature carbide slurry and roll it back and forth on the surface for 3-6 times. Then, use the tip of a hard bristle brush to dip the ultra-high temperature carbide slurry and evenly press dense protrusions on the surface. The density of dense protrusions is 50-80 / cm 3 .

[0041] Further preferably, the height difference between the protrusions and depressions on the surface of the texture roller is less than 0.3 mm.

[0042] Further preferably, the tip of the bristle brush is dipped into the rough layer slurry in an amount of no more than 2 g each time, and is dipped into the slurry again after pressing each point 5-10 times.

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

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

[0045] Further preferably, the ultrahigh temperature carbide powder B is at least one selected from zirconium carbide, hafnium carbide, tantalum carbide, and niobium carbide.

[0046] Further preferably, the metal in the metal powder B is selected from at least one of nickel, chromium and aluminum.

[0047] Further preferably, the third binder is selected from at least one of ethyl cellulose, methyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0048] Further preferably, the third dispersant is selected from at least one of polyethyleneimine and ammonium polyacrylate.

[0049] Further preferably, the third solvent is selected from at least one of ethanol, toluene, benzyl alcohol and pure water.

[0050] The preferred solution is to use the metal carbide ceramic mixed slurry to perform point pressing on the surface of the rough carbide layer. The process is as follows: use the tip of a hard-bristled brush to dip the metal carbide ceramic mixed slurry and evenly press dense protrusions on the surface of the rough carbide layer. The tip of the hard-bristled brush dips no more than 2g of the metal carbide ceramic mixed slurry each time, and re-dip the slurry after 5-10 times of point pressing.

[0051] The preferred solution is to use a metal carbide ceramic mixed slurry to press the surface of the rough carbide layer to obtain dense protrusions with a density of 10-20 / cm 3 .

[0052] In the present invention, sintering is first performed to obtain a rough carbide layer, which plays a protective role through the dense sintering of ceramics. Secondly, rough ceramic points are formed, which can play the role of points of mechanical interlocking structure. At the same time, they can fix the molten protective layer raw materials and promote metal-ceramic points and molten protective layer raw materials in the subsequent plasma spraying process for metallurgical bonding. Then, by point pressing of the metal carbide ceramic mixed slurry, rough metal-ceramic points and rough points for metallurgical bonding are formed in the subsequent protective layer preparation process. The combination of the two rough points can combine the joint effects of metallurgical bonding and mechanical bonding, improve the bonding strength between coatings, and avoid stress cracking between coatings caused by material property differences. The final performance is optimized by controlling the density of metal carbide protrusions within the scope of the present invention. If the metal carbide protrusions are too dense, the stress between the coatings will be very large. If they are too small, the bonding performance of the coatings will be reduced.

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

[0054] The solid content of the protective layer slurry is 40-60wt%. 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 aluminum oxide, 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 of the plasma spraying are: 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 surface of the graphite substrate; spray gun movement speed: 50-200 mm / s.

[0057] The present invention also provides a multi-layer composite ceramic coating with high temperature stability and corrosion resistance on the graphite surface prepared by the above preparation method. The multi-layer composite ceramic coating comprises, from bottom to top, a porous transition layer, a rough carbide layer, and a surface protective layer.

[0058] In a preferred embodiment, the material of the porous transition layer is composed of ultra-high temperature carbide C and silicon carbide, the rough carbide layer has a rough structure, and its material is ultra-high temperature carbide D, and the material of the surface protective layer is 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 aluminum oxide, 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 the present invention is to first apply an ultra-high temperature ceramic oxide slurry containing ultra-high temperature ceramic oxide powder and silicon carbide powder to the surface of a graphite substrate. During the heat treatment process, the ultra-high temperature oxide reacts with the graphite substrate to form a porous carbide, wherein the silicon carbide particles in the slurry can effectively prevent the carbide grains generated by the reaction from sintering and growing, and maintain a porous structure. The porous transition layer obtained by the above reaction, however, forms a dense rough carbide layer with a certain thickness on the basis of the porous transition layer. The porous transition layer has uniform micron pores and is metallurgically bonded to the graphite substrate, which can effectively alleviate the gap between the substrate and the rough carbide layer and the outermost protective layer due to thermal expansion. Excessive coefficient differences lead to stress cracking in the coating. Based on the rough carbide layer, a metal carbide-ceramic mixed slurry is first applied to the surface of the rough carbide layer for point pressing to obtain dense protrusions. The protective layer material is then sprayed. The protrusions formed by the metal carbide can form a point-to-point metallurgical bond with the protective layer prepared on the surface after high-temperature sintering. This further improves the bonding strength between the rough carbide layer and the protective layer while also alleviating the stress cracking and even shedding caused by the different thermal expansion coefficients between the different materials. The metal phase distributed on the rough sites diffuses into and out of the coating under high temperature conditions, 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 the complex environment of alternating high and low temperature cycles, the mechanical interlocking structure and the metallurgical bonding points work synergistically to effectively enhance the bonding between the coating and the substrate. Furthermore, the diffusion of carbon in the carbide coating is low, thus preventing the carbon in the substrate from adversely affecting the performance of the protective layer. When in a high-temperature corrosive environment, metal points will melt to form liquid phases. These liquid phases can promptly seal cracks and pores that may appear on the coating surface, further enhancing the protective performance of the coating, thereby making the multilayer composite ceramic coating provided by the present invention have high interface bonding strength and excellent antioxidant properties.

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

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

[0063] 2. The rough carbide layer in the present invention improves the bonding force between the coating and the protective layer with different physical and chemical properties through the mechanical interlocking structure and metallurgical bonding points, avoids the reaction between the graphite substrate and the protective layer, and can fully exert the performance of the protective layer.

[0064] 3. The present invention can prepare high-purity coatings by adjusting the raw material ratio and sintering process, and can be applied to fields with high purity requirements such as semiconductors. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 Schematic diagram of preparing a rough coating using the method of the present invention, which shows, from bottom to top, a substrate 100, a porous transition layer 200, a rough carbide layer 300, and a surface protection layer 400.

[0066] Figure 2 The scanning electron microscope secondary electron image of the cross section and surface of the transition layer of Example 1 is shown in FIG. Figure 2 (a) is a scanning electron microscope secondary electron image of the cross section of the transition layer provided in Example 1, Figure 2 (b) is a scanning electron microscope secondary electron image of the surface of the transition layer provided in Example 1.

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

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

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

[0070] Figure 6 is the coating failure diagram provided in Comparative Example 2, wherein Figure 6 (a) is the macroscopic image of the substrate after the coating falls off. Figure 6 (b) shows the peeled coating. DETAILED DESCRIPTION

[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 × 10 mm, polish the graphite surface with 600-mesh diamond sandpaper, and then ultrasonically clean the graphite with acetone and ethanol in sequence.

[0074] (2) 70 wt% tantalum oxide (primary 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) were weighed in proportion, and then ball-milled for 4 h using a planetary ball mill at a ball-to-material ratio of 5:1. After ball milling, an ultrahigh temperature ceramic oxide slurry was obtained; a fine brush was used to dip the ultrahigh temperature ceramic oxide slurry and then evenly brushed on the graphite surface 10 times. Each time the slurry was brushed, it was placed in a 50°C oven to dry the solvent in the slurry before the next brushing step was performed. The brushed tantalum oxide coating was about 40 μm.

[0075] (3) The graphite substrate coated with ultra-high temperature ceramic oxide slurry was 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 1800°C at a heating rate of 2°C / min and kept warm for 1 hour to obtain a porous transition layer on the graphite surface.

[0076] (4) 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 were weighed in proportion and then ball-milled for 4 h using a planetary ball mill at a ball-to-material ratio of 5:1 to obtain an ultrahigh temperature carbide slurry. First, a fine brush was used to dip the ultrahigh temperature carbide slurry and then evenly brushed on the graphite surface 8 times. Each brushing was placed in a 50°C oven to dry the solvent in the slurry before the next brushing step was performed. After brushing-drying, an ultrahigh temperature carbide layer of approximately 35 μm was obtained. Then, use a sponge brush to apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer, and control the brushing thickness to 0.2mm. Then use a texture roller (the height difference of the surface protrusions and depressions is less than 0.3mm, and the average height difference is about 0.25) to dip the rough layer slurry and roll it back and forth on the surface 4 times. Finally, use the tip of the hard-bristled brush to dip the ultra-high temperature carbide slurry and press dense protrusions evenly on the surface. The tip of the hard-bristled brush should not dip more than 2g of the coarse carbide slurry each time. After pressing 5-10 times, dip the slurry again and repeat the point pressing on the surface until the entire surface forms uniform point-pressed dense protrusions (the average height of the protrusions is about 0.3mm). The density of the protrusions is 50-80 / cm 3 .

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

[0078] (6) 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 were weighed in proportion, and then ball-milled 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 hard-bristled brush to dip the metal carbide slurry and evenly press dense protrusions on the surface of the rough layer. The tip of the hard-bristled brush should dip no more than 2g of the rough carbide slurry each time. After 5-10 times of pressing, re-dip the slurry and repeat the pressing on the surface until uniform dense protrusions (average height of the protrusions is about 0.3mm) are formed on the entire surface. The density of the protrusions is 10-20 / cm 3 .

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

[0080] The secondary electron scanning electron microscopy image of the prepared transition layer is shown in Figure 2 As shown, Figure 2 (a) is a scanning electron microscope secondary electron image of the cross section of the transition layer provided in Example 1, Figure 2 (b) is a secondary electron image of the surface of the transition layer provided in Example 1, obtained by a scanning electron microscope. Uniform micropores can be clearly seen in the image.

[0081] Figure 3 The figure shows a cross-section scanning electron microscope secondary electron image of the coating of Example 1. The outermost layer in the figure is the protective layer. It can be seen from the figure that the protective layer and the middle rough layer form a mechanical interlocking structure, and there is a porous transition layer between the rough layer and the bottom substrate.

[0082] Figure 4This is a macroscopic image of the multilayer composite ceramic coating prepared in Example 1. From the image, it can be seen that the coating is dense, smooth and has no 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 × 10 mm, polish the graphite surface with 600-mesh diamond sandpaper, and then ultrasonically clean the graphite with acetone and ethanol in sequence.

[0086] (2) 56 wt% tantalum oxide and 14 wt% hafnium oxide (primary particle size is 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) were weighed in proportion, and then ball-milled for 4 h using a planetary ball mill at a ball-to-material ratio of 5:1. After ball milling, a fine brush was used to dip the tantalum oxide slurry and then evenly brushed on the graphite surface 10 times. Each time the slurry was brushed, it was placed in a 50°C oven to dry the solvent in the slurry before the next step of brushing was performed. The tantalum oxide coating was about 40 μm.

[0087] (3) The graphite substrate coated with tantalum oxide slurry was 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 kept warm for 1 hour to obtain a porous transition layer on the graphite surface.

[0088] (4) 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 were weighed in proportion, and then ball-milled 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, a fine-bristled brush was used to dip the ultra-high temperature carbide slurry and then evenly brushed it on the graphite surface 8 times. Each brushing was placed in a 50℃ oven to dry the solvent in the slurry before the next brushing was performed. After brushing and drying, an ultra-high temperature carbide layer of about 35μm was obtained. Then, use a sponge brush to apply the ultra-high temperature carbide slurry to the surface of the ultra-high temperature carbide layer, and control the brushing thickness to 0.2mm or less. Then use a texture roller (the height difference of the surface protrusions and depressions is less than 0.3mm, and the average height difference is about 0.25) to dip the rough layer slurry and roll it back and forth on the surface 5 times. Finally, use the tip of a hard-bristled brush to dip the rough layer slurry and evenly press dense protrusions on the surface. The tip of the hard-bristled brush should not dip more than 2g of coarse carbide slurry each time. After pressing 5-10 times, dip the slurry again and repeat the point pressing on the surface until the entire surface forms uniform point-pressed dense protrusions (the average height of the protrusions is about 0.3mm). The density of the protrusions is 50-80 / cm 3 .

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

[0090] (6) 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 were weighed in proportion, and then ball-milled 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 hard-bristled brush to dip the metal carbide slurry and evenly press dense protrusions on the surface of the rough layer. The tip of the hard-bristled brush should dip no more than 2g of the rough carbide slurry each time. After 5-10 times of pressing, re-dip the slurry and repeat the pressing on the surface until uniform dense protrusions (average height of the protrusions is about 0.3mm) are formed on the entire surface. The density of the protrusions is 10-20 / cm 3 .

[0091] (7) 50 wt% hafnium carbide (1-5 μm), 40 wt% molybdenum silicide (1-5 μm), 8 wt% hafnium boride (1-5 μm) powder, 1.5 wt% binder (90 wt% PVA, 10 wt% CMC), and 0.5 wt% dispersant ammonium polyacrylate were ball-milled for 2-4 h. The mixed powder was then mixed with water to form a slurry with a solid content of 50 wt%. The slurry was then placed in a spray dryer for granulation to form spherical particles of about 45 μm. The granulated powder was then fed into a powder feeding device. A protective layer coating of about 200 μm was sprayed on the surface of the rough layer using atmospheric plasma spraying (the spraying process was: main gas Ar: 50 L / min; auxiliary gas H2: 8 L / min; arc current: 600 A; arc voltage: 70 V; power: 42 kW; powder feeding rate: 35 g / min; spraying distance: 100 mm; spraying angle: perpendicular to the substrate surface; spray gun moving speed: 150 mm / s) to obtain a multilayer composite ceramic coating consisting of a porous transition layer-rough carbide layer-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 × 10 mm, polish the graphite surface with 600-mesh diamond sandpaper, and then ultrasonically clean the graphite with acetone and ethanol in sequence.

[0095] (2) 63 wt% zirconium oxide and 7 wt% niobium oxide (primary particle size is 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) were weighed in proportion, and then ball-milled for 4 h using a planetary ball mill at a ball-to-material ratio of 5:1. After ball milling, a fine brush was used to dip tantalum oxide slurry and then evenly brushed on the graphite surface 10 times. Each time the slurry was brushed, it was placed in a 50°C oven to dry the solvent in the slurry before the next brushing step was carried out. The tantalum oxide coating was about 40 μm.

[0096] (3) Place the graphite substrate coated with tantalum oxide slurry in a high-temperature carbon tube furnace, heat it to 1000°C at a heating rate of 10°C / min, then heat it to 1300°C at a heating rate of 5°C / min, and then heat it to 1700°C at a heating rate of 2°C / min and keep it warm for 1 hour to obtain a porous transition layer on the graphite surface.

[0097] (4) 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 were weighed in proportion and then ball-milled 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, a fine-bristled brush was used to dip the ultra-high temperature carbide slurry and then evenly brushed it on the graphite surface 8 times. Each brushing was placed in a 50℃ oven to dry the solvent in the slurry before the next brushing was performed. After brushing and drying, an ultra-high temperature carbide layer of about 35μm was obtained. Then, use a sponge brush to apply the carbide slurry to the surface of the transition layer, and control the thickness of the brushing to be 0.2mm or less. Then use a texture roller (the height difference of the surface protrusions and depressions is less than 0.3mm, and the average height difference is about 0.25) to dip the rough layer slurry and roll it back and forth on the surface 6 times. Finally, use the tip of the hard-bristled brush to dip the rough layer slurry and press dense protrusions evenly on the surface. The tip of the hard-bristled brush should dip no more than 2g of coarse carbide slurry each time. After pressing 5-10 times, dip the slurry again and repeat the pressing on the surface until the entire surface forms uniform dense protrusions (the average height of the protrusions is about 0.3mm). The density of the protrusions is 50-80 / cm 3 .

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

[0099] (6) 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 were weighed in proportion, and then ball-milled 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 hard-bristled brush to dip the metal carbide slurry into the rough layer surface and press dense protrusions uniformly. The tip of the hard-bristled brush dips no more than 2g of the rough carbide slurry each time. After 5-10 presses, re-dip the slurry and repeat the press on the surface until uniform dense protrusions (average protrusion height of about 0.3mm) are formed on the entire surface. The density of the protrusions is 10-20 / cm 3 .

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

[0101] The samples containing the multi-layer composite ceramic coatings obtained in Examples 1-3 and Comparative Examples 2-3 were subjected to oxidation tests at 1500°C, and the time when the weight loss rate reached 3wt% or more or the coating fell off was determined 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 fell off in the comparative example are shown in Table 1.

[0102]

[0103] Comparative Example 1

[0104] Compared with Example 1, the difference is that step (2) and step (3) are not performed, that is, the porous transition layer is not prepared on the graphite surface. As a result, after the preparation of other coatings, the coating falls off from the surface of the graphite substrate due to the large difference in thermal expansion coefficient, such as Figure 5 shown.

[0105] Comparative Example 2

[0106] Compared with Example 1, the difference is that step (4) and step (5) are not performed, that is, a rough carbide layer is not prepared 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 is easy to diffuse at high temperature and react with the oxide in the surface protective layer, resulting in the coating failing to react and falling off after being used at 1500°C for 5 hours. Figure 6 As shown, Figure 6 (a) is the macroscopic image of the substrate after the coating falls off. Figure 6 (b) shows the peeled coating.

[0107] Comparative Example 3

[0108] Compared with Example 1, the difference is that step (6) is not performed, that is, the metal carbide coating is not brushed 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 relieve the stress therein, the coating begins to fall 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 by: The ultrahigh temperature ceramic oxide slurry is brushed on the surface of the graphite substrate, and a first heat treatment is performed to obtain a porous transition layer. The ultrahigh temperature carbide slurry is then brushed on the surface of the porous transition layer and dried to obtain an ultrahigh temperature carbide layer. The ultrahigh temperature carbide slurry is then used to roughen the surface of the ultrahigh temperature carbide layer. The second heat treatment is then performed to obtain a rough carbide layer. The metal carbide ceramic mixed slurry is then first used to perform point pressing on the surface of the rough carbide layer to obtain dense protrusions, and then a protective layer material is sprayed to obtain the result. The ultrahigh temperature ceramic oxide slurry comprises ultrahigh temperature ceramic oxide powder and silicon carbide powder; The ultrahigh temperature carbide slurry comprises ultrahigh temperature carbide powder A; The metal carbide ceramic mixed slurry comprises metal powder B and ultra-high temperature carbide powder B; The protective layer material comprises ceramic powder, and the ceramic powder 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 multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The graphite substrate is first ultrasonically cleaned and dried; The graphite in the graphite substrate is selected from any one of fine-grained graphite, high-purity graphite, and isostatic graphite. The thermal expansion coefficient of the graphite substrate is 2-8×10 -6 / ℃.

3. The method for preparing a high-temperature stable and corrosion-resistant multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The ultra-high temperature ceramic oxide slurry is composed of the following components by mass percentage: 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 a 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 binder 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 on the surface of the graphite substrate 4-16 times, with a total brushing thickness of 20-80 μm. After each brushing is completed, it is dried and completely dried and cooled before the next brushing cycle is carried out. The drying temperature is ≤60°C; The first heat treatment is performed in a vacuum environment, the temperature of the first heat treatment is 1500-1900° C., the time of the first heat treatment is 2-4 hours, and the heating rate is ≤10° C. / min.

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

5. The method for preparing a high-temperature stable and corrosion-resistant multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The ultrahigh temperature carbide slurry is composed of the following components by mass percentage: 50-80 wt% ultrahigh 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 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 ultrahigh temperature carbide powder A is composed of ultrahigh temperature carbide powder A1 with a particle size of 0.5-1 μm and ultrahigh temperature carbide powder A2 with a particle size of 15-20 μm, with the mass ratio of ultrahigh temperature carbide powder A1 to ultrahigh temperature carbide powder A2 being 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 binder 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 applied on the surface of the porous transition layer is 20-50 μm.

6. The method for preparing a high-temperature stable and corrosion-resistant multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The process of roughening with 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, and control the brushing thickness to be 0.2mm or less. Then, use a texture roller to dip the ultra-high temperature carbide slurry and roll it back and forth on the surface 3-6 times. Then, use the tip of a hard bristle brush to dip the ultra-high temperature carbide slurry and evenly press dense protrusions on the surface. The density of dense protrusions is 50-80 / cm 3 ; The height difference between the convex and concave surfaces of the texture roller is less than 0.3 mm; The tip of the bristle brush should be dipped into the rough layer slurry no more than 2g each time, and it should be dipped into the slurry again after pressing each point 5-10 times; The temperature of the second heat treatment is 2200-2500°C, and the time of the second heat treatment is 2-6 hours.

7. The method for preparing a high-temperature stable and corrosion-resistant multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The metal carbide ceramic mixed slurry is composed of the following components by mass percentage: 10-20 wt% of ultra-high temperature carbide powder B, 45-60 wt% of metal powder B, 0-10% of silicon powder, 1-5 wt% of a third binder, 0.1-1 wt% of a 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 binder 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 multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The process of applying point pressure to the surface of the rough carbide layer using the metal carbide-ceramic mixed slurry is as follows: use the tip of a hard-bristled brush to dip the metal carbide-ceramic mixed slurry into the rough carbide layer and evenly press the dense protrusions on the surface of the rough carbide layer. The tip of the hard-bristled brush dips no more than 2g of the metal carbide-ceramic mixed slurry each time, and re-dip the slurry after 5-10 times of pressing. The metal carbide ceramic mixed slurry is used to press the surface of the rough carbide layer to obtain dense protrusions with a density of 10-20 / cm 3 .

9. The method for preparing a high-temperature stable and corrosion-resistant multi-layer composite ceramic coating on a graphite surface according to claim 1, characterized in that: The protective layer material is obtained by ball-milling ceramic powder, a fourth binder, and a fourth dispersant for 2-4 hours to obtain a mixed powder, then mixing the mixed powder with water to obtain a protective layer slurry, and then granulating the protective layer slurry to obtain spherical particles of 20-100 μm, which are the protective layer material; The solid content of the protective layer slurry is 40-60wt%. 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 aluminum oxide, silicon oxide, yttrium oxide, and mullite; the silicide powder is selected from at least 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 the plasma spraying are: 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 surface of the graphite substrate; spray gun movement speed: 50-200 mm / s.

10. A multi-layer composite ceramic coating on a graphite surface that is stable at high temperatures and is corrosion-resistant, prepared by the method according to any one of claims 1 to 9, characterized in that: The multi-layer composite ceramic coating comprises, from bottom to top, a porous transition layer, a rough carbide layer, and a surface protection layer; The material of the porous transition layer is composed of ultra-high temperature carbide C and silicon carbide, the rough carbide layer has a rough structure, and its material is ultra-high temperature carbide D, the material of the surface protective layer is 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 aluminum oxide, silicon oxide, yttrium oxide, mullite, molybdenum silicide, tungsten silicide, ytterbium silicate, titanium nitride, tantalum nitride, and silicon nitride.

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