Seyed Allameh

On the measurement of the plasticity length scale parameter in LIGA nickel foils

This paper presents measurements of the plasticity length scale for polycrystalline LIGA nickel foils produced by electroplating in a sulfamate bath. The micro-bend test method developed by Stolken and Evans [Acta Mater. 46 (1998) 5109] is used to obtain a composite length scale parameter, lc that is primarily associated with rotational gradients. The micro-bend test utilizes the measurement of the curvature change associated with elastic spring-back of bent micro-beams in the extraction of a composite length scale parameter, lc. The length scale is measured to be ∼5.6 μm for foils between 25 and 175 μm thick. This is in the range (3–5 μm) reported from prior micro-bend experiments on nickel foils (loc. cit.). The measured length scale is also similar to values reported previously for polycrystalline copper wires using torsion tests in which rotation gradients dominate.

Mechanical properties of functionally graded hierarchical bamboo structures

This paper presents the results of a series of multi-scale experiments and numerical models concerning the mechanical properties of moso culm functionally graded bamboo structures. On the nano- and microscales, nanoindentation techniques are used to study the local variations in the Youngs moduli of moso culm bamboo cross-sections. These are then incorporated into finite element models in which the actual variations in Youngs moduli are used to model the deformation and fracture of bamboo during fracture toughness experiments. Similarly, the measured gradations in moduli are incorporated into crack bridging models that predict the toughening observed during resistance curve tests. The implications of the results are discussed for the bio-inspired design of structures that mimic the layered, functionally graded structure of bamboo.

An introduction to mechanical-properties-related issues in MEMS structures

A brief introduction to the issues related to the mechanical properties of MEMS components in terms of their functionality, types of loading and modes of operation is presented. At the microscale, for most cases, some mechanical properties including strength increase. However, surface phenomena such as adhesion/stiction become important as the aspect ratio of the components decreases. Subcritical crack initiation and growth in silicon, as the most common type of material used in MEMS devices under various types of loading is discussed. In addition, fatigue and creep behavior of MEMS components are described.

Surface topography evolution and fatigue fracture in polysilicon MEMS structures

This paper presents the results of an experimental study of the micromechanisms of surface topography evolution and fatigue fracture in polysilicon MEMS structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO/sub 2/ layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are characterized as functions of fatigue cycling before discussing the micromechanisms of fatigue fracture.

On the evolution of surface morphology of polysilicon MEMS structures during fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures.

Mechanical Properties of Au Films on Silicon Substrates

This paper presents the results of recent studies of the effects of film thickness on the mechanical properties of electron-beam (e-beam) deposited Au films on silicon substrates. Following a brief description of film microstructure and surface topography, film mechanical properties (Youngs modulus and hardness) are determined using nanoindentation techniques. The effects of stiff silicon substrates on the Youngs modulus are analyzed within the framework established by King [6 8]. The effects of indentation size on film hardness are also explained within the context of strain gradient plasticity theories and substrate effects. The plasticity length scale parameters are shown to scale with film thickness. The amount of material pile up is also shown to increase with decreasing film thickness, for a given ratio of indentation depth to film thickness. The implications of the work are discussed for applications of Au films on silicon substrates.

Material removal on lubricated steel gears with W-DLC-coated surfaces

The wear process that occurs in lubricated steel gears with thin, metal-containing diamond-like-carbon (Me-DLC) films deposited on the surface has been characterized using a variety of techniques that include the atomic force microscope and the focused ion beam imaging system. The profile of the tungsten-containing DLC (W-DLC) has been found to duplicate that of the original steel surface with peaks and valleys having amplitude up to 1 μm, superposed on a very thin (50 nm) Cr adhesion layer. Beneath the surface, imperfections are embedded in the steel. Wear occurs through the removal of the peaks by a polishing mechanism, leaving the valleys intact. When the peaks have been fully removed to create a plateau, the RMS roughness is approximately 35 nm. There are still pits corresponding to the original valleys up to 200 nm deep. When the remnant DLC becomes smaller than the amplitude of the peaks on the steel surface, the steel and the Cr adhesion layer become polished, causing the peaks to be eliminated, as well as the imperfections present in the subsurface. The resulting surface has RMS roughness with amplitude 25 nm, as well as small protuberances associated with the carbide particles in the steel.

Fatigue damage evolution in silicon films for micromechanical applications

In this paper we examine the conditions for surface topography evolution and crack growth/fracture during the cyclic actuation of polysilicon microelectromechanical systems (MEMS) structures. The surface topography evolution that occurs during cyclic fatigue is shown to be stressassisted and may be predicted by linear perturbation analyses. The conditions for crack growth (due to pre-existing or nucleated cracks) are also examined within the framework of linear elastic fracture mechanics. Within this framework, we consider pre-existing cracks in the topical SiO2 layer that forms on the Si substrate in the absence of passivation. The thickening of the SiO2 that is normally observed during cyclic actuation of Si MEMS structures is shown to increase the possibility of stable crack growth by stress corrosion cracking prior to the onset of unstable crack growth in the SiO2 and Si layers. Finally, the implications of the results are discussed for the prediction of fatigue damage in silicon MEMS structures.

An investigation of the effects of thickness on mechanical properties of LIGA nickel MEMS structures

This paper examines the effects of thickness on the mechanical properties of LIGA Ni MEMS structures plated from sulfamate baths. The as-plated LIGA Ni specimens of different thickness (50 μm, 100 μm and 200 μm) were utilized in the microtensile experiments. Optical microscopy, orientation imaging microscopy and scanning electron microscopy were used to characterize the microstructure of the LIGA Ni specimens. Fracture Modes obtained from specimens with different thickness were revealed by scanning electron microscopy. The effects of specimen thickness are then discussed within the context of strain gradient plasticity theories.

Surface topography evolution and fatigue fracture of polysilicon

This paper presents the results of an experimental stydy of the micromechanisms of fatigue crack nucleation and fatigue fracture in polysilicon MEMS Structures. The initial stages of fatigue are shown to be associated with stress-assisted surface topography evolution and the thickening of SiO2 layers that form on the unpassivated polysilicon surfaces and crack/notch faces. The differences in surface topography and oxide thickness are elucidated as functions of fatigue cycling before discussing the micromechanisms of crack growth and final fracture.