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The porous, cellulosic fiber networks in commercial paper are generally regarded as continuum materials when the sheet is large compared to the size of fiber agglomerates (flocs). In this manuscript we reveal that even when the average macroscopic tensile response of copy paper specimens is well described by orthotropic anisotropy, the local deformation characteristics inside a specimen and among different specimens are not uniform as predicted by continuum theories. Using a non-contact, high resolution full-field strain measurements, we present specimen orientation specific strain maps and deformation fields. The characteristic length scales of strain hot spots are obtained using thresholding and lineal path correlation and are compared to the characteristic size of flocs. We quantify the spatial distribution and extent of nonaffine (non-continuum) deformations during tensile loading of a commercial paper, and show how they are critical features of deformation and tearing mechanisms in paper.

The presence of debris, surface roughness, or other asperities on crack faces creates a geometry that is reminiscent of a lever (the crack face) on a fulcrum (the debris or surface asperity). This ‘cantilever effect’ is intuitive and is routinely forwarded as a mechanism for crack advance when compressive loads are applied. Recently this cantilever effect has also been linked to fatigue of micron-scale silicon films. Finite-element modelling was used in this study to evaluate different wedging configurations that would be likely to occur within micromachined silicon thin films. The results of the model clearly show that wedges or other asperities in the wake of a crack do not increase the magnitude of the stress intensity factor during compression.

Micromachined resonant fatigue characterization structures have been used by a variety of investigators to evaluate the stress-life fatigue behaviour of thin films. This work will review the design, testing and analysis of these versatile thin-film characterization structures. Subsequent discussion will illustrate how this material characterization approach has been used to evaluate the high-cycle fatigue behaviour of silicon films commonly used in microelectromechanical systems (MEMS). This work demonstrates that properly designed resonators can be used to monitor extraordinarily low fatigue crack growth rates (i.e. ≪ 10−10 m/cycle) relevant to these minute mechanical components.

A study has been made of high-cycle fatigue in 2um thick structural films of n+- type, polycrystalline silicon for MEMS applications.

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (/spl sim/20 /spl mu/m thick) single crystal silicon specimens were tested to failure in a controlled air environment (30/spl plusmn/0.1/spl deg/C, 50/spl plusmn/2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1/spl times/106 to 1/spl times/1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation.

The surfaces of worn components hold clues to the underlying wear mechanisms. Previous evidence suggested that the absolute wear rates of acetabular components in a hip simulator were related to mechanical behavior; we hypothesized that the surface morphology of the liners might also be sensitive to mechanical properties. A noncontact, three-dimensional surface topography measurement system based on white light interferometry was used to quantify the surface morphology of ultra-high molecular weight polyethylene, polytetrafluoroethylene, high-density polyethylene, and polyacetal liners, and their corresponding femoral heads, after 3 million cycles in a multi-directional hip simulator. Comparisons were made with the fatigue soaked and control (as machined) components. Statistically significant power law relationships were observed between the arithmetic mean surface roughness (Ra) of the worn acetabular liners and the volumetric wear rate in the hip simulator (p < 0.01, r2 = 0.52). Significant relationships were also observed between Ra and the elastic and large deformation mechanical behavior of the liner materials, measured directly from the wear-tested liners using the small punch test (p < 0.01, r2 = 0.54–0.81). The results support the hypothesis that wear mechanisms of acetabular liners during hip simulator testing are related to surface morphology in conjunction with the mechanical behavior of the polymeric materials. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 52, 447–459, 2000.

Previous reports suggest that Raman peaks in uniaxially loaded nanowires with diamond cubic and zinc blende crystal structures shift at rates that are significantly different from bulk specimens. We have investigated the first order Raman scattering from individual, free-standing, [111] oriented GaP nanowires ranging from 75 to 180 nm in diameter at uniaxial tensile stresses up to 5 GPa. All of the phonon modes were shifted to frequencies lower than previously reported for bulk GaP, and significant splitting of the degenerate transverse optical mode was observed. A general analysis method using single and double Lorentzian fits of the Raman peaks is presented and used to report more accurate values of the phonon deformation potentials (PDPs) that relate uniaxial strains to Raman peak shifts in GaP. A new set of PDPs determined from the nanowires revealed that the they have elastic moduli and failure strains that are consistent with bulk GaP. The analysis method eliminated the anomalous, inconsistent deformation behavior commonly reported in Raman-based strain measurements of nanowires, and can be extended to other materials systems with degenerate phonons.

The performance of structural films unde r cyclic loading conditions is a cri tical consideration when designing microelectromechanical systems (MEMS) based on silicon stru ctural films. Empirical and th eoretical studies have shown that silicon films are susceptible to fatigue at room temperat ure, but the underlying mechanistic origin is still an active topic of debate. This study characterized the fa tigue behavior of electrostatically-actuated, n + -type, 2 P m thick polycrystalline silicon films with a thin native oxide. Elect rostatically actuated resonators (natural frequency, f 0 ~ 40 kHz) were used to evaluate the stress-life fatigue behavior of the films in 30°C, 50% relative humidity (R.H.) air. These tests revealed delayed failure with increasing fatigue lives (up to 10 11 cycles) for decreasing stress amplitudes (down to 2.5 GPa). Long fatigue lives were as sociated with larger decreases in f 0 and very smooth failure origins that encompassed several grains. These findings are consis tent with cyclic degradation of silicon films occurring with in a surface reaction layer that forms upon exposure to the service environment and that evolves during fatigue loading. Keywords: silicon, fatigue, MEMS, reaction-layer fatigue

Recent research has established that for silicon structural films used in microelectromechanical systems (MEMS), the susceptibility to premature failure under cyclic fatigue loading originates from a degradation process that is confined to the surface oxide. In ambient air environments, a sequential, stress-assisted oxidation and stress-corrosion cracking process can occur within the native oxide on polycrystalline silicon (referred to as reaction-layer fatigue); for the structural films of micron-scale dimensions, such incipient cracking in the oxide can lead to catastrophic failure of the entire silicon component. Since the degradation process is intimately linked to the thin reaction layer on the silicon, modification of this surface and the access of the environment to it can dramatically alter the fatigue resistance of the material. The purpose of this paper is to evaluate the efficacy of modifying the fatigue behavior of polycrystalline silicon with alkene-based monolayers. Specifically, 2-μm thick polysilicon fatigue structures were coated with a monolayer film based on 1-octadecene and cyclically tested to failure in laboratory air. By applying the coating, the formation of the native oxide was prevented. Compared to the fatigue behavior of untreated polysilicon, the lives of the coated samples ranged from 105 to >1010 cycles at stress amplitudes greater than ∼90% of the ultimate strength of the film. The dramatic improvement in fatigue resistance was attributed to the monolayer inhibiting the formation of the native oxide and stress corrosion of the surface. It is concluded that the surprising susceptibility of thin structural silicon films to premature fatigue failure can be inhibited by such monolayer coatings.

2-μm thick structural films of polycrystalline silicon are shown to display “metal-like” stress-life fatigue behavior in room air, with failures occurring after > 1011 cycles at stresses as low as half the fracture strength. Using in situ measurements of the specimen compliance and transmission electron microscopy to characterize such damage, the mechanism of thin-film silicon fatigue is deduced to be sequential oxidation and moisture-assisted cracking in the native SiO2 layer. This mechanism can also occur in bulk silicon but it is only relevant in thin films where the critical crack size for catastrophic failure can be exceeded within the oxide layer. The fatigue susceptibility of thin-film silicon is shown to be suppressed by alkene-based self-assembled monolayer coatings that prevent the formation of the native oxide.

The surfaces of worn components hold clues to the underlying wear mechanisms. Previous evidence suggested that the absolute wear rates of acetabular components in a hip simulator were related to mechanical behavior; we hypothesized that the surface morphology of the liners might also be sensitive to mechanical properties. A noncontact, three-dimensional surface topography measurement system based on white light interferometry was used to quantify the surface morphology of ultra-high molecular weight polyethylene, polytetrafluoroethylene, high-density polyethylene, and polyacetal liners, and their corresponding femoral heads, after 3 million cycles in a multi-directional hip simulator. Comparisons were made with the fatigue soaked and control (as machined) components. Statistically significant power law relationships were observed between the arithmetic mean surface roughness (R(a)) of the worn acetabular liners and the volumetric wear rate in the hip simulator (p0.01, r(2) = 0.52). Significant relationships were also observed between R(a) and the elastic and large deformation mechanical behavior of the liner materials, measured directly from the wear-tested liners using the small punch test (p0.01, r(2) = 0.54-0.81). The results support the hypothesis that wear mechanisms of acetabular liners during hip simulator testing are related to surface morphology in conjunction with the mechanical behavior of the polymeric materials.

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, cyclic fatigue can only take place where there is some degree of toughening, implying that premature fatigue failure would not be expected in polycrystalline silicon where such toughening is absent. However, the fatigue failure of polysilicon is reported in the present work, based on tests on thirteen thin-film (2 μm thick) specimens cycled to failure in laboratory air (∼25°C, 30-50% relative humidity), where damage accumulation and failure of the notched cantilever beams were monitored electrically during the test. Specimen lives ranged from about 10 seconds to 34 days (5 × 105 to 1 × 1011 cycles) with the stress amplitude at failure being reduced to ∼50% of the low-cycle strength for lives in excess of 109 cycles.

A non-contact surface topography measurement system based on white light interferometry was used to quantify the surface morphology of the UHMWPE, PTFE, HDPE, and polyacetal liners before and after wear testing. We investigated the hypothesis that the wear rate of four polymeric materials in the hip simulator was related to quantitative metrics of the surface morphology (i.e., surface roughness). Nine common roughness parameters (PV, Ra, Rrms, Rsk, Rku, Rtm, Rz, R3z, and H) were evaluated in 28 acetabular liners. The surface morphology within a given material was reproducible from insert to insert. Statistical relationships were observed between the surface roughness of the acetabular liners and the volumetric wear rate (p < 1 × 10−6). However, the power law relationships accounted for less than 50% of the variability in the data, based on r2.

The experimentally measured elastic moduli and yield strengths of nanowires and nanofilaments vary widely in the literature and are often beyond the theoretical limits of the particular material. In this work, Si nanowires with very low defect densities were loaded in uniaxial tension to establish the origins of their apparently nonlinear constitutive behavior. The diameters of the nanowires ranged from 230 to 460 nm and the growth directions were primarily [112] with the exception of a [111] oriented nanowire. The resulting fracture strengths of the nanowires ranged from 3.88 to 10.1 GPa. The nonlinear constitutive behavior was accompanied by fracture surfaces with features that were not commonly observed in Si. A nonlinear continuum elasticity model and electron microscopy established that reports of unusual deformation behavior and fracture surface morphologies are a direct byproduct of the electron and ion beam deposited adhesives (Pt-based in this work) used to affix specimens in place for testing.

Ordered intermetallic films have a favorable combination of properties such as high strength, metallic electrical conductivity, good oxidation and corrosion resistance, and a high melting temperature and thermal stability that make them suitable for microelectromechanical systems (MEMS). One potential drawback to intermetallics is a lack of ductility at room temperature; however, the B2 compounds NiAl and RuAl show some ductility at room temperature, which has been shown to increase as the grain size decreases. Additionally, the fracture toughness of both materials is higher than those of Si and SiGe. It is also possible to deposit these materials at temperatures that make them compatible with complementary metal oxide semiconductor processing. The authors have shown that by controlling the Ar pressure during cosputtering, NiAl and RuAl thin films can be deposited near room temperature with stresses ranging from compressive to tensile, possibly eliminating the need for annealing. This article examines Ni–...

Previous studies of the mechanical properties of Si nanowires have not shown the size-dependent strengths that are expected for this prototypical brittle material. A potential source of the ambiguity in the literature is the development of tensile stresses during the large (nonlinear) deflections that were present during the flexure tests. In this work we show that size-dependent strengths can be observed in Si nanowires when they are evaluated using uniaxial tension loading conditions. Si nanowires with diameters ranging from 268 to 840 nm were fabricated using the vapor-liquid-solid method and were strained to failure in vacuum using a micromachined load frame. The smallest nanowires were the strongest but the magnitude of the size effect suggests that the flaw populations in Si nanowires are orientation-dependent.

The accuracy of instrumented indentation data relies heavily on the evaluation of experimental errors such as displacement drift. In spite of its importance, little attention has been given to the magnitude of an acceptable displacement drift rate, its relationship to a given set of test conditions, and how errors manifest themselves in force-displacement data. In this work we explored how drift rates that were acceptable for short-term tests caused artificial “abnormal” behavior that could have been interpreted as a true material response for a longer-term test. A critical review of the drift behavior of the nanoindentation system revealed that a useful metric for screening data quality was the nominal accumulated system drift as a fraction of the maximum penetration depth. Additionally, suggestions for drift management during nanoindentation tests were given.