A yttria gradient protective coating for graphite matrix and a preparation method and application thereof

Abstract

This invention discloses a yttrium oxide gradient protective coating for graphite substrates, its preparation method, and its applications. The yttrium oxide gradient protective coating comprises a silicon carbide layer, a transition layer, and a yttrium oxide layer sequentially stacked on a graphite substrate. The silicon carbide layer is formed by atmospheric plasma spraying of silicon carbide powder and graphite powder. The transition layer is formed by atmospheric plasma spraying of silicon carbide powder, graphite powder, and yttrium oxide powder. The yttrium oxide layer is formed by atmospheric plasma spraying of yttrium oxide powder. Furthermore, along the direction from the silicon carbide layer to the yttrium oxide layer, the content of silicon carbide and graphite in the yttrium oxide gradient protective coating gradually decreases. This invention, by introducing graphite powder to protect silicon carbide and combining the gradient structure design with atmospheric plasma spraying technology, significantly improves the interfacial bonding strength between the protective coating and the graphite substrate, and effectively reduces the risk of thermal cycling cracking of the coating, showing promising application prospects in the photovoltaic and semiconductor fields.

Description

Technical Field

[0001] This invention relates to the field of material surface coating technology, specifically to a yttrium oxide gradient protective coating for graphite substrate and its preparation method, which is particularly suitable for high-temperature tooling components such as thermal field components, load-bearing components, crucibles, and flow guiding components in the photovoltaic and semiconductor industries. Background Technology

[0002] Graphite materials are widely used in the photovoltaic and semiconductor industries for their high temperature resistance, good thermal conductivity, light weight, ease of processing, and good chemical stability, serving as thermal field components, load-bearing components, crucibles, flow guiding components, and other high-temperature tooling components. Especially under conditions requiring high temperature, vacuum, corrosive atmospheres, or high cleanliness, the surface condition of the graphite matrix has a significant impact on the service life and process stability of the components.

[0003] Yttrium oxide (Y₂O₃) coatings are an ideal choice for protecting graphite substrates due to their excellent high-temperature resistance, plasma corrosion resistance, and low-pollution properties. Currently, methods such as atmospheric plasma spraying (APS) for directly depositing Yttrium oxide coatings on graphite surfaces have been reported. However, due to the significant difference in thermal expansion coefficients between graphite and Yttrium oxide, and poor interfacial wettability, direct spraying easily leads to low coating bonding strength and high porosity, making it prone to cracking and peeling during thermal cycling, thus failing to meet the requirements for long-term stable operation.

[0004] To improve the interfacial bonding, patent CN121293017A discloses a technical approach of first forming a SiC transition layer on the graphite surface, and then depositing a yttrium oxide coating. This approach utilizes the good chemical and mechanical compatibility of SiC and graphite, as well as SiC's high hardness and chemical stability, to effectively improve the interfacial compatibility between the substrate and the surface layer, and enhance the yttrium oxide layer's resistance to plasma corrosion. However, the aforementioned two-layer structure of a single SiC transition layer + a single Y2O3 surface layer still faces the following technical bottlenecks: (1) There is still a significant change in composition and thermophysical properties (such as coefficient of thermal expansion and elastic modulus) between the SiC layer and the Y2O3 layer. During high-temperature service and thermal cycling, stress concentration is easily generated at the interface, which leads to coating cracking and delamination failure.

[0005] (2) When preparing coatings using solution precursor plasma spraying (SPPS) technology, the resulting coatings generally have a columnar crystalline structure. Open pores and microcrack channels easily form between the columnar crystals, making it easier for corrosive media (such as plasma, molten salt, etc.) to penetrate into the graphite matrix along the inter-column boundaries, thus reducing the overall density, interface shielding ability, and anti-stripping stability of the coating. In addition, SPPS uses liquid precursors, and the spraying process involves multiple steps such as atomization, solvent evaporation, precursor decomposition, and particle sintering, resulting in significant heat and material losses. The effective solid content actually deposited on the substrate is relatively low, leading to a deposition efficiency significantly lower than that of conventional atmospheric plasma spraying (APS).

[0006] Given the significant shortcomings of existing technologies in terms of interfacial bonding strength, thermal cycling stability, and preparation efficiency of yttrium oxide protective coatings on graphite substrates, there is an urgent need to develop a yttrium oxide protective coating with high interfacial bonding strength, strong anti-stripping ability, good thermal cycling stability, and high preparation efficiency to meet the increasingly stringent performance requirements of high-temperature tooling components in the high-end manufacturing field. Summary of the Invention

[0007] This invention addresses the shortcomings of current yttrium oxide protective coatings prepared on graphite substrates, such as low interfacial bonding strength, poor thermal cycling stability, and low preparation efficiency. It provides a gradient yttrium oxide protective coating for graphite substrates, its preparation method, and its applications. By introducing graphite powder as a compensating carbon source to protect silicon carbide, and by combining it with atmospheric plasma spraying technology to prepare a protective coating with a multi-layer gradient structure, the interfacial bonding strength between the protective coating and the graphite substrate can be significantly improved, and the risk of thermal cycling cracking of the protective coating can be effectively reduced. This coating can be used to prepare high-temperature tooling components such as thermal field components, load-bearing components, crucibles, and flow guiding components in the photovoltaic or semiconductor industries.

[0008] Specifically, the following technical solutions are provided: The present invention provides a yttrium oxide gradient protective coating for a graphite substrate, the yttrium oxide gradient protective coating comprising a silicon carbide layer, a transition layer and a yttrium oxide layer sequentially stacked on a graphite substrate; The silicon carbide layer is formed by atmospheric plasma spraying of silicon carbide powder and graphite powder; the transition layer is formed by atmospheric plasma spraying of silicon carbide powder, graphite powder and yttrium oxide powder; and the yttrium oxide layer is formed by atmospheric plasma spraying of yttrium oxide powder. Furthermore, along the direction from the silicon carbide layer to the yttrium oxide layer, the content of silicon carbide and graphite in the yttrium oxide gradient protective coating gradually decreases.

[0009] This invention addresses the technical challenges of graphite substrates being easily oxidized at high temperatures, exhibiting poor interfacial compatibility with protective coatings, and being prone to cracking during thermal cycling. It utilizes a mixture of silicon carbide and graphite powder as raw materials, leveraging high-temperature in-situ reaction and physical filling during atmospheric plasma spraying to achieve carbon loss compensation and localized carbon environment regulation. Furthermore, by incorporating a gradient structure design, it simultaneously resolves the chemical incompatibility and physical-thermal mismatch between the graphite substrate and the yttrium oxide coating. Specifically: During the preparation of silicon carbide layers using atmospheric plasma spraying, it was found that silicon carbide undergoes an oxidation reaction in the high-temperature plasma flame, with an oxidation rate of approximately 10%-20%. Some silicon carbide is converted into silicon dioxide, leading to the loss of the effective protective phase, silicon carbide. To address this issue, this invention introduces an appropriate amount of graphite powder into the silicon carbide powder as a compensating carbon source. The introduction of graphite powder allows it to undergo a carbothermic reduction reaction with silicon dioxide, thereby reducing the direct oxidation of silicon carbide, compensating for the carbon lost due to oxidation, and maintaining the content of the effective protective phase, silicon carbide, in the coating. Simultaneously, the graphite powder maintains a relatively stable local carbon environment under high temperatures, effectively inhibiting the decomposition of silicon carbide and thermodynamically reducing the driving force for silicon carbide oxidation, thus acting as a chemical buffer.

[0010] In addition, to further alleviate the problem of thermal expansion mismatch between layers (room temperature - 1000 ℃, the coefficient of thermal expansion of graphite is 2-5×10), -6 K -1 The coefficient of thermal expansion of silicon carbide is 4-4.5×10⁻⁶. -6 K -1 The coefficient of thermal expansion of yttrium oxide is 7.5-8.5×10⁻⁶. -6 K -1 By designing a gradient protective coating, a gradual transition in composition and coefficient of thermal expansion is formed, thereby progressively varying thermal stress and effectively avoiding stress concentration at the interface, thus suppressing cracking under thermal cycling. Simultaneously, this invention utilizes APS technology to achieve rapid melting, accelerated flight, and quenching and solidification of high-melting-point components (silicon carbide, yttrium oxide) upon impact with the substrate. This allows for independent control of the thickness, porosity, and microstructure of each layer, forming a dense interface of mechanical interlocking and localized diffusion, thereby further improving the interfacial bonding strength.

[0011] The synergistic effect of the aforementioned materials, structures, and processes significantly improves the interfacial bonding strength and thermal cycling stability of the overall protective coating.

[0012] Furthermore, the transition layer includes a first transition layer, a second transition layer, and a third transition layer stacked sequentially, wherein the first transition layer is bonded to the silicon carbide layer, and the third transition layer is bonded to the yttrium oxide layer.

[0013] In this invention, through a large number of experiments, it was unexpectedly discovered that when the number of transition layers is set to 3, the prepared protective coating has the best interfacial bonding strength and the best thermal cycling stability. If the number of transition layers is too small, the stress buffering effect is insufficient, while if the number of transition layers is too large, the interfacial bonding strength and thermal stability of the prepared protective coating will decrease. This is because too many transition layers will introduce too many interfacial defects and residual stress.

[0014] Further, the ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the first transition layer is preferably 3:1, and the mass ratio of silicon carbide powder to graphite powder is preferably 1:(0.2-0.3), for example 1:0.25; the ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the second transition layer is preferably 1:1, and the mass ratio of silicon carbide powder to graphite powder is preferably 1:(0.2-0.3), for example 1:0.25; the ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the third transition layer is preferably 1:3, and the mass ratio of silicon carbide powder to graphite powder is preferably 1:(0.2-0.3), for example 1:0.25.

[0015] Furthermore, the mass ratio of silicon carbide powder to graphite powder in the raw materials for preparing the silicon carbide layer is 1:(0.2-0.3), for example, 1:0.2, 1:0.25, 1:0.3, etc., including but not limited to the mass ratios listed above.

[0016] In this invention, the amount of graphite powder added is preferably controlled at 20%-30% of the mass of silicon carbide powder, more preferably 25%, which is the result of comprehensive optimization based on multiple factors such as carbon oxidation loss compensation, particle packing densification, coating structure uniformity, and thermal cycling performance. When the amount of graphite added is less than 20%, it cannot fully compensate for the 10%-20% oxidation loss of silicon carbide in the plasma flame, resulting in insufficient effective silicon carbide phase content, unstable local carbon environment, and decreased high-temperature oxidation resistance of the coating. When the amount of graphite added exceeds 30%, the excessive graphite will burn off in large quantities during the spraying process, forming interconnected pores, reducing the density of the coating, and the residual graphite, as a soft phase, will break the silicon carbide skeleton, resulting in a significant decrease in hardness and bonding strength.

[0017] Furthermore, the thickness of the yttrium oxide gradient protective coating is preferably 400-450 μm; the thickness of the silicon carbide layer is preferably 50-150 μm, for example 100 μm; the thickness of the transition layer is 50-150 μm, for example 100 μm; and the thickness of the yttrium oxide layer is preferably 200-250 μm, for example 200 μm.

[0018] Furthermore, the particle size of the silicon carbide powder is preferably 20-45 μm, and the particle size of the graphite powder is preferably 5-15 μm.

[0019] Furthermore, the particle size of the yttrium oxide powder is preferably 25-45 μm.

[0020] In this invention, the particle size of the raw material powder needs to be controlled between 5-45 μm to balance powder feeding stability, the heating state of the particles in the plasma flame, reaction efficiency, and final deposition quality. If the particle size of the raw material powder is too large, the particles will not heat up sufficiently in the plasma flame, and unreacted or semi-molten particles are likely to appear, which will affect the continuous deposition and uniformity of the coating, and reduce the density and interfacial bonding performance of the coating. If the particle size of the raw material powder is too small, the specific surface area of ​​the particles will increase significantly. Although this is beneficial for rapid heating, it is also more prone to overheating, scattering, and accelerated oxidation. In particular, it will further increase the carbon component burn-off in the flame, which is not conducive to maintaining the stability of the deposition layer composition.

[0021] Furthermore, as an auxiliary carbon source and compensating component, graphite powder needs to have a particle size smaller than that of silicon carbide, preferably 5-15 μm. This facilitates filling between larger silicon carbide particles, improving mixing uniformity, and increases the contact area, making it easier for graphite to participate in local reactions or form a stable carbon environment during spraying, thereby compensating for carbon loss at high temperatures. If the graphite powder particle size is too large (e.g., greater than 15 μm), it is prone to mixing segregation and insufficient local carbon source; if the particle size is too small (e.g., less than 5 μm), it is prone to agglomeration and increased adsorption, affecting powder feeding stability. Using the above particle size ratio is beneficial to improving the dispersion uniformity of the mixed powder, powder feeding stability, and reaction sufficiency during spraying, thereby obtaining a silicon carbide underlayer with a more uniform structure and denser composition.

[0022] A second aspect of the present invention provides a method for preparing the yttrium oxide gradient protective coating for a graphite substrate as described in the first aspect, comprising the following steps: S1. Mix silicon carbide powder and graphite powder evenly to obtain a mixed powder; S2. An atmospheric plasma spraying equipment equipped with a dual feeding system is adopted, wherein the first feeding system is used to transport the mixed powder, the second feeding system is used to transport yttrium oxide powder, and the spraying parameters are set. S3. First, the mixed powder is conveyed through the first feeding system and then deposited on the surface of the preheated graphite substrate by atmospheric plasma spraying to form a silicon carbide layer. S4. Adjust the powder feeding amount of each feeding system in the dual feeding system according to the preset mass ratio, and simultaneously convey the mixed powder and yttrium oxide powder. Then, deposit the transition layer on the surface of the silicon carbide layer away from the graphite substrate by atmospheric plasma spraying. S5. Finally, yttrium oxide powder is fed through the second feeding system and deposited on the surface of the transition layer away from the silicon carbide layer by atmospheric plasma spraying to form a yttrium oxide layer, thus obtaining the yttrium oxide gradient protective coating.

[0023] Further, in step S2, the spraying parameters are as follows: spraying current of 450-650 A, spraying voltage of 50-75 V, spray gun power of 25-45 kW, main gas of argon with a flow rate of 30-50 L / min, auxiliary gas of hydrogen and / or helium with a flow rate of 5-15 L / min, spraying distance of 90-100 mm, spray gun moving speed of 300-600 mm / s, and substrate preheating temperature of 280-320 ℃. These spraying parameters are consistent with commonly used control factors for atmospheric plasma spraying of ceramic coatings. Among these, the current, main gas flow rate, and spraying distance are all important parameters affecting the density and porosity of Y₂O₃-based APS coatings.

[0024] Further, in step S2, the powder feeding air flow rate of the first feeding system is preferably 3-8 L / min; the powder feeding air flow rate of the second feeding system is preferably 3-8 L / min.

[0025] Furthermore, in step S3, the graphite substrate is a pretreated graphite substrate, and the pretreatment includes the steps of cleaning and sandblasting the graphite substrate.

[0026] Furthermore, in step S3, the preheating temperature is 290-310 ℃, for example, 300 ℃.

[0027] Furthermore, after the coating is completed, the workpiece is controlled to cool slowly or in the furnace to avoid thermal shock cracks caused by sudden cooling; the coating surface can be lightly ground to meet the surface flatness requirements.

[0028] The third aspect of the present invention provides the application of the yttrium oxide gradient protective coating for graphite substrates described in the first aspect or the preparation method described in the second aspect for graphite substrates in the preparation of high-temperature tooling components for the photovoltaic or semiconductor industries.

[0029] The beneficial effects of this invention are: This invention provides a yttrium oxide gradient protective coating for a graphite substrate. By introducing an appropriate amount of graphite powder as a compensating carbon source during the preparation of the silicon carbide layer using atmospheric plasma spraying, the graphite powder reacts with the silicon dioxide produced by oxidation in a carbothermic reduction reaction, compensating for carbon loss and maintaining the effective silicon carbide phase content. Simultaneously, it forms a stable local carbon environment at high temperatures, inhibiting silicon carbide decomposition and thermodynamically reducing the oxidation driving force, thus acting as a chemical buffer. This solves the problem of reduced effective protective phase due to silicon carbide oxidation during spraying. Furthermore, by designing a protective coating with a gradual gradient in composition and coefficient of thermal expansion, a graded change in thermal stress is achieved from the graphite substrate to the yttrium oxide surface layer, effectively avoiding interfacial stress concentration and suppressing thermal cycling cracking. Moreover, utilizing the high-temperature rapid melting and quenching solidification characteristics of atmospheric plasma spraying technology, the thickness, porosity, and microstructure of each layer can be independently controlled, forming a dense mechanically interlocked and locally diffused interface, significantly improving the interfacial bonding strength between the coating and the substrate, as well as between each layer. Under the synergistic effect of the above materials, structures and processes, the yttrium oxide gradient protective coating provided by the present invention has high interfacial bonding strength with the graphite substrate and excellent thermal cycling stability. It is suitable for surface protection of graphite components in the photovoltaic and semiconductor fields, and helps to reduce the problems of coating cracking, peeling and shortened service life caused by temperature fluctuations.

[0030] This invention also provides a method for preparing the above-mentioned yttrium oxide gradient protective coating. The method employs atmospheric plasma spraying technology and utilizes dual-feed port control to achieve multi-level gradient deposition, which makes the process highly adjustable and allows for flexible adjustment of the thickness and composition path of each layer. This reduces the variable difficulty of multi-step spraying operations and improves the repeatability of the preparation process. In addition, the entire method is carried out in an atmospheric environment without the need for vacuum or special atmosphere protection, which significantly reduces equipment costs and process complexity, and facilitates industrial mass production. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the preparation of a yttrium oxide gradient protective coating using atmospheric plasma spraying according to the present invention; Figure 2 This is a schematic diagram of the structure of the yttrium oxide gradient protective coating prepared in this invention; Figure 3 The image shown is a scanning electron microscope (SEM) image of the cross-section of the yttrium oxide gradient protective coating prepared in Example 1 of this invention, with a magnification of 150x.

[0032] Wherein, 0 is the graphite substrate, 1 is the silicon carbide layer, 2 is the transition layer, 20 is the first transition layer, 21 is the second transition layer, 22 is the third transition layer, 3 is the yttrium oxide layer, 4 is the plasma gas, 5 is the cathode, 6 is the water-cooled anode, 7 is the feed port of the second feeding system, 8 is the insulator, and 9 is the feed port of the first feeding system. Detailed Implementation

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items. The terms “comprising” or “including” as used herein may also be replaced with the closed form “is” or “consisting of”.

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0035] Example 1: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate, specifically including the following steps: (1) The isostatic graphite substrate was processed into a 100 mm × 100 mm × 10 mm sample. The surface was cleaned with acetone, anhydrous ethanol and deionized water in sequence. After drying, it was roughened by corundum sand blasting. After sandblasting, it was cleaned again and the substrate was preheated to 300 ℃ before spraying.

[0036] Silicon carbide powder (particle size 20 μm) was lightly sieved through a 200-mesh sieve, and graphite powder (particle size 10 μm) was lightly sieved through a 325-mesh sieve to remove agglomerated particles and improve powder dispersibility. Subsequently, the powder was premixed and mechanically mixed at a mass ratio of 1:0.25 to obtain a uniformly dispersed and free-flowing mixed powder for later use.

[0037] (2) An atmospheric plasma spraying equipment with a dual feeding system is used. The first feeding system transports the mixed powder prepared in step (1), and the second feeding system transports yttrium oxide powder (particle size of 30 μm). The spraying parameters are set as follows: spraying current 600A, spraying power 32 kW, spraying distance 95 mm, main gas Ar flow rate 40 L / min, auxiliary gas H2 flow rate 8 L / min, powder feeding gas Ar flow rate of both the first and second feeding systems is 5 L / min, and spray gun moving speed is 400 mm / s.

[0038] (3) A silicon carbide layer with a thickness of 100 μm is deposited on the surface of the graphite substrate through the first feeding system.

[0039] (4) Then, the powder is fed synchronously through a dual feeding system. The powder feeding amount of each feeding system is adjusted according to the following mass ratio to prepare the first transition layer, the second transition layer and the third transition layer with a thickness of approximately 33 μm in sequence: First transition layer: The mass ratio of mixed powder to yttrium oxide powder is 3:1; Second transition layer: The mass ratio of mixed powder to yttrium oxide powder is 1:1; The third transition layer has a mass ratio of 1:3 between the mixed powder and the yttrium oxide powder.

[0040] (5) Then the first feed port is closed, and a pure yttrium oxide layer is deposited on the surface of the silicon carbide substrate through the second feed port. The layer thickness is 200 μm.

[0041] After spraying, the coating is cooled in the furnace, forming a yttrium oxide gradient protective coating with a total thickness of 400 μm on the graphite substrate.

[0042] Example 2: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (1), the mass ratio of silicon carbide powder to graphite powder in the powder mixing is 1:0.2.

[0043] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0044] Example 3: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (1), the mass ratio of silicon carbide powder to graphite powder in the powder mixing is 1:0.3.

[0045] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0046] Example 4: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (4), only one transition layer with a thickness of 100 μm is prepared, and the mass ratio of the mixed powder to the yttrium oxide powder in the transition layer is 1:1.

[0047] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0048] Example 5: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (4), two transition layers are prepared. In the first transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.7:0.3, and the layer thickness is 50 μm. In the second transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.3:0.7, and the layer thickness is 50 μm.

[0049] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0050] Example 6: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (4), four transition layers are prepared. In the first transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.8:0.2, and the layer thickness is 25 μm. In the second transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.6:0.4, and the layer thickness is 25 μm. In the third transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.4:0.6, and the layer thickness is 25 μm. In the fourth transition layer, the mass ratio of mixed powder to yttrium oxide powder is 0.2:0.8, and the layer thickness is 25 μm.

[0051] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0052] Example 7: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (1), graphite powder with a particle size of 1 μm is used instead of graphite powder with a particle size of 10 μm.

[0053] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0054] Example 8: This example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that in step (1), graphite powder with a particle size of 20 μm is used instead of graphite powder with a particle size of 10 μm.

[0055] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0056] Comparative Example 1: This comparative example relates to the preparation of a yttrium oxide protective coating for a graphite substrate. The only difference from Example 1 is that a 200 μm yttrium oxide layer is directly sprayed onto the surface of the graphite substrate after the treatment in step (1). The spraying process is the same as in Example 1, and a 200 μm thick yttrium oxide protective coating is formed on the graphite substrate.

[0057] Comparative Example 2: This comparative example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate, which differs from Example 1 only in that no transition layer is prepared.

[0058] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 300 μm formed on the graphite substrate.

[0059] Comparative Example 3: This comparative example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that pure silicon carbide powder is used instead of mixed powder. All other operations are the same, and a yttrium oxide gradient protective coating with a total thickness of 400 μm is formed on the graphite substrate.

[0060] Comparative Example 4: This comparative example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that the first feeding system delivers pure silicon carbide powder, and the second feeding system delivers a mixed powder prepared by mixing yttrium oxide powder and graphite powder at a mass ratio of 1:0.25.

[0061] All other operations were the same, resulting in a yttrium oxide gradient protective coating with a total thickness of 400 μm formed on the graphite substrate.

[0062] Comparative Example 5: This comparative example relates to the preparation of a yttrium oxide gradient protective coating for a graphite substrate. The only difference from Example 1 is that the second feeding system is used to transport the yttrium oxide precursor solution, and the coating is prepared using APS+solution precursor plasma spraying (SPPS) technology. The specific operation is as follows: (1) The isostatic graphite substrate was processed into a 100 mm × 100 mm × 10 mm sample. The surface was cleaned with acetone, anhydrous ethanol and deionized water in sequence. After drying, it was roughened by sandblasting with corundum sand. After sandblasting, the dust was cleaned again. The substrate was preheated to 300 ℃ before spraying.

[0063] Silicon carbide powder (particle size 20 μm) was lightly sieved through a 200-mesh sieve, and graphite powder (particle size 10 μm) was lightly sieved through a 325-mesh sieve to remove agglomerated particles and improve powder dispersibility. Subsequently, the powder was premixed and mechanically mixed at a mass ratio of 1:0.25 to obtain a uniformly dispersed and free-flowing mixed powder for later use.

[0064] Preparation of yttrium oxide precursor solution: Weigh yttrium nitrate hexahydrate and dissolve it in a mixed solvent of deionized water and anhydrous ethanol, wherein the volume ratio of deionized water to anhydrous ethanol is 7:3; control Y 3+ The total ion concentration was 0.5 mol / L, and the solution was magnetically stirred at room temperature for 2 h to obtain a clear and transparent precursor solution. Then, it was filtered through a 0.45 μm filter membrane to remove a small amount of insoluble matter and particulate impurities, and the yttrium oxide precursor solution was prepared for use.

[0065] (2) Using an atmospheric plasma spraying equipment, the mixed powder obtained in step (2) is transported through a powder feeding system and a silicon carbide layer with a thickness of about 100 μm is deposited on the surface of the preheated graphite substrate. The spraying parameters of this step are the same as those in Example 1.

[0066] (3) On the surface of the formed silicon carbide layer, three transition layers are deposited sequentially using an APS+SPPS combination method. The silicon carbide / graphite mixed powder is fed by the APS powder feeding system, and the yttrium oxide component is fed by the SPPS liquid injection system (during the SPPS process, the precursor solution is axially injected into the plasma flame through the injection pump. The droplets undergo atomization, solvent evaporation, precursor thermal decomposition, and in-situ generation of yttrium oxide particles in the high-temperature plasma flame before being deposited on the substrate surface). The spraying parameters for the SPPS stage are set as follows: the spraying current is 500 A, the spraying voltage is 55 V, the spray gun power is about 27.5 kW, the main gas Ar flow rate is 45 L / min, the auxiliary gas H2 flow rate is 8 L / min, the liquid feed rate is 20 mL / min, the spraying distance is 70 mm, and the spray gun moving speed is 400 mm / s.

[0067] By adjusting two feed rates to control the relative proportions of the mixed powder component and the yttrium oxide component in each layer, a first transition layer, a second transition layer, and a third transition layer, each with a thickness of approximately 33 μm, were prepared sequentially. Their specific compositions are as follows: First transition layer: The mass ratio of the mixed powder to the yttrium oxide component is 3:1; Second transition layer: The mass ratio of the mixed powder to the yttrium oxide component is 1:1; The third transition layer has a mass ratio of 1:3 between the mixed powder and the yttrium oxide component.

[0068] (4) After the third transition layer deposition is completed, stop the APS powder feeding for mixing powders, retain the yttrium oxide precursor solution feed, and continue to deposit a pure yttrium oxide layer on the surface of the third transition layer using the SPPS method. The layer thickness is about 200 μm. During the deposition of the transition layer and the yttrium oxide layer, pause for 10-20 s after each few sprays to avoid local overheating of the substrate.

[0069] (5) After the spraying is completed, the sample is placed in an air atmosphere and kept at 600 °C for 1 h to promote the further decomposition of the residual precursor and stabilize the coating structure. Then, it is cooled to room temperature with the furnace to form a gradient protective coating with a total thickness of about 400 μm on the graphite substrate surface.

[0070] The gradient protective coating prepared by the above method still has the same layered gradient structure as in Example 1. However, since the yttrium oxide-related components are deposited by SPPS, the transition layer and the surface yttrium oxide region are more likely to form columnar or feather-like stacking structures. There are relatively more fine pores and microcrack channels in the layer, and the droplet evaporation and precursor decomposition process will reduce the effective deposition efficiency.

[0071] Performance testing: The interfacial bonding strength and thermal cycling stability between the protective coatings and the graphite substrate prepared in the above embodiments and comparative examples were tested using the following methods: Interfacial bonding strength test: The interfacial bonding strength of each sample was tested according to ISO 14916:2017 to compare the adhesion performance of different structure coatings on graphite substrate.

[0072] Thermal cycling stability test: The sample was subjected to 50 temperature cycles between room temperature and 1000 ℃. The surface of the coating was observed for cracks, local peeling or delamination, and its thermal shock resistance was evaluated.

[0073] The test results are shown in the table below:

[0074] As shown in the table above, the yttrium oxide gradient protective coatings (Examples 1-8) prepared by introducing an appropriate amount of graphite powder as a compensating carbon source into silicon carbide powder and using atmospheric plasma spraying technology exhibit high interfacial bonding strength and excellent thermal cycling stability, far superior to Comparative Example 1, which directly prepares yttrium oxide coatings on the graphite substrate surface. Furthermore, Examples 1-8 show that the amount of graphite powder added, its particle size, and the number of transition layers all affect the interfacial bonding strength and thermal cycling stability of the prepared protective coatings. When the amount of graphite powder added is 25% of the mass of silicon carbide, the particle size is suitable, and the number of transition layers is 3, the prepared protective coating exhibits the best interfacial bonding strength and thermal cycling stability.

[0075] Furthermore, as shown in Example 1 and Comparative Examples 1 and 2, while the introduction of a silicon carbide layer can improve the interfacial adhesion and thermal cycling stability of the protective coating to some extent, the improvement effect is limited. However, by introducing a suitable number of transition layers between the silicon carbide layer and the yttrium oxide layer, this invention can further significantly improve the interfacial adhesion and thermal cycling stability of the protective coating. Compared to Example 1, the protective coating prepared without the introduction of graphite powder (Comparative Example 3) showed a significant decrease in both interfacial adhesion strength and thermal cycling stability. This further illustrates that the introduction of graphite powder plays a crucial role in the interfacial adhesion strength and thermal cycling stability of the protective coating prepared by this invention.

[0076] As can be seen from Examples 1 and Comparative Examples 4 and 5, the interfacial bonding strength and thermal cycling stability of the protective coating prepared by adding graphite powder to yttrium oxide powder are significantly worse than those of the protective coating prepared by adding graphite powder to silicon carbide powder. The protective coating prepared by APS+SPPS technology (Comparative Example 5) not only has low deposition efficiency, but also the overall density, interfacial shielding ability and thermal cycling stability of the prepared coating are inferior to those of the gradient protective coating prepared by the full APS process in Example 1.

[0077] The embodiments described above are merely preferred examples to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A yttrium oxide gradient protective coating for a graphite substrate, characterized in that, The yttrium oxide gradient protective coating comprises a silicon carbide layer, a transition layer, and a yttrium oxide layer sequentially stacked on a graphite substrate; The silicon carbide layer is formed by atmospheric plasma spraying of silicon carbide powder and graphite powder; the transition layer is formed by atmospheric plasma spraying of silicon carbide powder, graphite powder and yttrium oxide powder; and the yttrium oxide layer is formed by atmospheric plasma spraying of yttrium oxide powder. Furthermore, along the direction from the silicon carbide layer to the yttrium oxide layer, the content of silicon carbide and graphite in the yttrium oxide gradient protective coating gradually decreases.

2. The yttrium oxide gradient protective coating for a graphite substrate according to claim 1, characterized in that, The transition layer includes a first transition layer, a second transition layer, and a third transition layer stacked sequentially. The first transition layer is bonded to the silicon carbide layer, and the third transition layer is bonded to the yttrium oxide layer.

3. The yttrium oxide gradient protective coating for a graphite substrate according to claim 2, characterized in that, The ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the first transition layer is 3:1, and the mass ratio of silicon carbide powder to graphite powder is 1:(0.2-0.3). The ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the second transition layer is 1:1, and the mass ratio of silicon carbide powder to graphite powder is 1:(0.2-0.3). The ratio of the total mass of silicon carbide powder and graphite powder to the mass of yttrium oxide powder in the raw materials for preparing the third transition layer is 1:3, and the mass ratio of silicon carbide powder to graphite powder is 1:(0.2-0.3).

4. The yttrium oxide gradient protective coating for a graphite substrate according to claim 1, characterized in that, The mass ratio of silicon carbide powder to graphite powder in the raw materials for preparing the silicon carbide layer is 1:(0.2-0.3).

5. The yttrium oxide gradient protective coating for a graphite substrate according to claim 1, characterized in that, The thickness of the yttrium oxide gradient protective coating is 400-450 μm; And / or, the thickness of the silicon carbide layer is 50-150 μm; And / or, the thickness of the transition layer is 50-150 μm; And / or, the thickness of the yttrium oxide layer is 200-250 μm.

6. The yttrium oxide gradient protective coating for a graphite substrate according to claim 1, characterized in that, The particle size of the silicon carbide powder is 20-45 μm; And / or, the particle size of the graphite powder is 5-15 μm; And / or, the particle size of the yttrium oxide powder is 25-45 μm.

7. A method for preparing a yttrium oxide gradient protective coating for a graphite substrate as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Mix silicon carbide powder and graphite powder evenly to obtain a mixed powder; S2. An atmospheric plasma spraying equipment equipped with a dual feeding system is adopted, wherein the first feeding system is used to transport the mixed powder, the second feeding system is used to transport yttrium oxide powder, and the spraying parameters are set. S3. First, the mixed powder is conveyed through the first feeding system and then deposited on the surface of the preheated graphite substrate by atmospheric plasma spraying to form a silicon carbide layer. S4. Adjust the powder feeding amount of each feeding system in the dual feeding system according to the preset mass ratio, and simultaneously convey the mixed powder and yttrium oxide powder. Then, deposit the transition layer on the surface of the silicon carbide layer away from the graphite substrate by atmospheric plasma spraying. S5. Finally, yttrium oxide powder is fed through the second feeding system and deposited on the surface of the transition layer away from the silicon carbide layer by atmospheric plasma spraying to form a yttrium oxide layer, thus obtaining the yttrium oxide gradient protective coating.

8. The preparation method according to claim 7, characterized in that, In step S2, the spraying parameters are as follows: spraying current is 450-650 A, spraying voltage is 50-75 V, spray gun power is 25-45 kW, main gas is argon, main gas flow rate is 30-50 L / min, auxiliary gas is hydrogen and / or helium, auxiliary gas flow rate is 5-15 L / min, spraying distance is 90-100 mm, spray gun moving speed is 300-600 mm / s, and substrate preheating temperature is 280-320 ℃. And / or, the powder feeding air flow rate of the first feeding system is 3-8 L / min; And / or, the powder feeding air flow rate of the second feeding system is 3-8 L / min.

9. The preparation method according to claim 7, characterized in that, In step S3, the graphite substrate is a pretreated graphite substrate, and the pretreatment includes the steps of cleaning and sandblasting the graphite substrate.

10. The application of a yttrium oxide gradient protective coating for a graphite substrate as described in any one of claims 1-6, or a yttrium oxide gradient protective coating for a graphite substrate prepared by the preparation method as described in any one of claims 7-9, in the preparation of high-temperature tooling components for the photovoltaic or semiconductor industry.

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