Metallic thin film fatigue dominated by interface character

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Thin films are used in a wide variety of applications due to the unique properties which are imparted at the micro- and nanoscale. Metal films, for example, are ideal for electrical conduction in rigid and flexible electronics and sensors, reflectors for surface mirrors on spacecraft or micro-electrical-mechanical systems (MEMS). In most applications, thin films are cyclically loaded and fail through specific fatigue mechanisms. However, sub-micrometric thin metal films behave, and thus fail, differently than bulk materials. This has been attributed to the prominent presence of an interface to a substrate and to the free surface, which strongly affects the dislocation processes pertaining to bulk fatigue. What has not yet been examined is how the type of interface controls the fatigue behavior of thin films. Interfaces can be considered as hard, created with a rigid ceramic or metal substrate, soft, next to a polymer substrate, or films can have no interface and be free-standing. It is believed that the interface type is a dominant parameter that controls the deformation mechanisms and final failure of the thin metal films as a function of the microstructure, yield strength and cyclic stress amplitude.In order to obtain deformation and failure information on thin films and their specific interface, a thorough and systematic investigation is required. It is planned to use advanced in-situ micro-mechanical testing methods to examine the role of the interface type, hard, soft or no interface, on the damage formation and failure of thin metal films. Of note is the use of sophisticated in-situ bulge testing, X-ray diffraction techniques and transmission electron microscopy with cyclic mechanical testing to observe film deformation, possible grain growth, extrusion formation, film cracking or delamination. Direct observations will enable the decoupling of the microstructural response from the interface induced mechanisms. These advanced techniques and the expertise on thin film mechanical behavior is the foundation of the proposed collaboration between the Friedrich-Alexander University Erlangen-Nürnberg (Dr. Benoit Merle) and the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences (Dr. Megan Cordill). The unique synergies between the groups will notably allow the first systematic comparison of the fatigue behavior of similar samples in free-standing and in different supported conditions. The new knowledge about the interface specific 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.