FRACTURE BEHAVIOR OF METALLIC THIN FILMS AS EVALUATED BY BULGE-TESTS AND IN SITU TEM DEFORMATION EXPERIMENTS

摘要

Metallic thin films generally show a fracture toughness which is considerably lower than that of bulk samples. Although this has been evidenced by several groups, a conclusive understanding of this low fracture toughness is still missing and open questions related with the difficulty of reliably testing very thin films often remain. Bulge testing is a very suitable method allowing reliable investigations of the fracture toughness of thin films by introducing a slit in a freestanding membrane by focused ion beam (FIB) milling. With such tests the fracture toughness of silver and gold films in the thickness range of 100 nm have been determined to be around 2 MPa m~(1/2) confirming earlier results obtained with other testing techniques on similar metallic thin films. Recent investigations by Preiss et al. [1] gave an explanation for this extremely low fracture toughness based on in-situ observations of the crack tip region by atomic force microscopy (AFM). The AFM scans show stable crack growth mainly along grain boundaries and sliding of grains. Plastic deformation is localized in a very narrow corridor in front of the crack tip and a large plastic zone, as one would typically expect under plane stress, is not observed. We conclude that the spatial confinement of the plastic deformation is the primary reason for the low fracture toughness of metallic thin films.