Hardness and fracture toughness enhancement in transition metal diboride multilayer films with structural variations

The simultaneous enhancement of hardness (H) and fracture toughness (K _IC) through the formation of superlattice structures challenges the conventional belief that these quantities are mutually exclusive. Here, this approach has been applied to the transition metal diborides, whose inherent brittleness severely restricts their application potential. The mechanical properties of TiB ₂/TaB ₂ systems as a function of bi-layer period Λ are investigated, combining theoretical and experimental approaches. Density Functional Theory is used to investigate the structural stability and mechanical properties of stoichiometric hexagonal TiB ₂/TaB ₂ superlattices for Λ = 3.9 – 11.9 nm. The calculations predict the highest H = 38 GPa and K _IC (100) of 3.3 MPa.m ^1/2 at the value of Λ = 5.2 nm. Motivated by the theoretical results, multilayer films with Λ = 4–40 nm were prepared by direct current magnetron sputtering. Due to the sputtering effects, the deposited diboride films differ significantly from the view of stoichiometry and structure. A detailed structure investigation reveals TiB ₂/TaB ₂ in form of superlattices exhibiting coherent interfaces for Λ = 4 nm. For higher Λ, parts of TaB ₂ layers transform from the crystalline to the disordered phase. These transformations are reflected in the mechanical properties as measured by nanoindentation and micromechanical bending tests. The evolution of hardness follows Hall-Petch behavior, reaching a maximum of 42 GPa at Λ = 6 nm. Enhancing fracture toughness involves more complex mechanisms resulting in two K _IC maxima: 3.8 MPa.m ^1/2 at Λ = 6 nm and 3.7 MPa.m ^1/2 at Λ = 40 nm.