Method for spraying Si coating on graphite surface with thermal shock resistance and oxidation resistance at 1500 DEG C, coated graphite and application

Abstract

The invention provides a 1500-DEG C thermal shock and oxidation resistant plasma spraying method for a Si coating on the surface of graphite, coating graphite and application. The method comprises the following steps that the to-be-sprayed surface of a graphite matrix is pretreated; spraying the to-be-sprayed surface by adopting a plasma spraying technology to obtain a Si coating on the to-be-sprayed surface; carrying out heat treatment on the graphite matrix with the Si coating in a vacuum environment by adopting a vacuum heat treatment technology, so that part of solid silicon in the Si coating is molten into liquid silicon or silicon steam, and the liquid silicon or silicon steam permeates into the graphite matrix containing air holes under the action of capillary tubes, so that part of liquid silicon or silicon steam and the graphite matrix are subjected to chemical reaction; and thus, the gradient transition Si / SiC coating is obtained through in-situ growth on the to-be-sprayed surface of the graphite matrix. According to the method provided by the invention, the problem that a traditional method for spraying the Si coating on the surface of the graphite in a plasma manner cannot be used in an environment of 1500 DEG C is solved.

Description

Technical Field

[0001] The present application relates to the technical field of plasma sprayed Si coatings, and in particular to a method for plasma spraying Si coatings that are resistant to thermal shock and oxidation at 1500°C on graphite surfaces, coated graphite, and applications. Background Art

[0002] Graphite's oxidation behavior at high temperatures primarily manifests itself in an oxygen-containing environment above 400°C. This oxidation process occurs when oxygen diffuses near the graphite surface, adsorbs onto active sites, and reacts with carbon to form carbon dioxide or carbon monoxide. Graphite's crystal structure contains edge carbon atoms, which possess unsaturated chemical bonds and free π electrons. These atoms readily react with oxygen atoms, initiating oxidation. This oxidation process destroys the graphite's structure. Initially, oxidation of the edge carbon atoms reduces the carbon network plane. Subsequently, oxidation of the internal active carbon atoms also begins, breaking bonds one by one on the surrounding carbon atoms, forming oxides and ultimately forming corrosion pits within the carbon network plane. This oxidation behavior reduces the strength and performance of graphite, limiting its use in high-temperature environments.

[0003] To improve graphite's oxidation resistance, solution impregnation, matrix modification, and surface antioxidant coating are commonly used. Solution impregnation fills the graphite's internal pores with an impregnating agent and covers the surface, isolating it from oxidizing gases. However, the impregnating agent is volatile above 800°C, making it suitable only for low-temperature protection. Matrix modification involves adding antioxidants to the graphite matrix to reduce the oxidation rate. Surface antioxidant coating involves applying an antioxidant coating to the graphite surface to prevent oxidizing gases from entering. These methods can effectively delay oxidation reactions and improve graphite's high-temperature oxidation resistance.

[0004] The porosity and density of a coating significantly influence its performance, with coatings with lower porosity generally exhibiting better performance. Plasma-sprayed Si coatings typically exhibit lower porosity and higher density, contributing to improved oxidation and wear resistance. Si coatings are often used for protection in high-temperature environments, such as gas turbine engines and high-temperature furnaces, effectively preventing oxidation and corrosion of the substrate material.

[0005] Since pure silicon has a melting point of 1414°C, the melting point of the Si coating produced by plasma spraying is close to that of pure silicon. High temperatures accelerate the oxidation reaction of silicon. Especially in environments above 600°C, silicon reacts rapidly with oxygen to form silicon dioxide. The oxidation rate increases significantly with increasing temperature. After oxidation, a thin film of silicon dioxide forms on the silicon surface. This film protects against further oxidation, but it also affects the electrical conductivity and optical properties of silicon. During the high-temperature oxidation process, the silicon's crystal structure inevitably undergoes some damage, resulting in a decrease in the material's mechanical properties.

[0006] It can be seen that Si coatings plasma-sprayed on graphite surfaces cannot be used in an environment of 1500°C. Plasma-spraying SiC coatings on graphite surfaces can increase the service temperature, but when SiC powder is directly sprayed using plasma, it is easily decomposed in the high-temperature plasma flame, with the decomposition rate being at least 70%. Therefore, it is impossible to directly obtain a SiC coating with the required chemical composition and thickness according to the set spraying parameters. Summary of the Invention

[0007] In response to the problems in the background art, one of the objectives of this application is to provide a method for plasma-spraying a 1500°C thermal shock and oxidation-resistant Si coating on a graphite surface, thereby resolving the problem that conventional plasma-sprayed Si coatings on graphite surfaces cannot be used in a 1500°C environment. A second objective of this application is to provide a graphite substrate having a 1500°C thermal shock and oxidation-resistant coating on its surface. A third objective of this application is to provide an application for a graphite substrate having a 1500°C thermal shock and oxidation-resistant coating on its surface.

[0008] According to a first aspect of the present application, a method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance is provided. The method specifically comprises the following steps: S1. Pre-treating the surface of the graphite substrate to be sprayed; S2, spraying the surface to be sprayed using plasma spraying technology to obtain a Si coating on the surface to be sprayed; S3. The graphite substrate with the Si coating is heat-treated in a vacuum environment using a vacuum heat treatment technique, so that part of the solid silicon in the Si coating is melted into liquid silicon or silicon vapor, and penetrates into the graphite substrate containing pores through capillary action, so that part of the liquid silicon or silicon vapor chemically reacts with the graphite substrate, thereby in-situ growing a gradient transition Si / SiC coating on the surface to be sprayed of the graphite substrate.

[0009] Optionally, step S2 includes: S21, using Si powder with a purity greater than 99.99% as spray powder, and setting the spraying parameters of the plasma spraying equipment according to the thickness to be sprayed and the spraying powder; S22. Using a spray gun in the plasma spraying equipment, spraying the spraying powder onto the surface to be sprayed of the graphite substrate according to the spraying parameters, to prepare a Si coating of the thickness to be sprayed.

[0010] Optionally, the spraying parameters include: the working gas is a mixture of nitrogen and argon, the mass proportion of the argon is 50%~70%, and the flow rate of the mixture is 7SPLM~10SPLM; the working current is 100A~120 A; the working voltage is 130V~140 V; the powder feeding gas is argon, and the flow rate of the spraying powder is 4 g / min~10 g / min; the spraying distance is 250mm; and the spraying angle is 90°.

[0011] Optionally, the Si powder is spherical and has a particle size of 30 μm to 80 μm.

[0012] Optionally, in step S3, the graphite substrate with the Si coating is placed in a vacuum furnace, heat treatment parameters of the vacuum furnace are set, and the graphite substrate with the Si coating is heat treated in the vacuum furnace according to the heat treatment parameters.

[0013] Optionally, the heat treatment parameters include: heat treatment temperature of 1500°C to 1600°C; heating rate of less than 15°C / min; vacuum degree of at least 10 -3 Pa.

[0014] Optionally, step S1 includes: S11, roughening, degreasing and preheating the surface of the graphite substrate to be sprayed.

[0015] Optionally, in step S11, the roughening includes polishing the graphite substrate with sandpaper; and the preheating includes preheating the graphite substrate with a spray gun in a plasma spraying device; wherein the temperature of the preheating is 120°C~150°C.

[0016] According to the second aspect of the present application, a graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface is provided, comprising a graphite substrate, the surface of the graphite substrate having a gradient transition Si / SiC coating, and the Si / SiC coating is obtained according to the method for plasma spraying Si coating with a 1500°C thermal shock and oxidation resistant coating on the graphite surface provided in the first aspect of the present application.

[0017] According to the third aspect of the present application, an application of a graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface provided by the second aspect of the present application is provided, and a graphite substrate having a gradient transition Si / SiC coating on its surface is applied to a 1500°C environment.

[0018] Compared with the existing technology, this application has the following advantages: Compared to conventional plasma sprayed Si coatings, which are easily oxidized at high temperatures and cannot effectively prevent oxygen from further corroding the graphite substrate, directly sprayed SiC coatings are difficult to obtain due to the easy decomposition of SiC during the plasma spraying process, and thus cannot meet the high-temperature oxidation resistance requirements. The embodiments of the present invention utilize the advantages of plasma spraying technology to prepare coatings. Based on graphite material as the substrate, coupled with vacuum heat treatment technology, a gradient transition Si / SiC coating is directly prepared on the graphite surface. The gradient transition Si / SiC coating can alleviate the difference in thermal expansion coefficient and reduce thermal stress. The Si coating forms a good chemical bond with the graphite substrate during the sintering process. The coating has significant high-temperature thermal shock and oxidation resistance. Graphite with the Si / SiC coating on the surface can withstand high-temperature environments of 1500°C without being easily damaged or oxidized.

[0019] In this way, the embodiment of the present invention realizes the long-term service of graphite components in an extreme environment of 1500°C for the first time, fully utilizing the advantages of graphite materials such as easy processing, low material cost, wide source of raw materials, and excellent electrical and thermal conductivity, filling the technical gap in the use of graphite in an environment of 1500°C and expanding the application range of graphite materials.

[0020] The coated graphite provided in the embodiments of the present invention (components made of graphite material with a Si / SiC coating on the surface) is applied to high-temperature scenarios, such as industrial furnaces, electric heating components, aerospace thermal protection, etc., which overcomes the problem of easy oxidation of graphite at high temperatures. While maintaining the performance of the coating, it greatly reduces the cost of the substrate and improves the processing adaptability, and has stronger industrial application and promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0022] Figure 1 A flow chart showing the steps of a method for plasma spraying a Si coating on a graphite surface at 1500°C that is thermal shock and oxidation resistant, provided in an embodiment of the present application.

[0023] Figure 2 A scanning electron microscope image of a graphite substrate having a Si / SiC coating on its surface provided in Example 3 of the present application is shown. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its application or use. Based on the embodiments in the present application, any product that is identical or similar to the present application and is derived by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts falls within the scope of protection of the present application. In addition, all other embodiments obtained by ordinary technicians in this field without carrying out creative work fall within the scope of protection of the present application.

[0025] Where specific experimental steps or conditions are not specified in the examples, the conventional experimental steps or conditions described in the prior art in this field may be used. The reagents and other instruments used, for which the manufacturer is not specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely schematic illustrations of the embodiments of the present application and are not necessarily drawn to scale. Identical reference numerals in the figures represent identical or similar parts, and their repeated descriptions will be omitted. Some block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0026] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered as part of the description of this application.

[0027] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0028] It is understandable that high-temperature oxidation of graphite will bring many hazards, which are mainly reflected in the following aspects: Degradation of material properties: During the high-temperature oxidation process, the carbon atom structure within graphite is destroyed, resulting in a significant decrease in the mechanical strength of the graphite. For example, the compressive and tensile strength of graphite parts used in high-temperature furnaces will gradually weaken due to oxidation, which may eventually lead to component breakage or damage.

[0029] Poor conductivity: Graphite has good conductivity, but after high-temperature oxidation, an oxide layer (such as carbon dioxide or carbon monoxide) forms on its surface and inside. These oxidation products hinder the transmission of electrons, thereby reducing the conductive properties of graphite.

[0030] Reduced thermal conductivity: Graphite's thermal conductivity depends primarily on its internal carbon atomic structure. Oxidation destroys this structure, resulting in reduced thermal conductivity and affecting its heat dissipation effectiveness in high-temperature equipment.

[0031] Damage to structural integrity: High-temperature oxidation will cause corrosion pits to form on the graphite surface. Over time, these corrosion pits will gradually expand and deepen, eventually destroying the integrity of the graphite surface.

[0032] Increased internal pores: The oxidation reaction not only occurs on the surface of the graphite, but also diffuses into the interior, forming more pores. These pores reduce the density of the graphite and further weaken its structural strength.

[0033] Shortened service life: High-temperature oxidation accelerates the aging process of graphite, significantly shortening its service life in high-temperature environments. For example, in some high-temperature furnaces, graphite parts that have not been treated with anti-oxidation treatment may only be used for a few months or even weeks.

[0034] In related technologies, spraying Si coatings on graphite surfaces can only partially increase the graphite's service temperature, primarily providing thermal shock and oxidation protection in moderate-temperature environments. However, high-temperature oxidation of graphite still presents a challenge in extreme temperatures of 1500°C. While SiC coatings can operate in high-temperature environments and improve graphite's thermal shock and oxidation resistance, stable SiC coatings cannot be directly sprayed onto graphite surfaces using plasma spraying technology. Therefore, the oxidation problem of graphite in extreme high-temperature environments such as 1500°C cannot be effectively addressed using conventional plasma-sprayed Si or SiC coatings, making it difficult for graphite to effectively operate in extreme high-temperature environments of 1500°C, such as aerospace and thermal energy devices. Using plasma spraying technology to produce SiC-based functional coatings on graphite surfaces that can operate at extreme temperatures of 1500°C would enable graphite to enter the ranks of extremely high-temperature functional structural materials, pushing the boundaries of its application. Leveraging the abundant, low-cost, and widely available resources of graphite, graphite could replace some expensive, high-temperature resistant materials, offering significant engineering and industrial value.

[0035] In view of the problems in the related art, in the first aspect, the present application provides a method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance. Figure 1 The flowchart of the method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance provided by the embodiment of the present application is shown as follows: Figure 1 As shown, the method specifically includes the following steps: S1. Pre-treating the surface of the graphite substrate to be sprayed; In this step, pretreatment can remove impurities, oil stains, oxide layers, etc. on the surface of the graphite substrate by mechanical roughening, sandblasting, chemical cleaning, etc., thereby increasing the surface roughness, which is beneficial to the adhesion and density of the coating during the plasma spraying process in the subsequent step S2.

[0036] Preferably, step S1 includes: S11. The surface of the graphite substrate to be sprayed is roughened, degreased and preheated.

[0037] During the roughening process, polishing or other methods can be used to remove surface impurities and increase the surface roughness of the graphite matrix. For example, 1500# sandpaper can be used to polish the graphite matrix.

[0038] During the degreasing process, the graphite substrate is cleaned and soaked with an organic solvent or chemical cleaning agent to dissolve and remove impurities such as oil, dust, etc. on the surface of the graphite substrate, so that the substrate surface is in a clean state, and then washed with clean water and dried.

[0039] During the preheating treatment, the graphite substrate is preheated to remove adsorbed moisture and some volatile impurities on the surface, and at the same time, the temperature of the substrate surface can be increased, which is beneficial to reducing the temperature difference between the Si particles and the substrate during the subsequent step S2 plasma spraying process and reducing thermal stress.

[0040] The graphite substrate can be preheated by using a resistance heating furnace and an oven.

[0041] Preferably, the graphite substrate is preheated using a plasma spray gun; the preheating temperature is 120°C to 150°C. The plasma spray gun emits a plasma jet to preheat the graphite substrate surface. Directly applying the spray gun to the substrate surface before spraying provides a high-energy beam, enabling rapid, targeted heating of the graphite substrate surface to be sprayed. Furthermore, the preheating and spraying processes share the same plasma spray gun, eliminating the need for additional preheating equipment such as resistance heating furnaces and ovens, streamlining the process.

[0042] The spray gun's control system precisely adjusts parameters such as plasma gas flow, current, and voltage to maintain the graphite substrate's surface temperature between 120°C and 150°C. Graphite slowly oxidizes above 400°C in air, so controlling the preheating temperature in air within the range of 120°C to 150°C prevents premature thermal oxidation or stress deformation of the graphite substrate. It also effectively removes moisture and volatiles from the graphite surface, reduces the difference in thermal expansion coefficient between the substrate and the coating, and reduces internal stress during cooling and shrinkage, thereby enhancing the bonding strength between the coating and the substrate.

[0043] Therefore, the plasma jet preheating method has the advantages of high process integration, fast heating efficiency, and precise local temperature control. It can effectively improve the adhesion performance and structural integrity of the subsequent Si coating, and provide a favorable basis for the formation of high-quality gradient transition Si / SiC coatings.

[0044] S2. Spraying the surface to be sprayed with plasma spraying technology to obtain a Si coating on the surface to be sprayed; In plasma spraying technology, plasma spraying generates a high-temperature plasma jet by ionizing the working gas, and sends silicon powder into the plasma jet to be heated to a molten or semi-molten state. Then, under the action of high-speed airflow, the molten Si powder is accelerated through the nozzle of the spray gun to impact the surface to be sprayed of the graphite substrate, and quickly cooled to form a Si coating.

[0045] In this technology, suitable plasma spraying equipment is selected, including a spray gun, power supply, gas control system, and powder feeding system. Appropriate spraying parameters, such as the working gas, working gas flow rate, current, voltage, powder feeding rate, spraying distance, spray gun movement speed, and number of spraying cycles, are set based on the characteristics of the graphite substrate and silicon powder, as well as the thickness to be sprayed. For example, a mixture of argon and argon is used as the working gas. The pretreated graphite substrate is fixed to the spraying platform, and spraying is performed according to the set spraying parameters to obtain the desired spray thickness and good quality Si coating.

[0046] S3. A vacuum heat treatment technique is used to heat treat the graphite substrate with the Si coating in a vacuum environment, so that part of the solid silicon in the Si coating is melted into liquid silicon or silicon vapor, and penetrates into the graphite substrate containing pores through capillary action, so that part of the liquid silicon or silicon vapor chemically reacts with the graphite substrate, thereby in situ growing a gradient transition Si / SiC coating on the surface to be sprayed of the graphite substrate.

[0047] In this step, graphite will slowly oxidize when it is above 400°C in air, and will rapidly oxidize into CO or CO2 when it exceeds 500°C to 600°C, causing mass loss and surface damage. Similarly, the Si coating will also oxidize at high temperatures. Therefore, vacuum heat treatment technology is used to establish a vacuum environment. In a vacuum environment, the Si coating and the graphite substrate can be prevented from chemically reacting with oxygen or other gases, and only part of the liquid silicon or silicon vapor generated in the Si coating at high temperature is allowed to penetrate into the graphite substrate and react with the graphite substrate to form SiC, thereby achieving reactive penetration and in-situ carbonization of the Si coating.

[0048] Using vacuum heat treatment technology, the graphite substrate with Si coating is subjected to high-temperature treatment in a vacuum atmosphere, so that the sprayed solid silicon melts into liquid silicon, which penetrates into the porous graphite substrate through capillary action. Most of the liquid silicon or silicon vapor reacts chemically with the C in the substrate. As the thickness of the graphite substrate increases, the penetration ability of Si slowly decreases, and the content of infiltrated Si gradually decreases. At the same time, a small amount of Si does not react with C and is located in the outermost layer, thereby in situ generating a gradient transition structure Si / SiC coating on the graphite surface.

[0049] In this technology, the appropriate heat treatment temperature and time can be set based on the characteristics of the graphite substrate and the Si coating. By controlling the heat treatment temperature and time, the penetration depth and reaction degree of the silicon can be controlled, thereby achieving the formation of a gradient transition coating. The graphite substrate sprayed with the Si coating is placed in a vacuum heat treatment furnace. The furnace is first evacuated to a certain pressure to establish a vacuum environment. The furnace is then heated to the heat treatment temperature at a set heating rate, held at that temperature for a specified period of time, and then cooled to room temperature.

[0050] The Si / SiC coating obtained through steps S1-S3 can serve as a transitional bonding layer between the graphite substrate and the outermost ceramic coating subsequently sprayed. Due to its high compatibility with the graphite substrate, the Si / SiC coating improves mechanical and metallurgical bonding, preventing coating spalling. Furthermore, the Si / SiC coating has a thermal expansion coefficient between that of the graphite substrate and the ceramic coating, reducing thermal stress and delaying thermal fatigue cracking.

[0051] In summary, compared to conventional plasma sprayed Si coatings, which are easily oxidized at high temperatures and cannot effectively prevent oxygen from further corroding the graphite substrate, directly sprayed SiC coatings are difficult to obtain SiC coatings with ideal chemical composition and dense structure due to the easy decomposition of SiC during the plasma spraying process, and cannot meet the high-temperature anti-oxidation requirements. This embodiment utilizes the advantages of plasma spraying technology to prepare coatings. Based on graphite material as the matrix, coupled with vacuum heat treatment technology, a gradient transition Si / SiC coating is directly prepared on the graphite surface. The gradient transition Si / SiC coating can alleviate the difference in thermal expansion coefficient and reduce thermal stress. The Si coating forms a good chemical bond with the graphite substrate during the sintering process. The coating has significant high-temperature thermal shock and oxidation resistance. The graphite with Si / SiC coating on the surface can be easily damaged or oxidized in a high-temperature environment of 1500°C. Therefore, this embodiment achieves the long-term service of graphite components in an extreme environment of 1500°C for the first time, fully leveraging the advantages of graphite materials such as ease of processing, low material cost, wide raw material sources, and excellent electrical and thermal conductivity. This fills the technical gap in the use of graphite in 1500°C environments and expands the application range of graphite materials. Thus, the coated graphite provided by the embodiments of the present invention (components made of graphite material with a Si / SiC coating on the surface) is applied in high-temperature scenarios such as industrial furnaces, electric heating components, and aerospace thermal protection. This overcomes the problem of graphite's easy oxidation at high temperatures, significantly reduces substrate costs, improves processing adaptability, and has greater industrial application and promotion value while maintaining coating performance.

[0052] Furthermore, step S2 includes: S21, using Si powder with a purity greater than 99.99% as spray powder, and setting the spraying parameters of the plasma spraying equipment according to the thickness to be sprayed and the spraying powder; S22. Using a spray gun in a plasma spraying device, spraying the spray powder onto the surface to be sprayed of the graphite substrate according to spraying parameters to prepare a Si coating of a thickness to be sprayed.

[0053] In this embodiment, Si powder with a purity greater than 99.99% is selected as the spray powder, effectively reducing the impurity content in the coating and improving the coating's purity and quality. Furthermore, during the subsequent vacuum sintering process, the high-purity Si coating reacts with the graphite substrate to form SiC, preventing other impurities from escaping and reacting with the graphite substrate, resulting in a high-quality gradient transition Si / SiC coating.

[0054] Among them, the spraying parameters of the plasma spraying equipment are set according to the thickness to be sprayed and the spraying powder. The spraying parameters include powder feeding rate, powder feeding gas, working current, working voltage, working gas, spraying distance, spray gun moving speed, spray angle, etc. The spraying parameters are used to control the coating thickness and the melting state of Si powder, thereby obtaining a Si coating of the required thickness and good quality.

[0055] Preferably, the coating thickness can be set to 30 μm to 200 μm.

[0056] Preferably, the spraying parameters include: the working gas is a mixture of nitrogen and argon, the mass proportion of argon is 50%~70%, and the flow rate of the mixture is 7SPLM~10SPLM; the working current is 100A~120 A; the working voltage is 130V~140 V; the powder feeding gas is argon, and the flow rate of the spraying powder is 4 g / min~10 g / min; the spraying distance is 250mm; and the spraying angle is 90°.

[0057] In this embodiment, the flow rate of the mixed gas is determined by the spray gun power and structure, and is set at 7-10 SPLM to ensure stable plasma arc combustion. Argon is used as the primary gas to stabilize the plasma, while nitrogen is used as the auxiliary gas to increase temperature and regulate flame speed. An operating current of 100-120 A and an operating voltage of 130-140 V ensure sufficient melting of the Si powder without overheating-induced evaporation, decomposition, or oxidation. Argon is used as the powder feed gas, and the spray powder flow rate is 4-10 g / min to ensure efficient coating deposition and sufficient melting of all powder particles. A spray distance of 250 mm avoids excessive spraying distances, which can lead to increased particle cooling and a potential decrease in velocity, and close spraying distances, which can cause overheating or droplet dispersion upon impact. Within this spray distance, the molten particles optimally impact and spread, forming a dense coating. The spraying angle is set to 90°, and the molten particle flow is controlled to impact the substrate surface vertically and positively, thereby achieving sufficient flattening, improving powder adhesion, reducing shadow effects and spraying dead angles, reducing porosity, optimizing microstructure, and improving deposition efficiency.

[0058] Preferably, the Si powder is spherical and has a particle size of 30 μm to 80 μm. Spherical Si powder has good fluidity and can be evenly fed into the plasma spray gun through the powder feeding system. Spherical powder in the particle size range of 30 to 80 μm can be effectively melted in the plasma jet. This prevents undersized powder from being overheated or even vaporized, resulting in increased oxidation or spraying losses, and prevents oversized powder from being insufficiently melted, resulting in low deposition efficiency, and affecting the uniformity and density of the coating. After melting, the spherical Si powder will spread and solidify rapidly when it hits the surface of the graphite substrate at high speed, which is conducive to the formation of a continuous Si coating without obvious pores, providing good interface conditions for the subsequent Si infiltration reaction into the graphite.

[0059] During the specific implementation, the spraying parameters are set in the control system of the plasma spraying equipment. During the spraying process, the spray gun is perpendicular to the surface of the graphite substrate to be sprayed. The molten particle flow is sprayed perpendicularly on the surface of the substrate according to the set spraying parameters, and is accumulated layer by layer according to the set number of spraying times to form a Si coating with a thickness of 30μm~200μm and a smooth surface.

[0060] Furthermore, in step S3, the graphite substrate with the Si coating is placed in a vacuum furnace, heat treatment parameters of the vacuum furnace are set, and the graphite substrate with the Si coating is heat treated in the vacuum furnace according to the heat treatment parameters.

[0061] In this embodiment, a vacuum furnace is used to create a vacuum environment, effectively isolating oxygen, preventing oxidation of the Si coating and graphite substrate at high temperatures while also reducing interference from gaseous impurities. Heat treatment parameters are set to partially melt or vaporize the Si, allowing the liquid or gaseous Si to penetrate the micropores of the graphite surface under the influence of capillary forces and temperature gradients.

[0062] The control system of the vacuum furnace can achieve precise temperature and vacuum control. According to the coating thickness, composition and substrate material properties, the heat treatment parameters such as temperature, time, heating rate and vacuum degree can be flexibly adjusted to accurately control the penetration depth and reaction degree of silicon and optimize the oxidation resistance and thermal shock resistance of the gradient transition coating.

[0063] Preferably, the heat treatment parameters include: heat treatment temperature of 1500°C to 1600°C; heating rate of less than 15°C / min; vacuum degree of at least 10 -3 Pa.

[0064] In this embodiment, the heat treatment temperature is set to 1500℃~1600℃, which is higher than the melting point of Si (1414℃). At the same time, it is avoided that the temperature is too high, which may cause a large amount of Si evaporation and cracking of the sintered body. The vacuum degree is set to be greater than 10 -3 Pa, to avoid insufficient vacuum, which can easily lead to Si oxidation to form SiO2, reducing density and strength. The vacuum furnace holding time can be set according to the coating thickness and penetration depth. Set the heating rate to less than 15°C / min to control the penetration rate of liquid Si, avoid excessive reaction that may cause porosity or warping, and prevent thermal stress cracking of the substrate.

[0065] In summary, the present invention employs a two-step process: first, a Si coating is deposited on the graphite surface via plasma spraying. Subsequently, a vacuum heat treatment is performed, allowing the Si to infiltrate the graphite substrate through capillary action and react, resulting in an in-situ grown gradient transition Si / SiC coating on the graphite surface. The SiC reaction layer is at least 200 μm thick and has a hardness greater than 10 GPa. The coating remains intact at a heating rate of 300°C / min to 1500°C, protecting the graphite substrate and exhibiting performance characteristics such as thermal shock resistance, diffusion resistance, and oxidation resistance.

[0066] In the second aspect, a graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface includes a graphite substrate having a gradient transition Si / SiC coating on its surface, and the Si / SiC coating is obtained according to the above method of plasma spraying Si coating on the graphite surface at 1500°C thermal shock and oxidation resistant.

[0067] In a third aspect, the present application provides an application of a graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface, and applies a graphite substrate having a gradient transition Si / SiC coating on its surface to a 1500°C environment.

[0068] In order to enable those skilled in the art to understand the present application more clearly, the present application is now described in detail through the following examples.

[0069] Example 1 A method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance, comprising the following steps: 1. Use 1500# sandpaper to polish the surface of the graphite substrate to be sprayed to improve adhesion.

[0070] 2. Adjust the spraying distance to 250 mm and the spraying angle to 90° so that the plasma jet ejected from the spray gun is perpendicular to the substrate surface.

[0071] 3. Turn on the plasma spray gun, preheat the graphite body first, and control the surface temperature to 120℃.

[0072] 4. Select spherical Si powder with a purity greater than 99.99% and a powder diameter of 30μm.

[0073] 5. Set the coating thickness to 30 μm, start the plasma spray equipment, and set the following spray parameters: working gases: nitrogen and argon; total gas flow: 7 SPLM; argon ratio: 50%; operating current: 100 A; operating voltage: 130 V; powder feeder: 4 g / min; argon as the feed gas; a Si coating with a thickness of 30 μm was prepared. 6. Place the Si-coated graphite substrate into a vacuum furnace with a vacuum degree of 1×10 -3 Pa, heating rate of 10 ℃ / min, heating to 1500 ℃, keeping at 1500 ℃ for 2h, and obtaining gradient transition Si / SiC coating in situ grown on the surface of graphite substrate.

[0074] In this embodiment, the graphite substrate was taken out of the vacuum furnace and kept at 1500°C for 2 hours without any peeling of the coating or oxidation of the substrate. At the same time, the graphite substrate with the coating was subjected to corresponding high-temperature thermal cycle tests. When the temperature was increased to 1500°C at a heating rate of 300°C / min, the coating did not peel off, effectively protecting the graphite substrate and having the performance of thermal shock resistance, diffusion resistance, and oxidation resistance.

[0075] Example 2 Compared with Example 1, the difference is: A method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance, comprising the following steps: 1. Use 1500# sandpaper to polish the surface of the graphite substrate to be sprayed to improve adhesion.

[0076] 2. Adjust the spraying distance to 250 mm and the spraying angle to 90° so that the plasma jet ejected from the spray gun is perpendicular to the substrate surface.

[0077] 3. Turn on the plasma spray gun, preheat the graphite body first, and control the surface temperature to 140℃.

[0078] 4. Select spherical Si powder with a purity greater than 99.99% and a powder diameter of 50μm.

[0079] 5. Set the coating thickness to 50 μm, start the plasma spray equipment, and set the spray parameters as follows: working gases: nitrogen and argon; total gas flow: 8 SPLM; argon ratio: 50%; operating current: 110 A; operating voltage: 130 V; powder feeder: 6 g / min; argon as the feed gas; and produce a Si coating with a thickness of 50 μm. 6. Place the Si-coated graphite substrate into a vacuum furnace with a vacuum degree of 1×10 -3 Pa, heating rate of 10 ℃ / min, heating to 1500 ℃, keeping at 1500 ℃ for 2h, and obtaining gradient transition Si / SiC coating in situ grown on the surface of graphite substrate.

[0080] In this embodiment, the graphite substrate was taken out of the vacuum furnace and kept at 1500°C for 2 hours without any peeling of the coating or oxidation of the substrate. At the same time, the graphite substrate with the coating was subjected to corresponding high-temperature thermal cycle tests. When the temperature was increased to 1500°C at a heating rate of 300°C / min, the coating did not peel off, effectively protecting the graphite substrate and having the performance of thermal shock resistance, diffusion resistance, and oxidation resistance.

[0081] Example 3 A method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance, comprising the following steps: 1. Use 1500# sandpaper to polish the surface of the graphite substrate to be sprayed to improve adhesion.

[0082] 2. Adjust the spraying distance to 250 mm and the spraying angle to 90° so that the plasma jet ejected from the spray gun is perpendicular to the substrate surface.

[0083] 3. Turn on the plasma spray gun, preheat the graphite body first, and control the surface temperature to 150℃.

[0084] 4. Select spherical Si powder with a purity greater than 99.99% and a powder diameter of 80μm.

[0085] 5. Set the coating thickness to 100 μm, start the plasma spray equipment, and set the following spray parameters: working gases: nitrogen and argon; total gas flow: 8.5 SPLM; argon ratio: 50%; operating current: 120 A; operating voltage: 140 V; powder feeder: 6 g / min; argon as the feed gas; and produce a Si coating with a thickness of 100 μm. 6. Place the Si-coated graphite substrate into a vacuum furnace with a vacuum degree of 1×10 -3 Pa, heating rate of 10 ℃ / min, heating to 1500 ℃, keeping at 1500 ℃ for 2h, and obtaining gradient transition Si / SiC coating in situ grown on the surface of graphite substrate.

[0086] In this embodiment, the graphite substrate was taken out of the vacuum furnace and kept at 1500°C for 2 hours without any peeling of the coating or oxidation of the substrate. At the same time, the graphite substrate with the coating was subjected to corresponding high-temperature thermal cycle tests. When the temperature was increased to 1500°C at a heating rate of 300°C / min, the coating did not peel off, effectively protecting the graphite substrate and having the performance of thermal shock resistance, diffusion resistance, and oxidation resistance.

[0087] Example 4 A method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance, comprising the following steps: 1. Use 1500# sandpaper to polish the surface of the graphite substrate to be sprayed to improve adhesion.

[0088] 2. Adjust the spraying distance to 250 mm and the spraying angle to 90° so that the plasma jet ejected from the spray gun is perpendicular to the substrate surface.

[0089] 3. Turn on the plasma spray gun, preheat the graphite body first, and control the surface temperature to 150℃.

[0090] 4. Select spherical Si powder with a purity greater than 99.99% and a powder diameter of 80μm.

[0091] 5. Set the coating thickness to 200 μm, start the plasma spray equipment, and set the following spray parameters: nitrogen and argon as working gases; total gas flow rate of 9 SPLM; argon ratio of 70%; operating current of 120 A; operating voltage of 140 V; powder feed rate of 10 g / min, argon as the powder feed gas, to produce a Si coating with a thickness of 200 μm. 6. Place the graphite substrate with Si coating in a vacuum furnace with a vacuum degree of 1×10-3 Pa and a heating rate of 15℃ / min to 1500℃. Keep it at 1500℃ for 2h to obtain a gradient transition Si / SiC coating in situ grown on the surface of the graphite substrate.

[0092] In this embodiment, the graphite substrate was taken out of the vacuum furnace and kept at 1500°C for 2 hours without any peeling of the coating or oxidation of the substrate. At the same time, the graphite substrate with the coating was subjected to corresponding high-temperature thermal cycle tests. When the temperature was increased to 1500°C at a heating rate of 300°C / min, the coating did not peel off, effectively protecting the graphite substrate and having the performance of thermal shock resistance, diffusion resistance, and oxidation resistance.

[0093] Coating microstructure testing The graphite substrate with the gradient transition Si / SiC coating on the surface prepared in Example 3 was subjected to SEM scanning, and the following Figure 2 Scanning electron micrograph of a graphite substrate with Si / SiC coating is shown. Figure 2 Panel (a) shows the cross-sectional structure between the graphite substrate (graphite) and the SiC layer (reaction-sintered layer) formed thereon. The coating thickness is noted as 319 µm, representing the penetration depth of the SiC reaction layer. This indicates that during vacuum heat treatment, the plasma-sprayed Si coating partially reacts with the graphite substrate, transforming it into a dense, gradient-transition Si / SiC coating structure. The interface between the coating and the graphite substrate is clear, with no delamination or delamination, indicating good interfacial bonding between the Si / SiC coating and the graphite substrate. Figure 2 (b) shows a partial magnification of the SiC layer, illustrating its microstructure. The reaction-sintered layer exhibits a dense and uniform microstructure, free of significant pores or cracks. The dense SiC layer effectively blocks oxygen intrusion, improving the graphite matrix's service stability at extreme temperatures of 1500°C.

[0094] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0095] For the sake of simplicity, the method embodiments are described as a series of action combinations. However, those skilled in the art should be aware that this application is not limited by the order of the actions described, because according to this application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in this specification are all preferred embodiments, and the actions and components involved are not necessarily required by this application.

[0096] The above is a detailed introduction to the 1500°C thermal shock and oxidation resistant plasma spraying Si coating method, coated graphite and application provided by this application. Specific examples are used in this article to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method of this application and its core idea; at the same time, for general technical personnel in this field, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on this application.

Claims

1. A method for plasma spraying Si coating on graphite surface at 1500℃ with thermal shock resistance and oxidation resistance, characterized in that: The method comprises the following steps: S1. Pre-treating the surface of the graphite substrate to be sprayed; S2, spraying the surface to be sprayed using plasma spraying technology to obtain a Si coating on the surface to be sprayed; S3. The graphite substrate with the Si coating is heat-treated in a vacuum environment using a vacuum heat treatment technique, so that part of the solid silicon in the Si coating is melted into liquid silicon or silicon vapor, and penetrates into the graphite substrate containing pores through capillary action, so that part of the liquid silicon or silicon vapor chemically reacts with the graphite substrate, thereby in-situ growing a gradient transition Si / SiC coating on the surface to be sprayed of the graphite substrate.

2. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 1, characterized in that: Step S2 includes: S21, using Si powder with a purity greater than 99.99% as spray powder, and setting the spraying parameters of the plasma spraying equipment according to the thickness to be sprayed and the spraying powder; S22. Using a spray gun in the plasma spraying equipment, spraying the spraying powder onto the surface to be sprayed of the graphite substrate according to the spraying parameters, to prepare a Si coating of the thickness to be sprayed.

3. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 2, characterized in that: The spraying parameters include: the working gas is a mixture of nitrogen and argon, the mass proportion of the argon is 50%~70%, and the flow rate of the mixture is 7SPLM~10SPLM; the working current is 100A~120A; the working voltage is 130V~140V; the powder feeding gas is argon, and the flow rate of the spraying powder is 4 g / min~10 g / min; the spraying distance is 250mm; and the spraying angle is 90°.

4. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 2, characterized in that: The Si powder is spherical and has a particle size of 30 μm to 80 μm.

5. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 1, characterized in that: In step S3, the graphite substrate with the Si coating is placed in a vacuum furnace, heat treatment parameters of the vacuum furnace are set, and the graphite substrate with the Si coating is heat treated in the vacuum furnace according to the heat treatment parameters.

6. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 5, characterized in that: The heat treatment parameters include: heat treatment temperature of 1500℃~1600℃; heating rate less than 15℃ / min; vacuum degree of at least 10 -3 Pa.

7. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 1, characterized in that: Step S1 includes: S11, roughening, degreasing and preheating the surface of the graphite substrate to be sprayed.

8. The method for plasma spraying Si coating on graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to claim 7, characterized in that: In step S11, the roughening includes grinding the graphite substrate with sandpaper; and The preheating treatment includes preheating the graphite substrate using a spray gun in a plasma spraying device; wherein the temperature of the preheating treatment is 120° C. to 150° C.

9. A graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface, characterized in that: The invention comprises a graphite substrate with a gradient transition Si / SiC coating on the surface, wherein the Si / SiC coating is obtained by the method for plasma spraying a Si coating on a graphite surface at 1500°C with thermal shock resistance and oxidation resistance according to any one of claims 1 to 8.

10. An application of the graphite substrate having a 1500°C thermal shock and oxidation resistant coating on its surface as claimed in claim 9, characterized in that: The graphite substrate with a gradient transition Si / SiC coating on its surface was applied in a 1500°C environment.

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