Metallic thin film fatigue dominated by interface character
Final Report Abstract
Submicrometric metallic thin films are critical for a wide range of technical applications. They are either coated onto substrates, e.g. in integrated circuits and flexible electronics, or used in freestanding condition, e.g. in microsensors and micro-electrical-mechanical systems (MEMS). In most applications, thin films are cyclically loaded and fail through specific fatigue mechanisms. However, sub-micrometric thin metallic films behave, and thus fail, differently than bulk materials. This is generally thought to arise from the prominent presence of an interface to a substrate and to the free surface, which strongly affects the dislocation processes pertaining to bulk fatigue. In this project, we investigated the influence of different types of substrates (none, soft, hard) on the fatigue properties of model metallic thin films, and identified the mechanisms responsible for specific failure modes. We used a combination of bulge testing, electrical resistance monitoring, X-ray diffraction beamline experiments, transmission electron microscopy with cyclic mechanical testing to investigate film deformation, grain growth, extrusion formation, film cracking and delamination. In freestanding films, the high cycle fatigue was found to be governed by both dislocation glide and diffusion creep, with a relation between them varying with temperature and stress level. Approaching low cycle fatigue, the films exhibited mechanisms strikingly similar to polyimide supported films. From the experiments on supported films, the initiation of through-thickness cracks was found to be central to the failure of the films. In case of a hard interface, it was shown to be a result of a complex interplay of strain localization in both film and substrate. Despite lab access restrictions during the Covid19 pandemic, the cooperation between the project partners allowed for significant advancements in the field of characterization methods. We pioneered novel in situ approaches for the nanomechanical cyclic characterization of thin films, which will benefit future research endeavors.
Publications
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Linking through-thickness cracks in metallic thin films to in-situ electrical resistance peak broadening. Scripta Materialia, 212, 114550.
Gebhart, David D.; Krapf, Anna; Gammer, Christoph; Merle, Benoit & Cordill, Megan J.
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Creep-dominated fatigue of freestanding gold thin films studied by bulge testing. Materials Science and Engineering: A, 887, 145759.
Krapf, Anna; Gebhart, David D.; Gammer, Christoph; Cordill, Megan J. & Merle, Benoit
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Describing mechanical damage evolution through in situ electrical resistance measurements. Journal of Vacuum Science & Technology A, 41(2).
Gebhart, David D.; Krapf, Anna; Merle, Benoit; Gammer, Christoph & Cordill, Megan J.
