Toughness enhancement in TiN/WN superlattice thin films

Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.