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Increasing evidence demonstrates that some ceramics and semiconductors can possess increased ductility and fracture toughness at lower temperatures than previously thought. The present study focuses on the unlikely candidate of single-crystal silicon, which is studied using microscale bending beams at elevated temperature. A potential solution for the issue of focused ion beam (FIB) pre-notches is also presented, by re-propagating arrested cracks. This allows for proper measurement of stress intensity from nearly atomically sharp cracks, along with removing any potential influence of Gallium. This does not mean the crack is initially dislocation free but does represent an appropriate microstructure that might be experienced in a manufacturing setting. Through post mortem transmission electron microscopy (TEM) of thinned samples after testing between 300 and 873 K, dislocation nucleation is observed at temperatures near 500 K. Beyond 500 K, there is increased propensity toward dislocation enhanced toughening through either crack-tip shielding and/or generalized plasticity.

In this paper, the historical context of the development of what is now known as ‘Fracture Mechanics’ is selectively developed. We start from the safety and economic motivations, and review the essential efforts, over the centuries, to develop the ability to predict fracture and those factors leading up to final catastrophic events. The experimental and analytical quantitative aspects, and their interdependence, are emphasized. It is shown how these efforts were integrated to define the field we now know as Fracture Mechanics. The paper concludes with some thoughts on unmet needs and new directions.

Silicon is widely used at the small scale in environments where possible hydrogen can be introduced to the material. In this work, small scale mechanical testing with nanoindentation coupled with thermal gas hydrogen charging are used to determine activation volumes and fracture toughness in material with and without hydrogen charging. Hydrogen is found to increase microcracking in micropillars at the sub-micron scale. Additionally, it lowers and produces a large size dependence in fracture toughness not observed in single-crystal silicon material without hydrogen charging.

We present novel in-situ scanning electron microscope experiments exploring the fracture of silicon as a function of temperature at the microscale, from room temperature to 600 °C. Clear post mortem TEM observations of dislocation activity at and above 450 °C suggest that back stresses from crack-tip dislocation emission raise the applied stress intensity at initiation, as part of a brittle to ductile transition starting at 300 °C. This is in agreement with other microscale measurements; however, these experiments are particularly noteworthy in their ability to directly observe crack advance and perform post-mortem analysis to investigate dislocation activity.

The brittle to ductile transition (BDT) is difficult to predict without extensive fitting parameters or tuning to a particular material. Currently, predicting fracture through extensive fitting or computationally expensive algorithms is high in both cost and time required to capture the relevant deformation physics. Presented here is analysis using a comparatively high throughput analytical model to predict fracture behavior using relatively few key experimentally determined parameters: activation volume, shear stress, and activation energy. This approach could reduce the time scale to predict fracture and thus accelerate new materials discovery. The current work utilizes seminal studies to provide the inputs for validating our approach via two single crystal materials, Si and W, which both have marginal toughness at low temperatures. It is shown that knowledge of underlying deformation mechanisms (still in progress) coupled to rapid determination of physical quantities (shear stress, activation volumes, and dislocation shielding) promotes unique discovery and opportunities, including future application to polycrystalline materials and phenomena. The technique, using literature values for physical parameters, correlates well to experimental fracture behavior for these two different classes of materials, semiconductors and metals, offering new opportunities for broader study.

The ductile–brittle transition of nano/microscale silicon is explored at low-temperature, high stress conditions. A pathway to eventual mechanism maps describing this ductile–brittle transition behavior using sample size, strain rate, and temperature is outlined. First, a discussion of variables controlling the BDT in silicon is given and discussed in the context of development of eventual modeling that could simultaneously incorporate all their effects. For description of energy dissipation by dislocation nucleation from a crack tip, three critical input parameters are identified: the effective stress, activation volume, and activation energy for dislocation motion. These are discussed individually relating to the controlling variables for the BDT. Lastly, possibilities for measuring these parameters experimentally are also described.

Development of more accurate descriptions of dislocation motion requires understanding the actual effective stress driving it. Back stresses from dislocation pile-ups can work against the applied stress resulting in lower stresses acting on moving dislocations. This study presents calculations of back stress derived from in-situ compression of 26–39 nm sized single crystal silicon cubes inside the transmission electron microscope. These initially dislocation free particles exhibited yielding culminating in over 60% plastic strain. The back stress was calculated based on a pile-up model which, when subtracted from the applied stress, suggests a constant effective stress for continuing plasticity.

Silicon is one of the most technologically important materials, used extensively in electronics, solar cells, micro-electro-mechanical systems (MEMS) based devices and more. Yet its mechanical properties are not well understood at the nanoscale where it is often utilized. Experimental measurements under a variety of loading conditions are needed, and compression experiments are particularly lacking. Here, the elastic–plastic response of 20–65 nm cubic Si nanocubes under uniaxial compression is investigated. The purely elastic limit of these nanocubes is observed to be up to 0.07 true strain at 7 GPa true stress with an upper yield point of 0.20 true strain and 11 GPa true stress. Investigation of the nature of dislocations generated during deformation of these nanocubes using post-mortem analysis in the TEM provides evidence that leading partial dislocations are the dominant source of plasticity at this scale.

Today, the Mohs scale is used profusely throughout educational systems without any persuasive understanding of the fundamental principles. Why one mineral has a scratch hardness over the next culminating in a scale of 1 (chalk) to 10 (diamond) has no atomistic or structure-sensitive basis that explains this outcome. With modern computationally based atomistic and multiscale models, there is increasing promise of defining the pressure and rate-dependent parameters that will allow a fundamental understanding of the Mohs scale. This study principally addresses the combined fracture and plasticity parameters that qualitatively affect fracture at the nanoscale. A physical model wherein the crack tip under a scratch is shielded by dislocations is supported by molecular dynamics (MD) simulations in both ductile aluminum and brittle silicon carbide. Next, this model is applied to nanoindentation data from the literature to produce a ranking of Mohs minerals based on their fundamental properties. As such, what is presented here is a first step to address the flow and fracture parameters ultimately required to provide a figure of merit for scratch hardness and thus the Mohs scale.

Metallic thin films are widely used and relied upon for various technologies. Direct measurements of fracture toughness are rare for metallic thin films and existing methods for obtaining these measurements often do not provide characterization of the cracking process for determination of crack growth mechanisms. To rectify this, we explore a new technique which utilizes doubly clamped, in-situ three-point bend testing of micro-scale and nano-scale specimens. This is done by in-situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) mechanical testing for specimens with thicknesses of 2500 nm (SEM), 500 nm (SEM) and 100 nm (TEM). For in-situ TEM, a novel notching method is employed using the converged electron beam which achieves a notch radius of approximately 5 nm. Additionally, we present supporting characterization using Electron Backscatter Diffraction (EBSD) for 2500 nm thick specimens as a demonstration of the potential of this technique for understanding local deformation. Analysis of the acquired data presents several issues that require addressing, and recommendations for future improvements are given.

A number of recent mechanical property studies have sought to validate atomistic and multiscale models with matching experimental volumes. One such property is the ductile-brittle transition temperature (DBTT). Currently no model exists that incorporates both external and internal variables in an analytical model to address both length scales and environment. Using thermally activated parameters for dislocation plasticity, the present study attempts a small piece of this. With activation energy and activation volumes previously determined for single and polycrystalline Fe-3% Si, predictions of DBTT both with and without atmospheric hydrogen are made. These are compared with standard fracture toughness measurements similarly for samples both with and without atmospheric hydrogen. In the hydrogen-free samples, average strain rate varied by four orders of magnitude. DBTT shifts are experimentally found and predicted to increase 100 K or more with either increasing strain rate or exposure to hydrogen.

The mechanism of secondary hardening in MP35N (Co–35Ni–20Cr–10Mo) alloy due to exposures at elevated temperatures has been studied. It was observed that short exposure to elevated temperatures increased the ultimate tensile strength and yield stress while decreasing the elongation of MP35N wires. Upon aging at temperatures from 300 to 900 °C the elastic modulus increased although no changes in crystallographic orientation or microstructure were observed. The grain size and major texture components were unchanged following aging. Analytical scanning transmission electron microscope investigation showed that MP35N is hardened by preferential segregation of molybdenum to stacking faults and deformation twins. It also revealed that the concentration of molybdenum segregation was proportional to the amount of initial cold work before aging.

Measurements of submicron spheres and pillars of silicon single crystals have exhibited a strain-hardening capacity equal to or greater than their metallic counterparts. Stress–strain characteristics are reported for diameters ranging from 40 to 400 nm. Evaluations were performed with nanoindentation-based atomic force, scanning and transmission electron microscopies. Values of strain-hardening exponents up to unity in nanospheres are attributed to a size effect variation on the rate of increase of contact area with deformation. A surface-mediated dislocation nucleation concept is shown to be consistent with length scale effects partially modified by geometry as well as size. It is proposed, but not proven, that the modification relates to greater constraint in compact spheres as opposed to tall pillars.

Large scale molecular dynamics simulations of the compression of silicon nanospheres were performed with the Stillinger–Weber potential. Several defects were observed to cause the yielding, including dislocations, stacking faults and phase transformations. To better investigate dislocation interactions, spheres of increasing size comprised of up to one million atoms were simulated. The morphologies of the defects and the conditions under which they are formed are explored. A new and interesting route to dislocation formation is identified and examined in which perfect dislocations form on {1 1 0} planes as opposed to the expected {1 1 1} planes. The dislocations on {1 1 0} planes are observed to form through a pathway with an intermediate metastable state corresponding to a change in the atomic bonding. Density Functional based Tight Binding calculations reveal the feasibility of this pathway although the appearance of dislocations on the {1 1 0} plane in the molecular dynamics simulations is specific to the Stillinger–Weber potential.

Thin native oxide layers can dominate the mechanical properties of metallic thin films. However, to date there has been little quantification of how such overlayers affect yield and fracture during indentation in constrained film systems. To gain insight into such processes, electrical contact resistance was measured in situ during nanoindentation on constrained thin films of epitaxial Cr and polycrystalline Al, both possessing a native oxide overlayer. Measurements during loading of the films show both increases and decreases in current, which can then be used to distinguish between various sources of plasticity. Ex situ measurements of the oxide thickness are used to provide a starting point for elasticity simulations of stress in both systems. The results show that dislocation nucleation in the metal film can be differentiated from oxide fracture during indentation.

To understand the brittleness transition in low-toughness materials, the nucleation and kinetics of dislocations must be measured and modeled. One aspect overlooked is that the apparent activation energy for plasticity is modified at very high stresses. Coupled with state of stress and length scale effects on plasticity, the lowering of the brittle-to-ductile transition (BDT) in such materials can be partially understood. Experimental evidence in silicon single crystals in the length scale regime of 40 nm to 1 mm is presented. It is shown that high stress affects both length scale and temperature-dependent properties of activation volume and activation energy for dislocation nucleation and/or mobility. Nanoparticles and nanopillars of single-crystal silicon demonstrate unexpectedly high fracture toughness at low temperatures under compression. A thermal activation approach can model the three decades of size associated with the factor of three absolute temperature shift in the BDT.

“Smaller is stronger” is now a tenet generally consistent with the predominance of evidence. An equally accepted tenet is that fracture toughness almost always decreases with increasing yield strength. Can “smaller is tougher” then be consistent with these two tenets? It is taught in undergraduate engineering courses that one design parameter that allows for both increased strength and fracture toughness is reduced grain size. The present study on the very brittle semiconductor silicon proves this exception to the rule and demonstrates that smaller can be both stronger and tougher. Three nanostructures are considered theoretically and experimentally: thin films, nanospheres, and nanopillars. Using a simple work per unit fracture area approach, it is shown at small scale that toughness is inversely proportional to the square root of size. This is supported by experimental evidence from in situ electron microscopy nanoindentation at length scales of less than a micron. It is further suggested that dislocati...

Molecular dynamics has been used to simulate the uniaxial compression of single crystal silicon nanospheres using the Tersoff potential. The resulting yield behavior is shown to vary with changes in temperature, sphere size, and crystallographic orientation with respect to the loading direction. Only compression along the [1 0 0] crystallographic direction resulted in the formation of the β-Sn phase. A temperature dependent hardening response is observed in all orientations independent of the β-Sn phase transformation. Dislocation activity is detected at elevated temperatures in the largest sphere indicating a critical temperature and size for nucleation. Consequences of these dislocations to simulating strength properties at the nanoscale are discussed.

In order to characterize the wear and related mechanical behavior of materials from small volumes, a program employing nanoscratch and nanoindentation was performed. Nanoscratch techniques were used to generate square wear patterns with varying degrees of shear strain followed by nanoindentation tests to measure the mechanical properties within the deformation area. Results show a systematic increase in hardness with both the applied load and number of nanoscratch passes. An analytical approach was used to determine the stress-strain response and strain hardening behavior of electroformed nickel. The strain hardening exponent determined from this method follows the work hardening behavior established from previous tensile tests, supporting the use of a nanomechanics-based approach for evaluating the mechanical properties of wear-tested material.

Vapor-liquid-solid (VLS) Si nanotowers were coated with nanocrystalline SiC to form a Si-SiC core-shell composite. Due to a mismatch in the coefficients of thermal expansion (CTE), the Si core was under a compressive stress following the deposition. The composite tower was then cross-sectioned using focused ion beam milling, exposing the Si core. Indentation into the Si showed an increased toughness as a function of diameter compared to similar sized Si nanotowers and nanospheres. This result is explained through enhanced dislocation and phase transformation plasticity in the Si core from the CTE compressive stresses.

We use the ex situ and in situ deformation of silicon nano-spheres to investigate the mechanism(s) responsible for hardening, which consequently affects strength. With spheres in the 40–400 nm range, an inverse strength dependence was found. Applicability of linear, forest and/or exhaustion hardening are explored with the hypothesis that all three are of the same class limited by dislocation nucleation.

Because of its ease of implementation and insensitivity to indenter drift, dynamic indentation techniques have been frequently used to measure mechanical properties of bulk and thin film materials as a function of indenter displacement. However, the actual effect of the oscillating tip on the material response has not been examined. Recently, it has been shown that the oscillation used with dynamic indentation techniques alters the measured hardness value of ductile metallic materials, especially at depths less than 200 nm. The alteration in the hardness is due to the added energy associated with the oscillation which assists dislocation nucleation. Atomistic simulations on nickel thin films agree with experiments that more dislocations are nucleated during dynamic indents than with quasi-static indents. Through the analysis of quasi-static and dynamic indents made into nickel single crystals and thin films, a theory to describe this phenomenon is presented. This is coined the Nano-Jackhammer effect, a combination of dislocation nucleation and strain rate sensitivity caused by indentation with a superimposed dynamic oscillation.

Robust nanostructures for future devices will depend increasingly on their reliability. While great strides have been achieved for precisely evaluating electronic, magnetic, photonic, elasticity and strength properties, the same levels for fracture resistance have been lacking. Additionally, one of the self-limiting features of materials by computational design is the knowledge that the atomistic potential is an appropriate one. A key property in establishing both of these goals is an experimentally-determined effective surface energy or the work per unit fracture area. The difficulty with this property, which depends on extended defects such as dislocations, is measuring it accurately at the sub-micrometer scale. In this Full Paper the discovery of an interesting size effect in compression tests on silicon pillars with sub-micrometer diameters is presented: in uniaxial compression tests, pillars having a diameter exceeding a critical value develop cracks, whereas smaller pillars show ductility comparable to that of metals. The critical diameter is between 310 and 400 nm. To explain this transition a model based on dislocation shielding is proposed. For the first time, a quantitative method for evaluating the fracture toughness of such nanostructures is developed. This leads to the ability to propose plausible mechanisms for dislocation-mediated fracture behavior in such small volumes.

Decreasing scales effectively increase nearly all important mechanical properties of at least some “brittle” materials below 100 nm. With an emphasis on silicon nanopillars, nanowires, and nanospheres, it is shown that strength, ductility, and toughness all increase roughly with the inverse radius of the appropriate dimension. This is shown experimentally as well as on a mechanistic basis using a proposed dislocation shielding model. Theoretically, this collects a reasonable array of semiconductors and ceramics onto the same field using fundamental physical parameters. This gives proportionality between fracture toughness and the other mechanical properties. Additionally, this leads to a fundamental concept of work per unit fracture area, which predicts the critical event for brittle fracture. In semibrittle materials such as silicon, this can occur at room temperature when the scale is sufficiently small. When the local stress associated with dislocation nucleation increases to that sufficient to break bonds, an instability occurs resulting in fracture.

In ductile metals, sliding contact induces plastic deformation resulting in subsurfaces, the mechanical properties of which are different from those of the bulk. This article describes a novel combination of nanomechanical test methods and analysis techniques to evaluate the mechanical behavior of the subsurfaces generated underneath a wear surface. In this methodology, nanoscratch techniques were first used to generate wear patterns as a function of load and number of cycles using a Hysitron TriboIndenter. Measurements were made on a (001) single crystal plane along two crystallographic directions, and . Nanoindentation was then used to measure mechanical properties in each wear pattern. The results on the (001) single crystal nickel plane showed that there was a strong increase in hardness with increasing applied load that was accompanied by a change in surface deformation. The amount of deformation underneath the wear patterns was examined from focused ion beam cross-sections of the wear patterns.

Dislocation events are seen as excursions or pop-in events in the load–displacement curve of nanoindentation experiments. Two nanoindenters have been used to examine the difference between quasi-static and dynamic loading during indentation. Yield excursions were present in the load–displacement curves of both the statically and dynamically loaded single crystal nickel samples. Only one major excursion occurred in each quasi-static indent, nominally loaded at 100 μN/s while staircase yielding was observed under dynamic loading indentation with a 45 Hz oscillation of 2 nm superimposed on a 60 μN/s loading rate. Thermal activation analysis is used to explain the arrest and reinitiation of the yielding with activation volumes being modeled. For nanoindentation experiments differences between quasi-static and dynamic loading are described by the models presented. It is proposed that insight into the plastic deformation mechanisms associated with such plastic instabilities will provide one of the keys to length scale effects necessary to understanding nanostructures.

SiC–TiX (where ‘X’ includes oxides, carbides, or elemental Ti alone) multilayered nanocomposite coatings with layer thicknesses of 1 to 3 μm were deposited on molybdenum substrates through a hybrid process of nanoparticle impaction and chemical vapor deposition. Reactants are injected into a thermal plasma and undergo a rapid expansion through a converging nozzle, resulting in gas-phase particle nucleation and hypersonic impaction onto a substrate. While film growth is composed primarily of nanoparticles, excess vapor from the particle nucleation process forms a reactive boundary layer above the substrate and supports CVD growth. Embedded particles both inhibit grain growth in the matrix material during the deposition and enhance fracture toughness under compression. Consecutive deposition of SiC and TiX layers creates a film with layers of crystalline SiC nanoparticles embedded in a crystalline SiC matrix followed by Ti/TiO 2 /TiC/TiO composite layers. Both layers show an increased hardness without a change in the modulus compared to coarse-grained samples. While residual tensile stresses at the TiX–SiC interface suggest weak interlayer adhesion, the independent performance of the layers with respect to H 3 / E ⁎ 2 suggests unique microstructural benefits inherent to the deposition process. These results demonstrate the potential for high strength nanocomposites with enhanced wear properties.

Recent interest in high-strength nanocrystalline structures has prompted a call for understanding scale effects in deformation mechanisms. One aspect, producing nanostructures by severe plastic deformation, promoted the present examination of nanoposts undergoing large strain compression. On e-beam lithography produced nanoposts of both aluminum and permalloy with radii ranging from 50 to 150 nm, single and repeat compression tests produced measurements of flow stress and apparent activation volume. Flow stresses, continuously measured with a nanoindenter allowed theoretical assessment of the deformation mechanism. It was concluded that the high stresses (1–3 GPa) and small activation volumes (1–10 b 3 ) where b is the modulus of the Burgers vector, were consistent with dislocation nucleation. A traditional model is shown to give good first order accountability for these two face-centered cubic metals.

Dynamic indentation techniques are often used to determine mechanical properties as a function of depth by continuously measuring the stiffness of a material. The dynamics are used by superimposing an oscillation on top of the monotonic loading. Of interest was how the oscillation affects the measured mechanical properties when compared to a quasi-static indent run at the same loading conditions as a dynamic. Single crystals of nickel and NaCl as well as a polycrystalline nickel sample and amorphous fused quartz and polycarbonate have all been studied. With respect to dynamic oscillations, the result is a decrease of the load at the same displacement and thus lower measured hardness values of the ductile crystalline materials. It has also been found that the first 100 nm of displacement are the most affected by the oscillating tip, an important length scale for testing thin films, nanopillars, and nanoparticles.

SiC nanostructured coatings were deposited using a novel synthesis process combining chemical vapor deposition (CVD) with nanoparticle impaction. Indentation moduli (∼370 GPa) were similar to and hardnesses (∼39 GPa) exceeded the best commercially grown CVD films. Furthermore, film fracture toughness (∼ 6 MPa m 1/2 ) showed significant improvement to reported values in the literature. The nanoparticles were synthesized by injecting chemical reactants into a thermal plasma that undergoes a rapid expansion through a converging nozzle, resulting in a gas-phase nucleation. This purposefully runs counter to design criteria for CVD, where the reactant concentration is kept low to avoid gas-phase nucleation and a disordering of film growth. By limiting the residence time, the average particle size can be kept under 20 nm. However, these nanoparticle films suffer greatly from porosity. In the present paper, an increase in substrate temperature is used to enhance the amount of growth due to chemical vapor deposition. Particle formation was confirmed with in-situ particle size distribution measurements. The relative amount of chemical vapor to particle contributions to the film growth was investigated through observation of preferred growth orientation and microstructural characteristics. In addition, improvements in hardness and fracture toughness are presented and explained in terms of observed structural changes.

Nanoindentation and scanning probe microscopy techniques applied to deforming thin films, nanospheres, nanowires, and nanocrystalline structures have uncovered new mechanical property phenomena dependent on size scale. This overview addresses several segments — those associated with measurement by nanoindentation, as well as the resulting properties of elasticity and plasticity, and fracture toughness of semi-brittle crystals. Specific to volumes in indentation or compression, it is shown that both pressure and scale effects can become dominant for both elasticity and plasticity at sizes less than 100nm. A strong inverse relationship between yield strength and activation volume for both single crystal and nanocrystalline structures is reviewed. Equally strong is a relationship between fracture toughness, the number of shielding dislocations accommodating indentation prior to fracture, and the basic mechanical and physical properties of semi-brittle solids. The link between fracture toughness and plasticity in these semi-brittle materials is shown to be the activation volume for dislocation nucleation in these strong solids.

Electrically conductive coatings and composites are prepared from aqueous dispersions of conducting particles and polymer latex particles. Relatively small amounts of conductive particles are needed to develop electrical conductivity, because the particulate nature of the latex leads to a segregated network that lowers the percolation threshold. Several nanosized conductive fillers have been studied: carbon black, antimony-doped tin oxide, indium tin oxide and carbon nanotubes. The latex chosen for most studies was either a poly(vinyl acetate-co-acrylic) polydisperse latex, a poly(vinyl acetate) polydisperse latex, or monodisperse poly(vinyl acetate) latex. This paper reviews the effect of particle size, aggregation and aspect ratio on the microstructure and properties of conductive composites and coatings.

The present study focuses on the effects of surface orientation on the peculiarities of the earliest stages of nanoindentation-induced plasticity in sapphire (Al2O3) single crystal surfaces. The previous theoretical analyses do not account for all the experimentally observed trends. Additional considerations are required to bridge the gap between experimental results and theoretical predictions. Of key importance are accounting for the sense of twinning shear, the multiplicity of slip and twinning systems involved and an appropriate criterion for the transition from elastic to elastic-plastic regime. The present study supplements a continuum-based stress analysis with the above considerations and compares the resulting theoretical predictions with the experimental results for basal [C, (0001)], rhombohedral [R,(1102)] and prism [A,(1210) and M,(1010)] surfaces. Surface patterns of slip and twining are scrutinized in Part I. Previously unexplained features justified by the results obtained by the present a...

Part II of the present study focuses on the yield point phenomenon, a discontinuous transition from the apparently elastic to the elastic–plastic regime for basal [C, (0001)], rhombohedral [R, (1 012)] and prism [A, (12 10) and M, (101 0)] planes of sapphire (Al2O3) under spherical contacts. The yield point mechanisms are predicted by supplementing the analysis presented in Part I with a criterion for the yield point transition. The proposed criterion accounts for the low-symmetry structure of sapphire. The resulting theoretical predictions are compared with experimental results. This comparison focuses on the effects of surface orientation and loading rates on the yield point load and on the peculiarities of yield point mechanisms, as reflected in the acoustic emission (AE) signals associated with the yield point. For the C plane, the availability of pyramidal and prism slip is expected to be a limiting factor for the yield point transition. Depending on the loading rate, either basal slip or basal twinn...

Increasingly, the essential, robust character of many nanoscale devices requires knowledge of their fracture toughness. For most brittle materials the technique of choice has been indentation mechanics but little insight into the fracture mechanism(s) has resulted since these have generally been treated as brittle fracture dominated by the true surface energy. Linear elastic fracture mechanics approaches have been invoked to describe indentation fracture but do not address why the surface energy from fracture toughness is most often slightly or even substantially greater than the true surface energy. In the present study we invoke a crack extension force correlation that demonstrates why this is the case at least in fracture measurements based on indentation mechanics. The proposed correlation is different from previous ones in that it focuses on observations of indentation-induced dislocation activity prior to fracture. Allowing the resistance side of the crack extension force analysis to incorporate small amounts of plasticity gives a relationship that is consistent with 22 relatively brittle intermetallics, semiconductors and ceramics. This explains why measured strain energy release rates can be 2 to 5 times as large as surface energies measured in vacuum or calculated by pseudopotentials using the local density approximation.

Flexible, transparent and conductive polymer blend coatings were prepared from aqueous dispersions of poly(3,4-ethylenedixoythiophene)/poly(styrenesulfonate) [PEDOT/PSS] gel particles (∼80 nm) and latex (∼300 nm). The stable dispersions were deposited as wet coatings onto poly(ethylene terephthalate) substrates and dried at 80 °C. Microstructure studies using tapping mode atomic force microscopy (TMAFM) indicate that a network-like microstructure formed during drying at 0.03 volume fraction PEDOT/PSS loading. In this network-like structure, the PEDOT/PSS phase was forced into the boundary regions between latex. In addition, migration of the PEDOT/PSS particles towards coating surface is likely during drying of the aqueous dispersions. The addition of a small amount of dimethyl sulfoxide (DMSO) in dispersions altered the distribution of the PEDOT/PSS phase. As PEDOT/PSS concentration increases to 0.15 volume fraction, the coating surface is dominated by the PEDOT/PSS phase. The effect of DMSO on microstructure becomes less apparent as PEDOT/PSS concentration increases. The conductivity of the polymer blend coatings increases in a percolation-like fashion with a threshold of ∼0.02 volume fraction PEDOT/PSS. The addition of DMSO in dispersions enhanced the coating conductivity beyond the threshold by more than two orders of magnitude. The highest conductivity, ∼3 S/cm, occurs at 0.20 volume fraction PEDOT/PSS concentration. The polymer blend coatings have good transparency with only a weak dependence of transparency on wavelength due to the small refractive index difference between filler and matrix.

Hard, wear-resistant SiCN coatings were produced via a thermal plasma chemical vapor deposition process in a triple torch reactor. Vaporized hexamethyldisilazane (HMDSN) and nitrogen and/or hydrogen gases were dissociated through the argon plasma as additional reactants. Several SiCN coatings were synthesized with varying reactant flows and substrate temperature. Film composition, morphology, and mechanical performance were examined. Hardness and elastic modulus increased with decreasing N:H gas ratio, decreasing roughness, and the inclusion of nanocrystallites. A kinetic reaction mechanism determined that the atomic species primarily existed in the substrate boundary layer. The Damkohler number was calculated to predict the necessary reactant composition for morphologically smooth films.

An indentation size effect is a common occurrence during nanoindentation. Thin and thick gold films, deposited using sputter deposition and evaporation, illustrate this at depths less than 100 nm. The indentation size effect, however, has been observed to be independent of film thickness. It has been modeled using a combination of an indentation size effect model and a parabolic hardening model. At the near surface regime, the indentation size effect model is dominant, and at larger depths, the parabolic hardening model is dominant, taking into effect the film thickness. The described model, which is a combination of these two, fits the experimental data for the sputter-deposited films and the evaporated films.

Conventional wisdom and indirect studies suggest that the mechanical properties of nanoparticles can be considerably different than their bulk properties would predict. However, little is actually known about their mechanical behaviour because of the practical difficulties in investigating individual particles. Direct experimental studies of these properties require knowledge of the crystallographic orientation, size and microstructure of the nanoparticle in order to be complete. By deforming a single nanoparticle in the transmission electron microscope we have been able to determine each of these parameters of an isolated silicon nanoparticle a priori. With this approach, we could then directly examine dynamic deformation processes and demonstrate the first direct observation of plasticity-induced cleavage fracture of a silicon nanoparticle in compression.

Thin film adhesion energies can be calculated using buckles with the telephone cord geometry. The buckles can be measured through the point of inflection of the buckle or through the center of the buckle and modeled as a straight buckle of uniform width and height. In the tungsten-silica system, a unique delamination morphology involved a transition from the straight to telephone cord buckle. This structure was used to compare various measurement techniques. The stability range between the straight and telephone cord morphology has been shown to be broader than previously described and exhibits a smooth transition from the straight to telephone cord structure. Measurements of the straight-sided buckles produce adhesion energies which are within 15% of the values calculated when approximating the telephone cord as a straight-sided structure at the inflection point of the buckle.

Wear-resistant, hard Si–C–N coatings were synthesized in a triple torch plasma reactor using a thermal plasma chemical vapor deposition process. In this reactor, three dc plasma torches were angled so that their jets converge to form a highly chemically reactive region at the substrate. Vaporized hexamethyldisilazane (HMDSN) was injected through a central injection probe, while nitrogen or hydrogen gases were added through the torches to the argon plasma. Various dissociation, recombination and intermediate reactions were considered to determine what major species exist in the gas phase during the deposition of Si–C–N films. Reactant flow rates were varied to evaluate the thermodynamic equilibrium compositions across a linear temperature profile above the substrate and to identify the species that lead to the production of wear-resistant, hard Si–C–N films. A series of experiments were conducted at low HMDSN flows (∼ 1 sccm) and varying hydrogen and nitrogen flows. Films were characterized by micro X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Indentation tests were conducted on the polished film cross-sections, while wear tests were carried out on the film surfaces. At substrate temperatures below 1000 °C, amorphous Si–C–N films were deposited, while higher temperatures produced crystalline composite films of α- and β-Si 3 N 4 and α- and β-SiC. Films produced with hydrogen at low HMDSN flows displayed non-columnar morphology and therefore had higher wear-resistance, indicating the benefit of low reactant-to-plasma gas flow concentrations on film growth. At low HMDSN flows, low nitrogen-to-hydrogen ratios had also shown an increase in film linear density. Small variations in mechanical properties and wear were observed between films grown under low N:H flow ratio conditions (smooth film surfaces). Wear-resistance of films with columnar structures from high N:H conditions was significantly lower, while the hardness was unobtainable. This result indicates the importance of film morphology on mechanical performance.

Nanoindentation-induced dislocation emission at 5–7 nm displacements in ultra-thin films (12–33 nm) and nanocubes (40–60 nm) is used to examine deformation and plasticity models. Using the Tabor estimate, this displacement corresponds to a plastic strain of 3–5%. Load–displacement curves produced using nanoindentation show evidence of discretized, Burgers vector–length displacement steps, or excursions, which can be associated with individual dislocation emission events. Using these displacement steps and the residual plasticity present on unloading, theoretical hardening models are developed. Linear and parabolic hardening approaches are compared for ultra-thin films of nickel, cobalt, and Permalloy (Ni80Fe20), and also for silicon nanocubes. It is determined that the linear hardening model can predict the early trends of the experimental data while parabolic hardening may be more appropriate at later stages.

The fracture behavior and toughness of thin tetrahedral amorphous carbon (ta-C) films were examined using acoustic-sensing nanoindentation. Laser annealing was used to induce several levels of film stress ranging from −1.7 GPa (compressive) to +2.4 GPa (tensile). Indentations into the films generated radial cracks that extended into channel cracks with increasing indentation depth. Crack length was a function of both the indentation load and the sign and magnitude of the residual film stress, with tensile stresses promoting crack extension. Indentation fracture toughness models were used to estimate the fracture toughness of the ta-C films. Results obtained using an acoustic sensor embedded into the nanoindentation tip revealed a relationship between the released acoustic energy and the total crack length. These results were used to formulate an expression for the fracture toughness of the thin films directly from the acoustic signal, with comparable results to the standard indentation-based measurement.

As a natural response to the ongoing trend of device miniaturization, many effects of scaling on the properties of materials have become well documented. However, the mechanical properties of individual nanoparticles are not well understood and the direct observation of nanoparticle deformation has only recently been achieved. This work investigates the mechanical behavior of silicon nanospheres in the transmission electron microscope (TEM) using an in situ indentation sample holder. In situ TEM studies provide information which is not accessible by more traditional means, including particle orientation prior to deformation and the type and location of any preexisting defects. In this study, isolated nanoparticles were located and compressed between a diamond tip and a sapphire substrate. Here, the deformation behavior of individual particles is investigated and analogous strain fields between small particles are discussed.

Adhesion physics was relegated to the lowest echelons of academic pursuit until the advent of three seemingly disconnected events. The first, atomic force microscopy (AFM), eventually allowed fine-scale measurement of adhesive point contacts. The second, large-scale computational materials science, now permits both hierarchical studies of a few thousand atoms from first principles or of billions of atoms with less precise interatomic potentials. The third is a microelectronics industry push towards the nanoscale which has provided the driving force for requiring a better understanding of adhesion physics. In the present contribution, an attempt is made at conjoining these separate events into an updating of how theoretical and experimental approaches are providing new understanding of adhesion physics. While all material couples are briefly considered, the emphasis is on metal/semiconductor and metal/ceramic interfaces. Here, adhesion energies typically range from 1 to 100 J m −2 where the larger value is considered a practical work of adhesion. Experimental emphasis is on thin-film de-adhesion for 10 to 1000 nm thick films. For comparison, theoretical approaches from first principles quantum mechanics to embedded atom methods used in multi-scale modelling are utilized.

Nanodefects induced by nanoindentation on thin polystyrene (PS) films spin cast on silicon (Si) relax upon annealing at 110 °C. The relaxation process for low molecular weight PS is interpreted in terms of a curvature driven flow which leads to the measurement of a diffusion coefficient. The latter is compared with the expected Rouse predictions using (i) bulk T g bulk and (ii) surface T g surf glass transition temperature data, found in the literature. Deviations from the Rouse predictions are observed when T g bulk is used for the analysis of the data. On the contrary, excellent agreement with the Rouse model is reported when T g surf is used.

Nanoparticles in the range of 20 to 100 nm in size can be deposited, isolated, and individually probed for their mechanical properties. With a hypersonic plasma particle deposition technique, this has been successfully accomplished for silicon and titanium. We have already shown that silicon nanoparticles are superhard in the 30 to 50 GPa range after work hardening (Gerberich, W.W., Mook, W.M., Perrey, C.R., Carter, C.B., Baskes, M.I., Mukherjee, R., Gidwani, A., Heberlein, J., McMurry, P.H., Girshick, S.L., 2003a. Superhard silicon nanospheres. J. Mech. Phys. Solids 51, 979). At the same time when small nanospheres are compressed, a fraction of the plastic strain is reversed after unloading. Initially, the amount of reverse dislocation motion was small but appeared to accelerate once a threshold strain was reached. The cumulative reverse plastic strain from repeated loading of the same nanosphere appeared to increase from less than 0.04 to approximately 0.4 as cumulative strain increased from 0.2 to 0.6. For large strains then, it appears that a greater amount of plastic strain is recovered after unloading. This can at least partially be understood in terms of the enormous back stress developed at the small scale when dislocations are only a few nm apart. As the ramifications to nanoscopic features on MEMS, micromachines and magnetic recording devices is considerable, it is desirable to understand if a length scale can be developed for such phenomena. In terms of classic dislocation theory an attempt is made. Problems and prospects are discussed with regards to predictive models for hardness and reverse plasticity.