Mechanistic study of superlattice-enabled high toughness and hardness in MoN/TaN coatings

Abstract

Machining and forming tools exposed to challenging environments require protective coatings to extend their lifetime and reliability. Although transition metal nitrides possess excellent strength and resistance against chemical attacks, they lack ductility and are prone to premature failure. Here, by investigating structural and mechanical properties of MoN-TaN superlattices with different bilayer thickness, we develop coatings with high fracture toughness and hardness, stemming from the formation of a metastable tetragonally distorted phase of TaN up to layer thicknesses of 2.5 nm. Density functional theory calculations and experimental results further reveal a metal-vacancy stabilized cubic Ta0.75N phase with an increased Young’s modulus but significantly lower fracture toughness. We further discuss the influence of coherency strains on the fracture properties of superlattice thin films. The close interplay between our experimental and ab initio data demonstrates the impact of phase formation and stabilization on the mechanical properties of MoN-TaN superlattices.