Role Of Surface Stress Research Articles

On the role of surface stress in environment-assisted fracture

Abstract Environment effects in plasticity and fracture of metals, well studied for several decades, still pose many unanswered questions. A micro-mechanics explanation of how dislocation activity is influenced by the material surface state, that can answer these questions, has been elusive. We build on a recently discovered effect in metal cutting – organic monolayer embrittlement (OME) – wherein metal surfaces are rendered brittle by long-chain organic adsorbates, to explore how material state variables influence surface plasticity and fracture. In particular, cutting experiments with Al containing Self Assembled Monolayers (SAMs), show that the OME is controlled by surface stress (f) induced by the adsorbates. This is contrary to many instances of environment-assisted fracture which are usually attributed to surface energy changes, and wherein f is largely ignored. Other contributions include (a) a cantilever technique to measure surface stress, (b) demonstration of strong effect of SAM molecule chain length on f, (c) characterization of how dislocation activity at crack-tips is affected by adsorbate-induced f, and (d) large improvements in machining processes enabled by controlled environment-assisted fracture. We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC.

Surface stress can initiate environment-assisted fracture in metals.

Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.

Energetics of cracks and defects in soft materials: The role of surface stress

In classical fracture theory, the resistance to fracture is governed by the fracture toughness Gc, which is defined as the energy required to create new crack surfaces. To propagate a crack, the energy released to the crack tip (i.e. the energy release rate J) must overcome the fracture toughness Gc. Previous studies have demonstrated that surface stress can significantly modify the crack opening displacement and the stress field in soft solids and hence alter the energy release rate J, assuming small deformation. In this paper, we present a theoretical work on how surface stress affects fracture of soft solids under large deformation. We provide an expression of a modified J-integral for the energy release rate that accounts for surface stress. The expression is verified using large-deformation finite element analysis (FEA) with surface stress elements. Our results show that when the elastocapillary length, defined as the ratio of surface stress σ to the Young’s modulus of the solid E, is comparable to the relevant material size, surface stress noticeably blunts the crack opening profile and reduces the energy release rate.

Sample-size-dependent surface dislocation nucleation in nanoscale crystals

The finite-temperature mechanical strength of nanoscale pristine metals at laboratory strain rates may be controlled by surface dislocation nucleation, which was hypothesized to be only weakly dependent on the sample size. Previous studies on surface dislocation nucleation investigated factors such as surface steps, oxidation layers and surface diffusion, while the role of surface stresses and sample size remains unclear. Here we perform systematic atomistic calculations on the activation free energy barriers of surface dislocation nucleation in sub-50 nm nanowires. The results demonstrate that surface stresses significantly influence the activation processes of surface dislocation nucleation. This renders the strength strongly dependent on sample size; whether it is “smaller is stronger” or “smaller is weaker” depends on the combined effects of surface stress and applied axial stress, which can be universally explained in terms of the local maximum resolved shear stress. A linear relation between the activation entropy and activation enthalpy (Meyer-Neldel compensation rule) was found to work well across a range of stresses and sample sizes.

Role of Surface Stress on the Reactivity of Anatase TiO2(001).

In contrast with theoretical predictions in which anatase TiO2(001) and its (1 × 4) reconstructed surfaces are highly reactive, recent experimental results show this surface to be inert except for the defect sites. In this report, based on a systematic study of anatase TiO2(001)-(1 × 4) surface using first-principles calculations, the tensile stress is shown to play a crucial role on the surface reactivity. The predicted high reactivity based on add-molecule model is due to the large surface tensile stress, which can be easily suppressed by a stress-release mechanism. We show that various surface defects can induce stress release concomitantly with surface passivation. Thus the synthesis of anatase(001) surface with few defects is essential to improve the reactivity, which can be achieved, for example, via H2O adsorption. Our study provides a uniform interpretation of controversial experimental observations and theoretical predictions on anatase TiO2(001) surface and further proposes new insights into the origin of surface reactivity.

How does surface tension affect energy release rate of cracks loaded in Mode I?

The role of surface stress on fracture of elastic solids has been studied by two group of researchers with different conclusions. One group concludes that surface stress has no effect on energy release rate except that the singularity of the crack tip field is stronger than the usual inverse square root singularity dictated by linear elastic fracture mechanics. The other group concludes that the singularity of the stress field reduces to a logarithmic singularity, thus implying that the local energy release rate is zero. In this letter we resolve this paradox by examining the solution of a special case where surface stress is isotropic and independent of surface strain. We show that surface tension resists crack growth by lowering the applied energy release rate while retaining the inversely square root singularity of the elastic crack tip field. We demonstrate this idea by solving a perturbation problem where the capillary length is much smaller than the crack length. A closed form expression for the local energy release rate is obtained in the limit of small surface tension.

Study the Postbuckling of Hexagonal Piezoelectric Nanowires with Surface Effect

Piezoelectric nanobeams having circular, rectangular and hexagonal cross-sections are synthesized and used in various Nano structures; however, piezoelectric nanobeams with hexagonal cross-sections have not been studied in detail. In particular, the physical mechanisms of the surface effect and the role of surface stress, surface elasticity and surface piezoelectricity have not been discussed thoroughly. The present study investigated post-buckling behavior of piezoelectric nanobeams by examining surface effects. The energy method was applied to post-buckling of hexagonal nanobeams and the critical buckling voltage and amplitude are derived analytically from bulk and surface material properties and geometric factors.

MeV Au2+ ions induced surface patterning in silica

This paper reports the formation of self-organised surface morphological features on silica irradiated with MeV energy gold ions. Amorphous silica substrates were irradiated with 1.8MeV gold ions at normal incidence at room temperature to various doses in the range of 5×1016ions/cm2 to 2×1017ions/cm2. The formation of a periodic surface pattern with a wavelength of 1.35μm was observed at an irradiation dose of 1×1017ions/cm2. The observation of surface segregation of gold at around the same dose suggests possible role of surface stresses caused by the incorporation of metal atoms in the formation of the observed periodic surface morphology.

The role of surface stress in structural transitions, epitaxial growth and magnetism on thenanoscale

We review results on combined stress measurements by the crystal curvature technique andstructural investigations on different nanoscale systems. It is shown that stressmeasurements offer highly sensitive and accurate data which identify even subtle structuralchanges in the sub-monolayer coverage regime. We discuss the unique potential of stressmeasurements to complement structural investigations of atomic layers and at surfaces.Our examples reveal that stress measurements enhance, support and clarify theinterpretation of quantitative structural data. The role of surface stress and film stress forstructural transitions in epitaxial growth, surfactant-mediated growth, surfacereconstruction and adsorbate-induced spin reorientation transitions in monolayers isdiscussed.

Self-organization on Au(111) vicinal surfaces and the role of surface stress

Using scanning tunneling microscopy and low energy electron diffraction, we report on the long range spatial self-organization of four Au(111) vicinal surfaces. It is shown that the atomic structure of steps strongly affects the long range ordered morphology: periods on surfaces misoriented towards the [21̄1̄] azimuthal direction are about 100Å, whereas quasi-periods are about 2000Å when surfaces are misoriented towards the opposite azimuth. The stable structures are explained in terms of stress relief induced at the facet edges.

Time-resolved surface differential diffraction study of Ag electrodeposition on Au(111) and the role of surface stress

An insitu X-ray diffraction investigation using surface differential diffraction (SDD) was carried out to study the mechanism and kinetics of Ag electrodeposition on Au(111). The effect of the surface stress on SDD measurements employing a thin film electrode has been investigated, which reveals that stress-induced peak shifts can account for some inconsistency in previous SDD results. Potential dynamic and potential step modes of deposition were used and the adlayer spacing of Ag on Au(111) was determined as a function of potential and time, respectively. A structural change in adlayer spacing during Ag electrodeposition on Au(111) was observed in both modes of deposition, which is consistent with a transition from initial adsorption in a mixture of bridge and atop sites to three-fold hollow sites.

Hard elastic behavior in polyurethane foams

AbstractHard elastic behavior is characterized by high porosity and high recoverability from large strain, and initial Hookean elasticity was discovered in polyurethane foams containing styrene–acrylonitrile (SAN) copolymer particles. The presence of SAN particles introduces a heterogeneous morphology, and when the foam was strained in the SEM, it was observed that the struts became highly porous with profuse voiding nucleated by the SAN particles. It was found that these flexible polyurethane foams had a similar morphological structure in the strained struts as did the typical hard elastic materials. The phenomenon of stress depression, when foam specimens under stress were subjected to nonswelling liquids, was utilized to probe the role of surface stress in these hard elastic foams. An analytic methodology established for other highly porous hard elastic materials based on stress depression was utilized to obtain the average distance between voids in the struts. The calculated values were in good agreement with direct scanning electron microscopy observations, confirming that voiding initiated at the boundaries of SAN particles.

The Role of Surface Stress in the Faceting of Stepped Si(111) Surfaces

ABSTRACTThe nucleation and growth of facets on stepped Si(111) have been observed in real time using the newly developed technique of low-energy electron microscopy (LEEM). The results show that the growth of an isolated facet spontaneously stops at a well-defined size. This is a surprise as classical theories of the growth of linear facets predict a continuous growth, which would only be limited by collisions with neighboring facets. We review predictions that elastic interactions between surface regions of different structure can stabilize finite size domains and facets. We show that a model in which elastic relaxations caused by the facet boundaries stabilize a finite facet width is entirely consistent with the experimental observations.

Surface vibrations on Ni(110): The role of surface stress

Using inelastic electron scattering we have determined the dispersion of surface phonons on the clean Ni(110) surface along the [001] ( Γ ¯ Y ¯ ), the [ 1 1 ¯ 0 ] ( Γ ¯ X ¯ ), and the [ 1 1 ¯ 1 ] ( Γ ¯ S ¯ ) direction. A lattice dynamical calculation which takes surface relaxation into account shows good agreement with the experimental results for the resonance mode near Γ ¯ and for a longitudinal resonance along the [ 1 1 ¯ 0 ] direction when the interlayer surface force constants are scaled in relation to the experimentally determined lattice relaxation. Substantial deviations from the experimental data for the vertically polarized surfaces modes near X ¯ and Y ¯ remain however. They are attributed to the presence of (an anisotropic) surface stress on the Ni(110) surface and good overall agreement with the experimental results is found when a force field taking surface stress into account is used. With the help of the eigenvectors from the lattice dynamical model and using a fully dynamical theory for the inelastic electron scattering we have also calculated the spectra for the Y ¯ and X ¯ point and find good agreement with the experimental results at different impact energies. It is shown that bulk modes can contribute significantly to the spectrum even when one has no particular enhancement of the amplitude near the surface.

Surface vibrations on Ni(110): The role of surface stress

Surface vibrations on Ni(110): The role of surface stress

Surface vibrations on Ni(110); the role of surface stress

Using inelastic electron scattering we have determined the dispersion of surface phonons on the clean Ni (110) surface along the [OO1], [11̄O], and [11̄1] direction, i.e. along T Y , T X , and T S within the surface Brillouin zone (S.B.Z.). The experimental data are compared to lattice dynamical calculations using the slab method (ref. 1).

The role of surface stress in fracture

Because of surface atomic structure changes relative to the bulk, intrinsic mechanical stresses are found in a general free surface, including surfaces of internal cracks. This paper presents a theoretical study of the elastic fracture behavior under the influence of these stresses. The inter relationships between surface stress, free energy and substrate volume stress are derived from the principles of thermodynamics and mechanics. It is shown that 1 r and 1 √r (k-field) singularities can be generated in the near tip region by model stress distributions. The forces on cracks and near by dislocations are calculated. The crack force is found to depend solely on the cleavage work term, γ, provided that the crack moves in a self-similar way. The criteria for dislocation emission from crack tips is found to be altered by the presence of surface stresses, with possible implications for environmental embrittlement. However, because of the lack of data on surface stress parameters, no quantitative predictions are possible for specific materials.

The role of surface stress in the morphology of microbes.

The shapes of many prokaryotes can be understood by the assumption that the cell wall expands in response to tension created by the osmotically derived hydrostatic pressure. Different organisms have different shapes because wall growth takes place in different regions. A previous paper (Koch et al., 1981 a) considered the simplest case of prokaryotic growth, i.e. that of Streptococcus faecium. In the present paper, an elaboration of this theory is applied to two further cases - the more perfectly spherical cocci and the rod-shaped bacteria. These cases are more complex mathematically, because growth over a considerable fraction of the surface must be considered. Such diffuse growth cannot be treated analytically, but can be simulated on a computer or handled by geometric arguments. The spherical form of the cocci may result from either diffuse growth over their entire external surface, or from zonal growth in which the addition of new material only occurs in the immediate vicinity of the splitting septum. In the zonal model, it must be assumed that the least amount of previously laid down septal peptidoglycan consistent with wall growth is reworked in the formation of the new external wall. For Gram-positive rods, where the body of the rod is truly cylindrical, three kinds of growth zones are required: (1) the inward edge of the ingrowing septum, (2) the junction of septum and nascent pole, and (3) the cylindrical walls. Two modes for cylindrical elongation ara possible: (a) new wall is added in one or a few narrow annular zones, or (b) new wall material is added continuously all over the innermost surface and the outer layer is degraded. It is shown that the latter case applies to Bacillus subtilis. Also summarized in this paper are results, developed in more detail elsewhere, concerning the morphology of fusiform bacteria, Gram-negative rods and the hyphal tips of fungi.

Surface Stress on Hydrogen Absorption

Abstract The following is a discussion based on comments made by C. M. Hudgins1 on the paper entitled “The Role of Surface Stress on Hydrogen Absorption by 4340 Steel” co-authored by D. I. Phalen and D. A. Vaughan.2

Discussion: Surface Stress on Hydrogen Absorption by 4340 Steel

Abstract This brief note is a discussion of the paper “The Role of Surface Stress on Hydrogen Absorption by 4340 Steel” by D. I. Phalen and D.A. Vaughan, “Corrosion,” 24, 243–246 (1968) August.