Understanding the extremely low fracture toughness of freestanding gold thin films by in-situ bulge testing in an AFM

The fracture toughness of freestanding gold films with thicknesses between 60 nm and 320 nm was determined by bulge testing to be around 2 MPa m(1/2). This surprisingly low value confirms the trend also observed for other metals that thin films exhibit only a fraction of the bulk fracture toughness. In order to understand this behavior, the fracture process of freestanding gold films with a crack introduced by focused ion beam (FIB) milling was observed in-situ in an atomic force microscope (AFM). AFM scans of the crack tip region show stable crack growth mainly along grain boundaries. Plastic deformation is localized in a narrow corridor in front of the crack tip. A large plastic zone, as one would typically expect under plane stress, is not observed. Instead, strong local necking is evidenced. We conclude that the spatial confinement of the plastic deformation is the primary reason for the low fracture toughness of metallic thin films.