The atomic force microscope (AFM) provides a facilitating tool to investigate the atomic-scale friction properties of surfaces through the sliding of the scanning tip; therefore, the interaction between the tip and the surface should play an important role to determine the frictional behaviors. In this study, density functional theory (DFT) calculations have been carried out to perform the pushing down processes of a tip (10 atom Ir or Au tip) on the top, hollow, and bridge sites of the graphene/Ni(111) substrate. The calculation results indicate that the interactions between the tips and the graphene/Ni(111) substrate influence the adsorption energy remarkably, leading to the sequence of bridge < top < hollow for Ir and Au tips, which is totally different from the adsorption energy of an inert Ar atom, following the sequence of hollow < bridge < top. The strong interactions between the (Ir or Au) tip and the graphene/Ni(111) substrate will introduce novel frictional properties into the system, and an anomalous negative friction coefficient could be obtained. Further investigations show that these interactions arise from the hybridizations between the 2pz orbitals of C atoms and the 5dz2 orbitals of the tip apex atom.

•Hydrostatic pressure increases the corrosion potential.•Hydrostatic pressure induces ionization of H2O.•Hydrostatic pressure inhibits recombination of adsorbed hydrogen atoms to molecules.•Pressure induces hydrogen diffusion from surface to subsurface tetrahedral sites.•Hydrogen coverage at the sample surface increases with hydrostatic pressureArmco iron samples are exposed to pressures of 0.1–30 MPa to study hydrogen permeation during potentiostatic charging. Elecrochemical impedance spectroscopy (EIS) is performed to analyze the electrode/electrolyte interface reaction. According to EIS and first-principles calculations, the hydrostatic pressure decreases the distance between the adsorbed hydrogen atoms and the sample surface, which induces hydrogen atom diffusion into the subsurface by decreasing the diffusion barrier energy. Under high pressure, hydrogen molecules gather at the sample entry side and inhibit the hydrogen recombination reaction. Thus, the amount of adsorbed hydrogen and the hydrogen subsurface coverage increase.Download high-res image (164KB)Download full-size image

•Hydrogen permeation tests were performed under different hydrostatic pressure.•Steady-state current density and diffusivity increase with hydrostatic pressure.•Strong pressure effect for thin, but not thick samples.•Hydrostatic pressure reduces energy barrier for hydrogen absorption and desorption.•Hydrostatic pressure promotes hydrogen adsorption.A514 offshore structural steel was exposed to hydrostatic pressure ranging from 0.1 to 40 MPa to examine hydrogen permeation during galvanostatic hydrogen charging. Hydrostatic pressure decreases the energy barrier for hydrogen absorption and desorption, while increasing hydrogen adsorption. These effects are induced by both steady-state current density and the apparent diffusivity slight increase with hydrostatic pressure. Taking surface effect of hydrogen permeation into account, the intrinsic diffusivity of (3.2 ± 0.04) × 10−6 cm2/s is obtained, which is independent of the hydrostatic pressure.

Solid films are considered as typical model systems to study size effects on thermal vacancy concentration in nanomaterials. By combining the generalized Young–Laplace equation with the chemical potential of vacancies, a strict size-dependent thermodynamic model of vacancies, which includes the surface intrinsic elastic parameters of the eigenstress, Young's modulus and the geometric size of the solid films, was established. The vacancy concentration changes in the film with respect to the bulk value, depending on the geometric size and surface stress sign of the solid films. Atomistic simulations of Au and Pt films verified the developed thermodynamic model. These results provide physical insights into the size-dependent thermal vacancy concentration in nanomaterials.

Size-dependent thermodynamic model is presented to simultaneously analyze stress and vacancy concentration, along with their coupling behavior in spherical metal nanoparticles. A nanoparticle is treated as a composite with a core lattice and a surface shell, which both have their corresponding thermodynamic properties. Thermodynamic analysis of Ni, Cu, Au, and Pt particles shows that the vacancy concentration in the core and the surface shell could be significantly decreased for smaller particles. The apparent vacancy concentration in the whole nanoparticle reaches the maximum at the critical particle size of about 100 nm and is lower for larger or smaller particles. This result provides comprehensive understanding of the vacancy concentration in nanomaterials, which had been represented by two opposite views in the past decades.

First-principles calculations were conducted on armchair graphene nanoribbons (AGNRs) to simulate the elastic behavior of AGNRs with hydrogen-terminated and bare edges. The results show width-dependent elastic properties with a periodicity of three, which depends on the nature of edge. The edge eigenstress and eigendisplacement models are able to predict the width-dependent nominal Young’s modulus and Poisson’s ratio, while the Clar structure explains the crucial role of edges in the periodically modulated size-dependent elastic properties.

•The hydrogen atom prefers tetrahedral interstice site under the strain −3% to 6%.•The diffusion coefficient increases with the increasing compressive strain.•This phenomenon is explained by quantum electronic stress and zero point energy.•The quantum effect is dominant for the hydrogen atom diffusion in the strained Fe.In this paper, the diffusion of the hydrogen atom in α-Fe under an applied 3-axis strain was investigated by the thermodynamics analysis and density functional theory calculations. The quantum manifestations of the hydrogen diffusion were presented, and were well explained by putting the quantum electronic stress and the zero point energy of the hydrogen atom into the thermodynamics model. It indicates that the quantum effects are dominant for the hydrogen atom diffusion in the strained α-Fe. The diffusion barrier energy increases and the diffusion coefficient decreases with the decreasing compressive strain or increasing tensile strain. The charge density difference was also calculated to reveal the physical insight of the effect of strain on the hydrogen diffusion in α-Fe.

The surface elasticity and non-local elasticity effects on the elastic behavior of statically bent nanowires are investigated in the present investigation. Explicit solutions are presented to evaluate the surface stress and non-local elasticity effects with various boundary conditions. Compared with the classical Euler beam, a nanowire with surface stress and/or non-local elasticity can be either stiffer or less stiff, depending on the boundary conditions. The concept of surface non-local elasticity was proposed and its physical interpretation discussed to explain the combined effect of surface elasticity and non-local elasticity. The effect of the nanowire size on its elastic bending behavior was investigated. The results obtained herein are helpful to characterize mechanical properties of nanowires and aid nanowire-based devices design.

Finite element analyses including a cohesive zone model (CZM) were conducted to investigate the role of corrosion product films (CPFs) in stress corrosion cracking (SCC) for copper in an ammoniacal solution. It is found that a tensile CPF-induced stress generates near the interface between the CPF and the copper substrate at the substrate side in front of the notch tip for a U-shaped edge-notched specimens. The CPF-induced stress is superimposed on the applied stress to enhance emission and motion of dislocations. The peak opening stress (S11) increases with an increase in CPF thickness and a decrease in CPF Young’s modulus. Damage mechanics based on the CZM was applied to study the stress corrosion crack initiation and propagation by analyzing the stress redistributions and load–displacement curves. The results show that the crack initiates first in the CPF and then propagates to the copper substrate. The fracture strain of the specimen covered a CPF is lower than that without a CPF. Based on the simulation results, the mechanism of the CPF-induced SCC, which promoted the initiation and propagation of the stress corrosion cracks, was discussed.

Experimental and theoretical studies of water-molecule adsorption on BaTiO3 single-crystal surfaces are presented in this paper. The Fourier transform infrared spectrum shows that there are three types of energy-nonequivalent active modes for water-molecule adsorption on the in-plane-polarized BaTiO3(100) surface. The X-ray photoelectron spectroscopic results illustrate hydroxyl group on the surface, thereby indicating that the adsorbed water molecules are dissociated. The first-principles calculations of the 1/4-, 1/2-, and 1-monolayer water coverage demonstrate that H bonds are formed between the hydrogen of water and the surface oxygen of BaTiO3 and between the hydrogen of hydroxyl and the surface oxygen of BaTiO3, and the difference in the water adsorption behavior on the BaO- and TiO2-terminated surfaces. The calculation results are in good agreement with the experimental observations.

This paper theoretically proposed a series of formulae for a novel tensile testing method to extract the residual stress in the surface film on metallic substrates. Finite element calculations were conducted to verify the accuracy and reliability of the proposed formulae. From the tensile stress–strain curves of metallic substrates with and without a surface film, one can evaluate the residual stress in the film using these formulae. A tarnish film on a brass substrate formed after immersion in Mattsson’s solution was tested to demonstrate these methods, and the obtained residual stress showed a low difference below 6.8%.Highlights► A method to obtain the residual stress in film on metallic substrate is proposed. ► The theoretic analysis formulae of the testing method are proposed. ► Finite element method was conducted to verify the accuracy and reliability. ► Experiment of tarnish film on brass was tested to demonstrate the method.

The stress corrosion cracking (SCC) behaviour of H62 brass with various levels of pre-strains in Mattsson’s solution was investigated using electrochemical and SCC tests. The results revealed that the corrosion current densities increased and the work function decreased when the pre-strains increased, which facilitate anodic dissolution, thereby resulting in an increase of the film growth rate and the film-induced stress, ultimately resulting in the enhancement of film rupture and SCC susceptibility. Based on the obtained results, a film-rupture-induced SCC mechanism of brass in Mattsson’s solutions was proposed.Highlights► Potentiodynamic polarisation revealed the pre-strain enhanced anodic dissolution. ► SCC susceptibility of H62 brass increased with increasing film-induced stress. ► Film rupture frequency increased with increasing pre-strain, which promoted SCC. ► Film-rupture-induced SCC mechanism of brass in Mattsson’s solutions was proposed.

The size dependence of the tensile properties and necking effect of pure copper exposed to various heat treatments are investigated in this paper using round bar cross-section specimens through experimental and 3D finite element methods (FEM). The results show that the total and post-necking elongation increased dramatically as the specimen diameter increased. The size effects on necking and stress distribution of different diameter specimens are analysed. These results show that the post-necking deformation length is only dependent on the diameter of the specimens and is independent of the specimen’s gauge length, which results in the size dependence of the post-necking elongation. Moreover, FEM simulations show that the size dependence of the tensile properties can be eliminated when the ratio of the gauge length to the diameter is larger than 10.

Specimens with a rectangular cross-section are commonly used to measure the tensile properties of materials. However, the specimen size may influence the results. In this study, the tensile properties of FH550 and X80 steels were investigated using rectangular cross-section specimens with different thicknesses and the same width. Both an experimental study and a 3D finite element method (FEM) study have been conducted. The results show that the uniform elongation is independent of specimen size, but the post-necking elongation increases dramatically as specimen thickness increases, which was attributed to the stress distribution near neck region. Single factor analysis results show that both the yield and the ultimate tensile strength of the specimens are independent of the thickness when the specimen thickness is larger than 1 mm.Highlights► We proposed a FEM failure criterion to simulate the elongation of plate samples. ► The FEM simulation result showed good agreement with experiment result. ► We found that the elongation increases as the thickness of the specimen increase. ► The size effect was due to the stress distribution at neck region of different size.

Theoretical formula to determine flexible membrane deformation of microbridge test was derived based on the assumption that the membrane deformation is dominated by stretching, opposite to bending. Microbridge samples were fabricated from 0.17 μm thick free-standing graphene oxide membrane placed on patterned silicon substrates and tested with a nanoindentation system. The effective Young’s modulus was determined from the microbridge test during both loading and unloading. Combining the flexible membrane theoretical formula with deformable tension-shear model, a multiscale model was proposed to determine the shear modulus of graphene oxide membrane. The effect of temperature on Young’s modulus in different loading sequence was investigated. Polarized Raman spectroscopy and X-ray diffraction were utilized to characterize the self-stiffening behavior of graphene oxide membranes.

•Edge effects on mechanical properties of armchair MoS2 nanoribbons were investigated.•Structure changes of different width armchair MoS2 nanoribbons were obtained.•Tensile/compressive tests were conducted to determine elastic constants.•Mechanical properties are compared for two and three dimensional conditions.Edge effects on mechanical properties of armchair molybdenum disulfide nanoribbons were investigated using first principles calculations. The edge eigen-stress model was applied to explain the relaxation process of forming molybdenum disulfide nanoribbon. Edge effects on surface atoms fluctuation degree were obtained from each fully relaxed nanoribbon with different width. Changes of the relaxed armchair molybdenum disulfide nanoribbons structure can be expressed using hexagonal perimeters pattern. Based on the thickness change, relaxed armchair molybdenum disulfide nanoribbons tensile/compression tests were simulated, providing intrinsic edge elastic parameters, such as eigen-stress, Young's modulus and Poisson's ratio.

Solid films are considered as typical model systems to study size effects on thermal vacancy concentration in nanomaterials. By combining the generalized Young–Laplace equation with the chemical potential of vacancies, a strict size-dependent thermodynamic model of vacancies, which includes the surface intrinsic elastic parameters of the eigenstress, Young's modulus and the geometric size of the solid films, was established. The vacancy concentration changes in the film with respect to the bulk value, depending on the geometric size and surface stress sign of the solid films. Atomistic simulations of Au and Pt films verified the developed thermodynamic model. These results provide physical insights into the size-dependent thermal vacancy concentration in nanomaterials.