Internal Compressive Stress Research Articles

Structures, mechanical properties and thermal stability of TiN/SiN x multilayer coatings deposited by magnetron sputtering

Polycrystalline TiN/SiN x multilayer coatings with different modulation periods were deposited by reactive magnetron sputtering Ti and Si targets, respectively, and annealed at different temperatures in a vacuum furnace. Their structures and mechanical properties were characterized using X-ray reflectivity, X-ray diffraction and nanoindentation experiments. The results showed that both modulation period and annealing treatment had a significant influence on the structures and mechanical properties for multilayer coatings. As modulation period was below 5.03 nm the preferred orientation for TiN layer in multilayer coatings was (1 1 1), while above 10.05 nm the preferred orientation turned to (2 0 0). The hardness for coatings reached a maximum value of 38.27 ± 0.30 GPa as modulation period was 5.03 nm, but the internal compressive stress decreased with an increase in modulation period. It was found that thermal stability of TiN/SiN x multilayer coatings depended on modulation period, and large modulation period had a relatively high thermal stability. An enhancement in hardness for the as-deposited multilayer coatings, compared to either TiN or SiN x single layer film, could be attributed to the presence of internal compressive stress and the effect of interface strain and quantum trapping in the interfaces. The resistance to plastic deformation for the obtained coatings was reduced after annealing treatment.

A first principles study of the properties of Al:ZnO and its adhesion to Ag in an optical coating

A first principles density functional study of the atomistic properties of Al:ZnO and its adhesion to Ag is presented. Optical coatings often contain interfaces between ZnO (0001) and Ag (111) layers whose bonding can be improved by incorporating small amounts of Al into the ZnO but the underlying strengthening mechanism remains unclear. It is assumed that Al relaxes the internal compressive stress in the film but the situation is complicated by the presence of hydrogen and/or water which can adsorb on the ZnO surface during fabrication of the coating. Hydrogen and/or water are known to weaken the Ag/ZnO interface particularly when it is O terminated. In this paper it is shown that aluminum substitutes on Zn sites in ZnO and this does indeed reduce the internal stress in the layer under compression. However, it is also shown that Al segregates to the ZnO surface when it is O terminated (but not Zn terminated) and this reduces the propensity for hydrogen adsorption. Thus by eliminating some of the hydrogen from the ZnO surface which is more likely to be O terminated than Zn terminated under ambient conditions, the strength of the Ag/ZnO interface can be increased. The effect of aluminum incorporation into the ZnO layer is therefore twofold: it relaxes the residual stresses in the coating and also improves the chemical bonding at the metal/oxide interface by removing the weakening effects of gaseous adsorption. The changes in interfacial bonding are explained in terms of an electron redistribution and compensation model.

Water-Driven Assembly of Laser Ablation-Induced Au Condensates as Mesomorphic Nano- and Micro-Tubes

Reddish Au condensates, predominant atom clusters and minor amount of multiply twinned particles and fcc nanoparticles with internal compressive stress, were produced by pulsed laser ablation on gold target in de-ionized water under a very high power density. Such condensates were self-assembled as lamellae and then nano- to micro-diameter tubes with multiple walls when aged at room temperature in water for up to 40 days. The nano- and micro-tubes have a lamellar- and relaxed fcc-type wall, respectively, both following partial epitaxial relationship with the co-existing multiply twinned nanoparticles. The entangled tubes, being mesomorphic with a large extent of bifurcation, flexibility, opaqueness, and surface-enhanced Raman scattering, may have potential encapsulated and catalytic/label applications in biomedical systems.

Bifurcated Mechanical Behavior of Deformed Periodic Porous Solids

AbstractThe transformation of periodic microporous structures fabricated by interference lithography followed by their freezing below glass transition is described. Periodic porous microstructures subjected to internal compressive stresses can undergo sudden structural transformation at a critical strain. The pattern transformation of collapsed pores is caused by the stresses originated during the polymerization of acrylic acid (rubbery component) inside of cylindrical pores and the subsequent solvent evaporation in the organized microporous structure. By confining the polymerization of acrylic acid to localized porous areas complex microscopic periodic structures can be obtained. The control over the mechanical instabilities in periodic porous solids at a sub‐micron scale demonstrated here suggests the potential mechanical tunability of photonic, transport, adhesive, and phononic properties of such periodic porous solids.

Fracture of Layered Ceramics

Layered ceramics are foreseen as possible substitutes for monolithic ceramics due to their attractive mechanical properties in terms of strength reliability and toughness. The different loading conditions to which ceramic materials may be subjected in service encourage the design of tailored layered structures as function of their application. The use of residual stresses generated during cooling due to the different thermal strain of adjacent layers has been the keystone for the improvement of the fracture response of many layered ceramic systems, e.g. alumina-zirconia, alumina-mullite, silicon nitride-titanium nitride, etc. In this work, the fracture features of layered ceramics are addressed analysing two multilayered structures, based on the alumina-zirconia system, designed with tailored compressive residual stresses either in the external or internal layers. Contact strength and indentation strength tests have been performed to investigate the response of both designs to crack propagation. The experimental findings show a different response in terms of strength and crack growth resistance of both designs. While layered structures with compressive stresses at the surface provide a better response against contact damage compared to monoliths, a flaw tolerant design in terms of strength and an improved toughness through energy release mechanisms is achieved with internal compressive stresses. The use of layered architectures for automotive or biomedical applications as substitutes for alumina-based ceramics should be regarded in the near future, where reliable ceramic designs are needed.

Interfacial effects on the polarization of BiFeO 3 films

By considering an interfacial layer between the electrode and the BiFeO 3(BFO) layer, the polarization and the hysteresis behavior of BFO film are simulated. It is found that the non-ferroelectric interface will increase the coercive field, and remarkably suppress the polarization of the ultrathin film under low applied fields. Due to the competition between the interfacial effect and the internal compressive stress, the maximum polarization on the P – E loop of a BFO film can be independent of the film thickness under an adequate applied field.

Development of intergranular thermal residual stresses in beryllium during cooling from processing temperatures

The intergranular thermal residual stresses in texture-free solid polycrystalline beryllium were determined by comparison of crystallographic lattice parameters in solid and powder samples measured by neutron diffraction during cooling from 800 °C. The internal stresses are not significantly different from zero >575 °C and increase nearly linearly <525 °C. At room temperature, the c axis of an average grain is under ∼200 MPa of compressive internal stress, and the a axis is under 100 MPa of tensile stress. For comparison, the stresses have also been calculated using an Eshelby-type polycrystalline model. The measurements and calculations agree very well when temperature dependence of elastic constants is accounted for, and no plastic relaxation is allowed in the model.

X線法によるポリイミド基板スパッタ銅薄膜の変形挙動の評価

Copper thin films were fabricated by RF-magnetron sputtering under target power densities of 0.5 and 5W/cm^2. The polyimide film was used as a substrate to apply a plastic deformation to the copper thin film. Average grain sizes of the copper films obtained at 5W/cm^2 were 125nm and 230nm. On the other hand, the sizes at 0.5W/cm^2 were about 50nm. Load-controlled and strain controlled cyclic deformation tests were conducted on the polyimide films where the copper films deposited. Stress, full width at half maximum (FWHM) of diffraction profile, crystallite size, and microstrain of the copper films were measured during the deformation tests using X-ray method. In the load-controlled tests, the crystallite size decreased with applied strain during loading period, although the applied strain hardly affect crystallite size during unloading. On the other hand, the microstrain during unloading period was sensitive to the applied strain: the microstrain increased after initial rapid decrease. In the strain-controlled tests, the X-ray stress averaged over a strain cycle decreased with strain cycles. This is because the copper film was subjected to large plastic deformation which can cause internal compressive stress during unloading. The averaged value of the FWHM decreased also with strain cycles. This decrease in the FWHM was consistent with texture development during cyclic loading. On the other hand, the changes in the crystallite size and the microstrain during cyclic loading were insignificant.

Thin-Film Deposition of Silicon-Incorporated Diamond-Like Carbon by Plasma-Enhanced Chemical Vapor Deposition Using Monomethylsilane as a Silicon Source

We have deposited Si-incorporated diamond-like carbon (DLC) films by radio-frequency plasma-enhanced chemical vapor deposition using methane, argon, and monomethylsilane (MMS; CH3SiH3) as a silicon source, and have investigated the structural and mechanical properties of the films. The deposition rate and Si atomic fraction [Si/(Si+C)] in the DLC films increased with increasing MMS flow ratio. The Si fraction was approximately 13% at a MMS flow ratio [MMS/(MMS+CH4)] of 3%, showing that the deposition using a combination of CH4 and MMS produces films with high Si content compared with those deposited using conventional C and Si sources. The Si fraction was also found to increase with a decrease in Ar flow rate under a constant MMS flow ratio. Many particles composed mainly of Si, whose size was 0.3–1 µm in diameter, were observed on the surface when deposition was carried out at MMS flow ratios of 15 and 30%. Compressive internal stress in the films decreased with the MMS flow ratio and/or with the Ar flow rate. The decrease in internal stress is probably due to the relaxation of a three-dimensional rigid network by the formation of Si–C and Si–H bonds in the films as well as Ar+ ion bombardment.

Interaction between cracking, delamination and buckling in brittle elastic thin films

A discrete lattice based model for the interaction of cracking, delamination and buckling of brittle elastic coatings is presented. The model is unique in its simultaneous incorporation of the coating and of disorder in the interface and material properties, leading to realistic 3D bending (and buckling) behavior. Results are compared to the literature. In the case of cracking, the key role of a stress transfer correlation length ξ in establishing a scaling behavior for the brittle fracture of thin films is shown to extend to all geometrical and material properties involved. In the scaling regime of crack density in uniaxial tension cracking and delamination are found to occur simultaneously. In uniaxial tension of films with an internal biaxial compressive stress, the predicted initiation of buckles above delaminated areas near crack edges in the model is remarkably similar to experimental results.

Fatigue Behavior of Alumina–Zirconia Multilayered Ceramics

The influence of sustained and cyclic loading on the crack growth behavior of a multilayered alumina–zirconia composite exhibiting high internal compressive stresses is investigated. The study was conducted on precracked notched samples and focused on evaluating the static and cyclic fatigue resistance to crack extension beyond the first arresting interface (threshold) as well as the mechanisms involved during stable crack growth through the layered structure for each loading condition studied. Although it is found that the layered composite is prone to subcritical crack growth, the effectiveness of operative toughening mechanisms, i.e., compressive residual stresses as well as crack bifurcation and delamination at interfaces, is observed to be independent of the loading conditions. As a consequence, fatigue degradation of the multilayered ceramics studied is restricted to the intrinsic environmental‐assisted cracking of the individual layers, pointing them out as toughened composites practically immune to variable stresses and much less static and cyclic fatigue sensitive than other structural ceramics.

Internal stresses and stability of the tetragonal phase in zirconia thin layers deposited by OMCVD

Zirconia thin films were deposited by OMCVD (organo-metallic chemical vapour deposition) at various temperatures and oxygen partial pressures on a AISI 301 stainless steel substrate with Zr(thd) 4 as precursor. The as deposited 250 nm thin zirconia films presented a structure consisting of two phases: the expected monoclinic one and also an unexpected tetragonal phase. According to the literature, the stabilization of the tetragonal phase (metastable in massive zirconia) can be related to the crystallite size and/or to the generated internal compressive stresses. To analyze the effect of internal and external stresses on the thin film behaviour, in-situ tensile experiments were performed at room temperature and at high temperature (500 °C). Depending on the process parameters, phase transformations and damage evolution of the films were observed. Our results, associated to XRD (X-ray diffraction) analyses, used to determine phase ratios and residual stresses within the films, before and after the mechanical experiments, are discussed with respect to their microstructural changes.

Growth and structure of hydrogenated carbon films containing fullerene-like structure

Hydrogenated carbon films were prepared by magnetron sputtering of a titanium target in methane and argon atmosphere. The film grown at −800 V bias exhibits excellent mechanical properties with a hardness of 20.9 GPa and an elastic recovery as high as 85%. Its structure, characterized by high-resolution transmission electron microscopy, Raman spectrum, and x-ray photoelectron spectroscopy, can be described as fullerene-like structures uniformly dispersed in an amorphous carbon matrix. In order to reveal the evolution of fullerene-like structures in our films, different bias voltages were introduced. The results show that high bias voltage leads to the accumulation of high compressive internal stress in the film and promotes the evolution of fullerene-like structures. Although the film grown at −800 V bias presents high sp2 bonding content, it exhibits good mechanical properties with high hardness and high elasticity at the same time; we attribute it to the unique structure of the film, in which a fullerene-like structure, just like a molecule spring dispersed in the film, reserves the elastic energy during distortion through reversible bond rotation and bond angle deflection, while the amorphous carbon matrix restrains the relaxation of the rigid C–C network and compressive stress and restricts the slip of graphene sheets.

Diamond-like carbon films deposited on polycarbonates by plasma-enhanced chemical vapor deposition

Diamond-like carbon films were coated on optical polycarbonate using plasma-enhanced chemical vapor deposition. A mixture of SiH 4 and CH 4/H 2 gases was utilized to reduce the internal compressive stress of the deposited films. The structure of the DLC films was characterized as a function of film thickness using Raman spectroscopy. The dependence of G peak positions and the intensity ratio of I D/I G on the DLC film thicknesses was analyzed in detail. Other studies involving atomic force microscopy, ultraviolet visible spectrometry, and three adhesion tests were conducted. Good transparency in the visible region, and good adhesion between diamond-like carbon films and polycarbonate were demonstrated. One-time recordings before and after a DLC film was coated on compact rewritable disc substrates were analyzed as a case study. The results reveal that the diamond-like carbon film overcoating the optical polycarbonates effectively protects the storage media.

Optimal strength and toughness of Al 2O 3–ZrO 2 laminates designed with external or internal compressive layers

Layered ceramics have been proposed as an alternative choice for the design of structural ceramics with improved fracture toughness, strength and reliability. The use of residual compressive stresses, either at the surface or in the internal layers, may improve the strength as well as the crack resistance of the material during crack growth. In this work, two alumina–zirconia laminates designed with external (ECS-laminates) and internal (ICS-laminates) compressive stresses have been investigated using a fracture mechanics weight function analysis. An optimal architecture that maximises material toughness and strength has been found for each design as a function of geometry. From a flaw tolerant viewpoint, ECS-laminates are suitable for ceramic components with small cracks or flaws which are embedded in or near the potential tensile surface of the piece. On the other hand, the existence of large cracks or defects suggests the use of ICS-laminates to attain a more reliable response.

強ひずみ加工を施した Al-Mg 合金の陽極酸化後の耐孔食性に及ぼす熱処理の影響

The effect of annealing on the pitting corrosion resistance of anodized Al-Mg alloy processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol/L of AlCl3 and also by surface analysis. The degree of internal stress generated in anodic oxide films during anodization was evaluated with a strain gauge. The ECAP decreased the pitting corrosion resistance of anodized Al-Mg alloy. However, the pitting corrosion resistance was improved by annealing after the ECAP. The internal stress present in the anodic oxide films was compressive, and the stress was higher in the alloys with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Cracks occurred in the anodic oxide film on Al-Mg alloy during initial corrosion. The ECAP produces high internal stresses in the Al-Mg alloy; the stresses remain in the anodic oxide films, increasing the likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from the ECAP. The improvement in the pitting corrosion resistance of anodized Al-Mg alloy by annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films.

Effect of Annealing on the Pitting Corrosion Resistance of Anodized Aluminum-Magnesium Alloy Processed by Severe Plastic Deformation

The effect of annealing on the pitting corrosion resistance of anodized Al–Mg alloy processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol·L−1 of AlCl3 and also by surface analysis. The degree of internal stress generated in anodic oxide films during anodization was evaluated with a strain gauge. The ECAP decreased the pitting corrosion resistance of anodized Al–Mg alloy. However, the pitting corrosion resistance was improved by annealing after the ECAP. The internal stress present in the anodic oxide films was compressive, and the stress was higher in the alloys with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Cracks occurred in the anodic oxide film on Al–Mg alloy during initial corrosion. The ECAP produces high internal stresses in the Al–Mg alloy; the stresses remain in the anodic oxide films, increasing the likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from the ECAP. The improvement in the pitting corrosion resistance of anodized Al–Mg alloy by annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films.

Anisotropic Relaxation Behavior of Compressive Residual Stress in Delafossite CuAlO2

The anisotropic relaxation behavior of the compressive residual stress of delafossite film was identified to take place on silicon substrate, on which the film was grown. Experimental results suggest that in order to release the internal compressive residual stress of the film, CuO hillocks would be favored to grow on the film surface. It was also proposed that because of the structural anisotropic nature associated with the delafossite , the compressive residual stress was released first by breaking the O–Cu–O bonds of the dumbbell layers and subsequently by the diffusion of Cu and O atoms along the -axis direction on the close-packed Cu layers, suggesting that the -axis direction across the octahedral layers has a greater resistance to compressive residual stress.

Effect of substrate on the morphology, crystal structure and property of ZnO thin films

AbstractZnO thin films were deposited on glass, Si, SiO2 and NiO substrates with radio frequency (RF) magnetron sputtering using a ZnO target. ZnO films on glass and SiO2 substrates were relatively smooth and dense with similar surface morphology, while Si gave porous films, and the porosity increased with increasing film thickness. XRD analysis suggested that the ZnO films on Si, SiO2 and glass were highly oriented along the c‐axis. Films on glass and SiO2 had lower compressive internal stress than those on Si. The results of Hall effect measurements indicated that the conductivity of ZnO films on Si, SiO2 and glass increased with increasing film thickness. However, the changes of carrier concentration and mobility with film thickness are different for the different substrates. XPS showed that the Zn to O ratio on Si is higher than those on SiO2 and glass. The mechanism of the substrate effect is then briefly discussed on the basis of the experimental results. Copyright © 2007 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Characterising of internal stresses in duplex coating by FEM

The internal stresses in a duplex coating involving a prequenched layer are believed to change if it is exposed to thermal loading. To characterise the internal stresses in such a duplex coating, a gradient model of finite element method is set up. The initial stress within the substrate developed in as quenching and the internal stresses due to the tempering of the prequenched layer (TPQL) in such a duplex coating are calculated. The synthetical internal stresses in coating can be estimated by superposing uniform initial stresses developed during plating. The results indicate that the residual tensile stresses due to fabrication in coating will be decreased greatly, or even synthetical compressive internal stresses may arise in the coating.