On the spinodal nature of the phase segregation and formation of stable nanostructure in the Ti–Si–N system

The free energy of the mixed Ti–Si–N system is calculated according to a semi-empirical thermodynamic formula based on the sub-lattice model for different activities of nitrogen. The results show that the phase segregation in the ternary Ti–Si–N systems is of chemically spinodal nature at a nitrogen pressure and deposition temperature typically used for the deposition of the superhard nc-TiN/a-Si3N4 nanocomposites, i.e. pN2≥10−3pN2≥10−3 mbar and Tdep 550–600 °C, respectively. Only at much lower nitrogen pressure and much higher temperature, can the chemically spinodal decomposition be restrained. A simple estimate of the interfacial strain energy for a semi-coherent interface, as expected for this system, shows that this contribution is unlikely to hinder the system being also coherently spinodal. It is further shown that kinetic constraints, such as Tdep ≤ 300 °C and low pN2pN2, although within the range where the stoichiometric nitrides should be fully spinodally segregated, will kinetically hinder the system to reach the thermodynamically driven equilibrium.