Aqueous suspensions of colloidal silicas are readily silylated with chlorosilanes in the presence of acid and isopropyl alcohol without aggregation of the silica particles. By using chlorosilanes, spherical nanoparticles with controlled functionality can be made and transferred to an organic phase to provide stable, water free suspensions. The hydrophobic colloidal silica particles readily disperse into silicone polymers. In order to provide mechanical reinforcement comparable to traditional fumed silicas, a high filler loading of ca. 60 wt % mono-dispersed colloidal silicas need to be used.

Recently a unique pearl necklaced-shaped colloidal silicas become commercially available. These nanoparticles are successfully silylated and they maintain the original structures as evidenced by transmission electron microscopy (TEM). They are compounded into silicone polymers at ca. 30 wt % and they provide excellent viscosities and mechanical properties, especially the tear strengths. In this paper, we present the silylation, characterization and mechanical properties of these partially aggregated colloidal silica nanoparticles in silicone elastomers.

The structural evolution of polytetrafluoroethylene (PTFE) crystalline polymer latex films is studied at hundreds nanometer length scale by atomic force microscopy and Brillouin light scattering. In a controlled sintering process the transition is observed from the original particle distribution towards a ‘fibrillar’ structure of crystalline regions embedded in a disordered matrix. This transition is accompanied by a cross-over from localized acoustic excitations to propagating acoustic phonons, related to mesoscopic elastic properties. After sintering, a ‘mark’ of the original particulate structure persists, suggesting that filming of crystalline polymers may be analogous to sintering of ceramic powders. Films of crystalline polymers can thus be exploited as model systems to study the elasto-optical properties of granular and disordered media.

TiC and TiCN thin films were deposited by RF sputtering from a TiC target. For TiC thin films, the various sputtering pressure were carried out in order to observe the influence of this parameter on structural and mechanical properties. The experimental results show that a pressure of 1 Pa is necessary to obtain stoichiometric films with <111> texture. Lower pressures induce the formation of distorted titanium carbide, while RBS spectra show that the Ti/C ratio is constant for all these samples. Both the compressive stress and the hardness exhibited a maximum value for the lowest pressures. For TiCN thin films, the composition and the hardness were investigated as function of the N2 partial pressure (PN2 = 3% to 70%).

A novel experimental configuration is devised to measure the evolution of the deformation field and the corresponding hardening evolution within soft metallic films constrained by hard layers. The experimental configuration provides pure shear state within the constrained film. The material system utilized comprised ductile layer of tin based solder, encapsulated within relatively hard copper shoulders. Different tin-lead compositions are tested with grain size approaching the film thickness. The in-plane strain distribution within the film layer is measured by a microscopic digital image correlation system. The hardening evolution within such highly gradient deformation field is monitored qualitatively through a 2D surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order of 3-4 the grain size. A size-dependent macroscopic response on the layer thickness is observed. However, the corresponding film thickness is approximately 100-1000 times larger than those predicted by non-local continuum theories and discrete dislocation.

The properties of nanocrystals were studied by in-situ high-pressure powder diffraction technique using our methodology of the structural analysis, the “apparent lattice parameter” (alp) concept. The experiments were performed for nanocrystalline SiC and diamond powders using synchrotron and neutron sources and hydrostatic or isostatic pressure. Elastic properties of the samples were examined based on the measurements of the lattice parameters and of the reflection width, as well as of interatomic distances. The results show a dual nature of the properties (compressibilities) of the powders indicating a complex, core-shell structure of the grains.

Grain size can be controlled by varying the process conditions used to deposit the film and post growth processing. This study examines how the grain size will impact the mechanical properties, hardness and adhesion, of thin ductile films on brittle substrates. The grain structures of copper and tin films were examined using OIM and AFM. The interfacial fracture toughness of each film was calculated using a tungsten overlayer and mechanics based models. The hardnesses of the films were correlated to the measured interfacial fracture toughness, and demonstrate that increased hardness correlates to an increased sensitivity to the Mode II component of loading.

Plasticity is a significant contributor to the interfacial fracture resistance of multilayer thinfilm structures containing ductile layers. In this study we investigate the effect of a nanoscale elastic barrier layer separating the crack from a plastically deforming layer. The simulation procedure is multiscale in the sense that the simulation zone encompasses only a small region around the crack tip that includes the layers of interest and the bulk of the sample is modeled as applied boundary conditions on that region. The salient parameters governing the plasticity contribution to interfacial fracture energy, GC, including the barrier layer thickness and elastic properties, intrinsic fracture energy G0, and the maximum cohesive stress governing interface separation are reported. In addition, we explore the effect of residual stresses on debonding with particular attention to the the stress state in the ductile layer. It is shown that residual thin-film stresses can alter plasticity in the ductile layer and significantly influence the macroscopic fracture energy.

This paper discusses the interaction between mechanical properties of a thin layer and those of its substrate. The underlined questions are: how the substrate's mechanical properties affect those of the film; how the layer's thickness affects its mechanical properties. Nanoindentation tests were performed on polymer films of different thicknesses deposited onto a silica substrate. The mechanical response of such a system is known to be a composite response (film+substrate). Our aim is to understand the elastic behavior of the film inside this structure.

Model was used to estimate the elastic modulus of the film from the global measured value. For thin and “compliant” polymer layers deposited on “hard” substrates, an increase of the film elastic modulus was observed along indentation. We attribute this increase to the “anvil effect”: the bulk elastic modulus increases with the hydrostatic pressure which results from the compression of the film. In conclusion, one first interpretation is proposed to understand specific behavior of thin soft polymer layers on rigid substrates.

To measure the Ag/Co interfacial free energies, biaxial zero creep experiments were performed on Ag/Co multilayer films deposited on the Si (111) wafers. As the samples were heated from room temperature to 450°C, the residual stress in films, which was in situ monitored by substrate curvature technique, decayed gradually to zero due to the increasing plastic deformation in films. After held for several hours at 450°C, they reached a zero creep state while the equilibrium stresses were measured. The annealed element layers were immiscible, and exhibited the column grain distribution and (111) preferred orientations. Based on the Josell model, the free energy of Ag/Co (111) interfaces at 450°C was found to be 1.02 ± 0.17 J/m2.

Recent experimental work (J. Gray, R.Hull and J. Floro, App. Phys. Lett. Vol 81, No 13, 2002) has investigated the cooperative nucleation and growth of pits and islanding at pit edges in SiGe/Si(100) epitaxial films. The pits, which are observed to form under certain growth conditions, evolve along with the islands until a complete island wall is formed around the pit's periphery. Once formed the pit'island configuration exhibits a surprising stability, with no further lateral growth during continued deposition or annealing. The pits consists of low energy {501} facets (inverted {501} pyramid) and the islands surrounding the pit also exhibiting faceting. Elastic finite element analyses of these configurations were carried out to investigate the dependence of strain energy and surface stress on pit/island geometry. In particular, our results provide insight as to the mechanism responsible for the observed size stability of the pit/island structure once it has become fully developed. Surface stress gradients arising from the geometry are also found to be responsible for the observed initial island formation at the center of pit edges during growth.

Mechanical alloying techniques, which use plastic deformation to effect structural changes, are commonly used to prepare nanostructured metals with exemplary mechanical properties. When these nanostructured metals are subjected to further straining fully amorphous alloys can result, but there is little understanding of the atomic-scale mechanisms behind this amorphization. In the present work, we explore the final stages of such a mechanical alloying process via molecular simulations. Initial Cu-Zr nanolaminates are sequentially strained and consolidated, and the amorphization process is followed explicitly. The results are in qualitative agreement with existing experimental data, and provide insight into experimentally inaccessible features of the structural evolution.

Carbon black in the form of nanoparticle chains is used as a reinforcing filler in elastomers. However, the dynamics of the filler particles under tension and their role in the improvement of the mechanical properties of rubber are not well understood. We have studied experimentally the dynamics of isolated nanoparticle chain aggregates (NCAs) of carbon made by laser ablation, and also that of carbon black embedded in a polymer film. In situ studies of stretching and contraction of such chains in the transmission electron microscope (TEM) were conducted under different maximum values of strain. Stretching causes initially folded NCA to reorganize into a straight, taut configuration. Further stretching leads to either plastic deformation and breakage (at 37.4% strain) or to a partial elastic behavior of the chain at small strains (e.g. 2.3% strain). For all cases the chains were very flexible under tension. Similar reorientation and stretching was observed for carbon black chains embedded in a polymer film. Such flexible and elastic nature of NCAs point towards a possible mechanism of reinforcement of rubber by carbon black fillers.

Plasticity size effects are well known in a wide variety of situations where either the material microstructure or a strain gradient exist at small length scales. Several theories have been developed to describe changes in the work hardening behaviour under these conditions but none that predict a change in the initial yield stress. Careful studies by Chaudhri et al and Pharr et al have unambiguously demonstrated plasticity size effects in ductile metals. In those experiments indentation stress-strain curves were generated using spherical indenters with radii ranging from a few micrometres to several hundred micrometres and these were compared to data from conventional compression tests. Large radius indenters produced a single indentation stress-strain curve independent of indenter radius with a power law hardening coefficient equivalent to that in the compression tests. However, the indentation stress-strain curves appeared at progressively higher pressures for smaller radius indenters. In this paper we model those experiments using finite element analysis methods. By inputting the uniaxial stress-strain data to the model (effectively, using von Mises criterion) the indentation stress-strain curves for the macro size indenters are reproduced. However, the model shows no length scale dependence for any size of indenter. We show that by off-setting the compression stress-strain curve by increasing the initial yield stress and inputting this data to the model, the indentation behaviour of the smaller radius indenters can be modelled. The increase in yield stress with decreasing indenter radius is demonstrated for Cu, Wand Ir and is shown to be consistent with the initiation of yielding over a finite volume.

Three different sets of styrene-based ionomer homoblends containing ca. 5 mol% of ionic repeat units, i.e. poly(styrene-co-sodium itaconate) [P(S-co-ITANa)]/ poly(styrene-co-sodium methacrylate) [P(S-co-MANa)] ionomer blends, sodium sulfonated polystyrene [P(S-co-SSNa)]/P(S-co-MANa) ionomer blends, and P(S-co-ITANa)/P(S-co-SSNa) ionomer blends, were prepared and their dynamic mechanical properties were investigated. It was observed that with increasing itaconate content in the blend of P(S-co-ITANa)/P(S-co-MANa) ionomers the multiplet of the P(S-co-MANa) ionomer disrupted initially very rapidly, and ionic modulus of the blend increased drastically. In the case of P(S-co-ITANa)/P(S-co-SSNa) ionomer blends, again, the ionic modulus increased significantly with increasing itaconate content. In these two ionomer blend systems, the itaconate, having two ionic groups per ionic repeat unit, influenced the properties of the blends more noticeably than the other two ionic units, containing only one ionic group per ionic repeat unit. In the blend system of P(S-co-SSNa)/P(S-co-MANa), as expected, the cluster loss tangent peak shifted to higher temperature, ionic modulus decreased, but the ionic plateau extended more with increasing the ratio of the P(S-co-SSNa) content. However, it should be mentioned that when the ion contents of the methacrylate and sulfonate ionomers increased to over 6 mol%, at which the cluster phase of the P(S-co-MANa) ionomers is known to become dominant, compared to the matrix phase, the three loss tangent peaks were observed, which implies that methacrylate-rich and sulfonaterich phases exist together. This might be due to the fact that a copolymerization effect becomes stronger; that is, with increasing ion content the role of ionic units in random copolymer ionomers becomes more important, compared to the role of host non-ionic units. As a concluding remark, the properties of these three ionomer blend systems depend on the degree of clustering, type of ionic groups, and the number of ionic groups per repeat unit.

Composite thin films of yttria stabilized zirconia (YSZ) and alumina (Al2O3) have been synthesized using liquid fuel combustion chemical vapor deposition (CCVD) and pulsed laser deposition (PLD) in the NSF Center for Advanced Materials and Smart Structures (CAMSS) at North Carolina A&T State University. With the CCVD technique, addition of alumina was realized by adding the designated amount of aluminum-organic in the reagent solution; while with PLD, doping of alumina in YSZ was accomplished by alternative ablations of an YSZ target and an alumina target. Variations in morphology, surface roughness and nano-mechanical properties of the composite thin films of Al2O3/YSZ were characterized. Crystal size of the films processed by CCVD was much larger than that processed by PLD; surface roughness follows the similar tendency. Upon high-temperature annealing, crystals in the PLD processed thin films grew up to 300 nm. The effect of Al2O3 in YSZ thin films on their nano-mechanical properties was dependent on the film deposition techniques in our research. For the films deposited by CCVD, addition of Al2O3 improved the nano hardness and elastic modulus of YSZ thin films, while a decline was observed in the mechanical properties of the films deposited by PLD.

Nanomechanical mapping of individual multi-walled carbon nanotubes (MWNTs) has been undertaken to investigate intra-tube variations of mechanical response. Ultrasonic force microscopy has been used to measure the relative axial and radial variations of contact stiffness of individual MWNTs synthesized using chemical vapor deposition (CVD) and arc-discharge (AD) techniques. For CVD-based MWNTs the contact stiffness of the tube was seen to vary strongly across volume defects (axial variation of the tube radius) and is assumed to result from the high crystalline defect density associated with such radial variations. These observations support recent experimental data of effective Young's modulus inferred from electrostatically-induced nanotube vibration amplitudes.

Structural and mechanical properties of nanoscale TiN/Cu multilayers grown by dual ion beam sputtering with bilayer periods (A) ranging from 2.5 to 50 nm were studied. Both low-angle and high-angle X-ray diffraction (XRD) experiments have been employed to globally characterize the multilayers structure. The microstructure of the multilayers has been scrutinized by high resolution transmission electron microscopy (HRTEM). The effects of interface and bilayer thickness on hardness were investigated by depth-sensing nanoindentation technique. A small hardness increase with decreasing periodicity of the multilayers has been observed. The relationship between the hc/T ratio (hc is the contact depth and T is the total film thickness) and the hardness is established. The correlation between the microstructure and hardness is discussed.

The fracture strength of ultrananocrystalline diamond (UNCD) thin films, grown by microwaveplasma- enhanced chemical-vapor deposition (PECVD), was measured using the membrane deflection experiment (MDE) developed by Espinosa and coworkers. The data show that UNCD fracture strength appears to follow a Weibull distribution. Furthermore, we show that the Weibull parameters are highly dependent on the seeding process used in the growth of the films. When seeding was performed with micron-size diamond particles, using mechanical polishing of the substrate, the stress, resulting in a probability of failure of 67%, was found to be 1.74 GPa, and the Weibull modulus was 5.74. By contrast, when seeding was performed with nano-size diamond particles, using ultrasonic agitation, the stress, resulting in a probability of failure of 67%, increased to 4.13 GPa and the Weibull modulus was 10.76. The investigation highlights the role of microfabrication defects on material properties and reliability, as a function of seeding technique, when identical PECVD chemistry is employed. The parameters identified in this study are expected to aid the designer of MEMS/NEMS devices employing UNCD films.

Mechanical studies of semiconductor superlattices have shown that the onset of plastic deformation under an inhomogeneous stress is a process that takes place simultaneously across a finite volume of the order of a micron across. The ability to incorporate known internal stresses, and to vary the stress and thickness of individual layers in a semiconductor superlattice, is a very powerful tool, opening up new possibilities for investigations that cannot be achieved by varying external stresses on a specimen that is sensibly homogeneous. In this way, from the initial yield stress of single-crystal strained-layer superlattices under indentation, we demonstrated a new criterion, of which the key feature is that it is to be averaged over a finite volume. Here we show that designing samples with individual layers in bands to form low yield-stress material within the structure can give information about the size and position of the initial yield volume.

Although bulk silicon is ostensibly immune to cyclic fatigue and environmentally-assisted cracking, the thin film form of the material exhibits significantly different behavior. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Fatigue lives in excess of 1011 cycles have been observed at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using surface micromachined, resonant-loaded, fatigue characterization structures. Recent experiments have clarified the origin of the susceptibility of thin film silicon to fatigue failure. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models have been used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the nanometerscale silica layer on the surface of the silicon, via a process that we term ‘reaction-layer fatigue’. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the surface oxide layer. The growth of the oxide layer and the environmentally-assisted initiation of cracks under cyclic loading conditions are discussed in detail. Furthermore, the importance of interfacial fracture mechanics solutions and the synergism of the oxidation and cracking processes are described. Finally, the successful mitigation of reaction-layer fatigue with monolayer coatings is shown.