A new approach for bimaterial interface fracture toughness evaluation

A material configuration of central importance in composite materials or in protective coating technology is a thin film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficient of thermal expansion, chemical reactions, or other physical effects. Therefore, in general, the weakest link in this composite system often resides at the interface between the thin film and the substrate. In order to make multi-layered electronic devices and structural composites with long-term reliability, the fracture behavior of the material interfaces must be known. This project offers an innovative testing procedure of using a spiral notch torsion bar method for the determination of interface fracture toughness that is applicable to thin coating materials in general. The feasibility study indicated that this approach for studying thin film interface fracture is repeatable and reliable, and the demonstrated test method closely adheres to and is consistent with classical fracture mechanics theory.