The incoming alumina powder is processed into powder material according to the requirements of different products and forming methods. The particle size of the powder is typically below 1 μm. When producing high-purity alumina oxide ceramic products, in addition to ensuring that the alumina purity is above 99.99%, the powder must also be finely ground to achieve uniform particle size distribution.
For extrusion or injection molding, binders and plasticizers are introduced into the powder, generally consisting of 10-30% by weight of thermoplastic plastic or resin. These organic binders must be uniformly mixed with the alumina powder at 150-200°C to facilitate the forming process. For hot-pressing, binders are not required in the raw powder.
When semi-automatic or fully automatic dry pressing is used, the powder must meet special process requirements, such as being treated with spray granulation to improve powder flowability for automatic filling of mold cavities. In addition, 1-2% lubricants and binders (PVA) are added to reduce friction between the powder and the mold walls. For dry pressing, spray granulation with polyvinyl alcohol as a binder is required.
In recent years, a research institute in Shanghai has developed a water-soluble wax as a binder for Al₂O₃ spray granulation, which exhibits excellent fluidity when heated. The spray granulated powder must have good flowability, loose density, and an ideal particle size distribution to achieve high green body density.
There are several forming methods for alumina oxide ceramic products, such as dry pressing, slip casting, extrusion, cold isostatic pressing, injection molding, tape casting, hot pressing, and hot isostatic pressing. In recent years, other forming technologies have been developed, such as pressure filtration molding, direct solidification casting, gel casting, centrifugal slip casting, and solid freeform fabrication. Different product shapes, sizes, complexity, and precision requirements necessitate different forming methods. Here are some common forming methods:
Dry pressing technology for alumina oxide ceramic is limited to simple shapes where the wall thickness exceeds 1 mm and the length-to-diameter ratio does not exceed 4:1. The pressing can be uniaxial or biaxial, with hydraulic or mechanical presses. Presses can be semi-automatic or fully automatic, with maximum pressure up to 200 MPa and production rates reaching 15-50 pieces per minute. Since hydraulic presses provide uniform pressure, variations in powder filling result in height differences in pressed parts. Mechanical presses, on the other hand, vary pressure according to powder filling, leading to potential size differences after sintering. Therefore, uniform powder particle distribution is critical for consistent filling of the mold. Accurate filling significantly influences the dimensional accuracy of alumina oxide ceramic parts. Particle sizes greater than 60 μm and between 60-200 mesh ensure optimal flow and pressing results.
Slip casting is the earliest forming method used for alumina oxide ceramic. It uses plaster molds, which are low-cost and suitable for forming large and complex parts. The key to slip casting is the preparation of the alumina slurry, which is typically water-based with the addition of deflocculants and binders. The slurry is thoroughly ground, degassed, and poured into the plaster mold. The plaster mold’s capillary action absorbs water, solidifying the slurry in the mold. For hollow slip casting, excess slurry is poured out once the desired wall thickness is achieved. To minimize shrinkage, it is advisable to use high-concentration slurries.
Organic additives are also added to alumina slurries to form a double electric layer on the particle surface, ensuring a stable suspension without sedimentation. Furthermore, binders like polyvinyl alcohol, methyl cellulose, alginate, and dispersants such as polyacrylamide and gum arabic are added to facilitate the slip casting process.
Sintering is the process of densifying ceramic particles to form a solid material. This process removes voids between particles, as well as gases, impurities, and organic substances, allowing the particles to grow and bond, forming new material.
The most common heating equipment used for sintering is an electric furnace. In addition to conventional sintering (pressureless sintering), hot pressing and hot isostatic pressing are also used. Although continuous hot pressing increases production output, it is expensive due to equipment and mold costs. Additionally, since the heating is axial, product length is limited.
Hot isostatic pressing uses high-temperature, high-pressure gas as the pressure medium, allowing uniform heating in all directions. This method is well-suited for sintering complex shapes, offering improved material performance by 30-50% compared to cold pressing, and 10-15% compared to conventional hot pressing. Therefore, high-value-added alumina oxide ceramics or special components required for defense and military applications, such as ceramic bearings, reflectors, nuclear fuel, and gun barrels, often use hot isostatic pressing.
Furthermore, microwave sintering, arc plasma sintering, and self-propagating sintering technologies are currently under development.
Some alumina oxide ceramics materials require finishing after sintering. For example, products used as artificial bones must have a highly polished surface, like a mirror, to enhance lubrication. Due to the high hardness of alumina oxide ceramic materials, they require harder grinding and polishing materials, such as SiC, B₄C, or diamond. Typically, a series of abrasives are used, starting from coarse to fine, with final polishing using <1 μm Al₂O₃ powder or diamond paste. Laser processing and ultrasonic grinding and polishing techniques can also be employed.
Some alumina oxide ceramic parts may need to be packaged with other materials.
