Showing papers by "Eduard Arzt published in 2003"

Materials become insensitive to flaws at nanoscale: lessons from nature.

Abstract: Natural materials such as bone, tooth, and nacre are nanocomposites of proteins and minerals with superior strength. Why is the nanometer scale so important to such materials? Can we learn from this to produce superior nanomaterials in the laboratory? These questions motivate the present study where we show that the nanocomposites in nature exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (robustness). We further show that the widely used engineering concept of stress concentration at flaws is no longer valid for nanomaterial design.

From micro to nano contacts in biological attachment devices.

Abstract: Animals with widely varying body weight, such as flies, spiders, and geckos, can adhere to and move along vertical walls and even ceilings. This ability is caused by very efficient attachment mechanisms in which patterned surface structures interact with the profile of the substrate. An extensive microscopic study has shown a strong inverse scaling effect in these attachment devices. Whereas μm dimensions of the terminal elements of the setae are sufficient for flies and beetles, geckos must resort to sub-μm devices to ensure adhesion. This general trend is quantitatively explained by applying the principles of contact mechanics, according to which splitting up the contact into finer subcontacts increases adhesion. This principle is widely spread in design of natural adhesive systems and may also be transferred into practical applications.

Dislocation sources and the flow stress of polycrystalline thin metal films

Abstract: Plastic deformation of thin metal films has been investigated with the help of a discrete dislocation dynamics simulation. In particular, the operation of a Frank-Read source was studied in the confined geometry of a thin polycrystalline film. The results lead to a new model for the dependence of flow stress on film thickness and grain size. It is based on the assumption that the number of dislocation sources per grain is small and multiple activation is required. The model predicts a flow stress that scales with the inverse film thickness or grain size (whichever is smaller) and that is roughly four times higher than predicted by previous models, thereby giving better agreement with experimental data.

Methods for modifying the surfaces of a solid and microstructured surfaces with increased adherence produced with said methods

Abstract: Disclosed are methods for the surface modification of an object in order to increase the adherence of said object. The surface undergoes structuring, whereupon a plurality of projections are formed with a respective foot and head part. The head part possesses a front face pointing away from the surface. The size of each projection is such that all front faces have the same vertical height above the surface and form an adherent contact surface which is interrupted by reciprocal distances between the front faces.

Creep of aluminum-based closed-cell foams

Abstract: Metal foams creep when loaded mechanically at high homologous temperatures We have studied the creep behavior of closed-cell aluminum-based foams with relative densities of 0092, 0112, and 0163 Compressive creep tests were performed at 300 °C at strain rates ranging from 10−9 to 10−4 s−1 Special efforts were made to produce and characterize a bulk reference material exhibiting the same chemical composition Results show that the foams exhibit a lower creep strength and a higher stress exponent than predicted by the Gibson-Ashby model for regular foams The possible mechanisms responsible for this deviation are discussed A semi-empirical rate equation is established which describes the experimental data well

Microstructured surfaces with increased adherence

Abstract: Disclosed are methods for the surface modification of an object (10), increasing the adherence of said object. The surface (11) undergoes structuring, whereupon a plurality of projections (12) are formed with a respective foot and head part. The head part possesses a front face (13) pointing away from surface. The size of each projection (12) is such that all front faces (13) have the same vertical height above the surface (11) and form an adherent contact surface (14) which is interrupted by reciprocal distances between the front faces (13).

Plasticity-Related Phenomena in Metallic Films on Substrates

Abstract: Plastic deformation due to thermal stresses has been investigated for different metallic films deposited on Si or α-alumina substrates. We conducted post-mortem TEM and SEM investigations of samples that underwent thermal cycles in order to capture the microstructural changes imposed by thermal stresses. The ultimate goal is to determine the dominant plasticity mechanisms responsible for such changes. In-situ thermal cycles performed inside the TEM allowed direct and real-time observations of dislocation behaviour under stress. It is shown that dislocation density drops in Al/Si, Au/Si and in Cu/α-alumina thin film systems. Except in the case of pseudo-epitaxial Cu on sapphire, the interaction of dislocations with the interfaces (passivation, oxide, adhesion layer) is attractive and leads to the disappearance of interfacial dislocations. In this light, the generalized observation of high tensile stresses that arise in metallic films at the end of cooling is explained in terms of insufficient dislocation sources instead of classic strain hardening. Diffusional processes can substitute for a lack of dislocation, but the low relaxation strain rate that would be excpected should lead to high stresses during the cooling stages of thermal cycles.

Nucleation model for hcp martensite: Application to Co thin films

Abstract: The martensitic face-centered cubic (fcc) to hexagonal close-packed (hcp) transformation was studied in cobalt thin films on silicon substrates in order to understand effects of film constraints, texture and stress state The martensitic transformation was observed in situ with the substrate curvature method The transformation manifests itself as a stress drop revealing a positive work However, with increasing tensile stress, the start of the martensite transformation is shifted to lower temperature Thus, stress obstructs the transformation in spite of the positive sign of the transformation work A dislocation-based model for the martensite nucleation is presented and applied to Co thin films A distinction is made between a microscopic shape change and a macroscopic shape change The model predicts the stress-induced delayed nucleation due to the microscopic shape change The positive work results from the macroscopic shape change Within experimental error, model and experiments agree well

Parallel Glide: A Fundamentally Different Type of Dislocation Motion in Ultrathin Metal Films

Abstract: When confronted by severe geometric constraints, dislocations may respond in unforeseen ways. One example of such unexpected behavior is parallel glide in unpassivated, ultrathin (200 nm and thinner) metal films. This involves the glide of dislocations parallel to and very near the film/substrate interface, following their emission from grain boundaries. In situ transmission electron microscopy reveals that this mechanism dominates the thermomechanical behavior of ultrathin, unpassivated copper films. However, according to Schmid’s law, the biaxial film stress that evolves during thermal cycling does not generate a resolved shear stress parallel to the film/substrate interface and therefore should not drive such motion. Instead, it is proposed that the observed dislocations are generated as a result of atomic diffusion into the grain boundaries. This provides experimental support for the constrained diffusional creep model of Gao et al.[1], in which they described the diffusional exchange of atoms between the unpassivated film surface and grain boundaries at high temperatures, a process that can locally relax the film stress near those boundaries. In the grains where it is observed, parallel glide can account for the plastic strain generated within a film during thermal cycling. One feature of this mechanism at the nanoscale is that, as grain size decreases, eventually a single dislocation suffices to mediate plasticity in an entire grain during thermal cycling. Parallel glide is a new example of the interactions between dislocations and the surface/interface, which are likely to increase in importance during the persistent miniaturization of thin film geometries.

Microstructure of die-cast alloys Mg-Zn-Al(-Ca): A study by electron microscopy and small-angle neutron scattering

Abstract: The microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibrium τ phase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubic τ phase Mg32(Al, Zn, Ca)49 as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast c...

Vacuum holder for sucking and holding e.g. thin metal plate, has contact surface that comes in contact to target object in negative pressure state of suction chamber, and that has microstructures in cylindrical, thin plate or pin form

Abstract: The vacuum holder (10) has a contact surface (28) that comes in contact to a target object (32) in the negative pressure state of a suction chamber (26). The contact surface has microstructures formed in cylindrical shape, thin plate shape or pin form. The suction chamber is in fluid connection to a negative pressure generator (14). The suction chamber is formed between the sealing edges (22) of a suction disc (20) held in a suction disc retainer (12). An independent claim is included for the vacuum holder manufacture.

Mechanical Size-Effects and Dislocation Dynamics in Cu Thin Films

Abstract: In modern applications ranging from catalysis to microelectronic devices, Cu is frequently used in the form of thin films with sub-micron geometric and microstructural dimensions. In this sizeregime, mechanical deformation processes start to deviate from the well-known bulk behaviour [1]. While this is expected to lead to stress-induced changes of the electronic properties, with potential improvements in catalysis, the existence of high internal stresses may be detrimental to the mechanical reliability of these constrained material volumes.

3D simulation of the dislocation dynamics in polycrystalline metal thin films

Abstract: The plastic deformation of polycrystalline fcc metal thin .lms with thicknesses of 1 μm and less is investigated by simulating the dynamics of discrete dislocations in a representative columnar grain. The simulations are based on the assumption that dislocation sources or multiplication sites are rare and that sources have to operate several times to generate appreciable plastic deformation. This model is thoroughly tested by calculating the response of randomly distributed dislocation sources to an applied stress and comparing the results with experimental data. Stress–strain curves, the influence of boundary conditions, dislocation densities, work hardening rates and their dependence on the film thickness as well as the dependence on grain orientation are studied. The agreement between simulation and experiment is good and many aspects of thin film plasticity can be understood with the assumption that small-scale plastic deformation is source controlled rather than mobility controlled.

Verfahren zur oberflächenmodifizierung eines festkörpers und daraus hergestellte mikrostrukturierte oberflächen mit gesteigerter adhäsion

Abstract: Es werden Verfahren zur Oberflachenmodifizierung eines Objektes, um die Adhasionsfahigkeit des Objektes zu erhohen, beschrieben, wobei die Oberflache einer Strukturierung unterzogen wird, so dass eine Vielzahl von Vorsprungen gebildet wird, die jeweils mit einem Fussteil und einem Kopfteil gebildet werden, wobei der Kopfteil eine von der Oberflache wegweisende Stirnflache besitzt, wobei jeder Vorsprung mit einer Grosse derart gebildet wird, dass alle Stirnflachen die gleiche senkrechte Hohe uber der Oberflache besitzen und eine adharente, durch gegenseitige Abstande zwischen den Stirnflachen unterbrochene Kontaktflache bilden.

Surfaces microstructurees a plus forte adherence

Abstract: La presente invention concerne des procedes pour modifier la surface d'un objet (10) afin d'augmenter son pouvoir d'adherence. Selon cette invention, la surface (11) est soumise a un traitement structurant de facon a presenter une pluralite de protuberances (12) qui sont respectivement constituees d'une partie de pied et d'une partie de tete. La partie de tete possede une face (13) orientee dans la direction opposee a ladite surface. La taille de chaque protuberance (12) est telle que toutes les faces (13) presentent la meme hauteur verticale sur la surface (11) et forment une surface de contact (14) adherente, interrompue par des espacements mutuels entre les faces (13).