Carbon Nitride
The structure of carbon nitride (C3N4) was proposed in 1985. This compound, isostructural with Si3N4, was predicted to be harder than diamond. The calculated bond length was 1.47 Å, 5% shorter than the C-C bond length in diamond. Later calculations indicated that the shear modulus is 60% of that of diamond, and carbon nitride is less hard than c-BN.
Despite two decades pursuing this compound, no synthetic sample of C3N4 has validated the hardness predictions, that was attributed to the difficulty in material synthesis and C3N4's instability. Carbon nitride is only stable at a pressure that is higher than that of the graphite-to-diamond transformation. The synthesis conditions would require extremely high pressures because carbon is four- and sixfold coordinated. In addition, C3N4 would pose problems of carbide formation if they were to be used to machine ferrous metals. Although publications have reported preparation of C3N4 at lower pressure than stated, synthetic C3N4 was not proved superhard.
Read more about this topic: Superhard Materials