Superlayer Test for Interfacial Fracture Toughness Measurements

Knowledge of the mode-mixity (?) dependent interfacial fracture toughness (Γ) is needed to predict the interface delamination and the component reliability of thin-film structures. Mode-mixity, ?, is a measure of the relative shearing to tensile opening of the interface crack near the tip. Typically, Γ increases as ? increases, such that the delamination is less likely when the loading on the interface is shear-dominated. The measurement of mode-mixity dependent Γ has been a challenge for thin film interfaces. The single-strip superlayer test, developed by the authors, eliminates the shortcomings of current testing methods. This test employs a stress-engineered superlayer to drive the interfacial delamination between the thin-film and the substrate. An innovative aspect of the proposed test is to introduce a release layer of varying width between the interested interfaces to control the amount of energy available for delamination propagation. By designing a decreasing area of the release layer, it is possible to arrest the interfacial delamination at a given location, and the interfacial fracture toughness or critical energy release rate can be found at the location where the delamination ceases to propagate. Design, preparation, and execution of the test are presented. Results are shown for Ti/Si interfaces of different mode mixities.