Reduction Of Compressive Stress Research Articles

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.

Synthesis and mechanical properties of BCN coatings deposited by PECVD

BCN films were deposited by plasma enhanced chemical vapour deposition (PECVD) from chloridic precursors in an industrial-scale d.c. plasma CVD plant, where the power density (pd) at the substrate was varied, and the as-deposited films were analysed with respect to composition, structure and elastic properties. We find, independent of the power density applied to the substrate, evidence for sp 2 coordinated boron, which is consistent with the formation of a hexagonal or a rhombohedral BCN. Based on electron microscopy and electron diffraction we suggest a change in film texture from an orientation where the c-axis is parallel to the substrate surface at pd=0.5 W/cm 2 to a more randomly oriented polycrystalline structure at pd=2 W/cm 2. As the power density was increased a reduction in compressive stress was measured which may be a consequence of defect annihilation and subsequent relaxation. Furthermore, the texture–stress relationship suggested here is consistent with published thermodynamic calculations. The elastic modulus decreases from 115 to 40 GPa as the power density at the substrate is increased. This can be understood by the suggested change in texture as a consequence of the large elastic anisotropy of sp 2 coordinated BCN.

Internal stress reduction by incorporation of silicon in diamond-like carbon films

Si-containing diamond-like carbon (DLC) films deposited using CH 4+SiH 4 by an electron beam excited plasma (EBEP) CVD system were investigated for internal stress, dynamic hardness and structural properties. As the SiH 4 flow ratio was varied from 0 to 36.4%, the internal compressive stress linearly decreased from 2.5 to 1.0 GPa while the dynamic hardness remained nearly constant. From the correlations between the internal stress and the structural properties, it is suggested that SiH bonds formed by silicon incorporation can play the role of reducing the compressive stress. Also, the formation of SiH bonds is accompanied by a reduction in unbound hydrogen, which may cause the compressive stress reduction.

Stoichiometric cBN films deposited by electron-cyclotron-wave-resonance plasma sputtering of hBN

Electron-cyclotron-wave-resonance (ECWR) plasma of pure nitrogen was employed to deposit cBN thin films onto Si(100) substrates by hBN target sputtering. The ion current density and ion energy can be varied fairly independently. Deposition was achieved with ion energies between 70 and 230 eV. The cBN phase was identified with both Fourier transform-infrared spectroscopy (FT-IR) and high-resolution transmission electron microscopy (TEM). Thicker films with nearly 100% cBN phase in the upper layer appeared bright gray or transparent. A maximum thickness of 350 nm for the cBN layer was measured. The film growth follows the aBN→hBN→cBN sequence, with nanoarches being located at the hBN–cBN interface. Energy-dispersive X-ray (EDX) analysis verified the perfect stoichiometry of the deposits. Depending on the processing parameters the films displayed varied morphology. In particular, the surface of the film deposited at 74 eV exhibited large islands with diffuse boundaries on a compact base plane, suggesting the reduction of compressive stress at low ion energy.

STRAIN FIELDS IN DIAMONDLIKE AMORPHOUS CARBON

This paper reviews the fundamental ideas related to the concept of local atomic stresses and their application to the study of strain fields in ta-C. The calculations are based on Monte Carlo simulations within the empirical potential approach. We find that the stress distributions in ta-C are high inhomogeneous, both in the bulk as well as in the surface and interface regions. There is a close relationship between local stress and hybridization. The most probable stress state for fourfold sites is compression, while threefold sites prefer to be under tension. Local-atomic behavior dominates longer-ranged stress conditions. Interdiffusion following thermal annealing is an important factor for the reduction of compressive stress in ta-C films grown on silicon substrates.

High Resolution TEM of Post-Deposition Ion Implanted C-BN Films

Abstract Post-deposition ion implantation of boron nitride (BN) films is to be considered as a powerful method for improvement of adhesion between films and substrates and reduction of compressive stresses, which usually accompany the growth of cubic BN. The goal of this investigation is microcharacterization of as-deposited and ion-implanted BN films to better understand effect of ion implantation on microstructure of the films and film/substrate interface. High Resolution TEM (HF-2000 with field emission gun) has been utilized because of nanocrystalline dimensions of both cubic (c-) and hexagonal (h-) BN phases. BN films have been deposited on (100) oriented Si substrates by magnetically enhanced activated reactive evaporation2. Plasma source ion implantation has been performed to make surface modifications and to improve the adhesion of the films. X/S specimens for TEM and HRTEM have been prepared in a routine manner, using G-1 epoxy for coupling the specimens, Dimpler VCR D500I, and Gatan Precision Ion Polishing System for final thinning.

Syn-orogenic extensional and contractional deformation related to granite emplacement in the northern Tasman Orogenic Zone, Australia

The Hodgkinson Province of the northern Tasman Orogenic Zone comprises uniform, low-grade metasediments of mid-Palaeozoic age that host numerous Permian granite intrusions. Three widely developed, sequential deformations, D 2–D 4, are recognized on the basis of penetrative fabrics and folds, which show clear, regionally consistent overprinting relationships. D 3 and D 4 were contemporary with phases of granite emplacement as shown by relationships of S 3 and S 4 microstructures to porphyroblasts which grew in thermal aureoles adjacent to intrusive contacts. Granitic dyke relationships with S 3 and the deflection of S 3 and S 4 foliation trajectories around plutons support this view. Granites of D 3 and D 4 affiliation have a close spatial relationship and strong geochemical similarities, suggesting that they were closely associated in time. Isotopic dating of granites of D 4 association show that this deformational phase represents the Hunter–Bowen Orogeny which has been recognized as the terminal, Permian–Triassic episode of tectonism for the eastern sector of the Tasman Orogenic Zone. D 3 fabrics are best developed in zones of enhanced metamorphic grade, and are typically subhorizontal or of gentle inclination and associated with recumbent folds when not affected by subsequent deformation. D 3 is attributed to a phase of tectonism linked to the Hunter–Bowen Orogeny but preceding the main episode of compression. Uplift above tracts of rising magma, combined with the thermal weakening of crustal material and the reduction of compressive stress due to plate convergence, sponsored widespread ductile extension at a high crustal level. The zone of failure was a discontinuous carapace above the magmatic front. D 4 is attributed to the main episode of the Hunter–Bowen orogenic compression and involved crustal shortening with the imposition of upright structures and the widespread emplacement of granite. S 3 foliation surfaces have been reoriented in the vicinity of D 4 granite bodies, which behaved as rigid bodies during D 4 compression. The preparation of crustal pathways by D 3 granite was utilized by granites of the D 4 generation with the Hodgkinson Province displaying an intimate interplay of magmatism, stress regime and structural evolution over a time period of perhaps 15–20 Ma.

Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer-matrix composite

Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply [0/90] carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0° direction, R in this direction and R perpendicular to the fiber layers were measured. Upon static tension to failure, R in the 0° direction first decreased (due to increase of degree of 0° fiber alignment and fiber residual compressive stress reduction) and then increased (due to 0° fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 0° fiber alignment and delamination). Upon cyclic tension, R (0° decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing at a maximum stress of 57% of the fracture stress, R (0°) irreversibly increased both in spurts and continuously, due to 0° fiber breakage, which started at 15% of the fatigue life, while R (perpendicular to the fiber layers) irreversibly increased both in spurts and continuously, due to delamination, which started at 33% of the fatigue life. The peak R (0°) in a cycle irreversibly decreased, while the minimum R (perpendicular to the fiber layers) at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life, due to irreversible increase in the degree of 0° fiber alignment. R (0°) became noisy starting at 87% of the fatigue life, whereas R (perpendicular to the fiber layers) became noisy starting at 50% of the fatigue life. For a [90] unidirectional composite, R (0°) increased reversibly upon tension and decreased reversibly upon compression in the 0° direction, due to piezoresistivity.

Synthesis of cubic boron nitride films using a helicon wave plasma and reduction of compressive stress

A high density helicon wave plasma was used to synthesize a thin film of cubic boron nitride (c-BN) on the Si(100) substrate. Borazine (B 3N 3H 6) vapor was directly introduced into the reaction chamber where the plasma density was greater than 10 10–10 11 cm −3. The plasma density high enough to dissociate borazine effectively resulted in depositing a relatively good stoichiometric BN film without additional nitrogen sources. The bombardment of energetic ions with a high flux and energy is necessary for the growth of c-BN. The BN film synthesized at the condition of plasma density and rf substrate bias of 8.5 × 10 10 cm −3 and −350 V, respectively, was confirmed to be of cubic phase by both the FT-IR and TEM analyses. However, there existed an excessive compressive residual stress of 4–8 GPa in the c-BN film, which resulted in poor adhesion. The adhesion of c-BN film was significantly improved without a phase transformation through the post-heat treatment at 800 °C in N 2 atmosphere to relax the compressive residual stress.

Effect of in and Out of Plane Stresses During Indentation of Diamond Films on Metal Substrates

AbstractDiamond films grown on molybdenum substrates are indented with conical diamond indenters to depths more than 5 times the thickness of the film. This causes delamination and spalling of the film. The effects of variations in indenter angle and film thickness are demonstrated. Using a radius of the delaminated area to the residual contact radius, sharper indenters are shown to cause a 10 to 20% change in that ratio. For films greater than 5 μm thick, there is no apparent film thickness effect. Possibilities for this observation are cracking due to bending stresses or the interfacial crack kinking out of the interface due to a reduction in compressive stresses, leading to mixed mode loading. The strain energy release rate of the interface for an 800 nm diamond film on molybdenum is between 2.4 and 7 J/m2.

An electron spin resonance study of the effects of thermal nitridation and reoxidation on P b centers at (111) Si/SiO2 interfaces

Electron spin resonance of Pb centers is used to probe the dielectric/silicon interface in NH3-nitrided oxide (NO) and reoxidized nitrided oxide (RNO) dielectrics. The Pb spectra observed in the NO and RNO systems differ from Si/SiO2 systems only in the value of g⊥; the Δg⊥ suggests that the average value of the local strain at the defect site decreases upon nitridation, and is little changed by subsequent reoxidation. The relaxation of the Pb center structure appears to coincide with a reduction of compressive stress in the dielectric and an increase of tensile stress in the silicon substrate. These results suggest that differences in atomic scale strain due to nitrogen incorporation may be involved in the reduced radiation- and hot carrier-induced interface state generation that NO and RNO dielectrics exhibit.

Numerical simulation of leaflet flexure in bioprosthetic valves mounted on rigid and expansile stents

Recent studies suggest that flexural stresses induced during the opening phase may be responsible for much of the mechanical failures of bioprosthetic heart valves. Sharp leaflet bending is promoted by the mounting of valves on rigid stents that do not mimic the systolic expansion of the natural aortic root. We, therefore, hypothesized that flexural stresses could be significantly reduced by incorporating a flexible or expansile supporting stent into the valve design. Using our own non-linear finite element code (INDAP) and the pre- and post-processor modules of a commercial finite element package (PATRAN), we simulated the opening and closing behaviour a trileaflet bovine pericardial valve. The leaflets of this valve were assumed to be of uniform thickness, with a non-linear elastic behaviour adapted from experimentally obtained bending stiffness data. Our simulations have shown that during maximal systolic valve opening, sharp curvatures are induced in the leaflets near their commissural attachment to the supporting stent. These areas of sharp flexure experience compressive stresses of similar magnitude to the tensile stresses induced in the leaflets during valve closure. By incorporating a stent with posts that pivot about their base, such that a 10% expansion at the commissures is realized, we were able to reduce the compressive commissural stressing from 250 to 150 kPa. This was a reduction of 40%. Conversely, a simple pliable stent with stent posts that deflect inward and outward under load did not achieve a significant reduction of compressive stresses. This numerical analysis, therefore, supports the theory that (i) high flexural and compressive stresses exist at sites of sharp leaflet bending and may promote bioprosthetic valve failure, and (ii) that proper design of the supporting stent can significantly reduce such flexural stresses.

Refractive index behaviour of phosphorus-doped planar silica waveguides

The effect of annealing on the refractive index and the optical uniformity of undoped and phosphorus doped SiO2 films on silicon and quartz substrates is studied. The films are grown using low-pressure chemical vapour deposition. Vertical homogeneity of the refractive index is shown to be a relevant measure upon optimizing the annealing treatment for the film. High temperature annealing results in a decrease of the refractive index and an increase in the inhomogeneity of the refractive index in the vertical direction. This is due to the outdiffusion of phosphorus as well as the reduction of compressive stresses in the films. Stresses have also been considered to be the main reason for the refractive index difference of the order of 10-3 between films grown on silicon and on quartz substrates.

Intrinsic stress in hydrogenated amorphous silicon deposited with a remote hydrogen plasma

In hydrogenated amorphous silicon it is demonstrated that the maximum compressive intrinsic stress correlates with the optimum electronic properties. Undoped films were deposited over a range of temperatures in a remote hydrogen plasma (RHP) reactor and, for comparison, in a rf glow discharge (GD) system. The dependence of the stress on deposition temperature is qualitatively identical for the two reactors. Quantitatively, both the maximum compressive stress and the optimized electronic properties (e.g., minimum defect density) are obtained at 400 °C for the RHP films and near 250 °C for the GD films. Additionally, it is demonstrated that the transition from amorphous to microcrystalline silicon, induced by high hydrogen dilution, is accompanied by a reduction in compressive stress. Formation of compressive stress during RHP growth is ascribed to the insertion of hydrogen into the rigid silicon network immediately beneath the growing surface.