Nanoindentation study of amorphous-Co₇₉Zr₁₃Nb₈/Cr multilayers

The mechanical properties of Co₇₉Zr₁₃Nb₈/Cr multilayers were investigated using nanoindentation. The hardness is higher than the average value calculated by rule-of-mixture. The hardness and the resistance to plastic deformation characterized by the ratio of H³/E² vary similarly with periodicity (Λ). They all arrive to the maximum at Λ = 8 nm and decrease subsequently when the Λ increases. The hardness dependence on the Λ is fitted by Hall-Petch relation. The fitted index n is much lower than the normal value (~ 0.5) in many crystalline multilayers. The mutual restriction of shear band and dislocation in amorphous/crystalline structure, which is named structure barrier strengthening, should be main mechanism for the hardness enhancement. The SEM study of indents shows that the shear bands are distorted significantly at the smaller Λ (4 nm) and disappear at the larger Λ (> 20 nm). This morphology variation implies a potential improvement of plasticity caused by the restriction effect of the Cr crystalline layers on the shear bands propagation.