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 Ta 0.75 N 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.

Atomistic Mechanisms Underlying Plasticity and Crack Growth in Ceramics: A Case Study of Aln/Tin Superlattices

“…While the SL-induced hardness enhancement had already been reported for e.g. TiN/VN [8] or TiN/NbN SLs [9] and explained by hindered dislocation glide within/across interfaces [9,10] , Hahn's findings attracted attention to the SL-induced toughening [5,[11][12][13] . To date, however, atomic-level understanding of toughening effects is lacking.…”

“…Slip-induced softening is typically attributed to SLs with "too low" (easy glide across in- 2) , plotted on a log 10 scale as a function of the bilayer period. The dashed lines (inspired by measured trends of hardness [8,9,67] and toughness [5,7,11] vs. in nitride SLs) serve as guide to the eye. terfaces) or "too high" (easy glide within layers) bilayer periods [9,10] .…”

“…In particular, tension across a flawed region is a primary cause for crack growth and decohesion. Employing the concept of superlattice (SL) architecture [1][2][3][4][5][6] , Hahn et al . [7] showed that superior strength and fracture resistance do not have to be mutually exclusive.…”

Characteristics of the Structure, Mechanical, and Tribological Properties of a Mo-Mo2N Nanocomposite Coating Deposited on the Ti6Al4V Alloy by Magnetron Sputtering

“…Among the PVD techniques, cathodic and magnetron sputtering are often used to coat metal surfaces. Among the coatings deposited on the metallic substrates, the ones consisting of titanium, chromium, zirconium, molybdenum, and tantalum nitrides (characterized by high hardness, high tribological wear resistance, and good adhesion to the substrate) are widely used [6][7][8][9]. The addition of aluminum preserves the hardness at elevated temperatures [10].…”

“…Hard monolithic coatings have low fracture resistance, leading to premature failure. The most successful strategies to improve the fracture resistance of ceramics are based on the production of multilayer [9,26,27] and nanocomposite coatings [28,29]. The H 3 /E 2 ratio is an important parameter that allows the classification of coatings, in terms of resistance to plastic deformation.…”