Tailoring nanostructured Cu/Cr multilayer films with enhanced hardness and tunable modulus

Length-scale dependent hardness (H) and indentation modulus (E) are systematically investigated in nanostructured Cu/Cr multilayer films with a wide modulation period (λ) range from 5 nm to 250 nm. H is gradually increased with reducing λ down to ∼50 nm, whereafter a λ-independent effect is shown. The theoretical analysis of confined dislocation gliding and interface barrier strength quantitatively assess the variation of H. The indentation modulus E, however, exhibits a non-monotonic evolution with λ and attains a maximum at the same critical λ of ∼50 nm. This unusual variation in E is related to the formation of interfacial amorphous layers, and can be reasonably explained in terms of a competition between the enhancement effect induced by compressed out-of-plane interplanar spacing of the constituent layers and the reduction effect associated with free volume in the amorphous intermixing layer. This result offers great benefits in engineering ductile nanolaminates with high strength and low modulus by tailoring interfacial structure and/or introducing amorphous layers.