Material Properties
CBN crystals are typically black, brown, or dark red, featuring a sphalerite structure with excellent thermal conductivity. Their hardness ranks only second to diamond, making them an ultra-hard material often used in tooling and abrasives.
Boron nitride exhibits resistance to chemical erosion and is not attacked by inorganic acids or water. The boron-nitrogen bond breaks in hot concentrated alkalis. It begins to oxidize in air at temperatures above 1200°C. Its melting point is 3000°C, and it starts sublimating slightly below this temperature. Decomposition begins at approximately 2700°C under vacuum. It is slightly soluble in hot acids and insoluble in cold water, with a relative density of 2.25. Its compressive strength is 170MPa. It can be used at higher temperatures of up to 900°C in oxidizing atmospheres, and up to 2800°C in non-reactive reducing atmospheres, although its lubricating properties are poor at room temperature. Most properties of boron carbide are superior to carbon-based materials. For hexagonal boron nitride: low friction coefficient, excellent high-temperature stability, good thermal shock resistance, high strength, high thermal conductivity, low expansion coefficient, high resistivity, corrosion resistance, and permeability to microwaves or infrared radiation.
Material Structure
Hexagonal crystalline boron nitride, commonly known as graphite lattices, also has amorphous variants. Besides the hexagonal form, boron carbide has other crystal structures, including: rhombohedral boron nitride (abbreviated as r-BN, or triclinic boron nitride, with a structure similar to h-BN, which is formed during the conversion from h-BN to c-BN), cubic boron nitride [abbreviated as c-BN, or |3-BN, or z-BN (i.e., sphene boron nitride), known for its extreme hardness], and wurtzite boron nitride (abbreviated as w-BN, a hard state of h-BN under high pressure). People have also discovered two-dimensional boron nitride crystals similar to graphene (such as MoS2, a two-dimensional crystal).
