A comprehensive computational analysis is presented of the atomistic mechanisms of strain relaxation over a wide range of applied biaxial tensile strain in free-standing Cu thin films. The analysis is based on large-scale isothermal-isostrain MD simulations using slab supercells with cylindrical voids normal to the film plane and extending throughout the film thickness. Our analysis has revealed various regimes in the film's mechanical response as the applied strain level increases. Following an elastic response at low strain (≶ 2%), plastic deformation occurs accompanied by emission of screw dislocations from the void surface and threading dislocations from the film surfaces, in parallel with generation of vacancies due to slip of jogged dislocations. At the lower strain range following the elastic-to-plastic deformation transition (⋚ 6%), void growth is the major strain relaxation mechanism, while at higher levels of applied strain (≥ 8%), a subsequent transition leads to a new plastic deformation regime where void growth plays a negligible role in the film strain relaxation.

Surface morphology and features of pure copper heavily deformed by equal channel angular pressing were studied by means of an atomic force microscope (AFM) and scanning electron microscope (SEM) techniques. Surface characterization helps to develop a better understanding of microstructure evolution of materials subjected to any kind of mechanical loading and thermal treatment. The main objectives of this study are to exploit the capabilities of AFM to accurately perform an analysis of the surface features of nano-structured copper subjected to plastic deformation and heat treatment; and examination of the mechanical characteristics influenced by material's microstructure. It is shown that the AFM technique can be extremely useful for the investigations of a surface topography of materials.

A much better understanding of the contact mechanics can be obtained through finite element modeling. The experimentally determined tip shape function was included to reproduce the same contact area for a given contact depth as in the experiment. The fundamental material properties affecting pile-up are the ratio of the effective modulus to yield stress Eeff/Y and the work hardening rate.

The stability of interfaces in an ultrafine TiAl-(y)/Ti3Al-(α2) lamellar structure by straining at room temperature has been investigated using in-situ straining techniques performed in a transmission electron microscope. The purpose of this study is to obtain experimental evidence to support the previously proposed creep mechanisms in ultrafine lamellar TiAl alloys based upon the interface sliding in association with a cooperative movement of interfacial dislocations. The results have revealed that both the sliding and migration of lamellar interfaces can take place simultaneously as a result of the cooperative motion of interfacial dislocations.

A Cobalt-20wt.% Nickel polycrystal produced by electrodeposition has been investigated in planar and cross sections using a high resolution scanning electron microscope. The local texture, grain size, amount of phase and grain boundaries, were characterized by Electron Backscatter Diffraction (EBSD). The average grain size perpendicular to the grain growth direction was 400 nm. Parallel to it, a pronounced bimodal grain structure was observed with grains reaching more than 10 μm and grains of approximately 800 nm diameter.

Indentation techniques have been carried out to study the mechanical behavior of amorphous Al85Ni10La5 alloy powders produced by inert gas atomization. The present work reveals that this amorphous alloy undergoes a three-stage crystallization process in the temperature range of 250°C ~ 390°C, with a glass transition temperature of approximately 259°C. In this study, the influence of devitrification on mechanical response was investigated via indentation of the Al85Ni10La5 alloy powders annealed at various temperatures, for instance, at temperatures well below or close to glass transition (235°C, 245°C, 250°C), and well above glass transition (283°C). Moreover, the microstructure evolution and the formation of nanoscale crystallites were studied using TEM and XRD. The influence of devitrification on the indentation response was characterized, paying particular attention to shear band formation and variations in hardness. The hardening behavior was analyzed on the basis of a rule-of-mixtures approach by treating the partially crystallized alloy as a nanocomposite.

A triple torch plasma reactor was used to synthesize Si—C—N composite films via the thermal plasma chemical vapor deposition process. The argon-nitrogen plasma provided atomic nitrogen to carbon- and silicon-based reactants, which were injected through a central injection probe and ring configuration. Films were deposited with variations of the total nitrogen flow through the torches (1.5-4.5slm), reactant mixture (silicon tetrachloride and acetylene or hexamethyldisilazane) and substrate material (silicon and molybdenum). Micro X-ray diffraction was used to determine that both α-Si3N4 and β-Si3N4 were dominant in most of the depositions. Composites of silicon nitride and silicon carbide were synthesized on molybdenum. The bonding of amorphous phases was investigated using Fourier transform infrared spectroscopy, which indicated the presence of N—H, CHx and CΞ in various films. Indentation tests on the polished film cross-sections determined that large variations in hardness and elastic modulus existed for minor changes in film composition. Correlations between indentation results and scanning electron and optical microscope images showed that the mechanical properties greatly depend on the film morphology; the denser, smoother, and more crystalline films tended to display enhanced mechanical properties.

The present work analyses the effect of substrate deformation during the nanowire/nanotube bending test. An individual nanowire or nanotube is treated as a linear isotropic continuum. The substrate deformation is modeled by two coupled springs and the spring compliances arefunctions of the nanowire/nanotube diameter, and the Young moduli of the nanowire/nanotube and the substrates. An atomic potential is used to determine the adhesion between the nanowire/nanotube and its substrate. Consequently, a simple three dimensional Finite Element (FE) model is built to calculate the spring compliances. The load-displacement relation, which takes into account of substrate deformation, is derived in a closed form, which can be reduced to the load-displacement relations based on the simply-supported ends and the built-in ends. The numerical results indicate that the substrate deformation has a great influence on the determination of the Young modulus of a nanowire/nanotube from the bending test. The nanobridge test on carbon nanotubes is taken as an example to demonstrate the feasibility of the developed method.

With the semiconductor technologies continuously pushing the miniaturization limits, there is a growing interest in developing novel low dielectric constant (low-k) materials to replace traditional dense SiO2 based insulators. In order to survive the multi-step integration process and provide reliable material and structure for the desired integrated circuit (IC) functions, the new low-k materials have to be mechanically strong and stable. Thus the material selection and mechanical characterization are vital in the successful development of next generation low-k dielectrics. A new class of low-k dielectric materials, nanoporous pure-silica zeolite, is prepared in thin films using IC compatible spin coating process and characterized using depth sensing nanoindentation technique. The elastic modulus measurements of the zeolite thin films are found to be significantly higher than that of other porous silicates with similar porosity and dielectric constants. Correlations of the mechanical, microstructural and electrical properties are discussed in detail.

74Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by thermal annealing. Transmission electron microscopy (TEM) of as-grown samples reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding damage. Embedded nanocrystals experience large compressive stress relative to bulk, as measured by Raman spectroscopy of the zone center optical phonon. In contrast, ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals embedded in sapphire melt very close to the bulk melting point (Tm= 936 °C) whereas those embedded in silica exhibit a significant melting point hysteresis around Tm.

Nanoimprint is one of the most promising fabrication techniques for nanoelectronics. The main feature of this process is the compression of a thin film of a polymer resist by a rigid mold to produce a surface pattern. Hence, nanoimprint is essentially a mechanical forming process that depends greatly on the nanoscale mechanical behavior of the plastically deformed polymer film. Consequently, basic understanding of nanoimprint mechanics is imperative for improving pattern quality, reproducibility, and automation. The objective of this study was to elucidate the mechanical response of thin polymer films subjected to nanoindentation loading. Three deformation regimes were identified in the experiments performed with poly(methyl methacrylate) (PMMA) films of thickness in the range of 200-400 nm with a Berkovich tip of nominal radius of curvature equal to 100 nm. A three-layer model consisting of surface, intermediate, and interface layers was introduced to explain the mechanical response of the indented PMMA films. The spatial constraints imposed to the plastic flow of the interface layer by the rigid indenter and substrate surfaces produce a dynamic effect, demonstrated by the loading rate dependence of the deformation response. This phenomenon is of great importance to polymer plastic flow in nanoimprinting.

Epitaxial TiN/Cu bilayers and multilayers with periods L between 5 and 50 nm have been grown by ultrahigh vacuum ion beam sputtering on Si and MgO(001) substrates at room temperature. The deformation modes induced by a Berkovich nanoindent have been imaged using Focused Ion Beam – Transmission Electron Microscopy (FIB-TEM) and Atomic Force Microscopy (AFM). The observations suggest that the mechanical response of the multilayers is essentially governed by an extensive plastic flow inside the Cu layers, which is confined by a bending of the more rigid TiN layers. This specific deformation behavior, with no contribution of the interfaces as a barrier for dislocation motion could explain the absence of significant hardness enhancement in this system.

Mechanical milling at cryogenic temperatures (cryomilling) was applied to fabricate a composite powder with 20 wt. % B4C (submicron-to-several microns in size) in a nanocrystalline (NC) 5083 Al matrix. A uniformly blended powder with 50 wt. % cryomilled composite powder and 50 wt. % coarse-grained (CG) 5083 Al powder was degassed, cold isostatic pressed (CIPped) and extruded to form a composite with 10 wt. % B4C, 50 wt. % CG 5083 Al and balance NC 5083 Al. This tri-modal material was then tested for mechanical behavior under compressive and tensile load conditions at various temperatures. The composite exhibited an extremely high yield stress at room temperature, but limited ductility. Although the composite lost its strength at elevated testing temperatures rapidly, the retained strength was still much higher than that of the conventional 5083 Al. The composite exhibits its highest ductility of 26% at 200°C under tensile load. In compression, it plastically deformed uniformly at all the elevated temperatures (≥373 K) and did not fracture even when the deformation exceeded 30%. The microstructure of this composite, including the distribution of each phase, the grain sizes of the Al matrix, the interfaces between these three phases, and the fracture surfaces were characterized using transmission electron microscopy (TEM) and optical microscopy (OM) techniques. The relationship between the microstructures and mechanical properties was discussed.

Strain-rate jump tests in compression are carried out on nanostructured copper (grain size = 90 nm) at moderate temperatures (353K - 393K). Strain-rate sensitivity m is measured as a function of temperature, T, and strain rate, έ. Increasing temperature or decreasing strain rate induces an increase in the strain-rate sensitivity. For (έ, T) = (1×10-5 s-1, 393K), m is equal to 0.17 which is the highest value reported for nanocrystalline copper. These results of enhanced m are encouraging in terms of gain in ductility. The measurements emphasize the existence of a thermally activated mechanism different from the normal rate-controlling process observed for microcrystalline fcc metals.

The strong ion bombardment, applied during sputter deposition of diamond-like carbon films (DLC), which is needed to promote the growth of the sp3-bonded hard phase, inevitably is accompanied by compressive stress generation, thus limiting their maximum thickness. Gradient coatings with gradients in composition, constitution or properties are a well-known concept to manage such stress problems. The stepwise graded layer concept adjusts a graded constitution of the growing carbon film by a stepwise increase of the ion energy, i.e. the substrate bias voltage, during magnetron sputtering. To study the influence of the layer thickness on the expansion of the interface regions between the layers deposited with different bias voltage, samples with increasing deposition time of the top layer and thus thickness ratio were investigated by using the small angle cross-section nanoindentation method (SACS). It was revealed that the thickness of the interface regions is linearly dependent on the thickness ratio of the graded layers, which might be an evidence for stress-induced diffusion and relaxation processes in the carbon network. By using microindentation with a Berkovich indenter and ex-situ AFM-imaging it was found that all graded films exhibited higher Berkovich thresholds for crack development and thus better crack resistance than the hardest single-layer film and kept a high hardness value of about 4000 HV0.005.

We present surprising evidence that the fracture resistance of porous forms of poly(arylene) ether (PAE) films exhibit increasing fracture resistance with increasing porosity. Such behavior is in stark contrast to the fracture toughness of porous solids, which typically decrease markedly with increasing porosity. A fracture mechanics based model is presented to rationalize the increase in fracture toughness of the voided polymer film and explain the behavior in terms of the pore size and volume fraction. It is shown that a certain dependence of pore size and volume fraction is required to increase rather than decrease the fracture resistance. The research has implications for the optimum void size and volume fraction needed to enhance the fracture resistance of porous ductile polymer films.

In this work we focus upon theoretical exploration of the mechanical constitutive behavior of amorphous nanocomposites in terms of a multi-scale approach starting from the nanostructure. Local heterogeneous stress field and deformation are calculated based on the concept of eigenstrain and equivalent inclusion method. The overall elastoplastic constitutive model for amorphous nanocomposites is developed through homogenization averaging procedures. Explicit expressions of the effective elastic stiffness and yield strength of amorphous nanocomposites in terms of the constituents' properties and nanostructures are obtained. An interlayer phase between nanoparticles and the amorphous matrix is experimentally observed and incorporated in the proposed model. The interlayer thickness is treated as a characteristic length scale. Thus, the particle size effect on the nanocomposite properties is particularly investigated within continuum nanomechanics framework. It provides direct determination of the intrinsic mechanisms of the nanocomposite structure-property relationship at the nanoscale.

Several theories and program codes were developed for large-scale atomistic simulations with fully quantum mechanical description of electron systems. The fundamental concepts are generalized Wannier state and Krylov subspace. Test calculations were carried out with upto 106 atoms using a standard workstation. How electronic state is described in large-scale calculation was demonstrated on Si(001)-(2×1) surface. As a practical application, cleavage fracture of silicon was simulated with 10-nm-scale samples for investigating its nanoscale mechanical behavior. Discussions are focused on the unstable (001) cleavage mode and the stable (experimentally observed) (111)-(2×1) cleavage mode. As well as elementary surface reconstruction, step formation and bending in cleavage path were observed. These results were compared with experiments, such as scanning tunneling microscope (STM).

This paper provides a perspective on submicron-resolution three-dimensional x-ray structural microscopy for the investigation of materials microstructure and evolution on mesoscopic length scales. Microstructure and thermal/stress induced microstructure evolution at the length scale of the local crystal grains, grain boundaries, triple junctions, and dislocation patterning are known to play critical roles in determining the physical properties of materials. Large scale computer simulations and multi-scale modeling are making significant progress on the nanoscale and detailed mesoscale computations are becoming increasingly tractable as new methods are developed. X-ray structural microscopy with submicron resolution now provides a (previously missing) direct experimental link between theory and computations and the mesoscopic behavior of materials microstructure.