Computational and experimental studies on structure and mechanical properties of Mo–Al–N

Ab initio calculations show that with increasing N-vacancy content of Mo 1- x Al x N y solid solutions, the cubic structure is increasingly preferred over the wurtzite-type hexagonal structure. While Mo 1- x Al x N solid solutions, without N-vacancies, energetically favor the wurtzite-type structure over the whole composition range, Mo 1- x Al x N 0.5(1+ x ) and Mo 1- x Al x N 0.5 solid solutions energetically prefer the cubic structure up to ∼45 and 65 at.% Al at the metal sublattice. Detailed ab initio calculations in combination with detailed elemental and phase composition analyses and nanoindentation experiments of reactively sputtered Mo 1- x Al x N y coatings prove the formation of face-centered cubic structures for Al-contents x ≤ 0.57. These Mo 1- x Al x N y coatings exhibit an Al-dependent population of the nitrogen sublattice, following the MoN 0.5 –AlN quasi-binary tie line. For Al-contents x ≥ 0.79 the coatings crystallize in the wurtzite-type hexagonal phase, while in the intermediate composition range both phases, cubic and wurtzite-type hexagonal, coexist. As long as the cubic structure is maintained, the hardness increases from ∼33.0 to 38.4 GPa with increasing Al-content, but drops to ∼22 GPa for x ≥ 0.67, when the films contain hexagonal wurtzite-type phases.