Reduction Of Compressive Stress Research Articles

Molecular dynamics simulation for the influence of incident angles of energetic carbon atoms on the structure and properties of diamond-like carbon films

The influence of incident angles of energetic carbon atoms (0–60°) on the structure and properties of diamond-like carbon (DLC) films was investigated by the molecular dynamics simulation using a Tersoff interatomic potential. The present simulation revealed that as the incident angles increased from 0 to 60°, the surface roughness of DLC films increased and the more porous structure was generated. Along the growth direction of DLC films, the whole system could be divided into four regions including substrate region, transition region, stable region and surface region except the case at the incident angle of 60°. When the incident angle was 45°, the residual stress was significantly reduced by 12% with little deterioration of mechanical behavior. The further structure analysis using both the bond angles and bond length distributions indicated that the compressive stress reduction mainly resulted from the relaxation of highly distorted C–C bond length.

Influence of growth stress on the surface morphology of N-polar GaN films grown on vicinal C-face SiC substrates

In-situ stress measurements were used to monitor the growth of N-polar GaN films on vicinal C-face SiC substrates using a two-step temperature process. A reduction in compressive stress in the N-polar GaN and a corresponding decrease in surface roughness were observed as the initial growth temperature was reduced from 1000 °C to 900 °C. The results suggest that compressive stress in N-polar GaN promotes step bunching and macroscale roughness in films grown on vicinal substrates. The reduction in compressive stress is proposed to originate from tensile thermal stress induced by the temperature change in the two-step process.

Sonicated sol–gel preparation of nanoparticulate ZnO thin films with various deposition speeds: The highly preferred c-axis (002) orientation enhances the final properties

Zinc oxide (ZnO) thin films have been deposited onto glass substrates at various deposition speeds by a sonicated sol–gel dip-coating technique. This work studies the effects of deposition speed on the crystallisation behaviour and optical and electrical properties of the resulting films. X-ray diffraction (XRD) analysis showed that thin films were preferentially oriented along the (0 0 2) c-axis direction of the crystal. The transformation sequence of strain and stress effects in ZnO thin films has also been studied. The films deposited at a low deposition speed exhibited a large compressive stress of 0.78 GPa, which decreased to 0.43 GPa as the deposition speed increased to 40 mm/min. Interestingly, the enhancement in the crystallinity of these films led to a significant reduction in compressive stress. All films exhibited an average transmittance of greater than 90% in the visible region, with absorption edges at ∼380 nm. The photoluminescence (PL) measurements indicated that the intensity of the emission peaks varied significantly with deposition speed. The optical band gap energy (Eg) was evaluated as 3.276–3.289 eV, which increased with decreasing compressive stress along the c-axis. The energy band gap of the resulting ZnO films was found to be strongly influenced by the preferred c-axis (0 0 2) orientation.

A rapid process of YBa2Cu3O7−δ thin film fabrication using trifluoroacetate metal–organic deposition with polyethylene glycol additive

Trifluoroacetate metal–organic deposition (TFA-MOD) is a promising technique to fabricate YBa2Cu3O7−δ (YBCO) superconducting films. However, its slow pyrolysis process, which usually takes more than 10 h, constitutes a barrier for industrial production. In this study, polyethylene glycol (PEG) was utilized to reduce the stress generation inside the coated films when the strong pyrolysis reactions happen. With the addition of 30 wt% PEG2000 to the precursor solution, a smooth film surface could be obtained through a rapid pyrolysis process of 15 min. After the optimizations of the crystallization and oxygenation processes, mass percentage and molecular weight of PEG additive, YBCO thin films with Jc of about 4.5 MA cm−2 (77 K, self-field) could be routinely fabricated using (20–30) wt% PEG(1000–2000) additive with a total treatment time of about 2 h including the 15 min pyrolysis process time. The effects of PEG additive were discussed using one of the mechanisms of buckling formation. The reduction of compressive stress by PEG additive was suggested to be the reason for preventing buckling.

Alucone Interlayers to Minimize Stress Caused by Thermal Expansion Mismatch between Al2O3 Films and Teflon Substrates

Alucone films were employed as interlayers to minimize stress caused by thermal expansion mismatch between Al(2)O(3) films grown by atomic layer deposition (ALD) and Teflon fluorinated ethylene propylene (FEP) substrates. The alucone films were grown by molecular layer deposition (MLD) using trimethylaluminum (TMA), ethylene glycol (EG), and H(2)O. Without the alucone interlayer, the Al(2)O(3) films were susceptible to cracking resulting from the high coefficient of thermal expansion (CTE) mismatch between the Al(2)O(3) film and the Teflon FEP substrate. Cracking was observed by field emission scanning electron microscopy (FE-SEM) images of Al(2)O(3) films grown directly on Teflon FEP substrates at temperatures from 100 to 160 °C and then cooled to room temperature. With an alucone interlayer, the Al(2)O(3) film had a crack density that was reduced progressively versus alucone interlayer thickness. For Al(2)O(3) film thicknesses of 48 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >60 nm on 50 μm thick Teflon FEP substrates. For thinner Al(2)O(3) film thicknesses of 21 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >40 nm on 50 μm thick Teflon FEP substrates. Slightly higher alucone interlayer thicknesses were required to prevent cracking on thicker Teflon FEP substrates with a thickness of 125 μm. The alucone interlayer linearly reduced the compressive stress on the Al(2)O(3) film caused by the thermal expansion mismatch between the Al(2)O(3) coating and the Teflon FEP substrate. The average compressive stress reduction per thickness of the alucone interlayer was determined to be 8.5 ± 2.3 MPa/nm. Comparison of critical tensile strains for alucone films on Teflon FEP and HSPEN substrates revealed that residual compressive stress in the alucone film on Teflon FEP could help offset applied tensile stress and lead to the attainment of much higher critical tensile strains.

THE SCALING OF PULL‐APARTS AND IMPLICATIONS FOR FLUID FLOW IN AREAS WITH STRIKE‐SLIP FAULTS

Widths of pull‐aparts between strike‐slip fault segments appear to obey power‐law scaling relationships, at least up to the scale of hundreds of millimetres. Field data from the Liassic limestones of Somerset, UK, are presented and are discussed in relation to fluid flow through rock. The established scaling relationships are used to predict the contributions of different‐sized pull‐aparts to space created by strike‐slip faults. Fluid‐filled pull‐aparts are likely to be significant in hydrocarbon reservoirs affected by strike‐slip faulting because they are sub‐vertical tube‐like structures that may extend through reservoirs and top seals. There are, however, insufficient data to develop a model for permeability along pull‐aparts. Models of fluid flow in rock typically assume a limited range of fracture apertures, so models can be improved by incorporating information on the scaling relationships of the fractures and the contributions of different‐sized fractures to secondary porosity. Also, geomechanical models that assume planar strike‐slip faults are likely to underestimate migration because they do not consider the reduction of compressive stresses at pull‐aparts.

Stress reduction of diamond-like carbon by Si incorporation: A molecular dynamics study

The residual stress and atomic bond structure of Si-incorporated diamond-like carbon films were investigated by the molecular dynamics simulation using Tersoff interatomic potential. The effect of Si incorporation into amorphous carbon matrix was analyzed for the various Si concentrations ranging from 0 to 2.1at.%. The present simulation revealed that the incorporation of a small amount of Si significantly reduced the residual compressive stress: when the Si content was 0.54at.%, the minimal compressive stress of 1.4GPa was observed. Structural analysis using the radial distribution function and the bond angle distribution indicated that the compressive stress reduction resulted from the relaxation of highly distorted bond angles less than 109.5°.

Effect of Si interlayers on the magnetic and mechanical properties of Fe/Ge neutron polarizing multilayer mirrors

The neutron polarizing supermirror is one of the most important optical devices for polarizing neutron beams. To meet a variety of research demands, neutron polarizing supermirrors need to display high polarization efficiencies at low external magnetic fields. Fe/Si and Fe/Ge multilayers are typically used in neutron polarizing supermirrors because the contrast in scattering length densities almost vanishes for spin-down neutrons. The Fe/Si/Ge/Si multilayer, obtained by adding thin interlayers of Si to an Fe/Ge multilayer, is effective in reducing the external field strength necessary to achieve efficient neutron polarization. To gain insight into the mechanism that controls the required external field strength for a neutron polarizing supermirror, we investigated the magnetic and mechanical properties of Fe/Si, Fe/Ge, and Fe/Si/Ge/Si multilayers. The external field strength required to achieve efficient neutron polarization was found to be proportional to the compressive film stress. The compressive stress of the Fe/Si/Ge/Si multilayer was smaller by a factor of 4.4 and 2.7 than that of Fe/Si and Fe/Ge multilayers, respectively. These measurements and analyses showed that a reduction in the compressive film stress in the Fe/Si/Ge/Si multilayer permits the use of lower external field strength to achieve efficient neutron polarization. X-ray photoelectron spectroscopic studies showed that the formation of a Ge-Si solid solution in the Ge layer may explain the marked reduction in compressive stress in the case of the Fe/Si/Ge/Si multilayer. This study confirmed that a reduction in compressive film stress is very important for a high-performance neutron polarizing supermirror.

Effect of N/Ga flux ratio on transport behavior of Pt/GaN Schottky diodes

GaN films were grown on c-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). The effect of N/Ga flux ratio on structural, morphological, and optical properties was studied. The dislocation density found to increase with increasing the N/Ga ratio. The surface morphology of the films as seen by scanning electron microscopy shows pits on the surface and found that the pit density on the surface increases with N/Ga ratio. The room temperature photoluminescence study reveals the shift in band-edge emission toward the lower energy with increase in N/Ga ratio. This is believed to arise from the reduction in compressive stress in the films as is evidenced by room temperature Raman study. The transport studied on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and was found to be caused by the increase in pit density as well as increase in dislocation density in the GaN films.

The structural properties of Al doped ZnO films depending on the thickness and their effect on the electrical properties

In this study, the structural and electrical properties of AZO films with different film thickness deposited by r.f. magnetron sputtering were interpreted in relation with film growth process. The result shows that the grain size increases during film growth, which is accompanied by decrease of compressive stress, indicating the enhancement of crystallinity. The relationship between grain size and compressive stress follows the same tendency for the samples regardless of deposition temperature, which implies the strong dependencies between the grain size and the compressive stress. The XPS analysis shows that the defects such as chemisorbed oxygen and segregated Al2O3 cluster at grain boundary are reduced with increase of film thickness or deposition temperature, leading to increase of carrier concentration and mobility. The mobility increase is accompanied by grain size increase and compressive stress reduction, indicating the influences of grain boundary and crystallinity on the mobility.

Stress, Texture and Phase Transformation in Titanium Thin Films

{111} fiber textured face centered cubic (fcc) titanium has been found to coexist with the {0002} fiber textured hexagonal close packed (hcp) titanium in polycrystalline titanium (Ti) thin films (thickness: 144 nm to 720 nm) deposited on Si (100) substrate by magnetron sputtering. X-ray diffraction investigation confirms that relative phase fraction of such metastable fcc Ti phase decreases with increasing film thickness indicating thickness dependent fcc-hcp phase transformation of titanium. Texture development in hcp Ti phase was due to film microstructure (thickness effect) rather than the phase trans-formation. Diffraction stress analysis (by d-sin2 method) indicates that fcc to hcp phase transformation is also accompanied by the reduction of compressive stress in the hcp Ti phase with increasing film thickness. Strain energy calculations for both phases of titanium indicate that fcc Ti is a more stable phase compared to hcp Ti at relatively low film thickness (144 nm to 432 nm). It has been concluded that film stress favours fcc to hcp phase transformation towards the higher film thickness. Reverse transformation (hcp to fcc) occurs towards the lower film thickness.

Time-Dependent Behavior of Concrete-Filled Steel Tubular Arch Bridge

This paper develops a simplified method using a summation procedure and a related computer program to calculate the time-dependent behavior of a concrete filled steel tubular (CFST) arch bridge based on the geometric compatibility principle, a step-by-step time incremental process, and self-equilibrium equations. An experimental test on a scaled (1:5) segmental model of the main arch ribs of the Maocaojie Bridge was used to confirm the effectiveness of the proposed calculation method for evaluating the long-term behavior of CFST arch bridge under sustained load. It is concluded that: (1) the numerical results were in good agreement with the experimental results, demonstrating that the proposed analytical model is capable of predicting long-term effects for CFST arch bridges; (2) the stresses in the steel tubes increased, and the compressive stresses in the concrete decreased due to the effects of concrete creep and shrinkage. The maximum relaxation of the compressive stress in concrete due to concrete creep was 52.7% of the initial concrete stress, and the maximum increase of stress in the steel tubes was 27.3%; and (3) more than 90% of the total creep of the concrete took place in the first year. Subsequent creep of the concrete was limited because of the lack of water exchange between the structure and atmosphere and the reduction of compressive stress in the concrete.

Residual stresses and thermal fatigue in CrN hard coatings characterized by high-temperature synchrotron X-ray diffraction

The aim of this work is to analyze thermal fatigue in hard coatings/substrate composites (i) during slow heating and cooling and (ii) after local cyclic thermal laser pulse experiments. As a model system, CrN coatings with a thickness of 3 µm deposited on steel, hard metal and Si(100) substrates using reactive magnetron sputtering at a temperature of 350 °C are used. The coatings are at first characterized by means of in-situ high-temperature X-ray diffraction (XRD) using a commercially available temperature attachment and by applying heating and cooling rates of less than 0.3 °C/s. The treatment results in the expected reduction of intrinsic stresses which are independent of substrate material but strongly influenced by substrate roughness. To simulate local thermal fatigue, selected coating/substrate composites are thermally cycled using a laser beam of 6 mm in diameter in a temperature range of 50–850 °C applying up to 10 4 cycles and using heating and cooling rates of about 10 3 °C/s. Subsequently, laser cycled samples are analyzed using synchrotron XRD, scanning electron microscopy and focused ion beam technique. Laser pulses cause a reduction of compressive stresses in the coatings and a development of tensile stresses in the substrates accompanied by formation of cracks and ripples. The results show that the changes of the local macro- and micro-strains/stresses in the coatings and in the underlying substrates are strongly interlinked. The stress relaxation in the coatings is caused by recovery effects, by micro-cracks formed in the tensely-stressed coating and by plastic deformation of the metallic substrates.

Influence of ion bombardment on residual stresses in diamond-like carbon films

The dependence of internal residual stresses in thin diamond-like carbon films grown by the PECVD technique on the most important growth parameters such as the power of the exciting RF discharge and the substrate bias potential is considered. The results have shown that the mechanical stresses in films reach the uppermost value of 1.9 GPa at the smallest values of power and potential. The stress decreases with the growth of both parameters and has only a slight dependence on the film thickness in the range 0.1–1 μm. The bombardment of the obtained films by argon ions with energy of 300 keV and phosphorus ions with energy of 200 keV has resulted in the reduction of compressive stress with the ion dose growth down to its inversion. AFM study of the bombarded films has revealed significant changes in their surface morphology.

Residual stresses in thermally cycled CrN coatings on steel

CrN coatings were deposited on polycrystalline ferritic steel substrates at 350°C by magnetron sputtering using Cr targets in Ar + N 2 atmosphere. In order to simulate the thermal fatigue, the samples were repeatedly irradiated using a laser beam of 6mm in diameter. The thermal cycling was performed in the range of 50–650°C with up to 100 000cycles. Subsequently, the structures were characterized using high-energy synchrotron and high-temperature laboratory X-ray diffraction. The structures exhibit complex changes in the morphology and in residual stress state in the heated spot. The annealing results in the relaxation of compressive stresses in the coating and in the formation of high tensile stresses in the steel substrate. This effect decisively depends on the number of applied cycles. The reduction of compressive stress in the coating is caused by the annealing of point defects and by dimensional changes of the substrate due to its plastic deformation in the center of the irradiated spot. The plastic deformation of the substrate is also the probable reason for the ripples observed for samples cycled more than 3000 times. The presented approach allows a complex characterization of thermo-mechanical processes in coating-substrate composites and opens the possibility to understand phenomena related to the thermal fatigue of coated tools.

Effect of residual surface stress on the fatigue behavior of a low-alloy powder metallurgy steel

In this study we have characterized the fatigue behavior of a low-alloy powder metallurgy (P/M) sintered steel, composed mainly of martensite and bainite with 10.4% porosity. After each processing step, the residual axial surface stress was measured by X-ray diffraction. Significant compressive surface stresses were generated during the machining of the fatigue specimens. A heat-treatment at 175 °C after machining had no effect on these residual stresses, but polishing the surface resulted in a 20% reduction in compressive stresses. The residual surface stresses had no apparent effect on the fatigue behavior of the sintered steel. Rather, the fatigue behavior was controlled by sub-surface defects consisting of weakly bonded particles located in regions of high porosity.

New approach in depositing thick, layered cubic boron nitride coatings by oxygen addition—structural and compositional analysis

Cubic boron nitride (c-BN) can be produced by PVD and PA-CVD techniques by intensive ion bombardment leading to highly stressed films limiting its use in industrial applications. Various attempts have been undertaken to reduce the compressive stress of c-BN thin films. A significant reduction in compressive stress and a substantially improved adhesion was achieved by a new coating concept consisting of a two-step adhesion-promoting base layer, a compositional-graded nucleation layer obtained by a stepwise decrease of the oxygen content in the Ar/N 2/O 2 atmosphere and a low-stressed c-BN:O top layer with controlled oxygen addition. The four-layer c-BN:O film with a thickness of 3 μm was deposited by unbalanced radio frequency magnetron sputtering of a hot-pressed hexagonal boron nitride target on silicon substrates. The adhesion layer was deposited in a mixed Ar/O 2 atmosphere of 0.26 Pa with a stepwise increased nitrogen gas flow and a subsequent increase of the ion energy by increasing the substrate bias from 0 to − 250 V. The c-BN nucleation was gradually initiated by decreasing the O 2 gas flow. The present study was focused on the investigation of the morphology, the microstructure on the nanoscale, and the bonding structure using scanning electron microscopy (SEM), Fourier-Transmission infra-red spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) employing analytical scanning transmission electron microscopy (ASTEM). The HRTEM images revealed a four-layer coating consisting of a gradual nucleation of t-BN, on which a gradual nucleation of c-BN was achieved by decreasing the oxygen gas flow.

Development and biocompatibility of hard Ti-based coatings using plasma immersion ion implantation-assisted deposition

The hard Ti-based coating TiAlN has been produced by plasma immersion ion implantation-assisted deposition (PIIIAD) on austenitic stainless steel. In PIIIAD, the substrate is exposed to cathodic arc plasma comprising of metallic ions corresponding to the cathode composition, and the substrate is biased pulsed negative. The pulsed bias of the substrate renders alternate phases of deposition (during the bias off time) and implantation and deposition (during bias on time), thereby controlling deposition temperature, reduction of compressive stresses, increase of crystallinity and enhancing adhesion of coating to substrate. The deposition has been carried out in a neutral as well as reactive nitrogen environment, which leads to nitrogen-containing coatings. In addition to TiAlN, also, Ti, TiN and TiAl were produced to investigate the effect of the different elements. All the coatings are tested for their suitability as biocompatible coatings, and hence, the adherence of osteoblast progenitor cells to these coatings was investigated in correlation with deposition parameters and surface morphology. The results identify the coatings that are wear resistant and suitable for orthopaedic implants.

Low temperature annealing in tetrahedral amorphous carbon thin films observed by13CNMR spectroscopy

For the first time to our knowledge, the 1 3 C solid-state magic angle spinning (MAS) NMR spectrum of a 99% 1 3 C enriched tetrahedral amorphous-carbon (ta-C) thin film containing a high concentration of fourfold coordinated carbon species (82%) is reported along with measured NMR spectra for the ta-C film after low temperature annealing (650 °C). Differential changes are observed for the 1 3 C MAS NMR chemical shifts and linewidths of both the fourfold (diamondlike) and threefold (graphitelike) coordinated carbon species within the thin films with increasing annealing time; however, there was no change (′2%) in the relative fourfold content. These spectral changes are associated with the large compressive stress reduction (6-8 GPa) in the carbon film. Ab initio calculations of the 1 3 C NMR chemical shift, along with shift variations as a function of atomic volume are reported for amorphous carbon and crystalline diamond. Using the observed spectral variations in the solid-state 1 3 C MAS NMR, along with the ab initio chemical shift calculations, the effect of annealing on the ta-C films is discussed and related to current models of thermal stress relaxation in ta-C thin films.

Improved Tribological Properties of Diamondlike Carbon/Metal Nanocomposites

ABSTRACTOne of the many forms of carbon, diamondlike carbon (DLC), consists mainly of sp3-bonded carbon atoms. DLC coatings possess properties close to diamond in terms of hardness, atomic bonding, and chemical inertness. Unfortunately, DLC exhibits poor adhesion to metals and polymers. The adhesion of the DLC film is determined by internal stresses in the film and by interfacial bonding. This work involves processing, characterization and modeling of diamondlike composite films on metal and polymer substrates to improve adhesion and wear properties. A novel target design was adopted to incorporate metal (silver or titanium) atoms into DLC films during pulsed laser deposition. STEM of the DLC/metal nanocomposites has shown that the metals that do not form carbides (e.g., silver) form 2–10 nm inclusions within the DLC matrix. Wear resistance measurements made on these samples have demonstrated that DLC/metal nanocomposites possess exceptional wear resistance. Diamond-like carbon nanocomposites also exhibit significantly enhanced adhesion. Careful analysis of the Raman data also indicated a significant shift to shorter wavelength in DLC nanocomposite films, indicating a reduction in compressive stress within these films. The sp3 content of these films was studied using electron energy loss spectroscopy (EELS). By varying the metal concentration as a function of distance from the interface, we have created functionally gradient DLC nanocomposites. These DLC/metal nanocomposite coatings have multiple biomedical applications.