This article refers to engineering ceramics, may be different from the conventional understanding, it belongs to the part of advanced ceramics, is specifically used in some equipment, machines or components on some proprietary structural parts. The material itself has high strength, high hardness, wear resistance, corrosion resistance, high temperature resistance, insulation, superconductivity and other ceramic properties, in the high temperature sintering out, often need to carry out a series of post-processing processing, such as cutting, grinding, improve the dimensional accuracy of the product and surface roughness, to achieve a certain level of assembly conditions required, to become a product of customer needs. Engineering ceramics are mainly used in national defense, chemical industry, metallurgy, electronics, machinery, aviation, aerospace, biomedical and other fields.
Classification of engineering ceramics
Engineering ceramics have different classifications, we mainly according to the chemical composition, can be divided into oxide ceramics, nitride ceramics, carbide ceramics, boride ceramics, silicide ceramics, fluoride ceramics and sulfide ceramics. Different materials corresponding to the use of the function will also be different, here is a summary of common materials and functions of the corresponding table for reference:
Commonly used engineering ceramics
Below we will introduce in more detail several common engineering ceramics
Aluminum oxide ceramics
Alumina ceramics is a kind of alumina as the main body of inorganic non-metallic ceramic materials, chemical formula for Al2O3, commonly used 95% alumina ceramics for the pure white, 99% purity above alumina ceramics for the ivory yellow. Some special use of the scene, the need to add metal oxides as a coloring agent to change the color of the porcelain body, for example, black alumina ceramics are often considered to absorb visible light, the dye for FeO, MnO , CoO, NiO, Cr2O3; red alumina ceramics coloring agent for chromium oxide.
Alumina ceramics have a lot of homogeneous hetero crystals, according to the research reported more than 12 kinds of variants, but mainly α-al2o3, β-al2o3 and γ-al2o3 three kinds of crystals: α-al2o3 is the most stable crystalline form among the three kinds of forms. Alumina ceramics within the crystal structure are different, because of its properties are also different, but in 1300 ℃ high temperature calcination, almost completely transformed into α-al2o3 crystal type. Only α-al2o3 exists in nature, such as natural corundum, ruby, sapphire and other minerals.
Alumina ceramics have a high melting point, high temperature resistance, high hardness, excellent wear resistance, etc., and the electrical properties are particularly good. Due to these special properties, alumina is the most commonly used material in engineering ceramics. After sintering, basically only diamond grinding method is used to achieve dimensional accuracy and highly polished to achieve surface roughness.
Zirconia Ceramics
Normally white, odorless and tasteless crystals, yellow or gray when containing impurities, chemically inactive, and has a high melting point, high electrical resistivity, high refractive index and low coefficient of thermal expansion of the nature of the material, which makes it an important high-temperature-resistant materials, ceramic insulating materials and ceramic shading agent, but also the main raw material for artificial drilling. There are three crystalline states of pure ZrO2 at atmospheric pressure, and the relevant properties are as follows:
The three crystalline states have different physicochemical properties, in order to obtain the required crystalline form and performance in practical applications, usually adding different types of stabilizers made of different types of zirconia ceramics, such as Partially Stabilized Zirconia(PSZ), when the stabilizer for the Y2O3, MgO, CaO, respectively, expressed as Y-PSZ Mg-PSZ, Ca-PSZ, and so on. Tetragonal zirconia composed of sub-stabilized T- ZrO2 is called tetragonal zirconia polycrystal ceramics (TZP), and when the stabilizers are Y2O3 and CeO2, it is denoted as Y-TZP, Ce-TZP, and so on.
Zirconia is a very important kind of engineering ceramics, second only to alumina in terms of breadth of application, its main characteristics are as follows:
1. High density: 5.85g/cm3 or more, it is known as ceramics in the “ceramic steel” (Ceramic steel);
2. High temperature resistance: the highest temperature in atmospheric air up to 2200 ℃; zirconia fiber is currently the international top of a refractory fiber materials
3. Good abrasion resistance: hardness of 1300 HV, high finish, self-lubricating.
4. Coefficient of thermal expansion close to steel: (8.0~9.5) x 10-6/℃.
5. Low thermal conductivity: (2~3) W/m.K, which means very good thermal insulation efficacy, such as, etc. In aerospace and industrial gas turbines, with sprayed zirconia thermal barrier coating, not only can reduce its external heat 50~200 ℃, but also can make full use of the thermal energy to provide the efficiency of the engine.
6. High insulation: high volume resistance, zirconia products are suitable for a variety of high-current, high-voltage application scenarios.
Aluminum Nitride Ceramics
It is a non-oxidized ceramic material, there is no natural aluminum nitride in nature, after high-temperature synthesis and the formation of the former main manufacturing process include: ammonia and aluminum nitride reaction directly, crushed, graded aluminum nitride powder; or alumina and charcoal fully mixed in an electric furnace at 1700 ℃ reduction of aluminum nitride.
Aluminum nitride (AlN) has high thermal conductivity and electrical insulation, thermal conductivity of 170W / (m.k), 5-8 times that of alumina ceramics, can be used at 1200 ℃ in an extremely hot environment.
Typical Applications of Aluminum Nitride Products
1. High thermal conductivity, good insulation and mechanical properties: the products are widely used in high power packaging and multi-chip assembly of high heat dissipation substrate, in microelectronics, optoelectronics (optical storage interfaces and electronic substrates for the electrostatic layer), electric power electronics and power electronics have broad application prospects.
2. High-temperature resistance and good high-temperature strength, small coefficient of expansion, good thermal conductivity: can be used as a high-temperature structural components heat exchanger materials.
3. Characteristics of piezoelectric effect of aluminum nitride, epitaxial stretching of aluminum nitride crystals: used for surface acoustic wave detectors, and detectors will be placed on silicon wafers.
4. Its coefficient of thermal expansion is close to that of silicon, and it has excellent plasma resistance: it is often used in the manufacture of components for semiconductor processing equipment (wafer carriers) and for military applications.
5. Aluminum nitride ceramics are resistant to the corrosion of iron, aluminum and other metals and alloys: they can be used as crucibles and casting molds for the melting of metals such as Al, Cu, Ag, and Pb.
Mullite Ceramics
Mullite chemical formula for 3Al2O3.2SiO2, Al2O3-SiO2 binary system is the only stable compound, melting point 1800 ՞ C, with high-temperature mechanical properties, thermal conductivity and coefficient of thermal expansion and low density, good creep resistance and other advantages. The disadvantages are poor mechanical properties at room temperature and difficult sintering.
Mullite ceramics in the field of high-temperature structural ceramics and refractory materials are widely used and show good potential, can be used to manufacture thermocouple protection tubes, electrical insulation tubes, high-temperature furnace lining, but also can be used for the manufacture of polycrystalline mullite fibers, high-frequency device ceramic parts, such as high-frequency main voltage insulators, coil skeletons, capacitor shells, high-voltage switches, casing and other large-scale device containers.
Silicon Nitride Ceramics
It is a very important synthetic inorganic non-oxidizing structural ceramic material, it is a super-hard material, itself has the lubricity, and wear-resistant; strong corrosion resistance, except for hydrofluoric acid and hot phosphoric acid, and other inorganic acids do not react; high temperature oxidation resistance, and it is also able to resist hot and cold shocks, in the air can be resistant to more than 1,000 ℃ of thermal shock.
Silicon nitride (Si3N4) exists in three crystalline structures, namely, α, β and γ phases. α and β phases are the most common crystalline appearances of Si3N4, and can be prepared at atmospheric pressure. The γ phase can only be synthesized under high pressure and high temperature, and its hardness can reach 35GPa.
Silicon nitride ceramic materials are characterized by wear resistance, corrosion resistance, high temperature resistance, oxidation resistance, thermal shock resistance and low specific gravity, which are difficult to compare with general metal materials. Can withstand metal or polymer materials are difficult to handle the harsh working environment, has a wide range of application prospects. As an important member of the family of high-temperature structural ceramics, Si3N4 ceramics, compared with other high-temperature structural ceramics, such as oxide ceramics, carbide ceramics, etc. have more excellent mechanical properties, thermal properties and chemical stability. Therefore, it is considered to be the high temperature structural ceramics in the most potential application of the material.
Boron Nitride Ceramics
Boron nitride crystals typically have two crystalline structures: cubic boron nitride (CBN), which is second only to the world’s hardest diamond. Hexagonal boron nitride (WBN), which has a graphite structure, is a common white powder, also known as “white graphite”. Hexagonal at 1350~1800 ℃, 6.5MPa conditions can be transformed into cubic BN.
Hexagonal dominated crystalline phase BN material has processability and self-lubrication, and can be used as high temperature bearings, etc. It has good heat resistance, thermal stability, thermal conductivity, and high temperature dielectric strength, and is an ideal heat dissipation material, high temperature insulating material, metallurgical container and high temperature abrasive material.
Machinable Ceramics
Machinable ceramics is the use of ordinary metal cutting tools at room temperature can be machined to a certain shape, precision and surface quality of ceramic materials. Due to the complex crystal structure of engineering ceramic materials, at room temperature is difficult to slip, dislocation and plastic deformation, general ceramic materials are difficult to turn, milling, planing, grinding, drilling and other metal machining methods, the need to use diamond tools and other professional processing tools, so the processing cost of the latter accounted for more than 70% of the total cost, in order to reduce the cost of machining, machinable ceramics thus developed.
Although the machinability of ceramic processing performance is better, but does not mean that you can “arbitrary” processing, the use of traditional metal cutting tools and cutting parameters, the same will exacerbate the wear of the tool, machining accuracy and scrap rate is difficult to ensure, so even if the machinability of ceramic machining performance is good, you need to set up the appropriate cutting parameters and tool paths, choose the Suitable cutting tools, in order to effectively process the machinable ceramics.
According to the material composition, workable ceramics can be divided into workable glass ceramics, non-oxide workable ceramics and oxide workable ceramics. Among them,Machinable glass ceramics is a polycrystalline composite material with a white color. It is a glass-ceramic material with synthetic mica crystals as the main crystal phase. Corning’s “macro” series is a typical representative. Processable ceramics have high mechanical strength, excellent dielectric and thermal properties, and good chemical stability.
Engineering ceramics are widely used in mining, aerospace, pharmaceuticals, oil refining, food and chemical, packaging science, electronics, industrial and power transmission industries, etc. They are difficult to process due to their hardness and brittleness, therefore, customized machining of engineering ceramics requires highly specialized ceramic engineering knowledge and ceramic processing equipment. Jinghui Industry Ltd. has many years of experience in developing engineered ceramic components, and we have an experienced team and ceramic processing equipment to provide customized services at competitive prices.
