Ruth Schwaiger - Academia.edu (original) (raw)
Papers by Ruth Schwaiger
Extreme Mechanics Letters, 2016
The damage generated by AC currents at 100 Hz in interconnects has been studied and compared with... more The damage generated by AC currents at 100 Hz in interconnects has been studied and compared with mechanical fatigue damage in thin films. The nature of the damage under the two loading conditions is qualitatively similar, supporting the idea that the AC current damage comes from mechanical cycling due to temperature swings on the order of 100 K from Joule heating in the interconnects. In both cases, the damage forms as surface wrinkles within single grains grow in amplitude and extent with time. The possible threat to the reliability of microelectronic and microelectromechanical systems is further escalated by the observation that soft encapsulation layers do nothing to retard the formation of the damage. CP612, Stress-Induced Phenomena in Metallization: Sixth International Workshop, edited by S. P. Baker, M. A. Korhonen, E. Arzt, and P. S. Ho, pp. 119-132.
Bulk Nanostructured Materials, 2009
Extreme Mechanics Letters, 2015
Journal of Materials Science, 2015
ABSTRACT In this study, we evaluate the evolution of the interfacial processes in metallic slidin... more ABSTRACT In this study, we evaluate the evolution of the interfacial processes in metallic sliding contacts (i.e., aluminum alloys) in terms of their elemental composition, structural changes, and nanomechanical properties in order to understand the optimal running-in behavior leading to steady-state low friction and high wear resistance. Two different sliding conditions are used, resulting in low and high long-term friction and corresponding well with the low and high wear rates. Ex situ elemental analysis of these sliding experiments was performed by means of X-ray photoelectron spectroscopy. The mechanical properties were evaluated using nanoindentation and microcompression testing. While the elemental analysis revealed an increased oxide content for the near-surface region of the worn surfaces compared to the unworn material, the oxide content was higher for the experiments that resulted in an unfavorable tribological response (i.e., high friction and high wear). Similarly, the sub-surface grain-refined layer under these conditions was thicker compared to the experiment with a short running-in stage and low steady-state friction and wear. These observations correlated well with the nanoindentation and microcompression results, which show higher hardness and yield stress for the high friction and wear experiment. Correspondingly, low steady-state friction and wear were obtained with the formation of a thin and mechanically stable tribolayer.
Scripta Materialia, 2015
ABSTRACT Nanoscale metallic multilayers have long been known for their high strength and hardness... more ABSTRACT Nanoscale metallic multilayers have long been known for their high strength and hardness [1], [2] and [3] with layer thickness and interface structure as the controlling factors [3], [4] and [5]. The importance of metallic multilayers in the context of fatigue [6], [7] and [8] and wear [9] and [10] has also been demonstrated. Both the fatigue strength [7] and [8] and wear resistance [9] were observed to increase with decreasing layer thickness. Nanoscale Cu–Ni multilayer coatings very effectively suppressed fatigue crack initiation and showed evidence of reduced accumulated plasticity compared to monolithic coatings [6]. Recently it was suggested that a nanolayered structure that formed during wear testing of Cu90Ag10 alloys – in combination with Ag acting as a solid lubricant – was responsible for reduced wear rates [11]. Considering the high thermal stability of Cu–Nb multilayers [12] and their resistance to radiation damage [13], multilayers represent excellent candidates for applications under extreme loading and temperature conditions. However, in order to understand and exploit these excellent properties, the role of the interface in large strain deformation, plastic deformation mechanisms and deformation microstructures need to be investigated in greater detail for different types of interfaces.
Beilstein Journal of Nanotechnology, 2016
Physisorbed water originating from exposure to the ambient can have a strong impact on the struct... more Physisorbed water originating from exposure to the ambient can have a strong impact on the structure and chemistry of oxide nanomaterials. The effect can be particularly pronounced when these oxides are in physical contact with a solid substrate such as the ones used for immobilization to perform electron or ion microscopy imaging. We used helium ion microscopy (HIM) and investigated morphological changes of vapor-phase-grown MgO cubes after vacuum annealing and pressing into foils of soft and high purity indium. The indium foils were either used as obtained or, for reference, subjected to vacuum drying. After four days of storage in the vacuum chamber of the microscope and at a base pressure of p < 10(-7) mbar, we observed on these cubic particles the attack of residual physisorbed water molecules from the indium substrate. As a result, thin magnesium hydroxide layers spontaneously grew, giving rise to characteristic volume expansion effects, which depended on the size of the particles. Rounding of the originally sharp cube edges leads to a significant loss of the morphological definition specific to the MgO cubes. Comparison of different regions within one sample before and after exposure to liquid water reveals different transformation processes, such as the formation of Mg(OH)2 shells that act as diffusion barriers for MgO dissolution or the evolution of brucite nanosheets organized in characteristic flower-like microstructures. The findings underline the significant metastability of nanomaterials under both ambient and high-vacuum conditions and show the dramatic effect of ubiquitous water films during storage and characterization of oxide nanomaterials.
Extreme Mechanics Letters, 2015
ABSTRACT The search for light yet strong materials recently benefitted from novel high resolution... more ABSTRACT The search for light yet strong materials recently benefitted from novel high resolution 3D-printing technologies, which allow for fabricating lightweight porous materials with optimally designed micro-topologies. Architectural design improves mechanical properties significantly compared to stochastic porosity, as in foams. Miniaturization of the architectures offers to exploit material strengthening size-effects occurring at the nanoscale. However, these effects and their interaction with structural behavior are not yet well understood. We present tensile experiments of nanoscale alumina-polymer composite bars and cellular microarchitectures, applying 3D-printed push-to-pull mechanisms. The strength of alumina is found to strongly increase as the material thickness decreases. Below 50 nm thickness a plateau at about 5.5 GPa is reached, which is in the range of the theoretical strength. The characteristic low tensile strength of ceramics and its high variability seem not to hold at the nanoscale. Thus, when designed and fabricated appropriately, microarchitectures will facilitate carrying these size-effects beyond scales in future, allowing the use of ceramic materials far beyond what is possible to date.
Procedia Materials Science, 2014
ABSTRACT Polycrystalline tungsten at room temperature shows a brittle fracture behavior, which is... more ABSTRACT Polycrystalline tungsten at room temperature shows a brittle fracture behavior, which is also strongly influenced by the grain structure and texture as well as sample dimensions. To gain insight into the mechanical response of individual grains, an experimental program has been set up to test small scale samples under microbending starting with a notched tungsten single crystal oriented with the {110}<> crack system along the loading direction. Related to this experimental program a finite element study has been performed to analyze the crack propagation in such single-crystal tungsten micro cantilevers. The aim of the present numerical work is to investigate the influence of the single-crystal orientation on the fracture process.
Thin Solid Films, 2015
ABSTRACT Two different single-layers and a bi-layer Ni-Ti thin films with chemical compositions o... more ABSTRACT Two different single-layers and a bi-layer Ni-Ti thin films with chemical compositions of Ni45Ti50Cu5, Ni50.8Ti49.2 and Ni50.8Ti49.2 / Ni45Ti50Cu5 (numbers indicate at.%) determined by energy dispersive X-ray spectroscopy were deposited on Si (111) substrates using DC magnetron sputtering. The structures, surface morphology and transformation temperatures of annealed thin films at 500 °C for 15 min and 1 h were studied using grazing incidence X-ray diffraction, transmission electron microscopy (TEM), atomic force microscopy and differential scanning calorimetry (DSC), respectively. Nanoindentation was used to characterize the mechanical properties.
Nanostructure Science and Technology, 2006
... Page 7. Size Effects on Deformation and Fracture of Nanostructured Metals 33 ... In Situ Test... more ... Page 7. Size Effects on Deformation and Fracture of Nanostructured Metals 33 ... In Situ Testing Technique In situ experiments with direct observation in an optical or electron microscope can reveal important information about deformation mechanisms in materials. ...
Proceedings of the National Academy of Sciences, 2014
To enhance the strength-to-weight ratio of a material, one may try to either improve the strength... more To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m(3); only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina-polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m(3).
ABSTRACT Nanoindentation measurement capabilities at elevated temperatures have developed conside... more ABSTRACT Nanoindentation measurement capabilities at elevated temperatures have developed considerably over the last two decades. Commercially available systems can now perform stable indentation testing at temperatures up to ∼800 °C with thermal drift levels similar to those present at room temperature. The thermal management and measurement techniques necessary to achieve this are discussed here, with particular emphasis on systems featuring independent heating of both the indenter and the sample. To enable measurements at temperatures where oxidation of the indenter and/or sample are a concern, vacuum nanoindentation techniques have also been developed. A natural extension of testing in vacuo is elevated temperature nanoindentation in situ in the scanning electron microscope, and the additional requirements for and benefits of this are discussed. Finally, several new emerging testing techniques are introduced: thermal cycling/fatigue, interfacial thermal resistance measurement and small scale transient plasticity measurements.
Extreme Mechanics Letters, 2016
The damage generated by AC currents at 100 Hz in interconnects has been studied and compared with... more The damage generated by AC currents at 100 Hz in interconnects has been studied and compared with mechanical fatigue damage in thin films. The nature of the damage under the two loading conditions is qualitatively similar, supporting the idea that the AC current damage comes from mechanical cycling due to temperature swings on the order of 100 K from Joule heating in the interconnects. In both cases, the damage forms as surface wrinkles within single grains grow in amplitude and extent with time. The possible threat to the reliability of microelectronic and microelectromechanical systems is further escalated by the observation that soft encapsulation layers do nothing to retard the formation of the damage. CP612, Stress-Induced Phenomena in Metallization: Sixth International Workshop, edited by S. P. Baker, M. A. Korhonen, E. Arzt, and P. S. Ho, pp. 119-132.
Bulk Nanostructured Materials, 2009
Extreme Mechanics Letters, 2015
Journal of Materials Science, 2015
ABSTRACT In this study, we evaluate the evolution of the interfacial processes in metallic slidin... more ABSTRACT In this study, we evaluate the evolution of the interfacial processes in metallic sliding contacts (i.e., aluminum alloys) in terms of their elemental composition, structural changes, and nanomechanical properties in order to understand the optimal running-in behavior leading to steady-state low friction and high wear resistance. Two different sliding conditions are used, resulting in low and high long-term friction and corresponding well with the low and high wear rates. Ex situ elemental analysis of these sliding experiments was performed by means of X-ray photoelectron spectroscopy. The mechanical properties were evaluated using nanoindentation and microcompression testing. While the elemental analysis revealed an increased oxide content for the near-surface region of the worn surfaces compared to the unworn material, the oxide content was higher for the experiments that resulted in an unfavorable tribological response (i.e., high friction and high wear). Similarly, the sub-surface grain-refined layer under these conditions was thicker compared to the experiment with a short running-in stage and low steady-state friction and wear. These observations correlated well with the nanoindentation and microcompression results, which show higher hardness and yield stress for the high friction and wear experiment. Correspondingly, low steady-state friction and wear were obtained with the formation of a thin and mechanically stable tribolayer.
Scripta Materialia, 2015
ABSTRACT Nanoscale metallic multilayers have long been known for their high strength and hardness... more ABSTRACT Nanoscale metallic multilayers have long been known for their high strength and hardness [1], [2] and [3] with layer thickness and interface structure as the controlling factors [3], [4] and [5]. The importance of metallic multilayers in the context of fatigue [6], [7] and [8] and wear [9] and [10] has also been demonstrated. Both the fatigue strength [7] and [8] and wear resistance [9] were observed to increase with decreasing layer thickness. Nanoscale Cu–Ni multilayer coatings very effectively suppressed fatigue crack initiation and showed evidence of reduced accumulated plasticity compared to monolithic coatings [6]. Recently it was suggested that a nanolayered structure that formed during wear testing of Cu90Ag10 alloys – in combination with Ag acting as a solid lubricant – was responsible for reduced wear rates [11]. Considering the high thermal stability of Cu–Nb multilayers [12] and their resistance to radiation damage [13], multilayers represent excellent candidates for applications under extreme loading and temperature conditions. However, in order to understand and exploit these excellent properties, the role of the interface in large strain deformation, plastic deformation mechanisms and deformation microstructures need to be investigated in greater detail for different types of interfaces.
Beilstein Journal of Nanotechnology, 2016
Physisorbed water originating from exposure to the ambient can have a strong impact on the struct... more Physisorbed water originating from exposure to the ambient can have a strong impact on the structure and chemistry of oxide nanomaterials. The effect can be particularly pronounced when these oxides are in physical contact with a solid substrate such as the ones used for immobilization to perform electron or ion microscopy imaging. We used helium ion microscopy (HIM) and investigated morphological changes of vapor-phase-grown MgO cubes after vacuum annealing and pressing into foils of soft and high purity indium. The indium foils were either used as obtained or, for reference, subjected to vacuum drying. After four days of storage in the vacuum chamber of the microscope and at a base pressure of p &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 10(-7) mbar, we observed on these cubic particles the attack of residual physisorbed water molecules from the indium substrate. As a result, thin magnesium hydroxide layers spontaneously grew, giving rise to characteristic volume expansion effects, which depended on the size of the particles. Rounding of the originally sharp cube edges leads to a significant loss of the morphological definition specific to the MgO cubes. Comparison of different regions within one sample before and after exposure to liquid water reveals different transformation processes, such as the formation of Mg(OH)2 shells that act as diffusion barriers for MgO dissolution or the evolution of brucite nanosheets organized in characteristic flower-like microstructures. The findings underline the significant metastability of nanomaterials under both ambient and high-vacuum conditions and show the dramatic effect of ubiquitous water films during storage and characterization of oxide nanomaterials.
Extreme Mechanics Letters, 2015
ABSTRACT The search for light yet strong materials recently benefitted from novel high resolution... more ABSTRACT The search for light yet strong materials recently benefitted from novel high resolution 3D-printing technologies, which allow for fabricating lightweight porous materials with optimally designed micro-topologies. Architectural design improves mechanical properties significantly compared to stochastic porosity, as in foams. Miniaturization of the architectures offers to exploit material strengthening size-effects occurring at the nanoscale. However, these effects and their interaction with structural behavior are not yet well understood. We present tensile experiments of nanoscale alumina-polymer composite bars and cellular microarchitectures, applying 3D-printed push-to-pull mechanisms. The strength of alumina is found to strongly increase as the material thickness decreases. Below 50 nm thickness a plateau at about 5.5 GPa is reached, which is in the range of the theoretical strength. The characteristic low tensile strength of ceramics and its high variability seem not to hold at the nanoscale. Thus, when designed and fabricated appropriately, microarchitectures will facilitate carrying these size-effects beyond scales in future, allowing the use of ceramic materials far beyond what is possible to date.
Procedia Materials Science, 2014
ABSTRACT Polycrystalline tungsten at room temperature shows a brittle fracture behavior, which is... more ABSTRACT Polycrystalline tungsten at room temperature shows a brittle fracture behavior, which is also strongly influenced by the grain structure and texture as well as sample dimensions. To gain insight into the mechanical response of individual grains, an experimental program has been set up to test small scale samples under microbending starting with a notched tungsten single crystal oriented with the {110}<> crack system along the loading direction. Related to this experimental program a finite element study has been performed to analyze the crack propagation in such single-crystal tungsten micro cantilevers. The aim of the present numerical work is to investigate the influence of the single-crystal orientation on the fracture process.
Thin Solid Films, 2015
ABSTRACT Two different single-layers and a bi-layer Ni-Ti thin films with chemical compositions o... more ABSTRACT Two different single-layers and a bi-layer Ni-Ti thin films with chemical compositions of Ni45Ti50Cu5, Ni50.8Ti49.2 and Ni50.8Ti49.2 / Ni45Ti50Cu5 (numbers indicate at.%) determined by energy dispersive X-ray spectroscopy were deposited on Si (111) substrates using DC magnetron sputtering. The structures, surface morphology and transformation temperatures of annealed thin films at 500 °C for 15 min and 1 h were studied using grazing incidence X-ray diffraction, transmission electron microscopy (TEM), atomic force microscopy and differential scanning calorimetry (DSC), respectively. Nanoindentation was used to characterize the mechanical properties.
Nanostructure Science and Technology, 2006
... Page 7. Size Effects on Deformation and Fracture of Nanostructured Metals 33 ... In Situ Test... more ... Page 7. Size Effects on Deformation and Fracture of Nanostructured Metals 33 ... In Situ Testing Technique In situ experiments with direct observation in an optical or electron microscope can reveal important information about deformation mechanisms in materials. ...
Proceedings of the National Academy of Sciences, 2014
To enhance the strength-to-weight ratio of a material, one may try to either improve the strength... more To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m(3); only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina-polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m(3).
ABSTRACT Nanoindentation measurement capabilities at elevated temperatures have developed conside... more ABSTRACT Nanoindentation measurement capabilities at elevated temperatures have developed considerably over the last two decades. Commercially available systems can now perform stable indentation testing at temperatures up to ∼800 °C with thermal drift levels similar to those present at room temperature. The thermal management and measurement techniques necessary to achieve this are discussed here, with particular emphasis on systems featuring independent heating of both the indenter and the sample. To enable measurements at temperatures where oxidation of the indenter and/or sample are a concern, vacuum nanoindentation techniques have also been developed. A natural extension of testing in vacuo is elevated temperature nanoindentation in situ in the scanning electron microscope, and the additional requirements for and benefits of this are discussed. Finally, several new emerging testing techniques are introduced: thermal cycling/fatigue, interfacial thermal resistance measurement and small scale transient plasticity measurements.