Phase stabilities of self-organized nc-TiN/a-Si_3N_4 nanocomposites and of Ti_(1-x)Si_xN_y solid solutions studied by ab initio calculation and thermodynamic modeling

摘要

Bulk properties of stable binary fcc-TiN and hcp(β)-Si_3N_4, hypothetical fcc-SiN and hcp(β)-Ti_3N_4, and ternary Ti_(1-x)Si_xN_y 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-Ti_3N_4, and of the interaction parameters of ternary Ti_(1-x)Si_xN_y phases. Based on these data, Gibbs free energy diagrams of the quasi-binary TiN_y-SiN_y system are constructed in order to study the relative phase stability of the metastable ternary fcc- and hcp-Ti_(1-x)Si_xN_y 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 TiN_y-SiN_y system in fcc structure show that metastable fcc-Ti_(1-x)Si_xN coatings should undergo chemically spinodal decomposition into coherent fcc-TiN and fcc-SiN. Due to a high lattice mismatch between fcc-TiN and hcp-Si_3N_4, and to much higher lattice instability of fcc-SiN with respect to stable hcp-Si_3N_4, only about one monolayer of pseudomorphic SiN_y interfacial phase is stable.