Alumina fiber composite material for aerospace thermal protection and method of making the same

Abstract

The application discloses an alumina fiber composite material for aerospace thermal protection and a preparation method thereof. The composite material has a gradient multi-stage pore structure along a thickness direction, and comprises a heat receiving surface layer, an intermediate transition layer and a back heat surface layer with increasing porosities in sequence. The composite material comprises an alumina fiber reinforcement, a multi-stage pore alumina-mullite composite phase matrix and a nano CeO2 interface layer arranged on the surface of the alumina fiber reinforcement. Through the gradient multi-stage pore structure design, the heat receiving surface layer provides high strength and erosion resistance, the back heat surface layer provides high efficient heat insulation, and the transition layer realizes smooth transition of performance, thereby effectively solving the technical problem that heat insulation and mechanical performance are difficult to be considered in the prior art.

Description

Technical Field

[0001] This invention relates to the field of ceramic matrix composites technology, and more specifically to an alumina fiber composite material for aerospace thermal protection and its preparation method. Background Technology

[0002] Continuous alumina fibers, due to their high strength, high modulus, and excellent high-temperature resistance and oxidation resistance, are highly valuable in high-temperature thermal insulation and spacecraft thermal protection. Continuous alumina fiber-reinforced alumina composites, with their high specific strength, low density, high-temperature resistance, and oxidation resistance, have become a promising new generation of aerospace thermal structural composite materials. (Domestic) The mechanical properties of composite materials have approached international standards. The strength of Nextel-610 alumina fiber can reach 3.0-3.5 GPa, and the long-term operating temperature can reach 1300-1400℃. Due to its advantages such as low density, high strength and low thermal conductivity, the alumina fiber heat-sealing system can be used for a long time in high-temperature oxidizing environments.

[0003] However, existing alumina fiber composites still have the following significant shortcomings in aerospace thermal protection applications: (i) It is difficult to balance thermal insulation performance and mechanical properties. To improve thermal insulation performance, existing alumina fiber composites often require the introduction of higher porosity, but the increase in porosity inevitably leads to a significant decrease in mechanical strength. Although fiber porous ceramic composites have the characteristics of low density, low thermal conductivity and high compressive strength, how to obtain sufficient structural strength while maintaining low thermal conductivity to simultaneously meet the thermal insulation and load-bearing requirements of thermal protection systems is the core contradiction restricting the engineering application of this material.

[0004] (ii) Difficulty in controlling the interfacial bonding strength between fibers and matrix. Alumina fibers and alumina matrix have good chemical compatibility, but excessive interfacial bonding can lead to brittle fracture of the composite material, making it difficult for cracks to deflect and thus failing to effectively exert the toughening effect of the fibers.

[0005] (iii) Insufficient suppression of high-temperature radiative heat transfer. Above 1000℃, radiative heat transfer becomes the main mode of heat conduction. Although existing technologies attempt to introduce reflective screens or light-blocking agents to suppress radiative heat transfer, the introduction of reflective screens increases material density and interface complexity.

[0006] (iv) The problem of drying cracking in large-sized porous composite materials is prominent. During the drying process, porous ceramic composite materials are prone to cracking due to uneven volume shrinkage caused by moisture loss, which seriously affects the yield and dimensional stability of the material. The problem of drying cracking is particularly prominent when the material size is large. Although there are reports of using freeze-drying processes to obtain porous matrices in the existing technology, freeze-drying equipment is expensive and the cycle is long, which is not conducive to low-cost large-scale preparation.

[0007] Based on the above technical pain points, there is an urgent need to develop an alumina fiber composite material and its preparation method that has excellent thermal insulation and mechanical properties, high-temperature stability at the interface, high efficiency in suppressing radiative heat transfer, integrated gradient structure molding, and simple and reliable preparation process. Summary of the Invention

[0008] To address the shortcomings of the prior art, the present invention aims to provide an alumina fiber composite material for aerospace thermal protection and its preparation method, thereby at least solving the aforementioned problems.

[0009] This invention provides an alumina fiber composite material for aerospace thermal protection, wherein the composite material has a gradient multi-level pore structure along the thickness direction, including a heat-facing surface layer, an intermediate transition layer and a heat-reducing surface layer with successively increasing porosity; The composite material comprises an alumina fiber reinforcement, a hierarchical porous alumina-mullite multiphase matrix, and nanomaterials disposed on the surface of the alumina fiber reinforcement. Interface layer.

[0010] Preferably, the porosity of the heat-facing surface layer is 15%-25%, the porosity of the intermediate transition layer is 35%-50%, and the porosity of the heat-reducing surface layer is 50%-65%. The thickness of the heat-facing surface layer accounts for 20%-30% of the total thickness of the composite material, the thickness of the intermediate transition layer accounts for 40%-50% of the total thickness of the composite material, and the thickness of the heat-reducing surface layer accounts for 20%-30% of the total thickness of the composite material. The composite material has a multi-level pore size distribution, including micropores with a pore size <2nm, mesopores with a pore size of 2-50nm, and macropores with a pore size >50nm.

[0011] Preferably, the alumina fiber reinforcement has a fiber volume fraction of 15%-35% and a fiber diameter of 5-12 μm; The alumina fiber reinforcement is selected from one or more combinations of chopped fibers, two-dimensional alumina fiber lay-up, 2.5D woven preform, orthogonal triaxial woven preform, or three-dimensional needle-punched felt.

[0012] Preferably, the nano The interface layer is a nano-sized material grown in situ on the surface of alumina fibers via cerium nitrate solution impregnation-thermal decomposition. The coating, the nano The thickness of the interface layer is 50-200 nm, and the nanometers... The interfacial shear strength between the interface layer and the hierarchical porous alumina-mullite matrix is ​​40-80 MPa.

[0013] Preferably, the hierarchical porous alumina-mullite multiphase matrix further contains dispersed functional fillers, the functional fillers comprising: Nano-zirconia powder with an average particle size of 50-100 nm and a mass fraction of 5%-15% of the dry weight of the matrix; Silicon carbide nanowires, with a diameter of 50-150 nm, a length of 5-20 μm, and a mass fraction of 1%-5% of the dry weight of the matrix.

[0014] Preferably, the composite material further comprises a high-emissivity oxide thermal barrier coating disposed on the heat-facing surface of the composite material, the high-emissivity oxide thermal barrier coating including... , , Or one of the doped perovskite structure oxides, with a coating thickness of 50-150 μm.

[0015] On the other hand, the present invention also provides a method for preparing an alumina fiber composite material for aerospace thermal protection, comprising the following steps: Step 1: After heat treatment to remove the sizing agent from the alumina fiber preform, it is immersed in a cerium salt solution, followed by drying and heat treatment to grow nano-scale structures in situ on the alumina fiber surface. Interface layer; Step 2: Prepare gradient structure slurries for forming the heat-facing layer, intermediate transition layer and back-heat-facing layer respectively. The content of pore-forming agent in each layer of slurry increases sequentially from the heat-facing layer to the back-heat-facing layer. Step 3: Place the alumina fiber preform processed in Step 1 into an impregnation tank, and use the gradient structure slurry in the order of the heat-facing layer, the intermediate transition layer and the back heat-facing layer for gradient impregnation. After gelation, the composite material preform is obtained. Step 4: Dry the composite material preform; Step 5: Sinter the dried composite material blank to decompose the pore-forming agent into multi-level pores, and at the same time sinter the matrix to densify, to obtain the alumina fiber composite material.

[0016] Preferably, the gradient impregnation in step three adopts a vacuum-pressure alternating impregnation method, which alternates multiple impregnation-degassing cycles under vacuum conditions below -0.095MPa and pressure conditions of 0.5-1.0MPa; wherein, the heat-facing surface layer slurry undergoes 3-5 impregnation-degassing cycles, and the intermediate transition layer slurry and the back-heat-facing surface layer slurry undergo 2-3 impregnation-degassing cycles respectively.

[0017] Preferably, in step one, the temperature for heat treatment to remove the wetting agent is 600-800℃, and the time is 1-3 hours; the cerium salt solution is cerium nitrate with a concentration of 0.05-0.2 mol / L. The solution is soaked in an aqueous solution and then subjected to ultrasonic treatment for 30-60 minutes; the heat treatment temperature for in-situ growth is 500-700℃ and the time is 1-2 hours. In step two, the pore-forming agent is a mixture of polymethyl methacrylate (PMMA) microspheres and starch granules, wherein the PMMA microspheres have a particle size of 1-10 μm and the starch granules have a particle size of 10-50 μm; the amount of PMMA pore-forming agent added to the heat-facing surface layer slurry is 5%-10% of the total solid mass, the amount of PMMA pore-forming agent added to the intermediate transition layer slurry is 15%-25% of the total solid mass and also contains 5%-10% starch granules, and the amount of PMMA pore-forming agent added to the back-heating surface layer slurry is 25%-40% of the total solid mass and also contains 10%-20% starch granules; In step four, the drying is a gradient temperature drying, which includes: the first stage of drying at a temperature of 40-50℃ and a relative humidity of 80%-90% for 24-48 hours, the second stage of drying at a temperature of 80-100℃ for 12-24 hours, and the third stage of drying at a temperature of 120-150℃ for 6-12 hours. In step five, the sintering includes: heating to 400-500℃ at a rate of 1-3℃ / min and holding for 2-4 hours to completely decompose the pore-forming agent; then heating to 1200-1400℃ at a rate of 2-5℃ / min and holding for 1-3 hours to sinter and densify the matrix.

[0018] Preferably, the gradient structure slurry in step two further includes functional fillers dispersed therein, the functional fillers including nano-zirconia powder and silicon carbide nanowires; The nano The amount of powder added is 5%-15% of the total solid mass in the heat-facing surface layer slurry and the intermediate transition layer slurry, and the amount of nano-powder in the back-heating surface layer slurry is 5%-15%. The amount of powder added is 40%-60% of the heat-facing surface slurry; the amount of SiC nanowires added is 1%-5% of the total solid mass in each layer of slurry.

[0019] The beneficial effects of this invention are: This invention, through a gradient multi-level porous structure design, provides high strength and erosion resistance to the heat-facing surface layer, efficient heat insulation to the heat-reducing surface layer, and a smooth performance transition layer, effectively solving the technical problem of balancing heat insulation and mechanical properties in the prior art.

[0020] This invention utilizes nanotechnology. The in-situ grown interface layer has excellent high-temperature oxidation resistance and can be stably used for a long time in an oxidizing environment of 1400℃, overcoming the defects of traditional PyC interface and BN interface that are easily oxidized at high temperatures.

[0021] This invention uses a gradient vacuum-pressure alternating impregnation process to obtain a gradient structure in one step, avoiding the interlayer interface problem of traditional layered preparation-stacked composite processes, and greatly improving the integrity and reliability of the material. Detailed Implementation

[0022] The present invention will be further described below with reference to embodiments. The described embodiments are some, but not all, of the embodiments of the present invention.

[0023] This invention provides an alumina fiber composite material for aerospace thermal protection, comprising the following components: (1) Reinforcing body: Three-dimensional needle-punched alumina fiber preform with a fiber volume fraction of 15%-35% and a fiber diameter of 5-12μm. Continuous alumina fiber (preferably Nextel610 or domestic continuous alumina fiber of the same type) is used. The fiber preform form is selected from one or more combinations of chopped fibers (length 3-10mm), two-dimensional alumina fiber cloth lay-up, 2.5D woven preform, orthogonal triaxial woven preform or three-dimensional needle-punched felt.

[0024] (2) Matrix: A hierarchical porous alumina-mullite multiphase matrix, formed by mixing alumina sol (solid content 20%-30%) and mullite precursor sol (solid content 15%-25%) at a mass ratio of 1:0.3-1 and then undergoing sol-gel transformation. The mullite precursor sol is selected from one of the following: aluminum-silicon composite sol, a mixed sol of aluminum salt and silica sol, or a mixed sol of organoaluminum source and organosilicon source.

[0025] (3) Functional filler (uniformly dispersed in the matrix): Nano-zirconia ( The powder has an average particle size of 50-100nm and a mass fraction of 5%-15% of the dry weight of the matrix. It is used to suppress high-temperature radiative heat transfer. Silicon carbide nanowires (SiCNWs), with diameters of 50-150 nm and lengths of 5-20 μm, and a mass fraction of 1%-5% of the dry weight of the matrix, are used for synergistic toughening.

[0026] (4) Interface layer: Nano-cerium oxide grown in situ on the fiber surface ( A coating with a thickness of 50-200nm is used as a weak interface layer between the fiber and the matrix.

[0027] (5) Surface coating (optional): High emissivity oxide thermal barrier coating, selected from , , Or one of the doped perovskite structure oxides, with a coating thickness of 50-150 μm.

[0028] The structural design adopts a gradient multi-level porous structure: The composite material of this invention has a gradient multi-level porous structure along the thickness direction, specifically: Heat-facing surface layer (thickness accounts for 20%-30% of the total thickness of the composite material): porosity of 15%-25%, fiber volume fraction of 30%-35%, serving as an anti-erosion and structural load-bearing layer; Intermediate transition layer (thickness accounts for 40%-50% of the total thickness of the composite material): porosity of 35%-50%, fiber volume fraction of 20%-30%, to achieve heat flow transition; Back heat insulation layer (thickness accounts for 20%-30% of the total thickness of the composite material): porosity of 50%-65% and fiber volume fraction of 15%-20%, serving as a high-efficiency heat insulation layer.

[0029] The porosity described above covers a multi-level pore size distribution: micropores (pore size <2nm), mesopores (pore size 2-50nm), and macropores (pore size >50nm) coexist.

[0030] The gradient hierarchical porous structure is achieved by progressively changing the content of pore-forming agent and fiber content in the slurry. The pore-forming agent is a mixture of polymethyl methacrylate (PMMA) microspheres (particle size 1-10 μm) and starch granules (particle size 10-50 μm). The multi-level pore size distribution is formed by utilizing the decomposition characteristics of PMMA microspheres and starch granules at different temperatures (PMMA microspheres decompose at 350-450℃, and starch granules decompose at 250-350℃).

[0031] The method for preparing the composite material according to the embodiments of the present invention includes the following steps: Step 1: Preparation and pretreatment of alumina fiber preforms: (1) Cut the continuous alumina fibers into short sections (3-10 mm in length) or weave them into preforms in the form of two-dimensional cloth / 2.5D weaving / orthogonal triaxial weaving / three-dimensional needle-punched felt; (2) Heat-treat the fiber preform at 600-800℃ for 1-3 hours to remove the sizing agent from the fiber surface; (3) Immerse the heat-treated fiber preform in cerium nitrate solution with a concentration of 0.05-0.2 mol / L. Soak in an aqueous solution for 30-60 minutes, then remove and dry at 80-120℃ for 6-12 hours. (4) Heat-treat the dried fiber preform at 500-700℃ for 1-2 hours to decompose cerium nitrate into nanoparticles. Particles, grown in situ on the fiber surface, form Interface layer.

[0032] Step 2: Preparation of gradient structure slurry: (1) Preparation of A slurry (for heat-facing surface layer): Alumina sol (solid content 20%-30%), mullite precursor sol (solid content 15%-25%), and nano- Powder (average particle size 50-100nm, added at 5%-15% of total solid mass), SiC nanowires (diameter 50-150nm, length 5-20μm, added at 1%-5% of total solid mass), PMMA pore-forming agent (particle size 1-3μm, added at 5%-10% of total solid mass) are mixed, and a dispersant (selected from ammonium polyacrylate, ammonium polymethacrylate or polyvinylpyrrolidone, added at 0.5%-2% of total solid mass) and deionized water are added. The mixture is ball-milled for 4-8 hours, and the pH is adjusted to 3-4. Specifically, the mullite precursor solution is prepared by using aluminum sol as the aluminum source and silica sol as the silicon source, adjusting the pH of the silica sol to 1.6-3.0 with hydrochloric acid to obtain acidified silica sol; the acidified silica sol is added to the aluminum sol at a mass ratio of aluminum to silicon of 1:0.08-0.28, and mixed and stirred at room temperature for more than 4 hours.

[0033] (2) Preparation of B slurry (for transition layer): Similar to A slurry, but the amount of PMMA pore-forming agent (particle size 1-5μm) added is increased to 15%-25% of the total solid mass, and starch granules (particle size 10-30μm, added at 5%-10% of the total solid mass) are added at the same time. (3) Preparation of C slurry (for back heat surface layer): Similar to B slurry, but the amount of PMMA pore-forming agent (particle size 1-10μm) added is increased to 25%-40% of the total solid mass, and the amount of starch granules (particle size 10-50μm) added is increased to 10%-20% of the total solid mass. The content is reduced to 40%-60% of that of slurry A.

[0034] Step 3: Gradient vacuum-pressure alternating impregnation and molding: (1) With The alumina fiber preform of the interface layer is placed in a vacuum impregnation tank, and the vacuum is drawn to below -0.095MPa and held for 30 minutes; (2) First impregnation (heat-facing surface): Inject slurry A into the impregnation tank and perform 3-5 impregnation-degassing cycles alternately under vacuum (below -0.095MPa) and pressure (0.5-1.0MPa) conditions to fully impregnate the heat-facing side of the precast body with slurry A. (3) Secondary impregnation (transition layer): Inject slurry B into the impregnation tank and perform 2-3 impregnation-degassing cycles; (4) Three impregnations (back heat surface layer): Inject C slurry into the impregnation tank and perform 2-3 impregnation-degassing cycles; (5) Take out the impregnated preform and gel at 60-80℃ for 12-24h.

[0035] Step 4: Gradient Temperature Drying Process Gradient temperature drying process is adopted: (1) First stage (low temperature and high humidity stage): Slowly dry for 24-48 hours at a temperature of 40-50℃ and a relative humidity of 80%-90% to prevent surface cracking; (2) Second stage (medium temperature range): Heat to 80-100℃ and dry for 12-24 hours; (3) Third stage (high temperature stage): heat up to 120-150℃ and dry for 6-12 hours.

[0036] Step 5: Sintering and Hole Formation (1) Place the dried composite material preform into a high-temperature furnace and heat it to 400-500℃ at a rate of 1-3℃ / min. Hold it for 2-4 hours to completely decompose the PMMA microspheres and starch particles and form multi-level pores. (2) Continue to raise the temperature to 1200-1400℃ at a rate of 2-5℃ / min, and hold for 1-3 hours to sinter and densify the alumina and mullite matrix; (3) Cool down to room temperature at a rate of 2-5℃ / min.

[0037] Step Six: Surface Coating (Optional) (1) Atmospheric plasma spraying (APS) or slurry coating method is used to prepare the heat-facing surface of the composite material. High emissivity coating, coating thickness 50-150μm; (2) Heat treatment at 800-1000℃ for 1-2 hours to ensure good bonding between the coating and the substrate.

[0038] It should be noted that the gradient hierarchical porous structure creates numerous gas-solid interfaces within the composite material. Micropores (<2nm) effectively scatter phonons and reduce lattice thermal conductivity; mesopores (2–50nm) further increase interfacial thermal resistance; macropores (>50nm) reduce material density and overall thermal conductivity. These three factors work synergistically to greatly extend and block the solid-phase heat transfer path.

[0039] nanometer Scattering and Interface reflection. At high temperatures (>800℃), the proportion of radiative heat transfer increases dramatically. Nanoparticles uniformly dispersed in the matrix... The particles (refractive index approximately 2.2) exhibit strong scattering of infrared radiation. Simultaneously, the nanoparticles on the fiber surface... The coating (refractive index approximately 2.3) creates a refractive index abrupt change at the fiber / matrix interface, further reflecting and scattering infrared radiation. Together, these factors can significantly reduce radiative thermal conductivity at high temperatures.

[0040] The matrix formed by the sol-gel process seals the pores into an independent and non-connected state, effectively suppressing gas convection heat transfer at high temperatures.

[0041] Continuous alumina fibers act as reinforcement, bearing most of the load. When the matrix cracks, the crack propagates to the fiber / matrix interface. The interface layer provides a moderate interfacial bonding strength (interfacial shear strength of about 50-80 MPa), which is neither too strong to cause brittle fracture nor too weak to cause load transfer failure. The crack deflects along the interface, allowing the fiber to be pulled out and bridged, thus consuming the fracture energy.

[0042] SiC nanowires (nanoscale), alumina fibers (micrometer scale), and a porous matrix form a multi-scale reinforcement network spanning nano-micro-macro scales. SiC nanowires bridge microcracks, while alumina fibers bridge macrocracks, achieving full-scale toughening from the nanoscale to the macroscale.

[0043] The high density of the heat-facing layer provides structural strength and erosion resistance; the gradual porosity of the transition layer allows for a smooth transition of the coefficient of thermal expansion and elastic modulus along the thickness direction, reducing thermal mismatch stress; the high porosity of the heat-reducing layer provides flexible buffering. The gradient structure effectively avoids interlaminar delamination and thermal shock cracking caused by abrupt changes in physical properties in traditional multilayer composite materials.

[0044] and A semi-coherent interface is formed between them, with moderate bonding strength (interfacial shear strength of about 50-80 MPa), which can both transfer loads and induce interfacial debonding when cracks arrive.

[0045] It has a melting point as high as 2400℃ and does not react with other substances below 1400℃. It exhibits a significant reaction, superior to traditional BN coatings (which begin to oxidize at 600℃ in an aerobic environment) and pyrolytic carbon coatings (which begin to oxidize at 500℃ in an aerobic environment).

[0046] In-situ growth on fiber surface via cerium nitrate impregnation-thermal decomposition method This achieves uniform and dense coating coverage, and the process is simple and inexpensive.

[0047] The alumina fibers used in the various embodiments and comparative examples are Continuous alumina fibers (fiber diameter 5-12 μm) were used, with an alumina sol solid content of 20%-30% and a mullite precursor sol solid content of 15%-25%. All reagents used in the examples were commercially available analytical grade reagents.

[0048] Example 1: Step 1: Preparation and pretreatment of alumina fiber preforms: (1) A preform woven from continuous alumina fibers in the form of a three-dimensional needle-punched felt, with a fiber volume fraction of 35%; (2) Heat-treat the fiber preform at 700℃ for 2 hours to remove the sizing agent from the fiber surface; (3) Immerse the heat-treated fiber preform in cerium nitrate solution with a concentration of 0.1 mol / L. The sample was ultrasonically treated in an aqueous solution for 45 minutes, then removed and dried at 100°C for 8 hours. (4) The dried fiber preform was heat-treated at 600℃ for 1.5h to decompose cerium nitrate into nanoparticles. Particles, grown in situ on the fiber surface, form a thickness of approximately 100 nm. Interface layer.

[0049] Step 2: Preparation of gradient structure slurry: (1) Mix alumina sol (25% solid content) and mullite precursor sol (20% solid content) at a mass ratio of 1:0.5 (the sum of the masses of solid oxides contained in the two is denoted as the mass of the matrix solid M), and add nano Powder (10% of M), SiC nanowires (3% of M), PMMA pore-forming agent (8% of M), ammonium polyacrylate dispersant (1% of M) and deionized water were added, ball milled for 6 hours, and the pH was adjusted to 3.5. (2) Preparation of slurry B (for transition layer): Similar to slurry A, but the amount of PMMA pore-forming agent (particle size 3μm) added is 20% of the total solid mass, and starch granules (particle size 20μm, added at 8% of the total solid mass) are added at the same time. (3) Preparation of C slurry (for back heat surface layer): Similar to B slurry, but the amount of PMMA pore-forming agent (5μm particle size) added is 32% of the total solid mass, the amount of starch granules (30μm particle size) added is 15% of the total solid mass, and the amount of nano-sized pore-forming agent is 15% of the total solid mass. The content is 50% of slurry A.

[0050] Step 3: Gradient vacuum-pressure alternating impregnation and molding: (1) With The alumina fiber preform of the interface layer is placed in a vacuum impregnation tank, and the vacuum is drawn to below -0.095MPa and held for 30 minutes; (2) First impregnation (heat-facing surface): Inject slurry A into the impregnation tank to completely submerge the preform. Perform four impregnation-degassing cycles alternately under vacuum (below -0.095MPa) and pressure (0.8MPa) conditions to ensure that slurry A fully wets the heat-facing side and internal pores of the preform. After the cycle is completed, drain the remaining slurry A from the tank (the preform is left in the tank and no drying or heating curing treatment is performed on the preform). (3) Secondary impregnation (transition layer): Inject slurry B into the impregnation tank to completely submerge the preform and perform 3 impregnation-degassing cycles; during this process, slurry B penetrates from the back heat side and the outer periphery of the preform towards the center, and intermingles with slurry A remaining on the heat-facing side of the preform in the contact area, naturally forming an intermediate transition layer with gradually changing composition and porosity; after the cycle is completed, the remaining slurry B in the tank is discharged (the preform is still retained in the tank). (4) Three impregnations (back heat surface layer): Inject C slurry into the impregnation tank to completely submerge the preform, and perform three impregnation-degassing cycles to ensure that the C slurry fully wets the back heat surface of the preform; after the cycle is completed, drain the remaining C slurry from the tank. (5) Take the preform out of the tank after gradient impregnation. At this time, the preform has a wet gradient distribution along the thickness direction, consisting of A slurry enrichment zone (heat-facing side) — A / B slurry interpenetration zone (transition layer) — C slurry enrichment zone (back-heat-facing side). Then, perform gelation treatment at 70°C for 18 hours.

[0051] Step 4: Gradient Temperature Drying Process (1) First stage: Slow drying for 36 hours at a temperature of 45℃ and a relative humidity of 85%; (2) Second stage: Heat to 90℃ and dry for 18 hours; (3) Third stage: Heat to 135℃ and dry for 9 hours.

[0052] Step 5: Sintering and Hole Formation (1) The dried composite material preform was placed in a high-temperature furnace and heated to 450°C at a rate of 2°C / min. The temperature was maintained for 3 hours to allow the PMMA microspheres and starch particles to decompose completely. (2) Continue to heat up to 1300℃ at a rate of 3℃ / min and hold for 2h to sinter and densify the alumina and mullite matrix; (3) Cool down to room temperature at a rate of 3℃ / min.

[0053] The resulting composite material has the following characteristics: the thickness of the heat-facing layer is 25%, the porosity is 15%; the thickness of the intermediate transition layer is 45%, the porosity is 35%; the thickness of the heat-reducing layer is 30%, the porosity is 50%, and the density is 1.68 g / cm³.

[0054] Example 2: The difference from Example 1 is as follows: (1) The fiber preform is a two-dimensional alumina fiber cloth layup with a fiber volume fraction of 28%; (2) The concentration of cerium nitrate solution was 0.15 mol / L, and the solution was ultrasonically treated for 50 min. The interface layer is approximately 150 nm thick. (3) In slurry A, the amount of PMMA pore-forming agent (particle size 3μm) added is 10% of the total solid mass; in slurry B, the amount of PMMA pore-forming agent (particle size 4μm) added is 22% of the total solid mass and the amount of starch granules (particle size 25μm) added is 9% of the total solid mass; in slurry C, the amount of PMMA pore-forming agent (particle size 8μm) added is 35% of the total solid mass and the amount of starch granules (particle size 40μm) added is 18% of the total solid mass. (4) Nano The powder has an average particle size of 60 nm and is added at 12% of the total solid mass; the SiC nanowires have a diameter of 80 nm and a length of 15 μm and are added at 4% of the total solid mass. (5) The sintering temperature is 1350℃ and the holding time is 2.5h.

[0055] The resulting composite material has the following porosity: 20% for the heat-facing layer, 42% for the intermediate transition layer, and 58% for the heat-reducing layer. Its density is 1.45 g / cm³.

[0056] Example 3: The difference from Example 1 is as follows: (1) The fiber preform is formed by compression molding of chopped fibers (5 mm in length) with a fiber volume fraction of 18%; (2) The concentration of cerium nitrate solution was 0.08 mol / L, and the solution was ultrasonically treated for 35 min. The interface layer is approximately 70 nm thick. (3) In slurry A, the amount of PMMA pore-forming agent (particle size 1μm) added is 6% of the total solid mass; in slurry B, the amount of PMMA pore-forming agent (particle size 2μm) added is 16% of the total solid mass and the amount of starch granules (particle size 15μm) added is 6% of the total solid mass; in slurry C, the amount of PMMA pore-forming agent (particle size 10μm) added is 38% of the total solid mass and the amount of starch granules (particle size 45μm) added is 12% of the total solid mass. (4) Nano The powder has an average particle size of 90 nm and is added at 6% of the total solid mass; the SiC nanowires have a diameter of 120 nm and a length of 8 μm and are added at 2% of the total solid mass. (5) The sintering temperature is 1250℃ and the holding time is 1.5h.

[0057] The resulting composite material has the following porosity: 25% for the heat-facing layer, 50% for the intermediate transition layer, and 65% for the heat-reducing layer. Its density is 1.22 g / cm³.

[0058] Example 4: Based on Example 1, the following steps are further included: Step Six: Surface Coating: (1) Preparation of composite material on the heat-facing surface using atmospheric plasma spraying (APS) High emissivity coating, coating thickness 100μm; (2) Heat treatment at 900℃ for 1.5h to ensure good bonding between the coating and the substrate.

[0059] The resulting composite material has the following porosity: 15% for the heat-facing layer, 35% for the transition layer, and 50% for the heat-reducing layer. The heat-facing surface also has... Coating. Density 1.75 g / cm³.

[0060] Example 5: The difference from Example 1 is as follows: The cerium nitrate solution concentration was 0.05 mol / L, and the mixture was sonicated for 30 min. The interface layer is approximately 50 nm thick.

[0061] The resulting composite material: The interface layer is 50 nm thick, the porosity of the heat-facing layer is 15%, the porosity of the transition layer is 35%, and the porosity of the heat-reducing layer is 50%. The density is 1.68 g / cm³.

[0062] Example 6: The difference from Example 1 is as follows: The cerium nitrate solution concentration was 0.2 mol / L, and the mixture was ultrasonically treated for 60 min. The interface layer is approximately 200nm thick.

[0063] The resulting composite material: The interface layer is 200 nm thick, the porosity of the heat-facing layer is 15%, the porosity of the transition layer is 35%, and the porosity of the heat-reducing layer is 50%. The density is 1.68 g / cm³.

[0064] Example 7: The difference from Example 1 is as follows: (1) Nanoparticles in slurry A The amount of powder (average particle size 80 nm) added was 15% of the total solid mass, and the amount of SiC nanowires (diameter 100 nm, length 10 μm) added was 5% of the total solid mass; (2) Nanoparticles in slurry B The addition amount is 15% of the total solid mass, and the addition amount of SiC nanowires is 5% of the total solid mass; (3) Nanoparticles in C slurry The content is 50% (i.e. 7.5%) of slurry A, and the SiC nanowire addition is 5% of the total solid mass.

[0065] The resulting composite material: nano The addition amount is 15%, the SiC nanowire addition amount is 5%, the porosity of the heat-facing layer is 15%, the porosity of the transition layer is 35%, and the porosity of the back-heat-facing layer is 50%. The density is 1.85 g / cm³.

[0066] Example 8: The difference from Example 1 is as follows: (1) Nanoparticles in slurry A The amount of powder (average particle size 80 nm) added was 5% of the total solid mass, and the amount of SiC nanowires (diameter 100 nm, length 10 μm) added was 1% of the total solid mass. (2) Nanoparticles in slurry B The addition amount is 5% of the total solid mass, and the addition amount of SiC nanowires is 1% of the total solid mass; (3) Nanoparticles in C slurry The content is 50% (i.e. 2.5%) of slurry A, and the SiC nanowire addition is 1% of the total solid mass.

[0067] The resulting composite material: nano The addition amount is 5%, the SiC nanowire addition amount is 1%, the porosity of the heat-facing layer is 15%, the porosity of the transition layer is 35%, and the porosity of the back-heat-facing layer is 50%. The density is 1.58 g / cm³.

[0068] Example 9: The difference from Example 1 is as follows: (1) The fiber preform is a 2.5D woven preform with a fiber volume fraction of 30%; (2) The concentration of cerium nitrate solution was 0.12 mol / L, and the ultrasonic treatment lasted for 40 min. The interface layer is approximately 120 nm thick. (3) The sintering temperature is 1320℃ and the holding time is 2h.

[0069] The resulting composite material has the following porosity: 15% for the heat-facing layer, 35% for the transition layer, and 50% for the heat-reducing layer. Its density is 1.72 g / cm³.

[0070] Example 10: The difference from Example 1 is as follows: (1) The fiber preform is an orthogonal triaxial woven preform with a fiber volume fraction of 32%; (2) The concentration of cerium nitrate solution was 0.1 mol / L, and the treatment was ultrasonic for 45 min. The interface layer is approximately 100 nm thick. (3) The sintering temperature is 1280℃ and the holding time is 2h.

[0071] The resulting composite material has the following porosity: 15% for the heat-facing layer, 35% for the transition layer, and 50% for the heat-reducing layer. Its density is 1.75 g / cm³.

[0072] Comparative Example 1: The difference from Example 1 is that: The cerium nitrate impregnation and thermal decomposition treatment in steps (3) and (4) of step one are omitted. After the fiber preform is heat-treated at 700℃ to remove the sizing agent, it is directly used for subsequent impregnation.

[0073] The resulting composite material: None The interface layer has a porosity of 15% for the heat-facing surface, 35% for the transition layer, and 50% for the heat-reducing surface. The density is 1.68 g / cm³.

[0074] Comparative Example 2: The difference from Example 1 is that: (1) Omit the gradient preparation of the three slurries A, B, and C in step two, and prepare only one uniform slurry: PMMA pore-forming agent (particle size 3μm) is added at 20% of the total solid mass, starch granules (particle size 20μm) are added at 10% of the total solid mass, and nano-sized pore-forming agent is added at 10% of the total solid mass. The addition amount is 10% of the total solid mass, and the addition amount of SiC nanowires is 3% of the total solid mass; (2) In step three, only one impregnation is performed (using uniform slurry for 4 vacuum-pressure alternating impregnation-defoaming cycles), without stratified impregnation.

[0075] The resulting composite material has a uniform overall porosity (approximately 42%) and no gradient structure. Its density is 1.52 g / cm³.

[0076] Comparative Example 3: The difference from Example 1 is that: Step four does not employ gradient temperature drying, but instead uses traditional single-temperature drying: direct drying at 80℃ for 48 hours.

[0077] The resulting composite material has a porosity of 15% in the heat-facing layer, 35% in the transition layer, and 50% in the heat-reducing layer. However, there are obvious microcracks on the surface, resulting in a significant reduction in the yield.

[0078] The performance testing methods for Examples 1-10 and Comparative Examples 1-3 are as follows: (1) Density: The bulk density of the composite material was determined according to Archimedes' displacement method.

[0079] (2) Porosity: The porosity was determined in accordance with GB / T1966-2024 "Determination of apparent porosity and bulk density of porous ceramics".

[0080] (3) Thermal conductivity at room temperature: The thermal conductivity was measured by laser flash method (Netzsch LFA427), with a sample size of φ12.7mm×2mm and a test temperature of 25℃.

[0081] (4) High temperature thermal conductivity (1000℃): The test temperature was 1000℃ and the atmosphere was air.

[0082] (5) Bending strength: Three-point bending test was conducted in accordance with GB / T6569-2006 "Test Method for Bending Strength of Fine Ceramics". The sample size was 3mm×4mm×45mm, the span was 30mm, and the loading rate was 0.5mm / min.

[0083] (6) Compressive strength: The compressive strength of fine ceramics was tested according to GB / T8489-2006. The sample size was 5mm×5mm×5mm and the loading rate was 0.5mm / min.

[0084] (7) Thermal shock resistance: After the sample is kept at 1500℃ for 30 min, it is quickly immersed in room temperature (25℃) water for quenching. This is repeated 10 times to test the retention rate of bending strength after thermal shock.

[0085] (8) Yield: The percentage of samples without macroscopic cracks in each batch of 10 samples (size 100mm×100mm×20mm) prepared is counted.

[0086] (9) Interfacial shear strength: The fiber / matrix interfacial shear strength was determined by single fiber extrusion method.

[0087] Table 1 below shows some performance data for Examples 1-10 and Comparative Examples 1-3: Table 1 Example 1 1.68 0.062 0.21 95 105 91 95 65 Example 2 1.45 0.055 0.19 85 88 90 92 70 Example 3 1.22 0.045 0.16 68 68 88 90 60 Example 4 1.75 0.058 0.18 108 118 93 93 65 Example 5 1.68 0.064 0.23 82 90 86 95 42 Example 6 1.68 0.061 0.20 88 98 90 95 78 Example 7 1.85 0.058 0.17 102 115 92 94 65 Example 8 1.58 0.067 0.25 78 82 87 95 65 Example 9 1.72 0.063 0.22 98 108 91 94 65 Example 10 1.75 0.062 0.21 100 112 91 94 65 Comparative Example 1 1.68 0.078 0.32 52 52 72 95 120 Comparative Example 2 1.52 0.071 0.28 62 62 78 94 65 Comparative Example 3 1.66 0.085 0.30 58 58 75 55 63 In conclusion, Comparative Example 1 does not contain The interfacial layer exhibits a flexural strength of only 52 MPa, approximately 45% lower than that of Example 1 (95 MPa); its compressive strength is also only 52 MPa, more than 50% lower than that of Example 1 (105 MPa). The interfacial shear strength of Comparative Example 1 is as high as 120 MPa, indicating that an excessively strong interfacial bond is formed between the fiber and the matrix, leading to brittle fracture of the composite material and preventing the effective toughening effect of the fiber.

[0088] In contrast, the interfacial shear strength of Example 1 is 65 MPa, which is within the ideal range of 40-80 MPa. This ensures effective load transfer and induces interfacial debonding and fiber pull-out when cracks arrive, thereby significantly improving the mechanical properties of the composite material.

[0089] Furthermore, the thermal conductivity of Comparative Example 1 at room temperature was 0.078 W / (m·K), and at 1000 °C it was 0.32 W / (m·K), both significantly higher than that of Example 1 (0.062 W / (m·K) and 0.21 W / (m·K)). This is because... The interface layer (refractive index approximately 2.3) forms a refractive index abrupt change at the fiber / matrix interface, effectively reflecting and scattering infrared radiation, thus suppressing high-temperature radiative heat transfer. The thermal shock resistance retention rate of Comparative Example 1 is only 72%, far lower than the 91% of Example 1, indicating that... The interface layer also significantly improves the material's thermal shock resistance.

[0090] Comparative Example 2 is a uniform porosity composite material with no gradient structure. Its flexural strength is 62 MPa, which is about 35% lower than that of Example 1 (95 MPa); its compressive strength is also 62 MPa, which is significantly lower than that of Example 1 (105 MPa). The room temperature thermal conductivity of Comparative Example 2 is 0.071 W / (m·K), and its thermal conductivity at 1000℃ is 0.28 W / (m·K), both of which are higher than those of Example 1.

[0091] This demonstrates that the gradient structure provides structural strength and erosion resistance through the high density (15% porosity) of the heat-facing layer, while the high porosity (50%) of the heat-reducing layer provides efficient thermal insulation, and the transition layer achieves a smooth performance transition. The gradient structure also avoids interlaminar delamination and thermal shock cracking caused by abrupt changes in physical properties—Comparative Example 2 exhibits a post-thermal shock strength retention rate of 78%, significantly lower than the 91% of Example 1.

[0092] Comparative Example 3, using a traditional single-temperature drying process (direct drying at 80°C for 48 hours), had a yield of only 55%, far lower than the 95% of Example 1. The composite material obtained in Comparative Example 3 exhibited obvious microcracks on its surface, resulting in a flexural strength of only 58 MPa, a room temperature thermal conductivity as high as 0.085 W / (m·K), a thermal conductivity of 0.30 W / (m·K) at 1000°C, and a strength retention rate of only 75% after thermal shock.

[0093] This is because uneven volume shrinkage caused by moisture loss during single-temperature drying can easily lead to cracking in large-sized samples. However, the gradient temperature drying process (45℃ / 85%RH → 90℃ → 135℃ three-stage process) used in Example 1 of this invention achieves slow and uniform moisture loss through a low-temperature, high-humidity stage, effectively preventing surface cracking. The improved drying quality directly translates into improved mechanical and thermal insulation properties, fully demonstrating that gradient drying is a key technical step in ensuring material performance.

[0094] Example 5 ( The interfacial shear strength of the 50nm interface layer is 42MPa, which is still within the ideal range of 40-80MPa. However, proximity to the critical point may lead to insufficient load transfer efficiency, resulting in a flexural strength of only 82MPa. The strength retention rate after thermal shock resistance is 86%. Example 6 ( The interfacial shear strength of the 200nm interface layer was 78MPa, and the flexural strength was 88MPa, which is slightly better than that of Example 5 but lower than that of Example 1 (100nm, 65MPa, 95MPa). This indicates that... There is an optimal range for the thickness of the interface layer. If it is too thin, the interface bonding is too weak and the load transfer is insufficient; if it is too thick, the interface bonding is too strong and crack deflection is hindered. A thickness of around 100nm is ideal. The interface layer can achieve the best interfacial bonding strength and comprehensive mechanical properties.

[0095] Example 7 (nano) The thermal conductivity at room temperature (15% SiC nanowires, 5% SiC nanowires) is 0.058 W / (m·K), the thermal conductivity at 1000℃ is 0.17 W / (m·K), and the flexural strength is 102 MPa, all of which are the best among Examples 1-3. Example 8 (nanowires) The room temperature thermal conductivity of the 5% (5%) and SiC nanowire (1%) is 0.067 W / (m·K), the thermal conductivity at 1000℃ is 0.25 W / (m·K), and the bending strength is 78 MPa, which is relatively poor.

[0096] This indicates that nano It has a significant effect on suppressing high-temperature radiative heat transfer—with the advancement of nanotechnology. As the SiC content increased from 5% to 15%, the thermal conductivity at 1000℃ decreased from 0.25 W / (m·K) to 0.17 W / (m·K), a reduction of 32%. Simultaneously, SiC nanowires, acting as a nano-reinforcing phase, formed a multi-scale synergistic reinforcing network with micron-sized alumina fibers. When the SiC nanowire content increased from 1% to 5%, the flexural strength increased from 78 MPa to 102 MPa, an increase of 31%. As the overall porosity increased from approximately 33% on average in Example 1 to approximately 40% on average in Example 2 and then to approximately 47% on average in Example 3, the density decreased from 1.68 g / cm³ to 1.22 g / cm³, the room temperature thermal conductivity decreased from 0.062 W / (m·K) to 0.045 W / (m·K), and the thermal conductivity at 1000°C decreased from 0.21 W / (m·K) to 0.16 W / (m·K). However, the flexural strength also decreased from 95 MPa to 68 MPa, and the compressive strength decreased from 105 MPa to 68 MPa.

[0097] This indicates that the present invention, through gradient structure design, can achieve on-demand control of thermal insulation performance and mechanical properties within a density range of 1.22-1.68 g / cm³. The high density of the heat-facing surface layer ensures structural strength, while the high porosity of the heat-reducing surface layer ensures efficient thermal insulation. Users can select different porosity gradient combinations according to specific application scenarios.

[0098] Example 4 adds to Example 1. The high-emissivity coating exhibits improved flexural strength to 108 MPa (an increase of approximately 14%), improved compressive strength to 118 MPa, and reduced thermal conductivity at 1000℃ to 0.18 W / (m·K) (a decrease of approximately 14%). This is because... The coating has high emissivity, which can further dissipate the heat absorbed by the heat-facing surface outward through radiation, reducing the heat flow entering the material's interior. At the same time, the dense coating also acts as a surface sealant and erosion resistant agent, improving the material's surface strength and thermal shock resistance, with the strength retention rate after thermal shock increasing to 93%.

[0099] The flexural strengths of Examples 9 (2.5D weaving) and 10 (orthogonal triaxial weaving) were 98 MPa and 100 MPa, respectively, both higher than that of Example 1 (3D needle-punched felt, 95 MPa). This is because the fiber orientation in the woven preform is more ordered, resulting in higher load transfer efficiency. However, the 3D needle-punched felt preform has lower manufacturing costs, simpler processes, and better in-plane isotropy. Users can choose the appropriate preform form according to the structural requirements of specific application scenarios.

[0100] The overall performance of each embodiment of the present invention is significantly better than that of the comparative examples. Taking the optimal embodiment 7 as an example: density 1.85 g / cm³, room temperature thermal conductivity 0.058 W / (m·K), 1000℃ thermal conductivity 0.17 W / (m·K), flexural strength 102 MPa, and strength retention rate after thermal shock resistance 92%, the overall performance meets the application requirements of aerospace thermal protection materials.

[0101] In contrast, Comparative Example 1 (without) The flexural strength of the first comparative example (interface layer) is only 52 MPa, the thermal conductivity at 1000℃ is as high as 0.32 W / (m·K), and the strength retention rate after thermal shock is only 72%; the flexural strength of the second comparative example (without gradient structure) is only 62 MPa, and the thermal conductivity at 1000℃ is 0.28 W / (m·K); the yield of the third comparative example (traditional drying process) is only 55%, and the flexural strength is only 58 MPa. These three comparative examples are far inferior to the embodiments of the present invention in terms of mechanical properties, thermal insulation performance, and process reliability.

[0102] The composite material of this invention can be used as rigid thermal insulation tiles, thermal sealing structural components, or thermal insulation gaskets for thermal protection systems (TPS) of aerospace vehicles or hot-end components of engines. Specific application methods are as follows: (1) Based on the aerodynamic shape and heat flow distribution of the aircraft, the composite material is processed into heat insulation tiles / heat insulation panels of the required shape and size; (2) Install and fix the heat-facing surface (high density layer) towards the high-temperature airflow direction and the heat-reducing surface (high porosity layer) towards the aircraft structure body; (3) For those with Example 4 of the coating: The coating side faces the high-temperature heat flow directly, and the high emissivity of the coating is used to dissipate radiant heat outward. (4) The operating temperature range is from room temperature to 1400℃ (long-term use), and it can withstand high temperature of 1600℃ for short periods.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An alumina fiber composite material for aerospace thermal protection, characterized in that, The composite material has a gradient multi-level porous structure along the thickness direction, including a heat-facing surface layer, an intermediate transition layer and a heat-reducing surface layer with successively increasing porosity; The composite material comprises an alumina fiber reinforcement, a hierarchical porous alumina-mullite multiphase matrix, and nanomaterials disposed on the surface of the alumina fiber reinforcement. Interface layer.

2. The alumina fiber composite material according to claim 1, characterized in that: The porosity of the heat-facing surface layer is 15%-25%, the porosity of the intermediate transition layer is 35%-50%, and the porosity of the heat-reducing surface layer is 50%-65%. The thickness of the heat-facing surface layer accounts for 20%-30% of the total thickness of the composite material, the thickness of the intermediate transition layer accounts for 40%-50% of the total thickness of the composite material, and the thickness of the heat-reducing surface layer accounts for 20%-30% of the total thickness of the composite material. The composite material has a multi-level pore size distribution, including micropores with a pore size <2nm, mesopores with a pore size of 2-50nm, and macropores with a pore size >50nm.

3. The alumina fiber composite material according to claim 1, characterized in that, The alumina fiber reinforcement has a fiber volume fraction of 15%-35% and a fiber diameter of 5-12 μm; The alumina fiber reinforcement is selected from one or more combinations of chopped fibers, two-dimensional alumina fiber lay-up, 2.5D woven preform, orthogonal triaxial woven preform, or three-dimensional needle-punched felt.

4. The alumina fiber composite material according to claim 1, characterized in that, The nano The interface layer is a nano-sized material grown in situ on the surface of alumina fibers via cerium nitrate solution impregnation-thermal decomposition. The coating, the nano The thickness of the interface layer is 50-200 nm, and the nanometer... The interfacial shear strength between the interface layer and the hierarchical porous alumina-mullite matrix is ​​40-80 MPa.

5. The alumina fiber composite material according to claim 1, characterized in that, The hierarchical porous alumina-mullite multiphase matrix also contains dispersed functional fillers, which include: Nano-zirconia powder with an average particle size of 50-100 nm and a mass fraction of 5%-15% of the dry weight of the matrix; Silicon carbide nanowires, with a diameter of 50-150 nm, a length of 5-20 μm, and a mass fraction of 1%-5% of the dry weight of the matrix.

6. The alumina fiber composite material according to claim 1, characterized in that, The composite material also has a high-emissivity oxide thermal barrier coating disposed on the heat-facing surface of the composite material, the high-emissivity oxide thermal barrier coating comprising: , , Or one of the doped perovskite structure oxides, with a coating thickness of 50-150 μm.

7. A method for preparing an alumina fiber composite material for aerospace thermal protection according to any one of claims 1-6, characterized in that, Includes the following steps: Step 1: After heat treatment to remove the sizing agent from the alumina fiber preform, it is immersed in a cerium salt solution, dried and heat-treated to grow a nano-CeO2 interface layer in situ on the surface of the alumina fiber. Step 2: Prepare gradient structure slurries for forming the heat-facing layer, intermediate transition layer and back-heat-facing layer respectively. The content of pore-forming agent in each layer of slurry increases sequentially from the heat-facing layer to the back-heat-facing layer. Step 3: Place the alumina fiber preform processed in Step 1 into an impregnation tank, and use the gradient structure slurry in the order of the heat-facing layer, the intermediate transition layer and the back heat-facing layer for gradient impregnation. After gelation, the composite material preform is obtained. Step 4: Dry the composite material preform; Step 5: Sinter the dried composite material blank to decompose the pore-forming agent into multi-level pores, and at the same time sinter the matrix to densify, to obtain the alumina fiber composite material.

8. The preparation method according to claim 7, characterized in that, The gradient impregnation described in step three adopts a vacuum-pressure alternating impregnation method, which alternates multiple impregnation-degassing cycles under vacuum conditions below -0.095MPa and pressure conditions of 0.5-1.0MPa; wherein, the heat-facing surface layer slurry undergoes 3-5 impregnation-degassing cycles, while the intermediate transition layer slurry and the back-heat-facing surface layer slurry each undergo 2-3 impregnation-degassing cycles.

9. The preparation method according to claim 7, characterized in that: In step one, the heat treatment to remove the wetting agent is performed at a temperature of 600-800℃ for 1-3 hours; the cerium salt solution is cerium nitrate with a concentration of 0.05-0.2 mol / L. The solution is soaked in an aqueous solution and then subjected to ultrasonic treatment for 30-60 minutes; the heat treatment temperature for in-situ growth is 500-700℃ and the time is 1-2 hours. In step two, the pore-forming agent is a mixture of polymethyl methacrylate (PMMA) microspheres and starch granules, wherein the PMMA microspheres have a particle size of 1-10 μm and the starch granules have a particle size of 10-50 μm; the amount of PMMA pore-forming agent added to the heat-facing surface layer slurry is 5%-10% of the total solid mass, the amount of PMMA pore-forming agent added to the intermediate transition layer slurry is 15%-25% of the total solid mass and also contains 5%-10% starch granules, and the amount of PMMA pore-forming agent added to the back-heating surface layer slurry is 25%-40% of the total solid mass and also contains 10%-20% starch granules; In step four, the drying is a gradient temperature drying, which includes: the first stage of drying at a temperature of 40-50℃ and a relative humidity of 80%-90% for 24-48 hours, the second stage of drying at a temperature of 80-100℃ for 12-24 hours, and the third stage of drying at a temperature of 120-150℃ for 6-12 hours. In step five, the sintering includes: heating to 400-500℃ at a rate of 1-3℃ / min and holding for 2-4 hours to completely decompose the pore-forming agent; then heating to 1200-1400℃ at a rate of 2-5℃ / min and holding for 1-3 hours to sinter and densify the matrix.

10. The preparation method according to claim 7, characterized in that, The gradient structure slurry described in step two also contains functional fillers dispersed therein, including nano-zirconia powder and silicon carbide nanowires; the nano- The amount of powder added is 5%-15% of the total solid mass in the heat-facing surface layer slurry and the intermediate transition layer slurry, and the amount of nano-powder in the back-heating surface layer slurry is 5%-15%. The amount of powder added is 40%-60% of the heat-facing surface slurry; the amount of SiC nanowires added is 1%-5% of the total solid mass in each layer of slurry.