As a new ceramic material with excellent properties and great development potential, boron nitride includes five isomers: hexagonal boron nitride (H – BN), cubic boron nitride (C – BN), wurtzite boron nitride (W – BN), rhombic boron nitride (R – BN) and orthorhombic boron nitride (o – BN).
Figure 1 Boron Nitride Powder
Boron nitride powder has the properties of looseness, lubrication, light weight and easy moisture absorption. Its color is white, and the products are ivory white.
Development history of boron nitride at home and abroad:
Boron nitride (BN) ceramics was first discovered in 1842. Foreign research on BN materials began after the end of World War II, but it was not officially developed until 1955 when researchers solved the hot pressing method of BN. Since then, the American diamond company and United carbon company have first put into production, and more than 10 tons have been produced in 1960; Japan began to import a large number of BN products from the United States every year. Since then, three Japanese companies have also conducted research for several years, successfully trial produced in early 1969 and put into operation in 1970. Domestically, China began to study BN powder in 1963. It was successfully developed in 1966 and put into production and applied to China’s industry and cutting-edge technology in 1967.
Physical and chemical properties of boron nitride ceramics
(1) Thermal performance: boron nitride products can be used in an oxidizing atmosphere of about 1000 ℃ and an inert atmosphere of nearly 3000 ℃. This property is higher than that of mullite, alumina and silicon carbide. At the same time, it has high heat resistance and impact resistance, and will not crack under the rapid cooling and heating environment of 1500 ℃.
(2) Chemical stability: with the vast majority of metals such as solution iron, aluminum, copper, silicon, potassium arsenide, brass and cryolite, it does not work, and so does slag glass. Therefore, the container made of boron nitride material can be used as the vessel of the above substances. However, those containing B2O3 generally have hygroscopicity, which makes the rapid heating of the product in the process of use will infiltrate the absorbed water and produce cracks. One of the effective measures to improve this disadvantage can be to add some binders.
(3) Electrical properties: because the dielectric constant and dielectric loss of BN products are small, they can be widely used in high frequency range and low frequency range. Its dielectric strength is 4.5 times that of alumina, and its dielectric constant is half that of alumina. Its electrical properties are very different from that of graphite. Like oxide ceramics, it has no conductivity at high temperature. It is an electrical insulating material that can be used in a large temperature range.
(4) Machinability: the Mohs hardness of BN Ceramics is 2. It can be processed on lathes, milling machines and other processing equipment; It can also be processed into thin sheets, spiral bodies and various parts with complex shapes. BN has low mechanical strength. Although it is superior to graphite at room temperature, it is more brittle than oxide ceramics.
(5) Lubricity: BN powder is a hexagonal crystal system. It is a kind of white flake fine material, which is similar to graphite. It is commonly known as white graphite and has excellent lubrication.
(6) Light weight: BN is the material with smaller weight in ceramic materials.
(7) Nature of neutron absorption: BN has a large neutron absorption section.
2 application examples of boron nitride ceramics
At present, the research on BN mainly focuses on its hexagonal phase (H – BN) and cubic phase (C – BN). The former has lubricity, thermal conductivity and good high temperature performance. The latter is also in thermodynamic equilibrium at room temperature and pressure. The main application field of H – BN is as a raw material for the synthesis of cubic boron nitride.
(1) Application of hexagonal boron nitride
Hexagonal boron nitride is a kind of material with high temperature resistance, corrosion resistance, high thermal conductivity, high insulation and excellent lubrication performance. At present, simplifying the process, reducing the production cost and improving the service life of components are the active research directions of this kind of material.
① Using the excellent chemical stability of hexagonal boron nitride, it can be used as crucible for melting and evaporating metal, boat dish, liquid metal delivery pipe, rocket nozzle, base of high-power device, pipe for melting metal, pump parts, mold of cast steel, etc.
② The heat resistance and corrosion resistance of hexagonal boron nitride can be used to manufacture high-temperature components, lining of rocket combustion chamber, heat shield of spacecraft, corrosion-resistant parts of magnetic current generator and so on.
③ Using the insulation of hexagonal boron nitride, it is widely used in the insulators of high-voltage and high-frequency electricity and plasma arc, as well as the insulators of various heaters, heating pipe sleeves, high-temperature, high-frequency and high-voltage insulation and heat dissipation components, and the materials of high-frequency electric furnaces.
Figure 2 Insulating high purity boron nitride ceramics for electric furnace
④ Using the lubricity of hexagonal boron nitride, when boron nitride is used as lubricant, it can be dispersed in heat-resistant grease, water or solvent; Spray on the friction surface and form a dry mold after the solvent volatilizes; It is filled in the surface layer of resin, ceramic and metal as high temperature resistant self-lubricating composite. Boron nitride suspension oil is white or yellow, so it does not pollute fiber products in textile machinery, and can be widely used in the lubrication of synthetic fiber textile machinery.
⑤ Hexagonal boron nitride can also be used as an additive for various materials. Boron nitride fiber processed from boron nitride is a medium modulus high-function fiber. It is an inorganic synthetic engineering material, which can be widely used in chemical industry, textile industry, aerospace technology and other cutting-edge industrial departments.
(2) Application of cubic boron nitride:
(1) As abrasive material: CBN single crystal with small particles can be used as abrasive material. CBN abrasive tool is a kind of superhard material abrasive tool, which binds CBN abrasive particles into products with certain geometry with the help of binder.
CBN Abrasives have good grinding performance for ferrous metals, especially materials with high hardness, high toughness, high strength at high temperature and low thermal conductivity. Its metal grinding rate is 10 times higher than that of diamond, which solves the processing problem of hard, tough and hard grinding materials.
Cubic boron nitride grinding tool is used for high-speed and efficient grinding and honing, which can greatly improve the grinding efficiency; It has high grinding precision and long service life of grinding wheel, which saves many auxiliary time such as grinding wheel replacement and dressing, machine tool adjustment and workpiece detection.
(2) As tool material: PCBN overcomes the shortcomings of easy cleavage and anisotropy of CBN single crystal, and is mainly used to make tool materials. PCBN cutting tool is especially suitable for high-speed cutting and high-precision cutting. It has been widely used in CNC machine tools. It is suitable for the cutting of high hardness materials, especially for the processing of high hardness and difficult to process materials that can only be grinded before. It can realize turning instead of grinding and milling instead of grinding, which can obtain better workpiece surface quality and eliminate the annealing process, Greatly improve production efficiency.