Ruth Schwaiger

High-strength cellular ceramic composites with 3D microarchitecture.

Significance It has been a long-standing effort to create materials with low density but high strength. Technical foams are very light, but compared with bulk materials, their strength is quite low because of their random structure. Natural lightweight materials, such as bone, are cellular solids with optimized architecture. They are structured hierarchically and actually consist of nanometer-size building blocks, providing a benefit from mechanical size effects. In this paper, we demonstrate that materials with a designed microarchitecture, which provides both structural advantages and size-dependent strengthening effects, may be fabricated. Using 3D laser lithography, we produced micro-truss and -shell structures from ceramic–polymer composites that exceed the strength-to-weight ratio of all engineering materials, with a density below 1,000 kg/m3. 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/m3; 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/m3.

Fatigue in thin films: lifetime and damage formation

Two new techniques, developed for studying the fatigue behavior of thin metal films on substrates, are presented: The first technique involves deposition of Cu films onto elastic polyimide substrates. During cyclic tensile testing of the film–substrate composite, the film is subjected to tension–compression cycles, since it is plastically deformed, while the substrate undergoes only elastic deformation. Using this technique, it was found that, for 3 μm thick Cu films, the number of cycles to failure follows the phenomenological Coffin–Manson relationship. For the other technique, thin films, here Ag films with thicknesses ranging from 0.2 to 1.5 μm, are deposited onto micromachined SiO2 cantilever beams. The beams are then deflected with a frequency of 45 Hz using a nanoindentation system. A detailed investigation of the damage formation in both fatigued Cu and Ag films revealed surface roughening prior to failure. Extrusions and cracks are formed inside large grains and between small grains, respectively.

Size effects in the fatigue behavior of thin Ag films

Abstract Thin film dimensions and microstructure affect the microscopic processes responsible for fatigue. This work focuses on the characterization of such mechanisms and the resulting fatigue behavior. The fatigue behavior of 0.2–1.5 μm thick, Ag films on SiO 2 was investigated. The films were tested using cantilever microbeam deflection with respect to the influence of loading conditions. Extrusions similar to those observed in bulk material were found at the Ag film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to fatigue failure. Fatigue damage was observed to occur predominantly in (100)-oriented grains. Thinner films were more fatigue resistant and contained fewer, smaller extrusions than thicker films.

Approaching theoretical strength in glassy carbon nanolattices

The strength of lightweight mechanical metamaterials, which aim to exploit material-strengthening size effects by their microscale lattice structure, has been limited by the resolution of three-dimensional lithography technologies and their restriction to mainly polymer resins. Here, we demonstrate that pyrolysis of polymeric microlattices can overcome these limitations and create ultra-strong glassy carbon nanolattices with single struts shorter than 1 μm and diameters as small as 200 nm. They represent the smallest lattice structures yet produced--achieved by an 80% shrinkage of the polymer during pyrolysis--and exhibit material strengths of up to 3 GPa, corresponding approximately to the theoretical strength of glassy carbon. The strength-to-density ratios of the nanolattices are six times higher than those of reported microlattices. With a honeycomb topology, effective strengths of 1.2 GPa at 0.6 g cm(-3) are achieved. Diamond is the only bulk material with a notably higher strength-to-density ratio.

Cyclic deformation of polycrystalline Cu films

Fatigue impairs the reliability of macroscopic metallic components utilized in a variety of technological applications. However, the fatigue behaviour of thin metal films and small-scale components used in microelectronics and mechanical microdevices has yet to be explored in detail. The fatigue behaviour in submicrometre thin films is likely to differ from that in bulk material, since the volume necessary for the formation of dislocation structures typical of cyclic deformation in bulk material is larger than that available in thin films. The thin-film dimensions and microstructure, therefore, affect the microscopic processes responsible for fatigue. The fatigue behaviour of Cu films 0.4, 0.8 and 3.0 µm thick on polyimide substrates was investigated. The specimens were fatigued at a total strain amplitude of 0.5% using an electromechanical tensile-testing machine. This work focuses on the characterization of fatigue mechanisms and the resulting fatigue damage of thin Cu films. Extrusions similar to those observed in bulk material were found at the film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to thin-film failure. Thinner films were more fatigue resistant and contained fewer and smaller extrusions than thicker films did. A small thickness appears to inhibit void nucleation. This observation is explained in terms of vacancy diffusion and annihilation at free surfaces or grain boundaries. Transmission electron microscopy investigations confirmed that no long-range dislocation structures have developed during fatigue loading of the films investigated.

Defect structure in micropillars using X-ray microdiffraction

White beam x-ray microdiffraction is used to investigate the microstructure of micron-sized Si, Au, and Al pillars fabricated by focused ion beam (FIB) machining. Comparison with a Laue pattern obtained from a Si pillar made by reactive ion etching reveals that the FIB damages the Si structure. The Laue reflections obtained from the metallic pillars fabricated by FIB show continuous and discontinuous streakings, demonstrating the presence of strain gradients.

Damage behavior of 200-nm thin copper films under cyclic loading

Fatigue damage in 200-nm-thick Cu films was investigated and compared with the damage in thicker Cu films. The fatigued 200-nm-thick Cu films exhibited only a few, small extrusions and extensive cracking along twin and grain boundaries, whereas the thicker films showed many extrusions/intrusions and cracks lying along the extrusions rather than along the boundaries. This change in fatigue damage behavior with film thickness is attributed to the inhibition of dislocation mobility and the limited availability and activation of dislocation sources on the small length scale. It is argued that the decrease in film thickness and grain size inhibits the localized accumulation of plastic strain within grains, such as at extrusions/intrusions and in extended dislocation structures, and promotes the formation of damage such as cracks at twin and grain boundaries during fatigue. This effect is suggested as the likely cause for the increase in fatigue life with decreasing specimen dimensions.

Effect of film thickness and grain size on fatigue-induced dislocation structures in Cu thin films

This letter presents systematic experimental observations of fatigue damage and corresponding dislocation structures in thin Cu films as a function of film thickness made using transmission electron microscopy and focused-ion-beam microscopy. It is found that, in thick films and grains of at least 3.0 μm diameter, coarse surface extrusions and dislocation wall and cell structures occur whereas, in thin films or in small-diameter grains, finer extrusions occur but no clearly defined dislocation structures are present. This minimum required dimension of 3.0 μm for fatigue damage formation may be caused by constrained dislocation motion in small dimensions.

Fatigue behavior of polycrystalline thin copper films

Abstract Fatigue, a common damage and failure mechanism in bulk metals, is largely unexplored for thin films. In the present paper, we report on the fatigue behavior of Cu films with thicknesses in the range 0.4–3.1 μm on deformable substrates. Films thicker than 1 μm seem to behave like bulk Cu and follow a Manson-Coffin relationship with a fatigue exponent and ductility of about 0.5 and 20 %, respectively. For the sub-micron thick films, a clear size effect is observed: the damage morphology changes and the lifetime increases signaficantly. Based on a microscopical damage analysis, the following sequence for the fatigue damage evolution in the Cu films is suggested: (i) in large grains, extrusions at the film surface and voids at the interface to the substrate are formed, (ii) cracks are nucleated at these voads and grow towards the film surface, and (iii) cracks connect intergranularly to form a continuous pattern of cracks and extrusions in the film. It is argued that void nucleation is the result of ...

The attachment strategy of English ivy: a complex mechanism acting on several hierarchical levels

English ivy (Hedera helix L.) is able to grow on vertical substrates such as trees, rocks and house plaster, thereby attaching so firmly to the surface that when removed by force typically whole pieces of the climbing substrate are torn off. The structural details of the attachment process are not yet entirely understood. We studied the attachment process of English ivy in detail and suggest a four-phase process to describe the attachment strategy: (i) initial physical contact, (ii) form closure of the root with the substrate, (iii) chemical adhesion, and (iv) shape changes of the root hairs and form-closure with the substrate. These four phases and their variations play an important role in the attachment to differently structured surfaces. We demonstrate that, in English ivy, different mechanisms work together to allow the plants attachment to various climbing substrates and reveal the importance of micro-fibril orientation in the root hairs for the attachment based on structural changes at the subcellular level.