Matthew R. Begley

The mechanics of size-dependent indentation

Indentation tests at scales on the order of one micron have shown that measured hardness increases significantly with decreasing indent size, a trend at odds with the size-independence implied by conventional plasticity theory. In this paper, strain gradient plasticity theory is used to model materials undergoing small-scale indentations. Finite element implementation of the theory as it pertains to indentation modeling is briefly reviewed. Results are presented for frictionless conical indentations. A strong effect of including strain gradients in the constitutive description is found with hardness increasing by a factor of two or more over the relevant range of behavior. The results are used to investigate the role of the two primary constitutive length parameters in the strain gradient theory. The study indicates that indentation may be the most effective test for measuring one of the length parameters.

The Mechanical Response of Freestanding Circular Elastic Films Under Point and Pressure Loads

This paper provides a comprehensive description of the mechanical response of freestanding circular elastic films subjected to point and pressure loads. Regimes of behavior, such as plate, linear membrane, and nonlinear membrane, are identified in terms of two dimensionless variables that allow the creation of a single map that indicates appropriate closed-form solutions. This map provides a theoretical framework to design experiments and interpret film behavior for all orders of magnitude of: film thickness-to-span ratio, deflection, loads, prestretch, and elastic properties. The normalization approach provides the means to quickly identify appropriate simplifications to the nonlinear governing equations, and identify applicable analytical solutions. Numerical results are used to illustrate behavior in transition regions, e.g., the transition for a given plate thickness from small to large deflections under increasing load. Critical loads, thickness and prestretch are identified which indicate when asymptotic plate or membrane solutions are accurate. Asymptotic and numerical results are presented which illustrate finite-sized regions of bending-influenced deformation near point loads and clamped edges. Theoretical predictions for the width of these regions enable us to estimate the validity of analytical strain distributions, and in turn the maximum strains in the film. These results help avoiding brittle fracture or ductile yielding of the film by identifying physical parameters that limit strains to an acceptable level.

Nanoporous Gold: Fabrication, Characterization, and Applications

Nanoporous gold (np-Au) has intriguing material properties that offer potential benefits for many applications due to its high specific surface area, well-characterized thiol-gold surface chemistry, high electrical conductivity, and reduced stiffness. The research on np-Au has taken place on various fronts, including advanced microfabrication and characterization techniques to probe unusual nanoscale properties and applications spanning from fuel cells to electrochemical sensors. Here, we provide a review of the recent advances in np-Au research, with special emphasis on microfabrication and characterization techniques. We conclude the paper with a brief outline of challenges to overcome in the study of nanoporous metals.

Analysis of a wedge impression test for measuring the interface toughness between films/coatings and ductile substrates

Abstract The adhesion between thin elastic films/coatings and ductile substrates can be measured using a wedge test, wherein a long sharp edge is impressed through the film into the substrate. The resulting plastic deformation causes delamination between the film and the substrate. The extent of the delamination can be correlated with the crack driving force to determine the toughness of the interface. Results are presented that relate the energy release rate and mode-mixity to the stresses: both residual and those induced by the impression. Numerical and asymptotic results are presented for the strain transferred to the coating. The numerical results verify that the asymptotics provide accurate closed-form solutions when the delaminations exceed about seven contact widths. An example is provided for a thermal barrier coating system.

The fabrication of low-impedance nanoporous gold multiple-electrode arrays for neural electrophysiology studies

Neural electrodes are essential tools for the study of the nervous system and related diseases. Low electrode impedance is a figure of merit for sensitive detection of neural electrical activity and numerous studies have aimed to reduce impedance. Unfortunately, most of these efforts have been tethered by a combination of poor functional coating adhesion, complicated fabrication techniques, and poor fabrication repeatability. We address these issues with a facile method for reliably producing multiple-electrode arrays with low impedance by patterning highly adherent nanoporous gold films using conventional microfabrication techniques. The high surface area-to-volume ratio of self-assembled nanoporous gold results in a more than 25-fold improvement in the electrode-electrolyte impedance, where at 1 kHz, 850 kOmega impedance for conventional Au electrodes is reduced to 30 kOmega for nanoporous gold electrodes. Low impedance provides a superior signal-to-noise ratio for detection of neural activity in noisy environments. We systematically studied the effect of film morphology on electrode impedance and successfully recorded field potentials from rat hippocampal slices. Here, we present our fabrication approach, the relationship between film morphology and impedance, and field potential recordings.

Plasticity in fretting contact

The fretting problem of a cyclically loaded cylinder on a flat elastic–plastic surface is analyzed under the assumption of plane strain. Severe fretting conditions are modeled by applying a constant normal load and a cyclic tangential load to the cylinder and describing the contact behavior using a Coulomb friction law. Detailed numerical results are presented for the evolution of plastic strains in the substrate as a function of loading and friction coefficient. Shakedown maps and cyclic plastic strain behavior maps are created to describe the relative contribution of cyclic plasticity, ratcheting and shakedown as a function of loading. Cyclic plastic strain amplitudes are presented as a function of tangential load amplitude for various levels of normal pressure and friction using extensive finite element computations. Several plasticity models are considered: elastic/perfectly-plastic, isotropic strain hardening and kinematic strain hardening. The results indicate that, while the plastic strain amplitudes decrease with increased strain hardening, the qualitative behavior is insensitive to the choice of plasticity model. The results are compared with corresponding fully elastic analyses to highlight the role of plasticity during contact deformation and to evaluate the implications of using fully elastic analyses to predict crack nucleation.

Spherical impression of thin elastic films on elastic-plastic substrates

The mechanical behavior of thin elastic films deposited onto structural alloys plays a critical role in determining film durability. This paper presents analysis of an impression experiment designed to evaluate some of the relevant properties of these films. The modeling provides quantitative strain information which can be used to estimate the fracture toughness of the film, the static friction coefficient of the surface and the constitutive behavior of the substrate. Results are presented for radial and circumferential strain distributions in the film relevant to the interpretation of cracking patterns. Additionally, load-displacement curves are provided that may be used to evaluate the plastic properties of the substrate. To facilitate estimates of the film cracking strain through correlation with experiments, the radial strain distributions are presented as functions of impression depth, yield strain and hardening exponent.

Solid phase extraction of DNA from biological samples in a post-based, high surface area poly(methyl methacrylate) (PMMA) microdevice

This work describes the performance of poly(methyl methacrylate) (PMMA) microfluidic DNA purification devices with embedded microfabricated posts, functionalized with chitosan. PMMA is attractive as a substrate for creating high surface area (SA) posts for DNA capture because X-ray lithography can be exploited for extremely reproducible fabrication of high SA structures. However, this advantage is offset by the delicate nature of the posts when attempting bonding to create a closed system, and by the challenge of functionalizing the PMMA surface with a group that invokes DNA binding. Methods are described for covalent functionalization of the post surfaces with chitosan that binds DNA in a pH-dependent manner, as well as for bonding methods that avoid damaging the underlying post structure. A number of geometric posts designs are explored, with the goal of identifying post structures that provide the requisite surface area without a concurrent rise in fluidic resistance that promotes device failure. Initial proof-of-principle is shown by recovery of prepurified human genomic DNA (hgDNA), with real-world utility illustrated by purifying hgDNA from whole blood and demonstrating it to be PCR-amplifiable.

Adhesion of micro-cantilevers subjected to mechanical point loading: modeling and experiments

This paper presents experimental and theoretical results that characterize the adhesion of MEMS cantilevers by means of mechanical actuation. Micro-cantilever beams are loaded at various locations along the freestanding portion of the beam using a nanoindenter. Transitions between three equilibrium configurations (freestanding, arc-shaped, and s-shaped beams) and the response to cyclic loading are studied experimentally. The resulting mechanical response is used to estimate the interface adhesion energy (using theoretical models), and to quantify the energy dissipated during cyclic loading. The experiments reveal interesting behaviors related to adhesion: (i) path dependence during mechanical loading of adhered beams, (ii) history dependence of interfacial adhesion energy during repeated loading, and (iii) energy dissipation during cyclic loading, which scales roughly with estimated cyclic changes in the size of the adhered regions. The experimental results are interpreted in the context of elementary fracture-based adhesion and contact models, and briefly discussed in terms of their implications regarding the nature of adhesion and future modeling to establish adhesion mechanisms.

The role of initial flaw size, elastic compliance and plasticity in channel cracking of thin films

In this paper, we consider the effects of initial flaw size and plasticity in adjacent layers on the formation of channeling cracks in thin films. Fully three-dimensional finite element analyses are used to determine energy release rates as a function of flaw size for both contained through cracks and edge cracks intersecting free surfaces. The results indicate that substantially larger flaws are required to achieve steady state for edge flaws and when the substrate is more compliant than the film. For edge flaws, the crack length required to achieve steady state is significantly larger than the film thickness, in contrast to conventional wisdom, which assumes steady state is reached when the crack length exceeds only several film thickness. The effect of residual stress in adjacent ductile layers is illustrated for a two-layer system bonded to an elastic substrate. Residual stress in the middle layer promotes plasticity adjacent to the crack and leads to much larger energy release rates than similar scenarios with films on ductile substrates without residual stress. Comparisons are made between several methods for predicting energy release rates, with the goal of identifying the validity of 2-D steady state approximations. The results can be used to predict critical flaws sizes that lead to film failure and to identify potential susceptibility to inelastic cracking mechanisms.