Boron Nitride(hexagonal boron nitride，h-BN)

Unlike ceramic materials such as alumina, zirconia, silicon carbide, and silicon nitride, h-BN ceramics are soft materials with a Mohs hardness of only 2, the lowest hardness among all ceramic materials. The mechanical properties of h-BN, such as elastic modulus and flexural strength, are also relatively low. The elastic modulus and flexural strength of h-BN-based composite ceramics can be effectively improved by adding sintering aids and reinforcing phases.

h-BN ceramics have good high-temperature mechanical properties. They do not have the load softening phenomenon similar to graphite at high temperatures. In the temperature range from room temperature to 1000°C or even higher, their bending strength can remain unchanged or even improve. Therefore, they are very suitable for use in high-temperature structural parts, which can improve their reliability in high-temperature environments.

Since h-BN ceramics have low hardness and self-lubricating properties, they have good machinability and can be processed by conventional turning, milling, drilling, grinding, cutting and other methods to form components of various complex shapes. h-BN can also be added to other ceramics as a modified phase to reduce the hardness of the material and improve machinability.

H-BN has good high temperature stability.It has no obvious melting point under standard atmospheric pressure and sublimates at 3000℃. In nitrogen or argon atmosphere, the heat resistance temperature of h-BN can reach 2800℃ without softening; the heat resistance temperature can also reach 2000℃ in neutral reducing atmosphere; but it is easily oxidized in oxygen atmosphere to generate low-melting-point boron oxide, which makes its stability worse and the operating temperature is generally below 900℃.

H-BN has excellent thermal conductivity. The room temperature thermal conductivity of hot-pressed dense bulk ceramics can reach more than 50W/(m·K), which is second only to beryllium oxide and aluminum nitride among ceramic materials and close to the thermal conductivity of steel. It is worth noting that the thermal conductivity of h-BN ceramics does not decrease much with increasing temperature. Above 600°C, its thermal conductivity is higher than that of beryllium oxide. At 1000°C, the thermal conductivity in the direction perpendicular to the c-axis is about 27W/(m·K), which is higher than most electrical insulators.

Based on higher thermal conductivity, lower thermal expansion coefficient and elastic modulus, h-BN ceramics have excellent thermal shock resistance, and the material can remain intact even after repeated strong thermal shocks.

h-BN is a good room temperature and high temperature electrical insulator. In a dry environment, the resistivity of high purity h-BN ceramics can reach 1016~1018Ω·cm, and even at 1000℃, the resistivity can still be maintained at 104~106Ω·cm. h-BN has a very high breakdown voltage, which can reach 30~40kv/mm, so it is an ideal material for high frequency insulation, high voltage insulation, and high temperature insulation. In addition, the dielectric constant of h-BN is 4~5, and the dielectric loss is (2~8)x10-4, which is relatively small among ceramic materials.

h-BN has excellent chemical stability, is insoluble in cold water, and hydrolyzes very slowly in boiling water, producing a small amount of boric acid and ammonia. h-BN has considerable corrosion resistance to various inorganic acids, alkalis, salt solutions and organic solvents, has excellent stability in most acids and alkalis, and does not react with weak acids and strong bases at room temperature. The mass loss rate of the samples immersed in concentrated sulfuric acid, nitric acid, phosphoric acid and other solutions for 7 days is less than 10%, slightly soluble in hot acid, and can only be decomposed after being treated with molten sodium hydroxide, potassium hydroxide, etc. In addition, h-BN neither wets nor reacts to high-temperature melts of most metals and glasses, so it can be used as an ideal crucible material for melting various metals.

Due to the lamellar structure of h-BN crystals, h-BN ceramics formed by hot pressing and sintering usually show a certain degree of anisotropy in mechanical, thermal and other properties, and there are differences in the properties parallel and perpendicular to the hot pressing pressure direction.

Therefore, according to the needs of performance, through sintering powder morphology control, sintering process optimization, etc., lamellar h-BN grain-oriented textured ceramics can be prepared to have significant anisotropy and exert their performance characteristics in a certain direction.

In summary, hexagonal boron nitride has many excellent properties, which give it wide application potential in various fields.