Four modern theoretical models have been used to predict (H V ). It has been previously reported that the thermodynamic (T) model , which is based on thermodynamic properties and crystal structure, shows surprising agreement with available experimental data and is therefore recommended for hardness evaluation of superhard and ultrahard phases . The Lyakhov-Oganov (LO) model takes into account the topology of the crystal structure, the strength of covalent bonding, the degree of ionicity and directionality; however, in the case of ultrahard phases of light elements, this model gives underestimated hardness values . Empirical models, Mazhnik-Oganov (MO) and Chen-Niu (CN) , are based on elastic properties, namely bulk and shear moduli. Fracture toughness (K Ic ) was evaluated using the Mazhnik-Oganov model .