Heavy-element-alloying for toughness enhancement of hard nitrides on the example Ti-W-N

The low intrinsic fracture toughness of transition metal nitride thin films critically restrains their applicability as protective coatings. We therefore investigate the Ti1-xWxNy system to provide detailed theoretical and experimental insight into simultaneous hardening and toughening effects induced by heavy-element-alloying via an enhanced metallic bonding character. The combination of structural and chemical analyses - supported by density functional theory (DFT) calculations - demonstrates that the addition of W progressively increases the concentration of nitrogen vacancies in rocksalt (rs) structured Ti1-xWxNy. With increasing W content, the hardness H initially increases from 25.4 +/- 0.5 GPa (for TiN) to 31.1 +/- 0.8 GPa (for Ti0.55W0.45Ny) and then slightly decreases to 30.4 +/- 0.5 GPa (for Ti0.42W0.58Ny) - beautifully following classical solid solution hardening principles. Cube corner indentations yield a continuous increase in resistance against crack propagation and formation with increasing W content. The highest W containing coating studied here, Ti0.42W0.58Ny, even yields no radial crack formation but pile-up formation at the corners of the imprint - being an unambiguous sign for plastic flow. Although Ti0.62W0.38Ny exhibits the same growth morphology and columnar grain size ( similar to 10 nm wide and 100 nm long) as Ti0.42W0.58Ny - with a similar hardness of 31.0 +/- 0.6 GPa - this coating still exhibits (short) radial cracks (without pile-up formation). DFT-calculated charge density maps suggest that the superior toughness-related performance of Ti1-xWxNy (with respect to TiN, which showed a pronounced radial crack formation) is linked to a metallisation of the interatomic bonds, being most pronounced for balanced W and Ti contents and N vacancies. (C) 2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

The low intrinsic fracture toughness of transition metal nitride thin films restricts their application as protective coatings. In this study, we investigate the Ti1-xWxNy system and provide detailed theoretical and experimental insights into the simultaneous hardening and toughening effects induced by heavy-element-alloying via enhanced metallic bonding.