Views: 0 Author: Site Editor Publish Time: 2026-02-12 Origin: Site
1. Advanced ceramics have gradually become an important component of new materials.
Ceramics are materials and various products obtained by grinding, mixing, shaping and firing clay as the main raw material along with other natural minerals. They are collectively referred to as pottery and porcelain. The traditional concept of ceramics refers to all artificial industrial products made from inorganic non-metallic minerals such as clay. It includes various products formed by mixing, shaping and firing clay or mixtures containing clay. The main raw material of ceramics is silicate minerals derived from nature, so it belongs to the "silicate industry" along with glass, cement, enamelware, and refractory materials.
In a broad sense, ceramic materials refer to all materials other than organic and metallic materials, namely inorganic non-metallic materials. Ceramic products come in a wide variety. Their chemical composition, mineral composition, physical properties, and manufacturing methods often overlap and interweave with each other, without clear boundaries, yet they have significant differences in application. Therefore, it is difficult to rigidly categorize them into several systems. The classification methods vary, and there is no unified classification method internationally until now. According to the preparation techniques and application fields of ceramics, they can be classified into traditional ceramic materials and advanced ceramic materials.
Traditional ceramics: In the traditional sense, ceramics refer to various products made from clay and its natural minerals through processes such as grinding, mixing, molding, and firing. These products are usually called "ordinary ceramics" or traditional ceramics. Examples include household ceramics and building and sanitary ceramics.
Advanced ceramics: According to their chemical composition, they can be classified as oxide ceramics, nitride ceramics, carbide ceramics, boride ceramics, silicide ceramics, fluoride ceramics, sulfide ceramics, etc. According to their performance and application, they can be divided into two major categories: functional ceramics and structural ceramics. Functional ceramics mainly rely on the special functions of the materials, featuring electrical properties, magnetism, biological characteristics, thermal sensitivity, and optical properties, etc., including insulating and dielectric ceramics, ferroelectric ceramics, piezoelectric ceramics, semiconductors and their sensitive ceramics, etc.; Structural ceramics mainly rely on the mechanical and structural applications of the materials, having high strength, high hardness, high temperature resistance, corrosion resistance, and oxidation resistance, etc.
1.1 Structural ceramics: The most promising high-quality materials for extreme environment applications
Structural ceramics, due to their excellent mechanical and thermal properties, have become an important branch of ceramic materials, accounting for approximately 30% of the entire ceramic market. Over the past two decades, major national projects and cutting-edge technologies have also placed higher demands and challenges on ceramic materials and their preparation technologies: For instance, the silicon nitride ceramic bearings used in the liquid hydrogen and liquid oxygen turbopumps for rocket launches in the aerospace industry operate at high speeds without slippage under extremely low-temperature conditions, requiring high strength, good initial properties, wear resistance, and high surface machining accuracy; large-sized ceramic sealing rings used in nuclear power plant main pumps need long service life and high reliability, especially the silicon carbide ceramic reflectors used in earth satellite ground target monitoring for imaging, in addition to having high elastic modulus, low thermal expansion coefficient, and lightweight, also require high precision ultra-mirror surfaces and large size, which poses a challenge to the forming technology, sintering technology, and processing technology of large-sized structural ceramics. And the ceramic plugs of optical communication fiber connectors, with an inner hole of 125 micrometers, require extremely high surface smoothness, size accuracy, and concentricity.
In terms of mechanical properties, the high melting point and wide temperature range for use have laid the foundation for the application of ceramic materials in the field of structures. Organic materials are mostly bonded by molecular bonds, while metal materials are mainly bonded by metallic bonds. Ceramic materials are mainly bonded by ionic bonds and covalent bonds, so the melting point of ceramic materials is the highest. At the same time, the long-term service temperature of ceramic materials under load is also stable at above 1000℃. Compared with metal materials, the highest current service temperature is that of high-temperature alloys, which is below 1200℃. When subjected to load, the service temperature is above 1000℃.
Furthermore, the high strength and wear resistance of ceramic materials make them stand out in the selection of materials in the field of structures. Compared with organic materials and metal materials, under the same density, specific stiffness and cost conditions, ceramic materials have the highest strength. This determines that ceramic materials can be better applied in more demanding environments. Moreover, according to the powder metallurgy research institute of Central South University, the wear resistance of ceramic materials is equivalent to 266 times that of manganese steel and 171.5 times that of high chromium cast iron.
In terms of thermal properties, the excellent thermal conductivity, thermal expansion properties, and thermal shock resistance of ceramic materials give them an irreplaceable position in many application fields compared to other materials such as metals. Compared to organic materials, ceramic materials and metal materials have better thermal conductivity. However, under high-temperature conditions, the thermal expansion coefficient and thermal stress fracture resistance of ceramic materials are lower than those of metal materials, meaning that ceramic materials can withstand larger thermal shocks under high temperatures and are the best materials in extreme environments.
The major drawback of structural ceramic materials is their brittleness. Currently, the research and development of structural ceramic materials have shifted from the previous focus on single-phase and high-purity characteristics to a multi-phase composite direction, including ceramic matrix composites reinforced by fibers (or whiskers), self-reinforcing ceramic materials, and nano-composite ceramics, etc., which has greatly improved the performance of structural ceramic materials.
1.1.1 Oxide Ceramics
The atomic bonds of oxide ceramic materials are mainly ionic bonds, with some covalent bonds present. Therefore, they possess many excellent properties. Most oxides have high melting points, good electrical insulation properties, especially excellent chemical stability and oxidation resistance. They have been widely applied in the engineering field. According to their components, they can be classified into single oxide ceramics (such as alumina, beryllium oxide, titanium dioxide ceramics, etc.) and composite oxide ceramics (such as spinel MgO·Al2O3, mullite 3Al2O3·2SiO2, lead zirconate titanate PZT ceramics, etc.).
Alumina ceramics: The earliest developed and most widely applied structural ceramics
In terms of the preparation of alumina ceramics, the currently commercially available methods include the Bayer process, the chemical process, the sintering of plate-like corundum method, and the electro-fusion corundum method. Among them, the Bayer process is the most widely used. The Bayer process can produce metallurgical-grade and industrial-grade alumina powder with a purity of 99.5%, but it mainly contains impurities such as sodium oxide. Later, the chemical process emerged, which can produce high-purity fine alumina powder with a purity of 99.99% commonly classified as high-purity alumina or 4N alumina. Furthermore, depending on the manufacturing techniques and desired properties, alumina ceramics can be classified into various product forms such as high-wear-resistant plates, precision ceramic components, and transparent alumina ceramics including substrates, insulators, wear parts, and bioceramics.
In the application field, alumina ceramics can currently be used in mechanical fields for wear-resistant components like seals and nozzles, in the power sector for high-temperature resistant insulation components, and in the semiconductor field for ceramic substrates, etc.Additionally, they are widely utilized in biomedical implants, cutting tools, wear-resistant liners, and high-voltage insulation parts, demonstrating their versatility across multiple high-tech industries.
Zirconia ceramics: Enhancing the toughness of high-performance structural ceramics is the key to their production.
The traditional applications of zirconia mainly include being used as raw materials for refractory materials, coatings and glazes, etc. However, with a deeper understanding of the thermodynamic and electrical properties of zirconia ceramics, it has become possible for it to be widely used as high-performance structural ceramics such as bearings, valves, and cuttings blades and solid dielectric materials like oxygen sensors and solid oxide fuel cell (SOFC) electrolytes. Especially with the in-depth study of the phase transformation process of zirconia, zirconia ceramic toughening materials emerged in the 1970s, which significantly improved the mechanical properties of zirconia ceramic materials leading to yttria-stabilized zirconia (YSZ), magnesia-stabilized zirconia (MSZ), and partially stabilized zirconia (PSZ), particularly the high room-temperature toughness ranking first among ceramic materials.
On the preparation side, toughening is the core objective, and the most common method is adding stabilizers like yttria (Y₂O₃), magnesia (MgO), or ceria (CeO₂). Zirconia is entirely derived from zircon sand and baddeleyite ore. Zircon sand is mainly composed of zirconium silicate (ZrO₂·SiO₂), while baddeleyite ore is dominated by ZrO₂, with minor impurities such as SiO₂ and TiO₂.
In the past, zirconia was exclusively produced using these two natural minerals as raw materials. However, the easily sinterable zirconia micropowders used for engineering ceramics are manufactured from zirconium salts—which are themselves prepared from the aforementioned natural minerals.
Zirconia exists in three crystal forms: cubic (c), tetragonal (t) and monoclinic (m). Thermodynamic analysis shows that the pure monoclinic zirconia is stable below 1170℃; above this temperature, it transforms into the tetragonal phase. When the temperature reaches 2370℃, it further converts to the cubic phase, which remains stable until melting occurs at 2680–2700℃. This phase transition process is reversible and is central to the transformation toughening mechanism in advanced zirconia ceramics like Y-TZP (Yttria-Tetragonal Zirconia Polycrystal).
During cooling from high temperatures to the tetragonal-to-monoclinic transition point, phase transformation hysteresis takes place: the tetragonal phase (t-phase) does not convert to the monoclinic phase (m-phase) until approximately 1050℃—about 100℃ lower than the theoretical transition temperature. This process is known as martensitic transformation, accompanied by a 5%–9% volume expansion. Such volume change exceeds the elastic limit of ZrO₂ grains, leading to material cracking.
Therefore, from the perspectives of thermodynamics and crystal phase transformation, it is nearly impossible to prepare pure ZrO₂ materials. To avoid this phase transition, divalent oxides (CaO, MgO, SrO) and rare earth oxides (Y₂O₃, CeO₂) can be used as stabilizers to form a solid solution with ZrO₂, yielding a stable cubic crystal structure. Note that these stabilizer oxides can only exert a stabilizing effect when the radius of their metal ions differs from that of Zr⁴⁺ by less than 40%.
In the application field, zirconia (especially the reinforced type such as transformation-toughened zirconia and YSZ yttria-stabilized zirconia) ceramics have been widely used in various industrial and technical fields due to their excellent properties. Most importantly, with its outstanding mechanical properties and high-temperature resistance, it is used as a structural material in mechanical engineering (as ceramic tools, measuring tools, bearings, molds, seals, etc.), metallurgical industry (crucibles, refractory materials, continuous casting nozzles, compression supports, guide rollers, etc.), military industry (rocket insulation layers, bulletproof armor plates), chemical industry, textile industry, bioengineering including dental crowns, orthopaedic implants, and hip joint balls and daily life, etc.
Magnesium oxide ceramics: A key material in modern metallurgy industry
Magnesium oxide ceramics are a typical type of new ceramics and also belong to traditional refractory materials. Magnesium oxide itself has a strong resistance to the erosion of alkaline metal solutions. The prepared magnesium oxide ceramic crucibles including high-purity MgO crucibles and fused magnesia crucibles possess excellent chemical properties and stability in resisting metal erosion, and do not react with magnesium, nickel, uranium, aluminum, molybdenum, etc. Under oxidizing atmosphere or nitrogen protection, magnesium oxide ceramics such as furnace linings, thermocouple tubes, and insulating substrates can operate stably up to 2400℃. Therefore, magnesium oxide is a key material in advanced processes of modern metallurgical industry.
In terms of preparation, the raw materials come from minerals or seawater. During the sintering process, additives need to be added to adjust the properties. Magnesium-containing compounds are abundant in nature, existing in various mineral forms in the earth's crust and the ocean, such as magnesite, dolomite, willemite, and talc. Industrially, MgO commonly in the form of dead-burned magnesia or fused magnesia is mainly extracted from the above minerals. Recently, extraction from seawater has also been developed to produce seawater magnesia. When extracting MgO from minerals or seawater, most of the process involves first preparing magnesium hydroxide or magnesium carbonate, and then through calcination, it is decomposed into MgO to yield calcined magnesia. This MgO can be further processed by chemical treatment or heat treatment to obtain high-purity MgO suitable for advanced ceramic grades. After processing the MgO raw materials, the ingredients are mixed according to the composition. To promote sintering and make the grains slightly grow larger, and to reduce the hydration tendency during preparation, some additives such as TiO2, Al2O3, V2O3, etc. can be added. If high-purity MgO ceramics like transparent MgO ceramics or high-density electrical insulators are required, the method of promoting sintering and grain growth by adding additives cannot be used. Instead, the activation sintering method is adopted, that is, Mg(OH)2 is calcined at an appropriate temperature to obtain active MgO with many lattice defects, which is used to manufacture sintered magnesium oxide ceramics including crucibles, tubes, and substrates for specialized applications.
In the application field, the theoretical operating temperature of magnesium oxide ceramics including sintered magnesia and fused magnesia products can reach up to 2200℃, and they can be used continuously in the range of 1600℃ to 1800℃. Their high-temperature stability and corrosion resistance are superior to those of alumina ceramics. Moreover, they do not react with Fe, Ni, U, Th, Zn, Al, Mo, Mg, Cu, Pt, etc. Therefore, their application scope can include: crucibles or other refractory materials such as lining bricks and casting nozzles in the metallurgical industries under corrosive conditions such as those in steel and glass production. Magnesium oxide ceramics can be used as crucibles for metal smelting, and are also suitable for high-purity uranium and thorium smelting in the atomic energy industry; they can also be used as protective sleeves for thermocouples. By taking advantage of its property of allowing electromagnetic waves to pass through, it can be used as radar domes and infrared radiation projection window materials, as well as crucibles for smelting metals, alloys such as nickel alloys, radioactive metals uranium and thorium alloys, iron and its alloys, etc. It can also be used as raw materials for piezoelectric and superconducting materials, and is pollution-free, resistant to lead corrosion, etc. It can also be used as a ceramic sintering carrier like setter plates and kiln furniture, especially for ceramic products with corrosive and volatile substances at high temperatures such as β-Al2O3.
Beryllium oxide ceramics: The oxide ceramics with the highest thermal conductivity, but the toxicity of the powder limits its application.
BeO is the only hexagonal wurtzite structure among the alkaline earth metal oxides. Due to the wurtzite structure and strong covalent bond of BeO, along with its relatively low molecular weight, BeO has extremely high thermal conductivity, being about 10 times that of alumina. Its room-temperature thermal conductivity can reach 250 W/(m·K), comparable to that of metals, and its electrical properties, heat resistance, thermal shock resistance, and chemical stability are all excellent under high temperatures and high frequencies making it suitable for high-performance heat sinks and microwave window materials. However, the fatal drawback of BeO ceramics commonly referred to as beryllia ceramics is its extremely high toxicity. Long-term inhalation of BeO dust can cause poisoning and even endanger life, and it will also cause pollution to the environment, which greatly affects the production and application of BeO ceramic substrates and components such as laser tube housings and RF transistor packages.
In the application field, beryllium oxide ceramics like BeO heat spreaders and hermetic seal packages possess high thermal conductivity, high refractoriness, excellent nuclear properties, and excellent electrical properties. Therefore, they can be applied in advanced refractory materials and as neutron multipliers in fusion reactors, atomic energy reactors, and various high-power electronic devices and integrated circuits including radar systems and satellite communication modules, etc. However, the toxicity of beryllium oxide cannot be ignored. As countries around the world increasingly attach importance to environmental protection, the use of beryllium oxide ceramics despite its irreplaceable role in certain military and aerospace thermal management systems may be subject to certain restrictions and influences in the future.
Mullite: A general term for minerals composed of aluminum silicate.
Mullite is a high-quality refractory material. This type of mineral is relatively rare. Mullite is a mineral formed by the reaction of aluminum silicate at high temperatures. When aluminum silicate is heated artificially, mullite is formed. The natural crystal of mullite is slender needle-like and in a radiating cluster form. Mullite ore is used to produce high-temperature refractory materials. It is often used as a thermal barrier coating in C/C composite materials and is widely applied. Mullite is a stable binary solid solution in the AI2O3-SiO2 element system at normal pressure. The natural mullite with the chemical formula AI2O3-SiO2 is very rare and is usually synthesized by methods such as sintering or electrofusion.
The mullite used in large-scale industrial applications at high temperatures is classified into two major types based on its preparation method: electrically fused mullite commonly used in refractory bricks and monolithic castables and sintered mullite. Mullite is a high-quality refractory material. It was first discovered on the Isle of Mull in Scotland and was named after that location. The aluminum and silicon components of mullite form a range, and they can exist stably at normal temperature and pressure. Natural mullite is relatively rare and is usually prepared by heat treatment of aluminum-silicon compounds to produce engineered mullite aggregates and grains. The synthesis of mullite can be divided into solid-phase synthesis (including the traditional sol-gel (SSG) process), liquid-phase synthesis, and gas-phase synthesis. The mullite synthesized by solid-phase and liquid-phase methods can be classified as sintered mullite typically utilized in kiln furniture and ceramic filters and fused mullite according to the heating temperature and the composition of aluminum-silicon. Sintered mullite refers to heating the raw materials of synthesized mullite to a temperature that generates a small amount of liquid phase, promoting sintering without affecting the solid-phase sintering, and then holding at a temperature to allow the mullite to crystallize and grow, forming the desired mullite morphology and structure. While fused mullite is formed by heating the mixture of alumina and silica above the mullite melting point, and crystallizing during the cooling process to produce high-purity fused mullite grains for advanced refractory applications. The sol-gel method for preparing mullite is also known as chemical mullite, which is mullite obtained through chemical reactions, thermal decomposition, and mullite formation resulting in fine mullite powders for technical ceramics and coatings. The performance of mullite prepared by this method highly depends on the purity, uniformity, crystallization temperature, and density of the compound.
In the application field, the refractory new materials made of mullite are currently widely used in high-temperature equipment such as muffle furnaces, calcining furnaces, boilers, and rotary kilns. The use of mullite to manufacture high-temperature equipment not only ensures high temperature resistance, but also has a long service life and corrosion resistance. Mullite, when combined with other high-quality materials, can complement each other's advantages and form refractory materials with better performance. For example, using corundum-mullite composite ceramic kiln components, the resulting material has the advantages of a small thermal expansion coefficient, excellent thermal shock resistance, high refractoriness, and good high-temperature stability. In addition, the application of mullite in the field of electrical performance demonstrates its excellent as a base material. It has a very low dielectric constant and can handle high circuit densities. Mullite ceramics and mullite-based glass-ceramic composites are used as excellent functional materials for high-performance integrated circuits.
1.1.2 Nitrogen Ceramics
Nitride ceramics are ceramics formed by combining nitrogen with metals or non-metallic elements. They are a type of important structural and functional materials.
Nitride ceramics possess excellent mechanical, chemical, electrical, thermal and high-temperature physical properties. They have extensive applications in industries such as metallurgy, aviation, chemical engineering, ceramics, electronics, machinery and semiconductors. However, many nitrides composed of nitrogen and metal elements are unstable at high temperatures and prone to oxidation, thus they cannot exist freely in nature and can only be artificially synthesized. Currently, the main synthetic methods for nitrides include covalent bonding types such as boron nitride, aluminum nitride, and silicon nitride.
Aluminum nitride ceramics: The ideal structural material for circuit substrates and packaging in the microelectronics industry
Aluminum nitride (AlN), as a new type of ceramic material, has become one of the research hotspots in the field of new materials in recent years. Although AlN powder was synthesized and produced as early as over a hundred years ago, due to its inherent difficulty in sintering, there were not many studies on AlN in the following decades. In the 1950s, AlN ceramics such as basic crucibles and simple components were first produced, but at that time, their strength was very low, which limited their industrial application. In the 1970s, dense nitride aluminum ceramics including thermally conductive substrates and ceramic packages were prepared, and a series of excellent characteristics such as excellent thermal conductivity, reliable electrical insulation, high temperature resistance, corrosion resistance, low dielectric constant, and matching thermal expansion coefficient with silicon were revealed. Especially in recent years, with the rapid development of microelectronics technology, electronic devices are becoming increasingly multi-functional, miniaturized, and highly integrated high-power electronic devices generate a large amount of heat during operation. To avoid the failure of electronic devices due to overheating, a substrate such as direct-bonded copper (DBC) substrates and thick-film ceramic circuits with high thermal conductivity is needed to carry away the heat. AlN has excellent thermal conductivity and is an ideal material for the new generation of substrates specifically high-power LED substrates, RF/microwave packages, and semiconductor processing equipment components like electrostatic chucks and heater plates. Its excellent high-temperature corrosion resistance, high-temperature stability, high strength and hardness make it have great potential for application in high-temperature structural materials including protective tubes, heat exchangers, and components for aerospace systems.
Aluminum nitride, as a covalent compound, is difficult to undergo solid-phase sintering. Usually, the liquid-phase sintering method is adopted, that is, sintering aids capable of generating a liquid phase are added to the aluminum nitride raw material powder, and the dissolution generates a liquid phase to promote sintering.
As an artificially synthesized material, the preparation process of aluminum nitride ceramics usually involves first synthesizing aluminum nitride powder, and then sintering the obtained powder to form ceramics. Due to the high covalent component of the aluminum-nitrogen bond (Al-N) in aluminum nitride, the melting point of aluminum nitride is high, the self-diffusion coefficient is small, and the sintering activity is low. Therefore, it is a difficult-to-sinter ceramic material. According to the editor of China Powder Network, when the purity of aluminum nitride powder is high, it is very difficult to achieve complete densification through sintering, and there are pores in the ceramic grains or at the grain boundaries, which greatly limits the practical application of aluminum nitride ceramics. Introducing appropriate sintering aids can, on the one hand, react with the Al2O3 formed on the surface of AlN to generate a second phase with a lower melting point, due to the surface tension effect of the liquid phase, promoting the rearrangement of AlN grains and accelerating the densification process of the sintered body. On the other hand, the formed second phase, after cooling, precipitates and condenses on the grain boundaries, reducing the possibility of oxygen entering the lattice at high temperatures, and playing a role in purifying the lattice and improving the thermal conductivity. Currently, the commonly used sintering aids mainly include oxides and fluorides. Oxides mainly include Y2O3, Sm2O3, La2O3, Dy2O3, and CaO; while fluorides include CaF2, YF3, etc. Among them, Y2O3 has strong oxygen removal ability, good stability, and superior comprehensive performance, becoming the most commonly used sintering aid; while CaO, due to its lower liquid phase formation temperature, plays a more obvious role in low-temperature sintering.
In the application end, aluminum nitride ceramic has relatively high strength at room temperature and is not easily affected by temperature changes. It also has a relatively high thermal conductivity and a relatively low thermal expansion coefficient. It is an excellent heat-resistant material and heat exchange material. As a heat exchange material, it is expected to be applied in the heat exchangers of gas turbines. In addition, aluminum nitride ceramic is a high-temperature heat-resistant material. Its thermal conductivity is high, more than 5 times higher than that of alumina ceramic, and its expansion coefficient is low, consistent with that of silicon. The substrate manufactured using aluminum nitride ceramic as the main raw material producing direct-bonded copper (DBC) substrates and thick-film ceramic circuits has excellent characteristics such as high thermal conductivity, low expansion coefficient, high strength, corrosion resistance, excellent electrical performance, and good light transmission. It is an ideal large-scale integrated circuit heat dissipation substrate and packaging material. With the continuous upgrading of electronic information industry technology, the miniaturization and functional integration of PCB substrates have become a trend. The market's requirements for the heat dissipation performance and high-temperature resistance of heat dissipation substrates and packaging materials are constantly increasing. The performance of relatively ordinary substrate materials is difficult to meet market demands. The development of aluminum nitride ceramic substrates including automotive power module substrates and components for semiconductor manufacturing equipment has ushered in an opportunity.
Silicon nitride ceramics: One of the best materials in terms of comprehensive performance among advanced ceramics
With the development of contemporary science and technology, the requirements for structural materials in fields such as aviation and aerospace energy have become increasingly higher. The development and research of structural materials with excellent properties such as high temperature resistance, corrosion resistance, friction resistance, high strength, high hardness, and comprehensive mechanical performance have become extremely important. Si3N4 ceramics notably sintered silicon nitride (SSN) and reaction-bonded silicon nitride (RBSN) is one of the materials with the best comprehensive performance among advanced ceramics. Its electrical, thermal and mechanical properties are very excellent. It can be used up to 1400℃ in an oxidizing atmosphere and up to 1850℃ in neutral or reducing atmospheres. It not only highlights the advantages of general ceramic materials such as hardness, heat resistance, wear resistance and corrosion resistance, but also possesses advantages such as good thermal shock resistance, high-temperature creep resistance, good self-lubrication, excellent chemical stability performance, and relatively low density, low dielectric constant, low dielectric loss and other excellent dielectric properties.
The molecular weight of silicon nitride is 140.28. In terms of weight percentage, silicon accounts for 60.28% and nitrogen accounts for 39.94%. The electronegativities of these two elements are similar. In the crystal of silicon nitride, the Si-N bonds are mainly formed by covalent bonds (with ionic bonds accounting for only 30%), and the bonding strength is high. Silicon nitride has no melting point. It sublimates and decomposes at 1870°C under normal pressure and has a high vapor pressure and very low diffusion coefficient. The Si atoms and N atoms are bonded by very strong covalent bonds, resulting in the high strength, high hardness, high temperature resistance, and insulation properties of silicon nitride making it ideal for products like ball bearings, cutting tools, and turbocharger rotors. Due to the strong covalent bond between Si atoms and N atoms, the atomic diffusion is very slow at high temperatures, so additives such as yttria (Y₂O₃) and alumina (Al₂O₃) that form a liquid phase at high temperatures need to be added during the sintering process to promote diffusion and accelerate sintering densification.
The properties of silicon nitride ceramics are closely related to the sintering method. The high-temperature mechanical properties of silicon nitride largely depend on the intergranular glass phase. To improve the sintering performance of silicon nitride, sintering aids are added to the raw materials. At high temperatures, the sintering aids form a glass phase, which remains at the grain boundaries after cooling. The retention and exertion of this high-temperature property of silicon nitride can only be achieved through grain boundary engineering treatment. Otherwise, the softening of the intergranular glass phase at high temperatures will cause grain boundary slip, which has a significant impact on the high-temperature strength, creep and slow crack propagation in static fatigue. The speed of grain boundary slip is related to the properties (such as viscosity) of the glass phase, its quantity and distribution.
In the application field, Si3N4 ceramic including high-performance grades like hot-pressed silicon nitride (HPSN) and gas-pressure sintered silicon nitride (GPSN) is an important structural material. It is an extremely hard substance that is lubricious by nature and resistant to wear. Apart from hydrofluoric acid, it does not react with other inorganic acids. It has strong corrosion resistance and is resistant to oxidation at high temperatures. Moreover, it can withstand thermal shock and will not fracture even when heated to over 1000°C in the air and then rapidly cooled and heated again. Due to its excellent properties, Si3N4 ceramic is often used to manufacture mechanical components such as bearings specifically full ceramic bearings and hybrid ceramic bearings, turbine blades, mechanical sealing rings, and permanent molds. Among them, by taking advantage of Si3N4's light weight and high stiffness, it can be used to manufacture ball bearings, which have higher precision than metal bearings, generate less heat, and can operate in higher temperatures and corrosive media. The steam nozzles made of Si3N4 such as injector nozzles and wear-resistant liners have wear-resistant and heat-resistant properties. After being used in a 650°C boiler for several months, they show no obvious damage, while other heat-resistant and corrosion-resistant alloy steel nozzles can only be used for 1-2 months under the same conditions.
Boron nitride ceramics: A soft ceramic among ceramic materials, with excellent mechanical processing properties
Boron nitride was invented over 100 years ago. Its earliest application was hexagonal boron nitride [abbreviated as h-BN, or a-BN, or g-BN (i.e., graphite-type boron nitride)], which was used as a high-temperature lubricant and as a mold release agent in non-ferrous metal casting. h-BN not only has a similar structure to graphite but also has similar properties, and it is naturally white, so it is commonly known as white graphite. Boron nitride (BN) ceramics were discovered as a compound as early as 1842. Foreign countries conducted extensive research on BN materials after World War II, and it was developed only after the thermal pressing method of BN was solved in 1955. In 1957, a researcher successfully developed CBN cubic boron nitride abrasive grains, and in 1969, a certain company sold it under the brand name Borazon. In 1973, the United States announced the production of CBN cutting tools. In 1975, Japan introduced technology from the United States and also produced CBN cutting tools. In 1979, Sokolowski successfully used pulsed plasma technology to prepare c-BN films at low temperature and low pressure. In the late 1990s, people were able to use various physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods to prepare c-BN films. It has excellent thermal resistance, thermal stability, thermal conductivity, and high-temperature dielectric strength, making it an ideal heat dissipation material in the form of thermal interface pads and electrically insulating substrates and high-temperature insulating material. Boron nitride has good chemical stability and can resist the erosion of most molten metals. It also has good self-lubrication properties. The hardness of boron nitride products like hot-pressed BN components and pyrolytic BN coatings is low, allowing for mechanical processing with an accuracy of 1/100mm.
At the preparation stage, for covalent bond compounds, the common method is to add sintering aids. The commonly used sintering aids for BN include B2O3, Si3N4, ZrO2, SiO2, BaCO3, etc. Currently, there are many methods for preparing boron nitride powder. Based on their principles, they can be roughly divided into two categories: one is the synthesis method, which mainly includes high-temperature synthesis method, sol-gel synthesis method, template method and chemical vapor deposition method (CVD); the other is the exfoliation method, including liquid-phase ultrasonic exfoliation method, laser etching exfoliation method, mechanical ball milling method, etc. With the continuous deepening of research on boron nitride, the properties of some nanostructured boron nitride have gradually been discovered. On one hand, the nanopowders have a high specific surface energy and high sintering activity, which can effectively promote the densification of h-BN ceramics; on the other hand, using nanopowders as raw materials can reduce the sintering temperature, reduce the grain size of the ceramic sintered body, improve the toughness of the ceramic, and enhance the mechanical properties of h-BN ceramics for applications in high-temperature furnace fixtures and semiconductor processing jigs, laying the foundation for the industrial large-scale application of h-BN ceramics.
In the application field, boron nitride can be used to manufacture crucibles for melting semiconductors and high-temperature containers for metallurgy specifically boron nitride crucibles and break rings for continuous casting, semiconductor heat dissipation and insulation parts, high-temperature bearings, thermocouple sleeves, and glass forming molds, etc. The commonly produced boron nitride has a graphite structure and is commonly known as white graphite. The other type is diamond-like, similar to the principle of graphite transforming into diamond. Graphite-like boron nitride can be transformed into diamond-like boron nitride under high temperature (1800℃) and high pressure (800 Mpa). In this type of boron nitride, the B-N bond length (156 pm) is similar to that of diamond (154 pm), and its density is also similar to that of diamond. Its hardness is comparable to that of diamond, while its heat resistance is better than that of diamond. It is a new type of high-temperature-resistant super-hard material especially as cubic boron nitride (CBN) cutting inserts and grinding wheels and is used to make drills, tools and cutting tools.
Sialon Ceramics: Soft ceramics among ceramic materials, with excellent mechanical processing properties
Sialon is a compound formed by the combination of Si, Al, O, and N elements. It is transliterated as "Sialon". Sialon ceramics belong to the series of compounds of Si3N4-Al2O3-AlN-SiO2. They are a type of dense polycrystalline nitride ceramic developed on the basis of Si3N4 ceramics, formed by partially replacing the Si and N atoms in Si3N4 with Al and O atoms from Al2O3. Sialon ceramics were discovered by Oyama and researchers from Japan (in 1971) and Jack and Wilson from the UK (in 1972). During their research on various additives for silicon nitride ceramics, they discovered solid solutions in metal nitrides, namely the solid solution of Si3N4 in the SiO2-Al2O3 system, which effectively promoted sintering and thus led to the discovery of sialon (Sialon ceramics). The main categories of sialon ceramics include β’-sialon, α’-sialon, and O’-sialon, with the former two being the most common.
In the preparation process, when preparing sialon ceramics, ultrafine and high-α-phase Si3N4 powders should be selected. Appropriate process measures should be adopted to control the composition and structure of the grain boundary phases, so as to obtain materials with excellent performance. Since sialon ceramics have a wide solid solution range, the composition of sialon ceramics can be designed according to the predetermined performance by adjusting the component ratio of the solid solution. By appropriately adjusting the addition amount of additives, the optimal ratio of α-sialon and β-sialon can be obtained, and the material with the best strength and hardness combination can be achieved. Sialon ceramics are usually sintered without pressure or under hot pressing. They are sintered in an inert atmosphere at 1600-1800℃, and sintered bodies with close theoretical density can be obtained. The main additives are MgO, Al2O3, AlN, SiO2, etc. At the same time, adding Y2O3 and Al2O3 can obtain sialon ceramics with very high strength. Moreover, adding Y2O3 can reduce the sintering temperature of sialon ceramics. The manufacturing process of sialon ceramics under normal pressure is to mix Si3N4 powder with an appropriate amount of Al2O3 powder and AlN powder, and then sinter it in a N2 atmosphere at 1700℃. The properties of the solid solution vary depending on its composition and processing temperature.
On the Application Side
As a new type of high-temperature structural ceramic with outstanding performance, sialon ceramics boast broad application prospects in the military industry, aerospace industry, machinery industry, electronics industry and other fields.
Sialon ceramics feature high hardness and excellent wear resistance, and have been adopted in the machinery industry for manufacturing bearings, seals, welding sleeves, positioning pins and wear-resistant components. They can also be used as flow distributors for continuous casting, thermocouple protection tubes, crystal growers, crucibles, linings for the lower part of blast furnaces, drawing mandrels for copper-aluminum alloy tubes, as well as die materials for rolling, extrusion and die-casting processes.
In addition, sialon ceramics are applicable for making cutting tools—their red hardness outperforms that of WC-Co cemented carbides and alumina, enabling high-speed cutting even when the tool tip temperature exceeds 1000℃. They can also be fabricated into transparent ceramics (e.g., high-pressure sodium lamp tubes, windows for high-temperature infrared thermometers) and used as bioceramics for manufacturing artificial joints and other implants.
