Nanomechanical characterization of the fracture toughness of Al/SiC nanolaminates

Abstract

The fracture toughness of Al/SiC nanolaminates with different layer thicknesses (in the range 10 to 100 nm) was measured by means of micropillar splitting and bending of a notched beam. The crack plane was perpendicular to the layers in the former while notched beams with the notch parallel and perpendicular to the layers were milled in the latter. It was found that crack propagation parallel to the layers took place along the metal–ceramic interfaces and the toughness increased with the layer thickness due to the contribution of the plastic deformation of the Al layers. Crack propagation perpendicular to the layers showed evidence of crack deflection/arrest at the interface. The toughness in this orientation increased as the layer thickness decreased due to the higher density of interfaces except for the nanolaminates with 10 nm layer thickness. In this latter case, crack propagation took place along the weak columnar grain boundaries, leading to a marked reduction in toughness.

Introduction

Nanolaminates made up of alternating metallic and ceramic nanoscale thick layers are promising materials in numerous engineering applications, due to their outstanding mechanical properties including high strength and wear resistance [1], [2], [3], [4], [5], [6], as well as unique electrical and optical properties [7], [8]. It has been argued that the fracture toughness of metal–ceramics nanolaminates can be higher than that of conventional metal–ceramic composites due to several reasons. Firstly, cracking of the brittle ceramic layers can be delayed due their nanoscale dimensions, which limit the size of the pre-existing flaws that initiate fracture. Secondly, the energy dissipated by the plastic deformation of metallic layers can arrest cracks at the metal–ceramic interfaces. And finally, the large number of interfaces associated with the nanoscale dimensions of the layers can introduce substantial energy dissipation through crack deflection mechanisms [9].

Nevertheless, there is very little reliable information on the fracture toughness of nanolaminates because most of them are fabricated in the form of thin-films or coatings, and appropriate techniques to measure the fracture toughness of coatings are still lacking [10], [11], [12]. Several strategies have been proposed to measure fracture toughness at the microscale, most of them based on bending of cantilevers of different geometries, such as Chevron notch cantilevers [13], [14], clamped beams [15] or double cantilever beams [16], [17], [18]. These strategies are very time consuming because they require the fabrication of the beams by focused ion beam milling and ion-induced damage at the root of the pre-notch may introduce additional artifacts [19]. Another approach is based on the micropillar splitting method, developed by Sebastiani et al. [20], [21], which does not require the introduction of pre-notches. Instead, the cracks are introduced directly by a sharp pyramidal indenter and propagate in mode I.

In this investigation, both micropillar splitting and bending of notched cantilevers were used to determine the fracture toughness of Al/SiC nanolaminates with different layer thicknesses (in the range 10 nm to 100 nm).