Abstract: Ceramic materials have different properties due to different composition elements. As a structural material, they are widely used in various industries
（1） Preface
In the late 20th century, with the rise of many new technologies (such as electronic technology, space technology, laser technology, computer technology, etc.), as well as the development of basic theories (such as mineralogy, metallurgy, physics, etc.) and testing technologies (such as electron microscope technology, X-ray diffraction technology and various spectrum meters, etc.), people have a profound understanding of the relationship between material structure and performance. By controlling the chemical composition and microstructure of materials, many ceramic materials with different properties have been developed, such as various functional ceramics (electronic materials, optical fibers, sensitive ceramic materials) and high-temperature structural ceramics. Compared with traditional ceramic materials, its strength has been improved hundreds of times. In addition, ceramic materials have excellent characteristics of high temperature resistance, wear resistance, corrosion resistance, insulation, etc., which make them more and more widely used in many important fields.
Commonly used ceramic materials of the last process mainly include: compounds composed of metals (transition metals or similar metals) and non-metallic elements such as boron, carbon, silicon, nitrogen and oxygen, and compounds composed of non-metallic elements, such as boron and silicon carbide and nitride.
They can be divided into oxide ceramics, nitride ceramics, carbide ceramics, silicide ceramics and boride ceramics according to their different element compositions. In addition, glass ceramics have been widely used as structural materials in recent years.
（2） Oxide ceramics
The atomic bonding of oxide ceramic materials is mainly ionic bond, and there are some covalent bonds, so it has many excellent properties. Most oxides have high melting point, good electrical insulation performance, especially excellent chemical stability and oxidation resistance, and have been widely used in the field of the last process.
(1) Alumina ceramics
Alumina ceramics, also known as corundum ceramics, are generally α- A1203 is the main crystalline phase. There are different series according to A1203 content and additives. For example, according to the different content of A1203, it can be divided into 75 porcelain, 85 porcelain, 95 porcelain, 99 porcelain, etc; According to the different main crystal phases, it can be divided into mullite porcelain, corundum mullite porcelain and corundum porcelain; According to the different additives, it can be divided into chromium corundum, titanium corundum, etc.
Al203 ceramic is one of the refractory oxides with the most stable chemical properties and the highest mechanical strength; A1203 ceramics have no reaction with most molten metals, and only Mg, Ca, Zr and Ti can reduce them above a certain temperature; Hot sulfuric acid can dissolve A1203. Hot HCl and HF can also corrode A1203; The vapor pressure and decomposition pressure of A1203 ceramics are the minimum. Because of its excellent chemical stability, A1203 ceramics can be widely used in acid resistant pump impellers, pump bodies, pump covers, shaft sleeves, pipe linings and valves for acid delivery.
Al203 ceramics, which contain more than 95% alumina, have excellent electrical insulation properties and low dielectric loss, so they have very broad applications in electronics and electrical appliances.
The high hardness and wear resistance of A1203 ceramics have been widely used in the mechanical field. Such as manufacturing textile wear-resistant parts and tools. A1203 ceramic spark plugs are also widely used in various engines.
Transparent Al203 ceramics have good transmittance to visible light and infrared light, and have high high temperature strength, good heat resistance and strong corrosion resistance. It can be used to manufacture high-pressure sodium lamp tubes, infrared detection window materials, etc.
(2) Zirconia (Zr02) ceramics
Zr02 has two kinds of zirconium allotrope cubic structure (c phase), tetragonal structure (t phase) and monoclinic structure (m phase). According to the composition of the contained phases, Zr02 ceramics can be divided into stabilized Zr02 ceramics and partially stabilized Zr02 ceramics.
Ⅰ Stabilized Zr02 ceramics
Stable Zr02 ceramics are mainly composed of cubic phase, with high refractoriness, small specific heat and thermal conductivity. It is an ideal high temperature insulation material, which can be used as the lining of high temperature furnaces or as various heat-resistant coatings.
Stable Zr02 ceramic has good chemical stability. It can still resist the corrosion of acidic and neutral substances at high temperatures, but cannot resist the corrosion of alkaline substances. The metal elements of Group V, VI and VII in the periodic table do not react with them and can be used as crucibles for smelting this metal.
Pure Zr02 is a good insulator. Due to its obvious high-temperature ionic conductivity, it can be used as a heating element at 2000 ℃, a high-temperature electrode material, and a lamp to generate ultraviolet light.
In addition, the oxygen sensor can be made to measure the oxygen concentration by using the oxygen ion conduction characteristics of Zr02.
Ⅱ Partially stabilized Zr02 ceramics
Partially stabilized Zr02 ceramic is composed of t c dual phase structure, which has very high strength, fracture toughness and thermal shock resistance. It is called "ceramic steel". At the same time, its thermal conductivity coefficient is small, the thermal insulation effect is good, and the thermal expansion coefficient is relatively large, which is easy to match with metal parts. It is used in the cylinder inner wall, piston, cylinder cover plate parts in the ceramic engine developed recently.
Partially stabilized Zr02 ceramics can also be used as non lubricated bearings for mining and mineral industries, nozzles for sand blasting equipment, parts for powder metallurgy industry, and stamping dies for pharmaceuticals.
In addition, some stabilized Zr02 ceramics can also be used as various industrial and medical devices with high toughness and strength. For example, scissors and wool shears used in textile industry bobbin dropping machine, scissors used in tape production, and tools used in microelectronics industry can also be used as bioceramic materials because they do not react with organisms.
(3) MgO ceramics
The main crystal phase of MgO ceramics is MgO, which belongs to cubic sodium chloride structure. The melting point is 2800 ℃, the theoretical density is 3.58 g/cm2, the specific volume resistance is high at high temperatures, the dielectric loss is low, and the dielectric coefficient is 9.12. It has good electrical insulation and belongs to weak alkaline substances. MgO has strong corrosion resistance to alkaline metal slag and does not work with magnesium, nickel, uranium thorium, aluminum, molybdenum, etc. It can be used to prepare crucibles for smelting metal, molds for casting metal, protective tubes for high-temperature thermocouples, lining materials for high-temperature furnaces, etc.
（3） Nitride ceramics
Nitrides include non-metallic and metal element nitrides, which are high melting point substances. There are many kinds of nitride ceramics, but they are not natural minerals, but synthetic. Recently, nitride ceramics that are widely used in industry include silicon nitride (Si3N4), boron nitride (BN), aluminum nitride (AlN), titanium nitride (TiN), etc.
(1) Silicon nitride (Si3N4) ceramics
Si3N4 ceramic material has small thermal expansion coefficient, so it has good thermal shock resistance; Among ceramic materials, Si3N4 has high bending strength, high hardness, self-lubricating property, low friction coefficient, similar to oiled metal, and has great potential as a mechanical wear-resistant material; Si3N4 ceramic material has a high resistivity at room temperature, which can be used as a better insulating material; Si3N4 ceramics are resistant to the corrosion of all inorganic acids and some lyes except hydrofluoric acid, and are not soaked and corroded by molten metal alloys such as lead, tin, silver, brass and nickel; The silicon oxide film formed on the material surface during high temperature oxidation can prevent further oxidation, and the anti seizure temperature reaches 1800 ℃.
Si3N4 ceramics can be used as thermal mechanical materials, cutting tools, advanced refractory materials, as well as anti-corrosion and wear-resistant sealing parts.
(2) Aluminum Nitride (AlN) Ceramics
AIN belongs to covalent bond compound, hexagonal crystal system, fibrous zinc ore structure, white or gray, density 3.26g/cm2, no melting point, sublimation decomposition at 2200 ℃ - 2250 ℃, high thermal hardness, and does not soften and deform even before decomposition temperature. It has excellent thermal shock resistance. AlN has good corrosion resistance to Al and other molten metals, arsenic compounds, etc., especially to molten Al solution. In addition, it also has excellent electrical insulation and dielectric properties; However, AlN has poor high-temperature oxidation resistance and is easy to absorb moisture and hydrolyze in the atmosphere.
AlN can be used as crucible for molten metal, thermocouple protection tube, container for vacuum evaporation and plating, container for vacuum evaporation and plating of gold, heat-resistant brick, etc., especially as lining material of oxidizing electric furnace at about 2000 ℃; The thermal conductivity of AlN is 2-3 times higher than that of A1203, and its strength during hot pressing is higher than that of Al203. It can be used in occasions with high strength and high thermal conductivity, such as the substrate of large-scale integrated circuits.
(3) Boron Nitride (BN) Ceramics
There are hexagonal and cubic BN materials in boron nitride (BN) ceramics.
I Hexagonal BN
Hexagonal BN is self-lubricating and can be used for mechanical seals, high-temperature solid lubricants, and bearings made of metal and ceramic fillers. It has excellent heat resistance and can be used in oxidation atmosphere below 900 ℃ and nitrogen and inert atmosphere below 2800 ℃. Liuli BN has good corrosion resistance to acid, alkali and glass slag. It neither wets nor reacts with most molten metals, so it can be used as a crucible, vessel and other parts for smelting nonferrous metals, precious metals and rare metals. BN is both a good conductor of heat and an insulator of electricity. Its breakdown voltage is 4-5 times of that of alumina, and its dielectric constant is 1/2 of that of alumina. It can be used as insulation material for ultra-high voltage wires. BN is transparent to microwave and infrared, and can be used as a window for infrared and microwave transmission. BN has stable performance under ultra-high pressure and can be used as pressure transfer material and container. BN is the lightest ceramic material and can be used as high-temperature structural material for aircraft and spacecraft. In addition, BN can be used as an electroluminescent material due to its luminescent property. Amorphous carbon fibers coated with BN can be used for rocket nozzles, etc.
II Vertical force BN
Cubic BN is of sphalerite structure, with high chemical stability, good thermal conductivity and heat resistance. Its hardness is similar to that of synthetic diamond, so it is an excellent abrasive material. Compared with diamond, its most outstanding advantage is that it does not react with ferrous metals at high temperatures and can be used at 1400 ℃.
In addition to being directly used as abrasive, Lili BN can also be mixed with some metals or ceramics and sintered into blocky materials as various high-performance cutting tools.
（4） Carbide ceramics
Typical carbide ceramic materials include silicon carbide (SiC), boron carbide (B4C), titanium carbide (TiC), zirconium carbide (ZrC, etc. Carbides will be oxidized at very high temperatures, but the oxidation resistance of many carbides is better than that of high melting point metals such as W and Mo. Most carbides have good conductivity and thermal conductivity, and many carbides have very high hardness, especially the hardness of B4C is only inferior to diamond and cubic boron nitride, but the brittleness of carbides is generally large.
(1) Silicon carbide (SiC) ceramics
Silicon carbide has no melting point and decomposes at 2500 ℃ under normal pressure. The hardness of silicon carbide is very high, the Mohs hardness is 9.2-9.5, and the microhardness is 33400MPa, which is second only to a few substances such as diamond, Lili BN and B4C.
The thermal conductivity of silicon carbide is very high, about twice that of Si3N4; Its thermal expansion coefficient is about 1/2 of A1203; The bending strength is close to Si3N4 material, but the fracture toughness is smaller than Si3N4; It has excellent high temperature strength and high temperature creep resistance. The bending strength of hot pressed silicon carbide at 1600 ℃ is basically the same as that at room temperature; Good thermal shock resistance. It has high chemical stability and is insoluble in common acids and mixed acids.
Oxide and nitride bonded silicon carbide materials have been widely used in metallurgy, light industry, machinery, building materials, environmental protection, energy and other fields, such as furnace structure materials, flame barriers, furnace tubes, furnace, etc; The heating elements made of carbide materials are gradually heating the main elements in the oxidation atmosphere below 1600 ℃; High performance silicon carbide materials can be used for high temperature, wear-resistant, corrosion resistant mechanical parts; The silicon carbide material is also used to manufacture the high-efficient heat exchanger of rocket exhaust nozzle, and good results have been achieved; In addition, silicon carbide is an important candidate material for improving the performance of various high-temperature gas turbine components.
(2) Boron Carbide (BC) Ceramics
The remarkable feature of boron carbide is its high melting point (about 2450 ℃); Low specific gravity, its density is only 1/3 of steel; Low expansion coefficient; High thermal conductivity; High hardness and high wear resistance, its hardness is only lower than diamond and cubic BN; High strength and certain fracture toughness. The bending strength of hot pressed B4C is 400-600MPa, and the fracture toughness is 6.0MPa ml/2; Large thermoelectric EMF (100 μ V/k) is a high-temperature P-type semiconductor, which can be changed from P-type semiconductor to N-type semiconductor with the decrease of carbon content in B4C; It has high neutron absorption cross section.
The excellent performance of B4C, in addition to being widely used as abrasive, can also be used to make various wear-resistant parts, thermocouple elements, high temperature semiconductors, thermoelectric conversion devices on spacecraft, bulletproof armor, reactor control rods and shielding materials.
（5） Glass ceramic materials
The glass with a specific composition (containing nucleating agent) is subject to crystallization heat treatment, and a large number of tiny crystals are uniformly precipitated in the glass and further grow to form a dense microcrystalline phase. The glass phase is filled in the grain boundary, and the polycrystalline solid materials like ceramics are collectively referred to as glass ceramics, also known as glass ceramics.
(1) Low expansion glass ceramics
This kind of glass-ceramic is characterized by its microstructure as frame silicate, and the main crystalline phases are β Monoquartz β Monokalopyroxene β Potassium nepheline is characterized by low coefficient of thermal expansion (can be negative), high strength, good thermal stability, high service temperature, and can be made into transparent and turbid white. Low expansion coefficient is very beneficial for dimensional stability and thermal shock resistance of components, so it can be used as a component with high dimensional stability requirements on space shuttle. Low expansion glass ceramics are the largest glass ceramics produced at present. They are widely used to make various advanced cookers, observation windows for high-temperature operation, microwave oven covers, support rods for large astronomical telescopes and laser mirrors, laser components and important parts on space shuttles.
(2) Surface hardenable glass ceramics
The strength of glass ceramics is several times higher than that of ordinary glass, and the bending strength can reach 88-250MPa, but it still cannot meet the requirements in some special occasions, so the strength needs to be further improved. Since the failure of brittle materials mostly originates from surface microcracks, the method of introducing a thin layer of compressive stress on the surface of glass ceramics can be used to prevent the surface microcracks from expanding, thus improving the strength of materials. Two methods are usually used, one is to introduce the surface compressive stress layer by using the difference between the surface and internal thermal expansion, and the other is to introduce the surface compressive stress layer by using ion exchange.
(3) Machinable glass ceramics
Machinable glass-ceramic is easy to machine mainly because its main crystal phase is fluoromica structure. It has been found that there are mainly three types of fluoromica in machinable glass-ceramic: fluorophlogopite, tetrasilicofluoromepite and lithium mica. Because mica flakes are easy to cleave, this unique microstructure allows mica containing glass ceramics to be processed to precise size by ordinary drilling, sawing, turning, grinding, etc. Machinable glass ceramics with fluoromica as the main crystal phase have high thermal shock resistance, excellent insulation performance and high dielectric strength; Low dielectric loss. Alkaline earth mica machinable glass ceramics have higher strength and toughness, higher thermal stability (>1100 ℃) and insulation. Therefore, machinable glass ceramics have broad application prospects in electrical insulation, microwave technology, precision instruments, aviation and aerospace fields.
Ceramic materials composed of different elements have different application fields. In general, the development trend of ceramics in the future is mainly to explore materials with high ductility, ultra-high strength, ultra-high toughness, ultra-high hardness and high temperature resistance