Enhanced hardness and fracture toughness in diboride superlattice films: Ab initio and experimental study

Abstract

Superlattice architecture presents a promising strategy for the simultaneous enhancement of hardness and toughness in hard ceramic films. Here, we demonstrate the success of this approach in transition metal diboride films—materials whose inherent brittleness typically limits their applications. We combined Density Functional Theory (DFT)-based calculations and direct current magnetron sputtering to investigate the mechanical properties as a function of the bilayer period (Λ). Theoretical calculations for ZrB₂/TaB₂ cells (Λ = 1.4–8.2 nm) reveal a stabilizing effect with decreasing Λ and a significant increase in stiffness, peaking at Λ = 2.7 nm. Experimentally, ZrB₂.₆/TaB₁.₄ superlattice films deposited with Λ = 1.8–31.5 nm exhibit a structural transition characterized by the crystallization of disordered TaB₂₋y layers. This transition is accompanied by a remarkable increase in hardness from H = 34.1 ± 1.9 to 47.2 ± 2.3 GPa as Λ decreases to 3.4 nm. The hardening exceeds the estimations of Koehler's strengthening, suggesting multiple contributing effects: chemical bonding, boron diffusion at interfaces, Hall-Petch behavior, and compressive residual stress. At the same time, the fracture toughness increases to KIC = 4.6 ± 0.3 MPa·m¹/² at Λ = 1.8 nm, attributed to coherent stresses at the ZrB₂₊ₓ/TaB₂₋y interfaces. This research demonstrates the effectiveness of superlattice architectures in diboride films and highlights the crucial role of nanostructure and stoichiometry.