Arthur H. Heuer

Structural basis for the fracture toughness of the shell of the conch Strombus gigas

Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few per cent of the composite material, the fracture toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude. The judicious placement of the organic matrix, relative to the mineral phase, and the hierarchical structural architecture extending over several distinct length scales both play crucial roles in the mechanical response of natural composites to external loads. Here we use transmission electron microscopy studies and beam bending experiments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two energy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shells tougher middle layers at higher loads. Both mechanisms are intimately associated with the so-called crossed lamellar microarchitecture of the shell, which provides for ‘channel’ cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material. Despite a high mineral content of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a ‘ceramic plywood’, and can guide the biomimetic design of tough, lightweight structures.

Thin-film shape-memory alloy actuated micropumps

Micropumps capable of precise handling of low-fluid volumes have the potential to revolutionize applications in fields such as drug delivery, fuel injection, and micrototal chemical analysis systems (/spl mu/TAS). Traditional microactuators used in micropumps suffer from low strokes and, as a result, are unsuitable for achieving large fluid displacement. They also suffer low-actuation work densities, which translate to low forces. We investigate the use of the shape-memory effect (SMA) in sputter-deposited thin-film shape-memory alloy (SMA) titanium nickel (TiNi) as an actuator for microelectromechanical systems (MEMS)-based microfluidic devices, as it is capable of both high force and high strains. The resistivity of the SMA thin film is suitable for Joule heating, which allows direct electrical control of the actuator. Two micropump designs were fabricated-one with a novel complementary actuator and the other with a polyimide-biased actuator-which provided thermal isolation between the heated microactuator and the fluid being pumped. A maximum water flow rate of 50 /spl mu/l/min was achieved.

Fracture Properties of Human Enamel and Dentin

Work of fracture measurements and scanning electron microscope fractographs show that both enamel and dentin can best be considered as brittle materials with anisotropic fracture properties. Enamel is highly anisotropic, with the weakest path of fracture parallel to the enamel rods. Dentin is less anisotropic, with easiest fracture perpendicular to the dentinal tubules. A model is proposed to explain the fracture behavior of enamel.

The TiNi shape-memory alloy and its applications for MEMS

The shape-memory effect is a solid state phenomenon which exploits a reversible phase transformation to repeatedly achieve an initial shape, even after some deformation of the material. Numerous metal alloys exhibit this effect. One of the most widely used shape-memory alloys is TiNi, due to its large range of recoverable deformations and its relative ease of processing. In bulk and wire form, TiNi has been applied to a number of applications, and as a thin film, TiNi is an excellent material for use as a microactuator in microelectromechanical systems (MEMS), due to its large recovery forces and high recoverable strains. Several TiNi-actuated MEMS devices have already been reported.

Transformation-toughening in partially-stabilized zirconia (PSZ)

Abstract The significant fracture toughness of optimally-fabricated partially-stabilized zirconia (PSZ) is due to mechanical energy absorption in the vicinity of the tip of a propagating crack due to stressinduced phase transformations occurring within metastable precipitates. Eshelbys transformed-inclusion analysis has been used to model the transformation toughening: a transformation zone is calculated from an energy balance approach, the size of this zone determining the mechanical energy dissipated during crack propagation. The increase in apparent fracture surface energy predicted by the model is in tolerable agreement with experiment, although the model does not account for multiparticle interactions, interfacial energy changes, and transformation-induced twinning.

TiNi (shape memory) films silicon for MEMS applications

TiNi shape memory alloy in thin film form is an excellent candidate for MEMS microactuation. Using RF sputter deposition, thin films of TiNi (51.7 at% Ti-48.3 at% Ni) have been formed on silicon substrates and produced shape memory behavior at approximately 60/spl deg/C. Films were amorphous when deposited and were subsequently annealed at 515/spl deg/C for 30 min. to crystallize the shape memory microstructure. Excellent adherence was achieved onto silicon, SiO/sub 2/ and poly-silicon surfaces. Microfabrication was used to create TiNi diaphragms, which exhibited useful shape memory microactuation and other desirable mechanical properties. The diaphragms recovered greater than 2% strain when heated through the phase transformation temperature, providing a maximum work density of at least 5/spl times/10/sup 6/ J/m/sup 3/. This work density is higher than that of any other type of microactuation.

Electrostatically actuated failure of microfabricated polysilicon fracture mechanics specimens

Polysilicon fracture mechanics specimens have been fabricated using standard microelectromechanical systems (MEMS) processing techniques, and thus have characteristic dimensions comparable with typical MEMS devices. These specimens are fully integrated with simultaneously fabricated electrostatic actuators which are capable of providing sufficient force to ensure catastrophic crack propagation from blunt notches produced using micromachining. Thus, the entire fracture experiment takes place on–chip, without any external loading source. Fracture has been initiated using both monotonic and cyclic resonance loading. A reduction in the nominal toughness under cyclic loading is attributed to subcritical growth of sharp cracks from the micromachined notches in the fracture mechanics specimens. Fatigue fracture has been observed in specimens subjected to as many as 109 cycles, and environmental corrosion is implicated in at least some aspects of the fatigue.

Biomineralization and Eggshells: Cell-Mediated Acellular Compartments of Mineralized Extracellular Matrix

Publisher Summary This chapter summarizes cell biology, morphological organization, crystallography, chemical composition, process of mineralization, and biological function of avian eggshells. It also discusses emerging concepts of biomineralization and speculates about the mechanism of eggshell assembly and its implications for fabrication of polymer-ceramic composites. The eggshell is a microenvironmental compartment for housing developing embryos of a number of species. This unique microenvironment provides physical protection to the embryo and regulates gas, water, and ionic exchange. The avian eggshell can be characterized as a multilayered, polymer–ceramic composite. The three main layers include an outer mucous layer, an intermediate calcified zone, and an inner fibrous membrane layer. The shell membranes are the most internal layer of the eggshell and are formed by two nonmineralized fibrillar sublayers, the outer membrana testae externa, and the inner membrana testae interna or putaminis. The most external layer of the eggshell is referred to as the cuticle. This proteinaceous layer covers the entire calcified portion of the shell to a depth of about 10 μ m. The complementary use of biological, chemical, and crystallographic approaches demonstrates that the avian eggshell is a very promising model for the study of biomineralization. Avian eggshell is one of the most rapidly mineralizing biological systems known.

On a martensitic phase transformation in zirconia (ZrO2)-I. Metallographic evidence

The tetragonal-monoclinic phase transformation in ZrO2 single crystals and polycrystals has been studied in both the forward and reverse directions and found to be martensitic. A large hysteresis is involved in the transformation, the As and Ms temperature being ~1050 and ~950°C respectively; surface relief accompanies the transformation in both directions. A high-temperature metallographic study established the athermal and shear-type nature of the transformation and was consistent with a diffusionless reaction. Two different habit planes were found and their orientations have been determined by two-surface optical analysis as ~(106) and ~(010) (referred to the monoclinic phase). Fully transformed single crystals and polycrystals have been studied by transmission electron microscopy and the microstructural features resulting from the phase transformation analyzed. In addition to confirming the two habit planes, the electron microscopy indicated that the lattice invariant shear usually occurs by slip rather than by twinning. Deformation twins have been observed in all the electron microscope foils, however, and are believed to arise from accommodation stresses generated during the transformation.

Overview no. 45: On the nucleation of the martensitic transformation in zirconia (ZrO2)

Abstract The kinetics of the martensitic tetragonal ( t )→monoclinic ( m ) transformation in ZrO 2 are nucleation-controlled, as they are in metals and alloys, and classical and non-clasical models for martensitic nucleation are considered in this paper. Transmission electron microscopy studies of the several types of transformation-toughened ZrO 2 -containing ceramics now available do not reveal any obvious defects that could act as classical heterogeneous nuclei. This is true for t -ZrO 2 as a coherent precipitate phase, as an incoherent dispersed phase, as a fine-grain matrix phase, or as an incoherent precipitate phase in internally-oxidized alloys. On the other hand, nucleation via a Clapp-type localized soft mode mechanism at strain concentrations is consistent with all observations. Nucleation is always stress-assisted, even when the transformation occurs spontaneously. In these latter cases, the stresses arise from thermal expansion mismatch or thermal expansion anisotropy and are enhanced by the stress concentrations provided by particle or grain facets and corners.