The hardness of a thin film of silicon is studied in the framework of the 3D Hertzian contact theory. The anisotropy and the anharmonicity of silicon are taken into account for the first time. It is shown that the contribution of plasticity to the hardness of silicon is significant while we know that it possesses strong covalent bonds and dislocations must be thermally activated in this material. The semiconductor-metal phase transition, driven by the tetragonal shear strain superimposed on the non-hydrostatic pressure generated by a diamond indenter, is also studied. For this aim, the Landau theory of phase transitions and the contact theory are combined. The comparison with available nanoindentation experiments is made.

The microstructure, mechanical properties and deformation of bimodal structured nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk nanocrystalline alloys. Bimodal bulk nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions, show balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness compared to comparable conventional alloys and nanocrystalline metals.

Chemical Vapor Deposition (CVD) using hydrogen, methane, and nitrogen feed gases has proven to be useful in depositing well-adhered diamond films on metal substrates. These films have already found a market in the cutting tool industry as coatings on cobalt-cemented tungsten carbide (WC-Co) inserts. The purpose of this investigation is to examine how the thermal and chemical pre-treatments typically used for carbide inserts (to remove the cobalt binder near the surface) affect the structure and interfacial adhesion of the diamond coating. Removal of the cobalt binder phase in various pre-treatment methods has been shown to minimize its catalytic effect of graphite formation during diamond deposition by CVD. The diamond-coated inserts in our study were characterized using x-ray diffraction, Raman spectroscopy, atomic force microscopy, and Rockwell indentation testing. We use an unconventionally high methane concentration in the feedgas in order to saturate the growth surface with carbon, thereby limiting cobalt migration from the bulk to the surface and reducing the dissolution and diffusion of carbon atoms coming from the plasma. Nitrogen is used in the feedgas in order to provide a tough, single-layer nanocrystalline diamond film structure. In addition, a multi-layer (nano-/micro-/nano-crystalline) CVD diamond film was grown by controlling the flow of nitrogen in order to show its characteristics in comparison with the single layer nanocrystalline diamond film. The multilayer film on the thermally-treated insert shows enhanced interfacial adhesion and fracture toughness when compared to other pretreatments and diamond coatings. This was demonstrated by indentation tests using 1470 N load.

During processes of mechanical alloying the characteristic structural length scales of an alloy, including the phase domain size and the crystallite grain size, decrease gradually to a nanocrystalline or even amorphous final state. This method therefore allows a unique avenue to explore the structure-property relationship over several orders of magnitude in length scale. In this work we have considered an ideal equiatomic Ti-Zr system deformed through multiple cold-rolling passes to refine the structural length scales into the nanometer range. The variation of the hardness of the system with decreasing length scale is discussed in terms of traditional Hall-Petch scaling, chemical mixing and the phase evolution of the system, as well as other possible contributions to the hardness variations during processing.

Abstract Superhard nanocomposites, nc-MnN/a-XxNy (M = Ti, W, V, Zr, (Al1-xTix)N; X = Si, B) with hardness of 40–100 GPa are prepared by plasma CVD or PVD under a sufficiently high nitrogen activity and deposition temperature that allow the formation of a stable nanostructure by self-organization upon strong thermodynamically driven, spinodal phase segregation. These nanocomposites display an extraordinary combination of a high hardness, high elastic recovery, high resistance against brittle fracture and tensile strength of 5 to 40 GPa approaching the ideal strength of flaw-free materials. These properties can be understood in terms of conventional fracture physics scaled appropriately down to crystallite sizes of few nm. The interfacial monolayer of Si3N4 or BN with strong bonding to the nanocrystallites and high structural flexibility avoids grain boundary sliding. With increasing thickness of this interface the hardness decreases, possibly due to an increase of this “liquid-like” component in which plastic transformation can be triggered.

The residual microstrains in cryomilled Ni powders, processed under various cryomilling conditions, are measured by XRD and analyzed using a single line approximation (SLA) method. The results show that the average residual microstrains are in the range of 2×10-3 ∼ 6×10-3, and the residual microstrain on the (200) plane is higher than those on the other planes by 33 %. The measured microstrain is proposed to evolve from the introduction of N as impurity atoms in Ni lattice may be attributed to the evolution of the residual microstrain. The N atoms tend to stay in the octahedral sites of Ni, and the diameter of N atom is larger than that of the octahedral site of Ni by 48 %. Accordingly, a lattice strain field is expected around interstitial N atoms that are located at octahedral sites. By comparing the crystal structure around the octahedral site of Ni with that of Ni3N structure, the lattice strains are estimated to be in the range of 5 ∼ 15 %. The results show that the (200) plane has the lattice strains that are two times higher than those in other planes, and this is argued to be the reason for the measured anisotropy of residual microstrain in Ni after cryomilling.

Polyethylene terephthalate glycol (PETG) is a clear amorphous polymer, which is extensively used in flexible packaging. The dual packaging requirements of recyclability and long-term shelf life are often difficult to achieve. Meeting these needs become more urgent when considering food packaging for large volumes of soldiers positioned in different parts of the world. Our approach is to develop a high barrier PET packaging system via the Montmorillonite layered silicate (MLS) based nano technology. Prior research has indicated the significant impact of the polymer crystalline regions on the properties of the resultant nanocomposite. Therefore we must first investigate the amorphous PETG. We must also investigate the influence of increased matrix polarity on dispersion of the PETG by incorporating maleic anhydride (MA) onto the PETG backbone. The influence of the clay concentration and maleation are independently investigated. The glass transition of the as-processed and annealed samples are analyzed using Differential Scanning Calorimetry (DSC) while the thermal stability is determined using Thermogravimetric Analysis (TGA). Testing showed a slight depression in the glass transition temperature of PETG film when the MLS is introduced into the system. The nanocomposite films also demonstrated a lower thermal stability in relation to the neat PETG films. The barrier properties were determined on an in-house built calibration unit based on atomic mobility under high vacuum. X-ray diffraction and TEM were utilized to determine the dispersion of the MLS in PETG. The results indicate that the dispersion was concentration independent but maleation of the PETG led to a slight decrease in agglomeration. An increased ultimate tensile strength and modulus was observed in PETG nanocomposites. The barrier properties were improved by incorporating the MLS into the system. Maleation of the PETG resulted in significant yellowing of the nanocomposites.

Testing methods are reviewed that can be applied to the small sample sizes which result from many of the processing routes for preparation of nanocrystalline materials. These include the measurement of elastic properties on small samples; hardness, with emphasis on nanoindentation methods; the miniaturized disk bend test (MDBT); the automated ball indentation test (ABI); the shear punch test; and the use of subsize compression and tensile samples.

A great challenge associated with current investigations of the structural strength properties of nanometrically-scaled polycrystals and/or composite materials relates to quantitative description of the continuous transition in mechanical behaviors occurring when going over to such materials from those microstructural, and larger, scale material deformation properties seemingly well-understood and modeled during the research investigations of the previous century [1]. The consideration relates to whether the same dislocation generation/interaction mechanisms operating within the grain volumes of a conventional microstructured material, say, as compared in the same case for local deformations, dislocation or otherwise, at the grain boundary regions, are either additionally restricted from operation, or enhanced, for nanostructured materials. The deduced indication is that there should be a relatively smooth mechanical property transition, if any change at all, to be demonstrated here on the basis of available experimental and theoretical modeling results. The predicted smooth transition in behavior, however, may be upset, possibly, by failure to achieve the demanding quality control predictably needed for the combined considerations of: (1) structural characterization; (2) mechanical testing methods; and, (3) model computations, so as to allow quantification of the finer-scaled material behaviors. The combined property behavior is illustrated by application of effective low temperature grain boundary strengthening models at larger grain (or particle) sizes, then, transitioning to effective grain size weakening at the smallest grain sizes, with such weakening thought to occur because of otherwise normal high temperature diffusional or grain boundary weakening mechanisms being promoted somehow to become controlling in the practical “thought-to-be” low temperature regime.

The microstructural characteristics and mechanical behavior at room temperature of a nanostructured Al-Mg alloy, with grain sizes of approximately 100 nm, manufactured by a cryomilling process were investigated in the present study. The results reveal an asymmetry of yield strength between tension and compression. On the basis of the mechanical behavior results it appears that the presence of a few micron inclusions may have a negative effect on tensile behavior of the cryomilled nanostructured Al-Mg alloy.

Open-cell foam can be thought of as a network of interconnected struts. To study the contact stiffness behavior, one can imagine the foam boundary to be characterized by struts that are free at one end, while the base-end is connected to the interior of the foam. In previous studies, the base-end was assumed to be built into a rigid surface, i.e., the surface struts were constrained to have zero displacement where they would connect to the foam interior. In this study the assumption that the surface elements are built-in is relaxed by modeling the elastic behavior of the foam interior. The foam interior is modeled as a network of struts within a finite element formulation. The analysis is simplified by considering two-dimensional honeycomb structures. It is found that including the foam interior in the model results in a significant reduction in the predicted contact stiffness.

The problems of the creation of composite polymer-based materials with the use of amorphous nanosized molybdenum and molybdenum dioxide fillers with the particle size of less than 5 nm are considered. The copolymer of formaldehyde and dioxolane (CFD) and the polyphenylene sulphide (PPS) are chosen as polymeric matrixes. A comparative analysis of the tribological properties of composite materials with amorphous nanosized fillers and crystalline microsized analogues is made. It is determined that the use of isotropic nanosized amorphous fillers leads to improved wear-resistance up to three times for CFD-based composites and up to an order for PPS-based ones.

Raman spectroscopy is a technique widely used to study the vibrational modes of carbon nanotubes. The low-frequency Radial Breathing Modes (RBMs) are frequently used to characterize carbon nanotube samples. We report a Raman spectroscopic study on the strain-induced intensity variations of the RBMs of Single-Wall Carbon Nanotubes (SWNTs) in epoxy/SWNT composites. The RBM intensities have been found to vary significantly over a range of strain between -0.3% and 0.7%. The trend (increase or decrease) as well as the magnitudes of the intensity variation depends on the nanotube diameter and its chirality. Using tight-binding calculations, we have shown that these intensity variations can be explained entirely by resonance theory. Electronic density of states calculations confirm that the energy separation between the Van Hove singularities shifts with strain. The nanotubes are thus moved closer or further away from resonance, causing the intensity variations. It is demonstrated that through the use of resonance theory, a tentative chirality can be assigned to each type of SWNT from knowledge of its RBM position and the effect of strain upon the RBM intensity, thus determining its entire structure.

Carbon films deposited by filtered cathodic arc show a high compressive stress which limits their thickness because of delamination. We study three methods of relieving the stress in these films. We first determine the dependence of the stress on DC bias up to bias voltages of 1200V and show that the formula of Davis provides a good fit to the data including the stress maximum in the region of 150–200V and the progressive decrease in stress at higher voltages. In the second method, plasma immersion ion implantation (PIII) was used to create multilayer of alternating high density, high stress (PIII on) films and lower density, low stress (PIII off) films. This method enabled thicker structures to be produced. In the third method we made multilayers using amorphous silicon and carbon layers. Annealing of these layers showed that the stress could be reduced to very low values because of the ability of the silicon layers to absorb compressive stress by contracting after the annealing step. The microstructural effects of PIII were studied by transmission and scanning electron microscopy.

Nanocomposites and nanostructured polymers with unique opto-mechanical properties have been developed as smart coatings for use in a novel, high resolution, and non-contact strain-measuring application. Remote polarized Raman spectroscopy has been used to monitor optical strain sensitivity of deformed coatings (deformation micromechanics), and determine local strains on the micron scale directly from stress/strain induced Raman band shifts.

The research is aimed at providing a novel high-resolution non-contact technique for the determination of surface stresses and strains in a wide variety of engineering components used in both laboratory and in-the-field (external) applications.

We perform atomistic simulations to compute bending of freestanding nanoscale thin Si film induced by strained Ge islands. We show that a larger Ge dome island can induce smaller bending than a smaller hut island and explain the surprising experimental observation for growth of Ge islands on patterned silicon-on-insulator substrate (SOI) with Si template layer thinned down to nanometer scale. This counterintuitive bending behavior is caused by strain sharing between the film and the ultra thin substrate.

The use of load-displacement derivative behavior and power-law curve fitting is applied to find the location of events for a number of different materials during depth-sensing indentation. Load-displacement curves for Berkovich indentations on fused silica, fullerene thin film on sapphire, CdTe thin film on silicon, single crystal silicon, carbide derived carbon, and a polymethylmethacrylate/hydroxyapatite (PMMA/HA) particle composite are examined. The analysis is applied to quantify the location of different events that occur during material loading and unloading.

To take advantage of the benefits of chemical functionalization and the desirable properties of unfunctionalized SWNTs, this research studies the effect of removing functional groups from SWNTs dispersed in a polymer matrix. Chemical functionalization of single wall carbon nanotubes (SWNTs) is a method for disrupting rope structure and adding reactive species to the nanotube to improve interfacial bonding and load transfer in composites, but changes to the nanotube hexagon structure caused by chemical modifications are expected to have a detrimental effect on the SWNTs' intrinsic mechanical properties. Thus, composites containing defunctionalized SWNTs and polyethylene are analyzed to evaluate the effect of functional group removal on the mechanical properties. The mechanical properties are measured using tensile tests. Issues of defects in the SWNT structure, polymer degradation, and changes in the fiber/matrix bonding as a result of functionalization removal are studied using Raman spectroscopy, thermogravimetric analysis, infrared spectroscopy, and dynamic mechanical analysis.

This study applies NMR, SRA, MIP, SEM and electric resistance tests to examine the microstructure of the composite materials of nano concrete. The test results show that owing to the pozzolanic reaction activated by the nanopowders, the CaOH2 content within the composite materials is decreasing, filling the capillary and gel pores. This way, the microstructure of the composite materials is transformed in its arrangement, which makes concrete much more well-mixed and densified, enhancing the compressive strength. The permeability is cut down as well. In conclusion, it ends in much better durability.

Ceramic coatings can provide an ideal protection for MEMS (MicroElectroMechanical Systems) structures while imposing a great challenge in processing a prime, reliant coating due to their inherent brittleness and defects formed during processing. In an attempt to compensate for the weakness of the ceramic coating, we have developed a low-temperature solution precursor process to create strain-tolerant, protective bilayer coating consisting of an integrated ceramic-organic hybrid material. The top ceramic coating offers an inert, protective layer whereas the underlying nanometer scale self-assembled organic coating provides compliance for the overlying hard coating. Together, these bilayers minimize mechanical and thermal stresses. In addition, organic self-assembled monolayers(SAM) act as a ‘template’ by forming a proper surface functionality for the subsequent growth of hard ceramic coatings. Molecular level understanding of the microstructure and micromechanics involved in the synthesis and processing of the coating is systematically studied by a variety of characterization techniques such as XRD, AFM, SEM/EDS and nanoindentation. This work is also complemented by numerical simulation to provide a clearer understanding of the stress development in the ceramic coating and its interfacial properties.