A method for producing a low-expansion silicon-based ceramic core

Abstract

The application belongs to the field of inorganic nonmetallic materials, and particularly relates to a preparation method of a low-expansion silicon-based ceramic core, which comprises the following steps: carrying out surface modification on quartz glass micropowder by using a gamma-methacryloxypropyltrimethoxysilane coupling agent; introducing 3-8% zirconium oxide nanofibers as a stress relaxation phase; realizing more than 95% dispersion uniformity through high-energy ball milling mixing; precisely controlling the heating rate of each temperature interval by adopting gradient heating sintering to realize uniform conversion of cristobalite crystal phase; and finally removing the stress relaxation phase through alkali corrosion treatment to form a connected pore network with a porosity of 15%-30%. The application utilizes the stress-induced phase change characteristics and crack deflection toughening mechanism of zirconium oxide to make the core fracture toughness reach 5-8 MPa.m^(1 / 2), the thermal expansion coefficient is controlled within 6-10*10^(-6) / DEG C, the removal efficiency is improved by 50%-70%, and the dispersion uniformity reaches more than 95%.

Description

Technical Field

[0003] This invention belongs to the field of inorganic non-metallic materials, specifically a method for preparing a low-expansion silicon-based ceramic core. Background Technology

[0005] Ceramic cores are key forming components in the investment casting process of aero-engine and gas turbine blades. Their performance directly determines the forming complexity and dimensional accuracy of the blade's internal structure. Silicon-based ceramic cores, due to their excellent high-temperature stability and low thermal expansion characteristics, have become the preferred core material for casting large-sized complex hollow blades.

[0006] Existing technologies typically employ process methods such as adjusting powder gradation, adding sintering aids, or optimizing the heating regime to improve the sintering performance of silicon-based ceramic cores. These methods can, to a certain extent, control the sintering densification process of the cores and meet the core preparation requirements for conventional performance.

[0007] However, the above-mentioned processes all have insurmountable technical limitations. The introduction of sintering aids often leads to the enrichment of undesirable grain boundary phases, which weakens the material's high-temperature creep resistance. The optimization of powder gradation and heating curves is limited by the intrinsic crystal phase transformation characteristics of silicon source, and cannot fundamentally eliminate the accumulation of thermal stress caused by shrinkage mismatch. As a result, the core is prone to microcracks or geometric distortions due to the imbalance of thermal expansion coefficient gradient in the subsequent high-temperature casting environment. The synergistic control of low expansion characteristics and structural integrity has become a key technical problem that urgently needs to be solved in this field.

[0008] Therefore, the present invention provides a method for preparing a low-expansion silicon-based ceramic core. Summary of the Invention

[0010] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0011] The technical solution adopted by this invention to solve its technical problem is: a method for preparing a low-expansion silicon-based ceramic core, comprising the following steps:

[0012] Step 1: Pretreatment and surface modification of the silicon source matrix material. Quartz glass micron powder is placed in a ball mill for pretreatment, using a wet ball milling process to control the average particle size of the quartz glass micron powder within the range of 0.5 to 5 microns. This eliminates the adverse effects of large particle agglomerates on the uniformity of the crystal phase transformation during subsequent sintering. After ball milling, a silane coupling agent is added to the slurry system for surface modification. The silane coupling agent selected is γ-methacryloyloxypropyltrimethoxy The mechanism of action of the silane is that the methoxy group in the coupling agent molecule undergoes a condensation reaction with the silanol group on the surface of the quartz glass micron powder, forming a modified coating layer with a thickness of 2 to 5 nanometers on the surface of the micron powder. This modified coating layer can effectively reduce the hydrophilicity of the micron powder surface and improve the dispersion stability of the micron powder in the subsequent mixing process. At the same time, the silicon carbide crystal nuclei generated by the decomposition of the modified coating layer during the sintering stage can act as heterogeneous nucleation cores to promote the uniform precipitation of cristobalite, laying the foundation for the subsequent precise control of the crystal phase transformation rate.

[0013] Step 2: Introducing and pre-dispersing the stress relaxation phase material. Zirconia nanofibers are selected as the stress relaxation phase material and dispersed in anhydrous ethanol to prepare a suspension with a concentration of 5 to 15% by mass. The length of the zirconia nanofibers is controlled within the range of 200 to 800 nanometers, and the diameter is controlled within the range of 10 to 50 nanometers. The determination of this geometric size range is based on the following technical considerations: if the fiber is too long, it is easy to entangle and form stress concentration points when mixed with the matrix material; if the fiber is too short, it cannot effectively exert the crack deflection and bridging toughening mechanism; if the fiber diameter is too fine, the specific surface area is too large, making it difficult to disperse evenly in the matrix; if the fiber diameter is too coarse, the interfacial bonding strength with the matrix is ​​insufficient. The zirconia nanofiber suspension is dispersed using an ultrasonic dispersion device with a processing power of 800 to 1200 watts and a processing time of 15 to 30 minutes. The cavitation effect of ultrasound can effectively break the weak interaction forces between nanofibers, so that the nanofibers are fully dispersed into a single state, creating favorable conditions for subsequent uniform mixing with the silicon source matrix material.

[0014] Step 3: Mixing and dispersing the stress-relaxed phase with the silicon source matrix material. Surface-modified quartz glass micron powder and dispersed zirconia nanofiber suspension are added to a high-energy ball mill mixing device in a specific ratio. The amount of zirconia nanofiber added is controlled within 3 to 8% of the mass of the silicon source matrix material. The mixing process is carried out using planetary ball milling, with the milling speed set to 200 to 350 rpm and the milling time to 2 to 4 hours. Zirconia balls are used as the milling media to avoid introducing metallic impurities. During the milling process, the ball-to-material ratio is adjusted to control the uniformity of mixing. The ball-to-powder ratio was set to a range of 5:1 to 10:1. Under these parameters, the mechanical force of high-energy ball milling enabled zirconia nanofibers to be uniformly embedded in the aggregate structure of quartz glass micron powder. At the same time, the continuous mechanical force caused moderate breakage of the silane coupling agent modified coating layer. The resulting nanoscale fragments were transformed into silicon carbide nuclei during the subsequent sintering process. After mixing, the slurry was placed in a vacuum drying oven for drying. The drying temperature was controlled within the range of 60 to 80°C, and the drying time was 12 to 24 hours until the anhydrous ethanol solvent in the slurry was completely evaporated, resulting in a loose mixed powder.

[0015] Step 4: Molding and granulation of the mixed powder. The dried mixed powder is granulated with an organic binder system, which includes polyvinyl butyral, polyethylene glycol, and stearic acid. The mass fraction of polyvinyl butyral is 1.5 to 3%, the mass fraction of polyethylene glycol is 1 to 2%, and the mass fraction of stearic acid is 0.5 to 1%. The addition of the organic binder system can give the mixed powder good plasticity and molding properties. The granulation process is carried out by spray drying. The inlet air temperature of spray drying is controlled within the range of 150 to 180°C, and the outlet air temperature is controlled within the range of 80 to 100°C. The resulting granulated powder has a particle size distribution in the range of 80 to 200 micrometers and has good flowability and filling properties. The granulated powder is filled into a metal mold for pressing. The pressing pressure is controlled within the range of 30 to 80 MPa, and the holding time is 30 to 60 seconds. During the pressing process, the organic binder undergoes plastic flow, causing the powder particles to be tightly packed, forming a green sample with initial shape and strength.

[0016] Step 5: Perform gradient heating sintering of the core blank. Place the formed core blank in an atmosphere sintering furnace for gradient heating sintering. The sintering process is divided into multiple temperature ranges for precise control. Within the temperature range of room temperature to 600℃, the temperature is slowly increased at a rate of 2 to 5℃ per minute to cause the organic binder system to decompose and release. If the temperature is increased too quickly, the gases generated by the rapid decomposition of the organic binder cannot be discharged in time, resulting in porosity defects inside the core. Within the temperature range of 600 to 1000℃, the temperature is increased at a rate of 3 to 8℃ per minute. At this time, the silane coupling agent modified coating layer undergoes thermal decomposition and generates silicon carbide crystal nuclei. An interface begins to form between the zirconium oxide nanofibers and the quartz glass matrix. For surface bonding, the temperature is precisely controlled within the range of 1000 to 1200℃, with a heating rate of 1 to 3℃ per minute. This temperature range is the key stage for the transformation of quartz glass into cristobalite. Precise control of the heating rate enables the cristobalite crystal nuclei to nucleate uniformly and grow slowly, effectively avoiding uneven volume shrinkage caused by excessively rapid phase transformation. Within the range of 1200 to 1300℃, the temperature is increased at a rate of 1 to 2℃ per minute and held for 1 to 3 hours to complete the densification sintering of the core material. During the sintering process, the zirconia nanofibers maintain their fiber morphology and form a strong interfacial bond with the cristobalite matrix. After the holding period, the temperature is cooled to room temperature at a rate of 3 to 5℃ per minute.

[0017] Step Six: Alkali etching and cleaning of the sintered core. The sintered and cooled silicon-based ceramic core is placed in a sodium hydroxide aqueous solution with a concentration of 20 to 40% by mass. The etching temperature is controlled within the range of 80 to 120°C, and the etching time is 2 to 8 hours. The purpose of alkaline etching is to remove the stress relaxation phase components in the core and form interconnected pore channels inside the core. Zirconia nanofibers undergo a dissolution reaction in the alkaline environment to generate sodium zirconate, which is released into the alkaline solution. After alkaline etching, the core is removed and placed in deionized water for multiple ultrasonic cleanings to remove residual alkaline solution and dissolution products on the surface and inside the pores of the core. The cleaning process uses ultrasonic-assisted cleaning with an ultrasonic power of 200 to 500 watts and a cleaning time of 10 to 30 minutes. After cleaning, the core is placed in a drying oven for final drying. The drying temperature is controlled within the range of 80 to 120°C, and the drying time is 2 to 4 hours, resulting in a finished silicon-based ceramic core with a low coefficient of thermal expansion and an internal microporous structure.

[0018] Preferably, in the process of introducing zirconia nanofibers, the zirconia nanofibers have a metastable tetragonal zirconia phase structure. Tetragonal zirconia has stress-induced phase transformation characteristics. When the stress field at the crack tip reaches a critical value, tetragonal zirconia can undergo a phase transformation to a monoclinic phase, accompanied by a 3 to 5 percent volume expansion. This volume expansion effect can generate compressive stress at the crack tip, offsetting the effect of external tensile stress, thereby effectively inhibiting further crack propagation. At the same time, the phase transformation process absorbs a large amount of fracture energy, significantly increasing the resistance to crack propagation.

[0019] Preferably, in the surface modification treatment, the mass addition amount of the silane coupling agent is controlled within the range of 0.5 to 2% of the mass of the silicon source matrix material. If the amount of coupling agent added is too small, a complete modified coating layer cannot be formed on the surface of the micron powder, and the modification effect is not significant. If the amount of coupling agent added is too large, an excessively thick coating layer will be formed on the surface of the micron powder. This coating layer decomposes during sintering to generate too many silicon carbide crystal nuclei, causing the phase transformation behavior of the matrix material to deviate from the preset uniform distribution pattern, thereby affecting the stability of the final thermal expansion coefficient of the core.

[0020] Preferably, in the mixing and dispersion process, a dispersant is introduced simultaneously during the high-energy ball milling mixing to further improve the dispersion uniformity of zirconia nanofibers. The dispersant is polyvinylpyrrolidone, and the mass addition amount of the dispersant is controlled within the range of 5 to 15 percent of the mass of the zirconia nanofibers. Polyvinylpyrrolidone molecules are adsorbed on the surface of nanofibers through anchoring groups. Its long chain structure forms a steric hindrance effect in anhydrous ethanol medium, effectively preventing the re-aggregation between nanofibers. The introduction of the dispersant can enable the dispersion uniformity of zirconia nanofibers in the final core to reach more than 95 percent.

[0021] Preferably, in the molding and granulation process, the pressing process is carried out in a bidirectional pressing manner, with the pressing direction along the height direction of the core. This makes the filling density of the powder particles in the mold more uniform and consistent, avoiding the density difference between the upper and lower parts of the core caused by unidirectional pressing. After pressing, a holding pressure treatment is performed to allow the powder particles to undergo sufficient plastic deformation and rearrangement under pressure. Extending the holding pressure time allows the organic binder sufficient time to form a uniform lubricating film layer between the particles, reducing the risk of cracks on the core surface during subsequent demolding.

[0022] Preferably, in the gradient heating sintering process, the control of the heating rate in the temperature range of 600 to 1000°C is particularly critical. This temperature range is the main stage for the thermal decomposition of the silane coupling agent modified coating layer to form silicon carbide crystal nuclei. If the heating rate is too fast, the decomposition reaction of the modified coating layer will be too violent, resulting in too many silicon carbide crystal nuclei and uneven distribution. If the heating rate is too slow, the crystal nucleus formation time will be too long, resulting in too high nucleation density of cristobalite and too small crystal size, which will not be able to form an effective stress relaxation microstructure. Therefore, accurately controlling the heating rate within the range of 3 to 8°C per minute can enable the modified coating layer to decompose smoothly within an appropriate temperature window, generating a moderate number of silicon carbide crystal nuclei with uniform distribution.

[0023] Preferably, in the alkaline corrosion treatment, the concentration of the sodium hydroxide aqueous solution and the corrosion temperature need to be precisely matched according to the content of zirconia nanofibers in the core. When the zirconia nanofiber content is 3 to 5%, the sodium hydroxide concentration can be controlled in the range of 20 to 30% by mass and the corrosion temperature in the range of 80 to 100°C. When the zirconia nanofiber content is 5 to 8%, the sodium hydroxide concentration needs to be increased to the range of 30 to 40% by mass and the corrosion temperature needs to be increased to the range of 100 to 120°C to ensure that the zirconia nanofibers can be completely dissolved and removed. After the alkaline corrosion treatment, the porosity inside the core is controlled in the range of 15 to 30%. This porosity range can ensure that the core has good alkaline penetration channels in the subsequent casting process, which facilitates the complete removal of the core.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. The present invention discloses a method for preparing a low-expansion silicon-based ceramic core. By introducing zirconia nanofibers as a stress-relaxing phase and utilizing the stress-induced phase transformation characteristics of zirconia, a synergistic mechanism of crack deflection toughening and phase transformation toughening is established inside the core. This improves the fracture toughness of the core from less than 3 MPa·m^1 / 2 in the prior art to 5 to 8 MPa·m^1 / 2. This effectively overcomes the technical bottleneck of brittle ceramic materials lacking an effective stress relaxation mechanism. When the thermal stress generated during casting cooling is applied to the core, the zirconia nanofibers can absorb fracture energy through crystal phase transformation and generate compressive stress on the crack tip, significantly inhibiting the initiation and propagation of microcracks. This fundamentally solves the technical limitation of traditional low-expansion regulators that only reduce the coefficient of thermal expansion but cannot improve the load-bearing capacity of the material.

[0026] 2. The method for preparing a low-expansion silicon-based ceramic core according to the present invention, through the synergistic combination of silane coupling agent surface modification treatment and gradient temperature sintering process, precisely controls the nucleation rate and crystal growth rate of the transformation of quartz glass to cristobalite crystal phase, so that the cristobalite crystal phase is uniformly distributed throughout the core volume, effectively eliminating the volume shrinkage mismatch problem caused by uneven crystal phase transformation. The uniform distribution of the cristobalite crystal phase enables the thermal expansion coefficient of the core to be stably controlled within the range of 6 to 10 × 10^-6 per ℃ in the temperature range of 25 to 800℃. Compared with the prior art, the uniformity of thermal expansion coefficient distribution is improved by 30 to 40 percent, and the thermal stress distribution of the core during the casting cooling process is more uniform, which greatly reduces the risk of microcracks caused by the imbalance of thermal expansion coefficient gradient.

[0027] 3. The method for preparing a low-expansion silicon-based ceramic core according to the present invention involves removing zirconia nanofibers through alkaline etching, thereby forming a connected pore network structure inside the core. The porosity of this pore structure is precisely controlled within the range of 15% to 30%, and the pore size is distributed within the range of 10 to 50 micrometers. The uniformly distributed pore structure provides unobstructed channels for alkaline penetration during the subsequent casting process, thereby increasing the removal efficiency of the core after casting by 50% to 70%. At the same time, the presence of the pore structure does not affect the high-temperature load-bearing capacity and shape retention accuracy of the core during the casting process, ensuring the service performance of the core throughout the entire casting process.

[0028] 4. The method for preparing a low-expansion silicon-based ceramic core according to the present invention achieves uniform dispersion of zirconia nanofibers in a quartz glass matrix through synergistic optimization of high-energy ball milling mixing process and dispersant system, with a dispersion uniformity of over 95%. The uniform distribution of nanofibers ensures the spatial consistency of stress relaxation effect within the entire core volume, avoiding local stress concentration and performance fluctuations caused by uneven dispersion. At the same time, the mechanical force of planetary ball milling causes the silicon carbide nuclei formed by the moderate crushing of the silane coupling agent modified coating layer to be uniformly distributed, further enhancing the microstructure uniformity of the core and significantly improving the batch stability of the core product performance. Attached Figure Description

[0030] The invention will now be further described with reference to the accompanying drawings.

[0031] Figure 1 This is a flowchart of a method for preparing a low-expansion silicon-based ceramic core according to the present invention;

[0032] Figure 2 This is a flowchart of the key process stages of gradient heating sintering and alkaline corrosion treatment of the core green blank in this invention. Detailed Implementation

[0034] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0035] Example 1:

[0036] like Figure 1 and Figure 2 As shown, the preparation method of the low-expansion silicon-based ceramic core of the present invention includes six core process steps. Through the synergistic cooperation of stress-relaxed phase materials and gradient temperature sintering process, the low thermal expansion coefficient and high structural integrity of the ceramic core are synergistically controlled.

[0037] In step one, the silicon source matrix material undergoes pretreatment and surface modification. Quartz glass micron powder, as the silicon source matrix material, is placed in a ball mill for wet ball milling pretreatment. The ball mill uses a planetary ball mill with a polyurethane liner to prevent metal contamination. The milling media are zirconia balls, with a diameter distribution of 0.5 to 2 mm for large balls and 0.1 to 0.5 mm for small balls in a mass ratio of 3:7. The large balls primarily impact and break up large agglomerates in the quartz glass micron powder, while the small balls grind and shape the broken particles, narrowing the particle size distribution. The ball mill speed is set to 300 to 500 rpm, and the milling time is 4 to 8 hours. Under these parameters, the average particle size of the quartz glass micron powder decreases from the original 10 to 30 microns and is stably controlled within the target range of 0.5 to 5 microns.

[0038] During ball milling, deionized water is continuously introduced as the grinding medium. The amount of water added is controlled based on a slurry solid content of 60 to 70% by mass. Too low a slurry solid content will lead to a decrease in ball milling efficiency, while too high a slurry solid content will lead to an increase in slurry viscosity and affect the ball milling effect. After wet ball milling, the slurry is subjected to solid-liquid separation. The separation equipment is a vacuum filtration device or a centrifugal separator. The separated solid powder is placed in a forced-air drying oven for preliminary drying at a temperature of 80 to 100°C for 2 to 4 hours, so that the powder moisture content is reduced to below 1% by mass.

[0039] After ball milling pretreatment, γ-methacryloxypropyltrimethoxysilane is added to the slurry system as a surface modifier. The mass addition of the silane coupling agent is controlled within the range of 0.5% to 2% of the mass of the quartz glass micron powder. The specific addition amount needs to be matched and adjusted according to the subsequent sintering process parameters. Anhydrous ethanol is used as the solvent for dilution of the silane coupling agent. The mass-volume ratio of the coupling agent to anhydrous ethanol is 1:5 to 1:10. The diluted coupling agent solution is slowly added dropwise to the quartz glass micron powder slurry system under magnetic stirring at a stirring speed of 200 to 400 rpm. The dropping speed is controlled at 0.5 to 1 ml per minute to ensure that the coupling agent molecules can fully contact the surface of the micron powder. After the dropping is completed, the reaction continues under stirring. The reaction temperature is controlled within the range of 40 to 60°C, and the reaction time is 2 to 4 hours.

[0040] Under these conditions, the methoxy group in the coupling agent molecule undergoes a condensation and dehydration reaction with the silanol groups on the surface of the quartz glass micron powder, forming a uniform modified coating layer on the surface of the micron powder. The thickness of the modified coating layer is precisely controlled by the amount of coupling agent added. When the amount added is 0.5 to 1%, the thickness of the modified coating layer is about 2 to 3 nanometers.

[0041] When the addition amount is 1 to 2 percent, the thickness of the modified coating layer is about 3 to 5 nanometers. The modified coating layer can effectively reduce the hydrophilicity of the surface of micron powder, and increase the contact angle of the surface of micron powder from 20 to 40 degrees before modification to 60 to 80 degrees. This significantly improves the dispersion stability of micron powder in anhydrous ethanol medium during subsequent mixing, and prevents the powder particles from agglomerating and settling due to strong hydrophilicity.

[0042] The core technical point of this invention, which uses γ-methacryloxypropyltrimethoxysilane as a modifier, is that the silicon carbide precursor structure in the coupling agent molecule can be transformed into silicon carbide crystal nuclei during the thermal decomposition process in the subsequent sintering stage. The specific reaction mechanism is as follows: In the temperature range of 600 to 1000°C in step five, the modified coating layer undergoes thermal decomposition reaction, the coupling agent molecular skeleton breaks and undergoes carbonization reaction, generating nanoscale silicon carbide particles.

[0043] The silicon carbide nuclei have a size distribution of 5 to 20 nanometers and play the role of heterogeneous nucleation cores in the nucleation process of cristobalite. Since the silicon carbide nuclei are uniformly distributed on the surface of quartz glass micron powder, the nucleation sites of cristobalite are also uniformly distributed, thus ensuring the uniform precipitation of the cristobalite crystal phase throughout the entire core volume.

[0044] The density of silicon carbide nuclei is controlled by the amount of coupling agent added. When the amount of coupling agent added is 0.5 to 1%, the areal density of silicon carbide nuclei is about 5 to 10 per square micrometer; when the amount of coupling agent added is 1 to 2%, the areal density of silicon carbide nuclei is about 10 to 20 per square micrometer.

[0045] In step two, stress-relaxing phase material is introduced and pre-dispersed. This invention selects zirconia nanofibers as stress-relaxing phase material. The core technical feature of this material is that its crystal structure is metastable tetragonal zirconia.

[0046] Zirconia nanofibers are prepared by electrospinning combined with high-temperature sintering. In the preparation process, zirconium salt precursors such as zirconium oxychloride are first dissolved in an organic solvent to form a spinning solution. The solution is then used to form fiber filaments under a high-voltage electric field through an electrospinning device. Subsequently, the fibers are heat-treated at a temperature of 800 to 1000°C to crystallize them into tetragonal zirconium oxide.

[0047] The geometric dimensions of zirconia nanofibers are precisely controlled within a specific range: the fiber length is controlled within the range of 200 to 800 nanometers, and the fiber diameter is controlled within the range of 10 to 50 nanometers. The determination of this geometric dimension range is based on the following technical mechanism analysis: when the fiber length is less than 200 nanometers, although the fiber has good dispersion performance when mixed with the matrix material, its insufficient aspect ratio makes it unable to effectively exert the crack bridging toughening mechanism, and the crack deflection effect is also weakened accordingly.

[0048] When the fiber length is greater than 800 nanometers, the tendency for entanglement between fibers increases significantly. When mixed with the matrix material, entangled agglomerates are easily formed. These entangled agglomerates become stress concentration points after sintering, which weakens the mechanical properties of the material.

[0049] When the fiber diameter is less than 10 nanometers, the specific surface area of ​​the fiber increases dramatically, reaching 200 to 300 square meters per gram. The excessively large specific surface area causes the fiber to easily re-aggregate in anhydrous ethanol medium due to van der Waals forces, resulting in a significant decrease in dispersion stability.

[0050] When the fiber diameter is greater than 50 nanometers, the interfacial bonding area between the fiber and the matrix material is relatively reduced, and the interfacial bonding strength is insufficient to effectively transfer stress. The crack deflection effect and phase transformation toughening effect cannot be fully utilized.

[0051] The pre-dispersion treatment of zirconia nanofibers was carried out using an ultrasonic dispersion device; the zirconia nanofibers were dispersed in anhydrous ethanol medium to prepare a suspension with a concentration of 5 to 15% by mass.

[0052] The suspension preparation process is as follows: First, zirconia nanofibers are mixed with anhydrous ethanol at a mass percentage of 5 to 15. Then, the mixture is placed in a magnetic stirrer for initial stirring and dispersion at a speed of 500 to 800 revolutions per minute for 10 to 20 minutes, so that the nanofibers are initially and evenly dispersed in anhydrous ethanol.

[0053] After initial dispersion, the suspension is transferred to an ultrasonic dispersion device for processing. The power of the ultrasonic generator is set to 800 to 1200 watts, the ultrasonic frequency is set to 20 to 40 kHz, and the ultrasonic processing time is 15 to 30 minutes. The cavitation effect of the ultrasonic waves generates a large number of microbubbles in the suspension. These microbubbles expand rapidly under the negative pressure of the sound waves and collapse rapidly under the positive pressure, forming local high temperature and high pressure micro-regions.

[0054] The microjets and shock waves generated by cavitation can effectively break the van der Waals forces between nanofibers, allowing the entangled and agglomerated nanofibers to be fully dispersed into individual fibers. During ultrasonic dispersion, the temperature of the suspension needs to be controlled to not exceed 40°C to prevent the loss of anhydrous ethanol due to excessive temperature rise. When the temperature of the suspension exceeds 35°C, ultrasonic treatment needs to be stopped and water bath cooling should be performed. After dispersion, the dispersion state of the nanofibers in the suspension is detected by a dynamic light scattering particle size analyzer to ensure that the agglomeration index of the nanofibers is less than 1.5, indicating that the dispersion state meets the requirements of the subsequent mixing process.

[0055] The tetragonal zirconium oxide nanofibers used in this invention have the core technological advantage of stress-induced phase transformation characteristics; when the stress field at the crack tip reaches a critical value, the tetragonal zirconium oxide can undergo a phase transformation to the monoclinic phase.

[0056] This phase transformation process is accompanied by a 3 to 5 percent volume expansion effect. This volume expansion generates compressive stress on the crack tip, which counteracts the destructive effect of external tensile stress, thereby effectively inhibiting the further propagation of the crack.

[0057] The crystal phase transformation process requires the absorption of a large amount of fracture energy. According to the principle of phase transformation thermodynamics, the enthalpy of this phase transformation is about 5 to 10 kilojoules per mole. This makes crack propagation require overcoming additional energy barriers, and the resistance to crack propagation increases significantly.

[0058] The phase transformation toughening effect and crack deflection toughening effect of zirconia nanofibers have a synergistic mechanism: when the crack encounters the nanofiber during the propagation process, it tends to be deflected along the interface between the fiber and the matrix. The bending of the crack path prolongs the crack propagation distance and increases the fracture surface area, thereby consuming more fracture energy.

[0059] In step three, the stress-relaxed phase and silicon source matrix material are mixed and dispersed. The surface-modified quartz glass micron powder and the dispersed zirconia nanofiber suspension are added to a high-energy ball mill mixing device in proportion for mixing.

[0060] The high-energy ball mill mixing equipment uses a planetary ball mill. The working principle of the planetary ball mill is to use the combined motion of revolution and rotation to generate high-frequency impact and friction force in the grinding media inside the ball mill jar, thereby realizing the crushing, mixing and dispersion of materials.

[0061] The amount of zirconia nanofibers added was controlled within the range of 3 to 8% of the mass of quartz glass micron powder. When the amount added was less than 3%, the number of nanofibers in the matrix material per unit volume was insufficient, and the stress relaxation effect could not form an effective continuous network throughout the core volume. When the amount added was more than 8%, the spacing between the nanofibers was too small, which easily led to local agglomeration. Furthermore, too many nanofibers would interfere with the normal nucleation and growth of the cristobalite crystal phase, affecting the low expansion performance of the core.

[0062] The process parameters for the mixing process are set as follows: the ball mill speed is set to 200 to 350 revolutions per minute, the ball milling time is 2 to 4 hours, and zirconia balls are selected as the grinding media to avoid introducing metal impurities for contamination; the grinding media are zirconia balls with a diameter of 3 to 5 mm. Zirconia balls have the characteristics of high hardness, high wear resistance and good chemical stability, and can maintain sphericity without significant wear during long-term ball milling.

[0063] During ball milling, the mixing uniformity is controlled by adjusting the ball-to-powder ratio, which is set to a range of 5:1 to 10:1, meaning the ratio of the mass of zirconia balls to the total mass of the mixed powder is 5:1 to 10:1. When the ball-to-powder ratio is 5:1 to 7:1, the impact and shear forces of the milling media on the mixed powder are relatively mild, making it suitable for conditions with low nanofiber content or small matrix powder particle size. When the ball-to-powder ratio is 7:1 to 10:1, the force of the milling media is enhanced, making it suitable for conditions with high nanofiber content or large matrix powder particle size.

[0064] During the high-energy ball milling process, the continuous mechanical force produces multiple technical effects. Zirconia nanofibers are forcibly embedded into the aggregate structure of quartz glass micron powder under the high-speed impact of the ball milling media. Since the hardness of the nanofibers is much higher than that of quartz glass, the nanofibers will cut and break up the aggregates during the embedding process, causing the aggregates to be dispersed into smaller particle units. The ball milling mechanical force also acts on the silane coupling agent modified coating layer on the surface of the quartz glass micron powder, causing the modified coating layer to be moderately broken, forming nanoscale fragments with a size of 5 to 20 nanometers. These nanoscale fragments become the precursor morphology of silicon carbide crystal nuclei in the subsequent sintering process in step five.

[0065] After ball milling and mixing, the mixed powder is vacuum dried to remove anhydrous ethanol solvent. The vacuum drying equipment uses a vacuum drying oven, with the drying temperature controlled in the range of 60 to 80°C, the drying time being 12 to 24 hours, and the vacuum degree controlled in the range of -0.08 to -0.1 MPa. Under vacuum conditions, the boiling point of anhydrous ethanol is lowered, enabling rapid evaporation at a lower temperature. At the same time, the vacuum environment can effectively prevent the oxidative deterioration of zirconia nanofibers during the drying process.

[0066] After drying, a loose mixed powder is obtained, with a bulk density of 0.8 to 1.2 g / cm³ and a flowability index of 25 to 30 seconds per 50 g.

[0067] To further improve the dispersion uniformity of zirconia nanofibers in the mixed powder, the present invention preferably introduces a dispersant simultaneously during the high-energy ball milling process; the dispersant is polyvinylpyrrolidone, whose amide groups in its molecular structure can be anchored to the surface of zirconia nanofibers through hydrogen bonding to form a stable adsorption layer; the mass addition amount of the dispersant is controlled within the range of 5 to 15 percent of the mass of the zirconia nanofibers.

[0068] The dispersant is added before the ball milling process begins. First, the dispersant is dissolved in anhydrous ethanol to form a dispersant solution. Then, the dispersant solution is premixed with the zirconia nanofiber suspension before being added to the ball mill jar. After polyvinylpyrrolidone molecules are adsorbed onto the surface of nanofibers through anchoring groups, their long-chain structure forms a steric hindrance effect in the anhydrous ethanol medium. When two nanofibers approach each other, the long chains of polyvinylpyrrolidone on their surfaces will repel each other, effectively preventing the re-aggregation of nanofibers.

[0069] The introduction of dispersant can enable the dispersion uniformity of zirconia nanofibers in the final core to reach more than 95%. The dispersion uniformity is detected by metallographic microscope combined with image analysis software. Ten fields of view are randomly selected for fiber distribution statistics and the coefficient of variation is calculated. A coefficient of variation of less than 5% indicates that the dispersion uniformity meets the requirements.

[0070] In step four, the mixed powder is shaped and granulated. The dried mixed powder is then granulated with an organic binder system. The formulation and mass fraction range of the organic binder system are as follows: polyvinyl butyral mass fraction is 1.5 to 3%, polyethylene glycol mass fraction is 1 to 2%, and stearic acid mass fraction is 0.5 to 1%.

[0071] Polyvinyl butyral, as the main binder component in organic binder systems, functions to form a bridging structure between powder particles, binding loose powder particles into particle aggregates with a certain strength. Polyvinyl butyral has a molecular weight of approximately 50,000 to 200,000, and its polymer segments can form physical entanglement and hydrogen bonding between powder particles, thereby providing bonding strength.

[0072] Polyethylene glycol, as a plasticizer, lowers the glass transition temperature of the organic binder system, enabling the binder to have better plastic flow properties at lower temperatures, thus allowing it to better fill the gaps between powder particles during granulation.

[0073] Polyethylene glycol has a molecular weight of approximately 1,000 to 6,000 and good flexibility of its molecular chain segments, which can effectively reduce the viscosity of the system. Stearic acid, as a lubricant, forms a lubricating layer on the surface of powder particles, reduces the frictional resistance between particles, and improves the flow properties of granulated powder. Stearic acid has a melting point of 68 to 70°C and can soften and melt under the inlet air temperature conditions of spray drying to form a uniform lubricating film on the particle surface.

[0074] The granulation process is carried out by spray drying. The inlet air temperature of the spray drying equipment is controlled within the range of 150 to 180°C, and the outlet air temperature is controlled within the range of 80 to 100°C. The atomizer is a centrifugal atomizer with a rotation speed of 15,000 to 25,000 revolutions per minute.

[0075] The mixed powder and organic binder system is first premixed by dissolving the organic binder in anhydrous ethanol to form a binder solution. Then, the binder solution and the mixed powder are mixed in a mass ratio of 1:2 to 1:3. The mixing equipment is a high-speed shear mixer with a shearing speed of 3000 to 5000 revolutions per minute and a mixing time of 5 to 10 minutes to form a uniform slurry system.

[0076] The slurry is pumped into a spray drying tower via a peristaltic pump. Under the high-speed rotation of the centrifugal atomizing disc, it is thrown out to form tiny droplets. The droplets exchange heat and mass with the hot air inside the drying tower. The solvent in the droplets evaporates rapidly, and the solute precipitates on the surface of the droplets to form a shell. As drying proceeds, the shell shrinks inward and eventually forms hollow spherical particles. The granulated powder obtained by spray drying has a particle size distribution in the range of 80 to 200 micrometers, and the particle morphology is spherical or nearly spherical, with good flowability and filling properties. The angle of repose of the granulated powder is less than 35 degrees, the compressibility is less than 15 percent, and the loose density is 1.0 to 1.4 g per cubic centimeter.

[0077] The granulated powder is filled into a metal mold for pressing and molding. The metal mold is made of high-strength tool steel, and the mold surface is chrome-plated to improve wear resistance and demolding performance. The pressing and molding process is carried out by bidirectional pressing, with pressure applied bidirectionally along the height direction of the core.

[0078] The working principle of bidirectional compression is to apply pressure to the powder inside the mold by the upper and lower pressure heads at the same time, so that the pressure is transmitted from both the upper and lower directions to the center of the mold, thereby achieving uniform compression of the powder particles.

[0079] Unidirectional pressing leads to uneven pressure distribution of powder within the mold, with a significant pressure gradient along the pressing direction. The pressure is lowest at the bottom of the mold, resulting in a density difference of 5 to 10 percent between the top and bottom of the core. In contrast, bidirectional pressing can make the pressure distribution more uniform and consistent, controlling the density difference between the top and bottom of the core to within 2 percent.

[0080] The pressing pressure is controlled within the range of 30 to 80 MPa. The selection of the pressing pressure needs to be adjusted according to the compressibility of the granulated powder. When the pressing pressure is lower than 30 MPa, the porosity between the powder particles is too high, and the density of the core after sintering is insufficient. When the pressing pressure is higher than 80 MPa, the powder particles will be excessively broken, and the organic binder will be extruded and lost under high pressure.

[0081] The holding time is 30 to 60 seconds. The purpose of the holding operation is to allow the powder particles to undergo sufficient plastic deformation and rearrangement under pressure, maximizing the contact area between the particles. The organic binder undergoes plastic flow during the pressing process, forming a continuous bonding network between the particles, tightly packing and connecting the loose granulated particles together to form a green sample with initial shape and strength. The density of the green sample is 55 to 65 percent of the theoretical density, and the flexural strength is 2 to 5 MPa.

[0082] In step five, a gradient heating sintering treatment is performed on the core blank; the formed core blank is placed in an atmosphere sintering furnace for gradient heating sintering.

[0083] The atmosphere sintering furnace adopts resistance heating, and the furnace chamber is made of alumina ceramic fiber. The effective heating space of the furnace chamber is 300 mm in diameter and 400 mm in height. The temperature uniformity inside the furnace is ±5 degrees Celsius. The sintering process is divided into multiple temperature zones for precise control. The heating rate of each temperature zone is optimized according to the physicochemical changes within that temperature zone.

[0084] Within the temperature range of room temperature to 600℃, the temperature is slowly increased at a rate of 2 to 5℃ per minute. This temperature range is the main stage for the thermal decomposition of the organic binder system in the core preform. The thermal decomposition process of the organic binder is as follows: stearic acid first melts and evaporates within the temperature range of 150 to 200℃, and the molten stearic acid forms a lubricating layer between the powder particles; polyethylene glycol undergoes thermal decomposition within the temperature range of 200 to 350℃, and the molecular chain segments break down to generate small molecule compounds that evaporate and escape.

[0085] Polyvinyl butyral undergoes thermal decomposition in the temperature range of 300 to 600°C, with the main molecular chain breaking and accompanied by carbonization. The decomposition products include carbon dioxide, water vapor, and carbonaceous residue. If the heating rate is too fast, the organic binder will decompose rapidly, generating a large amount of gas. These gases cannot be discharged in time and form porosity defects inside the core, with a porosity of 5 to 10 percent, which seriously affects the density and mechanical properties of the core.

[0086] If the heating rate is too slow, the sintering cycle will be prolonged, affecting production efficiency and increasing energy costs. Therefore, the heating rate should be precisely controlled within the range of 2 to 5°C per minute to ensure that the gaseous products of the organic binder can be discharged smoothly and to avoid an excessively long sintering cycle.

[0087] Within a temperature range of 600 to 1000°C, the temperature is increased at a rate of 3 to 8°C per minute. Two key physicochemical changes occur within this temperature range: First, the silane coupling agent modified coating undergoes a thermal decomposition reaction, causing the coupling agent molecular skeleton to break and undergo a carbonization reaction to generate silicon carbide nuclei. The size of the silicon carbide nuclei is 5 to 20 nanometers, and the location of the nuclei is consistent with the location of the modified coating formed by the silane coupling agent in step one. Therefore, the silicon carbide nuclei are uniformly distributed in the quartz glass matrix.

[0088] An interfacial bond begins to form between zirconia nanofibers and the quartz glass matrix. Within this temperature range, trace amounts of zirconia on the surface of the zirconia nanofibers undergo a solid-state reaction with the quartz glass, generating a zirconium silicate transition layer at the interface. This transition layer, approximately 2 to 5 nanometers thick, enhances the interfacial bond strength between the nanofibers and the matrix. Controlling the heating rate within the 600 to 1000°C temperature range is crucial. An excessively rapid heating rate leads to an overly vigorous decomposition reaction of the modified coating, resulting in an excessive number and uneven distribution of silicon carbide nuclei. Conversely, an excessively slow heating rate results in a prolonged nucleus formation time, leading to an excessively high nucleation density and a small crystal size in the cristobalite, hindering the formation of an effective stress-relaxed microstructure.

[0089] Within the temperature range of 1000 to 1200℃, precise temperature control was performed at a heating rate of 1 to 3℃ per minute; this temperature range is the key stage for the transformation of quartz glass into the cristobalite crystal phase; cristobalite is one of the high-temperature crystal forms of SiO2, and its coefficient of thermal expansion is approximately 18 × 10⁻⁻⁻⁶. 6 Each degree Celsius, higher than 0.5 × 10⁻ of quartz glass. 6 Each degree Celsius, but the phase transformation of cristobalite is reversible, and a reverse transformation to quartz glass occurs during the cooling process.

[0090] The nucleation sites of cristobalite crystal nuclei mainly occur on the surface of silicon carbide crystal nuclei formed in step two. Silicon carbide crystal nuclei have a heterogeneous nucleation catalytic effect on the transformation of quartz glass to cristobalite. The nucleation rate and crystal growth rate of cristobalite depend on temperature and heating rate. The nucleation rate is highest in the temperature range of 1100 to 1150℃, and the crystal growth rate is highest in the temperature range of 1150 to 1200℃. Precise control of the heating rate can enable cristobalite crystal nuclei to nucleate uniformly and grow slowly, effectively avoiding uneven volume shrinkage caused by excessively fast crystal phase transformation rate.

[0091] During the phase transformation of cristobalite, the volume shrinkage rate is approximately 2 to 5%. If this volume shrinkage is uneven within the core, it will generate internal thermal stress, which can lead to core cracking in severe cases. By precisely controlling the heating rate within the range of 1 to 3°C per minute, the phase transformation of cristobalite can proceed in a slow and controllable manner. The volume shrinkage generated by the phase transformation can be absorbed by the surrounding ceramic matrix through plastic flow, thus avoiding stress concentration.

[0092] Within a temperature range of 1200 to 1300°C, the core material is heated at a rate of 1 to 2°C per minute and held for 1 to 3 hours to achieve densification sintering. Within this temperature range, the cristobalite crystals continue to grow, with the crystal size increasing from 100 to 200 nanometers to 500 to 1000 nanometers. At the same time, sintering necks grow and fuse between adjacent crystals, forming a continuous three-dimensional network structure.

[0093] Zirconia nanofibers maintain their fibrous morphology during sintering, and their crystal structure is stable tetragonal zirconia. No significant grain growth or phase transformation occurs at the sintering temperature of 1300℃. The nanofibers and the cristobalite matrix form a strong interfacial bond through the zirconium silicate transition layer at the interface. During the densification sintering process, the relative density of the core increases from 55 to 65 percent in the green stage to 92 to 96 percent, and the porosity decreases from 45 to 35 percent to 4 to 8 percent.

[0094] After the heat preservation is completed, the temperature is cooled to room temperature at a rate of 3 to 5°C per minute. During the cooling process, cristobalite will undergo a reverse transformation to quartz glass. The starting temperature of the reverse transformation is about 573°C. The cooling rate needs to be carefully controlled near this temperature point to avoid thermal stress caused by the phase change volume change.

[0095] In step six, the sintered core undergoes alkaline etching and cleaning. The sintered silicon-based ceramic core, cooled to room temperature, is placed in an aqueous sodium hydroxide solution with a concentration of 20 to 40% by mass. The etching temperature is controlled within the range of 80 to 120°C, and the etching time is 2 to 8 hours. The purpose of the alkaline etching is to remove the stress relaxation phase component of the zirconia nanofibers in the core, thereby forming interconnected pore channels inside the core.

[0096] The dissolution mechanism of zirconia nanofibers under alkaline conditions is as follows: Zirconia reacts chemically with sodium hydroxide to produce sodium zirconate and water, and the reaction equation is ZrO2 + 2NaOH → Na2ZrO3 + H2O; sodium zirconate is readily soluble in aqueous solution and can diffuse out from the pore channels inside the core; after alkaline corrosion treatment, the original positions occupied by zirconia nanofibers are transformed into pores, and these pores are interconnected to form a network structure;

[0097] The concentration and corrosion temperature of the sodium hydroxide aqueous solution need to be precisely matched according to the content of zirconia nanofibers in the core. When the content of zirconia nanofibers is 3 to 5%, the concentration of sodium hydroxide can be controlled in the range of 20 to 30% by mass and the corrosion temperature can be controlled in the range of 80 to 100°C. When the content of zirconia nanofibers is 5 to 8%, the concentration of sodium hydroxide needs to be increased to the range of 30 to 40% by mass and the corrosion temperature needs to be increased to the range of 100 to 120°C.

[0098] The principle for selecting corrosion parameters is to ensure that the zirconia nanofibers can be completely dissolved and removed within a specified time. When the sodium hydroxide concentration or corrosion temperature is too low, the dissolution reaction rate decreases and the corrosion time needs to be extended accordingly. However, excessive corrosion time will cause alkaline corrosion of the cristobalite in the core matrix. When the sodium hydroxide concentration or corrosion temperature is too high, the dissolution reaction rate accelerates, but it will cause excessive corrosion of the core matrix, affecting the dimensional accuracy and surface quality of the core.

[0099] After the alkaline etching treatment, the porosity inside the core is controlled within the range of 15% to 30%, and the pore size is distributed within the range of 10 to 50 micrometers. This porosity range can ensure that the core has good alkaline penetration channels during the subsequent casting process, which facilitates the complete removal of the core.

[0100] After the alkaline corrosion treatment is completed, the core is removed and placed in deionized water for multiple ultrasonic cleanings to remove residual alkaline solution and dissolution products on the surface and inside the pores of the core. The cleaning process uses ultrasonic-assisted cleaning with an ultrasonic frequency of 20 to 40 kHz, an ultrasonic power of 200 to 500 watts, and a cleaning time of 10 to 30 minutes.

[0101] The cavitation effect of ultrasound can generate micro-jets in the micropores and deep holes on the core surface, washing out the alkaline solution and sodium zirconate crystals remaining in the pores. The cleaning needs to be performed 3 to 5 times, and the deionized water should be replaced after each cleaning until the pH value of the cleaning water drops to the near-neutral range of 7 to 8, indicating that the alkaline residues have been fully removed.

[0102] After cleaning, the core is placed in a drying oven for final drying. The drying temperature is controlled within the range of 80 to 120°C, and the drying time is 2 to 4 hours, resulting in a silicon-based ceramic core product with a low coefficient of thermal expansion and an internal microporous structure.

[0103] To verify the effectiveness of the technical solution of this invention, a detailed description of a specific application example is given using actual aero-engine blade casting as a scenario. The internal cooling channel structure of aero-engine high-pressure turbine blades is complex, requiring the ceramic core to have high dimensional accuracy, excellent high-temperature load-bearing capacity, and good removal performance. Taking the internal cooling channel of a certain type of high-pressure turbine blade as an example, the minimum cross-sectional dimension of the channel is 0.8 mm, the maximum length dimension is 120 mm, and the wall thickness uniformity is required to be within ±0.05 mm.

[0104] The specific implementation process is as follows: Quartz glass micron powder with an average particle size of 2 micrometers is placed in a planetary ball mill for wet ball milling pretreatment. The ball milling speed is 400 revolutions per minute and the ball milling time is 6 hours, so that the particle size distribution is narrowed to a narrow distribution range of D50 of 2.0 micrometers and D90 of 4.5 micrometers. After the ball milling pretreatment is completed, γ-methacryloyloxypropyltrimethoxysilane is added for surface modification treatment. The amount of coupling agent added is 1.2% of the mass of quartz glass micron powder. The coupling agent is reacted at 50°C for 3 hours to form a modified coating layer with a thickness of about 3 to 4 nanometers on the surface of the micron powder.

[0105] Tetragonal zirconia nanofibers with a length of 400 to 600 nanometers and a diameter of 20 to 35 nanometers were selected as the stress relaxation phase material. The zirconia nanofibers were dispersed in anhydrous ethanol to prepare a suspension with a concentration of 10% by mass. This suspension was then treated using an ultrasonic dispersion device with a power of 1000 watts for 20 minutes to ensure thorough dispersion of the nanofibers. Modified quartz glass micron powder and the zirconia nanofiber suspension were added to a planetary ball mill in a specific ratio. The amount of zirconia nanofibers added was 5% of the mass of the quartz glass micron powder. The ball milling speed was 300 rpm for 3 hours, with a ball-to-powder ratio of 8:1. Polyvinylpyrrolidone (PVP) was added simultaneously as a dispersant during the mixing process, with the amount added being 10% of the mass of the zirconia nanofibers. After mixing, the mixture was dried in a vacuum drying oven at 70°C for 18 hours to obtain a loose mixed powder.

[0106] The dried mixed powder was granulated with an organic binder system containing 2.2% polyvinyl butyral, 1.5% polyethylene glycol, and 0.7% stearic acid by mass. Spray drying was used for granulation, with an inlet air temperature of 165℃ and an outlet air temperature of 90℃, resulting in spherical granulated powder with a particle size distribution ranging from 80 to 200 micrometers. The granulated powder was then filled into a metal mold for bidirectional pressing at a pressure of 50 MPa and a holding time of 45 seconds, yielding a core green sample. The density of the green sample was 60% of the theoretical density, and its flexural strength was 3.5 MPa.

[0107] The green core was placed in an atmosphere sintering furnace for gradient heating sintering according to the following temperature program: heating rate of 3°C per minute from room temperature to 600°C; heating rate of 5°C per minute from 600°C to 1000°C; heating rate of 2°C per minute from 1000°C to 1200°C; heating rate of 1.5°C per minute from 1200°C to 1300°C; and held at 1280°C for 2 hours. After holding, it was cooled to room temperature at a rate of 4°C per minute. After sintering, a dense ceramic core sample was obtained with a relative density of 94%.

[0108] The sintered core was placed in a 30% by mass sodium hydroxide aqueous solution and subjected to alkaline etching at 100°C for 5 hours to remove the zirconium oxide nanofibers that formed a connected pore structure. After the alkaline etching was completed, the core was ultrasonically cleaned with deionized water at a power of 300 watts for 15 minutes, for 5 times. Finally, it was dried at 100°C for 3 hours to obtain the final silicon-based ceramic core product.

[0109] Performance testing results of the finished core show that: the fracture toughness of the core reaches 6.5 MPa × m², which is 160% higher than the 2.5 MPa × m² of the traditional silicon-based ceramic core; the coefficient of thermal expansion is 7.5 × 10⁻⁶ per degree Celsius in the temperature range of 25 to 800℃, and the uniformity of distribution reaches 92%; the porosity of the core is 22%, and the pore size distribution is 10 to 45 micrometers; the room temperature flexural strength of the core is 45 MPa, and the high temperature flexural strength remains at 25 MPa at 1200℃.

[0110] Example 2:

[0111] To verify the effect of the amount of dispersant added in the technical solution of this invention on the final core performance, a comparative study on the amount of dispersant added was conducted based on a formulation system with a zirconia nanofiber addition of 5% by mass and an average particle size of 2 micrometers for quartz glass micron powder. The specific implementation process is basically the same as in Example 1, except that the amount of dispersant polyvinylpyrrolidone added is set at different levels.

[0112] In the first group of experiments, the amount of dispersant added was 5% of the mass of zirconia nanofibers. At this time, the adsorption layer formed by polyvinylpyrrolidone on the surface of the nanofibers was relatively thin, the steric hindrance effect was relatively weak, the dispersion uniformity test result was 87%, and the core fracture toughness was 5.8 MPa multiplied by the square root of m.

[0113] In the second group of experiments, the amount of dispersant added was increased to 10% of the mass of zirconia nanofibers. At this time, the adsorption layer thickness increased, the steric hindrance effect was enhanced, the dispersion uniformity was improved to 95%, and the core fracture toughness was improved to 6.5 MPa × m².

[0114] In the third group of experiments, the amount of dispersant added was further increased to 15% of the mass of zirconia nanofibers. At this point, the excessively thick adsorption layer decomposed too much residual carbonaceous material during sintering, which had an adverse effect on the high-temperature performance of the core. The high-temperature flexural strength decreased slightly, but the dispersion uniformity remained at 96%, and the core fracture toughness was 6.6 MPa × m². Considering both the dispersion effect and the core performance, the optimal parameter was a dispersant addition of 10%.

[0115] Example 3:

[0116] To verify the effect of the amount of zirconia nanofiber added in the technical solution of this invention on the final core performance, a comparative study on the amount of nanofiber added was conducted based on a formulation system with a dispersant addition of 10% of the mass of zirconia nanofiber and an average particle size of 2 micrometers for quartz glass micron powder. The specific implementation process is basically the same as in Example 1, except that the amount of zirconia nanofiber added is set at different levels.

[0117] In the first group of experiments, the amount of zirconia nanofibers added was 3%. At this time, the number of nanofibers in the matrix material per unit volume was small, the stress relaxation effect failed to form a complete network structure, the core fracture toughness was 5.2 MPa multiplied by the square root of m, and the porosity after alkaline corrosion treatment was 16%.

[0118] In the second group of experiments, the amount of zirconia nanofibers added was increased to 5%, the stress relaxation network structure tended to be complete, the core fracture toughness was significantly improved to 6.5 MPa multiplied by the square root of m, and the porosity after alkaline corrosion treatment was 22%.

[0119] In the third group of experiments, the amount of zirconia nanofibers added was further increased to 8%. Excessive nanofibers agglomerated during the sintering process, and the fracture toughness of the core increased to 7.2 MPa × m², but the coefficient of thermal expansion increased slightly to 8.5 × 10⁻⁶ degrees Celsius. The porosity after alkaline corrosion treatment increased to 28%. Considering both fracture toughness and coefficient of thermal expansion, the optimal parameter was 5% nanofiber addition.

[0120] Comparative example:

[0121] Comparative Example 1 uses a traditional silicon-based ceramic core preparation method, with quartz glass micron powder as the matrix material. Alumina powder is introduced as a low-expansion regulator, with the amount of alumina added being 15% of the mass of the quartz glass micron powder. After ball milling and mixing, the core is pressed and sintered at a temperature of 1250℃ for 2 hours. The fracture toughness of the core prepared in Comparative Example 1 is 2.5 MPa × m², and the coefficient of thermal expansion is 8.0 × 10⁻⁶ per degree Celsius. However, the uniformity of the coefficient of thermal expansion is only 65%. Microcracks are generated in the core during the casting and cooling process due to the uneven distribution of thermal stress.

[0122] Comparative Example 2 used the same matrix material and nanofiber addition amount as the present invention, but did not perform silane coupling agent surface modification treatment. Instead, it directly ball-milled and sintered quartz glass micron powder and zirconium oxide nanofibers. Due to the lack of silicon carbide heterogeneous nucleation cores generated by the decomposition of the silane coupling agent-modified coating layer during sintering, the nucleation position of cristobalite was uncontrollable, and the uniformity of crystal phase transformation was significantly reduced. The uniformity of thermal expansion coefficient distribution of the core prepared in Comparative Example 2 was only 70%, with a thermal expansion coefficient value of 9.2 x 10^-6 per degree Celsius, and there was an obvious thermal stress concentration area inside the core.

[0123] Table 1: Comparison of Embodiments of the Invention with Comparative Examples

[0124] Fracture toughness (MPa·m^1 / 2) 6.5 6.5 7.2 2.5 5.5 Coefficient of thermal expansion (×10^-6 / ℃) 7.5 7.5 8.5 8.0 9.2 Uniformity of thermal expansion coefficient distribution (%) 92 92 88 65 70 Porosity (%) 22 22 28 5 20 Flexural strength at room temperature (MPa) 45 45 42 38 40 High-temperature flexural strength (MPa) 25 25 23 18 22 Nanofiber dispersion uniformity (%) 95 95 96 — 82 Core removal efficiency improved (%) 60 60 68 benchmark 55

[0125] The above comparative data fully demonstrate that the composite ceramic material technology solution of the present invention, which adopts synergistic modification of nanofiber reinforcement and optimization of core removal process, is significantly superior to the comparative solutions in terms of fracture toughness, uniformity of thermal expansion coefficient distribution, room temperature and high temperature flexural strength, uniformity of nanofiber dispersion, and improved core removal efficiency.

[0126] Example 3 of the present invention exhibits a fracture toughness of 7.2 MPa·m^1 / 2 (comparative Example 1 is 2.5, and Comparative Example 2 is 5.5), a thermal expansion coefficient distribution uniformity of 88% (comparative Example 1 is 65%, and Comparative Example 2 is 70%), a room temperature flexural strength of 42 MPa (comparative Example 1 is 38 MPa), a high temperature flexural strength of 23 MPa, a nanofiber dispersion uniformity of 96% (comparative Example 2 is 82%), and a core removal efficiency that is 68% higher than the baseline (comparative Example 2 is 55% higher), demonstrating the best overall performance. Examples 1 and 2 also show excellent performance and are superior to the comparative examples in terms of porosity (22%), thermal expansion coefficient (7.5×10^-6 / ℃), room temperature flexural strength (45 MPa), and high temperature flexural strength (25 MPa).

[0127] Although Comparative Example 1 has low porosity, its fiber dispersion, toughness, and thermal expansion uniformity are all significantly insufficient. The synergistic effect of highly uniform nanofiber dispersion, gradient structure design, and optimized core removal process enables the composite ceramic material to achieve the optimal balance in terms of mechanical properties, thermophysical properties, and preparation efficiency.

[0128] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a low-expansion silicon-based ceramic core, characterized in that, Includes the following steps: Step 1: Quartz glass micron powder is placed in a ball mill as a silicon source matrix material for wet ball milling pretreatment to control the particle size range. γ-methacryloyloxypropyltrimethoxysilane is added to the ball milling pretreatment slurry system for surface modification treatment to form a modified coating layer on the surface of the micron powder. Step 2: Disperse the zirconia nanofibers in anhydrous ethanol medium for pre-dispersion treatment to obtain a zirconia nanofiber suspension; Step 3: The surface-modified quartz glass micron powder and the dispersed zirconia nanofiber suspension are added to a high-energy ball mill mixing device in a certain proportion for mixing and dispersion. After drying, a loose mixed powder is obtained. Step 4: Granulate the dried mixed powder with the organic binder system, fill the granulated powder into a metal mold and press it to obtain a core green sample; Step 5: Place the core blank in an atmosphere sintering furnace for gradient heating sintering, hold at the temperature, and then cool to room temperature; Step 6: The sintered and cooled core is subjected to alkaline etching treatment, followed by cleaning and drying to obtain a finished silicon-based ceramic core with a low coefficient of thermal expansion.

2. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, The mass addition amount of the γ-methacryloxypropyltrimethoxysilane is controlled within the range of 0.5 to 2 percent of the mass of the silicon source matrix material.

3. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, The length of the zirconia nanofibers is controlled within the range of 200 to 800 nanometers, and the fiber diameter is controlled within the range of 10 to 50 nanometers.

4. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, A dispersant was introduced during the high-energy ball milling mixing process to improve the dispersion uniformity of zirconia nanofibers.

5. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, The organic binder system includes polyvinyl butyral, polyethylene glycol, and stearic acid.

6. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, The temperature is slowly increased at a rate of 2 to 5°C per minute within the temperature range of room temperature to 600°C; at a rate of 3 to 8°C per minute within the temperature range of 600 to 1000°C; at a rate of 1 to 3°C per minute within the temperature range of 1000 to 1200°C; and at a rate of 1 to 2°C per minute within the temperature range of 1200 to 1300°C, and then held at that temperature. After holding at that temperature, the temperature is cooled to room temperature at a rate of 3 to 5°C per minute.

7. The method for preparing a low-expansion silicon-based ceramic core according to claim 6, characterized in that, The granulation process is carried out using spray drying, and the pressing and molding process is carried out using bidirectional pressing.

8. The method for preparing a low-expansion silicon-based ceramic core according to claim 6, characterized in that, Within a temperature range of 600 to 1000°C, the temperature is increased at a rate of 3 to 8°C per minute to cause thermal decomposition of the silane coupling agent modified coating layer and generate silicon carbide crystal nuclei.

9. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, The alkaline corrosion treatment is carried out using an aqueous solution of sodium hydroxide with a concentration of 20 to 40% by mass, and the corrosion temperature is controlled within the range of 80 to 120°C.

10. The method for preparing a low-expansion silicon-based ceramic core according to claim 1, characterized in that, A dispersant is introduced during the high-energy ball milling mixing process, granulation is carried out by spray drying, molding is carried out by bidirectional pressing, sintering is carried out by gradient heating, and stress-relaxed phases are removed by alkaline corrosion treatment to form a connected pore structure, resulting in a silicon-based ceramic core product with a low expansion coefficient and internal microporous structure.

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