From a single powder to alumina ceramic structural components: Exploring the "invisible assistants" behind them
Alumina ceramic components, with their excellent properties such as high hardness, high temperature resistance, corrosion resistance, and good insulation, are widely used in electronics, medical, aerospace and other fields. From precision electronic components to mechanical parts in harsh environments, alumina ceramic components can be found everywhere. However, simply adding water to alumina powder and firing it at high temperature cannot directly produce these high-performance ceramic components with excellent properties. Behind this, additives play a crucial role.
The pure alumina has a melting point as high as 2050℃, and the sintering temperature usually needs to be above 1600℃. This not only leads to high energy consumption, but also easily causes coarse grains and performance degradation. In addition, alumina itself is highly brittle, and the directly sintered ceramics are prone to cracking, making it difficult to meet the requirements of precision devices. The functions of additives are as follows:
① reducing the sintering temperature (energy saving and cost reduction);
② improving the density (reducing pores and enhancing strength);
③ optimizing the grain boundary structure (improving toughness and thermal shock resistance);
④ regulating electrical/thermal properties (such as insulation and thermal conductivity).
This article introduces several commonly used additives:
1. Fluxes: Reduce the sintering temperature and improve the density.
Alumina has a high melting point, and direct sintering not only consumes huge energy but also imposes extremely high requirements on equipment. The emergence of fluxes has effectively solved this problem. It is like a "temperature regulator", which can reduce the sintering temperature of alumina ceramics, making the sintering process more efficient and energy-saving.
(1) Titanium dioxide (TiO₂) is one of the common fluxes. During the sintering process, it can react with alumina to form a eutectic, thereby reducing the temperature at which the liquid phase appears. This is similar to cooking, where adding some special seasonings can make ingredients reach the ideal cooking state faster. Using titanium dioxide as a flux can not only reduce the sintering temperature but also improve the hardness of alumina ceramics to a certain extent. For example, in some alumina ceramic cutters used for cutting processes, adding an appropriate amount of titanium dioxide can make the cutters more wear-resistant while maintaining high hardness, thus extending their service life.
(2) Yttrium oxide (Y₂O₃) is also an important flux. It can inhibit the crystal phase transformation of alumina at high temperatures, maintaining the stability of the ceramic structure. For alumina ceramic products with extremely high requirements for thermal stability, such as the bases of electronic components used in high-temperature environments, adding yttrium oxide can endow ceramic components with good thermal shock resistance, making them less prone to cracking in environments with rapid temperature changes.
(3) Calcium oxide (CaO) also has the effect of reducing the sintering temperature. It can form a eutectic with alumina, reducing energy consumption, while preventing excessive grain growth, which is conducive to obtaining ceramics with a fine-grained structure. Ceramics with a fine-grained structure usually have higher strength and toughness. In some alumina ceramic parts that need to withstand large external forces, such as wear-resistant components of mechanical equipment, adding calcium oxide can enhance the performance of the parts.
2. Reinforcing and Toughening Agents: Improve fracture toughness and reduce brittleness.
Although alumina ceramics have high hardness, their toughness is relatively poor, making them prone to brittle fracture when subjected to external impact. The emergence of reinforcing and toughening agents is like putting on a layer of "armor" for alumina ceramic components, which can effectively enhance their strength and toughness.
Silicon carbide (SiC) is a commonly used reinforcing and toughening agent. Its particles are uniformly distributed in the alumina matrix. When the ceramic component is subjected to external forces, the SiC particles can hinder the propagation of cracks. This is similar to setting up obstacles on a road: when a crack encounters these particles, it changes its propagation direction, thereby consuming more energy and making the ceramic component less prone to fracture. In alumina ceramic bearings used in high-load environments, adding silicon carbide can significantly improve the bearing capacity and service life of the bearings.
Boron nitride (BN) is also a member of the reinforcing and toughening agent family. It can improve the friction performance of ceramics and enhance their strength and toughness. In some alumina ceramic components requiring good friction performance, such as ceramic rings used for sealing, adding boron nitride can reduce friction and wear between components, improve sealing effect, and enhance the strength of components to make them more durable. However, if the addition amount exceeds 10%, the hardness may be reduced, so it is necessary to balance lubricity and strength.
3. Functional Additives: Adjust electrical, thermal or optical properties
In addition to improving basic properties, functional additives can also endow alumina ceramic components with some unique properties to meet the needs of different fields.
(1) Rare earth oxides, such as lanthanum and cerium, are "magicians" for improving electrical properties. In some alumina ceramic components used in the electronics field, such as ceramic substrates for high-frequency circuits, adding rare earth oxides can adjust the dielectric constant and loss tangent of ceramics, enhance their electrical performance, make them more suitable for working in high-frequency environments, and expand their application scope in the electronics field.
(2) Colorants are the "artists" that add colors to alumina ceramic components. Colorants such as chromium oxide and cobalt oxide can react with alumina to produce ceramics in different colors. In some ceramic products with decorative requirements, such as ceramic tableware and decorative ornaments, adding colorants can make ceramic components more beautiful and diverse, meeting the aesthetic needs of different consumers.
4. Binders and Molding Aids: Improve powder fluidity and molding strength.
In the forming process of alumina ceramic components, binders and molding aids play a crucial role. They are like a group of "unsung heroes" silently transforming alumina powder into structural parts of various shapes.
Organic binders, such as polyvinyl alcohol (PVA) and polyacrylate, act like glue, bonding alumina powders together during green body forming to endow the green body with certain strength for subsequent processing and sintering. Take tape casting as an example: PVA can uniformly disperse alumina powders in a solvent to form a slurry with appropriate viscosity and fluidity, which is then cast into a green film through the tape casting process. After the green film dries, binders like PVA maintain its shape and structure, preventing deformation during subsequent processing.
Molding aids include lubricants, dispersants, etc. Lubricants can reduce the friction between the powder and the mold, minimize mold wear, and ensure uniform density of the green body. In dry pressing, adding an appropriate amount of lubricant can make the powder easier to fill the mold, improving the density of the green body. Dispersants, on the other hand, ensure uniform dispersion of the powder in the slurry to avoid agglomeration. In injection molding, dispersants play a particularly important role by enabling the slurry to have good fluidity, facilitating its injection into the mold to form complex-shaped ceramic components.
Additives play an indispensable role in the manufacturing process of alumina ceramic components. They comprehensively support the production of alumina ceramic components in various aspects, from reducing the sintering temperature and enhancing performance to endowing unique properties and shaping. With the continuous advancement of technology, more new types of additives may emerge in the future, bringing more possibilities for the performance improvement and application expansion of alumina ceramic components.