Generation Of Compressive Stresses Research Articles

Correlated Evolution of the Structural and the Optical Properties During the Growth of Cl-Doped Polycrystalline CdTe

A study of the structural and the optical properties has been carried out before, upon, and after the coalescence of Cl-doped CdTe islands. Coalescence is found to induce high tensile stresses with the formation of grain boundaries. These stresses are eventually relaxed through the generation of compressive stresses related to the segregation of Cl atoms in the vicinity of grain boundaries. These segregation phenomena also influence the compensation mechanisms: a preferential formation of β acceptor complexes instead of A centers is directly observed by cathodoluminescence measurements. Furthermore, the coalescence and the post-coalescence stresses contribute to an orientation-dependent driving force for the grain growth, which leads to a progressive texturation of the material.

Mechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe–Pd magnetic shape memory alloy films on MgO

Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning.

Microstructure and residual stresses in surface layer of simultaneously laser alloyed and burnished steel

The new hybrid method, which combines the laser alloying process with slide burnishing, was investigated and presented in the work. Both treatments were performed on the laser stage in one operation. The experiments were done on carbon steel alloyed with cobalt stellite. The alloying process was conducted with continuous laser CO 2, at different parameters. The single- and multiple-path processes were investigated. The microstructure, microhardness and residual stresses of surface layer after the laser alloying process and laser alloying combined with burnishing were presented. The results of the experimental studies have shown that the proposed new hybrid method allowed for the generation of compressive stresses in surface layer of the alloyed material. The structural analysis has proven that the burnishing process had caused deformation of grains in the 20–30 μm thick zone and increased microhardness of the surface zone material. The X–ray diffraction measurements of residual stresses in surface layer of the samples subjected to the alloying process and burnishing, both in cold and hot conditions, were performed. In the case of multiple-path laser alloying treatment the tensile stresses, approximately 500 MPa, were obtained at the surface. Multiple alloying combined with burnishing generated compressive stresses of about −600 MPa at the surface, substantially improving the surface layer.

Structure and stressed state of molybdenum layers in Mo/Si multilayers

Structure and stressed state of molybdenum layers in Mo/Si multilayer periodical compositions prepared by direct current magnetron sputtering have been investigated by methods of X-ray tensometry in grazing-incidence asymmetrical geometry, cross-sectional transmission electron microscopy and small-angle X-ray reflectometry. The level of symmetrical biaxial stresses depends non-monotonously on the thickness of Mo layers. Value and sign of stresses are defined by the following processes: bombardment of the growing surface of metallic layers by particles with enhanced energy, recrystallization of Mo grains and the formation of silicon-substitutional solid solution in molybdenum. The bombardment is accompanied by a formation of self-interstitial Mo atoms that sink at internal dislocation defects, an increase in quantity of lattice points in the film plane and a generation of biaxial compressive stresses. The recrystallization reduces the density of grain boundaries, at the same time decreases the average specific volume of condensed atoms and generates tensile stresses in Mo layers. Compared to those for pure bulk molybdenum, notably lower average lattice spacing was measured in unstressed sections for Mo layer thicknesses less than 20 nm. This indicates the formation of a solid solution of Si in molybdenum presumably in vicinity of Mo-on-Si interface.

Mechanisms of stress generation during bombardment of Ge with keV ions: experiments and molecular dynamics simulations

The present contribution focuses on the phenomenology and mechanisms of stress generation in Ge thin films during keV ion bombardment. Experimentally, amorphous Ge (a-Ge) thin films were grown from vapor, and subsequently bombarded with Ar+ ions with energies of up to 3 keV. Stress generation is monitored by a laser beam deflection method. In order to identify the underlying nanoscale physics, molecular dynamics simulations were performed, in which crystalline and (a-Ge) films of different densities were irradiated. Experiments and simulations both show generation of compressive stresses, which saturate at ≈200 MPa and can be attributed to generation of voids with sizes of approximately 1 nm several nanometres below the surface.

Thin Film Compressive Stresses due to Adatom Insertion into Grain Boundaries

Atomic simulations of the growth of polycrystalline Ni demonstrate that deposited atoms incorporate into the film at boundaries, resulting in compressive stress generation. Incorporated atoms can also leave the boundaries and thus relieve compressive stress. This leads to a complex interplay between growth stress, adatom incorporation, and surface structure. A simple, theoretical model that accounts for grain size effects is proposed and is in good agreement with simulation results.

Competition between tensile and compressive stress creation during constrained thin film island coalescence

Various analytical models have been proposed to predict the tensile stress created when discrete islands contact during a Volmer-Weber thin film growth. Past efforts to experimentally validate these models have been hindered by the stochastic nucleation of islands, which results in coalescence over a large distribution of times and length scales. To avoid this we systematically varied island geometries using electrodeposition of Ni islands on lithographically patterned conductive substrates (Au film on Si), which allowed for independent control of island size and growth rate. Using this technique, we previously demonstrated that most of the coalescence stress occurred after the initial contact of the neighboring islands, reaching a steady state when the film surface became nearly planar. In this work, we expand on these initial results to examine the kinetics of the coalescence process and to systematically evaluate the stress transition from discrete islands to a planar film. The steady state stress in planar films increased with growth rate, but asymptotically approached a limiting value for higher growth rates that depended on the island size. We attribute this to the competition between the kinetically limited compressive stress generation and tensile coalescence stress processes. The interaction of these mechanisms is consistent with both the observed transient stress evolution during the initial stages of island coalescence and the steady state stress evolution later in the process. The instantaneous stress at both the initial contact and at longer times decreased with increasing island size, as predicted in the literature. However, the existing models predict significantly larger grain size effects than those observed in these experiments.

Effect of changing rate of residual stress on thermal expansion behavior of magnesium borate whisker-reinforced aluminum composite

Magnesium borate whisker-reinforced aluminum matrix composite is a new aluminum matrix composite with high properties and low cost. In the study, the residual stress changing behaviors on heating and cooling of the composite were investigated through the coefficient of thermal expansion (CTE) curves. On heating, the tensile stress relaxation and compressive stress generation in the matrix of the composite results in the CTE of composite is smaller than that predicted by mixture rule, but the compressive stress relaxation in the matrix of composite causes the CTE of composite is larger than that predicted by mixture rule. The tensile and compressive stress changing rates with temperature are linear on heating in the matrix of the composite at lower and higher temperature ranges. The CTE of the composite on cooling is smaller than that on heating.

Stem-righting Mechanism in Gymnosperm Trees Deduced from Limitations in Compression Wood Development

In response to inclination stimuli, gymnosperm trees undergo corrective growth during which compression wood develops on the lower side of the inclined stem. High compressive growth stress is generated in the compression wood region and is an important factor in righting the stem. The aims of the study were to elucidate how the generation of compressive growth stress in the compression wood region is involved in the righting response and thus to determine a righting mechanism for tree saplings. Cryptomeria japonica saplings were grown at inclinations of 0 degrees (vertical) to 50 degrees. At each inclination angle, the growth stress on the lower side of the inclined stem was investigated, together with the degree of compression-wood development such as the width of the current growth layer and lignin content, and the upward bending moment. Growth stress, the degree of compression wood development, and the upward moment grew as the stem inclination angle increased from 0 to 30 degrees, but did not rise further at inclinations > 30 degrees. The results suggest the following righting mechanism for gymnosperm saplings. As the stem inclination is elevated from 0 to 30 degrees, the degree of compression wood development increases to force the sapling back to its original orientation; at inclinations > 30 degrees, the maximum degree of compression wood is formed and additional time is needed for the stem to reorient itself.

Focused ion beam induced deflections of freestanding thin films

Prominent deflections are shown to occur in freestanding silicon nitride thin membranes when exposed to a 50 keV gallium focused ion beam for ion doses between 10(14) and 10(17) ions/cm(2). Atomic force microscope topographs were used to quantify elevations on the irradiated side and corresponding depressions of comparable magnitude on the back side, thus indicating that what at first appeared to be protrusions are actually the result of membrane deflections. The shape in high-stress silicon nitride is remarkably flattopped and differs from that in low-stress silicon nitride. Ion beam induced biaxial compressive stress generation, which is a known deformation mechanism for other amorphous materials at higher ion energies, is hypothesized to be the origin of the deflection. A continuum mechanical model based on this assumption convincingly reproduces the profiles for both low-stress and high-stress membranes and provides a family of unusual shapes that can be created by deflection of freestanding thin films under beam irradiation.

Stress-Induced Tin Whisker Initiation Under Contact Loading

As electronic components become smaller, tin whiskers have an increasingly adverse effect on the assessment of the reliability of the components. In particular, the contact between components brings about whisker initiation near the contact area. Although a number of spontaneous tin whisker models have been proposed, a model of tin whisker initiation by contact loading has not yet been established. In this paper, the behavior of stress-induced tin whiskering by contact load was examined in a tin plating based on multiaxial creep theory. Creep properties were measured using a nanoindentation creep technique. The creep rate of Sn-10Pb is higher than that of Sn and affects the generation of compressive stresses after stress relaxation. Nonlinear finite-element analysis (FEA) reveals that the multiaxial compressive stress state appears after stress relaxation. The axial compressive stress increases even though the contact stress decreases. When multiaxial stresses are applied, not only axial stress but also the difference between stresses, affects the behavior of whisker formation. The axial compressive stress depends on the structures of the contact bodies, and the contact stress depends on the creep properties of the plating

Computational Evaluation of the Effects of Bone Ingrowth on Bone Resorptive Remodeling after a Cementless Total Hip Arthroplasty

In this study, we simulated a wide cortex separation from a cementless hip prosthesis using the bone resorption remodeling method that is based on the generation of high compressive stress around the distal cortical bone. Thereafter, we estimated the effect on late migration quantities of the hip prosthesis produced by the interface state arising from bone ingrowth. This was accomplished using cortical bone remodeling over a long period of time. Two-dimensional natural hip and implanted hip FEM models were constructed with each of the following interface statements between the bone and prosthesis: (1) non-fixation, (2) proximal 1/3, (3) proximal 2/3 and (4) full-fixation. The fixation interfaces in the fully and partially porous coated regions were rigidly fixed by bony ingrowth. The non-fixation model was constructed as a critical situation, with the fibrous or bony tissue not integrated at all into the implant surface. The daily load history was generated using the three loading cases of a one-legged stance as well as abduction and adduction motions. With the natural hip and one-legged stance, the peak compressive principal stresses were found to be under the criteria value for causing bone resorption, while no implant movement occurred. The migration magnitude of the stem of the proximal 1/3 fixation model with adduction motion was much higher, reaching 6%, 11%and 21%greater than those of the non-fixation, proximal 2/3 fixation and all-fixation models, respectively. The full-fixation model showed the lowest compressive principal stress and implant movement. Thus, we concluded that the late loosening and subsequent movement of the stem in the long term could be estimated with the cortical bone remodeling method based on a high compressive stress at the bone-implant interface. The change caused at the bone-prosthesis interface by bony or fibrous tissue ingrowth constituted the major factor in determining the extent of cortical bone resorption occurring with clinical loosening and subsequent implant movement.

Depth Profiling of Mechanical Properties on the Nanoscale of Single-Layer and Stepwise Graded DLC Films by Nanoindentation and AFM

AbstractThe strong ion bombardment, applied during sputter deposition of diamond-like carbon films (DLC), which is needed to promote the growth of the sp3-bonded hard phase, inevitably is accompanied by compressive stress generation, thus limiting their maximum thickness. Gradient coatings with gradients in composition, constitution or properties are a well-known concept to manage such stress problems. The stepwise graded layer concept adjusts a graded constitution of the growing carbon film by a stepwise increase of the ion energy, i.e. the substrate bias voltage, during magnetron sputtering. To study the influence of the layer thickness on the expansion of the interface regions between the layers deposited with different bias voltage, samples with increasing deposition time of the top layer and thus thickness ratio were investigated by using the small angle cross-section nanoindentation method (SACS). It was revealed that the thickness of the interface regions is linearly dependent on the thickness ratio of the graded layers, which might be an evidence for stress-induced diffusion and relaxation processes in the carbon network. By using microindentation with a Berkovich indenter and ex-situ AFM-imaging it was found that all graded films exhibited higher Berkovich thresholds for crack development and thus better crack resistance than the hardest single-layer film and kept a high hardness value of about 4000 HV0.005.

Mechanisms inducing compressive stress during electrodeposition of Ni

The evolution of stress during electrodeposition of Ni films on Au substrates has been investigated as a function of bath chemistry and deposition conditions to examine the microstructural origins of the compressive stress observed during deposition from an additive-free sulfamate bath. Three likely mechanisms for the generation of compressive stress in this system were investigated: interstitial hydrogen/impurity incorporation, capillarity stress, and a chemical-potential gradient driven atom incorporation model. Only the last model, the chemical-potential gradient model, could not be discounted as the active mechanism. However, further study is required to verify that this is the primary compressive stress generation mechanism.

The effect of saccharin addition on the mechanical properties and fracture behavior of electroless Ni–Cu–P deposit on Al

In a previous study, the effects of additive saccharin on internal stress, diffusion, and crystallization behaviors of electroless Ni–Cu–P deposits on Al were examined. In this study, tensile test results and microhardness measurements were used to investigate the effect of additive saccharin on the mechanical properties and fracture behavior of electroless Ni–Cu–P deposits on Al. An increase in the saccharin content of the plating solution from 0 to 12 g/L results in nodule growth and void elimination in the deposits. The denser nodules in the deposit also cause a decrease in the tensile stress and reveal the effect of ΔTCE on compressive stress generation. Consequently, the mechanical properties of Ni–Cu–P/Al deposits in terms of microhardness, yield strength, modulus of elasticity, and ultimate tensile strength were improved. The fracture behavior of the deposit changes from transnodular to internodular when the saccharin addition is above 4 g/L.

Structure?function relationships of four compression wood types: micromechanical properties at the tissue and fibre level

The mechanisms behind compressive stress generation in gymnosperms are not yet fully understood. Investigating the structure–function relationships at the tissue and cell level, however, can provide new insights. Severe compression wood of all species lacks a S3 layer, has a high microfibril angle in the S2 layer and a high lignin content. Additionally, special features like helical cavities or spiral thickenings appear, which are not well understood in terms of their mechanical relevance, but need to be examined with regard to evolutionary trends in compression wood development. Thin compression wood foils and isolated tracheids of four gymnosperm species [Ginkgo biloba L., Taxus baccata L., Juniperus virginiana L., Picea abies (L.) Karst.] were investigated. The tracheids were isolated mechanically by peeling them out of the solid wood using fine tweezers. In contrast to chemical macerations, the cell wall components remained in their original condition. Tensile properties of tissue foils and tracheids were measured in a microtensile apparatus under wet conditions. Our results clearly show an evolutionary trend to a much more flexible compression wood. An interpretation with respect to compressive stress generation is discussed.

Strength of Silicon Nitride after Thermal Shock

A water‐quenching technique was used to evaluate the thermal‐shock strength behavior of silicon nitride (Si3N4) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air‐exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si3N4 components before exposure to a thermal‐shock environment is recommended in practical applications.

Origin of compressive residual stress in polycrystalline thin films.

We present a model for compressive stress generation during thin film growth in which the driving force is an increase in the surface chemical potential caused by the deposition of atoms from the vapor. The increase in surface chemical potential induces atoms to flow into the grain boundary, creating a compressive stress in the film. We develop kinetic equations to describe the stress evolution and dependence on growth parameters. The model is used to explain measurements of relaxation when growth is terminated and the dependence of the steady-state stress on growth rate.

Periodicity as a Factor in the Generation of Isotropic Compressive Growth Stress Between Microfibrils in Cell Wall Formation during a Twenty-Four Hour Period

Summary Field emission scanning electron microscopy and energy-dispersive X-ray microanalysis were used to investigate the generation of growth stress in connection with the deposition of cell wall materials along the innermost surface of radial walls during secondary wall formation of differentiating tracheids of Cryptomeria japonica. Samples were collected when the turgor pressure of the tree was low (16:00 h), and when the pressure was high (6:00 h). In samples collected at 16:00 h, cellulose microfibrils deposited on the innermost surface of radial walls were clearly evident and amorphous material was rarely found. Conversely, in samples collected at 6:00 h, cellulose microfibrils were rarely observed and the amorphous material was prevalent. Cellulose microfibrils were evident, however, after removing the amorphous material with chlorite. The concentration of potassium on the inner surface of radial walls was greater at 6:00 h than at 16:00 h. After chlorite treatment, the concentration of potassium measured in the samples collected at 6:00 h decreased to a level equivalent to that in samples collected at 16:00 h and thus potassium was found to be associated with the amorphous material on the cellulose microfibrils. The amorphous material is probably a matrix of hemicellulose and monolignol, material that is abundant on the inner surface of developing cell walls, especially during expansion (as a result of high turgor pressure) of differentiating tracheids. A matrix of hemicellulose and lignin deposited in the expanded gaps between cellulose microfibrils and daily expansion of the gaps probably produces an isotropic compressive stress (growth stress). This paper demonstrates periodicity in cell wall formation as a result of the cycles of compressive growth in the differentiating cell wall.

Physics and applications of laser-shock processing

The first part of this article presents a review of the main process parameters controlling pressure generation in a confined mode. The effect of laser intensity, target material, laser pulse duration, and laser wavelength are, therefore, discussed. An optimized process can then be defined. The second part of this article deals with the surface modifications induced by laser-shock processing. The generation of residual compressive stresses is then highlighted. Finally, in the third part, the interest of laser-shock processing is discussed for several typical applications. A conclusion will present the future trends of this technique.