Ceramic materials used in engineering applications can be divided into traditional ceramics and special ceramics.

The main raw materials of traditional ceramics are clay (Al2O3·2SiO2·H2O), quartz (SiO2) and feldspar (K2O·Al2O3·6SiO2).

Special ceramic is also called modern ceramic, or advanced ceramic. According to different uses, it can be divided into structural ceramics and functional ceramics, such as piezoelectric ceramics, magnetic ceramics, capacitor ceramics, high temperature ceramics and so on.

High-temperature ceramics include oxide ceramics, carbide ceramics, boride ceramics and nitride ceramics.

Oxide ceramics:

Oxide ceramics properties:

The melting point is mostly above 2000 ℃ and the firing temperature is about 1800 ℃. At the firing temperature, the oxide particles are sintered rapidly, and the solid surface reaction occurs between the particles, resulting in the formation of large ceramic crystals (single phase), or a small amount of gas is produced.

The strength of oxide ceramics decreases with the increase of temperature, but keeps high strength below 1000 ℃, and changes little with temperature.

Pure oxidized ceramics are good structural materials for high temperature fire resistance, and ceramics will not produce oxidation under any circumstances.

Oxide ceramics include:

Alumina ceramics Al2O3

The structure of alumina is that O is closely arranged in hexagonal structure and Al occupies the interstitial position. There is very little pure alumina in nature. Depending on the amount of impurities, alumina can be red or blue. In actual production, the Al2O3 content of alumina ceramics can be divided into 75,95,99 and other kinds of porcelain.

The melting point of alumina is as high as 2050 ℃, and it has good oxidation resistance and high hardness, and the hardness of microcrystalline corundum red can reach 1200 ℃. Commonly used in the manufacture of metal wire drawing dies and cutting quenched steel tools.

Zirconia ceramics

The melting point of zirconia ceramics is above 2700 ℃ and can withstand the high temperature of 2300 ℃. The recommended temperature is 2000 ℃ ~ 2200 ℃. Therefore, it can be used as reactor insulation material. Zirconia as an additive can greatly improve the strength and toughness of ceramics and produce zirconia toughened ceramics (PSZ).

Zirconia toughened ceramics have multiphase structure and can have three different crystal structures at different temperatures and pressures, so that stress can induce phase transformation and phase transformation toughening under suitable conditions, and greatly improve the fracture toughness. Zirconia toughened alumina ceramic material has a strength of 1200 MPa and a fracture toughness of 15MPa m.

Zirconia toughened ceramics have the properties to meet the requirements of hot extrusion die, and have the characteristics of high temperature resistance, corrosion resistance and wear resistance, especially under high temperature and pressure and small permanent deformation. it is more suitable for hot extrusion die than tungsten carbide, nickel-based or cobalt-based cemented carbide.

The hardness of zirconia toughened ceramics is higher than that of metals, its toughness is higher than that of ordinary ceramics, and it has high chemical stability and resistance to high temperature at least 800 ℃. The temperature of the metal powder extrusion die can reach 600 ℃, and the copper rod extrusion die works at 950 ℃. The service life of the die made of zirconia toughened ceramic is tens of times longer than that of the cemented carbide die. The bending strength of PSZ containing magnesia can reach 400 MPa.

The disadvantages of zirconia toughened ceramics are non-ductility, low thermal conductivity and mismatch between thermal expansion and metal materials, which should be considered in design and use. Zirconia and its excellent performance in the production of wire drawing die, drawing die, etc., are often used in the process of deep drawing stainless steel.

Carbide ceramics:

Carbide ceramics include silicon carbide, boron carbide, cerium carbide, molybdenum carbide, niobium carbide, zirconium carbide, titanium carbide, vanadium carbide, tungsten carbide, tantalum carbide and so on. This kind of carbide has high melting point, hardness and wear resistance, but poor high temperature oxidation resistance (about 900C ~ 1000 ℃) and high brittleness.

It is mainly used as high temperature material or high power material in chemical industry, automobile industry, nuclear industry, microelectronics industry, laser and other fields. In mold manufacturing, it is often used in wear-resistant, corrosion-resistant wire drawing die, forming die, hot die-casting die, honeycomb ceramic mold and so on.

Carbide Ceramics Properties:

Carbide ceramics have high melting point. For example, the melting point of titanium carbide is 3460 ℃, that of tungsten carbide is 2720 ℃, and that of zirconium carbide is 3540 ℃.

The hardness of carbide ceramics is higher. For example, boron carbide is the hardest material after diamond and cubic boron nitride.

Good thermal conductivity and chemical stability. Carbide ceramics do not react with acid, individual metal carbide ceramics do not react with acid even if heated, and the most stable carbide ceramics are not even corroded by the mixture of nitric acid and hydrofluoric acid.

Carbide ceramics include:

Silicon carbide ceramics

The density of silicon carbide ceramics is 3.2 × 10 kg/m3, the bending strength is 200 ℃ 250 MPa, the compressive strength is (1000 ℃)1500 MPa, the hardness is 9.2, the thermal conductivity is very high, the thermal expansion coefficient is very small, and it is slowly oxidized at 900C ~ 1300 ℃.

Boron carbide ceramics

Boron carbide ceramics have high hardness and strong abrasive wear resistance, and their melting point is up to 2450 ℃. It will oxidize quickly at high temperature and react with hot or molten ferrous metals. Mainly used as abrasives, sometimes used in the manufacture of super-hard tool materials.

Boride ceramics

Properties of boride ceramics

Excellent high temperature characteristics. The melting point range is 1800 ℃ ~ 2500 ℃. It has high high temperature oxidation resistance and the service temperature is up to 1400 ℃. At the high temperature of 800 ℃, the bending strength almost does not decrease, and the decrease of hardness with the increase of temperature is smaller than that of other materials.

With high toughness. At room temperature, the fracture toughness KIC reaches 30 MN/m work, which is 6-8 times that of the representative engineering ceramic silicon carbide. When the grain size of B4C is refined to 5 μ m, the strength is 500 to 600 MPa, and when the grain size is less than 1 μ m, the strength is more than 1000 MPa.

High hardness and good wear resistance. The hardness is about 1000 HV and has high shear modulus. Chemical erosion resistance, difficult to volatilize, but poor high temperature corrosion resistance and oxidation resistance.

It is widely used in nuclear industry, aerospace and other fields. It is mainly used for high temperature bearings, internal combustion engine nozzles, various high temperature devices, devices dealing with molten non-ferrous metals, electrical contact materials, wear-resistant materials and tool materials, etc. In mold manufacturing, it is often used to manufacture mold structural components, heat-resistant components and so on.

Boride ceramics include:

Zirconium boride ceramics made of zirconium diboride (ZrB2), titanium diboride (TiB2), lanthanum hexboride (LaB6), titanium boride, chromium boride, molybdenum boride and tungsten boride, etc.

Nitride ceramics.

Properties of nitride ceramics:

The trisilicon tetra-nitride ceramic has the best anti-oxidation ability, starts active oxidation at 1400 ℃, and has good chemical corrosion resistance. Some also have special mechanical, dielectric or thermal conductivity.

Sintering is difficult. First, high quality powder raw materials are produced, and then ceramic products are made by nitriding reaction sintering, hot pressing sintering, hot isostatic pressing sintering and so on.

Nitride ceramics include:

Silicon tetra-nitride (Si3N4), boron nitride (BN), aluminum nitride (AlN), etc.

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tags: Semiconductors semiconductor industry