Study of spinodal decomposition and formation of nc-Al₂O ₃/ZrO₂ nanocomposites by combined ab initio density functional theory and thermodynamic modeling

Using ab initio density functional theory, the equilibrium properties, such as the total energy, the molar volume, the bulk modulus and its first derivative, as well as the formation enthalpy of monoclinic ZrO₂ and hexagonal α-Al₂O₃ phases, were calculated and compared with the published theoretical and experimental data. Based on the good agreement of these data, we calculated the lattice instabilities of hypothetical binary hexagonal Zr₂O₃ and monoclinic AlO₂, and the interaction parameters of ternary Zr_{1- x}Al_xO_y solid solutions. The binodal and spinodal diagrams were then constructed to predict the possibility of the formation of oxide-based nanocomposites which may display hardness enhancement above that of the solid solutions. It is shown that exponential dependence of the interaction parameter on temperature yields the most reliable results. The system should undergo spinodal phase segregation within the composition range that is relevant for the formation of hard or superhard nanocomposites with high thermal and oxidation stability, which are important for their applications.