Search

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.

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.

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.

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.

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.

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.

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.

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.

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.

Understanding the hardening response of small volumes is necessary to completely explain the mechanical properties of thin films and nanostructures. This experimental study deals with the deformation and hardening response in gold and copper films ranging in thickness from 10 to 400 nm and silicon nanoparticles with particle diameters less than 100 nm. For very thin films of both gold and copper, it was found that hardness initially decreases from about 2.5 to 1.5 GPa with increasing penetration depth. Thereafter, an increase occurs with depths beyond about 5–10% of the film thickness. It is proposed that the observed minima are produced by two competing mechanisms. It is shown that for relatively deep penetrations, a dislocation back stress argument reasonably explains the material hardening behavior unrelated to any substrate composite effect. Then, for shallow contacts, a volume-to-surface length scale argument relating to an indentation size effect is hypothesized. A simple model based on the superposition of these two mechanisms provides a reasonable fit to the experimental nanoindentation data.

This paper describes the nanoindentation technique for measuring sputter-deposited Au and Cu thin films’ mechanical properties at elevated temperatures up to 130 °C. A thin, 5-nm Pt layer was deposited onto the Cu film to prevent its oxidation during testing. Nanoindentation was then used to measure elastic modulus and hardness as a function of temperature. These tests showed that elastic modulus and hardness decreased as the test temperature increased from 20 to 130 °C. Cu films exhibited higher hardness values compared to Au, a finding that is explained by the nanocrystalline structure of the film. Hardness was converted to the yield stress using both the Tabor relationship and the inverse method (based on the Johnson cavity model). The thermal component of the yield-stress dependence followed a second-order polynomial in the temperature range tested for Au and Pt/Cu films. The decrease in yield stress at elevated temperatures accounts for the increased interfacial toughness of Cu thin films.

Length scales are essential to the understanding of small volume deformation and fracture in emerging technologies. Recent analysis by two groups at the atomistic (Horstmeyer and Baskes, 1999) and mesoscopic (Gerberich et al., 2002) levels have shown the importance of the volume to surface ratio to the indentation size effect (ISE) at small depths of penetration. We have interpreted this in terms of the plastic work under the contact and the surface work associated with the creation of new surface or the excess surface stress. Treating this as a modified Griffith criterion the case is made that this same length scale should apply to the delamination of thin films. By making this simple equivalency in length scales, an R-curve analysis for crack growth resistance, GR ,i n thin film delamination emerges. This recovers the classic σ 2 ysh/E term as well as the fact that interfacial toughness should scale with the square root of incremental crack growth. Here σys is yield strength, h is thickness and E is modulus of the film. As applied to thin Cu and Au films bonded to silicon substrates, the model is in good agreement.

Carbon fiducial marks are formed during thin film local delamination processes induced either by su- perlayer indentation forming circular blisters, or by residual stress relief through telephone cord blister formations. Hydrocarbons are sucked into the crack tip during the delamination process, outlining the crack tip opening angle (CTOA), which can be used to back calculate thin film adhesion using either elastic or plastic analyses presented here. Fiducial marks have been observed in two different thin films systems, namely Cu/SiO2 and TiWXNY/GaAs. TiWXNY/GaAs system also exhibited biaxial compressive stress-induced phone cord buckling delaminations. Surface AFM CTOA measurement approach is used to estimate the strain energy release rate increase along these phone cords delaminations.

Nanoindentation combined with atomic force microscopy was applied to measure the fracture toughness of polystyrene/glass interfaces. Film delamination occurs when the inelastic penetration depth approximately equals or exceeds the film thickness. The delamination size was accurately measured using atomic force microscopy. Using multilayer indentation and annular-plate analyses, the interfacial fracture toughness was then assessed. The values obtained from the two analyses are in good agreement with the fracture toughness of the interface being approximately 350 mJ/m2. By appropriate fracture surface characterization, it was shown that fracture occurs along the polystyrene/glass interface. Crack arrest marks were observed, and their possible cause discussed. On the basis of the morphology of the fracture surface, the fracture toughness was also evaluated using a process zone analysis. The result agrees well with those obtained from the other two analyses.

Films of poly(ether sulfone) have been implanted with 50-keV As+ in the dose range of 1015 to 1017 ions/cm2. Nanoindentation tests were then conducted on these films using a conical diamond tip with a 90° included angle, applying loads from 10 μN to 2 mN. The modulus and hardness were evaluated from the load–displacement data using the elastic unloading [J. Mater. Res. 7, 1564 (1992)] and the elastic–plastic unloading [J. Mater. Res. 13, 421 (1998)] models. The latter approach gave more reliable values for the mechanical properties since it is not as sensitive to creep-in effects. The implanted film showed as much as a twofold increase in hardness compared to the unimplanted polymer. However, the films with the highest dose did not exhibit the maximum values for the mechanical properties. Hardness and modulus values increased with increasing implantation dose up to 1 × 1017 ions/cm2 but dropped at higher doses, presumably due to a combination of sputtering of material and surface roughening. The dose dependence of the mechanical properties is observed to have the same trend as are reported for the electrical properties.

Plastic zone evolution in Al–2 wt% Si metal films on silicon and sapphire substrates was studied using nanoindentation and atomic force microscopy (AFM). AFM was used to measure the extent of plastic pileup, which is a measure of the plastic zone radius in the film. It was found that the plastic zone size develops in a self-similar fashion with increasing indenter penetration when normalized by the contact radius, regardless of film hardness or underlying substrate properties. This behavior was used to develop a hardness model that uses the extent of the plastic zone radius to calculate a core region within the indenter contact that is subject to an elevated contact pressure. AFM measurements also indicated that as film thickness decreases, constraint imposed by the indenter and substrate traps the film thereby reducing the pileup volume.

Evaluation of the plasticity effects in fracture along ductile/brittle interfaces requires appropriate models for plastic dissipation in a ductile component. For thin ductile films, constitutive properties appropriate to the small volumes involved are essential for adequate modeling. Here, yield stress is of primary importance. With nanoindentation, one can obtain both a large strain flow stress as well as the far field yield stress representing the small strain elastic-plastic boundary. Using these to estimate an appropriate plastic strain energy density, the crack tip plastic energy dissipation rates associated with the interfacial crack extension can be estimated for a ductile film. With the preceding analysis, plasticity effects on the interfacial toughness have been evaluated for external measures of strain energy release rates as obtained from indentation tests using the axisymmetric bilayer theory. Comparison involved RF sputtered 200- to 2000-nm-thick Cu interlayers between oxidized silicon and sputtered tungsten. Experimental values for the Cu/SiO2 interface increased with Cu film thickness from 1 to 15 J/m2. This was in qualitative agreement with the theoretical predictions for plastic energy dissipation rates. In contrast, first-order estimates suggest that the observed interfacial toughness increases cannot be attributed to either mode mixity effects or increased intrinsic interfacial fracture energies. As such, crack tip plasticity is identified as the dominant mechanism for increasing interfacial toughness.

The effect of carbon black content on the mechanical and electrical properties of carbon-black-filled poly(vinylpyrrolidone) composites was determined. Experimental data show a drop in modulus when the volume of carbon black exceeds 25%, coincident with pore formation documented by scanning electron microscopy. This behavior is consistent with surpassing the critical pigment volume concentration. Electrical conductivity, however, does not show a discontinuous change in behavior at 25 vol% carbon black and continues to increase through a carbon black loading of 35 vol%. A qualitative model of microstructural evolution is presented to explain the observed differences in electrical and mechanical behavior.

Sputtered copper and tungsten thin films both with and without tungsten and chromium superlayers were tested by using nanoindentation probing to initiate and drive delamination. The adhesion energies of the films were calculated from the induced delaminations using the analysis presented in “Quantitative adhesion measures of multilayer films: Part I. Indentation mechanics.” Copper films ranging in thickness from 150 to 1500 nm in the as-sputtered condition had measured adhesion energies ranging from 0.2 to 2 J/m2, commensurate with the thermodynamic work of adhesion. Tungsten films ranging in thickness from 500 to 1000 nm in the as-sputtered condition had measured adhesion energies ranging from 5 to 15 J/m2. The superlayer was shown to induce radial cracking when under residual tension, resulting in underestimation of the adhesion energy when the film was well adhered. Under conditions of weak adherence or residual compression, the superlayer provided an excellent means to induce a delamination and allowed an accurate and reasonably precise quantitative measure of thin film adhesion.

In this study we combined nanoscratch testing with a multilayer sapphire and aluminum nitride single-substrate system to determine the effects of interface composition and structure on susceptibility to fracture of hard, thin tantalum nitride films. Nanoindentation tests showed that the elastic moduli of the tantalum nitride and aluminum nitride films, as well as the sapphire substrate, were essentially equal at 400 GPa. On both portions of the substrate, these tests also showed that near surface hardness was near 35 GPa. Nanoscratch tests triggered long blisters and circular spalls on both the sapphire and aluminum nitride portions of the substrate. The blisters showed that the tantalum nitride film was subjected to a compressive residual stress of {minus}6.7 GPa. The spalls showed that failure occurred along the tantalum nitride film-substrate interface regardless of substrate composition. Most importantly, the blisters and spalls showed that the mode I component of the fracture energies were essentially equal on both substrate materials at a value near 3.1 J/m{sup 2}. These energies are on the order of the energies for metallic bonding. {copyright} {ital 1999 Materials Research Society.}

A simple superposition solution for a point-loaded elastic plate on a soft substrate is proposed. The solution considers a “drumhead” being elastically bent into a compliant substrate that is viscoelastic-plastic. With simplifying assumptions it is found that the drumhead and substrate support loads proportional to δ1/2 and δ3/2, respectively, where δ is the vertical point displacement. At fixed displacement, relaxation proceeds at high loads, but if sufficiently unloaded, recovery or increased load results with time. Qualitative verification of the time-dependent drumhead solution is shown by relaxation and recovery data on polycarbonate covered by polysiloxane, composite or diamondlike carbon (DLC) coatings, and films.

Nanostructured silicon carbide films have been deposited on molybdenum substrates by hypersonic plasma particle deposition. In this process a thermal plasma with injected reactants (SiCl4 and CH4) is expanded through a nozzle leading to the nucleation of ultrafine particles. Particles entrained in the supersonic flow are then inertially deposited in vacuum onto a temperature-controlled substrate, leading to the formation of a consolidated film. In the experiments reported, the deposition substrate temperature Ts has ranged from 250°C to 700°C, and the effect of Ts on film morphology, composition, and mechanical properties has been studied. Examination of the films by scanning electron microscopy has shown that the grain sizes in the films did not vary significantly with Ts. Micro-X-ray diffraction analysis of the deposits has shown that amorphous films are deposited at low Ts, while crystalline films are formed at high Ts. Rutherford backscattering spectrometry has indicated that the films are largely stoichiometric silicon carbide with small amounts of chlorine. The chlorine content decreases from 8% to 1.5% when the deposition temperature is raised from 450°C to 700°C. Nanoindentation and microindentation tests have been performed on as-deposited films to measure hardness, Young's modulus and to evaluate adhesion strength. The tests show that film adhesion, hardness and Young's modulus increase with increasing Ts. These results taken together demonstrate that in HPPD, as in vapor deposition processes, the substrate temperature may be used to control film properties, and that better quality films are obtained at higher substrate temperatures, i.e. Ts≈700°C.

The current study is centered on elastic-plastic solid interaction with hydrogen. Here, the environment isfree hydrogen, from either external or internal origins providing as such aggressive effects. In this context, near surface displacement occurred, beside microcracking onset or growth, significant interfacial weakening, as critical forms of mechanical degradation. Metastable austenitic stainless 316L steel was selected, in order to provide a comprehensive study on bulk surfaces. Globalfindings on hydrogen effects were supplemented by nanoscale information. Only for the nanosection, Ti/Cu thinfilms were also included, namely an additional small-volume case. Samples have been charged with hydrogen under lowfugacity conditions and the outcoming effects have been sorted out by mechanical response tracking assisted by contact mechanics methodology. Nanoindentation and continuous scratch tests were utilized supplemented by Scanning Probe Microscopy (SPM) visualization. Local resolution provided remarkable input to the globalfindings, in terms of dislocation nucleation aspects, near surface modification, plastic localization and microfracture onset. In thin layers, the effective work of the adhesion was reduced indicating significant degradation that could be expressed quantitatively. Global/local benefits of the stainless steel system under study made it possible to apply multiscale models describing complex micro­mechanical processes. Рассматривается взаимодействие упруго­-пластического твердого вещества с водоро­дом. Средой служит свободный водород от внешнего или внутреннего источника, что создает агрессивный эффект. В результате происходило приповерхностное смещение, кроме начала образования микротрещин или их роста и значительного межфазного раз­упрочнения, что является основными причи­нами потери механической прочности. Для всестороннего изучения внутренней струк­туры поверхности была выбрана метастабильная аустенитная нержавеющая сталь 316Л. Общие данные о действии водорода были дополнены информацией на наноуровне. Для получения данных на нано­уровне были изучены тонкие пленки Ti/Cu, т.е. проведены испытания на малом объеме материала. Образцы обрабатывали водоро­дом в условиях низкой летучести, а ре­зультаты классифицировали по механи­ческому отклику методом контактной меха­ники. Применяли наноиндентирование и непрерывное царапанье с использованием сканирующей микроскопии. Результаты ло­кальных исследований послужили значи­тельным вкладом в общие выводы, включая зарождение дислокаций, приповерхностную модификацию, начало пластической локали­зации и микроразрушения. Эффективная работа адгезии в тонких слоях уменьшилась, что свидетельствует о существенном сниже­нии механических свойств, выражаемом ко­личественно. Преимущества глобального и локального подходов при изучении нержа­веющей стали позволили использовать многоуровневые модели, описывающие комплексные микромеханические процессы.

The use of nanoindentation techniques to measure nanoscale mechanical behavior is a new path of interest to researchers today. Load drops and displacement excursions can be utilized to measure activation volumes for dislocation events in single crystals, thin films, and nanoposts. Through the introduction of a new length-scale parameter, the dislocation wall spacing, a mechanism describing staircase yielding, is presented. The dislocation wall spacing can also be used to estimate activation volumes. Molecular dynamics simulations of nickel film indentation have been used to validate the origin of staircase yielding and also show consistent dislocation wall spacings. Additionally, stress relaxation experiments have been used to estimate activation volumes.

A commercial piezoelectric acoustic emission transducer has been used in conjunction with nanoindentation techniques to study the relationship between acoustic emission signals and discrete physical events to identify the type and strength of an event. Indentations into tungsten and iron single crystals have been used to study dislocation generation and passive film failure. In addition, indentations made into a thin nitride film on sapphire have been used to cause film delaminations. Parameters such as signal rise time and frequency for a piezoelectric sensor are related to sample geometry, and not to the type of event which caused the acoustic emission signal. As a possible calibration for acoustic emission sensors, the most meaningful parameter is the acoustic emission energy, which has been shown to scale with the elastic energy released during the event. The measured values of elastic energy released correspond very closely to those calculated using Hertzian contact mechanics.

We have modified the microscratch test to create a near plane strain loading condition. In the Microwedge Scratch Test (MWST), a wedge-shaped diamond indenter tip is drawn along a fine line (i.e., narrow strip of film), while simultaneously being driven into the line. We compare microwedge scratching of zone 1 (voided grain boundaries) and zone T (metallurgical grain boundaries) thin film specimens of sputtered tungsten on thermally grown SiO2. Symptomatic of its weak grain boundaries, the zone 1 film displays three separate crack systems. Because of its superior grain boundary strength, the zone T film displayed only one of these—an interfacial crack system. By correlating fracture phenomena to signature events in the load-displacement curve, we develop governing equations for propagating interfacial cracks, including expressions for strain energy release rate, bending strain, and mode mixity. Grain boundary fracture causes zone 1 films to spall before a stable crack is formed. Zone T films survive the bending strains, and hence adhesions may be inferred from stable crack growth mechanics. We conclude by contrasting and comparing experimental results for plane strain indentation versus plane strain scratching.

A new method for evaluating modulus and hardness from nanoindentation load/ displacement curves is presented. As a spherical indenter penetrates an elastoplastic half-space, the elastic displacement above the contact line is presumed to diminish in proportion to the total elastic displacement under the indenter. Applying boundary conditions on the elastic and plastic displacements for elastic and rigid plastic contacts leads to an expression that can be best fit to the entire unloading curve to determine E*, the reduced modulus. Justification of the formulation is presented, followed by the results of a preliminary survey conducted on three predominantly isotropic materials: fused quartz, polycrystalline Al, and single crystal W. Diamond tips with radii ranging from 130 nm to 5 μm were used in combination with three different nanoindentation devices. Results indicate that the method gives property values consistent with accepted values for modulus and hardness. The importance of surface roughness and indentation depth are also considered.

Continuous microindentation has been carried out on an iron–3% silicon single crystal in 1 M sulfuric acid. The ability of the material to support elastic loading is directly linked to the presence of thermally grown oxide films and passive films applied through potentiostatic control of the sample. When the passive film is removed, either by chemical or electrochemical means, the iron alloy can no longer sustain pressures on the order of the theoretical shear strength of iron. Instead, the metal behaves in a traditional elastic-plastic manner when no film is present. The oxide film at the edges of the indentation can sustain applied tensile stresses up to 1.2 GPa prior to failure. Indentation in materials undergoing dissolution must account for the rate of material removal over the remote surface and the resulting plastic deformation around the contact of the indentation.

We have investigated mechanical probing of a precracked fine line structure as a new type of thin film fracture mechanics specimen. An idealized mechanics analysis is first presented. Experimentally, two types of precracks are formed. A thin carbon layer to which other layers weakly adhere creates a “processed precrack” by integrated circuit processing techniques. An “indented processed precrack” is formed by precision alignment of a sharp microwedge. The processed precrack is found to reduce the critical tangential load by 50% from a non-precracked line, while the indented processed precrack lowers the load by 200%. From this, a reasonable value of adhesion may be directly calculated. Crack path behavior is observed to depend on strength of the interface. In the case of a weak interface, the crack remains in the interface as it extends. For a strong interface, it kinks into the substrate if the crack is initially short, but remains in the interface if it is initially long. Given the experimental evidence, the mechanics are slightly modified to quantitatively model the experimental data.

The atomic force microscope (AFM) has been used to observe and characterize for the first time surface steps and grooves on the faces of Cr2O3 grains formed as an oxide scale on Ni−30Cr and Ni−30Cr−0.5Y alloys during high temperature oxidation. The very high spatial resolution of the AFM is required to characterize these features. We propose that these surface features, whose dimensions are in the range of nanometers and tens of nanometers, may be interpreted as evidence of highly localized plastic deformation of the oxide scale. The size and spacing of the steps and grooves are consistent with models of plastic deformation based on slip bands derived from dislocation climb or dislocation glide. Mechanical twinning and the models for stress-driven surface instability are also possibly responsible for some surface features. The addition of yttrium to the alloy seemed to enable enhanced plastic deformation of the scale. The strain corresponding to the observed features, estimated by simple models, could relax a significant part of oxide growth and thermal stresses.

Mechanical properties of cold-worked molybdenum, grade 4 titanium, and an α-β titanium alloy are measured with tensile tests and by indentations using conical indenters with 105, 120, and 137 deg included angles. The extent of plastic deformation and pileup around an indentation is measured using profilometry. Various models predicting the extent of plastic deformation and pileup are compared to the actual measured values. As inferred from indentation, the calculated yield strength of the material from the mean pressure does not correlate well to the yield strength measured by tensile testing. The plastic zone size surrounding an indentation can also be used to determine the yield strength of the material, and this does correlate to the yield strength measured by tensile tests. Furthermore, the extent of plastic deformation is relatively independent of the included angle of the indenter for the range of materials used in this system. Models predicting the amount of pileup at the edges of the indentation appear to approach but overestimate the actual amount of pileup in the materials tested.

Using a continuous microscratch technique, the adhesion strengths of Pt, Cr, Ti, and Ta2N metallizations to NiO and Al2O3 substrates have been characterized. The practical work of adhesion was determined as a function of both thickness and annealing conditions. For all except the Ta2N films, the practical work of adhesion increases nonlinearly from a few tenths of a J/m2 to several J/m2 as the thickness of the thin film is increased, indicating that a greater amount of plastic work is expended in delaminating thicker films. Further, the practical work of adhesion also increases with increasing annealing temperature, indicating stronger bonding at the interface. In the limit that the film thickness tends to zero, the plastic energy dissipation in the film tends to zero. As a result, the extrapolation to zero thickness yields the true work of adhesion for that system.

The interfacial fracture energy of Ti/Al2O3 composites was measured with and without a diffusion barrier at different bonding temperatures by using four-point bending tests. It was found that the interfacial fracture energy increases with increasing bonding temperature up to 950 °C. When the bonding temperature was further raised to 1000 °C, the interfacial fracture energy drops. The decrease of the interfacial fracture energy is due to the formation of the continuous intermetallic compound, Ti3Al, at the interface between Ti and Al2O3. By using a diffusion barrier, the interfacial fracture energy decreases from 25.4 to near O J/m2 and 32.9 to 8.7 J/m2 for applied bonding temperatures of 800 and 900 °C, respectively. This is because the diffusion barrier reduced the diffusion of Al across the interface and into the Ti, thereby preventing a strong chemical bond at the interface. For the composite bonded at 900 °C, the crack propagation was found to occur at the interface between the Ti and Al2O3. The interfacial failure was found to be in the Ti3Al reaction layer for the composite processed at 1000 °C. With a diffusion barrier, the crack propagation path follows several interfaces. Evaluation of the processing temperature on the mechanical properties of the Ti was also obtained by using a nanoindentation technique.

Nanomechanical devices constitute an important and growing field, as they allow for new understanding of the mechanical properties at interfaces and surfaces. As an example, a newly developed nanoindentation device has been used to accomplish μN load indents into GaAs. First, it is shown that a plastic zone can be measured and is comparable to theory. Also, it is shown that the rate of indentation affects both the depth and upset zone of low load indents, implying a strain-rate sensitivity effect at room temperature. This is reinforced by observation of what appears to be a glide-based relaxation process.

Under shear stress, silicon nanocrystals become amorphous through an intermediate hexagonal crystalline phase.

Teflon amorphous fluoropolymer (TAF) multi-walled carbon nanotube (MWCNT) suspensions have the potential for creating conductive coatings on insulating films for numerous applications. However, there are few studies on polymer MWCNT suspension properties and even fewer that use Teflon. To define mechanical and electrical property relationships, bilayer films of TAF-MWCNT were created with differing concentrations of MWCNTs. Nanoindentation revealed that addition of 8 wt% MWCNTs to TAF increased the elastic modulus by about 25% and hardness by about 15%. Conducting indentation showed 8 wt% MWCNT films exhibit uniform stable conductance once indentation depth exceeds several hundred nanometers. Films with lower concentrations of CNTs were insulating. The two techniques provide a unique description of structure property relationships in this suspension film system.

Localized heating of metals and alloys using a focused laser beam in ambient atmosphere produces dielectric oxide layers that have characteristic optical appearances including different colors. Nanoindentation probed the deformation and fracture of laser-fabricated oxides on 304L stainless steel. Conductive nanoindentation measured electrical contact resistance (ECR) of the same colored oxides indicating a correlation between laser exposure, conductance during loading, current-voltage (I-V) behavior at constant load, and indentation response. Microscopy and X-ray diffraction examined the microstructure and chemical composition of the oxides. Combining techniques provides a unique approach for correlating mechanical behavior and the resulting performance of the films in conditions that cause wear.

This paper presents the development of a higher-order direct boundary integral-displacement discon- tinuity method for crack propagation in layered elastic materials. The method is based on the dual boundary integral equations of linear elasticity which are solved by means of a quadratic boundary element formulation. The analytical solution for a point force within a bonded half-plane region is used to derive the kernel functions of the boundary integral equations. Square-root displacement-discontinuity elements are used to model the crack tips, and stress intensity factors may be computed using the numerically predicted values of the displacement discontinuity components at the midpoints of these crack-tip elements. An algorithm based on the maximum tensile-stress criterion is then developed and incorporated into the boundary element model to predict the paths of cracks propagating in layered elastic materials. In the experimental part of this study, crack profiles for straight-through-cracked, compact-tension specimens of the anodically bonded silicon/Pyrex glass system are measured by profilometry. The plane strain prediction of the crack-propagation path is compared with the experimentally measured crack profiles. Consistent with the prediction, the interfacial crack is observed to kink away from the strong, anodicaUy-bonded interface and propagate into the more compliant glass layer. The predicted initial kink angle of 26 ° agrees very well with the average measured value of 280 . The measured path of the crack is also in very good agreement with the predicted path over about the first 120 microns of crack growth with increasing deviation observed beyond that.

The mechanical properties of the interfaces in an Al2O3 fiber reinforced β-21S Ti alloy have been evaluated by using fiber pushout tests. The Al2O3 fibers were coated with a refractory metal and Y2O3 which served as a diffusion barrier during the HIPing used to produce the metal matrix composites. By doing fiber pushout tests, the interfacial fracture was found to occur at the interface between the refractory metal and the Y2O3. The interfacial shear strength and interfacial frictional stress were measured to be 323 and 312 ± 2 MPa, respectively. The interfacial frictional stress, which is due to asperity interlocking during the fiber sliding, was correlated to the surface roughness of the coated Al2O3 fiber obtained with the aid of an atomic force microscope. The measured surface roughness of 18.8 ± 2.2 nm was related to the frictional stress through Hutchinson's model.9 The frictional coefficient between the Al2O3 fiber and the Ti matrix is calculated to be 0.32 ± 0.02.

Direct observations of dislocation arrangements about a range of microindentations into the [100] face of an Fe−3 wt. % Si single crystal have been accomplished. Dislocations of both large loop character initiated from the indenter and small loop character initiated as secondary reactions are found. Analysis of these allows the various contributions to the plastic strain gradient around the indentation to be assessed, with the experimental observations being reasonably consistent with continuum models.

Continuous microindentation tests performed on the electropolished surfaces of single crystal Fe (3 wt.% Si), known to have a thin passivation film, show a sharp discontinuity at a load of 1.8 mN. To this point, there was no apparent plastic deformation in the metal in that the loading and unloading curves exactly overlay each other. Stresses at the discontinuity were close to the theoretical strength of the metal. Elastic contact theories of Hertz and Love reproduced the elastic portion of the load-displacement curves. On removing the passivation film with a HCl solution, indentation tests yielded strengths nearly two orders of magnitude smaller. The strength recovered to near its initial value after the liquid evaporated and the passivation film re-formed.

An interconnected set of observations assesses current equilibrium models of the ductile-brittle-transition temperature (DBTT). This involvesin situ transmission electron microscopy (TEM) studies of crack-tip dislocations in single and polycrystals and bulk fracture toughness tests at various temperatures. Beyond KI values of 8 MPa · m1/2 in both iron-base single and polycrystals, large numbers of redundant dislocations are created, as postulated recently by Weertman. [38] Still, the necessary shielding dislocations, as required by equilibrium, can be detected at values as high as 20 and 40 MPa · m1/2 byex situ TEM and electron channeling, respectively. In addition, the close approach of dislocations to the crack tip in some of the studies, as opposed to others, suggests that large dislocation free zones (DFZ) are a thin-film artifact. However, a failure criterion based partly on the Rice-Thomson model’21 is both consistent with the absence of a large DFZ and observed fracture toughness variations with test temperature. It is emphasized that this toughness transition is entirely in the semibrittle regime where cleavage is the failure mode. Nevertheless,Klc values increase from 3 to 60 MPa·m1/2 with an increase in test temperature.

Using a dislocation mechanics-based crack-tip shielding model, a zeroth-order estimate of the grain size and yield strength dependence of fracture toughness is made for mild steel. This model, appropriate to cleavage fracture in the lower shelf regime, is shown to predict the fracture toughness determined over a temperature regime of 100 to 240 K in both quasi-static and dynamic tests and for two different grain sizes. Two grain size terms in the proposed model result. One is associated with grain boundary blockage effects on crack-tip shielding, while the other is proposed to affect the far-field stresses and, indirectly, the local crack-tip stress field. Differences between the present approach and the classic Cottrell-Petch model are in how the friction stress and grain size affect local stresses at the crack tip rather than at a carbide or a grain boundary.

The adhesion of as-sputtered Pt thin films to NiO single crystals has been characterized by a continuous microscratch technique. In these experiments, a conical indenter was driven into a 1.2 μm thick Pt film at a rate of 15 nm/s, and across the sample surface at a rate of 0.5 μm/s, until a load drop was observed indicating that the film had delaminated. Using the width of the scratch track at the point at which the film delaminated from the substrate, the critical load required for delamination, and the area of the delaminated region, a model has been developed to determine the work of adhesion of the Pt/NiO system. This model uses an elastic contact mechanics approach to relate the stresses acting in a scratch experiment to the strain energy released during film delamination. Using this model, the work of adhesion and hence the interfacial fracture toughness have been determined to be 0.023–0.06 J/m2 and 0.07–0.11 MPa$\sqrt m$, respectively. These values are in reasonable agreement with those determined by other methods for metal-ceramic systems.