Hexagonal Boron Nitride Vs. Cubic Boron Nitride

Boron nitride polymorphs are considered a major technological and scientific promise. Part of the reason is their incredible phase stability at high temperatures, growth kinetics, extreme hardness, and pressure resistance. The two most common polymorphs are hexagonal (white graphite-like) and cubic boron nitride (diamond-like).

This hexagonal boron nitride vs cubic boron nitride expedition explains the differences between the polymorphs based on the following:

Structure and Bonding

Boron nitride is a ceramic compound synthesized by chemically reacting boric acid, nitrogen, and boron oxide. This guide intentionally starts by looking at this compound to better explain its polymorphs’ structure and bonding properties.

Boron, to start with, is a member of group III and part of the p Block in the periodic table. Meaning, its oxidation state is characteristically very stable. Boron forms compounds that are deficient in electrons, making them useful catalysts.

But in this case, boron forms several compounds with nitrogen, resulting in bonds similar to carbon-carbon bonds. For instance, carbon-carbon and boron-nitrogen bonds are isoelectronic, meaning both bonds have the same number of electrons. Carbon, boron, and nitrogen also have similar atomic radii.

Nitrogen is atomic number 7, meaning it has 7 protons and 7 electrons. Boron, on the other hand, has 5 protons and 5 electrons. The two atoms combine to form 12 protons and 12 electrons, just as in a carbon-carbon bond, where each atom has 6 protons and 6 electrons.

Boron nitride has the same number of electrons as carbon allotropes—graphite and diamond. This demonstrates its ability to form different crystal structures, a process called polymorphism. The difference in structures results from the circumstances surrounding the chemical reactions, including pressure, temperature, etc.

This is where hexagonal boron nitride and cubic boron nitride come in. Wurtzite boron nitride is also a boron nitride polymorph but is barely used. Hexagonal and cubic boron nitride are named based on their structures. One forms hexagonal layers whereas the other forms three-dimensional cubic layers.

Hexagonal boron nitride (h-BN) is analogous to white graphite while cubic boron nitride compares with diamond. h-BN forms layers that overlap each other, yet are weakly attached as in graphite. The layers’ weak bonds give h-BN a characteristic soft yet stable form that makes it a valuable additive in cosmetics. This feature also contributes to its industry use as a lubricant

Cubic boron nitride (c-BN), on the other hand, forms a giant covalent structure in all directions, resembling diamond’s tetrahedral arrangement. It is the second hardest material after diamond. Each boron atom bonds with four nitrogen atoms. Likewise, each nitrogen atom bonds with four boron atoms to form strong covalent bonds.

The strong bonds binding the atoms and strong forces binding the layers give cubic boron nitride a hard structure. It is therefore used as a cutting tool, giving a higher output performance than traditional cutting tools. c-BN is also grouped among the most unreactive materials, hence its use as an insulator or coating agent.

Stability and Pressure Resistance

The atomic structure of boron nitride provides chemists with an invaluable compound for industry use. Case in point, h-BN layers comprise a network of (BN)3 rings forming covalent bonds. Each layer is bonded to another by van der Waals forces not strong enough to prevent sliding through. It is hence an efficient solid lubricant, a key element in dental cement, cosmetics (that is skincare and makeup products), and paints.

Cubic boron nitride is quite different, mostly applied as an abrasive. The polymorph has the second strongest bonds, making it incredibly wear-resistant. This feature contributes to its toughness under high pressure and temperature conditions. Besides, it is insoluble in nickel, iron, and other alloys under high-temperature conditions. Diamond falls short of this property and dissolves.

Hexagonal boron nitride also demonstrates poor wettability up to temperatures as high as 900 °C. The material can also be applied in the production of alloys, resins, rubbers, ceramics, etc, making them inherently lubricant.

Thermal Conductivity

Cubic boron nitride has a higher thermal conductivity compared to h-BN. This is explained by its symmetric and isotropic properties. h-BN also has a higher number of atoms in its unit cell, undermining its thermal conductivity.

This doesn’t throw hexagonal boron nitride off the grid though. Its thermal conductivity is higher than most materials and ceramics – 300 - 2,000 W m1 K1 at room temperature. Whereas its cubic counterpart has a whopping 1300 W/mK  thermal conductivity.

As such, hexagonal BN is applied in metamaterials and metadevices while c-BN’s chemical inertness and optical characteristics are leveraged in thermal management industries.

The chart below shows the direct distinction between hexagonal and cubic boron nitride based on particular characteristics:

Conclusion

While hexagonal and cubic boron nitride exhibit a few similarities, each has a unique property that shapes its industrial influence. The core difference being h-BN’s soft yet stable nature and c-BN’s hardness, you want to take out h-BN when in need of a cosmetic additive and c-BN when looking for a resilient abrasive.