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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.

The break up of a thin polystyrene (PS) film applied on silicon (Si) upon heating above the bulk glass transition temperature (Tg) of the polymer is studied using atomic force microscopy (AFM). In a 17-nm thick PS film rupturing occurs with the formation of indent-like surface disturbances pointing toward the Si substrate and leads to the standard nucleated dewetting process. Indent growth toward the substrate is accompanied by an increase of the indent radius at the level of the free, unperturbed, PS surface and a decrease of the radius of curvature at the indent bottom. The two radii are comparable and are 1–3 orders of magnitude larger than the film thickness. The area induced in the system upon indent formation was measured for several indents and found to be almost negligible comparing to the total area of the system. Rupturing (growing indents) is discussed with respect to the opposing leveling (healing indents) process of artificially induced indents. Also, a comparison of the indent growth rate toward the substrate, calculated using previously published data, with the dewetting rate of an expanded dry patch along the substrate is provided, which suggests that the former is about 2–3 orders of magnitude lower than the latter.

Small volume deformation can produce two types of plastic instability events. The first involves dislocation nucleation as a dislocation by dislocation event and occurs in nanoparticles or bulk single crystals deformed by atomic force microscopy or small nanoindenter forces. For the second instability event, this involves larger scale nanocontacts into single crystals or their films wherein multiple dislocations cooperate to form a large displacement excursion or load drop. With dislocation work, surface work, and stored elastic energy, one can account for the energy expended in both single and multiple dislocation events. This leads to an energy balance criterion which can model both the displacement excursion and load drop in either constant load or fixed displacement experiments. Nanoindentation of Fe-3% Si (100) crystals with various oxide film thicknesses supports the proposed approach.

A concept for the deformation resistance of small volumes under contact subjected to displacements in the 2–100 nm regime is proposed. In terms of an energy balance criterion, the external work minus stored elastic energy is consumed by surface energy and plastic energy absorption. The surface energy is interpreted in terms of new area of contact or new area created by slip step emergence or oxide film fracture. The plastic energy absorption is interpreted in terms of dislocation work. As such, this is analogous to a contact mechanics Griffith criterion for the hardness or flow strength of crystalline metals, semiconductors or oxides. Derivations of supporting loads and stresses provided for nanoindenter tips, nanospheres, nanocubes and thin-walled nanoboxes are presented. For comparison, measured hardness and flow strengths of Al, Au, Fe–3%Si, Ti, W and Si nanostructures are reported in the 2–100 nm length scale regime. It is proposed that Si and Ti nanospheres experience a minimum in contact resistance as dictated by a balance in surface energy dominance at small contact and dislocation hardening dominance at large contact. The variation of a strength controlling coefficient, α , with material suggests a length scale dependence on shear and/or pressure activation volumes.

In elastic plastic solids, approaching the sub micron scale, critical experiments indicated significant differences in the mechanical response. Thus, mainly in small volume behavior a length scale issue is introduced with implications on the basic understanding of deformation and fracture processes. The current study is centered on the mechanical response of silicon particles in the range of 20-50 nm on sapphire substrate. Monotonic and cyclic mechanical tests have been performed by contact mechanics methodology at ambient temperature. Mechanical information and visualization assisted by scanning probe microscope-based nano indentation alluded to a model founded on dislocation dynamic effects. This facilitated developments regarding the length scale subject in the light of fatigue concepts and structural integrity aspects.

We study the leveling process of nanoindentation-induced defects on polystyrene (PS) films (h = 100−150 nm) spin cast on silicon (Si) surfaces. Leveling occurs upon annealing above the glass transition temperature (Tg) of bulk PS, resulting in a flat polymer surface. The dynamics of the process is recorded with an AFM for PS molecular weights lower and comparable to the critical entanglement point. Diffusivity values measured from the recorded curvature driven flow are found to be in a very good agreement with corresponding self-diffusivity values of bulk PS.

The mechanical properties of antimony-doped tin oxide (ATO) nanoparticle/poly (vinyl acetate-co-acrylic) (PVAc-co-acrylic) coatings with various ATO contents were investigated using depth-sensing indentation. These coatings were prepared from aqueous dispersions of ATO and PVAc-co-acrylic latex. Three types of methods, including a prolonged load holding time, analysis of the pull-off portion of the unloading curve, and dynamic indentation, were used to characterize the mechanical properties of these composite coatings. As compared to dynamic indentation, quasistatic conventional indentation even with a prolonged load holding time and analysis of the pull-off portion of unloading curves generate more scattered coating modulus data. This is due to the effect of creep deformation and inconsistency of the pull-off portion dimension, respectively. The results obtained using dynamic indentation are more reliable because the technique minimizes the effect of creep deformation using a combination load including static and dynamic components. The dynamic indentation results indicate that the addition of the ATO nanoparticles made the composite coatings stiffer and more elastic solid–like. For example, the storage indentation modulus of the PVAc-co-acrylic coating is ∼1 GPa and tan δ is ∼1.6; the addition of 0.50 volume fraction of ATO increased the modulus to ∼5 GPa and reduced the tan δ to ∼0.01. POLYM. ENG. Sci. 45:207–216, 2005. © 2005 Society of Plastics Engineers.

A quantitative comparison between spontaneous dewetting and particle nucleation for thin (thickness = 17nm) polystyrene (PS) films on nonwettable silicon (Si) surfaces is presented both experimentally and theoretically. Performing experiments in a class 100 clean room, we found that ∼ 23% of the observed dry patches formed because of dust particles, while the majority of the holes formed via the well known spontaneous dewetting process. The result was verified qualitatively by diffusion theory, which, however, predicted a diminished role for the airborne particles, leading to the conclusion that pre-existing particles on the Si surfaces and/or the polymer solutions contribute substantially to the dewetting process. The driving force of particle motion into the polymer film is examined by placing aluminum oxide (Al2O3) particles on PS films. Finally, the effect of particle geometry is studied by placing gold (Au) disks on the free surface of PS films. An optically continuous PS film is found to be present around the periphery of the disk particles, even after the completion of the dewetting process in the rest of the sample. An attempt to explain dewetting inhibition at the vicinity of the micro-disks, on the basis of molecular interactions developed in the system Au/PS/Si, is finally presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 138–145, 2005

Si–Ti–N coatings with various compositions were deposited on molybdenum substrates using hypersonic plasma particle deposition (HPPD). In this method, vapor phase precursors (TiCl4, SiCl4 and NH3) are dissociated in a DC plasma arc and the hot gas is quenched in a rapid nozzle expansion to nucleate nanoparticles. These nanoparticles are then accelerated in hypersonic flow, causing them to deposit by ballistic impaction on a substrate placed downstream of the nozzle. Films of 10–25 Am thickness were deposited at rates of 2–10 Am/min, depending on reactant flow rates, at substrate temperatures ranging from 200 to 850 8C. When the reactant gases were premixed the coatings consisted of nc-TiN, nc-TiSi2, nc-Ti5Si3 and amorphous Si3N4. For the unpremixed reactants case, the coatings consisted of free Si, nc-TiN and amorphous Si3N4. Hardness of as-deposited films was evaluated by nanoindentation of polished film cross-sections. Measured hardness values, averaged over 10–15 locations for each film, ranged from 16–24 GPa. In separate experiments with the same conditions, particle size distributions were measured by placing a sampling probe at the same location as the film substrate. The sampled aerosol was rapidly diluted and delivered to a scanning mobility particle sizer (SMPS). In-situ particle size distribution measurements confirmed that the coatings were formed by impaction of nanoparticles in the 5–15 nm range, with higher reactant flow rates producing larger particles. Focused ion beam (FIB) milling was used to observe film cross-section and porosity. For as-deposited films containing pores, in-situ plasma sintering was used to densify the film without grain growth. D 2004 Published by Elsevier B.V.

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.

A study of ultra-thin gold films and thin-walled nanoboxes has confirmed that length scales in terms of dislocation spacing can predict flow stress. Initial stages of deformation conform to linear hardening with average dislocation spacing controlled by the number of geometrically necessary dislocations in a pile-up. Later stages of deformation exhibit parabolic behavior with Taylor hardening interpreted in terms of a dislocation density described by the total line length of prismatic loops per unit volume. Comparisons of 20 and 40 nm thick planar films could be made to 205 nm high hollow gold nanoboxes with a wall thickness of 24 nm. These highly constrained, ultra-thin planar films demonstrated increased hardness from about 2 to 10 GPa with strains of 20 percent while less constrained nanoboxes increased from 0.8 to 4 GPa for the same strain magnitude.

Interfacial fracture energies of thin films may be calculated using many different techniques. Nanoindentation and stressed overlayers are by far the most common and more reliable of the testing techniques. They depend on mechanics-based models to calculate the interfacial fracture energy of an interface using only the site specific material properties and the dimensions of the delaminated region, either in spontaneous buckle or indentation-induced blister form. This study will focus on four adhesion measurement techniques: spontaneous buckles, stressed overlayer-induced buckles, and nanoindentation-induced blisters with and without stressed overlayers, to demonstrate that the techniques will produce similar results for the measurement of adhesion energy. Films of tungsten (W), platinum (Pt), and titanium (Ti) on SiO/sub 2/ (amorphous glass) substrates are examined and values of interfacial fracture energies reported. Results of interfacial fracture energy calculated from spontaneous buckles and indentation-induced blisters compare well to one another and values are reported for the aforementioned films.

We have conducted an experimental study of acrylic pressure-sensitive adhesive-like networks (PSA-LN) with the goal of establishing deconvolution of interfacial and bulk processes in adhesion. For contact adhesion testing, four types of cylindrically shaped samples were created, each synthetically modified to attain distinct bulk and surface properties. Introduction of small amounts of polar comonomers during synthesis increased the intrinsic adhesion energy, G o , from 65 mJ/m2 for an unmodified acrylic PSA-LN to 129, 158, and 218 mJ/m2 for PSA-LNs modified with 10 wt% of acrylic acid, amino acrylate, and acrylonitrile comonomers, respectively. Following a reversed trend, the critical rate of separation, ν* (below which deadhesion is an intrinsically interfacial process) was more than halved from 496 nm/s for the unmodified PSA-LN to 201, 188, and 212 nm/s for those modified with the same three comonomers (same order, respectively). The adhesion energy was found to be dependent upon the crack propagation...

Stress development during drying is a critical factor that affects the final structure and properties of a coated fiber or spherical product. Stress development during drying of the coating is due to nonuniform shrinkage and physical constraints. In this study, a large deformation elasto-viscoplastic model is developed to predict stress development in drying fibers and spheres after the coatings solidify. From the model, stress evolution in the drying fibers/spheres can be predicted by a partial differential equation of diffusion in one dimension, a first-order partial differential equation of pressure distribution, and two ordinary differential equations on local evolution of the stress-free state. The system of equations is solved by the Galerkin/finite element method in the one dimensional axial/spherical symmetric coatings. Solutions show changes in solvent concentration and viscous stress as the coating dries. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3934–3944, 2003

Nanoindetation is a powerful technique for measuring mechanical properties of thin films. First applied over 20 years ago in the hard drive industry, it is now commonly used for other applications. This paper describes nanoindentation techniques for measuring thin films mechanical properties, including elastic modulus, hardness, adhesion and fracture toughness as applied for modern microelectronics reliability. Elastic, plastic and adhesion properties of Cu interconnects are discussed, including the influence of film microstructure, thickness and grain size. Elastic, fracture and adhesion properties of advanced low-K dielectrics also discussed along with the current challenges of nanoindentation data interpretation and analysis as applied for advanced electronic materials.

Transparent, conductive composite coatings were fabricated from suspensions of poly(vinyl acetate-acrylic) (PVAc-co-acrylic) copolymer latices (50–600 nm) and nanosized antimony-doped tin oxide (ATO) particles (∼15 nm). The suspensions were deposited as coatings onto poly(ethylene terephthalate) substrates and dried at 50 °C. Microstructure studies using field emission scanning electron microscopy and tapping-mode atomic force microscopy (TMAFM) indicated that the latex particles coalesced during drying and forced the ATO particles to segregate into the boundaries between the latex particles. Low phase contrast was observed with TMAFM; this result was consistent with the presence of PVAc-co-acrylic in the ATO-rich phase of the composite. The conductivity of the composite coatings followed a percolation power-law equation, with the percolation threshold between 0.05 and 0.075 volume fractions of ATO and the critical conductivity exponent ranging from 1.34 to 2.32. The highest direct-current conductivity of the composite coatings was around 10−2 S/cm. The optical transmittance and scattering behavior of the coatings were also investigated. Compared with the PVAc-co-acrylic coating, the composite coatings had lower transparency because of the Rayleigh scattering. The transparency of the composite coatings was improved by a reduction in the coating thickness. The best transparency for the coatings with a direct-current conductivity of approximately 10−2 S/cm was around 85% at a wavelength of 600 nm. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1744–1761, 2003

Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20– 50 nm radii range, a prediction of observed hardnesses in the range of 20– 50 GPa is made. The ramifications of this to computational materials science studies on identical volumes are discussed.

DLC film interfacial fatigue and changes in friction coefficient phenomena were investigated during quasi-static nanoindentation/scratch while the sample surface was excited ultrasonically. An original experimental setup combines nanoindenter, SPM scanner and ultrasonic transducer operating in an elastic standing wave mode. Nanoindentation loading–unloading curves revealed delamination induced excursions after a certain number of ultrasonic loading cycles. Drastic changes in quasi-statically monitored friction coefficient during nanoscratch were correlated with sample vibration amplitude and loading force. Resulting indentation/scratch film delamination effects and surface wear patterns were examined by an in situ SPM technique. Ultrasonically excited nanoindentation and nanoscratch techniques can be considered as fast and qualitative methods to characterize critical tribo surfaces for data storage devices.

The semiconductor industry is gradually moving from well-established AlySiO technology to the new Cuylow-k interconnects, 2 which brings a challenge in terms of poor thermal andyor mechanical properties of low-K dielectric films.Extensive nanoindentation studies have been undertaken on organo-silicate glass (OSG) low-K films to explore their mechanical and fracture properties.A cube corner indentation method was used to measure the fracture toughness of the OSG films, which ranges from 0.01 to 0.05 MPaOm .Film fracture was also observed during superlayer indentation adhesion testing.Interfacial cracks kinked into the film 1y2

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.

Fracture and contact mechanics can synergistically contribute to the fundamental understanding essential to scaling of small volumes. Consider two levels of scale in terms of contact area: (i) light wear of tribological contacts at the nanometre level; (ii) delamination of hard protective coatings or ductile metal films at the micrometre level. We first briefly address two phenomena-one showing that oxide film fracture is responsible for yield excursions during nanoindentation and the other showing that analysis of acoustic events can sort out some of the discontinuous events of yield and fracture on the nanometre scale. These concern mostly the fracture process concomitant with a localized plasticity event. However, they do not address the fundamental aspects of what controls the size of the yield event or what controls the size of a localized fracture event, for example delamination. We propose that one way of understanding these two levels of scale and their interconnectivity is through a volume-surface area concept. At present, we show two levels of understanding for contacts to single-crystal surfaces and how these provide insight and potentially quantification of light wear contact.

Carbon fiducial marks are formed during thin-film local delamination processes induced either by superlayer indentation forming circular blisters, or by residual stress relief through telephone cord blister formation. Hydrocarbons are sucked into the crack tip during the delamination process, outlining the crack-tip-opening angle, 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-film systems, namely Cu/SiO2 and TiW x N y /GaAs. Cu/SiO2 delamination blisters have been cross-sectioned using the focused-ion-beam method, and high-resolution scanning electron microscopy of the cross-sections revealed crack renucleation ahead of the original crack tip. This is attributed to the stress redistribution process due to the dislocation shielding mechanism. At stress intensity levels of 0.33 MPa m½, it is found that four emitted dislocations can account for crack arrest, with renucleation of ...

Carbon fiducial marks are formed during thin film local delamination processes induced either by indentation, forming circular blisters, or by residual stress relief through telephone cord blister formations. Hydrocarbons are sucked into the crack tip during the delamination processes, outlining the crack tip opening angle, which can be used to back calculate thin film adhesion using elastic or plastic analyses presented in the paper.

Nanoindentation-induced deformation and fracture of thin polystyrene (PS) films on glass substrates were characterized using visible-light microscopy and atomic force microscopy (AFM). Two film thicknesses, 2 and 3.5 μm were studied. It was difficult to induce delamination in the 2-μm film while the 3.5-μm film delaminated easily under indentation loads of 150 mN and higher. AFM cross-section analysis of the deformation and fracture geometry revealed that the ratio of the delamination radius to contact radius was between 3 and 4. Analysis of the fracture surface on the glass side indicates that substrate cracking acts as a trigger for initiation and propagation of interfacial cracks. Crack-arrest marks and process-zone marks were also observed by AFM imaging. The interfacial fracture toughness, or practical work of adhesion, was evaluated following two methods based on the indentation-induced delamination and a process-zone analysis. The fracture toughness was found to be approximately 0.6 J/m 2 for the 3.5-μm PS film on glass. AFM examination of the glass surface after indentation also showed fine flow lines around the indentation impression, indicating plastic deformation of glass.

Using atomic force microscopy (AFM) we study the dynamics of the dewetting process of polystyrene (PS) films from, (i) atomically smooth silicon (Si) wafers; (ii) sputtered (rough) Si surfaces; and (iii) sputtered (rough) gold (Au) surfaces. Deviations from the circular geometry of the developed dry patches are observed when sputtered surfaces are used as substrates and they are more pronounced for the Au surfaces. It is concluded that the dry patch shape is affected by both the roughness and the nature of the substrate. Kinetic measurements performed using Si surfaces as substrates show that the dewetting rate decreases with substrate roughness. The dewetting dynamics of unentangled PS from Au surfaces shows no deviation from kinetic laws, established for Si substrates. That is the radius of the dry patch scales linearly with time and the dewetting rate is proportional to the ratio of the surface energy over the viscosity of the polymer. PS dewets faster from Au than from a Si surface. A comparison of the dewetting rates of PS from the two surfaces (Si and Au) is found to be in relatively good agreement with respective contact angle measurements.

For very shallow indentations in W, Al, Au, and Fe-3wt%Si single crystals, hardness decreased with increasing depth irrespective of increasing or decreasing strain gradients. As such, strain gradient theory appears insufficient to explain the indentation size effect (ISE) at depths less than several hundred nanometers. Present research links the ISE to a ratio between the energy of newly created surface and plastic strain energy dissipation. Also, the contact surface to plastic volume ratio was nearly constant for a range of shallow depths. Based on the above, an analytical model of hardness versus depth provides a satisfactory fit to the experimental data and correlates well with embedded atom simulations.

As a coating solidifies by drying, it tends to shrink. It is liquid enough in early stages that any shrinkage stress is rapidly relieved by viscous flow. It becomes solid enough in later stages to support elastic stress, which results from shrinkage inhibited by adherence to the substrate. Stress can relax by viscous creep in the coating. Thus, the stress level is an outcome of competing shrinkage and relaxation. A theoretical model of diffusion and mass transfer, large shrinkage-induced deformation and stress, together with yielding and post-yielding viscous deformation, was developed to predict stress evolution in drying of polymer coatings after solidification. The coupled equations of diffusion and stress development are solved by the Galerkin/finite-element method. This model is used in polymer coatings to study the effect of a grooved substrate and embedded particles to the stress and drying, and to simulate cantilever deflection method used to measure stress experimentally.

There are more than 200 different methods for measuring adhesion, suggesting it to be material, geometry and even industry specific. This availability has exploded at least partly due to the arrival of dissimilar material interfaces and thin films and the ease with which microfabrication techniques apply to silicon technology. Having an eye toward those tests utilized for thin films, this paper reviews only a few of these techniques. The emphasis is on measuring thin film adhesion from the standpoint of fracture mechanics, when the film is mechanically or by other means removed from the substrate, and the amount of energy necessary for this process is calculated per unit area of the removed film. This tends to give values approaching the true work of adhesion at small thickness and greater values of the practical work of adhesion at larger thickness, all being in the 30–30,000 nm range. The resulting large range of toughnesses is shown to be dependent on the scale of plasticity achieved as controlled by film thickness, microstructure, chemistry and test temperature. While the tests reviewed largely address the measurement of elastic strain energy release rates, we also briefly address a few theoretical models which are specific to the resistance side of the delamination equation. The weight of the evidence suggests for ductile metallic films that the major extrinsic variables are film stress, extent of delamination, thickness and temperature while the major intrinsic ones are modulus, yield strength, the thermodynamic work of adhesion and one or more length scales. For some 25 film/substrate multilayers, with emphasis on Al, Au and Cu, the comparison of several theoretical models as to how the extrinsic and intrinsic variables intertwine is made.

Using Atomic Force Microscopy (AFM) we study the effect of nanoindentation induced defects on 50 and 120 nm thick unentangled polystyrene (PS) films, spin cast on silicon (Si) substrates. Indents with residual depths of penetration less than the film thickness level upon heating above the glass transition temperature (Tg) of bulk PS. The resulting leveling process is discussed in terms of a diffusion process driven by the curvature gradient. Calculated diffusivity values are close to the self-diffusivity of bulk PS.

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.

The electrical and mechanical behaviors of carbon black-filled, poly(vinyl acetate) latex-based polymer composites were examined. These composites were found to exhibit percolation thresholds in electrical conductivity near 2 vol% carbon black due to their segregated microstructures. Storage modulus and ultimate tensile strength (UTS) both exhibited discontinuities at 10 vol% carbon black, corresponding to a critical pigment volume concentration. Drying composites at 60°C rather than room temperature produced a higher percolation threshold and better mechanical properties at carbon black loadings above 10 vol% carbon black. A figure of merit was proposed to assess the balance of electrical conductivity, storage modulus and UTS. The figure of merit exhibited a peak value at 10 vol% for composites dried at room temperature and was shifted to higher carbon black concentrations when composites were dried at 60°C.

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.

Nanoindentation of single crystals has been a topic of several recent investigations. This is a result of their ability to withstand near theoretical stresses without showing signs of plastic deformation. When plasticity occurs, it produces a yield point, a sudden discontinuous increase in indenter displacement and decrease in contact pressure. This study combines atomic force microscopy (AFM) with nanoindentation to focus on the roles that oxide and asperities play in the yield point process. Time dependent and instantaneous yield point properties were investigated for single crystals of tungsten and Fe–3 wt% Si in variable temperature and variable humidity environments. AFM observations indicate that the presence of asperities has a dramatic effect on the time dependent yield point properties. AFM measurements also provide evidence for plasticity in the absence of a yield point, suggesting that dislocation nucleation can occur well before a yield point is observed. Measurements on the dependenc...

A series of surfactant-free monodisperse latices were prepared with varying glass transition temperatures (Tg) and particle sizes using methyl methacrylate (MMA) and butyl acrylate (BA) monomers. Latices with Tgs between −11 and 105°C were synthesized by varying the MMA to BA ratio during batch polymerization. The particle size of latices with a Tg of 19°C was varied by independently changing temperature, initiator concentration and ionic strength of the aqueous reaction medium. These latices provide a model matrix starting material for carbon black-filled electrically conductive polymer composites. When latex with a number average particle diameter (Dn) of 930 nm is substituted for a solution-based polymer the percolation threshold of the final conductive composite drops from 14.5 to 1.45 vol% carbon black.

A simplified electrochemical etching technique has been developed to produce conical tungsten tips with radii of curvature ranging between 0.05 and 3 μm. These tips were characterized using scanning electron microscopy. Following characterization, the tungsten tips were mounted into tip holders and used in nanoindentation experiments on bulk aluminum, polystyrene, and polyethylene samples. Calculated modulus values for the polymers, based upon indentation data and known tip radii, agreed very well with modulus obtained using more macroscopic techniques (e.g., stress-strain apparatus). In the case of aluminum, calculated modulus was far lower than expected due to tip deformation during indentation. Calculated hardness values for the polymers tended to rise with indentation depth due to pressure effects that lead to densification and/or phase transformation. These results were also compared to modulus values, on the same three samples, obtained using a diamond tip with a comparable radius of curvature. For ...

We study four processes responsible for the dewetting onset of thin (thickness < 100 nm) polystyrene (PS) films spin cast on nonwettable silicon (Si) surfaces. (i) Spontaneous dewetting is initiated by indentlike disturbances formed at the free PS surface, upon heating above the glass transition temperature (Tg) of PS. Indents become steeper as they grow toward the Si substrate, resulting in an increase of the Laplace pressure which opposes dewetting. (ii) Nanoindentation-induced surface defects either grow toward the substrate, similar to the spontaneously formed indents, or heal, resulting in a flat film surface, depending on their curvatures. Artificial indents with curvatures comparable to the ones measured for the spontaneous indents grow. Healing indents correspond to higher curvatures. (iii) Particles initiate dewetting as they sink inside the film and toward the substrate. Performing our experiments in a controlled clean room environment, we found that ∼23% of the developed dry patches were formed...

We study the effect of nanoindentation-induced indents on polystyrene (PS) films (thickness = h > 100 nm), spin-cast on silicon substrates. Indents with residual depths of penetration, zD, comparable to the film thicknesses leveled (healed) upon heating above the glass transition temperature (Tg) of bulk PS, resulting in a flat polymer surface. Deep indents which clearly penetrated the films and damaged the substrate dewetted from the silicon surface. The healing rate, dzD/dt, was measured for several molecular weight PS films and found to scale with the curvature of the hole bottom, HD, as (dzD/dt) ∼ . Measurements were performed using atomic force microscopy. The healing rate was found to be, in general, higher than the dewetting rate of the laterally expanded deep indents.

Continuous developments into the understanding of hydrogen/deformation interactions have been facilitated by experimental advances and improved computational capabilities. Interactive fracture problems have enjoyed the assistance of fracture mechanics theory and methodology. Regarding hydrogen embrittlement (HE) the current study leans toward a generic hydrogen enhanced decohesion model (HEDE), allowing quantitative engagement with experiments. These include degradation in terms of fracture criteria, subcritical slow crack growth, alternating ductile/brittle transition, threshold values and fine scale fracture surface assessments. In this context the HEDE model remains illuminating, physically based, and experimentally consistent. However, one of the main points of contention, if not confusion, has been the use of thermodynamically based vs. kinetically based arguments to define the threshold and crack growth regimes. We give our view of the general process which we believe to have greater applicability to time-dependent interactive processes. In this context, the description applies to some aspects of stress corrosion cracking and liquid metal embrittlement as well. Following a brief overview regarding HE in the bulk, the paper centers on new avenues of exploring small volume problems. As such this investigation was assisted by nanomechanical testing and high resolution observations. Here, mainly two examples are described in polycrystalline FCC systems. For example, it is shown that hydrogen increases the load onset for dislocation nucleation in a metastable austenitic stainless steel by about a factor of two or more whereupon the yield point recovers to a value near 200 μN after hydrogen outgasses. In a different set of indentation experiments, it is shown that hydrogen decreases both the practical and true works of adhesion at Cu/Ti/SiO2 interfaces by about 50%. Besides evaluating interfacial strengths, the nanomechanical approach should allow additional critical experiments of hydrogen/deformation interactive effects.

We study the effect of nanoindentation induced defects on thin (h = 50 nm) and thick (h = 100 nm) polystyrene (PS) films, spin cast on nonwettable silicon (Si) substrates. Indents with depths of penetration higher than the film thicknesses were imposed. Upon heating above the bulk glass transition temperature (Tg) of PS, some of the indents healed, resulting in a flat polymer surface, and some grew laterally and dewetted the substrate. Our goal is to establish a criterion that can predict the final result (healing or growing) of the indent evolution. We find that the applied force during the nanoindentation and the substrate area exposed to the air after the imposition of an indent can only be used as crude rules, since they do not provide a “cutoff” border, that distinguishes the growing from the healing indents. On the contrary the excess surface energy (ΔFγ) of the system, that is added to the initially flat coating with the formation of an indent, provides a clear critical value, ΔFγ,crit, which separ...

The percolation thresholds of carbon black–polymer composites have been successfully lowered using particulate polymer starting materials (i.e., latex and water-dispersible powder). Composites prepared using carbon black (CB) and commercial poly(vinyl acetate) (PVAc) latex exhibit a percolation threshold near 2.5 vol % CB. This threshold value is significantly lower than that of a comparable reference composite made from poly(N-vinylpyrrolidone) (PNVP) solution and the same CB, which exhibits a sharp rise in electrical conductivity near 15 vol % CB. This dramatic difference in critical CB concentration results from the segregated microstructure induced by the latex during composite film formation. Carbon black particles are forced into conductive pathways at low concentration because of their inability to occupy volume already claimed by the much larger latex particles. There appears to be good qualitative agreement between experimental findings and current models dealing with conductive behavior of composites with segregated microstructures. Lack of quantitative agreement with the models is attributed to the polydispersity of the polymer particles in the latex. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 692–705, 2001

Free-standing polyimide films are manufactured by the chemical imidization of linear, soluble polymeric precursors. The reactive solution is coated onto a heated substrate, peeled off after partial imidization, and then dried and cured as a free-standing film. Adhesive bonds to the cast side of the final film more strongly than to the air side. Near-surface elastic moduli of film samples were measured with a nanoindentation setup. Samples were annealed at different final temperatures. The air side of the samples annealed at 400 °C had a higher modulus of 1.4 GPa than the 0.8 GPa of the casting side. This difference diminished as the annealing temperature was raised to 460 °C. Polyamic acid and polyimide exhibit phase transitions from disordered, isotropic solutions to ordered, liquid-crystalline states. A theoretical model of drying and curing demonstrates formation of a gradient in conversion and ordering: the air side vitrifies at a lower solvent content, lower conversion, and higher ordering; the casting side, at a greater solvent content, higher conversion, and less ordering. Subsequent high-temperature drying and curing of the free-standing films removes solvent, completes reaction, and nematically orders both sides. However, longer times and higher temperature annealing are needed to bring the two sides to their common equilibrium state of nematic order. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1824–1838, 2001

As a solvent-cast polymeric coating dries, each part reaches a concentration at which it solidifies and develops elastic modulus. Thereafter, as further solvent departs, that part shrinks out-of-plane, but not in-plane, if the coating adheres to its substrate. Hence, it develops in-plane elastic stress. If the stress grows large enough, the stress-free state may yield, which reduces the final stress level. A theoretical model of diffusion and mass transfer, large shrinkage-induced deformation, and elastic stress, together with yielding and postyielding viscous deformation, was developed to predict stress evolution in one-dimensional drying of polymer coatings. Concentration varies only perpendicularly to the substrate, the coating shrinks only in that direction, and the stress varies only in that direction but is in-plane isotropic. The predictions are compared with measurements of evolving stress in various solvent-cast polymer coatings and aqueous gelatin coatings by a cantilever-deflection method. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1000–1013, 2001

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 indentation of standard viscoelastic solids, that is, the three-element viscoelastic material, by an axisymmetric, flat-ended indenter has been investigated theoretically. Under the boundary conditions of flat-punch indentation of a viscoelastic half-space, the solutions of the equations of viscoelastic deformation are derived for the standard viscoelastic material. Their generality resides in their inclusion of compressible as well as incompressible solids. They cover the two transient situations: flat-punch creep test and load-relaxation test. In experimental tests of their applicability, nanoindentation and microindentation probes under creep and relaxation conditions yielded a modulus from 0.1 to 1.1 GPa and viscosity from 1 to 37 Gpa · s for a crosslinked glassy polyurethane coatings. For bulk polystyrene, the values vary from 1 to 2 GPa and from 20 to 40 Gpa · s, respectively. The analysis here provides a fundamental basis for probing elastic and viscous properties of coatings with nanoindentation or microindentation tests. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 10–22, 2000

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.