Stability and elasticity of metastable solid solutions and superlattices in the MoN-TaN system: First-principles calculations

摘要

In order to develop design rules for novel nitride-based coatings, we investigate trends in thermodynamic, structural, elastic, and electronic properties ofMo1-xTaxNsingle-phase alloys together with (MoN)(1-x)/(TaN)(x) superlattices. Our calculations predict that hexagonal Mo1-xTaxN are the overall most stable ones, followed by the disordered cubic solid solutions and superlattices. The disordered cubic systems are energetically clearly favoured over their ordered counterparts. To explain this unexpected phenomenon, we perform an in-depth structural analysis of bond-lengths and angles, revealing that the disordered phase is structurally between the NaCl-type and the hexagonal NiAs-type modifications. Similarly, the bi-axial coherency stresses in MoN/TaN break the cubic symmetry beyond simple tetragonal distortions, leading to a new tetragonal zeta-phase (P4mm, #129). Both zeta-MoN and zeta-TaN have lower formation energy than their cubic counterparts. Unlike the cubic TaN, the zeta-TaN is also dynamically stable. The hexagonal alloys are predicted to be extremely hard, though, much less ductile than the cubic polymorphs and superlattices. (C) 2018 Elsevier Ltd. All rights reserved.