Resolving the hardness–toughness trade-off dilemma of metal/ceramic multilayer films by introducing gradient structure

Equal-period modulated metal/ceramic multilayers have shown promise in enhancing the toughness of ceramic thin films. However, this toughness enhancement typically comes at the sacrifice of hardness, limiting their potential applications. To tackle this issue, this study designed and fabricated two gradient-structured multilayer variations using Ta/TaB₂: one with a higher ceramic layer fraction near the surface (M2) and the other with a converse structure (M3). A conventional equal modulation period Ta/TaB₂ multilayer film (M1) served as a reference. M2 exhibited superior performance, with a 30% hardness increase and significant toughness enhancement compared to M1. Conversely, M3 experienced failure due to excessive thermal stress from its unique gradient structure. Finite element simulations revealed that M2's structure could alleviate in-plane stress and enhance loading uniformity, thus enhancing the film's toughness. These findings suggest that a well-designed gradient structure holds promise for concurrently improving the hardness and toughness of metal/ceramic multilayer films.