Effects of layer thickness on the mechanical behavior of oxidation-strengthened Zr/Nb nanoscale multilayers

TL;DR

This study investigates how layer thickness influences the mechanical properties and oxidation behavior of Zr/Nb nanoscale multilayers, revealing that thinner layers exhibit higher strength and toughness due to suppressed damage mechanisms.

Contribution

It provides new insights into the effect of layer thickness on oxidation-induced damage and mechanical performance of Zr/Nb multilayers, highlighting the benefits of nanoscale layering.

Findings

Thinner layers (L=10 nm) show higher strength and toughness.

Oxidation causes volumetric expansion leading to cracks in thicker layers.

Residual stresses from oxidation increase hardness after annealing.

Abstract

The mechanical behaviour and deformation mechanisms of magnetron-sputtered Zr/Nb nanoscale multilayers were analysed as a function of the periodicity (L = 10 and 75 nm) and annealing time at 350C (2-336 hrs). The strength of the as-deposited multilayers was independent of the layer thickness and it was controlled by the co-deformation of the Zr and Nb layers. Annealing led to transformation of the Zr layers into ZrO2 after a few hours, while the Nb layers oxidised at a much slower rate. The volumetric expansion associated with the oxidation led to the formation of cracks at the interfaces and within the ZrO2 layers for the multilayers with L = 75 nm but not in the case of L = 10 nm. The nanoindentation hardness increased significantly after annealing due to the contribution of the residual stresses associated with the volume increase due to oxidation and to the higher strength of the oxides. The strength increase after annealing, as measured by micropillar compression tests, was smaller than that measured by nanoindentation, as it did not include the contribution of the residual stresses, which were relieved during micropillar fabrication. The nanolaminate with L = 10 nm presented the highest strength and toughness as damage during oxidation was suppressed, and a deformation mechanism controlled by the formation of shear bands.