Comparison of growth stress measurements with modelling in thin iron oxide films

High temperature oxidation of metals leads to residual stresses both in the metal and in the growing oxide. In this work, the evolution of this residual stresses is theoretically predicted in the growing oxide layers. The origin of these stresses is based on a microstructural model. Using experimental results providing from the oxidation kinetics, and an analysis proposed to describe the growth strain occurring in the thin layers, a set of equations is established allowing determining the stresses evolution with oxidation time. Then, the model is compared with experimental results obtained on both α-Fe and phosphated α-Fe, oxidised at different temperatures. Numerical data are extracted from experiments either with an asymptotic formulation or with an inverse method. These two methods give good agreement with experiments and allow extracting the model parameters.     

One-dimensional dynamics of nano-scale oxidation

A continuum model is proposed to describe the process of scanned probe oxidation in the presence of a thin water layer on the surface of a substrate. The model describes the electric field and ion transport in both the liquid and the oxide layers and incorporates the reaction mechanism at the substrate/oxide interface. Further, the influence of the space charge due to ions trapped near the substrate/oxide interface is taken into account.Separation of time scales for the chemical reactions and ion transport as well as the asymptotic limit in terms of a small aspect ratio of the oxide layer are used to reduce the complex system of partial differential equations to a one-dimensional system of ordinary differential equations. The analytical solution of the reduced system results in the evolution equation for the oxide thickness. Numerical simulations of the evolution equation predict features of oxide growth that qualitatively agree with the experimental observations. A parametric study is conducted to determine the influence of the key operating and material parameters on the thickness of the oxide, the electric field, and ion concentration in the system.     

Effects of the electric field shape on nano-scale oxidation

An axisymmetric continuum model for oxide growth by the scanned probe oxidation technique is presented. The model includes equations describing the electric fields, hydroxyl and hydrogen ion concentrations, and oxide free boundaries defining the system. The governing system of partial differential equations tracks ion transport in the liquid and oxide layers and includes reactions at the substrate/oxide interface. Further, space charge effects are considered near the substrate/oxide interface. Two liquid configurations, semi-infinite layer and hemispherical drop of liquid, are examined to determine the potential in the liquid region. The AFM tip is modeled as either a point or a ring source of charges. The asymptotic limit of a small aspect ratio (height to width) oxide feature is used to reduce the governing equations to a quasi-one-dimensional system to determine the ion transport. The solution of the reduced system leads to an evolution equation for the oxide thickness and radius. Numerical simulations of the evolution equation predict oxide features that qualitatively agree with experimental observations. A parametric study is conducted to determine the influence of key operating and material parameters on the thickness and radius of the oxide, and the electric and ion concentration fields in the system.     

Stress influence on high temperature oxide scale growth: Modeling and investigation on a thermal barrier coating system

In thermal barrier coating (TBC) systems, an oxide layer develops at high temperature below the ceramic coating, leading in the long term to the mechanical failure of the structure upon cooling. The mechanism of stress-affected oxidation likely to induce the growth of a non-uniform oxide scale detrimental to the TBC lifetime was investigated. A continuum thermodynamics formulation is derived accounting for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. It specially includes the contributions of the large volumetric strain and the mass consumption associated with metal oxidation. A continuum mechanics/mass diffusion framework is used along with the developed formulation for the interface evolution to study the growth of an oxide layer coupled with local stress development. The implementation of the model has required the development of a specific simulation tool, based on a finite element method completed with an external routine for the phase boundary propagation. Results on an electron-beam physical vapor deposited (EB-PVD) TBC case are presented. The processes resulting in a non-uniform oxide scale growth are analyzed and the main influences are discussed.     

Enhanced fatigue performance of porous coated Ti6Al4V biomedical alloy

Biofunctional coatings are necessary to improve integration of titanium implants in the host tissue but they may be detrimental for the implant fatigue properties. This study presents an attempt towards enhancement of the in vitro fatigue strength of plasma electrolytic oxidation coated Ti6Al4V alloy by applying shot peening process prior to coating. The electrolytic oxidation was performed in calcium acetate and calcium glycerophosphate electrolytes that allowed formation of porous oxide coatings with high surface free energy and apatite like ability. A deformed surface layer coupled with induced residual compressive stresses seem to affect oxide growth rate and fatigue behavior of the titanium alloy.     

Stress in dielectric contact layers on metals

Several sources of stress in dielectric contact layers (e.g. growing oxide films) are discussed and order-of-magnitude estimates are given for the accompanying strains. The strain distributions in isotropic layers are deduced for stresses produced by Coulomb forces, volume changes associated with diffusing defects, orienting molecular force (epitaxial) effects, and electrochemical potential gradients producing diffusion currents and growth of the layer.     

Stress analysis of local blisters coupling Raman spectroscopy and X-ray diffraction. Correlation between experimental results and continuous damage modelling for buckling in an iron oxide/phosphated iron system

In this present work, local stress development in the iron oxide layers growing on phosphated α-Fe at 400 °C in ambient air is investigated by Raman spectroscopy. Coupled with X-ray diffraction it enables to obtain directly local stresses’ maps in the oxide layers. Use of Raman spectroscopy allows obtaining better accuracy on mechanical behaviour at local scale. This characterisation technique is very useful to study systems developing mechanical heterogeneities on surface, especially in case of buckling phenomenon. Investigations on particular local blisters have been done to measure some characteristic lengths at local scale. From local measurements, we are able to evaluate general effect of buckling from simplified scale transition. So, a macroscopic approach has been performed to calculate global stress evolution of the oxide layer, based on continuous damage mechanics. Consequently, it leads to good comparison between modelling and experimental values (global stresses versus oxidation time) in α-Fe₂O₃ oxide.     

Crystal plasticity modeling of irradiation growth in Zircaloy-2

Abstract

A physically based reaction-diffusion model is implemented in the visco-plastic self-consistent (VPSC) crystal plasticity framework to simulate irradiation growth in hcp Zr and its alloys. The reaction-diffusion model accounts for the defects produced by the cascade of displaced atoms, their diffusion to lattice sinks and the contribution to crystallographic strain at the level of single crystals. The VPSC framework accounts for intergranular interactions and irradiation creep, and calculates the strain in the polycrystalline ensemble. A novel scheme is proposed to model the simultaneous evolution of both, number density and radius, of irradiation-induced dislocation loops directly from experimental data of dislocation density evolution during irradiation. This framework is used to predict the irradiation growth behaviour of cold-worked Zircaloy-2 and trends compared to available experimental data. The role of internal stresses in inducing irradiation creep is discussed. Effects of grain size, texture and external stress on the coupled irradiation growth and creep behaviour are also studied and compared with available experimental data.     

Interface Separation in Residually-Stressed Thin-Film Structures

Plasticity is a significant contributor to the interfacial fracture resistance of multilayer thin-film structures containing ductile layers. Salient parameters affecting plasticity contributions to interfacial fracture energy including the ductile layer thickness, yield strength, and the maximum cohesive stress governing interface separation, have been reported. However, the effects of residual stresses in the thin-film layers on such plasticity contributions have not been considered. We explore the effect of residual stresses on debonding with particular attention to the relationship between the stress state in both ductile and elastic layers and the resulting macroscopic debond energy. Using multiscale simulations it is shown that residual thin-film stresses can alter plasticity in the ductile layer and significantly influence the macroscopic fracture energy. A simple yield-based model to account for this behavior is proposed.     

Relation between a linear thermal expansion coefficient and residual stresses

A method of determining the residual stresses in metallic sheets is proposed. It consists in finding the strain field induced by central point heating and calculating the field of residual stresses using a functional relation between a linear thermal expansion coefficient and residual stresses.     

Effect of Residual Stresses on Delamination Cracks in Fiber Reinforced Composites

In the processing of cross-ply fiber reinforced materials, residual stresses, as well as possible transverse cracking may arise. These affect the stress field about a delamination between two layers. In this investigation, the effect of residual stresses resulting from curing and transverse cracks is examined. A 0°/90°/0° ply system is considered with a delamination assumed between one of the 0° and 90° layers. The residual stresses along the interface without the delamination are calculated. First, this analysis is done neglecting the transverse cracks in the 90° layer. Then, the transverse cracks are included and several methods are employed to calculate the residual stresses. These include the shear lag method, a semi-analytic method and the finite element method. It is seen that the latter two methods produce similar results. By means of the superposition principle, the stress intensity factors resulting from the residual stresses are obtained for the delamination. Use is made of the conservative M-integral with tractions along the crack faces.     

Biaxial stresses, surface roughness and microstructure in evaporated TiO2 films with different deposition geometries

The residual stresses, surface roughness and microstructure in titanium oxide films prepared by electron-beam evaporation and deposited with different geometries were investigated, with particular focus on the in-plane anisotropy of the biaxial stresses and microstructures. Thin films were deposited with various deposition angles on B270 glass substrates and silicon wafers. Two different types of deposition geometries were studied. The residual stress in the thin films was examined by a phase-shifting Twyman-Green interferometer. The optical constants, biaxial stress and surface roughness were found to be related to the evolution of the anisotropic microstructures in the films. The results revealed that the anisotropic stresses that developed in the evaporated titanium oxide films were dependent upon the deposition geometry and microstructure of the films.     

二氧化硅生长新的动力学模型

本文中提出了一个同时适用于描述极薄及厚二氧化硅层生长规律的新的氧化模型。该模型假定在氧化过程中,二氧化硅层内的总净电荷具有指数分布。本文考虑了此氧化层电荷的影响,并推得了硅热氧化的新关系式。此新关系式不论是对非常薄或厚氧化层都与实验结果符合得很好。对于厚氧化层此式则过渡到熟知的Deal-Grove关系。利用本模型还可满意地解释外加电场对氧化速率的影响。 关键词：     

Violet luminescence in Ge nanocrystals/Ge oxide structures formed by dry oxidation of polycrystalline SiGe

Nanocrystals of Ge surrounded by a germanium oxide matrix have been formed by dry thermal oxidation of polycrystalline SiGe layers. Violet (3.16 eV) luminescence emission is observed when Ge nanocrystals, formed by the oxidation of the Ge segregated during the oxidation of the SiGe layer, are present, and vanishes when all the Ge has been oxidized forming GeO₂. Based on the evolution of the luminescence intensity and the structure of the oxidized layer with the oxidation time, the recombination of excitons inside the nanocrystals and the presence of defects in the bulk oxide matrix are ruled out as sources of the luminescence. The luminescence is attributed to recombination in defects at the Ge sub-oxide interface between the Ge nanocrystals and the surrounding oxide matrix, which is GeO₂.     

Comparison between grating rosette and strain gage rosette in hole-drilling combined systems

Hole-drilling method is a most common technique for measuring residual stresses in various materials and structures, in which the strain gage rosette is usually used to obtain the strain distributions caused by deformation. To avoid disadvantages in the strain gage system, different strain measure methods have been proposed in the previous references. In this paper, an optical technique called as grating rosette and Moiré interferometry is presented and compared with the strain gage one. It is found that higher measure accuracy can be achieved as the hole drilling is combined with the new optical technique instead of the strain gage rosette.     

Surface modification by oil jet peening in Al alloys, AA6063-T6 and AA6061-T4: Residual stress and hardness

The life of structural members that experience cyclic loading is improved by the introduction of surface compressive residual stresses. A high-pressure oil jet is used for the introduction of surface compressive residual stresses in aluminum alloys, AA6063-T6 and AA6061-T4. The peening machine designed and developed in the laboratory is capable of generating high pressures using hydraulic oil. The magnitude of residual stress developed depends upon the stand-off distance and yield strength of the material. A hardened layer up to a depth of about 350 μm was developed in the materials investigated. The residual stresses and surface hardening induced are comparable to that produced by other peening processes. An analytical model is proposed to predict the impact pressure.     

Change of surface properties of high-chromium cast iron under conditions of laser treatment and wear in a pellet stream

We investigate the influence of laser treatment on the formation of residual stresses relative to the changing structure-phase composition in the surface layers of high-chromium cast iron with 16% chromium. We show that appreciable tensile stresses are produced in the region of the laser action and that their distribution depends on whether the laser treatment was or was not accompanied by surface melting. The produced residual stresses are responsible for the formation of a large number of cracks. Preheating to 400°C lowers the level of the tensile residual stresses and prevents crack formation. A pellet stream acting on the surface produces cold-work hardening layers in which the tensile stresses change into compressive ones. The depth, hardness, and magnitude of the compressive residual stresses depend on the method used to work harden the cast iron and on the angle of attack of the pellet as it acts on the surface.Translation of Preprint No. 195, Lebedev Institute of Physics, Academy of Sciences of the USSR.     

Residual stresses deriving from holographic interferometry data on a base of inverse problem solution

A transition model, which is capable of obtaining both membrane and bending residual stress components from initial experimental information, is developed for thin-walled plane structures. The determination of residual stresses is based on the combined implementing of the hole-drilling method and reflection hologram interferometry. Required input data are obtained by simultaneous measurements on through hole distortions in two principal strain directions on opposite sides of thin plane specimen. These sides are faces of the drill entrance and exit. Superimposed residual stresses field, which consists of both membrane and bending components, is a reason for the various deviations of each specific fringe pattern from an ideal form. This fact is a clear experimental indication of the bending stress contribution in a total stress field. Two ways of decomposition of superimposed residual stresses field are proposed and analysed in detail. Emphasis is laid on a careful quantitative formulation of the inverse problem needed for an accurate deriving both membrane and bending residual stress components. It is shown that an availability of two-side initial data is both an essential and necessary condition of such a formulation. Detailed analysis of an accuracy of the results obtained is performed. This analysis is based on a wide set of both actual interferograms and analogous reference fringe patterns related to superimposed residual stress field under study. Comparing residual stress values obtained proceeding from one-side and two-side data are presented for different types of superimposed field of interest.     

连续梯度的功能层对燃料电池在初始还原过程中曲率及残余应力的影响

连续梯度的电极由于其相对于多层梯度电极能更加有效地缓解电极和电解质的热失配及改善界面黏接而受到特别的关注. 本文通过建立含连续梯度的阳极功能层的阳极支撑固体氧化物燃料电池的力学模型, 研究了连续梯度的阳极功能层对阳极支撑固体氧化物燃料电池半电池在初始还原过程中曲率及残余应力的影响. 结果表明电池的曲率在初始还原过程中随还原程度的增大而逐渐增大. 连续梯度的阳极功能层的引入不能同时改善电池的曲率和残余应力, 即连续梯度的阳极功能层在缓解应力的同时会导致曲率的增大, 反之亦然. 含有连续梯度的阳极功能层的电池在部分还原状态下, 梯度层/阳极支撑界面处具有最大的拉应力容易导致电池受损, 实际中应保证电池被完全还原.     

Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.