Fracture mechanism of metallic film with nano to sub-micron thickness on polycrystalline substrate

Understanding the fracture behaviors of ultrathin metallic films on polycrystalline substrates is essential to preventing failures of extensive coated components. This study investigates the fracture mechanisms of niobium films with nano to sub-micron thickness deposited on stainless steels, serving as a model system. Cracks of the 100-nm films are demonstrated to be primarily induced by out-of-plane dislocation slips in the substrate. However, as the film thickness increases to 500 nm, our findings reveal that the deposited films no longer solely succumb to substrate plasticity. Instead, they influence substrate deformation by forming an interface-affected zone with intense heterogeneous interactions. In this zone, both the accumulation of geometrically necessary dislocations and a transition from dislocation to twinning accommodated plasticity occur. The extensive deformation twinning creates ultrahigh steps, ultimately resulting in film cracking. These in-depth microscopic insights pave the way for advancements in ultrathin coatings and offer valuable knowledge for future research. © 2024 Acta Materialia Inc. Published by Elsevier Ltd.