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 (∼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.