Metallic thin film fatigue dominated by interface character

Microelectronics, micro-electrical mechanical systems (MEMS), thermal barrier coatings and corrosion protection are comprised of thin films on substrates subjected to cyclic loads. Repeated loads induce different mechanisms which lead to failure of the device or coating. Failure of thin films include plastic deformation, extrusion formation, cracking or delamination. A dominating parameter that controls how the thin film fails which has not been examined is the type of interface which forms between the film and substrate. The type of interface can be characterized by the substrate. For example, a film deposited on a rigid ceramic or metal substrate creates a hard interface and on a compliant polymer substrate generates a soft interface. A third type of interface is the lack of an interface, or a free-standing film. In order to study the failure of the various thin films and interfaces cyclic loading is performed with advanced in-situ techniques that allow the observation of failure. The use of sophisticated micro-mechanical testing methods including bulge testing, X-ray diffraction and transmission electron microscopy allow for mechanisms such as grain growth, dislocation pile-up to form extrusions, or cracking to be observed first hand. With a thorough and systematic investigation of interface dominated fatigue, interface specific failure mechanisms will be identified that lead to failure. The new knowledge about failure mechanisms will be used to generate mechanism based models for thin film failure as well as provide improved design criteria for fatigue resistant thin film applications.

Microelectronics, micro-electromechanical systems, thermal barrier coatings and corrosion protection are comprised of thin films on substrates subjected to cyclic loads. Repeated loads induce different mechanisms which lead to failure of the device or coating. Failure of thin films includes plastic deformation, extrusion formation, cracking or delamination. A dominating parameter that controls how the thin film fails has been examined during the project is the type of interface which forms between the film and substrate (none, soft, hard). To study the role of the interface on the failure, cyclic loading was performed with advanced in-situ techniques that allowed the observation of failure. Sophisticated micro-mechanical testing methods including bulge testing, in-situ electrical resistance and transmission electron microscopy were used to observe grain growth, dislocation motion, and cracking. The team developed a new method to watch deformation processes in free-standing films (no interface) with a combination of bulge testing and transmission electron microscopy. The soft and hard interfaces (Au-polyimide, Au-Cr-Polyimide) were studied with electrical resistance. Using only the electrical signal, cracking in the films can be detected. The new knowledge gained about failure mechanisms will be used to provide improved design criteria for fatigue resistant thin film applications.