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FOREWORD

J. Eng. Mater. Technol. 2005;127(4):357. doi:10.1115/1.2020037.
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The behavior or materials at the nanoscale and the behavior of nanocomponents such as nanowires and nanotubes have been subjects of increasing research in recent years. The idea of dedicating a special issue of the Journal of Engineering Materials and Technology (JEMT) to original research on nanomaterials and nanomechanics was conceived in 2003 and the project began in 2004. The objective is to provide a survey of current research activities and compile a collection of some of the latest research results.

Topics: Nanomaterials
Commentary by Dr. Valentin Fuster

SPECIAL SECTION ON NANOMATERIALS AND NANOMECHANICS

J. Eng. Mater. Technol. 2005;127(4):358-368. doi:10.1115/1.1924561.

Simulations of nanoindentation in single crystals are performed using a finite temperature coupled atomistic/continuum discrete dislocation (CADD) method. This computational method for multiscale modeling of plasticity has the ability of treating dislocations as either atomistic or continuum entities within a single computational framework. The finite-temperature approach here inserts a Nose-Hoover thermostat to control the instantaneous fluctuations of temperature inside the atomistic region during the indentation process. The method of thermostatting the atomistic region has a significant role on mitigating the reflected waves from the atomistic/continuum boundary and preventing the region beneath the indenter from overheating. The method captures, at the same time, the atomistic mechanisms and the long-range dislocation effects without the computational cost of full atomistic simulations. The effects of several process variables are investigated, including system temperature and rate of indentation. Results and the deformation mechanisms that occur during a series of indentation simulations are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):369-373. doi:10.1115/1.1924562.

A thermodynamic framework is presented for the theory of Viscoplasticity Based on Overstress (VBO) developed by Krempl and co-workers (Krempl, E., and Ho, K., 2001, in Lemaitre Handbook of Materials Behavior Models, Academic Press, New York, pp. 336–348; 2000, in Time Dependent and Nonlinear Effects in Polymers and Composites, ASTM STP 1357, Schapery, R. A., and Sun, C. T., eds., ASTM, West Conshohocken, PA, pp. 118–137; Cernocky, E. P., and Krempl, E., 1979, Int. J. Non-Linear Mech., 14, pp. 183–203; Gomaa, 2004, Int. J. Solids Struct., 41, pp. 3607–3624), for anisotropic materials and small deformations. A Caratheodory-based approach is applied to demonstrate the existence of entropy and absolute temperature, as previously described by Hall (2000, Compos. Sci. Technol., 60, pp. 2581–2599). The present framework indicates that the stress rate-dependent term in the established growth law for the equilibrium stress cannot contribute to the dissipation, and is therefore referred to here as the elastic equilibrium stress rate. A new temperature rate-dependent term is obtained for the same growth law, which is also required to be dissipationless. These terms are therefore identified with dissipationless changes of the stored energy and∕or entropy. In general, the traditional, and thermodynamically justified, forms for the potential functions that arise in the present nonequilibrium treatment lead to dissipationless contributions from internal variable growth law terms that are linear in the rates of the controllable variables. Similar indications, without first establishing entropy and absolute temperature existence, were noted in the modeling of Lehmann (1984, in The Constitutive Law in Thermoplasticity, T. Lehmann, ed., Springer, New York).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;127(4):374-382. doi:10.1115/1.1867983.

Molecular dynamics calculations are performed to study the effect of deformation sequence and history on the inelastic behavior of copper interfaces on the nanoscale. An asymmetric 45 deg tilt bicrystal interface is examined, representing an idealized high-angle grain boundary interface. The interface model is subjected to three different deformation paths: tension then shear, shear then tension, and combined proportional tension and shear. Analysis shows that path-history dependent material behavior is confined within a finite layer of deformation around the bicrystal interface. The relationships between length scale and interface properties, such as the thickness of the path-history dependent layer and the interface strength, are discussed in detail.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;127(4):383-392. doi:10.1115/1.1867980.

When two material surfaces are brought into contact, the classical Amonton’s law predicts a monotonically increasing relation between the frictional force and the normal pressure. An abnormal friction law refers to the case where the friction force declines as the normal pressure increases. We investigate the possibility of abnormal tribological behavior for two surfaces coated with aligned multiwalled nanotube rafts. Part I of the investigation is devoted to the case when two contacting nanotube rafts are aligned to each other, while part II is aimed at more general case of arbitrarily oriented nanotube rafts. The analysis in part I is based on the JKR theory of adhesion and linear elasticity for aligned multiwalled carbon nanotube raft configuration. It gives rise of several interesting predictions. First, two surfaces covered by aligned nanotubes can adhere when bringing into a pressureless contact. Second, the aligned multiwalled nanotube rafts exhibit a detachment work that declines with the contacting pressure. Third, in contrast to the Amonton’s law, the frictional force would decline as the normal pressure increases.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;127(4):393-399. doi:10.1115/1.1867982.

The analysis in part I of this series is extended to the case of inclined and randomly distributed multiwall carbon nanotube (MWCNT) rafts that are brought into contact. The MWCNTs are modeled by elastic and cylindrically anisotropic materials. The JKR theory of adhesion is adopted. With the incorporation of three-dimensional contact configuration that features the inclined contact, we are able to show that the abnormal tribological behavior, in drastic contrast to the classical Amonton’s law, persists for both inclined and randomly oriented rafts.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):400-407. doi:10.1115/1.1925288.

A self-consistent scheme is used to describe the behavior of nanocrystalline F.C.C. materials. The material is approximated as a composite with two phases. The inclusion phase represents the grain cores while the matrix phase represents both grain boundaries and triple junctions. The dislocation glide mechanism is incorporated in the constitutive law of the inclusion phase while a thermally activated mechanism accounting for the penetration of dislocations in the grain boundaries is incorporated in the constitutive law of the matrix phase. The model is applied to pure Cu and the results are compared with various experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):408-416. doi:10.1115/1.2019865.

There are significant efforts to develop continuum theories based on atomistic models. These atomistic-based continuum theories are limited to zero temperature (T=0K). We have developed a finite-temperature continuum theory based on interatomic potentials. The effect of finite temperature is accounted for via the local harmonic approximation, which relates the entropy to the vibration frequencies of the system, and the latter are determined from the interatomic potential. The focus of this theory is to establish the continuum constitutive model in terms of the interatomic potential and temperature. We have studied the temperature dependence of specific heat and coefficient of thermal expansion of graphene and diamond, and have found good agreements with the experimental data without any parameter fitting. We have also studied the temperature dependence of Young’s modulus and bifurcation strain of single-wall carbon nanotubes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;127(4):417-422. doi:10.1115/1.1924558.

First principle, tight binding, and semi-empirical embedded atom calculations are used to investigate a tetragonal phase transformation in gold nanowires. As wire diameter is decreased, tight binding and modified embedded atom simulations predict a surface-stress-induced phase transformation from a face-centered-cubic (fcc) ⟨100⟩ nanowire into a body-centered-tetragonal (bct) nanowire. In bulk gold, all theoretical approaches predict a local energy minimum at the bct phase, but tight binding and first principle calculations predict elastic instability of the bulk bct phase. The predicted existence of the stable bct phase in the nanowires is thus attributed to constraint from surface stresses. The results demonstrate that surface stresses are theoretically capable of inducing phase transformation and subsequent phase stability in nanometer scale metallic wires under appropriate conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):423-433. doi:10.1115/1.1928915.

Molecular dynamics simulations are carried out to analyze the structure and mechanical behavior of Cu nanowires with lateral dimensions of 1.45–2.89 nm. The calculations simulate the formation of nanowires through a “top-down” fabrication process by “slicing” square columns of atoms from single-crystalline bulk Cu along the [001], [010], and [100] directions and by allowing them to undergo controlled relaxation which involves the reorientation of the initial configuration with a 001 axis and {001} surfaces into a new configuration with a 110 axis and {111} lateral surfaces. The propagation of twin planes is primarily responsible for the lattice rotation. The transformed structure is the same as what has been observed experimentally in Cu nanowires. A pseudoelastic behavior driven by the high surface-to-volume ratio and surface stress at the nanoscale is observed for the transformed wires. Specifically, the relaxed wires undergo a reverse transformation to recover the configuration it possessed as part of the bulk crystal prior to relaxation when tensile loading with sufficient magnitude is applied. The reverse transformation progresses with the propagation of a single twin boundary in reverse to that observed during relaxation. This process has the diffusionless nature and the invariant-plane strain of a martensitic transformation and is similar to those in shape memory alloys in phenomenology. The reversibility of the relaxation and loading processes endows the nanowires with the ability for pseudoelastic elongations of up to 41% in reversible axial strain which is well beyond the yield strain of the approximately 0.25% of bulk Cu and the recoverable strains on the order of 8% of most bulk shape memory materials. The existence of the pseudoelasticity observed in the single-crystalline, metallic nanowires here is size and temperature dependent. At 300 K, this effect is observed in wires with lateral dimensions equal to or smaller than 1.81×1.81nm. As temperature increases, the critical wire size for observing this effect increases. This temperature dependence gives rise to a novel shape memory effect to Cu nanowires not seen in bulk Cu.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):434-443. doi:10.1115/1.1924559.

Manipulating the strain distribution along the surface of a substrate has been shown experimentally to promote spatial ordering of self-assembled nanostructures in heteroepitaxial film growth without having to resort to expensive nanolithographic techniques. We present here numerical studies of three-dimensional modeling of self-assembly in Si-Ge systems with the aim of understanding the effect of spatially varying mismatch strain-fields on the growth and ordering of quantum dots. We use a continuum model based on the underlying physics of crystallographic surface steps in our calculations. Using appropriate parameters from atomistic studies, the (100) orientation is found to be unstable under compressive strain; the surface energy now develops a new minimum at an orientation that may be interpreted as the (105) facet observed in SiGeSi systems. This form of surface energy allows for the nucleationless growth of quantum dots which start off via a surface instability as shallow stepped mounds whose sidewalls evolve continuously toward their low-energy orientations. The interaction of the surface instability with one- and two-dimensional strain modulations is considered in detail as a function of the growth rate. One-dimensional strain modulations lead to the formation of rows of dots in regions of low mismatch—there is some ordering within these rows owing to elastic interactions between dots but this is found to depend strongly upon the kinetics of the growth process. Two-dimensional strain modulations are found to provide excellent ordering within the island array, the growth kinetics being less influential in this case. For purposes of comparison, we also consider self-assembly of dots for an isotropic surface energy. While the results do not differ significantly from those for the anisotropic surface energy with the two-dimensional strain variation, the one-dimensional strain variation produces profoundly different behavior. The surface instability is seen to start off initially as stripes in regions of low mismatch. However, since stripes are less effective at relaxing the mismatch strain they eventually break up into islands. The spacing of these islands is determined by the wavelength of the fastest growing mode of the Asaro-Tiller-Grinfeld instability. However, the fact that such a growth mode is not observed experimentally indicates the importance of accounting for surface energy anisotropy in growth models.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):444-450. doi:10.1115/1.1925290.

In AFM measurements of surface morphology, the locality is a traditional assumption, i.e., the load recorded by AFM is simply the function of the distance between the tip of AFM and the point on a sample right opposite the tip [Giessibl, F. J., 2003, “Advances in Atomic Force Microscopy  ,” Rev. Mod. Phys., 75, pp. 949–983]. This paper presents that nonlocality effect may play an important role in atomic force microscopic (AFM) measurement. The nonlocality of AFM measurement results from two different finite scales: the finite scale of the characteristic intermolecular interaction distance and the geometric size of AFM tip. With a coupled molecular-continuum method, we analyzed this nonlocality effect in detail. It is found that the nonlocality effect can be formulated by a few dimensionless parameters characterizing the ratio of the following scales: the characteristic intermolecular interaction distance between the AFM tip and the sample, the characteristic size of the tip and the characteristic nano-structure and∕or the nanoscale roughness on the surface of a sample. The present work also suggests a data processing algorithm—the approaching method, which can reduce the nonlocality effect in AFM measurement of surface morphology effectively.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):451-456. doi:10.1115/1.1925292.

Nanocomposite thin films which consist of 50nmAl2O3 nanoparticles in a copper metal matrix were deposited on a silicon wafer. The thickness of the nanocomposite thin films was about 3microns and the volume density of the nanoparticles was between 3% and 5%. The films were synthesized using electrocodeposition. The grain size of the nanocomposite film was significantly smaller than the grain size of control films of pure copper. Electron backscatter diffraction (EBSD) experiments indicate that neither the nanocomposite thin films nor the control films exhibits a crystallographic texture. Nanoindentation experiments show that the hardness of the nanocomposite thin film is approximately 25% higher than the hardness of the control films of pure copper. A prototype of a microchannel array in the nanocomposite thin film was made using standard microelectromechanical (MEMS) fabrication technology. It is expected that the enhanced mechanical properties exhibited by nanocomposite thin films have the potential to improve the reliability of various MEMS devices which rely on thin metal films. The results presented herein lay the groundwork for future studies in which the size, volume density, morphology, distribution as well as type of nanoparticle in the nanocomposite will be systematically and independently varied in order to optimize mechanical properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):457-461. doi:10.1115/1.2020014.

Highly disordered, ion-processed silicon is studied using a molecular dynamics simulation with empirical interatomic potentials. The surface free energy density, stress-strain relations, and continuum surface features of silicon, bombarded in the simulations to relatively high fluence by medium energy argon ions, are computed statistically by preparing multiple randomized ion-bombarded specimens. The surface-free energy per unit area for the ion-bombarded silicon is about 1.76Jm2, much lower than the 2.35Jm2 corresponding to a (001) unrelaxed, crystalline silicon surface. A stress-strain curve is obtained computationally by performing a constant strain test on the ion-bombarded specimens and by calculating stresses from the interatomic forces acting across different cross sections in the sample. The resulting tensile elastic modulus of the material, while slightly elevated due to the prominence of the free surface in the thin layer, is in good agreement with available experimental data. The surface is characterized using an interatomic potential-based C2 continuous sampling method.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):462-467. doi:10.1115/1.2019898.

We investigate the interactions between substitutional carbon atoms on the defect free, (2×1) reconstructed Si(001) surface, and bring evidence that the interaction energy differs significantly from the inverse-cube distance dependence that is predicted by the theory of force dipoles on an elastic half-space. Based on Tersoff potentials, we also calculate the interactions between carbon atoms and dimer vacancies. The calculations indicate that dimer vacancies (DVs) are strongly stabilized by fourth-layer C atoms placed directly underneath them. By use of simple model Monte Carlo simulations, we show that the computed interactions between carbon atoms and DVs lead to self-assembled vacancy lines, in qualitative agreement with recent experimental results.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. Eng. Mater. Technol. 2005;127(4):468-475. doi:10.1115/1.2019944.

This paper presents results from an analytical and experimental study of the effect of temperature and mixed-mode ratio on the interlaminar fracture toughness in glass-cloth∕epoxy laminates. Mode I, mode II, and mixed-mode tests were conducted by the double-cantilever beam, end-notched flexure, and mixed-mode bending test methods at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). A finite element model was used to perform the delamination crack analysis. Mode I, mode II, and mixed-mode energy release rates at the onset of delamination crack propagation were computed using the virtual crack closure technique. The fracture surfaces were examined by scanning electron microscopy to correlate with the interlaminar fracture properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(4):476-482. doi:10.1115/1.2019983.

In order to develop a procedure for evaluating the degradation of impact toughness in directionally solidified Ni-base superalloy CM247LC, which is commonly used for advanced gas turbine blades, the change in small punch (SP) fracture energy due to thermal aging has been investigated. The SP testing technique has been applied to materials aged under various aging conditions, and correlation with the results of Charpy V-notch impact tests has been examined. The experimental results reveal that SP fracture energy at room temperature decreases with aging at 800 °C and is uniquely correlated with high-temperature Charpy impact toughness. The current experiment has shown that the SP testing technique is useful in evaluating the degree of thermal aging embrittlement, one of the parameters required for remaining-life prediction of aged components.

Commentary by Dr. Valentin Fuster

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