Material Properties
CBN is typically black, brown, or dark red in crystal form, with a sphalerite structure and excellent thermal conductivity. Its hardness is second only to diamond, making it a superhard material commonly used in cutting tools and abrasives.
Boron nitride possesses chemical inertness, being unaffected by inorganic acids and water. The boron-nitrogen bond breaks down in hot concentrated alkalis. It begins to oxidize in air 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 acid, insoluble in cold water, with a relative density of 2.25. Its compressive strength is 170MPa. It can withstand 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 materials. For hexagonal boron nitride, it has a very low friction coefficient, excellent high-temperature stability, good thermal shock resistance, high strength, high thermal conductivity, low expansion coefficient, high resistivity, corrosion resistance, and can transmit microwaves or infrared radiation.
Material Structure
Hexagonal crystalline forms of boron nitride, commonly known as graphite lattices, also exist in amorphous variants. Besides the hexagonal form, boron carbide has other crystal structures, including: rhombohedral boron nitride (abbreviated as r-BN, also known as trigonal boron nitride, with a structure similar to h-BN, which is formed during the conversion of h-BN to c-BN), cubic boron nitride [abbreviated as c-BN, or |3-BN, or z-BN (i.e., sphene-type 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 graphite (similar to MoS2, a two-dimensional crystal).
