Silica-based ceramic core and method for producing the same

Abstract

This invention discloses a method for preparing a silicon oxide-based ceramic core, comprising: uniformly mixing zircon powder and alumina micropowder in a specific ratio to obtain a mixed powder; uniformly mixing the mixed powder and single-particle-size quartz glass powder in a specific ratio to obtain a refractory powder; weighing a plasticizer and refractory powder according to a calculated weight ratio and stirring them uniformly to obtain a ceramic slurry; forming a ceramic blank from the ceramic slurry; embedding the ceramic blank in pre-fired filling powder and sintering it at a set heating rate to obtain a ceramic core; and strengthening the ceramic core. This invention, through alumina micropowder doping technology and an optimized sintering process, achieves the ability to ensure the ceramic core performance meets the requirements for single-crystal turbine blade applications even when using single-particle-size quartz glass powder as the matrix material. By reducing the types of raw materials, it offers significant advantages in controlling the quality and stability of the ceramic core performance.

Description

Technical Field

[0001] This invention belongs to the field of ceramic core preparation technology, and particularly relates to a silicon oxide-based ceramic core and its preparation method. Background Technology

[0002] Ceramic cores (referred to as ceramic cores) are essential transitional components in the fabrication of hollow turbine blades, with silicon oxide-based cores being the most commonly used type. Quartz glass powder, as the main material for ceramic cores, has a significant impact on its performance due to its particle size distribution and impurity content. To improve the mechanical properties of ceramic cores, traditional silicon oxide-based core preparation methods typically involve mixing 2-4 types of quartz glass powder with different particle sizes to achieve a particle size distribution that yields good mechanical properties. However, the more types of raw materials used, the higher the impurity content and the greater the fluctuation in the particle size distribution of the mixed powder, leading to poor quality stability of ceramic cores in industrial mass production. Furthermore, research indicates that quartz glass micropowder (particle size less than 2μm) is the main source of high-temperature viscous flow in silicon-based cores, significantly affecting their high-temperature creep resistance. The ball milling process inevitably introduces micropowder into the quartz glass, further exacerbating the quality fluctuations in silicon-based cores produced from multi-particle-size quartz glass.

[0003] In summary, how to prepare ceramic cores with excellent mechanical properties while ensuring stable quality in industrial mass production is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] The main objective of this invention is to provide a silicon oxide-based ceramic core and its preparation method. By using alumina micro powder doping technology and an optimized sintering process, it is possible to ensure that the ceramic core performance meets the requirements for single-crystal turbine blades even when using quartz glass powder of a single particle size as the matrix material. Due to the reduction in the types of raw materials, it has a great advantage in controlling the quality stability of the ceramic core performance.

[0005] Therefore, the present invention provides a method for preparing a silicon oxide-based ceramic core, characterized in that it includes:

[0006] A specific ratio of zircon powder and alumina micro powder is mixed evenly to obtain a mixed powder;

[0007] A refractory powder is obtained by uniformly mixing a specific ratio of mixed powder and a single-size quartz glass powder.

[0008] Weigh the plasticizer and refractory powder according to the calculated proportions, and stir them evenly to obtain a ceramic slurry;

[0009] The ceramic slurry is used to make a ceramic blank;

[0010] The ceramic blank is embedded in pre-fired filling powder and sintered at a set heating rate to obtain a ceramic core.

[0011] The ceramic core undergoes a strengthening treatment; the weight matching of each powder is as follows:

[0012] 1) Total weight of refractory powder = Weight of ceramic slurry * (75-85)%

[0013] 2) Total weight of zircon powder and alumina powder: quartz glass powder = (10-30)% : (90-70)%

[0014] 3) Weight of alumina micro powder = Total weight of zircon powder and alumina micro powder * (0.5-1.5)%;

[0015] The plasticizer is a mixture of paraffin wax, beeswax and polyethylene in a mass ratio of 50:(40-47):(3-10):(1-3);

[0016] The average particle size of the zircon powder is 5-10 μm, and the composition meets the requirements of ZrO2≥66wt.% and Al2O3≤0.2wt.%.

[0017] The average particle size of the quartz glass powder is 25-35 μm, and the composition satisfies SiO2 ≥ 99.9 wt.%.

[0018] The average particle size of the alumina micro powder is 2-5 μm, and the composition meets the requirement of Al2O3 ≥ 99.7 wt.%.

[0019] The sintering process for ceramic green bodies is as follows: hold at 500-600℃ for 3-4 hours, hold at 800-900℃ for 1-2 hours, hold at 1050-1150℃ for 2-3 hours, and hold at 1180-1240℃ for 5-7 hours.

[0020] Specifically, the mixing process of zircon powder and alumina micro powder is as follows: mix in a ball mill at a speed of 150-250 rpm for 10-15 minutes. In order to ensure that the particle size distribution of the powder is not changed during the mixing process, the weight ratio of grinding balls to powder in the ball mill shall not exceed 1:1.

[0021] The mixing process of quartz glass powder and mixed powder is as follows: mix in a ball mill at a speed of 100-200 rpm for 10-15 minutes. In order to ensure that the particle size distribution of the powder is not changed during the mixing process, the weight ratio of grinding balls to powder in the ball mill should not exceed 1:1.

[0022] Specifically, the plasticizer preparation process is as follows, and the slurry preparation process is as follows:

[0023] The refractory powder is dried in an oven at 100-150℃ for at least 2 hours, and the drying process continues during the slurry preparation.

[0024] Add all the plasticizer to the mixer, set the temperature to 120±5℃ and the speed to 15-25rpm, until the plasticizer is completely melted and stirred.

[0025] Add the refractory powder that has been dried for more than 2 hours to the molten plasticizer in 4-6 portions every 30-60 minutes, with the weight of the refractory powder decreasing each time. Continue stirring for 30-50 hours to make a ceramic slurry.

[0026] In this application, because the plasticizer accounts for a relatively low proportion of the slurry during the slurry preparation process, the refractory powder should be added in multiple intervals to ensure that the powder and liquid plasticizer are fully wetted and mixed evenly. Otherwise, problems such as powder agglomeration and slurry clumping may easily occur. The powder must be continuously dried at a temperature of 100-150℃ before being used for mixing. If the powder is cooled to room temperature before adding the plasticizer, the temperature of the plasticizer will drop sharply, leading to plasticizer clumping, poor slurry uniformity, and affecting the quality of the final core.

[0027] Specifically, the ceramic slurry is transferred into a core press and injection molded to form a ceramic blank.

[0028] Specifically, in industrial production, ceramic core blanks are typically embedded in a bed of alumina powder for sintering to maintain their geometry before achieving sintering strength. The filling powder is industrial-grade alumina powder. However, industrial alumina powder inevitably contains alkali metal oxide impurities, which diffuse to the surface of the ceramic core during sintering, causing over-firing and cracking of the core surface. Therefore, pre-firing of the filling powder is necessary.

[0029] The pre-firing procedure for the filler powder is as follows: Fresh filler powder is placed in any high-temperature resistant sagger, and the ceramic blank is pre-embedded in the sagger. The temperature is increased to 600℃ at 3℃ / min and held for 3-5 hours. Then, the temperature is further increased to 1200-1250℃ at 5℃ / min and held for at least 6 hours. This process is repeated 4-5 times until the surface of the sintered ceramic blank is free of cracks.

[0030] Specifically, industrial-grade alumina micro powder is used as raw material. Before use, it is calcined at 1350℃ for at least 3-5 hours to convert the metastable γ-Al2O3 in the alumina powder into α-Al2O3. At the same time, some impurity elements can be burned off to reduce the impact of raw material composition fluctuations on sintering quality.

[0031] Specifically, the plasticizer preparation process is as follows: Heat the mixer to 130℃, add all the paraffin wax to the mixer and set the speed to 20-30 rpm; after the paraffin wax has completely melted, add all the polyethylene and stir for at least 2 hours until the polyethylene has completely melted; reduce the mixer temperature to 100℃, add all the beeswax and adjust the speed to 15 rpm and continue stirring for about 30 minutes, turn off the heating power, and cool to room temperature to obtain the plasticizer.

[0032] This application also provides a silicon oxide-based ceramic core prepared by the above-described method.

[0033] Principles and advantages

[0034] The quartz glass powder used in this invention is of medium particle size. It innovatively incorporates a specific proportion of alumina micropowder, along with a special sintering process. The alumina micropowder reacts with silicon dioxide at high temperatures to form aluminosilicates, promoting sintering of the quartz glass and improving the core's bending strength. Simultaneously, the alumina micropowder acts as a [SiO4] tetrahedral network modifier, promoting devitrification of the quartz glass to increase the amount of cristobalite crystallization during silicon-based core sintering. The cristobalite precipitated during sintering serves as nucleation sites for the core during the casting and heating process, lowering the onset temperature of the quartz glass's transformation to the structurally stable cristobalite phase and accelerating the transformation rate. This ensures that before unacceptable macroscopic flexural deformation occurs in the ceramic core, the amorphous quartz glass transforms into the structurally stable cristobalite phase, thus improving the creep resistance of the silicon-based core.

[0035] Sintering is a crucial process in silicon-based core fabrication. The design principle of the sintering regime is to control the degree of sintering based on the set formula. If the degree of sintering is too low, the sintering strength will be insufficient; if the degree of sintering is too high, there will be too many phase changes, resulting in crystal transformation and cracking during cooling, which also deteriorates the mechanical properties. Therefore, the sintering regime must be adapted to the core formula to control the appropriate degree of sintering. In this invention, after the plasticizer is burned off at 500℃, two heat preservation steps are added based on two considerations: 1) The crystallization initiation temperature of cristobalite is usually above 1150℃. Setting heat preservation steps below this temperature can homogenize the internal temperature of the saggers in different areas of the furnace, thereby ensuring that there is no significant difference in the amount of crystallization of the cores embedded in each sagger. This also ensures better consistency in the quality of ceramic cores in the same batch. If temperature homogenization is achieved by extending the final heat preservation time, it will cause differences in the crystallization time of the cores in each sagger, resulting in some cores reaching the crystallization temperature first and crystallizing more, leading to... Overheating occurs, and some cores have insufficient crystallization and are underheated. Furthermore, due to the addition of alumina micropowder to promote crystallization, crystallization is more sensitive to temperature, and the differences in crystallization in different regions of the core are more severe. 2) The transformation of amorphous quartz glass into crystalline cristobalite phase is a solid-state phase transition. There is a nucleation and incubation period slightly below the crystallization temperature (approximately 1050-1150℃). In this invention, a heat preservation step is set at a temperature of 1050-1150℃ to control the cristobalite nucleation and incubation period in an isothermal state, so as to ensure the stability of the quartz glass crystallization process and precisely control the amount of crystallization within the most favorable range.

[0036] This application, through the use of alumina micro powder doping technology and optimized sintering process, has for the first time achieved the ability to ensure that the performance of ceramic cores meets the requirements for single-crystal turbine blades even when using quartz glass powder of a single particle size as the matrix material. Due to the reduction in the types of raw materials, it has a great advantage in controlling the quality and stability of ceramic core performance. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a process flow diagram of the method for preparing silicon oxide-based ceramic cores according to the present invention;

[0039] Figure 2 These are diagrams showing the surface crack patterns of sintered green blanks made from landfill powder at different pre-firing times.

[0040] Figure 3 This is a sintering process diagram of the method for preparing silicon oxide-based ceramic cores according to the present invention;

[0041] Figure 4 This is a sintering process diagram of the method for preparing silicon oxide-based ceramic cores in Example 1;

[0042] Figure 5 These are microscopic images of the ceramic core prepared in Example 1;

[0043] Figure 6 This is a sintering process diagram of the method for preparing silicon oxide-based ceramic cores in Example 2;

[0044] Figure 7 These are microscopic morphology images of the ceramic core prepared in Example 2;

[0045] Figure 8 This is a backscattering image of the fracture surface of the ceramic core prepared in Example 3 after the bending strength test at 1550℃.

[0046] Figure 9 These are the X-ray diffraction patterns of the ceramic cores obtained in Examples 1, 2, and 1;

[0047] in, Figure 5 (b) is Figure 5 (a) Enlarged view of the circled area. Figure 7 (b) is Figure 7 (a) Enlarged view of the circled area. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Key mechanical properties of silica-based ceramic cores include flexural strength and resistance to high-temperature creep (the ability to remain undeformed at high temperatures, i.e., stiffness). Flexural strength depends on the sintering strength between the powder particles; the more fully sintered, the higher the core strength. The root cause of creep deformation is fine quartz glass powder, which becomes molten glass at high temperatures, undergoing viscous flow and causing core flexural deformation. Methods to improve high-temperature creep of quartz glass include reducing the content of viscous, flowing fine powder or adding auxiliary components to accelerate the transformation of the glass liquid phase into a structurally stable crystalline phase, preventing the glass liquid phase from undergoing viscous flow before transforming into a stable crystalline phase, thus reducing high-temperature core deformation.

[0050] Traditional silicon-based cores typically use a mixture of coarse, medium, and fine powders in specific proportions to create a particle size distribution. The coarse powder forms the core matrix framework, reducing the content of fine powder that can undergo viscous flow and thus helping to resist high-temperature deformation. The medium powder fills the gaps between the coarse powders, increasing density. The fine powder is used to promote liquid-phase sintering and improve sintering strength. However, using different gradations of quartz glass powder raw materials inevitably leads to an increase in the impurity content of the raw materials and large fluctuations in the particle size distribution of the mixed powder, resulting in poor quality stability of ceramic cores in industrial mass production.

[0051] If the core matrix material is entirely coarse powder, its sintering strength is low and its bending strength is insufficient because coarse powder is difficult to sinter. If it is entirely fine powder, the core strength is the highest, but the high content of fine powder makes it extremely prone to deformation at high temperatures. Cores made entirely of medium powder have slightly insufficient strength and resistance to high-temperature deformation, and their overall performance does not meet the application requirements. This application proposes an innovative solution to address the above problems.

[0052] See Figure 1 A method for preparing a silicon oxide-based ceramic core includes the following steps:

[0053] 1. Preparation of plasticizer

[0054] Weigh out paraffin wax, beeswax, and polyethylene in a mass ratio of 50:(40-47):(3-10):(1-3) and set aside. Heat the mixer to 130℃, add all the paraffin wax to the mixer and set the speed to 20-30 rpm; after the paraffin wax has completely melted, add all the polyethylene and stir for at least 2 hours until the polyethylene is completely melted; reduce the mixer temperature by 100℃, add all the beeswax and adjust the speed to 15 rpm and continue stirring for about 30 minutes, turn off the heating power, and cool to room temperature to obtain the plasticizer.

[0055] 2. Pre-calcination of alumina micro powder

[0056] Using industrial alumina powder as raw material, calcining at 1350℃ for at least 3-5 hours converts the metastable γ-Al2O3 in the alumina powder into α-Al2O3, while also removing some impurity elements.

[0057] 3. Powder Specification Requirements

[0058] (1) Average particle size of zircon powder (D) 50 The micrometer size is 5-10 μm, and the composition satisfies ZrO2 ≥ 66 wt.% and Al2O3 ≤ 0.2 wt.%.

[0059] (2) Average particle size of quartz glass powder (D) 50 The micrometer size should be controlled within the range of 25-35 μm, and the composition should meet the requirements of Table 1.

[0060] Table 1. Composition requirements for quartz glass powder (wt.%)

[0061] Ingredient Index Content Standard <![CDATA[Na2O]]> <![CDATA[≤50ⅹ10 -4 ]]> <![CDATA[Li2O]]> <![CDATA[≤1ⅹ10 -4 ]]> <![CDATA[K2O]]> <![CDATA[≤30ⅹ10 -4 ]]> <![CDATA[Fe2O3]]> <![CDATA[≤10ⅹ10 -4 ]]> <![CDATA[Al2O3]]> <![CDATA[≤50ⅹ10 -4 <!-- 4 -->]]> <![CDATA[SiO2]]> ≥99.9

[0062] (3) Average particle size of industrial alumina powder (D) 50 The micrometer size should be controlled within the range of 2-5 μm, and the composition must meet the requirements of Table 2.

[0063] Table 2. Composition requirements for industrial alumina micro powder (wt.%)

[0064] Ingredient Index Content Standard <![CDATA[Al2O3]]> ≥99.7 <![CDATA[Fe2O3]]> ≤0.01 <![CDATA[SiO2]]> ≤0.015 <![CDATA[Na2O]]> ≤0.15 <![CDATA[α-Al2O3]]> ≥94

[0065] 4. Refractory powder mixing

[0066] Refractory powders include quartz glass powder as the matrix material, zircon powder as the mineralizer, and alumina micro powder as the additive.

[0067] (1) Calculation method for the weight of each powder:

[0068] ① Total weight of refractory powder = weight of raw materials * (75-85)%

[0069] ②Total weight of zircon powder and alumina powder:quartz glass powder = (10-30)% : (90-70)%

[0070] ③Weight of alumina micro powder = Total weight of zircon powder and alumina micro powder * (0.5-1.5)%

[0071] For example, if 100kg of slurry is prepared, the total weight of the refractory powder is 75-85kg, of which the weight of quartz glass powder is (75-85)kg*(90-70)%, the weight of alumina micro powder is (75-85)kg*(10~30%)*(0.5~1.5)%, and the rest is zircon powder.

[0072] Specifically, the plasticizer weight ratio is (15-25)% of the ingredient weight. The purpose is to control the filling performance of the slurry and the porosity of the core. If the plasticizer weight is less than 15% of the ingredient weight, the slurry will have extremely poor fluidity and will not be able to be made into a green blank with ideal quality by injection molding. If the plasticizer weight is more than 25% of the ingredient weight, the plasticizer will burn off and leave too high a porosity in the core, which will greatly reduce the bending strength of the core.

[0073] Zircon powder (ZrSiO4) is structurally stable below 1650℃, exhibits higher strength than quartz glass and cristobalite, and does not undergo solid-phase reaction with quartz glass, nor does it affect the crystallization of quartz glass. The main function of zircon powder as a mineralizer added to silicon-based cores is heterogeneous strengthening. The zircon powder in this invention has an average particle size of 5-10 μm, significantly smaller than the average particle size of quartz glass powder (25-35 μm), thus allowing it to disperse diffusely among quartz glass particles, pinning crack propagation, thickening the glassy liquid phase between particles, and benefiting both flexural strength and creep resistance. The proportion of zircon powder needs to be controlled within a suitable range; too low a proportion leads to insufficient strengthening effect, while too high a proportion disrupts the integrity of the sintered quartz glass network, reducing mechanical properties.

[0074] (2) Mixing zircon powder and alumina micro powder

[0075] Weigh out zircon powder and alumina micro powder according to the calculation results, and mix them in a ball mill at a speed of 150-250 rpm for 10-15 minutes. In particular, in order to ensure that the particle size distribution of the powder is not changed during the mixing process, the weight ratio of grinding balls to powder in the ball mill should not exceed 1:1.

[0076] One of the innovations of this invention is that zircon powder must be mixed separately with alumina powder before being mixed with quartz glass powder. This is because alumina powder can increase the glass liquid phase content during the sintering process of quartz glass powder, promoting the sintering of quartz glass powder through liquid phase mass transfer and enhancing the mineralizing effect of zircon powder. Therefore, it is essential to ensure that alumina powder and zircon powder are fully and uniformly mixed; otherwise, the desired effect of this invention may not be achieved.

[0077] (3) Mixing of quartz glass powder and zircon powder / alumina micro powder

[0078] Weigh out the zircon powder / alumina micro powder mixture and quartz glass powder according to the calculated weight ratio, and mix them in a ball mill at a speed of 100-200 rpm for 10-15 minutes. In particular, in order to ensure that the particle size distribution of the powder is not changed during the mixing process, the weight ratio of grinding balls to powder in the ball mill should not exceed 1:1.

[0079] 5. Slurry preparation

[0080] (1) Weigh the plasticizer prepared in step 1 and the refractory powder mixed in step 4 according to the calculated proportion. Dry the refractory powder in an oven at 100-150℃ for at least 2 hours and continue to dry it during the slurry preparation process.

[0081] (2) Add all the plasticizer to the mixer, set the temperature to 120±5℃ and the speed to 15-25rpm, until the plasticizer is completely melted and stirred.

[0082] (3) Add the refractory powder that has been dried for more than 2 hours to the molten plasticizer in 4-6 portions every 30-60 minutes, with the weight of powder decreasing each time. That is, half of the total powder can be added the first time, one-quarter the second time, and the remaining powder can be added evenly in 2-3 portions. Stir continuously for 30-50 hours to make a ceramic slurry.

[0083] During slurry preparation, because the plasticizer accounts for a relatively low proportion of the slurry, the refractory powder should be added in multiple intervals to ensure that the powder and liquid plasticizer are fully wetted and mixed evenly. Otherwise, powder agglomeration and slurry clumping may occur. The powder must be continuously dried before being added to the plasticizer, maintaining a temperature of 100-150℃. If the powder is cooled to room temperature before adding the plasticizer, the plasticizer temperature may drop sharply, leading to plasticizer clumping, poor slurry uniformity, and affecting the final core quality.

[0084] 6. Forming the unfinished body

[0085] The slurry obtained in step 5 is transferred into a core press and ceramic blanks are produced by injection molding.

[0086] 7. Pre-calcination of landfill powder

[0087] In industrial production, ceramic core blanks with quartz glass powder as the matrix material are typically embedded in a bed of alumina powder for sintering to maintain the geometry of the blank before it develops sintering strength. However, industrial alumina powder inevitably contains alkali metal oxide impurities, which diffuse to the surface of the ceramic core during sintering, causing over-firing and cracking of the core surface. Therefore, pre-firing of the embedded powder is necessary.

[0088] Pre-burning process:

[0089] (1) Pack the new landfill powder into any high-temperature resistant crucible, with the pre-embedded weight ratio in the crucible being 10-30%.

[0090] Within the range, the green blank prepared in step 6 is heated to 600℃ at 3℃ / min and held for 3-5 hours, and then heated to 1200-1250℃ at 5℃ / min and held for at least 6 hours.

[0091] (2) Repeat step (1) 4-5 times until the surface of the sintered green body is free of cracks, such as Figure 2 As shown.

[0092] 8. Sintering of green blanks

[0093] (1) The ceramic blank is placed into a sagger and covered with industrial alumina powder pre-fired in step 7 and compacted.

[0094] (2) Place the sagger in any sintering furnace and sinter it under atmospheric conditions. The sintering curve is shown in the figure. Figure 3As shown.

[0095] Sintering is a crucial process in silicon-based core fabrication. The design principle of the sintering regime is to control the degree of sintering based on the set formula. If the degree of sintering is too low, the sintering strength will be insufficient; if the degree of sintering is too high, there will be too many phase changes, which will lead to crystal transformation and cracking during cooling, thus deteriorating the mechanical properties. Therefore, the sintering regime must be adapted to the core formula to control the appropriate degree of sintering.

[0096] The alumina micropowder doped with this invention is particularly sensitive to the sintering degree of the quartz glass powder. Figure 2 The sintering curve shown is another innovation of this invention. It was optimized by the inventors based on the component ratios given in steps 2 and 3, especially the particle size distribution of the single quartz glass powder and the characteristics of the alumina micro powder doping formulation of this invention. It integrates the inventors' numerical simulation results of the sintering process, the phase analysis results after sintering, and the mechanical property test results. Its feature is that after the plasticizer is burned off at 500℃, two heat preservation steps are added. The purpose is to improve the consistency between the actual temperature curve in the sagger and the preset sintering curve before the quartz glass crystallizes, and to homogenize the temperature in each sagger in the furnace. In addition, the heat preservation at 1050~1150℃ can also control the nucleation and incubation period of cristobalite in an isothermal state, so as to ensure the stability of the quartz glass crystallization process and to precisely control the amount of crystallization within the most favorable range, ensuring that the green blank is fully sintered, so as to achieve good comprehensive sintering performance.

[0097] More specifically, the design of the final sintering temperature and holding time provided by the present invention is related to the doping ratio of alumina micro powder. The higher the doping amount of alumina micro powder, the lower the final sintering temperature and holding time need to be appropriately reduced, taking the lower limit value recommended by the present invention; the lower the doping amount, the higher the final sintering temperature and holding time should be appropriately extended, taking the upper limit value recommended by the present invention.

[0098] 9. Remove the sintered ceramic core, remove the powder and flash from the surface, perform high-temperature strengthening treatment with ethyl silicate hydrolysate, and perform room-temperature strengthening treatment with phenolic resin to obtain the ceramic core.

[0099] Example 1 (Case 1)

[0100] A method for preparing a silicon oxide-based ceramic core includes the following steps:

[0101] 1. Weigh paraffin wax, beeswax, and polyethylene in a mass ratio of 50:45:5. Heat the mixer to 130°C, add all the paraffin wax to the mixer and set the speed to 25 rpm. After the paraffin wax has completely melted, add all the polyethylene and stir for at least 2 hours until the polyethylene is completely melted. Reduce the mixer temperature to 100°C, add all the beeswax and adjust the speed to 15 rpm and continue stirring for about 30 minutes. Turn off the heating power and cool to room temperature to obtain the plasticizer.

[0102] 2. Using average particle size (D) 50 Grade I zircon powder with a particle size of 8 μm and an average particle size (D) 50 Alumina micropowder with a particle size of 4.5 μm was mixed in a ball mill at 200 rpm for 10 min to obtain a mixed powder (zircon powder doped with alumina micropowder). During mixing, the weight ratio of grinding balls to powder in the ball mill did not exceed 1:1, the alumina micropowder accounted for 1.5% of the mixed powder by mass, and the zircon powder composition met the requirements of ZrO2 ≥ 66% and Al2O3 ≤ 0.2%. Industrial alumina powder was used as the raw material. Before mixing, it was calcined at 1350℃ for at least 3-5 hours to convert the metastable γ-Al2O3 in the alumina powder to α-Al2O3, while also removing some impurities. The composition of the industrial alumina powder must meet the requirements in the table below (wt.%):

[0103] Ingredient Index Content Standard <![CDATA[Al2O3]]> ≥99.7 <![CDATA[Fe2O3]]> ≤0.01 <![CDATA[SiO2]]> ≤0.015 <![CDATA[Na2O]]> ≤0.15 <![CDATA[α-Al2O3]]> ≥94

[0104] 3. Weigh quartz glass powder and zircon powder doped with alumina micropowder at a mass ratio of 80:20. Mix them in a ball mill at 150 rpm for 10 minutes. During mixing, the weight ratio of grinding balls to powder in the ball mill should not exceed 1:1. The average particle size of the quartz glass powder should be 30 μm. The composition of the quartz glass powder should meet the requirements in the table below (wt.%):

[0105] Ingredient Index Content Standard <![CDATA[Na2O]]> <![CDATA[≤50ⅹ10 -4 ]]> <![CDATA[Li2O]]> <![CDATA[≤1ⅹ10 -4 ]]> <![CDATA[K2O]]> <![CDATA[≤30ⅹ10 -4 ]]> <![CDATA[Fe2O3]]> <![CDATA[≤10ⅹ10 -4 ]]> <![CDATA[Al2O3]]> <![CDATA[≤50ⅹ10 -4 ]]> <![CDATA[SiO2]]> ≥99.9

[0106] 4. Weigh the plasticizer prepared in step 1 and the refractory powder mixed in step 3 at a mass ratio of 22:78.

[0107] 5. Melt and stir the plasticizer weighed in step 4 in a mixer at 120±5℃, add it to the refractory powder weighed in step 4 in 4-6 portions, and continue stirring for 45 hours to make a ceramic slurry.

[0108] 6. Transfer the slurry obtained in step 5 into a core pressing machine and use injection molding to form a ceramic blank.

[0109] 7. Fill the corundum-mullite sagger with the new filler powder. Pre-embed 20% by weight of the green blank prepared in step 5 in the sagger. Heat to 600℃ at 3℃ / min and hold for 4 hours. Then continue to heat to 1230℃ at 5℃ / min and hold for at least 6 hours. Repeat the sintering process 5 times until the surface of the standard sample is free of cracks.

[0110] 8. Place the ceramic blank into a sagger, cover it with the industrial alumina powder pretreated in step 7, and compact it with vibration.

[0111] 9. Place the sagger in a muffle furnace for sintering; the sintering curve is as follows: Figure 4As shown, the temperature is increased to 500℃ at 1℃ / min and held for 3 hours, then increased to 900℃ at 5℃ / min and held for 1 hour, then increased to 1100℃ at 3℃ / min and held for 3 hours, then increased to 1180℃ at 3℃ / min and held for 5 hours, and then cooled in the furnace.

[0112] 9. Remove the sintered ceramic core, remove surface powder and flash, perform high-temperature strengthening treatment with ethyl silicate hydrolysate, and perform room-temperature strengthening treatment with phenolic resin to obtain the ceramic core.

[0113] 10. The performance test results of the ceramic core are as follows: room temperature flexural strength 27.3 MPa, 1550℃ flexural strength 20.48 MPa, hot deformation at 1550℃ / 30min measured by the single-point method 0.47 mm, and linear shrinkage during firing 0.50%. The microstructure of the prepared ceramic core after sintering is as follows. Figure 5 As shown, the X-ray diffraction pattern of the ceramic core is as follows: Figure 9 As shown, from Figure 5 It can be seen that alumina micropowder is dispersed in the ceramic core matrix. As a [SiO4] tetrahedral network modifier, the alumina micropowder promotes the devitrification of quartz glass. Figure 9 As can be seen from the data, due to the appropriate proportion of alumina micro powder, the core sintering degree of this case is high after sintering at 1180℃ for 5 hours. During the sintering process, more cristobalite crystal phase is precipitated. The core has more nucleation particles during the casting and heating process. Therefore, the bending strength and creep resistance of the ceramic core are greatly improved, and the ceramic core meets the requirements for use in single-crystal turbine blades.

[0114] Example 2

[0115] Unlike Example 1, the alumina micro powder accounted for 0.5% of the mixed powder by mass. The temperature was increased to 500°C at 1°C / min and held for 3 hours, then increased to 900°C at 5°C / min and held for 1 hour. Afterwards, the temperature was increased to 1150°C at 3°C / min and held for 3 hours, and finally increased to 1240°C at 3°C / min and held for 6 hours. The sintering curve is as follows: Figure 6 As shown, the remaining processes are the same as in the example.

[0116] The performance test results of the ceramic core are as follows: room temperature flexural strength 29.84 MPa, 1550℃ flexural strength 19.67 MPa, hot deformation at 1550℃ / 30min measured by the single-point method 0.53 mm, and sintering linear shrinkage 0.55%. The ceramic core meets the requirements for use in single-crystal turbine blades. The microstructure of the prepared ceramic core after sintering is shown in the figure. Figure 7 As shown, from Figure 7 As can be seen, the alumina micropowder is dispersed in the ceramic core matrix. The X-ray diffraction pattern of the ceramic core is as follows: Figure 9 As shown, from Figure 9As can be seen from the graph of Case 2, this case has a high degree of core sintering due to the appropriate proportion of alumina micro powder and the relatively high sintering temperature (1240℃) for 6 hours. During the sintering process, more cristobalite crystal phases are precipitated, and there are more nucleation particles in the core during the casting and heating process. Therefore, the bending strength and creep resistance of the ceramic core are greatly improved, and the ceramic core meets the requirements for use in single-crystal turbine blades.

[0117] Example 3

[0118] Unlike Example 1, the zircon powder has an average particle size of 5 μm, the quartz glass powder has an average particle size of 30 μm, and the alumina powder has an average particle size of 5 μm. Quartz glass powder and zircon powder doped with alumina micropowder are weighed at a mass ratio of 85:15. The plasticizer prepared in step 1 and the refractory powder mixed in step 3 are weighed at a mass ratio of 20:80. The alumina micropowder accounts for 1% of the mass of the mixed powder. The final firing temperature is 1200℃. All other processes are the same as in Example 1.

[0119] The sintering performance test results of the prepared ceramic core are as follows: room temperature flexural strength 28.33 MPa, 1550℃ flexural strength 21.79 MPa, hot deformation at 1550℃ / 30 min measured by the single-point method 0.32 mm, and sintering linear shrinkage 0.46%. The backscattered image of the fracture surface of the prepared silicon-based core after the 1550℃ flexural strength test is shown below. Figure 8 As can be seen from the figure, the white zircon powder is dispersed inside the silicon-based core, which can pin the crack propagation and thicken the glass liquid phase between particles, thereby greatly improving the bending strength and creep resistance of the ceramic core.

[0120] Comparative Example 1

[0121] Unlike Example 1, the alumina micropowder accounted for 0.2% of the mixed powder by mass. All other processes were the same as in Example 1. The prepared ceramic core exhibited a room temperature flexural strength of 18.39 MPa, a flexural strength at 1550°C of 14.71 MPa, a heat deformation of 2.33 mm at 1550°C / 30 min measured using the single-point method, and a firing linear shrinkage rate of 0.27%. Figure 9 As shown, in this case, due to the low content of alumina micro powder, the core sintering degree was low after sintering at 1180℃ for 5 hours. Very few cristobalite crystal phases were precipitated during the sintering process, and there were few nucleation particles in the core during the casting and heating process. Therefore, the bending strength and creep resistance of the ceramic core were not ideal, and the thermal deformation was too large, which did not meet the requirements for use in single-crystal turbine blades.

[0122] Comparative Example 2

[0123] Unlike Example 1, the sintering process was carried out by raising the temperature to 500°C at 1°C / min and holding for 3 hours, then raising the temperature to 900°C at 5°C / min and holding for 1 hour, then raising the temperature to 1150°C at 3°C / min and holding for 3 hours, and then raising the temperature to 1250°C at 3°C / min and holding for 4 hours. The rest of the process was the same as in Example 1.

[0124] The prepared ceramic core exhibited a room temperature flexural strength of 20.71 MPa, a flexural strength of 13.96 MPa at 1550℃, a thermal deformation of 0.78 mm measured by the single-point method at 1550℃ for 30 min, and a firing linear shrinkage rate of 0.71%. In this case, the addition of 1.5% alumina micropowder and the selection of an excessively high sintering temperature resulted in the precipitation of an excessive amount of cristobalite phase. During cooling, a cristobalite crystal transformation occurred, causing numerous microcracks within the core and reducing its flexural strength, thus failing to meet the requirements for single-crystal turbine blade applications.

[0125] Comparative Example 3

[0126] Unlike Example 1, during sintering, the temperature was increased to 500℃ at a rate of 1℃ / min and held for 3 hours, then directly increased to 1180℃ at a rate of 3℃ / min and held for 5 hours before being cooled in the furnace. The prepared ceramic core had a room temperature flexural strength of 21.63 MPa, a flexural strength at 1550℃ of 16.61 MPa, a heat deformation of 1.91 mm at 1550℃ / 30 min measured by the single-point method, and a firing linear shrinkage rate of 0.44%. This sintering regime, due to the lack of two holding steps, resulted in a lower degree of sintering and crystallization (the actual holding time or sintering time for the initial crystallization and sintering was shorter), leading to under-firing and consequently lower flexural strength and creep resistance. The excessive deformation also failed to meet the application requirements.

[0127] Comparative Example 4

[0128] Unlike Example 1, quartz glass powder and zircon powder doped with alumina micropowder were weighed at a mass ratio of 60:40, with alumina micropowder accounting for 0.2% of the mixed powder mass. The rest of the process was the same as in Example 1. The prepared ceramic core had a room temperature flexural strength of 21.03 MPa and a flexural strength of 15.67 MPa at 1550°C. This was because the amount of zircon powder added was too high, which caused some adjacent quartz glass particles to be blocked by zircon powder and unable to sinter together, thus breaking the integrity of the sintering network and reducing the flexural strength.

[0129] The above embodiments are merely illustrative examples to clearly illustrate the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a silicon oxide-based ceramic core, characterized in that, include: A specific ratio of zircon powder and alumina micro powder is mixed evenly to obtain a mixed powder; A refractory powder is obtained by uniformly mixing a specific ratio of mixed powder and a single-size quartz glass powder. Weigh the plasticizer and refractory powder according to the calculated proportions, and stir them evenly to obtain a ceramic slurry; The ceramic slurry is used to make a ceramic blank; The ceramic blank is embedded in pre-fired filling powder and sintered at a set heating rate to obtain a ceramic core. The ceramic core undergoes a strengthening treatment; the weight matching of each powder is as follows: 1) Total weight of refractory powder = weight of ceramic slurry * (75-85)% 2) Total weight of zircon powder and alumina powder: quartz glass powder = (10-30)% : (90-70)% 3) Weight of alumina micro powder = Total weight of zircon powder and alumina micro powder * (0.5-1.5)%; The plasticizer is a mixture of paraffin wax, beeswax, and polyethylene; The zircon powder has an average particle size of 5-10 μm and a composition that satisfies ZrO2 ≥ 66 wt.% and Al2O3 ≤ 0.2 wt.%. The average particle size of the quartz glass powder is 25-35 μm, and the composition satisfies SiO2 ≥ 99.9 wt.%. The alumina micro powder has an average particle size of 2-5 μm and a composition satisfying Al2O3 ≥ 99.7 wt.%. The sintering process for ceramic green bodies is as follows: hold at 500-600℃ for 3-4 hours, hold at 800-900℃ for 1-2 hours, hold at 1050-1150℃ for 2-3 hours, and hold at 1180-1240℃ for 5-7 hours.

2. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that, The mixing process of zircon powder and alumina micro powder is as follows: mix in a ball mill at a speed of 150-250 rpm for 10-15 minutes. During the mixing, the weight ratio of grinding balls to powder in the ball mill does not exceed 1:

1.

3. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: The mixing process of quartz glass powder and mixed powder is as follows: mix in a ball mill at a speed of 100-200 rpm for 10-15 minutes. During the mixing, the weight ratio of grinding balls to powder in the ball mill does not exceed 1:

1.

4. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: The slurry preparation process is as follows: The refractory powder is dried in an oven at 100-150℃ for at least 2 hours, and the drying process continues during the slurry preparation. Add all the plasticizer to the mixer, set the temperature to 120±5℃ and the speed to 15-25rpm, until the plasticizer is completely melted and stirred. Add the refractory powder that has been dried for more than 2 hours to the molten plasticizer in 4-6 portions every 30-60 minutes, with the weight of the refractory powder decreasing each time. Continue stirring for 30-50 hours to make a ceramic slurry.

5. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: The ceramic slurry is transferred into a core press and ceramic blanks are produced by injection molding.

6. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: The pre-firing procedure for the filler powder is as follows: Fresh filler powder is placed in any high-temperature resistant sagger, and the ceramic blank is pre-embedded in the sagger. The temperature is increased to 600℃ at 3℃ / min and held for 3-5 hours. Then, the temperature is further increased to 1200-1250℃ at 5℃ / min and held for at least 6 hours. Repeat steps 4-5 times until the surface of the sintered ceramic blank is free of cracks.

7. The method for preparing a silicon oxide-based ceramic core according to claim 6, characterized in that: The landfill powder is industrial-grade alumina powder.

8. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: Before mixing, alumina micro powder needs to be calcined at 1350℃ for at least 3-5 hours to convert the metastable γ-Al2O3 in the alumina powder into α-Al2O3.

9. The method for preparing a silicon oxide-based ceramic core according to claim 1, characterized in that: The plasticizer preparation process is as follows: Heat the mixer to 130℃, add all the paraffin wax to the mixer and set the speed to 20-30 rpm; after the paraffin wax has completely melted, add all the polyethylene and stir for at least 2 hours until the polyethylene has completely melted; reduce the mixer temperature by 100℃, add all the beeswax and adjust the speed to 15 rpm and continue stirring for 30 minutes, turn off the heating power, and cool to room temperature to obtain the plasticizer.

10. A silicon oxide-based ceramic core, characterized in that, The silicon oxide-based ceramic core was prepared using the method described in any one of claims 1-9.

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