Multiple cracking of amorphous films on ductile substrates

The film–substrate architectures are a promising design for improving tensile ductility of metallic glasses. Recent experiments have shown that cracking of amorphous metallic glass films is the major fracture mode of the film–substrate systems subjected to tensile loading. In this study, calculations are carried out for multiple cracking of an amorphous metallic glass film bonded to a ductile substrate. The pressure sensitive yielding, plastic dilatancy and post-yield strain softening caused by evolution of free-volume of the amorphous film are accounted for; furthermore, the competition between brittle cleavage and shear band induced cracking in the amorphous film is considered in the numerical analyses. It is found that for thin films, the multiple cracking behavior is associated with development of shear bands, and small crack spacing involves interaction of multiple shear bands, which can further activate the mechanism of substrate confinement and leads to large critical strain for film cracking. The multiple cracking in thick amorphous films is governed by brittle cleavage mechanism, which suppresses plastic deformation in the film and gives rise to small critical strain for film cracking. For the amorphous film with intermediate thickness, the shear band induced cracking dominates in the case of small crack spacing, while brittle cleavage mechanism is responsible for the multiple cracking of amorphous films with large crack spacing. The findings of this study shed new lights on the design of reliable film–substrate systems.