Cubic boron nitride, abbreviated as CBN, is a synthetic superhard material. In 1957, researchers at the American company GE first synthesized cubic boron nitride using artificial methods under ultra-high temperature and high pressure conditions. In 1966, China successfully synthesized its first cubic boron nitride (CBN). However, natural cubic boron nitride has never been discovered, so cubic boron nitride is considered a synthetic product and does not exist in nature.
Until 2009, American scientists, together with their peers from Chinese and German research institutions, found this mineral in the chromium-rich rocks of the ancient oceanic crust on the Qinghai-Tibet Plateau in China. In August 2013, the International Mineralogical Association officially recognized this new mineral—cubic boron nitride.
Cubic boron nitride has an atomic structure that is highly similar to the carbon atomic structure in diamonds, giving it high density and extremely high hardness. CBN is a compound composed of nitrogen and boron atoms, with a chemical composition of 43.6% boron and 56.4% nitrogen. From the crystal structure perspective, cubic boron nitride is composed of a face-centered cubic lattice of B atoms and a face-centered cubic lattice of N atoms staggered along the diagonal by 1/4, making its crystal structure close to that of diamonds, with quite similar lattice constants.
Cubic boron nitride is the second hardest material in nature, only second to diamonds. Its Mohs hardness is 9.7, and its Vickers hardness is as high as 7500, which gives it unparalleled advantages in the field of material processing.
Cubic boron nitride has good thermal conductivity, with a thermal conductivity coefficient of 79.54w/m•k. Although lower than diamonds, it is far higher than high-speed steel, cemented carbide, and other ceramic materials, helping to maintain tool stability during high-speed cutting processes.
The thermal stability of cubic boron nitride is better than that of synthetic diamonds and can withstand cutting temperatures above 1200°C, making it perform excellently in high-temperature processing environments.
Cubic boron nitride has high chemical inertness and high chemical stability in neutral and reducing gas media against acids and alkalis, making it especially suitable for processing iron-based metals.
The synthesis of cubic boron nitride single crystals mainly adopts the static high-pressure catalyst method, using hexagonal boron nitride and different catalysts as raw materials, synthesized under high-temperature (1400℃~1800℃) and high-pressure (4~8GPa) conditions.
In the early stages, China mainly used metallic magnesium as a catalyst for synthesizing cubic boron nitride. Later, mainly metal nitride borides were used, such as Li3N, Mg3N2, Ca3N2. The selection of these catalyst materials and the optimization of synthesis conditions have enhanced the color and quality of cubic boron nitride, mostly resulting in pale yellow, amber, or colorless transparent crystals with complete crystal shapes, smooth crystal faces, and high single particle compressive strength.
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