Question

Cubic boron nitride is used as an abrasive in machining steel as an alternative to diamond, whereas hexagonal boron nitride is frequently used as a lubricant in electronic and high-temperature applications. Explain how these different properties result from different crystal structures and bonding in this material. (Hint: think about the different crystal structures of carbon!)

Answer 1

• Cybic form of boron nitride:

It is the second hardest material in the world after diamond. This is because of its crystal structure very similar to diamond which is made up of carbons. Here as we can see Boron and Nitrogen form alternate bonding giving a tetrahedral structure similar to diamond. This makes the material hard and also chemical inert and hence, it s used for abrasive purposes.

• Heragonal boron nitride

This has a slippery solid like structure similar to graphite!

Here, alternate boron and nitrogen atoms are linked to form hexagonal rings, just like the carbon atoms in graphite.

Since the forces betwen two layers is very weak, they have excellent lubricant properties and also high temperature stability.