Micro and Nanomechanics, Volume 5

Adhesion is an important phenomenon in the context of MEMS for which the surface forces become dominant in comparison with the body forces. Because the magnitudes of the adhesive forces strongly depend on the surface interaction distances, which in turn evolve with the roughness of the contacting surfaces, the adhesive forces cannot be determined in a deterministic way. To quantify the uncertainties on the structural stiction behavior of a MEMS, this work proposes a “stochastic multi-scale methodology”. The key ingredient of the method is the evaluation of the random meso-scale apparent contact forces, which homogenize the effect of the nano-scale roughness and are integrated into a numerical model of the studied structure as a random contact law. To obtain the probabilistic behavior at the structural MEMS scale, a direct method needs to evaluate explicitly the meso-scale apparent contact forces in a concurrent way with the stochastic multi-scale approach. To reduce the computational cost, a stochastic model is constructed to generate the random meso-scale apparent contact forces. To this end, the apparent contact forces are parameterized by a vector of parameters before applying a polynomial chaos expansion in order to construct a mathematical model representing the probability of the random parameters vector. The problem of micro-beam stiction is then studied in a probabilistic way.

To improve the integrity of densely stacked multilayers in microelectronic systems, e.g., Light Emitting Diodes (LED), and thereby overcome the currently experienced problems related to interface failure during manufacturing of such devices, accurate identification of interface properties is essential. The behavior of the interface is only measurable through kinematic information from adjacent materials.The goal of this research is to identify interface parameters by Integrated Digital Image Correlation (IDIC), in which experimental images of a deformation process are correlated by utilizing the mechanical response from finite element (FE) simulations. An interface is herein modeled by cohesive zone (CZ) elements exhibiting constitutive traction-separation laws. The versatility of FE simulations and the kinematic richness of the full-field measurements are thereby exploited.Comprising an elastic hinge system, a small-scale mechanical test-setup is designed from two 3-axes (XYZ) piezo stages, with which micrometer displacements and realistic interface loading conditions (shear, normal, and mixed-mode loading) can be applied to an LED specimen. This allows to, in a well-controlled manner, mechanically mimic interface delamination that is typically induced during fabrication steps by thermal expansion. This setup and the IDIC method are integrated to identify the CZ parameters of the critical interface of an LED specimen.

This work focused on the synthesis, and microstructural and mechanical characterizations of copper-30 %zinc-6 %aluminum (Cu30Zn6Al) alloy. The alloy was first synthesized using an induction furnace, then homogenized for different durations, and heated and quenched using one-step and two-step methods. Optical microscope and scanning electron microscope were employed for the microstructure characterization. Microhardness testing was performed to obtain the Vickers hardness of the alloy after the quenching processes. The increase of homogenization duration from 2 to 12 h increases the average grain size from about 50 μm to about 100 μm when the samples were homogenized at 500 °C. The microhardness of the alloy varied in the range of about 40 HV to about 75 HV after experiencing the quenching processes.

Lath martensite is the key constituent in advance steels that provides the overall strength. Martensite is known as hard and brittle, but recent evidence shows significant plasticity before fracture. The exactly reason is undisclosed but should relate to the underlying lath microstructure. Therefore, we studied the influence of sub-block and block boundaries on martensite plasticity through uni-axial tensile testing of individual micro-constituents, i.e. single block specimens and specimens with a single through-thickness block boundary parallel, perpendicular and at 45° to loading.

A unique micro-tensile methodology was developed included micro-specimen fabrication with minimal FIB damage, EBSD at top and bottom surfaces, a home-built highly-sensitive uni-axial tensile tester, and in situ microscopic slip trace analysis. Interestingly, all specimens showed extensive plasticity before fracture and no cleavage, however, strong differences are observed. Detailed analysis of the rich experimental data shows that not only the block but also the sub-block boundaries show boundary strengthening following a Hall–Petch relation, in the case that the easiest slip systems are crossed by these boundaries. However, for boundaries oriented under 45°, often easy glide is observed along the boundary reducing the strength, possibly caused by retained austenite films at the boundaries. TEM analysis of the boundary structure is ongoing.

This work focused on ascertaining the effect of pile-up during indentation of thin films on substrates. Conventional understanding has postulated that differences in contact area resulting from pile-up or sink-in significantly alter the extraction of material properties. In this work, the specific case of pile-up with compliant, plastically deforming films on stiff, non-plastically deforming substrates was studied. Several literature methods to assess pile-up were leveraged, and a new technique was developed and validated to quantify projected pile-up. Indentation testing was performed on gold films of multiple thicknesses on several ceramic-based substrates. The results indicated that the degree of pile-up was solely a function of indent depth into the film. Pile-up was not influenced by film thickness or substrate elastic modulus. In other words, the pile-up development was insensitive to the presence of the substrate and how it contributes to the composite’s elastic properties. In such case, if the elastic response of the film/substrate composite was independent of the degree of pile-up, then elastic data acquired from unloading did not require a contact area correction. The findings are confirmed using the Zhou–Prorok model for extracting film elastic properties for both gold and platinum films.

Size effects in micron sized pillars loaded in compression, fabricated out of metal single crystals with FIB milling has been reported by several authors. The size effect manifests in a significant amplification of the flow stress with decrease in diameter. Amorphous glassy polymers are widely used in micro-structures, such as in many micro-electro-mechanical system (MEMS) based devices in electronics and biological applications. But their mechanical behaviours at these length scales have not been adequately explored. In this work, we show that micropillars of an amorphous material, polystyrene (PS), also exhibit significant size effects under compression. The pillars are fabricated by methods that do not involve focused ion beam (FIB) milling and are therefore, free of ion damage. Micropillars of diameters ranging from 38 to 190 μm have been fabricated using a microfabrication technique and they are deformed in-situ in a FESEM under compression. The stress strain plots are obtained by fitting the load displacement behaviour using well calibrated constitutive model. Results suggest that the yield stress of PS intrinsically depend on the diameter.

Strong temperature and thickness dependent mechanical properties of Ti/Ni multilayer thin films have been observed with layer thickness from 200 nm to 6 nm and annealing temperature from room temperature to 500 °C. The as-deposited case follows the traditional trend of dislocation mediated-strengthening to grain boundary mediated-softening with decreasing layer thickness. Initial thermal strengthening of multilayers is achieved by annealing induced grain boundary relaxation. This strengthening is found to increase with decreasing layer thickness and increasing annealing temperature. Further strengthening could be achieved due to solid solution of diffused atoms and Ti-Ni intermetallic precipitates for multilayers with thin layer, while obvious softening has been observed for multilayers with thick layer due to recrystallization and grain growth.

Nanostructured electrodes have shown great potential in the development of Li-ion batteries with higher energy and power densities and longer cycle life. A fundamental understanding of the mechano-electrochemical behavior during charging/discharging cycles is essential for optimal and reliable design. Previous work has utilized in situ experimental techniques in an electron microscope to directly visualize material response during the reaction cycles. Unfortunately, the present in situ test methods are limited to room temperature and, as a result, the effect of temperature on charging/discharging cycles is not well understood. These electrochemical processes are intrinsically temperature sensitive, particularly for nanostructured electrodes. Here we present a novel microdevice that allows high resolution in situ observation of mechano-electrochemical response of nanomaterials in a scanning electron microscope with controlled temperature. The microdevice consists of built-in microcircuits for concurrent heating and temperature measurement during in situ experiments. To demonstrate these unique capabilities, we present the design, microfabrication and thermal characterization of this new class of microdevice.

Recent developments in high rate micro-mechanical testing have used miniaturized Kolsky (Split-Hopkinson) Bars as the basic loading technique. These methods, which employ optical instrumentation instead of strain gages, have been used to test samples in the 20–50 μm size range at rates as high as 1 M/s. However, difficulties attributed to machining and alignment of such small systems make higher strain-rates difficult to achieve. In this work, we investigate an alternate method in which a small compression sample will be loaded rapidly against a transparent elastic half space. As the specimen deforms, it exerts what is essentially a point load on the half space, resulting in an indentation at the contact point. This indentation is measured with a displacement interferometer and in principle can be used to calculate the force applied to the sample. This can then be used to determine the stress–strain response of the specimen. Because loading plates can be made and aligned to great precision, this method has the potential to reach strain-rates as high as 3 M/s provided the force can be correlated to measured displacement with sufficient accuracy. This paper presents preliminary experimental and numerical results that illustrate the challenges in implementing such an approach.

To gain a better understanding of the dynamic response of thermo-electro-mechanically coupled materials, in particular stiffness and mechanical damping, an experimental apparatus and method called Broadband Electromechanical Spectroscopy (BES) has been developed. The motivation for creating BES was to study the behavior of ferroelectric materials (and other electro-active materials) over wide ranges of frequencies of simultaneously-applied electric fields and mechanical stresses under accurate temperature control. By precisely controlling electric fields and mechanical loading, the effect of microstructural changes, in particular domain switching in ferroelectrics, on the material’s dynamic mechanical response is measured. Experiments show large increases in mechanical damping during domain switching that can be tuned by appropriate electrical loading. Results obtained using the new capabilities brought by BES can be used to better understand the damping associated with domain wall motion to be able to create ceramic materials with both high stiffness and high damping—an elusive combination of properties in typical engineering materials. The design and use of the apparatus along with results obtained from tests on a particular ferroelectric, viz. lead zirconate titanate, are presented.

Granular media support a rich array of acoustic phenomena that stem from their complex microstructure and highly nonlinear particulate interactions. While such complexity enables granular media’s unique properties, it simultaneously makes predicting their dynamic response challenging. An approach that has yielded insights into the dynamics of macroscale granular media is to study ordered and reduced-dimensional granular systems. Such simplified granular systems are commonly referred to as “granular crystals.” Granular media composed of micro- and nanoscale particles are predicted to be analogous in many ways to their macroscale counterparts, however, many phenomena that are negligible at macroscales, such as interparticle adhesion, become critical at sub-microscales. In this chapter, we review several recent studies of the contact-based dynamics of microscale granular crystals. This includes measurements of the interactions of guided and bulk acoustic waves with the contact resonances of self-assembled microsphere monolayers. These works have implications for the broader study of microscale granular materials, provide new ways to study microscale contact mechanics, and may result in a new class of materials for passive wave tailoring, which can be rapidly and inexpensively fabricated in large scales via self-assembly.