Decrease In Compressive Stress Research Articles

Long-wave in-plane buckling of elastoplastic square honeycombs

In this study, two formulae derived for very long-wave in-plane buckling of elastic square honeycombs are extended and examined in the elastoplastic case. To this end, microscopic buckling and post-buckling behavior of elastoplastic square honeycombs subject to in-plane compression are numerically analyzed using a homogenization theory of the updated Lagrangian type. It is thus demonstrated that very long-wave buckling occurs just after the onset of macroscopic instability if periodic length is sufficiently long, and that plasticity can cause the localization of microscopic buckling accompanied by a significant decrease in macroscopic compressive stress. Then, the very long-wave buckling stresses computed are predicted by incorporating the effect of plasticity in the two formulae of elastic square honeycombs. It is shown that the resulting formulae are successful in predicting the very long-wave buckling stresses in the plastic range as well as in the elastic range.

Extracellular post-translational modifications of collagen are major determinants of biomechanical properties of fetal bovine cortical bone

Mechanical behavior of bone depends on its mass and architecture, and on the material properties of the matrix, which is composed of a mineral phase and an organic component mainly constituted of type I collagen. Mineral accounts largely for the stiffness of bone, whereas type I collagen provides bone its ductility and toughness, i.e., its ability to undergo deformation and absorb energy after it begins to yield. The molecular mechanisms underlying the effect of alterations in type I collagen on bone mechanical properties are unclear. We used an in vitro model of fetal bovine cortical bone specimens ( n = 44), where the extent of type I collagen cross-linking was modified by incubation at 37°C for 0, 60, 90 and 120 days, keeping constant the architecture and the mineral content. At each incubation time, the following parameters were determined: (1) the bone concentration of enzymatic (pyridinoline; PYD and deoxypyridinoline, DPD) and non-enzymatic (pentosidine) crosslinks by HPLC, (2) the extent of aspartic acid isomerization of the type I collagen C-telopeptide (CTX) by ELISA of native (alpha CTX) and isomerized (beta CTX) forms, (3) the mineral density by DXA, (4) the porosity by micro-computed tomography and (5) the bending and compressive mechanical properties. Incubation of bone specimens at 37°C for 60 days increased the level (per molecule of collagen) of PYD (+98%, P = 0.005), DPD (+42%, P = 0.013), pentosidine (+55-fold, P = 0.005), and the degree of type I collagen C-telopeptide isomerization (+4.9-fold, P = 0.005). These biochemical changes of collagen were associated with a 30% decrease in bending and compressive yield stress and a 2.5-fold increase in compressive post-yield energy absorption ( P < 0.02 for all), with no significant change of bone stiffness. In multivariate analyses, the level of collagen cross-linking was associated with yield stress and post-yield energy absorption independently of bone mineral density, explaining up to 25% of their variance. We conclude that the extent and nature of collagen cross-linking contribute to the mechanical properties of fetal bovine cortical bone independently of bone mineral density.

Mechanical Performance of Auxetic Polyurethane Foam for Antivibration Glove Applications

In this study the static and dynamic characteristics of conventional open cell polyurethane (PU), of auxetic (negative Poisson's ratio) and of iso-density foams were analysed. The specimens were produced from conventional gray open-cells polyurethane foam with 30–35 pores/inch and 0.0027 g/cm3density, by means of process which has been previously defined by the authors. Poisson's ratio measurements were performed under quasi-static conditions using an MTS 858 servo-hydraulic test machine and a video image acquisition system. For the auxetic foams the results suggested similar behaviour to that previously reported in the literature, with significant increases in stiffness during compressive loading, and a significant dependence of the Poisson's ratio on the applied strain. Transmissibility tests, performed in accordance with the ISO 13753 procedure for antivibration glove materials, suggested a strong dependence of the transmissibility on the foam manufacturing parameters. Within the frequency range from 10 to 31.5 Hz the transmissibility was found to be greater than 1, while it was less than 1 at all frequencies greater than 31.5 Hz. The transmissibility results were similar to the mean values for 80 resilient materials tested by Koton et. al., but were higher than the five best materials (not all polymeric) identified by the same researchers. In this study it has been suggested that the resilient behaviour of glove isolation materials should also be evaluated in terms of the indentation characteristics. A simple, linear elastic, Finite Element simulation was therefore performed, and the indentation results suggested that auxetic foams offer a significant decrease in compressive stresses with respect to conventional PU foams.

658 高周波焼入焼戻し材の疲労に伴う表面圧縮残留応力の挙動(機械材料・材料加工,人体のコンピュータモデルの作成とその応用,特別講演)

Decrease of surface compressive residual stresses in an perfectly induction hardened and tempered steel S45C was confirmed to be small during rotating bending fatigue process. Before fatigue, the surface compcessive residual stress firstly relaxed in room temperature for about 5 years, and secondly relaxed in 80℃ for about 35 months or 120℃ for about 83 days, after that, relaxed by 200℃ tempering.

Infrared spectroscopy of Si–O bonding in low-dose low-energy separation by implanted oxygen materials

The effects of the processing conditions on the formation of buried oxide (BOX) layers in low-dose low-energy separation by implanted oxygen materials were investigated by using infrared spectroscopy and transmission electron microscopy. In as-implanted samples, the Si–O–Si stretching frequency increases either with increasing the oxygen dose or with decreasing implantation energy because the oxide composition becomes stoichiometric. However, the plateau frequencies were observed above a certain dose due to the compressive stress in the BOX layers. Upon ramping up to 1100 °C, the compressive stress decreases. Annealing beyond 1100 °C, the out diffusion of oxygen atoms was detected.

Mechanisms for the behavior of carbon films during annealing

The effect of annealing on $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thick single and multilayer amorphous carbon $(a\text{\ensuremath{-}}\mathrm{C})$ films prepared by filtered cathodic arc is investigated. Single layer films, with a $s{p}^{2}$ to $s{p}^{3}$ bonding fraction of approximately $50%$ increase their level of compressive stress following annealing. Multilayer films---consisting of alternating layers of high $s{p}^{3}$ fraction (tetrahedral amorphous carbon, $\mathrm{ta}\text{\ensuremath{-}}\mathrm{C}$) and intermediate $s{p}^{3}$ fraction show a decrease in compressive stress following annealing. Using cross-sectional transmission electron microscopy, we show that the single layer films and the intermediate $s{p}^{3}$ layers in the multilayer films develop a strong preferred orientation with graphite-like layers aligned perpendicular to the film surface. The $\mathrm{ta}\text{\ensuremath{-}}\mathrm{C}$ layers in the multilayer films develop the opposite preferred orientation near their top interfaces. We conclude that these preferred orientation effects are linked to the stress profile of the structures. We propose an underlying mechanism for the annealing effects of $a\text{\ensuremath{-}}\mathrm{C}$ films based on ab initio calculations. In order to minimize total energy, intermediate $s{p}^{3}$ films will either decrease their $s{p}^{3}$ fraction and generate stress or increase their $s{p}^{3}$ fraction and relieve stress. On the other hand, high $s{p}^{3}$ films retain their high $s{p}^{3}$ fraction following annealing.

Effects of Compressive Stress on Corrosion-Protective Quality and Its Maintenance under a Corrosive Environment for TiN Films Deposited by Reactive HCD Ion Plating

Titanium nitride films were deposited onto SUS304 substrates under various film thickness, deposition temperatures, and substrate bias voltages. Their protective quality was evaluated by electrochemical testing in accordance with the critical passivation current density (CPCD) method. Two types of tests were employed to evaluate corrosive behavior of coated substrates: a high-temperature and high-pressure corrosion test; and a measurement of the change in anodic current density with immersion time. A scanning electron microscope was used to examine surface morphology and fractured cross-sections of the films. Residual stress in the films was determined by the sin2 ψ method. An increase in film thickness engendered high protective quality. That protective quality was improved with increasing deposition temperature; micrometer-order pores were observed on all parts of films deposited at lower deposition temperatures, whereas few pores existed on films deposited at higher temperatures. This result indicates that these pores are one factor influencing overall protective quality. A film deposited with no substrate bias voltage displayed morphology with a typical columnar-structure; it also demonstrated complete protective quality. As the bias voltage increased, protective quality deteriorated, whereas an excess increase in the bias voltage gave rise to a slightly higher protective quality. Films with lower compressive stress had only a few pores and possessed higher protective quality, suggesting that pore formation originates in compressive stress. Corrosion tests indicated that the coated substrates corroded more rapidly as compressive stress in the film increased. The effect of compressive stress on maintenance of corrosion-protective quality was treated quantitatively. The rate of increase in the exposed area of the substrate was estimated from variation of an anodic current density with immersion time. This evaluation indicates that a decrease in compressive stress contributes greatly to maintenance of protective quality.

PACVD TiN/Ti–B–N multilayers: from micro- to nano-scale

Multilayer coatings with compositionally modulated structures in the nanometer range such as superlattices often show improved mechanical properties compared to homogenous materials. Various physical vapor deposition (PVD) methods have been utilized to deposit superlattice coatings, while chemical vapor deposition (CVD) is not suitable due to interdiffusion at high deposition temperatures. The aim of this work was to investigate the potential of plasma-assisted chemical vapor deposition (PACVD) for multilayers with extension to layer thicknesses in the nanometer range. TiN/Ti–B–N multilayers have been deposited in an industrially sized deposition chamber to optimize mechanical and tribological properties. Different layer numbers (4, 16, 32, 64, 128 and 300) of TiN and a 23 at.% B containing Ti–B–N coating have been alternately deposited by varying gas flow rates resulting in a total coating thickness of 2–3 μm. Electron energy-loss spectroscopy (EELS) measurements showed separated 5-nm thick layers for a total of 300 layers. A decrease in compressive stress has been observed for a decreasing individual layer thickness, which enhanced coating adhesion on tool steels. For the 4-layer coating, the friction behavior is determined by the higher chlorine containing low-friction TiN layer resulting in a constant steady-state friction coefficient of 0.2, whereas stable values of approximately 0.8 were reached for higher layer numbers. The optimum wear resistance was obtained for 128 layers, which is attributed to a combination of well-defined interfaces and moderate compressive stresses.

Temperature dependence of in-plane stresses in sublayers of Al/AlN/Al 2O 3(0 0 0 1) structure

Al and AlN sublayers with thickness of 100 and 500 nm, respectively, deposited on sapphire using pulsed laser ablation are characterized by elevated-temperature X-ray diffraction. Pole figure measurements performed at room temperature indicate heteroepitaxial growth of Al on AlN and AlN on sapphire with Al(1 1 1)[1 1 ̄ 0]|| AlN(0 0 0 1)[1 1 2 ̄ 0 ] and AlN(0 0 0 1)[1 0 1 ̄ 0]|| Al 2 O 3 (0 0 0 1)[1 1 2 ̄ 0 ] , respectively. The diffraction measurements in the temperature range of 25–550°C allow to resolve in-plane residual stresses for both sublayers simultaneously. The heating up of the structure is accompanied by a decrease of room temperature tensile stress and a subsequent plastic deformation in Al sublayer in the temperature range of 350–550°C while, in the AlN sublayer, a decrease of room temperature compressive stress as well as a post-annealing stress relaxation are observed. The temperature behavior of the stresses in the structure can be interpreted by the mismatch of thermal expansion coefficients between sublayers and substrate.

The constitution and properties of cubic boron nitride thin films: a comparative study on the influence of bombarding ion energy

Boron nitride thin films were deposited by unbalanced radio frequency magnetron sputtering of a hot-pressed hexagonal boron nitride target in a mixed argon/nitrogen atmosphere of 0.2 Pa. The intensity of ion bombardment during deposition was varied by a direct current substrate bias between 0 and −500 V. Thin films containing more than 85% of the sp 3-bonded cubic phase (c-BN), as confirmed by infrared spectroscopy, were produced in a wide voltage range from −200 to −400 V. The high c-BN content can be maintained even at an appreciably reduced bias of −80 V when a nucleation layer is formed in advance. In that case a decrease of compressive stress as well as an alteration of surface structure of the deposited c-BN films was observed. An analysis in light of electron energy loss spectroscopy indicates moreover the existence of an sp 2-coordinated surface layer with an ion-energy-dependent thickness. The results are discussed within the frame of the subplantation model.

Thin CNx overcoats deposited using various sputtering gases

This work presents systematic studies of the effect of various sputtering gases (Ar, Kr, and Xe) on the roughness, density, coverage, stress, and nanomechanical properties of CNx thin films. CNx (1–11 nm) was deposited on Si (100) substrates using dc magnetron sputtering. A three-parameter roughness analysis was carried out using atomic force microscopy. Film thickness and density were measured by x-ray reflectivity. Nanoindentation hardness and modulus were measured using a Hysitron TriboScope® nanomechanical system, while stress measurements were performed using a Flexus thin film stress measurement system. The coverage of CNx is assessed using a pinhole-decoration technique. As the mass of the sputtering gases increases, the density and hardness of CNx increases, while the deposition rate, surface roughness, and compressive stress decrease. Using Kr or Xe, a 3-nm-thick dc-sputtered CNx overcoat displaying good coverage is obtained.

Shear heating of a fluid‐saturated slip‐weakening dilatant fault zone 1. Limiting regimes

The one‐dimensional model of Rudnicki and Chen [1988] for a slip‐weakening dilating fault is extended to include shear heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of shear heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the shear heating is exactly balanced by heat flux from the fault zone. In the absence of shear heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the shear heating on the total shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.

In situ characterization of thin film growth: Boron nitride on silicon

Real-time ellipsometry (RTE) in combination with particle flux measurement is applied to ion beam assisted deposition of boron nitride (BN) films. RTE is used as a tool for process diagnostic to improve the deposition stability. A novel technique for the determination of absolute density depth profiles from dynamic growth rate data and film forming particle flux is employed. From real-time cantilever curvature measurement and simultaneously recorded film thickness data instantaneous stress depth profiles are derived with a depth resolution in the nm range. The synergistic effects on the information obtained from RTE, particle flux, and cantilever bending data are demonstrated. The density of turbostratic BN (tBN) is found to increase slightly with film thickness while the compressive stress decreases, indicating an increasing quality and/or size of crystallites in the course of film growth. Refractive index and density depth profiles in cubic BN (cBN) films correspond perfectly to structural information obtained from dark field transmission electron microscope graphs. The established tBN/cBN two-layer model is found to be a crude approximation that has to be replaced by a three-layer model including nucleation, grain growth, and coalescence of cBN. The instantaneous compressive stress in a homogeneous tBN film is found to decrease, while the density increases during growth. The instantaneous compressive stress depth profiles in cBN films are more complex and not easy to understand but reliable information on the structural evolution during growth can be extracted.

110 シリコン上にスパッタしたアルミニウム薄膜の気孔形成と硬度に関する MD 解析

Relation between pore formation and hardness of Al thin film sputtered on Si substrate is investigated by molecular dynamics simulations. Vacancy ratio and size introduced in thin films are varied and these effects to hardness are discussed. Effects to packing ratio and residual stress are also investigated. As vacancy ratio is increased, Both packing ratio and compressive stress which exist in the film without vacancy are decreased. As for hardness, films are softened because of the decrease of residual compressive stress. However, it is also implied that distribution and size of pore also influence film hardness. These results qualitatively agree with experimental results.

Residual stress in sputtered gold films on quartz measured by the cantilever beam deflection technique

With resonator applications in mind, the residual stress in sputtered gold electrodes on quartz has been investigated with respect to various deposition rates (2, 10, and 50 A/s), pressures (1.0 and 3.0 10(-3) mbar), deposition temperatures (80 degrees C and room temperature (RT)), film thicknesses (approx. 400 to 800 A), and substrate smoothnesses (lapped and polished), using the cantilever beam deflection method. Samples were monitored for 4 weeks at room temperature followed by 13 weeks of annealing at 85 degrees C. The initial stress (ranging from -180 to -60 MPa) was compressive for all samples but turned tensile (a few megaPascals) in some of the samples after annealing. A significant decrease in initial compressive stress appeared with samples coated at an elevated temperature. From samples prepared at lower pressure and differing only in film thickness and substrate roughness, an increased compressive stress was found in thicker films and on rougher surfaces. The stress relaxation has been fitted to an exponential expression, and an attempt to relate the stress to a frequency shift (typically a few parts per million for ordinary, 100-mum thick AT blanks) has been made. With the help of transmission electron microscopy (TEM) the film morphology was investigated and related to the deposition parameters and aging. Judging from the increase in compressive stress and grain refinement with increased deposition rate and decreased pressure, the atomic peening mechanism is the most likely reason for the induced stress. Rutherford backscattering spectrometry (RBS) was employed to rule out the inclusion of argon (below or around 0.5%) as an explanation. From the vague, but clearly discernible, trend toward faster RT stress relaxation with higher initial stress, together with the finer film morphology, the relief mechanism is believed to be stress-promoted grain boundary diffusion.

Three dimensional elastic-plastic finite element modelling of small fatigue crack growth under a single tensile overload

The effects of a single tensile overload on the stress, strain and displacement fields for a three dimensional semi-circular extending crack have been studied using an elastic-plastic finite element model. The results show that the application of a single tensile overload caused an immediate drop in the crack opening stress levels, but with further crack extension the opening stress levels rapidly rose to values higher than those before the application of the overload. It is found that the introduction of a single tensile overload increases the local residual compressive stress ahead of the crack front and, with the growth of the crack, the magnitude of this local compressive stress decreases. The results also show that the introduction of a single tensile overload increases the crack opening displacement, and that with further crack extension the crack opening displacement decreases rapidly to a value less than that before the application of the overload. The predicted crack opening displacements from finite element analysis were compared with those measured from direct in-situ SEM observations, and good agreement was obtained.

Study of tin films by filtered cathodic vacuum arc techniques

Thin TiN was grown on Si wafers (100) using a filtered cathodic vacuum arc technique. The films were characterised by their structural and mechanical behaviour. It was found that the properties of the deposited films are strongly dependent on the bias applied to the substrate. An increasing negative substrate bias results in a decrease in film compressive stress from 7·5 to 3·5 GPa and in hardness from 27·5 to 18·5 GPa. X-ray diffraction shows a preferred alignment from the (200) to the (111) direction with increasing negative sub strate bias. Atomic force microscopy results also show increasing surface roughness and grain size.

Determination of residual stresses in three-layer alumina composites using an indentation technique

The distribution of residual stresses in three-layer reaction bonded alumina (RBAO) composites with 17.5 vol % zirconia in the inner layer and 40 vol % mullite in the outer layers was determined by an indentation technique. It could be shown that the outer layers are subject to a residual compressive stress while the inner layer is under a tensile stress. With increasing outer layer thickness, the residual compressive stress decreases, and the residual tensile stress increases. Compressive stresses up to 550 MPa were measured in this study. It was found that the residual stresses are not uniform throughout the layers. The magnitude of the stresses decreases with increasing distance from the interfaces. This is attributed to creep during cooling from the sintering temperature.

Stresses in Al/TiW/Si(100) contacts during thermal cycling

Stresses in Al-1.0 wt.% Si/TiW thin films structure on Si(100) substrate were determined during thermal cycles by the curvature measurement technique. It was found that during heating the thermal strains are elastic up to about 90°C with compressive yield stress of about 200 MPa and biaxial elastic modulus of about 187 GPa. Above 90°C the films undergo plastic deformation in which the compressive stress decreases linearly with increasing temperature. During the third and fourth thermal cycles TiSi and TiSi 2 are formed at the Al/TiW interface leading to a decrease in the compressive stress by about 130 MPa. The influence of composition and microstructure changes on the stresses is discussed.

Stress relaxation and stability in thick amorphous carbon films deposited in layer structure

We have developed amorphous carbon films in sequential layers and studied their intrinsic stress, composition, and stress relief mechanisms. The films were deposited by sputtering either thin, with fixed bias voltage Vb or thick, with alternative (positive-layer A/negative-layer B) Vb. In situ spectroscopic ellipsometry and stress studies were used and supported by nanoindentation and x-ray reflectivity measurements. The films deposited with fixed negative (positive) Vb exhibit a thickness-dependent increase (decrease) of compressive stresses up to 200 Å and saturate at 6(1) GPa. In thick films deposited with alternative Vb the results demonstrate that: (1) the same modulation in stress values and sp3 content versus film thickness exists, supporting their direct interrelation; (2) the A layers (rich in sp2 sites) promote the stress relaxation of the films during a compositional rearrangement when a B layer is deposited; and (3) this process develops thick and stable films with lower stresses.