TY - BOOK

T1 - Metallic Thin Film Fatigue Dominated by the Interface Character

AU - Gebhart, David

N1 - no embargo

PY - 1800

Y1 - 1800

N2 - Metallic thin films are integral to a multitude of technological applications, including flexible electronics, photovoltaics, and micro-electromechanical systems (MEMS) where they are frequently subjected to mechanical and thermomechanical stresses. Understanding the fatigue behavior and damage mechanisms of these films, especially the influence of interfaces, is crucial for enhancing their reliability and longevity. This thesis investigates the fatigue properties and mechanical damage evolution in gold thin films on polyimide substrates, both with and without a chromium interlayer, as well as in freestanding Au films. Utilizing in-situ electrical resistance measurements during cyclic loading, we present the first qualitative report on the importance of full width at half maximum (FWHM) analysis in cyclic loading investigations. This new concept provides access to crack opening characteristics, which could not be tracked before, analyzing only resistance maxima and minima within each cycle. An increase in the resistance peak width correlates with a transition from necking to through thickness crack formation, and FWHM values can be used to detect through thickness cracks. We also reveal that electrical resistance data can be correlated to three domains of damage propagation and to grain growth. This method is simple to implement and offers a cost-effective alternative to traditional, time-intensive inspection techniques. A case is made for more extensive use of electrical data, emphasizing its ease of acquisition and the potential for quick in-situ analysis once data processing methods are established. Furthermore, we employ in-situ transmission electron microscopy (TEM) in a cross-sectional specimen to observe dislocation activities and associated fatigue properties at the nanoscale. In the Au/Cr bilayer system, dislocation wall structures inside an ultrafine-grained Au film evolved into what appears to be a geometrically necessary boundary (GNB) parallel to the Cr-Au interface. The GNB acted as an obstacle to dislocation motion, with activated slip systems terminating at its junction, significantly impeding dislocation mobility and enhancing fatigue resistance. These findings suggest that such geometrically necessary structures may play a significant role in damage accumulation in bilayer samples where a strain gradient is present, potentially increasing fatigue life. In freestanding Au films, intermittent TEM and 4D scanning transmission electron microscopy (4D-STEM) reveal substantial out-of-plane grain rotation, grain boundary migration, and grain growth in regions surrounding propagating cracks initiated from a notch. Crack propagation follows a mixture of intergranular and transgranular fracture modes. Orientation mapping demonstrates that deformation localization and crack propagation are linked to the local grain orientation, specifically the misorientation between neighboring grains. Cyclic deformation leads to out-of-plane grain rotation around the crack tip, highlighting this as an important deformation mechanism for freestanding metal thin films. Overall, this work advances the understanding of interface-related damage mechanisms in metallic thin films and underscores the critical role of microstructural characteristics in their fatigue behavior. The findings contribute valuable insights for the design of more reliable thin-film components in flexible electronic devices and other applications where mechanical integrity is crucial. The observed mechanisms, such as the formation of GNBs and the impact of grain orientation on crack propagation, may lead to future tailoring of multilayer systems and microstructures to enhance the fatigue lifetime of thin films.

AB - Metallic thin films are integral to a multitude of technological applications, including flexible electronics, photovoltaics, and micro-electromechanical systems (MEMS) where they are frequently subjected to mechanical and thermomechanical stresses. Understanding the fatigue behavior and damage mechanisms of these films, especially the influence of interfaces, is crucial for enhancing their reliability and longevity. This thesis investigates the fatigue properties and mechanical damage evolution in gold thin films on polyimide substrates, both with and without a chromium interlayer, as well as in freestanding Au films. Utilizing in-situ electrical resistance measurements during cyclic loading, we present the first qualitative report on the importance of full width at half maximum (FWHM) analysis in cyclic loading investigations. This new concept provides access to crack opening characteristics, which could not be tracked before, analyzing only resistance maxima and minima within each cycle. An increase in the resistance peak width correlates with a transition from necking to through thickness crack formation, and FWHM values can be used to detect through thickness cracks. We also reveal that electrical resistance data can be correlated to three domains of damage propagation and to grain growth. This method is simple to implement and offers a cost-effective alternative to traditional, time-intensive inspection techniques. A case is made for more extensive use of electrical data, emphasizing its ease of acquisition and the potential for quick in-situ analysis once data processing methods are established. Furthermore, we employ in-situ transmission electron microscopy (TEM) in a cross-sectional specimen to observe dislocation activities and associated fatigue properties at the nanoscale. In the Au/Cr bilayer system, dislocation wall structures inside an ultrafine-grained Au film evolved into what appears to be a geometrically necessary boundary (GNB) parallel to the Cr-Au interface. The GNB acted as an obstacle to dislocation motion, with activated slip systems terminating at its junction, significantly impeding dislocation mobility and enhancing fatigue resistance. These findings suggest that such geometrically necessary structures may play a significant role in damage accumulation in bilayer samples where a strain gradient is present, potentially increasing fatigue life. In freestanding Au films, intermittent TEM and 4D scanning transmission electron microscopy (4D-STEM) reveal substantial out-of-plane grain rotation, grain boundary migration, and grain growth in regions surrounding propagating cracks initiated from a notch. Crack propagation follows a mixture of intergranular and transgranular fracture modes. Orientation mapping demonstrates that deformation localization and crack propagation are linked to the local grain orientation, specifically the misorientation between neighboring grains. Cyclic deformation leads to out-of-plane grain rotation around the crack tip, highlighting this as an important deformation mechanism for freestanding metal thin films. Overall, this work advances the understanding of interface-related damage mechanisms in metallic thin films and underscores the critical role of microstructural characteristics in their fatigue behavior. The findings contribute valuable insights for the design of more reliable thin-film components in flexible electronic devices and other applications where mechanical integrity is crucial. The observed mechanisms, such as the formation of GNBs and the impact of grain orientation on crack propagation, may lead to future tailoring of multilayer systems and microstructures to enhance the fatigue lifetime of thin films.

KW - thin film

KW - fatigue

KW - data processing

KW - in-situ

KW - cracks

KW - TEM

KW - Dünnschicht

KW - Ermüdung

KW - Datenverarbeitung

KW - in-situ

KW - Risse

KW - TEM

M3 - Doctoral Thesis

ER -