Phase stabilities and spinodal decomposition in the Cr1−xAlxN system studied by ab initio LDA and thermodynamic modeling: Comparison with the Ti1−xAlxN and TiN/Si3N4 systems

The total energies and lattice constants of binary hexagonal close-packed (hcp)- and face-centered cubic (fcc)-CrN, AlN and ternary Cr0.5Al0.5N phases are calculated using the Vienna Ab-initio Simulation Program. The calculated total energies of the structures are then used to calculate the lattice stabilities of binary hcp- and fcc-CrN and AlN, and the interaction parameters of the ternary hcp- and fcc-Cr1−xAlxN solution phases. These results are used in the sublattice thermodynamic model to construct the Gibbs free energy diagram of the immiscible quasi-binary CrN–AlN system at different temperatures. Based on these results, we discuss the relative phase stability of the metastable ternary hcp- and fcc-Cr1−xAlxN solid solutions over the entire range of compositions. The predictions are compared with and supported by the published results from physical and chemical vapor deposition experiments. The constructed Gibbs free energy diagrams show that metastable fcc-Cr1−xAlxN coatings may undergo spinodal decomposition into coherent fcc-CrN and fcc-AlN, but there is a relatively large barrier for a direct formation of the stable hcp-AlN. A comparison with the Ti1−xAlxN and TiN–Si3N4 systems shows that the phase segregation will be more difficult and, therefore, the solid solution more stable in the Cr1−xAlxN case.