Lyakhov-Oganov approach considers topology of the crystal structure, strength of covalent bonding, degree of ionicity and directionality. The fracture toughness (K Ic ) was evaluated using Mazhnik-Oganov model which is the only model that allows such predictions. The results on mechanical properties of the considered phases of the Si-C-N system are presented in Table . It is worth noting that hardness and bulk moduli (B 0 ) calculated in the framework of the thermodynamic model are in perfect agreement with available experimental data . As follows from our findings, Vickers hardness of Si 2 C 2 N 2 is close to that of cubic silicon carbide, a conventional superabrasive, whereas hardness of tetragonal Si 2 C 4 is slightly lower.