Competing mechanisms in fracture of amorphous films resting on ductile substrates

The architecture consisting of a thin film resting on a substrate is a common feature in a wide variety of applications. Understanding of the failure mechanisms is essential for design of reliable film/substrate systems. Recent experiments showed that shear banding induced fracture and brittle cracking of films are two major fracture modes in the structure composed of an amorphous metallic glass film bonded to a metal substrate. In this study, we carry out numerical analyses of fracture in amorphous films resting on ductile substrates, with the aim to unveil the mechanisms governing the competition between the two fracture modes observed in the experiments. In the calculations, the brittle cracking of amorphous films is modeled using the cohesive zone fra-mework and the shear banding induced fracture in films is characterized by a simple local fracture criterion. The results show that brittle cracking of amorphous films occurs in the case of thin films, while the shear banding induced fracture dominates in the case of thick films. For the films with intermediate thickness, the mixed fracture modes are observed. Such a film thickness effect predicted by numerical simulations is consistent with the experimental observations. We further reveal that the film thickness effect is related to substrate constraint. In addition, it is identified that the cohesive strength and toughness of amorphous films also play an important role in controlling the fracture modes of the film/substrate systems.