A method for preparing an aluminum oxide ceramic by dry pressing

Abstract

The application discloses a preparation method of alumina ceramic by dry pressing, and belongs to the technical field of ceramic preparation. The method comprises the following steps: S1, powder pretreatment and granulation: high-purity alpha-alumina powder is granulated to obtain spherical granulated powder; S2, mold pretreatment and powder filling: the spherical granulated powder is sieved and then filled into a mold cavity and vibrated to a dense packing state; S3, multi-stage gradient pressing: pressure is applied to the mold, and the pressing process is divided into three stages: a pre-pressing stage, in which the pressure is slowly increased to 20-50 MPa at a rate of 5-15 MPa / s, and the pressure is kept for 10-30 seconds; a main pressing stage, in which the pressure is rapidly increased to 80-90% of the final forming pressure at a rate of 10-30 MPa / s; and a fine pressing stage, in which the pressure is slowly increased to the final forming pressure at a rate of 2-8 MPa / s, and the pressure is kept; S4, controllable-speed demolding: after pressure relief, the green body is smoothly separated from the mold cavity; and S5, green body post-treatment and sintering: after drying, the green body is placed in a high-temperature sintering furnace for glue removal and sintering to obtain alumina ceramic.

Description

Technical Field

[0001] This invention relates to the field of ceramic preparation technology, and in particular to a method for preparing alumina ceramics by dry pressing. Background Technology

[0002] Alumina ceramics are widely used in machinery, electronics, chemical industry, and biomedicine due to their high hardness, excellent wear resistance, good chemical stability, and insulation properties. Dry pressing is one of the most common and cost-effective methods for preparing simple-shaped alumina ceramic parts. The process mainly includes powder treatment, mold filling, unidirectional or bidirectional pressure forming, and demolding.

[0003] However, traditional dry pressing processes have several inherent defects that limit the performance of the prepared ceramics: First, poor powder flowability and particle agglomeration easily lead to uneven density distribution within the green body, resulting in stress concentration; second, during demolding, due to elastic aftereffects and friction between the powder and the mold wall, the green body is prone to defects such as delamination and microcracks; third, traditional processes have relatively crude control over key parameters such as molding pressure, holding time, and demolding rate, resulting in poor product consistency and reliability. These defects are amplified during subsequent sintering, ultimately affecting the mechanical properties and reliability of the ceramic products. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing alumina ceramics by dry pressing, which can improve the uniformity of alumina ceramic green body, reduce forming defects, and improve the quality of alumina ceramics.

[0005] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0006] A method for preparing alumina ceramics by dry pressing, the method comprising the following steps:

[0007] S1. Powder pretreatment and granulation:

[0008] High-purity α-alumina powder was ball-milled with 0.5–3.0 wt% sintering aid, 1.0–5.0 wt% organic binder and 0.1–1.0 wt% lubricant. After the mixture was ball-milled and homogenized, it was granulated by spray drying to obtain spherical granulated powder with good flowability. The particle size distribution of the spherical granulated powder was between 80 and 150 μm.

[0009] S2. Mold pretreatment and powder filling:

[0010] The cavity and punch surface of the metal mold are polished with high precision and coated with a layer of nano-diamond dispersion as a release agent. The spherical granulated powder prepared above is sieved and uniformly filled into the mold cavity. Then it is vibrated for 20 to 60 seconds to allow the spherical granulated powder to reach a dense packing state in the mold cavity.

[0011] S3. Multi-stage gradient pressure molding:

[0012] Pressure is applied to a mold containing spherical granulated powder. The pressurization process is divided into three stages:

[0013] The first stage is the pre-compression stage, where the pressure is slowly increased to 20-50 MPa at a rate of 5-15 MPa / s and held for 10-30 seconds to allow the powder particles to be initially rearranged and most of the air to be expelled.

[0014] The second stage: the main pressure stage, where the pressure is rapidly increased at a rate of 10-30 MPa / s to 80-90% of the final molding pressure, of which the final molding pressure is 100-300 MPa;

[0015] The third stage: the fine pressing stage, where pressure is slowly increased to the final molding pressure at a rate of 2-8 MPa / s, and held at this pressure for 60-180 seconds to allow the pressure to be fully transmitted, the particles to undergo plastic deformation, and the densification balance to be achieved.

[0016] S4. Controllable rate demolding:

[0017] After depressurization, the upper punch retracts at a rate of 0.1 to 0.5 mm / s, while the lower punch ejects the blank at the same rate as the upper punch, achieving smooth separation of the blank from the mold cavity and eliminating stress caused by sudden changes in demolding speed.

[0018] S5. Post-treatment and sintering of green blanks:

[0019] The demolded green body is dried at 80–120°C for 12–24 hours to remove residual moisture. Then it is placed in a high-temperature sintering furnace and heated to 600°C at a rate of 2–5°C / min in air atmosphere and held for 1–2 hours to completely remove the organic binder. Finally, it is heated to 1550–1650°C at a rate of 3–10°C / min and held for 2–4 hours. It is then cooled in the furnace to obtain dense alumina ceramic.

[0020] Preferably, in step S1, the purity of the high-purity α-alumina powder is greater than or equal to 99.5%.

[0021] Preferably, in step S1, the sintering aid is MgO or Y2O3, the organic binder is polyvinyl alcohol or polyethylene glycol, and the lubricant is stearic acid or zinc stearate.

[0022] Preferably, in step S2, the vibration is mechanical vibration or ultrasonic-assisted vibration.

[0023] The above technical solution has the following beneficial effects:

[0024] High density and uniformity: Excellent flowability spherical powder is obtained through spray granulation. Combined with vibration filling and multi-stage gradient pressurization, the uniformity of powder filling and density uniformity of green body are improved. The relative density of green body can reach 58-65%, effectively reducing density gradient.

[0025] Low defects: Optimized low-speed demolding process and mold pretreatment significantly reduce demolding friction and internal stress caused by elastic aftereffects, reducing the generation of delamination and microcracks;

[0026] High product performance and consistency: The finely controlled process parameters ensure the consistency of the quality of each green blank in mass production. The alumina ceramic products obtained after sintering have a relative density of ≥99% and a three-point bending strength of ≥380 MPa, significantly improving the reliability index.

[0027] Operability and universality: It is easy to modify and set parameters on traditional dry pressing equipment, without involving expensive equipment, with controllable costs, and suitable for large-scale industrial production. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] To address the problems of uneven green body density, easy formation of delamination and microcracks, and poor product consistency in the preparation of alumina ceramics by existing dry pressing processes, a high-quality alumina ceramic preparation method with optimized process parameters and strong operability is provided.

[0030] A method for preparing alumina ceramics by dry pressing includes the following steps:

[0031] S1. Powder pretreatment and granulation:

[0032] High-purity α-alumina powder was ball-milled with 0.5–3.0 wt% sintering aid, 1.0–5.0 wt% organic binder and 0.1–1.0 wt% lubricant. After the mixture was ball-milled and homogenized, it was granulated by spray drying to obtain spherical granulated powder with good flowability. The particle size distribution of the spherical granulated powder was between 80 and 150 μm.

[0033] Specifically, the powder is first pretreated, and then spray-granulated to obtain spherical granulated powder with good flowability. This involves weighing high-purity α-alumina powder, sintering aid, organic binder, and lubricant, and then ball-milling them together. The amount of sintering aid is 0.5–3.0 wt%, specifically 0.5 wt% or 3.0 wt%. The amount of organic binder is 1.0–5.0 wt%, specifically 1.0 wt% or 5.0 wt% of the total powder mass. The amount of lubricant is 0.1–1.0 wt%, specifically 0.1 wt% or 1.0 wt% of the total powder mass. The total mass percentage of the high-purity α-alumina powder, sintering aid, organic binder, and lubricant is 100 wt%. The resulting spherical granulated powder has a particle size distribution between 80 and 150 μm.

[0034] Among them, the purity of high-purity α-alumina powder is greater than or equal to 99.5%, specifically 99.8%, the sintering aid is MgO or Y2O3, the organic binder is polyvinyl alcohol (PVA) or polyethylene glycol (PEG), and the lubricant is stearic acid or zinc stearate;

[0035] S2. Mold pretreatment and powder filling:

[0036] The cavity and punch surface of the metal mold are polished with high precision and coated with a layer of nano-diamond dispersion as a release agent. The spherical granulated powder prepared above is sieved and uniformly filled into the mold cavity. Then it is vibrated for 20 to 60 seconds to allow the spherical granulated powder to reach a dense packing state in the mold cavity.

[0037] Specifically, the surface of the cavity and punch of the metal mold is polished with high precision and coated with a layer of nano-diamond dispersion as a release agent to facilitate demolding after pressing. The spherical granulated powder is sieved and then evenly filled into the mold cavity. Then it is vibrated for 20 or 60 seconds to make the spherical granulated powder reach a dense packing state in the mold cavity. The vibration is mechanical vibration or ultrasonic-assisted vibration, which is completed by mechanical vibration equipment or ultrasonic-assisted vibration equipment.

[0038] S3. Multi-stage gradient pressure molding:

[0039] Pressure is applied to a mold containing spherical granulated powder. The pressurization process is divided into three stages:

[0040] The first stage is the pre-compression stage, where the pressure is slowly increased to 20-50 MPa at a rate of 5-15 MPa / s and held for 10-30 seconds to allow the powder particles to be initially rearranged and most of the air to be expelled.

[0041] The second stage: the main pressure stage, where the pressure is rapidly increased at a rate of 10-30 MPa / s to 80-90% of the final molding pressure, of which the final molding pressure is 100-300 MPa;

[0042] The third stage: the fine pressing stage, where pressure is slowly increased to the final molding pressure at a rate of 2-8 MPa / s, and held at this pressure for 60-180 seconds to allow the pressure to be fully transmitted, the particles to undergo plastic deformation, and the densification balance to be achieved.

[0043] Specifically, the process of applying pressure can be divided into three stages, as follows:

[0044] The first stage is the pre-compression stage, where the pressure is slowly increased to 20 MPa or 50 MPa at a rate of 5 MPa / s or 15 MPa / s, and held for 10 or 30 seconds to allow the spherical granulated powder particles to initially rearrange and expel most of the internal air.

[0045] The second stage is the main pressure stage, where the pressure is rapidly increased to 80% or 90% of the final molding pressure at a rate of 10MPa / s or 30MPa / s, where the final molding pressure is 100MPa or 300MPa.

[0046] The third stage is the fine pressing stage, where pressure is slowly increased to the final molding pressure at a rate of 2MPa / s or 8MPa / s, and held at this pressure for 60 seconds or 180 seconds to allow the pressure to be fully transmitted, the particles to undergo plastic deformation, and a densification equilibrium to be achieved.

[0047] Excellent flowability spherical powders are obtained by spray granulation. Combined with vibration filling and multi-stage gradient pressurization, the uniformity of powder filling and the density uniformity of green body are improved. The relative density of green body can reach 58-65%, effectively reducing the density gradient.

[0048] S4. Controllable rate demolding:

[0049] After depressurization, the upper punch retracts at a rate of 0.1 to 0.5 mm / s, while the lower punch ejects the blank at the same rate as the upper punch, achieving smooth separation of the blank from the mold cavity and eliminating stress caused by sudden changes in demolding speed.

[0050] After the multi-stage gradient pressure molding process, the pressure is released, and the upper punch is controlled to retract at a rate of 0.1 mm / s or 0.5 mm / s. At the same time, the lower punch ejection mechanism controls the lower punch to eject the alumina ceramic blank at the same rate as the upper punch, so as to achieve smooth separation of the blank from the mold cavity and eliminate the stress caused by the sudden change in demolding speed.

[0051] The optimized low-speed demolding process and mold pretreatment significantly reduced the internal stress caused by demolding friction and elastic aftereffect, thus reducing the generation of delamination and microcracks.

[0052] S5. Post-treatment and sintering of green blanks:

[0053] The demolded green body is dried at 80-120℃ for 12-24 hours to remove residual moisture. Then it is placed in a high-temperature sintering furnace and heated to 600℃ at a rate of 2-5℃ / min in air atmosphere and held for 1-2 hours to completely remove the organic binder. Finally, it is heated to 1550-1650℃ at a rate of 3-10℃ / min and held for 2-4 hours. It is then cooled in the furnace to obtain dense alumina ceramic.

[0054] Specifically, the demolded alumina ceramic blank is dried at 80℃ or 120℃ for 12 hours or 24 hours. Specifically, it can be dried at a stable temperature of 80℃ for 24 hours or at a temperature of 120℃ for 12 hours to remove residual moisture. Then, it is placed in a high-temperature sintering furnace, and the temperature inside the sintering furnace is raised to 600℃ at a heating rate of 2℃ / min or 5℃ / min under an air atmosphere and held for 1 hour or 2 hours to completely remove the organic binder. Finally, the temperature is raised to 1550℃ or 1650℃ at a rate of 3℃ / min or 10℃ / min and held for 2 hours or 4 hours. Specifically, the temperature can be raised to 1550℃ at a rate of 3℃ / min and held for 4 hours, or raised to 1650℃ at a rate of 10℃ / min and held for 2 hours. The blank is then cooled in the furnace to obtain a dense alumina ceramic.

[0055] The precisely controlled process parameters ensure the consistency of the quality of each green blank in mass production. The alumina ceramic products obtained after sintering have a relative density of ≥99% and a three-point bending strength of ≥380 MPa, significantly improving reliability. They are easy to modify and set parameters on traditional dry pressing equipment, do not involve expensive equipment, have controllable costs, and are suitable for large-scale industrial production.

[0056] Example

[0057] A method for preparing alumina ceramic by dry pressing, comprising the following steps: preparing alumina ceramic sheets with a diameter of 50 mm and a thickness of 5 mm.

[0058] S1. Take 1000g of α-Al2O3 powder with 99.8% purity, add 1.5wt% MgO as sintering aid, 3.0wt% polyvinyl alcohol as binder, and 0.5wt% stearic acid as lubricant. Mix by wet ball milling for 6 hours. The slurry is spray dried at an inlet temperature of 200℃ and an outlet temperature of 90℃ to obtain spherical granulated powder with an average particle size of 120μm.

[0059] S2. Coat the mold cavity with nano-diamond release agent, fill it with 18.5g of granulation powder, and mechanically vibrate it for 40 seconds at a frequency of 50Hz and an amplitude of 0.5mm.

[0060] S3. Employ a bidirectional pressurization mode for multi-stage pressurization:

[0061] Pre-compression: Increase the pressure to 40 MPa at a rate of 10 MPa / s and hold the pressure for 20 seconds;

[0062] Main pressure: Increase the pressure to 180 MPa at a rate of 20 MPa / s (i.e., 90% of the final molding pressure of 200 MPa).

[0063] Fine pressing: Finally, increase the pressure to the final molding pressure of 200 MPa at a pressurization rate of 5 MPa / s and hold the pressure for 120 seconds;

[0064] S4. Control the demolding rate to 0.3 mm / s to smoothly eject the blank;

[0065] S5. After drying the green body at 100℃, it is sintered in a muffle furnace: the temperature is increased to 600℃ at 3℃ / min and held for 1.5h to remove the binder, and then increased to 1600℃ at 5℃ / min and held for 3h to complete the sintering preparation of ceramics.

[0066] The alumina ceramic sheet prepared above has a smooth surface without cracks, and the measured relative density is 99.2%, and the three-point bending strength is 395 MPa.

[0067] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing alumina ceramics by dry pressing, characterized in that, The method includes the following steps: S1. Powder pretreatment and granulation: High-purity α-alumina powder was ball-milled with 0.5–3.0 wt% sintering aid, 1.0–5.0 wt% organic binder and 0.1–1.0 wt% lubricant. After the mixture was ball-milled and homogenized, it was granulated by spray drying to obtain spherical granulated powder with good flowability. The particle size distribution of the spherical granulated powder was between 80 and 150 μm. S2. Mold pretreatment and powder filling: The cavity and punch surface of the metal mold are polished with high precision and coated with a layer of nano-diamond dispersion as a release agent. The spherical granulated powder prepared above is sieved and uniformly filled into the mold cavity. Then it is vibrated for 20 to 60 seconds to allow the spherical granulated powder to reach a dense packing state in the mold cavity. S3. Multi-stage gradient pressure molding: Pressure is applied to a mold containing spherical granulated powder. The pressurization process is divided into three stages: The first stage is the pre-compression stage, where the pressure is slowly increased to 20-50 MPa at a rate of 5-15 MPa / s and held for 10-30 seconds to allow the powder particles to be initially rearranged and most of the air to be expelled. The second stage: the main pressure stage, where the pressure is rapidly increased at a rate of 10-30 MPa / s to 80-90% of the final molding pressure, of which the final molding pressure is 100-300 MPa; The third stage: the fine pressing stage, where pressure is slowly increased to the final molding pressure at a rate of 2-8 MPa / s, and held at this pressure for 60-180 seconds to allow the pressure to be fully transmitted, the particles to undergo plastic deformation, and the densification balance to be achieved. S4. Controllable rate demolding: After depressurization, the upper punch retracts at a rate of 0.1 to 0.5 mm / s, while the lower punch ejects the blank at the same rate as the upper punch, achieving smooth separation of the blank from the mold cavity and eliminating stress caused by sudden changes in demolding speed. S5. Post-treatment and sintering of green blanks: The demolded green body is dried at 80–120°C for 12–24 hours to remove residual moisture. Then it is placed in a high-temperature sintering furnace and heated to 600°C at a rate of 2–5°C / min in air atmosphere and held for 1–2 hours to completely remove the organic binder. Finally, it is heated to 1550–1650°C at a rate of 3–10°C / min and held for 2–4 hours. It is then cooled in the furnace to obtain dense alumina ceramic.

2. The method for preparing alumina ceramics by dry pressing according to claim 1, characterized in that, In step S1, the purity of the high-purity α-alumina powder is greater than or equal to 99.5%.

3. The method for preparing alumina ceramics by dry pressing according to claim 1, characterized in that, In step S1, the sintering aid is MgO or Y2O3, the organic binder is polyvinyl alcohol or polyethylene glycol, and the lubricant is stearic acid or zinc stearate.

4. The method for preparing alumina ceramics by dry pressing according to claim 1, characterized in that, In step S2, the vibration is mechanical vibration or ultrasonic-assisted vibration.