Preparation method of alumina ceramic ball with yttrium oxide as sintering aid

Abstract

The application discloses a method for preparing alumina ceramic balls by using yttrium oxide as a sintering agent, and belongs to the technical field of advanced ceramic material preparation. The method uses a yttrium nitrate aqueous solution as a precursor, realizes uniform adsorption of yttrium ions on the surface of alumina particles under the condition that pH is 4.0-6.0 and temperature is 40-60 DEG C, in-situ generates a nano yttrium oxide coating layer after calcination at 500-700 DEG C, and prepares a composite powder with the addition amount of yttrium oxide being 0.3-0.6 wt%; after spray granulation and cold isostatic pressing, the composite powder is sintered in a weak reducing atmosphere in two stages, i.e., densification at 1580-1620 DEG C and then fast cooling at 8-12 DEG C / min to 1350-1400 DEG C, and then furnace cooling. The application solves the industry pain points that yttrium oxide additives are not uniformly dispersed and are easy to generate a brittle phase at grain boundaries, reduces the sintering temperature by more than 30 DEG C, and the prepared ceramic balls have high density, excellent mechanical properties and wear resistance, a wide process window, and are suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of advanced ceramic material preparation technology, specifically relating to a method for preparing alumina ceramic balls using yttrium oxide as a sintering aid. It can be widely used in the preparation of wear-resistant grinding media and wear-resistant structural parts in fields such as powder grinding, mining and metallurgy, and machining. Background Technology

[0002] Alumina ceramic balls, due to their high hardness, excellent wear resistance, corrosion resistance, and chemical stability, are widely used as grinding media in the powder processing industry and as wear-resistant structural components under various working conditions, making them one of the most widely used categories in the field of advanced ceramic materials. Traditional pure-phase alumina ceramics require long-term sintering at temperatures above 1650℃ to achieve densification. This not only results in extremely high production energy consumption and equipment wear but also easily leads to abnormal alumina grain growth, forming a coarse-grained structure. This directly degrades the mechanical properties, impact resistance, and wear resistance of the ceramic material, failing to meet the requirements of high-end applications.

[0003] To reduce the sintering temperature of alumina ceramics, the industry commonly uses the technique of adding sintering aids. Among them, yttrium oxide, as a rare earth sintering aid, can form a low-melting-point liquid phase at the alumina grain boundaries, promoting ion diffusion and compaction of the green body. It is a widely used aid system. However, in existing technologies, yttrium oxide additives are mostly added by directly mixing nanoparticles and alumina powders through physical ball milling. This method has inherent defects that cannot be overcome: First, nano-yttrium oxide powders have extremely high surface energy and are prone to agglomeration during ball milling, making it impossible to achieve uniform dispersion in the alumina matrix. Locally excessive yttrium oxide will undergo a solid-phase reaction with alumina during high-temperature sintering, generating a hard and brittle yttrium aluminum garnet (YAG) crystalline phase. This phase easily becomes a crack source inside the material, leading to a significant decrease in the crushing strength, impact resistance, and wear resistance of the ceramic balls. Second, this process is extremely sensitive to changes in the amount of yttrium oxide added and sintering process parameters, with a very narrow process control window. It is prone to problems such as large fluctuations in product performance between batches and low yield, making it unsuitable for the needs of large-scale stable production.

[0004] To address the aforementioned issues, the industry has developed technical solutions such as improving yttrium oxide dispersion through liquid-phase coating and controlling grain size through optimized sintering processes. However, existing solutions fail to resolve the fundamental contradiction in the application of yttrium oxide additives: the improvement of sintering aid effect and the suppression of brittle grain boundary phases cannot be simultaneously achieved. To improve the sintering aid effect and lower the sintering temperature, it is necessary to increase the amount of yttrium oxide added or improve its dispersion uniformity. However, uneven dispersion of yttrium oxide or a slightly higher addition amount will inevitably lead to the formation of brittle grain boundary phases, degrading product performance. Furthermore, there is a widespread technical bias in the field that "the effective addition amount of yttrium oxide as a sintering aid needs to be ≥0.8wt% to achieve a significant low-temperature sintering aid effect" and that "rapid cooling will cause residual internal stress inside alumina ceramics, reducing the mechanical properties and thermal shock resistance of the product." This results in existing technologies consistently failing to achieve both high performance and high batch stability of alumina ceramic balls under the premise of low additive addition and low sintering temperature. Summary of the Invention

[0005] In view of this, the present invention proposes a method for preparing alumina ceramic spheres using yttrium oxide as a sintering agent, in order to solve the core problems in the application of yttrium oxide sintering aids in the prior art, such as the inability to simultaneously achieve the sintering aid effect and grain boundary properties, the narrow process window, and the inability to unify product performance and batch stability.

[0006] The technical solution of the present invention is implemented as follows: The present invention provides a method for preparing alumina ceramic balls using yttrium oxide as a sintering agent, including four core steps: chemical coating preparation of composite powder, slurry preparation and spray granulation, cold isostatic pressing, and two-stage atmosphere sintering.

[0007] In some embodiments, the chemical coating preparation of the composite powder specifically involves: using a yttrium nitrate aqueous solution with a concentration of 0.08-0.12 mol / L as a yttrium precursor, and mixing it with high-purity α-alumina powder with a purity ≥99.8% and an average particle size of 0.8-1.2 μm in the liquid phase via wet ball milling, controlling the pH of the mixing system to be 4.0-6.0 and the temperature to be 40-60℃, so that yttrium ions are uniformly adsorbed on the surface of alumina particles; after mixing, the slurry is dried, and then calcined at 500-700℃ for 1.5-2.5 h to decompose the yttrium salt, generating a uniform nano-yttrium oxide coating layer in situ on the surface of alumina particles, thus obtaining yttrium oxide / alumina composite powder; wherein the amount of yttrium oxide added, calculated as Y2O3, is 0.3-0.6% of the weight of the alumina powder; the ball-to-particle ratio of the wet ball milling is 5:1, and the grinding media is 3-5 mm zirconia balls. The isoelectric point of α-alumina is 8-9. In a weakly acidic environment with a pH of 4.0-6.0, the surface of alumina particles is positively charged and can dissociate with the γ-rays from yttrium nitrate. 3 + Hydrated ions form stable electrostatic adsorption and hydrogen bonding, and with constant temperature control at 40-60℃, Y can be promoted. 3+YO ions undergo a condensation reaction with hydroxyl groups on the surface of alumina to form YO-Al chemical bonds, achieving uniform ion-level adsorption rather than mechanical adhesion through physical mixing. This fundamentally eliminates the agglomeration problem of nano-yttrium oxide powder. The 5:1 ball-to-powder ratio ensures sufficient dispersion of alumina powder, preventing adsorption sites from being covered due to particle agglomeration and ensuring the uniformity of yttrium ion adsorption. The addition amount of 0.3-0.6wt% of yttrium oxide is the critical equilibrium range between sintering aid effect and grain boundary brittle phase suppression, verified by thermodynamic calculations and extensive experiments. Under uniform distribution, this addition amount can form a continuous and uniform low-melting-point sintering aid liquid phase at the alumina grain boundaries, achieving low-temperature densification. At the same time, the Y2O3 concentration at the grain boundaries is lower than the critical molar ratio for YAG phase formation, completely avoiding the formation of the YAG brittle phase. This solves the inherent contradiction between sintering aid effect and performance degradation in existing technologies from the raw material end.

[0008] In some embodiments, the wet ball milling mixing time is 10-14 hours, and the drying temperature is 110-130°C. A ball milling time of 10-14 hours ensures sufficient contact between yttrium ions and the surface of alumina particles, achieving adsorption equilibrium, while avoiding excessive grinding of alumina particles and excessively wide particle size distribution caused by prolonged ball milling. A drying temperature of 110-130°C allows for stable evaporation of moisture in the slurry, avoiding solute migration and uneven yttrium element distribution caused by rapid drying.

[0009] In some embodiments, the amount of yttrium oxide added, calculated as Y2O3, is 0.4-0.5% of the weight of the alumina powder. This range is the optimal equilibrium range, which can reduce the sintering temperature by more than 30°C, completely eliminate the formation of the YAG phase, and simultaneously take into account raw material costs and product performance, thus meeting the economic requirements of large-scale production.

[0010] In some embodiments, the slurry preparation and spray granulation specifically involve mixing the yttrium oxide / alumina composite powder with a dispersant, a binder, and deionized water to prepare a ceramic slurry with a solid content of 55-60%, followed by spray drying to obtain spherical granulated powder. The 55-60% solid content range allows the ceramic slurry to possess both excellent flowability and suspension stability. This avoids the problems of low spray granulation efficiency and insufficient particle density caused by excessively low solid content, while also avoiding the problems of excessive slurry viscosity, composite powder sedimentation and agglomeration, and damage to the coating layer structure caused by excessively high solid content. This ensures that the uniform distribution of yttrium oxide is not disrupted during spray granulation, providing a stable raw material for subsequent molding.

[0011] In some embodiments, the dispersant is ammonium polyacrylate, added at 0.3-0.5% of the weight of the yttrium oxide / alumina composite powder; the binder is polyvinyl alcohol, added at 1.0-1.5% of the weight of the yttrium oxide / alumina composite powder. Ammonium polyacrylate can form a uniform adsorption layer on the surface of the composite powder particles through electrostatic steric hindrance, effectively preventing secondary agglomeration of the composite powder during slurry preparation and ensuring the long-term stability of the slurry. Polyvinyl alcohol, as a water-soluble binder, can be uniformly distributed inside and on the surface of the granulated powder particles during spray granulation, ensuring both the flowability and adhesion of the granulated powder during molding, improving the green body strength, and decomposing smoothly during subsequent debinding without leaving impurities that affect the ceramic sintering performance.

[0012] In some embodiments, the inlet temperature of the spray dryer is 230-250℃, the outlet temperature is 95-105℃, and the particle size distribution of the resulting spherical granulated powder is 80-120 mesh. These inlet and outlet temperature parameters enable rapid and uniform drying of the slurry droplets, avoiding uneven yttrium oxide distribution caused by solute migration during the drying process. Simultaneously, they allow for precise control of the residual moisture content of the granulated powder at 0.5-1.0%, ensuring the molding performance of the granulated powder. The 80-120 mesh particle size distribution enables uniform and tight filling of the spherical elastic mold, avoiding defects such as localized porosity and density gradients in the green body after molding, and ensuring uniform shrinkage during the subsequent sintering process from the molding end.

[0013] In some embodiments, the cold isostatic pressing specifically involves filling the spherical granulated powder into a spherical elastic mold, and using a cold isostatic press to hold the pressure at 150-180 MPa for 50-70 seconds to obtain a spherical green body with uniform density. The spherical elastic mold is a rubber mold or a polyurethane mold. Cold isostatic pressing can apply uniform compaction force to the granulated powder in the mold through isotropic high pressure, completely avoiding the problems of green body density gradient and unidirectional uneven compaction that are prone to occur in dry pressing. This ensures the uniformity of density throughout the spherical green body, thereby achieving uniform linear shrinkage during subsequent sintering and avoiding sintering defects such as local stress concentration, abnormal grain growth, and cracking. It also forms a synergistic effect with the atomic-level uniform distribution of the front-end composite powder, laying the foundation for precise control of the subsequent microstructure. Rubber molds and polyurethane molds have excellent elasticity and pressure resistance, and can uniformly transmit pressure under high pressure, adapting to the molding requirements of spherical green bodies. They are commonly used elastic mold materials in industrial production.

[0014] In some embodiments, the spherical green body undergoes a debinding process before sintering. This debinding process specifically involves heating the green body to 580-620°C in air at a rate of 0.8-1.2°C / min and holding it at that temperature for 1.5-2.5 hours. This slow heating regime ensures the stable decomposition and gradual removal of the binder and dispersant, avoiding concentrated decomposition of organic matter and blistering or cracking defects caused by rapid heating. The debinding temperature of 580-620°C allows for complete oxidative decomposition and removal of organic matter, while avoiding pre-sintering of the green body due to excessively high debinding temperatures, thus ensuring normal grain boundary migration and uniform densification of the green body during subsequent sintering.

[0015] In some embodiments, the two-stage atmosphere sintering specifically involves: placing the debinding spherical green body in a sintering furnace, heating it to 1580-1620℃ at a heating rate of 3-5℃ / min under a weakly reducing atmosphere, and holding it at that temperature for 2-4 hours to complete densification sintering; after holding, rapidly cooling it to 1350-1400℃ at a rate of 8-12℃ / min, holding it at that temperature for 0.5-1 hours, and then cooling it to room temperature with the furnace to obtain alumina ceramic spheres; the weakly reducing atmosphere is a nitrogen-hydrogen mixed atmosphere, wherein the volume percentage of hydrogen is 4-6%, the atmosphere flow rate during sintering is 2-5L / min, and a slight positive pressure of 0.02-0.05MPa is maintained inside the furnace. A weakly reducing atmosphere with a hydrogen content of 4-6%, combined with a gas flow rate of 2-5 L / min and slight positive pressure control, can stably maintain a low oxygen partial pressure environment inside the furnace, effectively increasing the oxygen vacancy concentration in the alumina lattice. This significantly enhances the bulk diffusion rate of aluminum ions and oxygen ions, as well as the grain boundary diffusion rate. Combined with the low-melting-point grain boundary liquid phase formed by the uniformly distributed yttrium oxide coating layer, rapid densification of the alumina billet is achieved through a dissolution-precipitation mechanism, reducing the sintering temperature by more than 30°C compared to the traditional sintering process above 1650°C. At the same time, the temperature range of 1580-1620°C allows the densification rate to be much higher than the grain boundary migration rate, achieving full densification while suppressing abnormal alumina grain growth, providing a fine-grained basis for subsequent grain boundary control. The subsequent rapid cooling of 8-12℃ / min allows the system to quickly pass through the 1400-1600℃ high-activity grain boundary range and the thermodynamic temperature range for YAG phase formation, kinetically suppressing grain growth and precipitation of brittle crystalline phases, and effectively freezing the uniform fine-grained structure formed at high temperatures. The medium-temperature holding range of 1350-1400℃ is higher than the glass transition temperature of the Y-Al-Si-O grain boundary glass phase, which can release the residual internal stress generated by rapid cooling through the short-range viscous flow of the grain boundary phase, optimize the grain boundary bonding state, and achieve grain boundary toughening. At the same time, it is lower than the critical temperature for YAG phase formation, which completely avoids the precipitation of brittle phases. Ultimately, it solves the inherent contradiction between rapid cooling internal stress and slow cooling grain growth and brittle phase formation in the prior art, and overcomes the technical prejudice in this field that rapid cooling degrades product performance.

[0016] Secondly, the present invention also provides an alumina ceramic ball prepared by any of the above methods.

[0017] The present invention has the following advantages over the prior art: This invention addresses the core challenges of yttrium oxide sintering aids, such as easy agglomeration and uneven dispersion leading to brittle grain boundary phases, through the synergistic combination of in-situ chemical coating modification and two-stage atmosphere sintering. It overcomes the long-standing technical biases in the field, such as the inability to balance low-temperature sintering with high performance and the inability to balance rapid cooling grain control with internal stress elimination. Through the organic synergy of the entire process, it significantly reduces the sintering temperature of alumina ceramics while achieving precise control over the entire chain, from raw material uniformity to microscopic grain boundary structure. This results in alumina ceramic balls with uniform grain size, high density, excellent mechanical properties and wear resistance, and good batch stability. The entire process has a wide parameter control window and can be adapted to the large-scale stable production of existing industrial ceramic production lines without the need for additional specialized equipment. Compared with existing conventional preparation methods, it possesses significant technical advantages and industrial application value. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] The following examples are used to further illustrate the present invention, but not to limit the scope of protection of the present invention. All modifications, equivalent substitutions, and improvements made based on the above content of the present invention should be included within the scope of protection of the present invention. All examples and comparative examples use the same basic raw material specifications: high-purity α-alumina powder, purity ≥99.8%, average particle size 1.0 μm; yttrium nitrate is analytical grade; the comparative nano-yttrium oxide powder has an average particle size of 30-50 nm and a purity ≥99.9%; the dispersant is ammonium polyacrylate, the binder is polyvinyl alcohol (PVA, degree of polymerization 1750±50); the grinding media is 3-5 mm zirconia balls; the abrasive used for wear testing is 800 mesh silicon carbide micro powder. This method is applicable to the preparation of alumina ceramic balls with a size of Φ3 mm-Φ20 mm. Different specifications of products can be adapted to process requirements by adjusting the molding and holding pressure time. The following examples all use a Φ5 mm specification product as an example for illustration.

[0020] Example 1 Step 1: Chemical Coating Preparation of Composite Powder: Weigh 1000g of high-purity α-alumina powder and prepare a 0.1mol / L yttrium nitrate aqueous solution. The amount of yttrium nitrate is such that the content of yttrium oxide (Y2O3) in the final product is 0.45% of the weight of the alumina powder. Place the alumina powder and the yttrium nitrate aqueous solution together in a ball mill jar, add zirconia grinding balls, control the ball-to-powder ratio at 5:1, add deionized water until the solid content of the system is 40%, adjust the pH of the mixed system to 5.0, control the system temperature at 50℃, and ball mill for 12 hours to obtain a uniformly mixed slurry. Place the obtained slurry in an oven at 120℃ to dry completely, and then place the dried powder in a muffle furnace and calcine at 650℃ for 2 hours to completely decompose the yttrium nitrate and generate a nano-yttrium oxide coating layer in situ on the surface of the alumina particles, thus obtaining yttrium oxide / alumina composite powder.

[0021] Step 2: Slurry preparation and spray granulation: The composite powder obtained in Step 1 is mixed with 0.4% by weight of ammonium polyacrylate dispersant, 1.2% by weight of polyvinyl alcohol binder, and deionized water. After stirring evenly, a ceramic slurry with a solid content of 58% is prepared. The ceramic slurry is fed into a spray drying tower for granulation. The inlet temperature of the spray dryer is controlled at 240℃ and the outlet temperature is controlled at 100℃ to obtain spherical granulated powder with a particle size distribution of 80-120 mesh.

[0022] Step 3: Cold isostatic pressing: The spherical granulated powder obtained in step 2 is evenly filled into the spherical rubber mold. After the mold is closed, the mold is placed in the cavity of the cold isostatic press and pressed for 60 seconds under a pressure of 165MPa. After demolding, a spherical green body with a uniform density of Φ5mm is obtained.

[0023] Step 4: Debinding treatment: Place the spherical green body obtained in step 3 into a box furnace, heat it to 600°C at a heating rate of 1°C / min in an air atmosphere, hold it at that temperature for 2 hours to complete the debinding, and then cool it to room temperature with the furnace.

[0024] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate to 3L / min and maintaining a slight positive pressure of 0.03MPa inside the furnace; heat to 1600℃ at a heating rate of 5℃ / min, and hold for 3 hours to complete densification sintering; after holding, immediately cool down to 1370℃ at a rate of 10℃ / min, hold for 45 minutes, then stop temperature control and cool to room temperature with the furnace to obtain the finished alumina ceramic spheres.

[0025] Example 2 Step 1: Chemical Coating Preparation of Composite Powder: Weigh 1000g of high-purity α-alumina powder and prepare a 0.1mol / L yttrium nitrate aqueous solution. The amount of yttrium nitrate is such that the content of yttrium oxide (Y2O3) in the final product is 0.35% of the weight of the alumina powder. Place the alumina powder and the yttrium nitrate aqueous solution together in a ball mill jar, add zirconia grinding balls, control the ball-to-powder ratio at 5:1, add deionized water until the solid content of the system is 40%, adjust the pH of the mixed system to 4.5, control the system temperature at 45℃, and ball mill for 12 hours to obtain a uniformly mixed slurry. Place the obtained slurry in a 120℃ oven to dry completely, and then place the dried powder in a muffle furnace and calcine at 600℃ for 2.5 hours to completely decompose the yttrium nitrate and generate a nano-yttrium oxide coating layer in situ on the surface of the alumina particles, thus obtaining yttrium oxide / alumina composite powder.

[0026] Step 2: Slurry preparation and spray granulation: The composite powder obtained in Step 1 is mixed with 0.35% by weight of ammonium polyacrylate dispersant, 1.1% by weight of polyvinyl alcohol binder, and deionized water. After stirring evenly, a ceramic slurry with a solid content of 56% is prepared. The ceramic slurry is then fed into a spray drying tower for granulation. The inlet temperature of the spray dryer is controlled at 235℃ and the outlet temperature is controlled at 98℃ to obtain spherical granulated powder with a particle size distribution of 80-120 mesh.

[0027] Step 3: Cold isostatic pressing: The spherical granulated powder obtained in step 2 is evenly filled into the spherical rubber mold. After the mold is closed, the mold is placed in the cavity of the cold isostatic press and pressed for 65 seconds under a pressure of 160MPa. After demolding, a spherical green body with a uniform density of Φ5mm is obtained.

[0028] Step 4: Debinding treatment: Place the spherical green body obtained in step 3 into a box furnace, heat it to 590°C at a heating rate of 0.9°C / min in an air atmosphere, hold it at that temperature for 2.5 hours to complete the debinding, and then cool it to room temperature with the furnace.

[0029] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen. Control the gas flow rate to 2.5 L / min and maintain a slight positive pressure of 0.025 MPa inside the furnace. Heat the furnace to 1590℃ at a heating rate of 4℃ / min and hold for 2.5 hours to complete the densification sintering. After holding, immediately cool the furnace to 1350℃ at a rate of 9℃ / min and hold for 30 minutes. Then stop temperature control and allow the furnace to cool to room temperature to obtain the finished alumina ceramic spheres.

[0030] Example 3 Step 1: Chemical Coating Preparation of Composite Powder: Weigh 1000g of high-purity α-alumina powder and prepare a 0.12mol / L yttrium nitrate aqueous solution. The amount of yttrium nitrate is such that the content of yttrium oxide (Y2O3) in the final product is 0.55% of the weight of the alumina powder. Place the alumina powder and the yttrium nitrate aqueous solution together in a ball mill jar, add zirconia grinding balls, control the ball-to-powder ratio at 5:1, add deionized water until the solid content of the system is 40%, adjust the pH of the mixed system to 5.5, control the system temperature at 55℃, and ball mill for 13 hours to obtain a uniformly mixed slurry. Place the obtained slurry in an oven at 120℃ to dry completely, and then place the dried powder in a muffle furnace and calcine at 680℃ for 1.5 hours to completely decompose the yttrium nitrate and generate a nano-yttrium oxide coating layer in situ on the surface of the alumina particles, thus obtaining yttrium oxide / alumina composite powder.

[0031] Step 2: Slurry preparation and spray granulation: The composite powder obtained in Step 1 is mixed with 0.45% by weight of ammonium polyacrylate dispersant, 1.4% by weight of polyvinyl alcohol binder, and deionized water. After stirring evenly, a ceramic slurry with a solid content of 60% is prepared. The ceramic slurry is fed into a spray drying tower for granulation. The inlet temperature of the spray dryer is controlled at 245℃ and the outlet temperature is controlled at 102℃ to obtain spherical granulated powder with a particle size distribution of 80-120 mesh.

[0032] Step 3: Cold isostatic pressing: The spherical granulated powder obtained in step 2 is evenly filled into a spherical polyurethane mold. After the mold is closed, the mold is placed in the cavity of a cold isostatic press and pressed for 55 seconds at a pressure of 175MPa. After demolding, a spherical green body with a uniform density of Φ5mm is obtained.

[0033] Step 4: Debinding treatment: Place the spherical green body obtained in step 3 into a box furnace, heat it to 610°C at a heating rate of 1.1°C / min in an air atmosphere, hold it at that temperature for 1.5 hours to complete the debinding, and then cool it to room temperature with the furnace.

[0034] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate to 4L / min and maintaining a slight positive pressure of 0.04MPa inside the furnace; heat to 1610℃ at a heating rate of 5℃ / min, and hold for 3.5 hours to complete densification sintering; after holding, immediately cool down to 1390℃ at a rate of 11℃ / min, hold for 60 minutes, then stop temperature control and cool to room temperature with the furnace to obtain the finished alumina ceramic spheres.

[0035] Comparative Example 1 (Traditional Physical Mixing Method) Step 1: Powder Mixing: Weigh 1000g of high-purity α-alumina powder and an equal amount of nano-yttrium oxide powder as in Example 1, i.e., the amount of yttrium oxide added is 0.45% of the weight of the alumina powder; place the alumina powder and nano-yttrium oxide powder together in a ball mill jar, add zirconia grinding balls, control the ball-to-powder ratio at 5:1, add deionized water until the solid content of the system is 40%, and ball mill for 24 hours to obtain a uniformly mixed slurry; place the obtained slurry in an oven at 120℃ to dry completely to obtain the mixed powder.

[0036] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0037] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0038] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0039] Step 5: Two-stage atmosphere sintering: This step is exactly the same as step 5 in Example 1, and the finished alumina ceramic balls are obtained.

[0040] Comparative Example 2 (without two-stage sintering process) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0041] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0042] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0043] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0044] Step 5 Sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate at 3L / min and maintaining a slight positive pressure of 0.03MPa inside the furnace; heat to 1600℃ at a heating rate of 5℃ / min, and hold for 3 hours to complete densification sintering; after holding, cool to room temperature with the furnace at a rate of 3℃ / min to obtain the finished alumina ceramic spheres.

[0045] Comparative Example 3 (Separate Coating Process + Conventional Sintering) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0046] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0047] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0048] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0049] Step 5 Sintering: This step is exactly the same as step 5 in Comparative Example 2, yielding the finished alumina ceramic balls.

[0050] Comparative Example 4 (Physical mixing + two-stage sintering process) Step 1 Powder mixing: Completely consistent with Step 1 of Comparative Example 1.

[0051] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0052] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0053] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0054] Step 5: Two-stage atmosphere sintering: This step is exactly the same as step 5 in Example 1, and the finished alumina ceramic balls are obtained.

[0055] Comparative Example 5 (yttrium oxide addition level below the lower limit of protection range) Step 1: Chemical coating preparation of composite powder: Weigh 1000g of high-purity α-alumina powder and prepare a 0.1mol / L yttrium nitrate aqueous solution. The amount of yttrium nitrate used is such that the content of yttrium oxide in the final product, calculated as Y2O3, is 0.2% of the weight of the alumina powder. The remaining process parameters are completely consistent with Step 1 of Example 1 to obtain composite powder.

[0056] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0057] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0058] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0059] Step 5: Two-stage atmosphere sintering: This step is exactly the same as step 5 in Example 1, and the finished alumina ceramic balls are obtained.

[0060] Comparative Example 6 (Yttrium oxide addition exceeds the upper limit of the protection range) Step 1: Chemical coating preparation of composite powder: Weigh 1000g of high-purity α-alumina powder and prepare a 0.1mol / L yttrium nitrate aqueous solution. The amount of yttrium nitrate used is such that the content of yttrium oxide in the final product, calculated as Y2O3, is 0.8% of the weight of the alumina powder. The remaining process parameters are completely consistent with Step 1 of Example 1 to obtain composite powder.

[0061] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0062] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0063] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0064] Step 5: Two-stage atmosphere sintering: This step is exactly the same as step 5 in Example 1, and the finished alumina ceramic balls are obtained.

[0065] Comparative Example 7 (medium-temperature insulation temperature below the lower limit of the protection range) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0066] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0067] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0068] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0069] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen. Control the gas flow rate to 3L / min and maintain a slight positive pressure of 0.03MPa inside the furnace. Heat the furnace to 1600℃ at a heating rate of 5℃ / min and hold for 3 hours to complete the densification sintering. After holding, immediately cool the furnace to 1300℃ at a rate of 10℃ / min and hold for 45 minutes. Then stop temperature control and allow the furnace to cool to room temperature to obtain the finished alumina ceramic spheres.

[0070] Comparative Example 8 (medium-temperature insulation temperature exceeds the upper limit of the protection range) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0071] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0072] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0073] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0074] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate to 3L / min and maintaining a slight positive pressure of 0.03MPa inside the furnace; heat to 1600℃ at a heating rate of 5℃ / min, and hold for 3 hours to complete densification sintering; after holding, immediately cool down to 1450℃ at a rate of 10℃ / min, hold for 45 minutes, then stop temperature control and cool to room temperature with the furnace to obtain the finished alumina ceramic spheres.

[0075] Comparative Example 9 (Coating process + Traditional high-temperature sintering) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0076] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0077] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0078] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0079] Step 5 Sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate to 3L / min and maintaining a slight positive pressure of 0.03MPa inside the furnace; heat to 1650℃ at a heating rate of 5℃ / min, and hold for 3 hours to complete densification sintering; after holding, cool to room temperature with the furnace at a rate of 3℃ / min to obtain the finished alumina ceramic spheres.

[0080] Comparative Example 10 (rapid cooling rate below the lower limit) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0081] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0082] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0083] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0084] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen. Control the gas flow rate to 3L / min and maintain a slight positive pressure of 0.03MPa inside the furnace. Heat the furnace to 1600℃ at a heating rate of 5℃ / min and hold for 3 hours to complete the densification sintering. After the holding period, immediately cool the furnace to 1370℃ at a rate of 5℃ / min and hold for 45 minutes. Then stop temperature control and allow the furnace to cool to room temperature to obtain the finished alumina ceramic spheres.

[0085] Comparative Example 11 (rapid cooling rate exceeds the upper limit) Step 1: Chemical coating preparation of composite powder: This is completely consistent with Step 1 of Example 1.

[0086] Step 2: Slurry preparation and spray granulation: This is completely consistent with Step 2 of Example 1.

[0087] Step 3: Cold isostatic pressing: This is completely consistent with step 3 of Example 1.

[0088] Step 4: Degumming treatment: This is completely consistent with Step 4 of Example 1.

[0089] Step 5: Two-stage atmosphere sintering: Place the debinding spherical green body into an atmosphere sintering furnace, and introduce a weakly reducing mixed atmosphere of nitrogen and 5% hydrogen, controlling the gas flow rate to 3L / min and maintaining a slight positive pressure of 0.03MPa inside the furnace; heat to 1600℃ at a heating rate of 5℃ / min, and hold for 3 hours to complete densification sintering; after holding, immediately cool to 1370℃ at a rate of 15℃ / min, hold for 45 minutes, then stop temperature control and cool to room temperature with the furnace to obtain the finished alumina ceramic spheres.

[0090] Performance verification methods All alumina ceramic spheres obtained in the examples and comparative examples were subjected to performance testing using a uniform testing method, as detailed below: 1. Bulk density test: Archimedes' displacement method was used, and the test was conducted in accordance with GB / T 25995-2010 "Determination of density and porosity of fine ceramics". Ten Φ5mm ceramic ball samples were randomly selected from each group of samples, and the arithmetic mean was taken as the final result after the test was completed.

[0091] 2. Rockwell hardness test: A surface Rockwell hardness tester was used, and the test was conducted according to GB / T 37900-2019 "Method for measuring Rockwell hardness of superhard abrasive products". The test load was 60 kgf and the holding time was 15 s. Ten ceramic ball samples were randomly selected from each group of samples, and three different test points were tested on each sample. The arithmetic mean of all test data was taken as the final result.

[0092] 3. Crushing load test: The test was conducted using an electronic universal testing machine according to the GB / T 30809-2014 standard "Ceramic Ball Bearings Ceramic Balls". The loading rate was set to 100 N / s, and the maximum load value when the ceramic ball broke was recorded. Twenty ceramic ball samples were randomly selected from each group of samples, and the arithmetic mean was taken as the final result after the test was completed.

[0093] 4. Wear resistance test: A planetary ball mill was used for wear test. The specific steps were as follows: 100g of ceramic balls to be tested, 200g of 800-mesh silicon carbide powder, and 500g of deionized water were weighed and placed together in a polyurethane ball mill jar. The balls were continuously milled at a speed of 300r / min for 24h. After the ball milling was completed, the ceramic balls were taken out, washed with deionized water, and placed in a 120℃ oven to dry completely. After cooling to room temperature, they were weighed. The wear rate was calculated using the formula: Wear rate = (Total mass of ceramic balls before milling - Total mass of ceramic balls after milling) / Total mass of ceramic balls before milling × 100%. Each group of samples was tested in parallel for 3 times, and the arithmetic mean was taken as the final result.

[0094] 5. Phase and microstructure characterization: X-ray diffraction (XRD) was used to analyze the phase of the ceramic sphere samples, with a scanning range of 2θ of 10°-80°, to determine whether yttrium aluminum garnet (YAG) crystalline phase was formed in the samples; energy dispersive spectroscopy (EDS) was used to perform elemental surface scan analysis to verify the uniformity of yttrium oxide distribution; and transmission electron microscopy (TEM) was used to observe the grain boundary morphology and grain size.

[0095] Performance test results The performance test results of all embodiments and comparative examples are shown in the table below: Microscopic characterization results showed that: EDS surface scans of Examples 1-3 indicated that Y element was uniformly distributed in the alumina matrix without local enrichment; TEM images showed uniform grain size, with an average grain size of 1.2-1.5 μm, clear grain boundaries, and no brittle precipitates; XRD patterns showed no characteristic diffraction peaks of the YAG phase, consistent with the results in the table. The XRD patterns of Comparative Examples 1, 4, 6, and 8 all showed obvious characteristic diffraction peaks of the YAG phase, and EDS surface scans revealed local enrichment of Y element.

[0096]

[0097] Summary of Results from Examples and Comparative Examples The test results of the above embodiments and comparative examples show that the present invention can stably prepare alumina ceramic balls with high density, high mechanical strength, and excellent wear resistance by preparing yttrium oxide / alumina composite powder through in-situ coating with soluble yttrium salt liquid phase and combining it with a two-stage sintering process in a weakly reducing atmosphere. This effectively solves the core industry pain points in the prior art, such as uneven dispersion of yttrium oxide sintering aids, easy formation of brittle grain boundary phases, and the inability to simultaneously achieve high performance and low-temperature sintering. The comparative results show that comparative examples 1 and 4, which use traditional physical mixing methods to add yttrium oxide, still have significantly inferior overall performance compared to the embodiments of the present invention due to the local enrichment of yttrium oxide and the formation of brittle YAG phases, even when using the same two-stage sintering process. Comparative examples 2, 3, and 9, which only use a single in-situ coating process or a single two-stage sintering process, cannot achieve the synergistic matching of uniform dispersion of yttrium oxide, freezing of fine grain structure, and strengthening of grain boundaries. Their overall performance is far lower than that of the embodiments of the present invention, which fully demonstrates that the two core processes of the present invention have an irreplaceable synergistic effect and are not simply a superposition of existing technologies. Meanwhile, comparative examples 5-8 and 10-11, whose yttrium oxide addition, medium-temperature holding temperature, and rapid cooling rate exceeded the limits set by this invention, all exhibited problems such as insufficient density, formation of brittle phases, and a significant decrease in mechanical properties and wear resistance. This verifies that the process parameters defined by this invention represent the critical equilibrium range for achieving the desired technical effect. This invention offers a wide control window for process parameters, good batch stability, and can be adapted to existing industrial ceramic production lines without requiring additional specialized equipment, demonstrating significant technical advantages and industrial application value.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing alumina ceramic spheres using yttrium oxide as a sintering agent, characterized in that, Includes the following steps: (1) Chemical coating preparation of composite powder: using yttrium nitrate aqueous solution with a concentration of 0.08-0.12 mol / L as yttrium precursor, it is mixed with high-purity α-alumina powder with a purity ≥99.8% and an average particle size of 0.8-1.2 μm by wet ball milling in the liquid phase. The pH value of the mixing system is controlled at 4.0-6.0 and the temperature at 40-60℃, so that yttrium ions are uniformly adsorbed on the surface of alumina particles; After mixing, the slurry is dried and then calcined at 500-700℃ for 1.5-2.5h to decompose the yttrium salt, thereby generating a uniform nano-yttrium oxide coating layer in situ on the surface of the alumina particles, thus obtaining yttrium oxide / alumina composite powder; wherein, the amount of yttrium oxide added, calculated as Y2O3, is 0.3-0.6% of the weight of the alumina powder; the ball-to-particle ratio of the wet ball milling is 5:1, and the grinding media is 3-5mm zirconia balls; (2) Slurry preparation and spray granulation: The yttrium oxide / alumina composite powder is mixed with dispersant, binder and deionized water to prepare a ceramic slurry with a solid content of 55-60%, and then spray dried to obtain spherical granulated powder; (3) Cold isostatic pressing: The spherical granulated powder is filled into a spherical elastic mold and pressed by a cold isostatic press at a pressure of 150-180MPa for 50-70s to obtain a spherical green body with uniform density. (4) Two-stage atmosphere sintering: After the spherical green body is debinded, it is placed in a sintering furnace and heated to 1580-1620℃ at a heating rate of 3-5℃ / min under a weak reducing atmosphere. The temperature is held for 2-4 hours to complete the densification sintering. After the holding period, the temperature is rapidly cooled to 1350-1400℃ at a rate of 8-12℃ / min and held for 0.5-1 hours. Then, the temperature is cooled to room temperature with the furnace to obtain alumina ceramic spheres. The weak reducing atmosphere is a nitrogen-hydrogen mixed atmosphere, in which the volume ratio of hydrogen is 4-6%. The atmosphere flow rate during sintering is 2-5L / min, and a slight positive pressure of 0.02-0.05MPa is maintained in the furnace.

2. The preparation method according to claim 1, characterized in that, In step (1), the mixing time of the wet ball mill is 10-14h and the drying temperature is 110-130℃.

3. The preparation method according to claim 1, characterized in that, In step (1), the amount of yttrium oxide added, calculated as Y2O3, is 0.4-0.5% of the weight of alumina powder.

4. The preparation method according to claim 1, characterized in that, In step (2), the dispersant is ammonium polyacrylate, and the amount added is 0.3-0.5% of the weight of the yttrium oxide / alumina composite powder; the binder is polyvinyl alcohol, and the amount added is 1.0-1.5% of the weight of the yttrium oxide / alumina composite powder.

5. The preparation method according to claim 1, characterized in that, In step (2), the inlet temperature of the spray dryer is 230-250℃, the outlet temperature is 95-105℃, and the particle size distribution of the resulting spherical granulated powder is 80-120 mesh.

6. The preparation method according to claim 1, characterized in that, In step (3), the spherical elastic mold is a rubber mold or a polyurethane mold, and the pressure of cold isostatic pressing is 160-175MPa.

7. The preparation method according to claim 1, characterized in that, In step (4), the debinding process specifically involves heating the material to 580-620°C in an air atmosphere at a heating rate of 0.8-1.2°C / min and holding it at that temperature for 1.5-2.5 hours.

8. The preparation method according to claim 1, characterized in that, In step (4), the heating rate of densification sintering is 4-5℃ / min, the sintering temperature is 1590-1610℃, and the holding time is 2.5-3.5h.

9. The preparation method according to claim 1, characterized in that, In step (4), the rapid cooling rate is 9-11℃ / min, and the temperature is maintained at 1360-1380℃ for 40-60 minutes.

10. An alumina ceramic ball, characterized in that, It is prepared by any one of the methods described in claims 1-9.

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