Boron nitride, as a high-performance and promising new ceramic material, encompasses five isomers: Hexagonal Boron Nitride (h-BN), Cubic Boron Nitride (c-BN), Wurtzite Boron Nitride (w-BN), Rhombohedral Boron Nitride (r-BN), and Orthorhombic Boron Nitride (o-BN).
Boron nitride powder features loose texture, lubricity, light weight, and moisture absorption. It has a clean white color, with products appearing ivory white.
Boron Nitride Development History: Domestic and International
Boron nitride (BN) ceramics were discovered as a compound in 1842. Research on BN materials abroad began after World War II, but it was not until 1955 that the formal development of BN was achieved when researchers solved the thermal pressing method. Subsequently, the American Diamond Company and Union Carbide Corporation were the first to start production, producing over 10 tons by 1960. Japan began importing large quantities of BN products from the US annually, and three Japanese companies also conducted several years of research, successfully trial-producing in early 1969 and commencing production in 1970. domestically, China began researching BN powder in 1963, achieved development in 1966, and started production and application in industrial and advanced technologies in 1967.
1. Physical and chemical properties of boron nitride ceramic
(1) Thermal Properties: Boron nitride products can be used in an oxidizing atmosphere around 1000°C and in an inert atmosphere up to nearly 3000°C. This property makes them superior to refractory materials like mullite, alumina, and silicon carbide in terms of耐火 ability. Additionally, they exhibit excellent resistance to thermal shock, preventing cracking even in rapid cooling and heating conditions up to 1500°C.
(2) Chemical Stability: The material is inert to most metals such as iron, aluminum, copper, silicon, potassium arsenide, brass, and cryolite, as well as slag glass. Therefore, containers made of boron nitride can be used for these substances. However, materials containing B2O3 are hygroscopic, which can cause absorbed water to be released upon rapid heating during use, leading to cracking. One effective measure to improve this deficiency is to add some binder.
(3) Electrical Properties: Due to the low dielectric constant and dielectric loss of BN products, they can be widely used in both high-frequency and low-frequency ranges. Its dielectric strength is 4.5 times that of alumina, and the dielectric constant is half that of alumina. The electrical properties differ significantly from those of graphite, and like oxide ceramics, it is non-conductive at high temperatures. It is an electrical insulating material that can be used over a wide temperature range.
(4) Machining Properties: BN ceramic has a Mohs hardness of 2. It can be machined on lathes, milling machines, and other processing equipment; it can also be formed into thin sheets, spirals, and various 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, a fine white thin-film material, similar to graphite and commonly known as white graphite, with excellent lubricity.
(6) Lightweight: BN is one of the lighter materials among ceramics.
(7) Property of neutron absorption: BN has a large neutron absorption cross-section.
Applications of Boron Nitride Ceramic
Current research on h-BN focuses on its hexagonal phase and cubic phase (c-BN). The former boasts lubricity, thermal conductivity, and excellent high-temperature properties. The latter is also in a thermodynamic equilibrium state at room temperature and pressure. The primary application of h-BN remains as a raw material for synthesizing cubic boron nitride.
(1) Applications of Hexagonal Boron Nitride
Hexagonal boron nitride is a material with excellent high-temperature resistance, corrosion resistance, high thermal conductivity, high insulation properties, and lubricity. Currently, research in this field is actively focused on simplifying processes, reducing production costs, and extending the service life of the components.
Utilizing the excellent chemical stability of hexagonal boron nitride, it can be used as crucibles, boats, liquid metal transfer tubes, rocket nozzles, bases for high-power devices, pipes for melting metal, pump components, and molds for cast steel.
② Leveraging the high-temperature and corrosion-resistant properties of hexagonal boron nitride, it can be used to manufacture high-temperature components, rocket combustion chamber linings, thermal shields for spacecraft, and corrosion-resistant parts for electromagnetic flow machines.
③ Leveraging the insulating properties of hexagonal boron nitride, widely used as insulators for high-voltage, high-frequency electrical applications, plasma arcs, various heater insulators, heating tube sleeves, and high-temperature, high-frequency, high-voltage insulating heat dissipating components, as well as materials for high-frequency furnace applications.
④ Leveraging the lubricity of hexagonal boron nitride, when used as a lubricant, it can disperse in heat-resistant grease, water, or solvents; sprayed onto the friction surface, it forms a dry film after the solvent evaporates; and filled in resins, ceramics, and metal surfaces as high-temperature self-lubricating composite materials. Boron nitride suspension oil is white or yellow, thus it does not contaminate fiber products on textile machinery, and can be used in large quantities for lubrication in synthetic fiber textile machinery.
⑤ Hexagonal boron nitride can also be used as an additive for various materials. Boron nitride fibers, processed from boron nitride, are medium modulus high-performance fibers, an inorganic synthetic engineering material that can be widely used in the chemical industry, textile industry, aerospace technology, and other advanced industrial sectors.
(2) Application of Cubic Boron Nitride:
(1) As Tool Material: Small-grained cBN single crystals can be used as tool material. cBN tools are made by bonding cBN grains into products of certain geometric shapes with the help of a binder, serving as ultra-hard material tools.
cBN cutting tools excel in grinding black metals, especially those with high hardness, great toughness, high strength at elevated temperatures, and low thermal conductivity. They offer superior grinding performance, with a metal removal rate 10 times that of diamond, effectively addressing the processing challenges of hard and tough materials.
Cubic Boron Nitride (CBN) abrasive tools are used for high-speed grinding and honing operations, significantly enhancing grinding efficiency; they offer high grinding accuracy, extended wheel life, and save considerable time on wheel replacement, machine adjustment, and workpiece inspection.
(2) As Cutting Tool Material: PcBN overcomes the shortcomings of cBN single crystals, such as easy cleavage and anisotropy, and is mainly used for manufacturing cutting tools. PcBN cutting tools are particularly suitable for high-speed machining and precision cutting, and have been widely applied in CNC machine tools. They are ideal for cutting hard materials and are especially suited for processing high-hardness, difficult-to-machine materials that were previously only grindable. This enables the replacement of grinding with turning and milling, resulting in better workpiece surface quality, elimination of the annealing process, and a significant increase in production efficiency.
