Bulk properties of stable binary fcc-TiN and hcP(beta)-Si3N4, hypothetical fcc-SiN and hcp(beta)-Ti3N4, and ternary Ti1-xSixNy phases are calculated by ab initio method. The values of total energies are then used for thermodynamic calculations of the lattice instabilities of hypothetical binary phases of fcc-SiN and hcp-Ti3N4, and of the interaction parameters of ternary Ti1-xSixNy phases. Based on these data, Gibbs free energy diagrams of the quasi-binary TiNy-SiNy system are constructed in order to study the relative phase stability of the metastable ternary fcc- and hcp-Ti1-xSixNy phases over the entire range of compositions. The results are supported by the published data from chemical and physical vapor deposition experiments. The constructed Gibbs free energy diagram and phase selection diagram of quasi-binary TiNy-SiNy system in fcc structure show that metastable fcc-Ti1-xSixN coatings should undergo chemically spinodal decomposition into coherent fcc-TiN and fcc-SiN. Due to a high lattice mismatch between fcc-TiN and hcp-Si3N4, and to much higher lattice instability of fcc-SiN with respect to stable hcp-Si3N4, only about one monolayer of pseudomorphic SiNy interfacial phase is stable. (C) 2007 Elsevier B.V. All rights reserved.