Decomposition mechanism of Al_1-xSi_xN _y solid solution and possible mechanism of the formation of covalent nanocrystalline AlN/Si₃N₄ nanocomposites

Using a combined ab initio density functional theory (DFT) and thermodynamic modeling, we study the stability of a variety of phases and the possible mechanism of the decomposition of the Al_{1- x}Si_xN_y solid solution and formation of nanocrystalline AlN/Si₃N₄ nanocomposites, which have been experimentally investigated in a number of recent publications. It is shown that the linear and exponential dependence of the interaction parameter on temperature yields reliable results. The hexagonal close-packed (hcp)(ZnS) to hcp(β) phase transition points occur at x ∼ 0.36. The calculated temperature-composition diagrams show that spinodal decomposition mechanism is unlikely in this system because of too small de-mixing energy, which is comparable with the interfacial energy of semi-coherent interfaces. Thus, the decomposition should occur by nucleation and growth, accompanied by a phase transformation from the unstable hcp(ZnS)-SiN to stable hcp(β) or amorphous Si₃N₄, which probably limits the achievable hardness enhancement of the nanocomposites as compared with the nanocrystalline TiN/a-Si₃N₄ ones (where a indicates X-ray amorphous, and the stoichiometry Si₃N₄ symbolizes the fact that Si is fourfold coordinated to nitrogen, as in stoichiometric silicon nitride).