Surface Tensile Stress Research Articles

Chain Length Effect on Surface Stress of Alkanethiolates Adsorbed onto AU(111) Surface: a van der Waals Density Functional Study

ABSTRACTFirst-principles calculations were employed to investigate the adsorption-induced surface stress of self-assembled alkanethiolate monolayers on a Au(111) surface as a function of the alkyl chain length. A recently developed fully nonlocal van der Waals density functional was used to accurately account for the chain-chain interactions. We found that surface charge redistribution produces compressive surface stress, while chain-chain interactions produce tensile surface stress. The stress induced by surface charge redistribution is about one order of magnitude greater than that of chain-chain interactions. We observed that the chain-chain interactions play an important role in determining the molecular configuration during adsorptions, and also contribute significantly to the induced anisotropic tensile (positive) surface stress. As the chain length increases the tensile stress increases at a rate of ∼ 0.32 (∼ 0.18) N/m for the direction perpendicular (parallel) to the chain tilt direction.

Structural integrity of direct metal laser sintered parts subjected to thermal and finishing treatments

Direct Metal Laser Sintering (DMLS) is a rapid manufacturing technology that is starting to be used for the manufacturing of functional components. Therefore, properties of DMLS manufactured parts must be carefully analyzed to determine if they satisfy technical requirements under fatigue conditions.In this work, the structural integrity of Maraging Steel, Inconel 718 and CoCr alloy DMLS parts subjected to different thermal and finishing (shot peening and surface polishing) treatments has been characterized in order to determine the optimum stabilization treatment with regard to structural integrity. For this characterization, surface residual stresses, microstructure, porosity and hardness of the parts have been studied.In general, DMLS process origins detrimental surface tensile stresses in Inconel 718 and CoCr alloy components whereas in Maraging Steel parts DMLS results in light surface compressive stresses that improve in general with the application of subsequent thermal and mechanical treatments. Moreover, surface polishing homogenizes the final stresses resulting, besides, in very compressive surface residual stresses.

Diffusion-induced stresses of electrode nanomaterials in lithium-ion battery: The effects of surface stress

Nanomaterials offer large reaction surfaces making for high-rate lithium-ion transfer and fewer constraints to avoid fracture. Nevertheless, surface effect arises inevitably due to so high surface-to-volume ratio. Accordingly, the fundamental framework of surface stress is involved to study diffusion-induced stresses within electrode nanoparticles in this work. As simple one-dimension models, solid and hollow nanowire electrode particles are investigated. The results show that surface tensile stress produces compressive stresses through the electrode materials, especially reducing maximum tensile stress, which may become a resistance to brittle fracture. Owing to high special surface area, it is demonstrated that diffusion-induced stresses for hollow materials are largely reduced compared to solid electrode materials. The influences of surface modulus on diffusion-induced stresses are much stronger under generalized plane strain condition in comparison with plane strain condition. Analysis based on the Tresca criterion indicates that shear failure may occur at the inner surface with decreasing radius.

Analytical model of dislocation nucleation on a near-surface void under tensile surface stress

We have analyzed in detail the mechanism leading to tip growth on a surface which operates via nucleation of dislocations on a near-surface void under tensile surface stress. We derived a simplified analytical model describing the relevant physical factors related to the observed linearity between the void radius and the maximum depth of the void for the growth to occur. The model is based on the direct numerical calculation of atomic level stresses in the simulated system. Based on the present model we can estimate this maximum depth for a void of a certain size under a given stress in the size range which is beyond the feasibility of the molecular dynamics simulation method.

The Corrosion and Oxidation Resistance Improvement of DZ4 Ni-Based Superalloy Irradiated by High Intensity Pulsed Ion Beams

In this paper, the DZ4 Ni-based superalloy was irradiated by high intensity pulsed C ion and H ion beam. The parameters of ion beam were: accelerating voltage 250 kV, pulse duration 70 ns, ion current density 160 A/cm2. Irradiation pulses were 2, 5, 10 and 15. The morphology of DZ4 before and after irradiation was observed by scanning electron microscope (SEM). The phase structure and stress state were analyzed by X-ray diffraction (XRD). The results show that there were some craters on the surface of the sample after irradiation, as the irradiation time increased, the crater boundary became blurred and even disappeared. The surface tensile stress produced by initial irradiation could be relieved after several irradiations pulses. The corrosion resistance of irradiated DZ4 alloy was improved greatly. The continuous and dense α-Al2O3 layer that formed on irradiated DZ4 surface at 1000°C atmosphere increased the high temperature oxidation resistance.

Fatigue response of Udimet 720 following minimum damage wire electrical discharge machining

The paper presents experimental data for the fatigue behaviour of Udimet 720 nickel based superalloy following wire electrical discharge machining using minimum damage generator technology. Comparative data for flank milled samples are also given together with associated micrographs detailing workpiece crack initiation and subsurface integrity. Despite a marginally higher S–N response for the milled specimens compared to the wire machined samples subject to a finishing regime, statistical analysis suggested no significant difference in performance at the 5% level. In contrast, wire cut samples fatigue tested after only roughing, showed a markedly lower response commensurate with the greater surface roughness, the presence of surface microcracks and tensile residual stresses approaching 600MPa.

Effect of cutting parameters in the surface residual stresses generated by turning in AISI 4340 steel

Machining processes induce a residual stress state in the machined part that, depending on its magnitude and sign, can be very detrimental to the service life of the machined components. This way, tensile residual stresses favour crack nucleation and propagation, leading to a reduction of fatigue life and corrosion and wear resistance. The final residual stress state in a piece depends on the material of the component and on the cutting parameters employed: cutting speed, cutting feed, depth of cut, kind of cutting tool (geometry, coating of the tool, etc), wear of the tool, lubrication, etc. Nevertheless, there is some lack of agreement in the literature regarding the specific tendency of residual stresses with each cutting parameter (feed and cutting speed) and there is not much literature about the effect of tool geometry and tool characteristics (coating). The present paper aims to put some light in this lack of agreement and complement the studies found in the literature.In this work, it has been studied the effect on the final surface stress state in AISI 4340 steel of cutting speed, feed, tool nose radius, geometry of the tool chip breaker and coating of the cutting tool. For this study surface residual stresses have been measured, by means of X-ray diffraction, in AISI 4340 steel bars subjected to turning tests using different cutting speeds (between 200 and 300m/min), different cutting feeds (between 0.075 and 0.200mm/rev), and cutting tools with two nose radius (0.4 and 0.8mm), two different surface states (one coated by CVD and the other without coating) and two different geometries of the chip breaker. In all cases surface tensile residual stresses have been measured, tending to be more or less tensile (and consequently more or less detrimental to the service life of the machined component) depending on the cutting conditions and the characteristics of the cutting tool.In this work, not only the magnitude but also the orientation of the principal residual stresses has been determined. This is not commonly encountered in the literature, in spite of its significance: the direction of maximum residual stress is a critical direction because if it coincides with the direction of the stresses acting on the component as a result of the nominal load (load suffered by the component during its normal service), the service life of the part will be significantly affected, leading to possible premature failure of the component. That is the reason why knowing the orientation and not only the magnitude of principal residual stresses is of huge importance.

Relief of surface stress at steps during displacive adsorption of As on Si(111)

The displacive adsorption of As on 1 × 1 Si(111) is examined using surface electron microscopy. High temperature adsorption experiments are used to reduce the As chemical potential and identify regions of enhanced As incorporation in the vicinity of steps. This is explained by a reduction in tensile surface stress associated with 1 × 1 As:Si(111). Cycling the incident As flux on and off creates surface roughening via respective displacive adsorption and evaporation of As. This roughness facilitates increased 1 × 1 As:Si(111) coverage during As deposition due to surface stress relief at steps.

Optimization of DNA Hybridization Efficiency by pH-Driven Nanomechanical Bending

The accessibility and binding affinity of DNA are two key parameters affecting the hybridization efficiency in surface-based biosensor technologies. Better accessibility will result in a higher hybridization efficiency. Often, mixed ssDNA and mercaptohexanol monolayers are used to increase the hybridization efficiency and accessibility of surface-bound oligonucleotides to complementary target DNA. Here, no mercaptohexanol monolayer was used. We demonstrate by differential microcantilever deflection measurements at different pH that the hybridization efficiency peaks between pH 7.5 and 8.5. At low pH 4.5, hydration and electrostatic forces led to tensile surface stress, implying the reduced accessibility of the bound ssDNA probe for hybridization. In contrast, at high pH 8.5, the steric interaction between neighboring ssDNA strands was decreased by higher electrostatic repulsive forces, bending the microcantilever away from the gold surface to provide more space for the target DNA. Cantilever deflection scales with pH-dependent surface hybridization efficiency because of high target DNA accessibility. Hence, by changing the pH, the hybridization efficiency is adjusted.

Origin of enhanced Ge interdiffusion at the initial stage of Ge deposition on Si(5 5 12)-2 × 1: Tensile stress induced by substrate chain structures

By combined investigation of STM and synchrotron PES on Ge/Si(5 5 12)-2×1 at 530°C, it has been found that, in addition to the upward-relaxed surface Si atoms, a subsurface Si atom is also readily replaced by an arriving Ge atom at the initial adsorption stage. Such enhanced interdiffusion is due to a unique character of one-dimensional chain structures of the reconstructed substrate, such as π-bonded and honeycomb chains not existing on other low-index Si surfaces such as Si(001)-c(4×2) and Si(111)-7×7, applying a tensile surface stress to the neighbouring subsurface atoms. Interdiffusion of Ge having lower surface energy induces adsorption of the displaced Si atoms on the surface to form sawtooth-like facets composed of (113)/(335) and (113)/(112) with arriving Ge atoms until the surface is filled with those facets. Such displacive adsorption is the origin of high Si concentration of formed facets.

Influence of zirconia framework thickness on residual stress profile in veneering ceramic: Measurement by hole-drilling

ObjectivesFramework design is reported to influence chipping in zirconia-based restorations, which is an important cause of failure of such restorations. Residual stress profile in the veneering ceramic after the manufacturing process is an important predictive factor of the mechanical behavior of the material. The objective of this study is to investigate the influence of framework thickness on the stress profile measured in zirconia-based structures. MethodsThe stress profile was measured with the hole-drilling method in bilayered disc samples of 20mm diameter with a 1.5mm thick veneering ceramic layer. Six different framework thicknesses from 0.5mm to 3mm were studied. Two different cooling procedures were also investigated. ResultsCompressive stresses were observed in the surface, and tensile stresses in the depth of most of the samples. The slow cooling procedure was found to promote the development of interior tensile stresses, except for the sample with a 3mm thick framework. With the tempering procedure, samples with a 1.5mm thick framework exhibited the most favorable stress profile, while thicker and thinner frameworks exhibited respectively in surface or interior tensile stresses. Significance: The measurements performed highlight the importance of framework thickness, which determine the nature of stresses and can explain clinical failures encountered, especially with thin frameworks. The adequate ratio between veneering ceramic and zirconia is hard to define, restricting the range of indications of zirconia-based restorations until a better understanding of such a delicate veneering process is achieved.

Multi-axial fatigue initiation at inclusions and subsequent crack growth in a bainitic high strength roller bearing steel at uniaxial experiments

The behaviour of inclusion initiated fatigue was studied for a high strength bearing steel with a bainite micro-structure. The analysis included experiments and numerical simulations. It was realized that the stress-state was multi-axial in the matrix material that met the inclusion also for a uniaxial far field stress. Fatigue initiation risk at the interface between the inclusion and matrix material was therefore predicted with the Findley multi-axial critical plane criterion. The fatigue parameters were determined from independent experiments on smooth specimens with tensile surface stress gradients. Crack growth from the inclusion to final rupture was modelled as a penny shaped crack with closure compensated effective material parameters. The growth simulations suggested that the majority of the fatigue life was consumed as fatigue crack initiation at the non-metallic inclusion.

Nanomechanical Thermal Analysis of Photosensitive Polymers

A few nanograms of poly(methyl methacrylate) (PMMA) or poly(vinyl cinnamate) (PVCN) was coated onto one side of a silicon cantilever, and the photodegradation of PMMA or photo-cross-linking of PVCN was investigated as a function of UV irradiation time. After UV irradiation, the resonance frequency and deflection of the polymer-coated silicon cantilevers were measured as a function of temperature, and these properties were found to be related to changes in the modulus and surface stress of the coated polymers, respectively. A decrease in the modulus and tensile surface stress was observed for PMMA under UV exposure due to photodegradation, whereas an increase in the modulus of PVCN was observed due to photo-cross-linking. In addition, the influence of UV exposure on the glass transition of the PMMA and PVCN was investigated using deflection measurements. Whereas a single glass transition was observed for PMMA, two distinctive glass transitions were observed for PVCN due to the copresence of cross-linked an...

Stress and structure at the NiO/Ag(001) interface

We present a combined stress and structural analysis of the growth of epitaxial NiO monolayers on Ag(001). Our experimental results indicate an unexpectedly complex interface formation, where a fraction of the first NiO monolayer (ML) is embedded into the Ag surface. This interface formation induces a tensile surface stress change. Subsequent deposition leads to a layer-by-layer growth of NiO up to 5 ML. Here, the average film stress is compressive $\ensuremath{-}5.8$ GPa, and it corresponds quantitatively to the misfit-induced stress. Our density functional calculations complement the experimental results by identifying the proper O-Ag bonding geometry in the ML regime from two indistinguishable options as provided by the analysis of surface x-ray diffraction data.

Microstructure Characterization of White Layer Formed by Hard Turning and Wire Electric Discharge Machining in High Carbon Steel (AISI 52100)

White layers, formed during hard turning and wire electric discharge machining, were characterized by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Different cutting speeds and flank wear were utilized in order to obtain different thermally and/or plastically deformed white layer. Since the white layer after wire electric discharge machining is mainly thermally induced, it was used as a reference structure. In the investigation, both bainitic and martensitic structures were studied. With a constant flank wear of 0.175 mm the thickness of the white layer increased from 1.5 μm to 3 μm as the cutting speed was increased. In both processes the white layer were characterized by nanosized grains and surface tensile residual stresses. M3C carbides were observed in the hard turned white layer, indicating that the time and temperature needed for completely dissolving the carbides were not reached during cutting. For both materials the white layers formed by wire electric discharge machining consisted of ~ 30 vol. % of retained austenite. Observation regarding the volume fraction of the retained austenite in the white layer formed by hard turning for martensitic material showed an increase in the volume fraction of retained austenite from ~ 2 - 3 vol. % to ~ 6 vol. %, while this observation was not seen in the white layer formed in the bainitic material.

The stress relief mechanism in laser irradiating on porous films

Stress relief effect is believed to be the main reason why the porous structure films have high laser induced damage threshold. But there is little systematic theoretical investigation of this view. This essay adopts spherical coordinate model to calculate the temperature and stress distribution in the film and analyzes the local stress redistribution after phase transformation or crush happens. The influences of porosity and pore radius on stress redistribution have been studied. The results show that: for the case of only local crush occurrence, the changes of porosity and pore radius have little influence on the stress relief effect. For the films with low porosity, the formed thermal stress cannot be alleviated effectively. When local phase transformation happens, the permeation action of high pressure fluid can reduce the local high pressure in the phase transformation zone; on the other hand, it can make the high pressure fluid permeate to film's surface and cause surface tensile stress increase. The final effect is the competitive result of these two mechanisms. Overall, when the permeation action of phase transformation materials happens, the higher porosity or larger pore radius, the higher capability of films to resist laser damage.

Surface stress changes in the Ir(001)/H system: Density functional theory study

Density functional theory calculations of the surface energy and surface stress changes associated with the formation of the Ir(001)(5 \ifmmode\times\else\texttimes\fi{} 1)-hex clean-surface reconstruction and its transformation to a (5 \ifmmode\times\else\texttimes\fi{} 1)-added row (AR) structure in the presence of adsorbed atomic hydrogen have been performed and compared with experimental results. The calculations clearly show that the clean-surface (1 \ifmmode\times\else\texttimes\fi{} 1)-to-(5 \ifmmode\times\else\texttimes\fi{} 1)-hex reconstruction is not driven by a reduction in tensile surface stress. While the surface energy is reduced by this transformation, the surface stress increases; proper convergence of the calculated surface stress requires the use of slabs containing at least nine atomic layers, due to the substantial subsurface relaxations associated with the (5 \ifmmode\times\else\texttimes\fi{} 1)-hex phase. By contrast, the H-induced reconstruction leads to a calculated reduction in the tensile surface stress in the range 1.76--2.06 Nm${}^{\ensuremath{-}1}$ for an H coverage range of 0.6--0.8 ML, in excellent agreement with the experimentally determined value of 1.7 Nm${}^{\ensuremath{-}1}$.

Moisture-induced stresses perpendicular to grain in cross-sections of timber members exposed to different climates

In variable humidity conditions, wood absorbs or desorbs moisture from the air. Unless the change in humidity is very slow, this will develop moisture gradients in the wood sections. These gradients will develop stresses due to constrained swelling or shrinkage strains. These stresses are named moisture-induced stresses. The present paper investigates the main parameters affecting such moisture-induced stresses, including the type of climate, the size of the timber cross-section, and the type of protective coating. A first attempt to identify moisture-induced stresses in different European climatic regions was made. For each climatic region, relative humidity and temperature histories were identified, and characteristic and mean values of yearly and daily variations were calculated. Using a finite element model implemented in Abaqus, the moisture content and stress distribution were computed on different timber cross-sections exposed to the climatic regions and protected with different types of coating. A Fickian moisture transfer model was used to compute the moisture distribution, and a mechanical model for time-dependent behaviour of wood was implemented to calculate the corresponding stress distribution. The variation of moisture was found to result in stresses of magnitudes that would probably cause cracking of wood in the perimeter of any uncoated cross-section size. The use of a protective coating, however, reduced considerably the moisture-induced stresses, and can be regarded as an effective protective measure to avoid cracking due to humidity variations. Considering European climates, Northern climates were found to result in higher surface tensile stresses than Southern climates.

Test Research on the Surface Residual Stress of Dimensional Ultrasonic Vibration Grinding for Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2(n)</sub>

Based on the research for the structure of the ceramic nanocomposites’ intragranular for Al2O3/ZrO2(n)，we did the test by the workpiece two-dimensional vibration grinding(WTDUVG), and focus on analyzing the characteristic and the effect element of the two-dimensional ultrasonic vibration grinding ceramic surface residual stress by the XRD diffraction. The result show that ceramic dimensional ultrasonic vibration grinding surface tensile stress is less than the same conventional grinding (CG) surface under tensile stress; two-dimensional ultrasonic vibration grinding surface residual compressive stress than conventional ground surface residual stress under the same grinding. Material removal mechanism of the grinding nature of the surface residual stress, when the material removaled by ductile deformation, grinding surface equal residual stress; when the material removaled by brittle- ductile mixed mode, the grinding surface tensile stress reduced, because the fracture of the ground surface, tensile stress released. As a results, the grit size of grinding wheel, Grinding depth and workpiece mechanical properties are the main technology factors affected the nature and size of the residual stress of ground surface.

In-situ surface stress and magnetic property of CoCrX( X = Pt, Ta)/CrTi bilayer during sputter-deposition

The measurement of the force per width of CoCrX(X = Pt, Ta)/CrTi bilayer showed that both the surface structure state and surface stress of CoCrX magnetic layer can be controlled by regulating the average film stress of CrTi underlayer through a variation of deposition conditions such as deposition rate and temperature as well as underlayer thickness. As the tensile surface stress of CoCrX magnetic layer was more, its coercivity was also more. This was because not a dynamic phase decomposition with a compressive ledge due to high adatoms mobility but an epitaxial film growth with a tensile ledge due to low adatoms mobility. Thus, the development of the crystallographic texture of CoCrX alloy can largely improved its coercivity. It is believed that the principal mechanism determining the coercivity of CoCrX/CrTi film is the optimized development of the crystallographic texture through an evolution of the surface stress of CrTi underlayer.