Effect of layer thickness on the mechanical behaviour of oxidation-strengthened Zr/Nb nanoscale multilayers

The effect of bilayer thickness (L) reduction on the oxidation-induced strengthening of Zr/Nb nanoscale metallic multilayers (NMM) is investigated. Zr/Nb NMMs with L = 10 and 75 nm were annealed at 350 A degrees C for a time ranging between 2 and 336 h, and the changes in structure and deformation behaviour were studied by nanoscale mechanical testing and analytical electron microscopy. Annealing led to the transformation of the Zr layers into ZrO2 after a few hours, while the Nb layers oxidised progressively at a much slower rate. The sequential oxidation of Zr and Nb layers was found to be key for the oxidation to take place without rupture of the multilayered structure and without coating spallation in all cases. However, the multilayers with the smallest bilayer thickness (L = 10 nm) presented superior damage tolerance and therefore structural integrity during the oxidation process, while for L = 75 nm the volumetric expansion associated with oxidation led to the formation of cracks at the interfaces and within the ZrO2 layers. As a result, the nanoindentation hardness increase after annealing was significantly higher for the nanolaminate with L = 10 nm. Comparison between nanoindentation and micropillar compression behaviour of the oxidised NMMs demonstrates that the hardness increase upon oxidation arises from the contribution of the residual stresses associated with the volume increase due to oxidation and to the higher strength of the oxides.