The impact of nitrogen content and vacancies on structure and mechanical properties of Mo–N thin films

Based on a combined computational and experimental study, we show that besides the thermodynamically stable β-MoN0.5 and δ2-MoN phases, also metastable γ-MoNx and its ordered relative γ′-MoNx can be synthesized by physical vapor deposition. The formation of the NaCl-based γ-MoNx phase is favored for nitrogen concentrations between 23 and 34 at.% (i.e., x = 0.30–0.53). Higher nitrogen contents (close to the 3:2 stoichiometry, hence, MoN0.67) favor the ordering of the vacancies at the nitrogen sublattice (hence, γ′-MoNx). The highest hardness of ∼33 GPa is obtained for single-phase cubic-structured γ-MoN0.53 coatings, whereas the ordered γ′-MoN0.67 coatings are slightly softer with a hardness of ∼28 GPa.