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The amorphous boron nitride (a-BN) variety, which is analogous to amorphous carbon, is very soft and can be used as a lubricant or additive. Hexagonal boron nitride (h-BN) has a structure similar to graphite, is also very soft and is used as a lubricant, while cubic boron nitride (c-BN), which has an atomic arrangement similar to diamond, is much harder and more chemically and thermally stable.
The crystal structure of hexagonal boron nitride consists of parallel planes that are bonded to each other by weak Van der Waals forces. This structure allows sliding movement between the parallel planes, and the resulting shear strength is low but compressive strength is high.
While the a-BN and h-BN polymorphs have a Vickers hardness that is about half that of diamond, c-BN has a hardness that can rival or surpass that value because of its greater stability to heat and transition metals. Bulk c-BN has a hardness of 100 gigapascals, and polycrystalline c-BN with grain sizes less than 10 nm has a Vickers hardness that is slightly lower but still very high.
The h-BN and a-BN polymorphs can be densified into ceramic shapes by hot-pressing with boron oxide or other sintering additives. The resulting h-BN and a-BN based ceramics have mechanical properties that are comparable to those of graphite, with a specific gravity between 1.45 and 1.48 g/cm3. Those materials can be used to make bullets that increase effective barrel life and increase intervals between bore cleaning, among other uses.