Graphite crucible silicon infiltration resistant coating based on gradient boron nitride-silicon carbide composite structure and preparation method thereof

Abstract

The invention discloses a graphite crucible silicon infiltration resistant coating based on a gradient boron nitride-silicon carbide composite structure and a preparation method thereof. The coating is composed of hexagonal boron nitride nanosheets, beta-type silicon carbide nanowires, yttrium aluminum garnet precursor sol and the like, and effective blocking of 1500-1600 DEG C silicon melt is achieved through gradient structural design and an in-situ self-repairing mechanism. The preparation process comprises the steps of h-BN activation, S C carbon layer deposition, slurry mixing and spraying and segmented heat treatment. The molten silicon contact angle of the coating reaches 158 + / -2 degrees at the temperature of 1550 DEG C, through cracks do not exist after 50 times of thermal shock circulation, the coating is effective after being continuously used for 48 batches under the industrial working condition, the problems of silicon infiltration interface adhesion and thermal shock failure of a traditional graphite crucible are solved, and the coating has remarkable industrial application value.

Description

Technical Field

[0001] The present invention relates to the technical field of high-temperature composite material manufacturing, and in particular to a graphite crucible anti-silicon infiltration coating based on a gradient boron nitride-silicon carbide composite structure and a preparation method thereof. Background Art

[0002] In the field of high-temperature preparation of carbon-ceramic composite materials, graphite crucibles, as the core supporting device of the silicon infiltration process, have long faced the common problems of molten silicon erosion and interface adhesion. When a carbon-carbon preform coated with silicon powder is placed in a graphite tooling for a high-temperature reaction, the excess silicon melt forms a chemical bond with the graphite matrix during the cooling and solidification stage. This strong interfacial bonding force causes the disassembly process to be accompanied by tearing damage to the graphite surface. Even more serious is that there is a significant mismatch between the thermal expansion characteristics of the silicon-carbon reaction products and the graphite matrix. Under the action of periodic thermal loads, the microcracks initiated at the interface gradually expand into through-going cracks, eventually causing layered exfoliation failure of the tooling structure. This failure mode forces the graphite crucible to be scrapped after a short service cycle, becoming a prominent bottleneck restricting large-scale production.

[0003] Traditional protective technologies often focus on physical isolation or chemical passivation strategies. The former uses carbon-based coatings to form a dense barrier layer. While this can temporarily slow the penetration of molten silicon, the rigid coating is prone to brittle cracking under thermal shock conditions, accelerating the preferential diffusion of the melt along the cracks. The latter relies on vapor deposition to produce a ceramic protective film. While exhibiting excellent thermal stability, it is limited by the complex vacuum deposition process and high equipment investment, and is more likely to damage the intrinsic structure of the graphite substrate due to high-energy particle bombardment. Such technologies, either due to insufficient protective effectiveness or lack of economic viability, struggle to meet the stringent requirements of industrial production for the long-term service life of tooling.

[0004] In the existing technology, some studies have attempted to improve the anti-silicon infiltration performance of graphite crucibles by coating with a single material. For example, when using silicon carbide coating, due to the large difference in thermal expansion coefficient between silicon carbide and graphite (the thermal expansion coefficient of silicon carbide is about 4.5×10 -6 / K, graphite is about 2.7×10 -6 / K), microcracks are easily generated in the coating during temperature cycles above 1500°C, causing silicon melt to penetrate along the cracks. While boron nitride coating has good resistance to molten metal corrosion, its bonding with the graphite substrate is weak, making it prone to coating shedding after multiple thermal shocks. Summary of the Invention

[0005] In response to the above technical problems, the present application provides a composite coating system based on the principles of solution chemistry and structural adaptation. This technology constructs a gradient protective layer on the graphite surface through a conventional spraying process, cleverly utilizes the in-situ phase change behavior of the material components, and spontaneously forms an interface transition zone with a gradual change in thermal expansion coefficient during the heat treatment process. The surface layer of the coating is constructed with graphite-like layered materials to form a silicon-phobic barrier, the middle layer maintains the high-temperature structural integrity through a nanowire reinforcement mechanism, and the bottom layer is achieved by stress buffering coupling with the graphite matrix with the help of carbon phase reconstruction. What is more important is that the coating can trigger self-repairing behavior during service, and dynamically repair micro-damage through the controlled reaction of active components and silicon melt, thereby giving the graphite crucible extraordinary thermal shock fatigue resistance while simplifying the preparation process.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted in this application is: a graphite crucible anti-silicon infiltration coating based on a gradient boron nitride-silicon carbide composite structure, comprising the following components in mass percentage:

[0007] Hexagonal boron nitride (h-BN) nanosheets: 38-45%, lateral size 0.5-3 μm, thickness ≤ 50 nm, surface oxygen content < 5 at%;

[0008] β-SiC nanowires: 15-20%, diameter 80-150nm, aspect ratio > 50, and surface coated with a 2-3nm amorphous carbon layer;

[0009] Yttrium aluminum garnet (YAG) precursor sol: 25-30%, wherein the molar ratio of yttrium (Y) to aluminum (Al) is 3:5, and the solid content is 40%;

[0010] Polyvinylpyrrolidone dispersant: 1.5-2.5%;

[0011] Deionized water: balance.

[0012] In order to better realize the present invention, further, the hexagonal boron nitride (h-BN) nanosheets are treated with hydrofluoric acid etching, and the etching conditions are 200 ° C. in a hydrofluoric acid steam reactor for 30 minutes to form activated hexagonal boron nitride (h-BN) nanosheets.

[0013] In order to better implement the present invention, further, the solvent of the yttrium aluminum garnet (YAG) precursor sol is an ethanol / acetylacetone mixed solution with a volume ratio of 1:1-2:1.

[0014] In order to better implement the present invention, further, the amorphous carbon layer on the surface of the β-type silicon carbide nanowire is deposited at 800° C. by introducing an ethylene / argon mixed gas with a volume ratio of 1:9 into a tube furnace.

[0015] In order to better realize the present invention, the coating further has a gradient structure, and the relative content of hexagonal boron nitride (h-BN) nanosheets gradually decreases from the coating surface to the interface with the graphite crucible substrate, and the relative content of the composite phase of β-type silicon carbide nanowires and yttrium aluminum garnet (YAG) gradually increases.

[0016] In order to solve the above technical problems, the present invention also provides a method for preparing the above-mentioned graphite crucible anti-silicon infiltration coating, comprising the following steps:

[0017] Step 1: Treat hexagonal boron nitride (h-BN) nanosheets in a hydrofluoric acid vapor reactor at 200°C for 30 minutes;

[0018] Step 2: introducing ethylene / argon mixed gas into the β-SiC nanowires in a tube furnace and depositing a carbon layer at 800°C to form carbon-coated β-SiC nanowires;

[0019] Step 3: Mix the YAG precursor sol with the polyvinyl pyrrolidone dispersant and pre-disperse it at a magnetic stirring speed of 600 rpm for 1 hour;

[0020] Step 4: Add activated hexagonal boron nitride (h-BN) nanosheets and carbon-coated β-silicon carbide nanowires stepwise, using zirconia balls with a diameter of 5-10 mm, and mix at 350 rpm in a planetary ball mill for 4 hours to form a uniform and stable suspension, i.e., a slurry;

[0021] Step 5: Adjust the slurry viscosity to 120-150 cP by adding deionized water or ethanol / acetylacetone mixture, and use a compressed air spray gun to evenly coat the slurry on the inner wall of the graphite crucible at a spray pressure of 0.3 MPa;

[0022] Step 6: First, dry at 80°C for 2 hours to remove the solvent; then, under argon protection, heat to 1450°C at a heating rate of 10°C / min and keep warm for 1 hour to crystallize the yttrium aluminum garnet precursor sol; finally, introduce 3 vol% methane / argon mixture and perform carbon-based interface repair at 1350°C for 30 minutes.

[0023] In order to better implement the present invention, further, in step 5, the spraying distance between the compressed air spray gun and the inner wall of the graphite crucible is 15-20 cm.

[0024] In order to better implement the present invention, further, in step 6, the drying process is carried out in a blast drying oven, and the purity of the argon gas is ≥99.99%.

[0025] In order to better realize the present invention, further, in step 6, before introducing methane / argon mixed gas for carbon-based interface repair, the pressure in the furnace is first reduced to 10 -3Pa, and then introduce methane / argon mixed gas.

[0026] The technical solution provided by the present invention has the following beneficial effects compared with the prior art:

[0027] 1. In this invention, the molten silicon contact angle: 158±2° at a steady state of 1550°C, effectively blocking the infiltration of molten silicon. Thermal shock cycle performance: After 50 cycles of rapid cooling and heating at 1550°C, no through-cracks greater than 5μm were observed.

[0028] 2. In the present invention, effective protection is maintained after 48 batches of continuous use in an industrial-grade silicon infiltration furnace, which is significantly improved compared to the life of uncoated graphite crucible (average service life of 9 batches).

[0029] 3. The high content of hexagonal boron nitride (h-BN) nanosheets in the coating forms a lotus leaf-like silicophore structure on the surface. Tests have shown that the contact angle with molten silicon at 1550°C reaches 158±2°, far exceeding that of traditional protective coatings. This makes it difficult for molten silicon to infiltrate and penetrate the coating, effectively blocking direct contact between the molten silicon and the graphite crucible, preventing damage to the graphite substrate caused by the silicon-carbon reaction, and significantly reducing the depth of silicon penetration into the graphite crucible.

[0030] 4. In the present invention, the gradient structural design makes the thermal expansion coefficient of the coating gradually change from the surface layer to the inner layer, effectively alleviating the thermal stress caused by temperature changes. The yttrium aluminum garnet (YAG) skeleton reinforced by β-type silicon carbide nanowires is interspersed, giving the coating good mechanical properties, and the bending strength is increased to 320MPa. In the thermal shock cycle test, the rapid cooling and heating method was adopted. The coating sample was kept at 1550℃ for 15 minutes and then quickly immersed in room temperature silicone oil for cooling. After 50 cycles, no through cracks greater than 5μm appeared, which is significantly better than traditional single-material coatings and can maintain structural integrity under high-temperature cycling conditions.

[0031] 5. In the present invention, actual operating conditions have shown that in an industrial-grade silicon infiltration furnace, with a loading capacity of 8 carbon-carbon preforms per batch, process parameters of 1520°C × 6 hours, and argon protection, the coating of the present invention maintained effective protection after 48 continuous batches of use, while the average service life of an uncoated graphite crucible was only 9 batches. This is because the YAG phase in the coating reacts with trace amounts of SiO2 during the periodic silicon infiltration process to form an yttrium silicate glass phase (Y2Si2O7), which can dynamically fill microcracks with a repair efficiency of 82%, achieving in-situ self-repair, thereby greatly extending the service life of the graphite crucible and reducing production costs.

[0032] 6. The coating can be prepared using a conventional spray coating process, eliminating the need for complex vacuum deposition equipment and requiring only standard spray coating equipment, thus lowering the technical barrier to industrial application. Furthermore, the raw materials, such as common hexagonal boron nitride nanosheets and β-silicon carbide nanowires, are relatively low-cost. Furthermore, the preparation process involves simple pretreatment and mixing of the raw materials, reducing process steps and energy consumption, thus demonstrating excellent economic efficiency and promising industrialization prospects.

[0033] 7. In the present invention, the nanocarbon film produced by methane cracking in the bottom layer of the coating effectively compensates for the thermal expansion coefficient mismatch between the coating and the graphite substrate, and the thermal expansion coefficient gradient is from 5.2 to 4.1×10 -6 / K forms a strong bond between the coating and the inner wall of the graphite crucible, avoiding peeling of the coating during use and further improving the protective reliability of the coating.

[0034] 8. In this invention, by precisely controlling the mass percentages of the coating components and the preparation process parameters, the coating composition and structure can be flexibly adjusted to meet the requirements for the graphite crucible's resistance to silicon infiltration under different operating conditions. In particular, adjusting the ratio of h-BN nanosheets and β-silicon carbide nanowires can optimize the coating's silicophobicity and mechanical properties; while varying the heat treatment regime can regulate the degree of crystallization and self-healing properties of the YAG phase. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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 those skilled in the art, other drawings can be obtained based on these drawings without paying any creative labor.

[0036] Figure 1 It is a process flow chart of the preparation method of the present invention. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Generally, the components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0038] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for protection, but merely represents selected embodiments of the present application. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments in the present application without making any creative efforts shall fall within the scope of protection of the present application.

[0039] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0040] In the description of this application, it should be noted that if the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. appear, the orientation or position relationship indicated is based on the orientation or position relationship shown in the accompanying drawings, or the orientation or position relationship in which the product of the application is usually placed when in use. It is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, it cannot be understood as a limitation on this application. In addition, if the terms "first", "second", etc. appear in the description of this application, they are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0041] Furthermore, the use of terms such as "horizontal" and "vertical" in the description of this application does not necessarily imply that a component must be absolutely horizontal or suspended, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical" and does not mean that the structure must be completely horizontal, but rather that it can be slightly tilted.

[0042] It should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood broadly. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. A person of ordinary skill in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0043] The present invention discloses a graphite crucible anti-silicon infiltration coating based on a gradient boron nitride-silicon carbide composite structure, comprising the following components in percentage by mass:

[0044] Hexagonal boron nitride (h-BN) nanosheets: 38-45%, lateral size 0.5-3 μm, thickness ≤ 50 nm, surface oxygen content < 5 at%;

[0045] β-SiC nanowires: 15-20%, diameter 80-150nm, aspect ratio > 50, and surface coated with a 2-3nm amorphous carbon layer;

[0046] Yttrium aluminum garnet (YAG) precursor sol: 25-30%, wherein the molar ratio of yttrium (Y) to aluminum (Al) is 3:5, and the solid content is 40%;

[0047] Polyvinylpyrrolidone dispersant: 1.5-2.5%;

[0048] Deionized water: balance.

[0049] The hexagonal boron nitride (h-BN) nanosheets are etched with hydrofluoric acid at 200° C. in a hydrofluoric acid steam reactor for 30 minutes to form activated hexagonal boron nitride (h-BN) nanosheets.

[0050] The solvent of the yttrium aluminum garnet (YAG) precursor sol is an ethanol / acetylacetone mixed solution with a volume ratio of 1:1-2:1.

[0051] The amorphous carbon layer on the surface of the β-type silicon carbide nanowire is obtained by introducing an ethylene / argon mixed gas with a volume ratio of 1:9 into a tube furnace and depositing it at 800°C.

[0052] The coating has a gradient structure. From the coating surface to the interface with the graphite crucible substrate, the relative content of hexagonal boron nitride (h-BN) nanosheets gradually decreases, and the relative content of the composite phase of beta-type silicon carbide nanowires and yttrium aluminum garnet (YAG) gradually increases.

[0053] The following steps are involved:

[0054] Step 1: Treat hexagonal boron nitride (h-BN) nanosheets in a hydrofluoric acid vapor reactor at 200°C for 30 minutes;

[0055] Step 2: introducing ethylene / argon mixed gas into the β-SiC nanowires in a tube furnace and depositing a carbon layer at 800°C to form carbon-coated β-SiC nanowires;

[0056] Step 3: Mix the YAG precursor sol with the polyvinyl pyrrolidone dispersant and pre-disperse it at a magnetic stirring speed of 600 rpm for 1 hour;

[0057] Step 4: Add activated hexagonal boron nitride (h-BN) nanosheets and carbon-coated β-silicon carbide nanowires stepwise, using zirconia balls with a diameter of 5-10 mm, and mix at 350 rpm in a planetary ball mill for 4 hours to form a uniform and stable suspension, i.e., a slurry;

[0058] Step 5: Adjust the slurry viscosity to 120-150 cP by adding deionized water or ethanol / acetylacetone mixture, and use a compressed air spray gun to evenly coat the slurry on the inner wall of the graphite crucible at a spray pressure of 0.3 MPa;

[0059] Step 6: First, dry at 80°C for 2 hours to remove the solvent; then, under argon protection, heat to 1450°C at a heating rate of 10°C / min and keep warm for 1 hour to crystallize the yttrium aluminum garnet precursor sol; finally, introduce 3 vol% methane / argon mixture and perform carbon-based interface repair at 1350°C for 30 minutes.

[0060] The spraying distance between the compressed air spray gun and the inner wall of the graphite crucible is 15-20 cm.

[0061] The drying process is carried out in a blast drying oven, and the purity of argon is ≥99.99%.

[0062] Before introducing methane / argon mixed gas for carbon-based interface repair, reduce the pressure in the furnace to 10 -3 Pa, and then introduce methane / argon mixed gas.

[0063] The specific analysis is as follows:

[0064] The invention discloses a silicon carbide (SiC) single-layer coating for resisting silicon penetration of a graphite crucible, adopts a spraying process, and has a sintering temperature of 1200°C.

[0065] In a 1500°C silicon infiltration environment, the coating showed through cracks after 10 thermal cycles, with an average service life of 9 batches and a silicon contact angle of 120°.

[0066] Comparison between the present invention and the prior art:

[0067]

[0068] Bottlenecks of traditional technology: The existing single coating cannot solve the three major problems of "silicon melt adhesion", "thermal shock cracking" and "weak interface bonding" at the same time.

[0069] The existing SiC coating has a thermal expansion coefficient that does not match that of graphite (SiC is 4.5×10 -6 / K, graphite is 2.7×10 -6 / K), cracks will inevitably occur during the 1500℃ cycle, leading to silicon penetration.

[0070] The present invention constructs a synergistic protection system of "silicon repellency-anti-cracking-buffering" through a multi-dimensional design of "gradient structure + surface modification + self-repair", achieving long-term protection in a wide temperature range (1500-1600℃).

[0071] Those skilled in the art generally believe that "anti-siliconization = high hardness material stacking". The existing technology emphasizes the wear resistance of SiC, but ignores the problem of thermal stress. However, the present invention takes the opposite approach and uses h-BN (thermal expansion coefficient 2.0×10 -6 / K) and SiC-YAG gradient ratio, so that the thermal expansion coefficient increases from 5.2 to 4.1×10 -6 / K gradient, this "soft-hard" gradient design has no precedent in the existing technology.

[0072] Generally, when making improvements in existing technologies, people usually consider "increasing the SiC thickness" or "adding second-phase particles." However, it is impossible to conclude through logical reasoning that "reducing the surface hard phase (SiC) and increasing the soft phase (h-BN)" can actually improve thermal shock resistance, because this contradicts the conventional perception that "hard materials are more wear-resistant."

[0073] Synergistic effect of h-BN activation and SiC coating, h-BN etching: In the prior art, hydrofluoric acid treatment is usually used to clean ceramic surfaces (HF is disclosed in the prior art to clean SiC), but it has never been used for interface activation of h-BN nanosheets. The present invention removes the B2O3 oxide layer on the h-BN surface by etching, so that the surface energy is increased from 0.5 J / m 2 Increased to 1.2J / m 2 .

[0074] SiC carbon coating: In the existing technology, carbon coating is mostly used for nanowire anti-oxidation (such as C-coated SiC for lithium batteries), but the present invention innovatively applies it to "silicon infiltration protection". The 2-3nm carbon layer just forms a "sacrificial layer" and preferentially reacts with silicon to generate SiC at 1500℃, protecting the internal SiC nanowires. This "active protection" mechanism exceeds conventional expectations in the field.

[0075] In the present invention, the nanocarbon film (thickness 5-10nm) generated by methane cracking at 1350°C acts like an "elastic gasket", filling the micro gaps between the coating and graphite, thereby increasing the interface bonding strength from 12MPa to 35MPa. This "in-situ growth buffer layer" method has never been tried in the prior art, and the effect far exceeds expectations (the bonding strength of the SiC transition layer in the prior art is only 18MPa).

[0076] The qualitative changes in the performance indicators of the present invention are shown in the following table:

[0077]

[0078] Single material limit: When h-BN is used alone, it will fall off after 20 thermal shocks; when the SiC-YAG composite layer is used alone, the silicon contact angle is only 130°.

[0079] The combined effect of the present invention is that the gradient structure makes the silipophobia of h-BN and the strength of SiC-YAG complement each other, while methane repairs and fills microcracks. The combination of the three produces a "1+1+1>3" effect - the contact angle increases by 21.5%, and the number of thermal shocks increases by 2.5 times. This synergistic effect cannot be derived using existing technologies.

[0080] The gradient structure is completely different from the traditional "single material stacking" idea, and the combination of various technical features (h-BN activation, SiC coating, methane repair) is not a simple superposition, but a synergistic enhancement is achieved through precise proportioning and process control; breaking the inherent cognition of "high hardness = high protection", achieving performance transcendence through material function gradient, and providing a new technical path for high-temperature protective coatings.

[0081] Example 1

[0082] Raw material pretreatment

[0083] Activation of h-BN nanosheets: 40 g of h-BN nanosheets (lateral size 0.5-3 μm, thickness ≤ 50 nm) were placed in a hydrofluoric acid vapor reactor and treated with HF vapor (pressure 0.1 MPa) at 200°C for 30 minutes to remove the surface B2O3 oxide layer and obtain activated h-BN (surface oxygen content < 5 at%).

[0084] Carbon coating of SiC nanowires: 18 g of β-type SiC nanowires (diameter 80-150 nm, aspect ratio > 50) were placed in a tube furnace and introduced with an ethylene / argon mixture with a volume ratio of 1:9 (total flow rate 500 mL / min). The mixture was deposited at 800°C for 2 hours to form a 2-3 nm amorphous carbon coating.

[0085] Slurry preparation

[0086] 28 g of YAG precursor sol (Y:Al molar ratio 3:5, solid content 40%, solvent ethanol / acetylacetone = 1:1, volume ratio) was mixed with 2 g of polyvinyl pyrrolidone dispersant and pre-dispersed under magnetic stirring at 600 rpm for 1 hour.

[0087] Activated h-BN and carbon-coated SiC nanowires were added stepwise, 50 g of deionized water was added to adjust the viscosity, and the mixture was mixed in a planetary ball mill at 350 rpm using 8 mm zirconia balls for 4 hours to form a uniform slurry (viscosity 130 cP).

[0088] Coating preparation

[0089] The slurry was evenly sprayed on the inner wall of the graphite crucible using a compressed air spray gun (spraying pressure 0.3 MPa, spraying distance 18 cm), with the thickness controlled at 50-80 μm.

[0090] After spraying, it was dried in a forced air drying oven at 80°C for 2 hours, and then under the protection of high-purity argon (purity ≥99.99%), the temperature was raised to 1450°C at 10°C / min and kept for 1 hour to allow the YAG sol to crystallize; finally, the pressure in the furnace was reduced to 10Pa, and a 3vol% methane / argon mixture was introduced, and repaired at 1350°C for 30 minutes to form a carbon-based interface layer.

[0091] Performance Testing

[0092] Contact angle of molten silicon: tested in a high-temperature furnace at 1550°C, the contact angle is 158°, and the molten silicon is completely non-wetting.

[0093] Thermal shock cycle: After 50 cycles of rapid cooling and heating at 1550℃ (after keeping warm for 15 minutes, immerse in room temperature silicone oil), the coating has no through cracks.

[0094] Industrial application: After 48 batches of continuous use in a 1520℃ silicon infiltration furnace, the coating remained intact and the graphite substrate was not damaged by silicon penetration.

[0095] Example 2

[0096] Composition gradient design

[0097] Surface layer: h-BN nanosheets 45%, SiC nanowires 15%, YAG sol 25%, dispersant 2%, and deionized water as the balance (silicon-repellent layer).

[0098] Middle layer: h-BN nanosheets 38%, SiC nanowires 20%, YAG sol 30%, dispersant 2%, and deionized water as the balance (reinforcement layer).

[0099] Interface layer: h-BN nanosheets 30%, SiC nanowires 20%, YAG sol 35%, dispersant 2%, and deionized water balance (buffer layer).

[0100] Preparation process

[0101] The layered spraying method is adopted: first spray the interface layer slurry (viscosity 150cP), then spray the intermediate layer (viscosity 130cP) after drying, and finally spray the surface layer (viscosity 120cP), with a total thickness of 80-100μm. The heat treatment process is the same as that in Example 1. After methane repair, the thermal expansion coefficient of the coating is increased from 5.2×10 -6 / K gradient to inner layer 4.1×10 -6 / K.

[0102] Performance Comparison

[0103] Thermal stress test: Flexural strength reaches 320MPa, a 60% increase compared to a single SiC coating (200MPa).

[0104] Interface bonding strength: Pull-out tests showed a bonding strength of 35 MPa, a 40% increase compared to the non-gradient coating (25 MPa).

[0105] Example 3

[0106] Comparative experiment

[0107] Experimental group: h-BN nanosheets were treated with HF vapor at 200° C. for 30 minutes (same as Example 1).

[0108] Control group: untreated h-BN nanosheets (surface oxygen content 12 at%).

[0109] Slurry properties

[0110] The experimental group slurry had better dispersibility and no precipitation after standing for 24 hours; the h-BN in the control group was severely agglomerated and required additional dispersant.

[0111] Coating effect

[0112] The contact angle of molten silicon in the experimental group was 158°, while that in the control group was only 140°, and the silicon repellency decreased by 11%.

[0113] Thermal shock cycle number: The experimental group had no cracks after 50 cycles, while the control group had through cracks after 30 cycles, indicating that the activation treatment significantly improved the interface bonding stability.

[0114] Comparative Example 1: Single SiC Coating (Prior Art)

[0115] Preparation method

[0116] 40% SiC particles (particle size 1-5 μm), 30% silica sol, 25% deionized water, and 5% dispersant are mixed, sprayed, and sintered at 1200° C. for 2 hours.

[0117] Performance flaws

[0118] Molten silicon contact angle: 120°, slight wetting of molten silicon.

[0119] Thermal shock cycle: Through cracks appeared after 10 cycles at 1500℃.

[0120] Industrial life: Failure after an average of 9 batches of use, the silicon penetration depth reaches 0.5mm, and the graphite matrix is ​​severely damaged by tearing.

[0121] Comparative Example 2: Non-gradient h-BN / SiC composite coating

[0122] Preparation method

[0123] 40% of h-BN nanosheets, 20% of SiC nanowires, 30% of YAG sol and 2% of dispersant were uniformly mixed and sprayed without gradient. The heat treatment was the same as that in Example 1.

[0124] Performance Comparison

[0125] Thermal shock cycle: Micro cracks appeared after 30 cycles at 1550°C, which is 2 times better than that of a single SiC coating, but still inferior to the gradient structure (no cracks after 50 cycles).

[0126] Interface bonding strength: 22MPa, 37% lower than that of gradient coating (35MPa), and the coating edge is easy to peel off.

[0127] Effect analysis of implementation examples

[0128] The synergistic effect of the gradient structure: the surface h-BN is silicophore-repellent, the middle layer is SiC-YAG reinforced, and the interface layer is a carbon film buffer. The three achieve a gradual change in the thermal expansion coefficient through the composition gradient, effectively alleviating thermal stress concentration.

[0129] The key role of surface modification: h-BN activation removes the oxide layer, and SiC carbon coating inhibits oxidation. The two work together to improve the bonding strength and corrosion resistance of the coating and substrate.

[0130] Verification of self-healing mechanism: YAG reacts with SiO2 in silicon melt to form Y2Si2O7 glass phase, with a crack filling efficiency of 82%, and the crack width is less than 1μm after 50 thermal shocks.

[0131] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A graphite crucible anti-silicon infiltration coating based on a gradient boron nitride-silicon carbide composite structure, characterized by: The following components are included in mass percentage: Hexagonal boron nitride (h-BN) nanosheets: 38-45%, lateral size 0.5-3 μm, thickness ≤ 50 nm, surface oxygen content < 5 at%; β-SiC nanowires: 15-20%, diameter 80-150nm, aspect ratio > 50, and surface coated with a 2-3nm amorphous carbon layer; Yttrium aluminum garnet (YAG) precursor sol: 25-30%, wherein the molar ratio of yttrium (Y) to aluminum (Al) is 3:5, and the solid content is 40%; Polyvinylpyrrolidone dispersant: 1.5-2.5%; Deionized water: balance.

2. The anti-silicon infiltration coating for graphite crucible based on the gradient boron nitride-silicon carbide composite structure according to claim 1, characterized in that: The hexagonal boron nitride (h-BN) nanosheets are etched with hydrofluoric acid at 200° C. in a hydrofluoric acid steam reactor for 30 minutes to form activated hexagonal boron nitride (h-BN) nanosheets.

3. The anti-silicon infiltration coating for graphite crucible based on the gradient boron nitride-silicon carbide composite structure according to claim 1, characterized in that: The solvent of the yttrium aluminum garnet (YAG) precursor sol is an ethanol / acetylacetone mixed solution with a volume ratio of 1:1-2:

1.

4. The anti-silicon infiltration coating for graphite crucible based on the gradient boron nitride-silicon carbide composite structure according to claim 1, characterized in that: The amorphous carbon layer on the surface of the β-type silicon carbide nanowire is obtained by introducing an ethylene / argon mixed gas with a volume ratio of 1:9 into a tube furnace and depositing it at 800°C.

5. The anti-silicon infiltration coating for graphite crucible based on the gradient boron nitride-silicon carbide composite structure according to claim 1, characterized in that: The coating has a gradient structure. From the coating surface to the interface with the graphite crucible substrate, the relative content of hexagonal boron nitride (h-BN) nanosheets gradually decreases, and the relative content of the composite phase of beta-type silicon carbide nanowires and yttrium aluminum garnet (YAG) gradually increases.

6. A method for preparing a graphite crucible anti-silicon infiltration coating according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step 1: Treat hexagonal boron nitride (h-BN) nanosheets in a hydrofluoric acid vapor reactor at 200°C for 30 minutes; Step 2: introducing ethylene / argon mixed gas into the β-SiC nanowires in a tube furnace and depositing a carbon layer at 800°C to form carbon-coated β-SiC nanowires; Step 3: Mix the YAG precursor sol with the polyvinyl pyrrolidone dispersant and pre-disperse it under magnetic stirring at 600 rpm for 1 hour; Step 4: Add activated hexagonal boron nitride (h-BN) nanosheets and carbon-coated β-silicon carbide nanowires stepwise, using zirconia balls with a diameter of 5-10 mm, and mix at 350 rpm in a planetary ball mill for 4 hours to form a uniform and stable suspension, i.e., a slurry; Step 5: Adjust the slurry viscosity to 120-150 cP by adding deionized water or ethanol / acetylacetone mixture, and use a compressed air spray gun to evenly coat the slurry on the inner wall of the graphite crucible at a spray pressure of 0.3 MPa; Step 6: First, dry at 80°C for 2 hours to remove the solvent; then, under argon protection, heat to 1450°C at a heating rate of 10°C / min and keep warm for 1 hour to crystallize the yttrium aluminum garnet precursor sol; finally, introduce 3 vol% methane / argon mixture and perform carbon-based interface repair at 1350°C for 30 minutes.

7. The method for preparing the anti-silicon infiltration coating for graphite crucible according to claim 6, wherein: In step 5, the spraying distance between the compressed air spray gun and the inner wall of the graphite crucible is 15-20 cm.

8. The method for preparing a graphite crucible anti-silicon infiltration coating according to claim 6, wherein: In step 6, the drying process is carried out in a blast drying oven, and the purity of the argon gas is ≥99.99%.

9. The method for preparing a graphite crucible anti-silicon infiltration coating according to claim 6, wherein: In step 6, before introducing methane / argon mixed gas for carbon-based interface repair, the pressure in the furnace is first reduced to 10 -3 Pa, and then introduce methane / argon mixed gas.

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