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

Abstract

Ab initio calculations show that with increasing N-vacancy content of Mo1-xAlxNy solid solutions, the cubic structure is increasingly preferred over the wurtzite-type hexagonal structure. While Mo1-xAlxN solid solutions, without N-vacancies, energetically favor the wurtzite-type structure over the whole composition range, Mo1-xAlxN0.5(1+x) and Mo1-xAlxN0.5 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 Mo1-xAlxNy coatings prove the formation of face-centered cubic structures for Al-contents x <= 0.57. These Mo1-xAlxNy coatings exhibit an Al-dependent population of the nitrogen sublattice, following the MoN0.5eAlN 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.