Size Dependent Strengthening of Highly Textured Cu-BN Multilayers

Nanostructured metal/ceramic multilayers via tailoring layer thickness opens a pathway to design high strength and toughness composite materials. In this work, we employed a combination of Berkovich and Spherical nanoindentation to study the in-depth mechanical properties and strain hardening/softening mechanism of highly textured Cu/c-BN multilayers. Berkovich nanoindentation is able to measure the hardness, which is proportional to flow stress (σ_f), while Spherical nanoindentation successfully captures the yield stress (σ_y) and post-yield extrusion behavior. Both yield stress and flow stress increase with decreasing layer thickness (h), and reach their maximum (σ_y = 15 GPa; σ_f = 21 GPa) when h = 5 nm. The multilayers show significant softening when h is less than 5 nm. Microscopic characterization confirms that the swap of deformation mechanisms is responsible for the size-dependent deformation behaviors. Plastic extrusion of Cu layers and brittle fracture of c-BN layers accompanied by dislocation pile-up are dominant when h > 5 nm, while Cu and c-BN layers are co-deformed when h < 5 nm. The findings provide insights into the design of advanced metal/ceramic nanolayered composites with excellent mechanical properties.