Interfacial Cohesion Research Articles

Haloing, flocculation, and bridging in colloid-nanoparticle suspensions

Integral equation theory with a hybrid closure approximation is employed to study the equilibrium structure of highly size asymmetric mixtures of spherical colloids and nanoparticles. Nonequilibrium contact aggregation and bridging gel formation is also qualitatively discussed. The effect of size asymmetry, nanoparticle volume fraction and charge, and the spatial range, strength, and functional form of colloid-nanoparticle and colloid-colloid attractions in determining the potential-of-mean force (PMF) between the large spheres is systematically explored. For hard, neutral particles with weak colloid-nanoparticle attraction qualitatively distinct forms of the PMF are predicted: (i) a contact depletion attraction, (ii) a repulsive form associated with thermodynamically stable "nanoparticle haloing," and (iii) repulsive at contact but with a strong and tight bridging minimum. As the interfacial cohesion strengthens and becomes shorter range the PMF acquires a deep and tight bridging minimum. At sufficiently high nanoparticle volume fractions, a repulsive barrier then emerges which can provide kinetic stabilization. The charging of nanoparticles can greatly reduce the volume fractions where significant changes of the PMF occur. For direct and interfacial van der Waals attractions, the large qualitative consequences of changing the absolute magnitude of nanoparticle and colloid diameters at fixed size asymmetry ratio are also studied. The theoretical results are compared with recent experimental and simulation studies. Calculations of the real and Fourier space mixture structure at nonzero colloid volume fractions reveal complex spatial reorganization of the nanoparticles due to many body correlations.

Design of multimaterial processed by powder metallurgy: Processing of a (steel/cemented carbides) bilayer material

A bimaterial, consisting of tough and hard layers, was designed to be processed by powder metallurgy. A materials selection software lead to choose steel and cemented carbide for the tough and hard components. To insure the interlayer compatibility, Fe was selected as base of the tough part and as binder of the hard part. The compositions and the temperature range enabling the simultaneous consolidation of the Fe base by transient liquid phase sintering and of the WC base by liquid phase sintering were defined from thermodynamics data. Experiments lead to define a treatment in two steps: presintering (1080 °C) for the partial densification of the Fe base then sintering (1280–1320 °C) to achieve densification and cohesion of the bimaterial. The sintering temperature increase provided a higher liquid fraction leading to interface cohesion but coarse porosity development. Bimaterial, with fair mechanical properties, were obtained by presintering at 1080 °C then sintering at 1280 °C.

Influence of aluminium on the valence electron density of the interface between the bond-coat and the thermally grown oxide of thermal barrier coatings

The interface cohesion greatly affects the longevity of thermal barrier coatings (TBCs). The adherence and stress of the interface between the bond-coat and the thermally grown oxide (TGO) are considered as crucial properties governing the life times of TBCs. In this paper, the electron density of the bond-coat/TGO interface of TBCs was calculated with the empirical electron theory in solid and molecules. The calculation results of the electron density of the bond-coat/TGO interface with various bond-coat aluminium contents were analyzed in the views of interface bonding force, interface stress and interface stability. It is shown that the suitable aluminium content of the bond-coat should be in the ranges of 8–10 wt% and about 2 wt% considering the bond-coat/TGO interface only. Taking the bond-coat/top-coat interface of TBCs into account, combining the consideration of the TGO forming ability, the suitable aluminium content becomes about 8 wt% and a little less than 8 wt%, which accords better with actual TBC applications. The deduction not only can be a useful reference to the composition design of the bond-coat of TBCs, but also shows the necessity of considering the electron density parameters of the interfaces of bond-coat/top-coat and bond-coat/TGO simultaneously in the composition design of the bond-coat of TBCs.

Influence of minor elements additions on microstructure and properties of 93W-4·9Ni-2·1Fe alloys

Effects of elements rhenium and chromium additions on properties and microstructure of 93W-4·9Ni-2·1Fe alloys were investigated. Optical microscope (OM), scanning electron microscope (SEM) and EDAX energy spectrometer were used to characterize the microstructure and compositions of the alloys, respectively. The tensile strength and elongation of alloys were evaluated using the quasi-static tensile testing machine, and the relative densities of the alloys were evaluated using the Archimedes water immersion method. The experimental results indicated that when elements Re and Cr were in the range of 0–1·0 wt.%, relative density, elongation, tensile strength of 93W-4·9Ni-2·1Fe alloys varied from 99·4%, 26·4%, 997·2 MPa without Re additions to 99·5%, 8·6%, 1161·2 MPa with 1·0 wt.% Re addition, respectively. Rhenium generated solid-solution strengthening, grain refinement, reducing ductile tearing and increasing transcrystalline fracture, which resulted in the ductility reduction and the strength increase of the heavy alloys. With the increase of Cr content from 0–1·0 wt.%, the tensile strength, relative density and elongation of 93W-Ni-Fe alloy reduced from 997·2 MPa, 99·3%, 15% to 844·4 MPa, 95·2%, 5·7%, respectively. Element Cr formed interphases with elements W, Ni, Fe and O and gathered along the interface of the alloys, which induced interfacial cohesion and resulted in lower mechanical properties of 93W-Ni-Fe alloys.

Effect of Press Processing on Fracture Behavior of HA/PLLA Biocomposite Material

Press process was performed in order to toughen HA/PLLA bio-composite material. Mode I fracture properties and bending mechanical properties of forged and unforged HA/PLLA were then evaluated to assess the effect of press process on these properties. FE-SEM was also used to characterize the toughening mechanism due to press. Testing results showed that the bending strength and modulus slightly increased due to press; on the other hand, the bending absorbed energy and the mode I fracture property dramatically improved. FE-SEM observation of the fracture surfaces showed that ductile deformation and fracture of PLLA fibrils were enhanced and in addition, interfacial cohesion strength was improved. These microstructural modifications appear to result in the toughness improvement.

Photoelectrochemical imaging of metal-scale decohesion on titanium thermally oxidised in oxygen

Important compressive stress developed in TiO 2 films thermally grown on pure titanium in oxygen is known to induce scale decohesion at the metal–oxide interface. If such a phenomenon is identified in the literature to play a key role on life time of many materials used at high temperatures, no data at the microscopic level can be found at this time. In the present work using photoelectrochemical microscopy, new information in the form of two specific images ICI (interface cohesion image) and SQI (structural quality image) could be obtained. Such images revealed the level of scale decohesion induced by growth stress as well as the structural quality of the oxide layer.

Real Space Structure and Scattering Patterns of Model Polymer Nanocomposites

Microscopic polymer liquid state theory is employed to study the real space pair correlation functions and collective scattering structure factors of melt polymer nanocomposites composed of hard spheres and adsorbing homopolymers over length scales ranging from monomeric to macroscopic. Increasing filler volume fraction has a profound effect on the polymer matrix, inducing oscillatory reorganization on a length scale commensurate with the nanoparticle diameter. The near contact interfacial monomer−filler pair correlations are suppressed by nanoparticle addition, but on larger scales power law correlations emerge with filler imprinted oscillatory features. Increased nanoparticle volume fraction also substantially changes interfiller packing, inducing a transition from a highly bridged or sterically stabilized type of organization in the infinite dilution limit to more diffuse liquidlike packing characterized by many-particle clustering. Distinctive modifications of the collective polymer structure factor include an increase in the osmotic compressibility, the emergence of a large bound layer or microphase-separation-like scattering peak on a length scale controlled by filler size indicative of distinct bound polymer layers, and a local rarefaction and suppression of the coherence of the monomer cage scale packing. All the real and Fourier space correlations depend in distinctive manners on the physical and chemical variables (filler size, volume fraction, strength and spatial range of the interfacial cohesion) and proximity to the contact aggregation and bridging phase separation boundaries. The bulk modulus of the nanocomposite generically softens with the addition of fillers corresponding to enhanced total density fluctuations.

Influence of yttrium on the interface valence electron density of thermal barrier coatings

Most of the bonding layers of thermal barrier coatings contain some yttrium to improve the physical consistency of the substrate and the ceramics layer. But the reason of the improvement and the proper yttrium content are not clear. In this paper, the valence electron densities ρ h k l and ρ u v w of the two sides and their difference Δ ρ min of the bonding layer/ceramic layer interface of thermal barrier coatings are calculated with the empirical electron theory in solids and molecules at various bonding layer yttrium content. The results show the following. The addition of yttrium has beneficial effect on the decrease of the interface stress because it decreases the Δ ρ min. The addition of yttrium can also increase the valence electron density ρ h k l or ρ u v w of the interface and so increase the interface cohesion force. The most effective yttrium content is at 0.4 wt% or so. The deductions accord with the actual coating, so the method can be applied to design the composition of the bonding layer of thermal barrier coatings. Furthermore, the calculation and the analysis methods of the interface valence electron densities can also be extended to other crystal coatings or composites with special orientation relationship, no matter the interface is a alloy/alloy one, a ceramics/alloy one or a ceramics/ceramics one.

Wear behavior of nano structured Al (Zn)/Al 2O 3 and Al (Zn)–4Cu/Al 2O 3 composite materials synthesized by mechanical and thermal process

Nanocrystalline materials have high potential for use in structural applications in which enhanced mechanical characteristics are required. Al–Zn/Al 2O 3 and Al–Zn–Cu/Al 2O 3 have been prepared via reaction sintering of partially reacted oxide mixtures derived from a high-energy ball milling process. The sintered samples have been characterized by XRD. Pin on disc type apparatus has been used for determining the wear rate. It is proposed in this study that the wear resistance of the milled composites is better than that of the unmilled composites, since Al 2O 3 particle size plays the main role in bearing the external load, and there is a good interfacial cohesion between Al 2O 3 particles and the matrix. On the basis of microscopic observations and analysis of the wear surface a change in mode of wear from mild to severe has been proposed at different loads.

Triarylamine siloxane anode functionalization/hole injection layers in high efficiency/high luminance small-molecule green- and blue-emitting organic light-emitting diodes

High efficiency/high luminance small-molecule organic light-emitting diodes (OLEDs) are fabricated by combining thin, covalently bound triarylamine hole injection/adhesion interlayers with hole- and exciton-blocking/electron transport interlayers in tris(8-hydroxyquinolato)aluminum(III) (Alq) and tetrakis(2-methyl-8-hydroxyquinolinato)borate (BQ4−)-based OLEDs. Green-emitting OLEDs with maximum luminance ∼85000cd∕m2, power and forward external quantum efficiencies as high as 15.2lm∕W and 4.4±0.5%, respectively, and turn-on voltages ∼4.5V are achieved in devices of the structure, ITO∕N,N′-diphenyl-N,N′-bis(p-trichlorosilylpropylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD-Si2)/1,4-bis(1-naphthylphenylamino)biphenyl (NPB)/Alq doped with N,N′-di(3-heptyl)quinacridone (DIQA)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)∕Li∕AgMg. Also, bright and efficient blue-emitting OLEDs with turn-on voltages ∼5.0V, maximum luminance ∼30000cd∕m2, and ∼5.0lm∕W and 1.6±0.2% power and external forward quantum efficiencies, respectively, are achieved in devices of the structure, ITO∕TPD-Si2∕NPB∕BQ4−∕BCP∕Li∕Al. TPD-Si2 interlayers are fabricated by spin casting N,N′-diphenyl-N,N′-bis(p-trichlorosilylpropylphenyl)(1,1′-biphenyl)-4,4′-diamine onto the ITO surface, while BCP interlayers are introduced by thermal evaporation. The excellent OLED performance is attributed to the differing functions of the above two interlayers: (1) The TPD-Si2 layer has a direct impact on hole injection by reducing the injection barrier and improving interfacial cohesion, and an indirect but strong effect on electron injection by altering internal electric fields. (2) The BCP layer, doped with lithium, directly reduces the electron injection barrier. Incorporation of both interlayers in OLED structures affords synergistically enhanced hole/electron injection and recombination efficiency. The results demonstrate a strategy to enhance OLED performance and an alternative strategy to increase electron density in electron-limited devices.

Cladding of stainless steel on aluminum and carbon steel by interlayer diffusion bonding

An Al–Cu–Mg interlayer alloy is used in cladding the sheets of 304L stainless steel/L2Y2 aluminum alloy and 304L stainless steel/Q235A carbon steel. Effects of cladding temperature and time on the interfacial bonding of the two sheets are studied. Experimental results show that the Interfacial cohesion of the substrates and the clad stainless steel is closely related to the thickness of the diffusion layers in the substrates and the clad, and controlled by diffusion layer thickness in 304L side.

Oxidation behavior of in situ synthesized TiB/Ti-Al composite

The oxidation behavior of an in situ synthesized TiB reinforced Ti-Al-8(vol%)B composites (TMCs) and Ti-Al were studied at 823, 873, and 923 K in atmospheric air. The oxidation kinetics follows a parabolic rate law. The oxidation rate decreases gradually as the oxidation proceeds. Scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry, and transmission electron microscopy (TEM) were used to identify oxidation products and characterize oxide scale morphology. The oxide scale formed on TMCs was predominantly rutile TiO2 and α-Al2O3. No B2O3 and other oxides were observed within the oxide scale. The in situ synthesized TiB particles can increase the oxidation resistance of TMCs. It is attributed to the enhanced alumina-forming tendency, the formation of thin and dense oxidation, and the interface cohesion and the clean interfacial microstructure before and after oxidation between the reinforcements and the titanium matrix alloy.

Oxidation of in situ synthesized TiC particle-reinforced titanium matrix composites

The oxidation behavior of TiC particle reinforced titanium matrix composites (TMCs) was studied in a temperature range 550–650 °C in atmosphere. The in situ oxidation observation at the very initial stage was investigated by high-temperature optical microscopy in air. The oxide layer of long-term oxidation behavior was examined by scanning electron microscopy (SEM) combined with an energy dispersive X-ray spectroscopy unit (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The oxidation kinetics follows a parabolic rate law. The oxidation rate decreases gradually as the oxidation proceeds. The oxidation of the composite took place firstly on Ti because of the higher reactivity of Ti and O 2 than that of TiC and O 2. However, the TiC reinforcement can decrease the overall oxidation rate at 550, 600, and 650 °C. It is attributed to the formation of thin and dense oxidation, the enough strong interface cohesion between reinforcements and the titanium matrix alloy.

The Effect of Diverse Forming Conditions on the Morphology of Hydroxyapatite Spongy Bioceramics

The importance of ceramics in medicine has accelerated and become increasingly more sophisticated, in part due to advances in cell biology and tissue engineering. Improvements in ceramic processing technologies have also led to a great interest in bioceramics. Nowadays, bioceramics are used extensively to repair defective tissues and organs. This paper discusses the manufacturing, chemical, and physical characterization of three-dimensional porous scaffolds developed for the repair of skeletal defects in tissue-engineering applications. Our attention was focused on the manufacturing of porous bioceramic three-dimensional scaffolds that, while allowing loading transmission through the bulk material, improve interface cohesion between natural bone and a synthetic scaffold. We have tested different manufacturing processes for the mixing of the raw materials, a Ca-P ceramic and two different kinds of polymeric powders, namely, a cellulose-derived polymer and a polyamide, in terms of the final product’s morphological resemblance to spongy bone tissue. Scanning electron microscopy (SEM) was used to visualize the scaffold morphology, and pore size distribution was analyzed in a mercury intrusion porosimeter.

High-temperature OM investigation of the early stage of (TiC+TiB)/Ti oxidation

The initial oxidation behavior of titanium matrix composites (TMCs) was studied in a temperature range 550 to 650∘C in a flow of purified oxygen at atmospheric pressure using thermogravimetry. The oxidation kinetics very initially follows approximately a linear rate law and then a parabolic rate law. The oxidation rate decreases gradually as the oxidation proceeds. The initial in situ oxidation was investigated by high-temperature optical microscopy in air. The oxide layer was examined by X-ray diffraction and scanning electron microscopy combined with an energy dispersive X-ray spectroscopy unit. It was found that the reaction products are predominantly rutile. The reinforcements of TiB and TiC can result in a decrease in the overall oxidation rate at 550, 600, and 650∘C. This is attributed to the interface cohesion and the clean interfacial microstructure between reinforcements and the titanium matrix alloy, which is strong enough such that the reinforcements can act as barriers to solid-state diffusion.

Photoelectrochemical and Raman imaging studies of chemical and mechanical properties of thermally grown titanium oxide scales

The present work focuses on thin TiO2 scales (up to 3 μm). The use of both micro-photoelectrochemistry (MPEC) and Raman imaging aims to get chemical and mechanical information at a micron scale. In particular, the MPEC technique allows images of the history of strain and adhesion levels of the oxide/metal interface to be realised. A set of photocurrent images recorded at different applied potentials allows different images to be constructed, Structural Quality Image (SQI) and Interface Cohesion Image (ICI). In addition, Raman spectroscopy images the chemical nature of the scale, topography of the surface and local stress level in the scale. Combining both techniques is a powerful tool in elucidating the mechanisms of thermal oxidation.

The effect of interfacial properties on the cohesion of highly filled composite materials

It has long been recognized that the cohesion of composite materials, in low confinement, is strongly affected by the properties of the interfacial transition zone (ITZ) between inclusions and matrix. While the effect of the ITZ on the elasticity properties of composites has been studied by many authors in the context of linear homogenization methods, the upscaling of the cohesion strength of highly filled composite materials has not been addressed. This is the focus of the non-linear homogenization procedure developed in this paper, which is based on the separation of the heterogeneous material system in phases of constant strength properties, a non-linear elastic representation of the limit stress state in each phase, and the definition of appropriate effective strain quantities that capture the morphological features of the microstructure. Applied to a three phase composite model composed of rigid inclusion, interface zone and matrix, the model provides a quantitative means of studying the effect of the interface cohesion and the interface volume fraction on the composite cohesion. In particular, we identify a critical interface-to-matrix cohesion ratio, below which the composite cohesion is smaller than the one of the matrix. Furthermore, the model lends itself readily to the study of the degradation of the interfacial properties in composite materials. This is shown for non-degraded and chemically softened cement-based materials, for which we provide conclusive evidence (1) that the interface strength properties of mortar are far more affected by chemical degradation than the one of the cement paste matrix; and (2) that chemical degradation does affect the mechanical strength performance of the cement paste not only through a change of volume proportions (i.e. increase of porosity), but as well through a pure chemical softening of the solid’s cohesion.

Microscale wear of vitrified abrasive materials

The study of bonding hard materials such as aluminium oxide and cubic boron nitride (cBN) and the nature of interfacial cohesion between these materials and glass is very important from the perspective of high precision grinding. Vitrified grinding wheels are typically used to remove large volumes of metal and to produce components with very high tolerances. It is expected that the same grinding wheel be used for both rough and finish machining operations. Therefore, the grinding wheel, and in particular its bonding system, is expected to react differently to a variety of machining operations. In order to maintain the integrity of the grinding wheel, the bonding system that is used to hold abrasive grains in place will react differently to forces that are placed on individual bonding bridges. This paper examines the role of vitrification heat treatment on the development of strength between abrasive grains and bonding bridges, and the nature of fracture and wear in vitrified grinding wheels that are used for precision grinding applications.

Oxidation Behavior of <i>In Situ</i> Synthesized TiB/Ti Composite in Air Environment

Isothermal oxidation behavior of in situ synthesized TiB/Ti composites has been investigated. Samples of titanium matrix composites reinforced with 0, 5 and 8 vol% TiB particulates were oxidized at 823, 873, and 923 K for 300 h in atmospheric air. Scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry, and transmission electron microscopy (TEM) were used to identify oxidation products and characterize oxide scale morphology. Oxidation was observed to follow parabolic kinetics. Rate of oxidation decreased with the increase of TiB reinforcements. The oxide scale formed on TiB/Ti composites was rutile-type TiO2. No other oxides were observed within the oxide scale. The increased oxidation resistance due to the addition of the in situ synthesized TiB reinforcement was attributed to the improved tendency for the formation of thin and dense oxide avoiding crack and spallation, the strong enough interfacial cohesion and the clean interfacial microstructure between the reinforcements and the titanium matrix alloy.

界面酸化過程を考慮した界面結合モデルに基づく遮熱コーティングのはく離強度評価法の提案

Thermally grown oxide layer formation between ceramic top coating layer and metal bond coating layer in “Thermal Barrier Coating (TBC)” is known to cause reduction of interface cohesion. This cohesion reduction is practically associated with a serious damage such as coating spallation due to thermal cycling, prolonged isothermal exposure and thermo-mechanical fatigue. Interface crack mechanics have been used widely to evaluate such interface strength. In the mechanics, an ideal interface condition was considered, because of the difficulty of mathematic treatment for stress field solution. However, actual interface strength is predominated by the effect of interface oxidation growth by Al diffusion process and interface instinctive problem such as a wavy interface geometry.In this study, the interface cohesive model, which is based on a spring connection between the interface nodes in the top coating and the substrate, is developed for evaluating TBC interface delamination strength with taking into account coating instinctive characterization, such as thermally grown oxide formation. Application of finite element analysis included with the developed model to the TBC delamination test by four-point bending is shown. Also, delamination energy, which is defined by energy stored in the interface spring until interface fracture surface formation, is proposed newly as TBC delamination strength.