Due to its high hardness, high temperature resistance, corrosion resistance, and excellent electrical insulation, alumina ceramics are widely used in high-end manufacturing fields such as aerospace, electronic packaging, medical devices, and wear-resistant parts. In these applications, the forming process is a critical step that determines the final performance, dimensional accuracy, and reliability of the ceramic. Forming refers to the process of preparing alumina powder into a green body with a certain shape and size. To avoid deformation or cracking during subsequent drying or sintering, the green body is required to have as high a density and good uniformity as possible.
Classification of Alumina Ceramic Forming Methods
Ceramic forming processes can be classified according to the flow characteristics of the blank into dry forming and wet forming, with wet forming further divided into plastic forming and colloidal forming.
Dry Forming
In dry forming, the blank contains little or no water (less than 6%), and the content of other binders or lubricants generally does not exceed 1%–2%. The process involves placing the powder into a mold and applying external mechanical force to shape the powder. The ceramic green body is held together by friction between ceramic particles, maintaining a certain size and shape. The green body is a composite system consisting of the blank, liquid phase (binder), and air.
01 Dry Pressing
Dry pressing involves first mixing the powder with water or a binder to form granules, then placing the granulated powder into a mold and applying pressure to form a green body with a certain strength and shape. The advantages of dry pressing include simple process, easy operation, suitability for large-scale industrial production, high green density, and small shrinkage of the sintered product. However, because dry pressing uses uniaxial pressing, the green body exhibits significant density non-uniformity, which becomes more pronounced as the sample thickness increases. Moreover, agglomerates in the powder cannot be eliminated during powder preparation, making it difficult to produce fine ceramics.
02 Isostatic Pressing
Isostatic pressing involves placing powder into a deformable rubber mold and then applying equal pressure from different axes via a gaseous or liquid medium to form the part. Compared with dry pressing, isostatic pressing uses multi‑axial pressing, which solves the density non‑uniformity problem of dry‑pressed green bodies and reduces the travel distance of powder particles, thereby increasing the forming rate. However, isostatic pressing has disadvantages such as high equipment cost, complex maintenance, and susceptibility to defects (e.g., surface irregularities, compression cracks). For large‑sized products, isostatic pressing is prone to "elephant foot" or density gradients from the outside inward. Currently, alumina ceramics produced by isostatic pressing in China are mainly used for ceramic radomes, high‑frequency terminal insulating ceramic sleeves, etc.
Traditional Wet Forming
Compared with dry forming, wet forming can avoid powder agglomeration, reduce or even eliminate defects in the green body, and improve the reliability of ceramic products, offering a reliable route for producing fine ceramics or ceramic films. Wet forming generally includes the following steps: (1) synthesis or mixing of ceramic powder; (2) preparation of ceramic slurry; (3) solidification of the slurry; (4) drying to remove solvent; (5) sintering to obtain the ceramic. The key points in wet forming are the preparation of the ceramic slurry and the drying of the green body.
01 Slip Casting
Slip casting involves uniformly dispersing ceramic powder in a liquid medium to form a ceramic slurry, then pouring the slurry into a plaster mold. The capillary force of the plaster mold absorbs the solvent from the slurry, causing the ceramic to solidify and form a green body with a certain shape and size. Slip casting requires the slurry to have excellent fluidity and stability. In addition, the mold must have good permeability to facilitate removal of the liquid medium from the green body. Slip casting has advantages such as simple process, low production cost, and easy process control. However, the forming time is long, the green density and strength are low, and defects such as cracking and pinholes are prone to occur. Based on slip casting, researchers have developed improved methods such as centrifugal slip casting, pressure slip casting, and filter pressing.
02 Tape Casting
The ceramic slurry used in tape casting is a viscous, poorly flowing mixture of ceramic powder, plasticizer, dispersant, and solvent. After de‑airing, the slurry is placed on a tape casting machine, spread by a doctor blade to a certain thickness onto a carrier tape, dried, and peeled off to form a thin film. The green body is then machined according to product dimensions. The slurry for tape casting requires uniform dispersion of the ceramic powder without agglomerates or undissolved binder. During green body preparation, tape casting is prone to issues such as uneven thickness, rough surface, scars, and defects. Tape casting is mainly used to produce thin ceramic plates, widely applied in integrated circuit substrates, substrate materials for substrates, capacitors, and low‑voltage varistors.
Novel Colloidal Forming Processes
Colloidal forming involves uniformly dispersing ceramic powder in a solvent to obtain a high‑solids, low‑viscosity ceramic slurry, which is then poured into a mold and solidified by various methods to obtain a green body with a certain shape and size. In recent years, significant progress has been made in such forming methods, leading to several derived processes, such as gelcasting, direct coagulation casting, and spontaneous coagulation casting.
01 Gelcasting
Gelcasting combines traditional wet forming with polymer chemistry. Organic monomers undergo polymerization under certain conditions to form a three‑dimensional polymer network, which locks the ceramic particles and solvent molecules in the network, causing the ceramic slurry to solidify and form a green body. This method not only inherits the advantages of traditional wet forming (slip casting, injection molding) but also largely overcomes their problems, making it a practically valuable ceramic forming process.
The greatest challenge in gelcasting is preparing a high‑solids (>50 vol.%) yet low‑viscosity ceramic slurry. The fluidity of the slurry is mainly determined by the dispersibility and stability of the powder in the solvent, which can be achieved by selecting an appropriate dispersant and controlling the slurry pH. Another challenge is controlling the solidification of the slurry. The polymerization conditions of the selected organic monomer and cross‑linker should be easily controllable. The organic content in the green body should be as low as possible, and the resulting green body should have high strength. The organic addition in gelcasting is generally less than 5 wt.%, so no separate binder burnout step is required. The obtained green body has high strength and uniform structure. Gelcasting offers advantages such as near‑net‑shape forming, the ability to produce complex‑shaped products, and the ability to produce large‑sized products.
02 Direct Coagulation Casting
Direct coagulation casting is a novel ceramic forming technique developed based on gelcasting. It uses methods such as enzyme catalysis, pH control of the ceramic slurry, or electrolyte concentration control to adjust the double‑layer repulsion between ceramic particles, destabilize the suspension stability of the ceramic slurry, and cause in‑situ solidification of the ceramic particles to form a green body. The mechanism of liquid‑solid transformation involves adjusting the van der Waals attraction and the electrostatic repulsion generated by the double layer. By changing the magnitudes of van der Waals force and electrostatic repulsion, the solidification of the slurry is controlled: when van der Waals attraction dominates, the slurry tends to solidify; when electrostatic repulsion dominates, the slurry tends to disperse. The solids loading of the slurry for direct coagulation casting should be no less than 55 vol.% so that solidification can be achieved by adjusting the slurry pH. The solidification conditions are easily controllable and achievable. Direct coagulation casting produces green bodies with high density, good uniformity, low organic content, and the ability to form complex‑shaped products. However, the process is complex, the green strength is relatively low, and the method lacks universality.
03 Spontaneous Coagulation Casting
Spontaneous coagulation casting is a novel ceramic forming process developed by the Shanghai Institute of Ceramics in 2011. It uses water‑soluble isobutylene‑based copolymers as both dispersant and binder to prepare ceramic green bodies. Spontaneous coagulation casting is similar to gelcasting in slurry preparation, forming method, and green body characteristics. The difference is that in spontaneous coagulation casting, the water‑soluble isobutylene‑based copolymer acts as both dispersant and binder, achieving solidification through bridging by long molecular chains. In contrast, gelcasting achieves solidification through polymerization of monomer molecules and cross‑linkers to form a three‑dimensional network that traps ceramic particles. The advantages of spontaneous coagulation casting include low organic content, room‑temperature solidification, simple operation, and low production cost.