Adhesion Problems Research Articles

Partial Constraint Singularities in Elastic Rods

We present a unified classical treatment of partially constrained elastic rods. Partial constraints often entail singularities in both shapes and reactions. Our approach encompasses both sleeve and adhesion problems, and provides simple and unambiguous derivations of counterintuitive results in the literature. Relationships between reaction forces and moments, geometry, and adhesion energies follow from the balance of energy during quasistatic motion. We also relate our approach to the balance of material momentum and the concept of a driving traction. The theory is generalizable and can be applied to a wide array of contact, adhesion, gripping, and locomotion problems.

Hyperbolic hemivariational inequalities controlled by evolution equations with application to adhesive contact model

A system which couples an abstract hemivariational inequality of hyperbolic type and an evolution equation in a Banach space is studied. The global existence of the system is established by exploiting the Rothe method. An application to a dynamic adhesive viscoelastic contact problem with friction is provided for which results on existence and regularity of weak solutions are proved.

An approximate JKR solution for a general contact, including rough contacts

In the present note, we suggest a simple closed form approximate solution to the adhesive contact problem under the so-called JKR regime. The derivation is based on generalizing the original JKR energetic derivation assuming calculation of the strain energy in adhesiveless contact, and unloading at constant contact area. The underlying assumption is that the contact area distributions are the same as under adhesiveless conditions (for an appropriately increased normal load), so that in general the stress intensity factors will not be exactly equal at all contact edges. The solution is simply that the indentation is δ=δ1−2wA′/P″ where w is surface energy, δ1 is the adhesiveless indentation, A′ is the first derivative of contact area and P′′ the second derivative of the load with respect to δ1. The solution only requires macroscopic quantities, and not very elaborate local distributions, and is exact in many configurations like axisymmetric contacts, but also sinusoidal waves contact and correctly predicts some features of an ideal asperity model used as a test case and not as a real description of a rough contact problem. The solution permits therefore an estimate of the full solution for elastic rough solids with Gaussian multiple scales of roughness, which so far was lacking, using known adhesiveless simple results. The result turns out to depend only on rms amplitude and slopes of the surface, and as in the fractal limit, slopes would grow without limit, tends to the adhesiveless result – although in this limit the JKR model is inappropriate. The solution would also go to adhesiveless result for large rms amplitude of roughness hrms, irrespective of the small scale details, and in agreement with common sense, well known experiments and previous models by the author.

Improving friction stir weldability of Al/Mg alloys via ultrasonically diminishing pin adhesion

Formation of intermetallic compounds (IMCs) during friction stir welding (FSW) of aluminum/magnesium (Al/Mg) alloys easily results in the pin adhesion and then deteriorates joint formation. The severe pin adhesion transformed the tapered-and-screwed pin into a tapered pin at a low welding speed of 30mm/min. The pin adhesion problem was solved with the help of ultrasonic. The weldability of Al/Mg alloys was significantly improved due to the good material flow induced by mechanical vibration and the fragments of the IMCs on the surface of a rotating pin caused by acoustic streaming, respectively. A sound joint with ultrasonic contained long Al/Mg interface joining length and complex mixture of Al/Mg alloys in the stir zone, thereby achieving perfect metallurgical bonding and mechanical interlocking. The ultrasonic could broaden process window and then improve tensile properties. The tensile strength of the Al/Mg joint with ultrasonic reached 115MPa.

SOLUTION OF ADHESIVE CONTACT PROBLEM ON THE BASIS OF THE KNOWN SOLUTION FOR NON-ADHESIVE ONE

The well-known procedure of reducing an adhesive contact problem to the corresponding non-adhesive one is generalized in this short communication to contacts with an arbitrary contact shape and arbitrary material properties (e.g. non homogeneous or gradient media). The only additional assumption is that the sequence of contact configurations in an adhesive contact should be exactly the same as that of contact configurations in a non-adhesive one. This assumption restricts the applicability of the present method. Nonetheless, the method can be applied to many classes of contact problems exactly and also be used for approximate analyses.

Improving Joint Formation and Tensile Properties of Dissimilar Friction Stir Welding of Aluminum and Magnesium Alloys by Solving the Pin Adhesion Problem

Friction stir welding (FSW), as a solid-state welding technology invented by TWI in 1991, has potential to join dissimilar Al/Mg alloys. In this study, the pin adhesion phenomenon affecting joint quality during FSW of 6061-T6 aluminum and AZ31B magnesium alloys was investigated. The adhesion phenomenon induced by higher heat input easily transformed the tapered-and-screwed pin into a tapered pin, which greatly reduced the tool’s ability to drive the plasticized materials and further deteriorated joint formation. Under the condition without the pin adhesion, the complex intercalated interlayer at the bottom of stir zone was beneficial to mechanical interlocking of Al/Mg alloys, improving tensile properties. However, the formation of intermetallic compounds was still the main reason of the joint fracture, significantly deteriorating tensile properties. Under the welding speed of 60 mm/min without the pin adhesion phenomenon, the maximum tensile strength of 107 MPa and elongation of 1.2% were achieved.

Inverse characterization of vegetable fibre-reinforced composites exposed to environmental degradation

The characterization of composite materials, particularly those reinforced with vegetable fibres, presents additional challenges, like the influence of the manufacturing process, fibre-matrix adhesion problems, progressive stiffness reduction or material degradation due to environmental elements (moisture, heat and UV) exposure.The present work studies the suitability of inverse characterization, based on modal properties, to determine the mechanical properties of jute fibre-reinforced composite plates, subjected to environmental exposure to simulate outdoors application conditions. With this method both the elastic and the damping properties of composite specimens are found, which highlights its great usefulness in the prediction of mechanical properties.It was found that this non-destructive method can better estimate material properties than the rule of mixtures, reflecting manufacturing, processing conditions and also the influence of moisture absorption and UV degradation on the mechanical properties of these natural fibre composites. This method is an important tool since it can be performed in the final parts where the results account for all external influences.

Self-Organized Growth of Crystallographic Macropores in Multicrystalline Zn by Nanoscale Sculpturing

The self-organized formation of crystallographically oriented macropores in multicrystalline Zn is presented in the present paper. These pores are obtained by pulsed-potential electrochemical etching in an aqueous KCl electrolyte. They exhibit the typical characteristics of crystallographically-oriented pores like growth along certain crystallographic directions, formation of pore domains, branching of pores, and intersections of pores, as observed for e.g. III-V semiconductors such as InP. The pore walls of the pores in Zn are entirely composed of metallic Zn unlike the amorphous metal oxide ones observed in other metals like Al (AAO) or Ti (TiO2). The main pore growth direction in Zn is along <0001>. The branching of side pores occurs along [] and [] in 90° angles from the root pore forming a pore geometry reminding remotely on a monkey puzzle tree. Because of their crystallographic nature of the pores they grow under various angles into the multicrystalline Zn surface depending on the underlying crystal orientation. Like this they form a perfectly suitable structure for mechanical interlocking with polymers. When filled with polymer, the root pores together with the branched side pores act as ideal hooks anchoring the polymer in Zn solving the well-known problem of poor adhesion of polymers and paint on Zn surfaces.

Two-Dimensional Adhesive Contact Problem for Graded Coating Based on a New Multi-Layer Model

In this paper, the contact problem with adhesion for graded coating indented by a cylindrical indenter is considered. A multi-layered model is developed to model functionally graded material (FGM) coating with arbitrarily varying elastic modulus. By using the transfer matrix method and Fourier integral transform technique, the problem is reduced to a Cauchy singular integral equation. The numerical calculation is conducted to achieve the effect of the gradient index of the FGM coating and the adhesive parameter on the pull-off force and the relation between the normal load and the maximum indentation depth. The results show that the gradient index of coating can effectively change the behavior of adhesive contact.

UV-laser cleaning and surface characterization of an aerospace carbon fibre reinforced polymer

In the recent aerospace generations, composite materials have shown that they can fulfil the increasing demand for structural efficiency. In this context adhesive bonding could be used to max out the material properties of carbon fibre reinforced polymers (CFRP). However, investigations have shown that residues of release agents on the CFRP surface can cause adhesion problems. Thus, a reliable surface cleaning procedure for removing residues prior to bonding is necessary.Analytical surface investigations of an Aerospace CFRP material revealed the thickness and morphology of the predominant adhesion-weakening contamination. The optical absorptions of the epoxy material and the release agent characterised by UV–vis measurements were considered to develop a cleaning model based on a ns-pulsed ultra-violet laser with 266nm wavelength. This removal mechanism leads to chipping and peeling of the release agent layer and could be supported with light microscopy, scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) investigations. Single lap shear tests of the CFRP samples in the pristine and contaminated state, pre-treated with UV-laser as well as with the reference process (grinding) show that the laser pre-treated samples lead to the highest joint strength in the study. The failure mode changed from adhesion failure in the pristine state to a failure in the fibre-resin interface after laser treatment. Comparing a laser parameter with the focus on a smooth cleaning process and a laser parameter that lead to a surface with exposed carbon fibres, the surfaces treated with the smooth laser parameter showed the higher joint strength. The measured joint strength was 100% higher than with grinding, which underlines the high potential of laser technology for CFRP surface pre-treatment.

Wet chemical synthesis of nanostructured semiconductor layers for thin-film solar thermoelectric generator

The work is devoted to the creation of the nanostructured bismuth sulfide (Bi2S3), copper iodide (CuI) and lead sulfide (PbS) semiconductor layers for thin-film solar thermoelectric generator by means of wet chemical synthesis, namely, via Chemical Bath Deposition (CBD) or via Successive Ionic Layer Adsorption and Reaction (SILAR). Ways to overcome the problems of poor adhesion of Bi2S3 layers to glass, mica and polyimide substrates and to remove restrictions deal with low thickness limits for Bi2S3 and PbS have been shown. Optical properties of the smooth and continuous ∼1.0–1.2μm thick Bi2S3, CuI and PbS films researched, which were deposited on the solid glass surfaces and on the flexible mica or polyimide substrates. The obtained films have polycrystalline structures of the corresponding bulk semiconductors with grain sizes of several tens of nanometers. Studies of their temperature dependent resistivities ρ, Seebeck coefficients S and power factors P=S2/ρ are depicted, which confirmed that the obtained thin-film materials are promising for the solar heat transforming into electricity.

Structural and dielectric properties of hot-pressed poly(vinylidene fluoride)-based composites

The characterization of ceramic/polymer composites was performed on zinc oxide/poly(vinylidene fluoride) – ZnO/PVDF and barium titanate/poly(vinylidene fluoride) composites with varying filler concentration in order to evaluate the main interactions responsible for the composite dielectric behavior. The materials, poly(vinylidene fluoride) and its composites, were melt-blended using a two-roller mixer and then hot-pressed. The permittivity of composites was enhanced compared with that of the pure poly(vinylidene fluoride) with the addition of 20 w/w% of ZnO particles. However, samples with 40 or 60 w/w% of ZnO registered a diminution in the real permittivity values which was connected to particle-matrix adhesion problems. On the other hand, barium titanate composites presented a more homogeneous morphology with less presence of voids and a better adhesion between the filler and the polymer, where real permittivity increased with the addition of barium titanate particles.

Experimental and numerical investigation of nanoparticle releasing in AFM nanomanipulation using high voltage electrostatic forces

Atomic Force Microscopes (AFMs) have been widely used as nanomanipulators due to their versatility to work with a broad range of materials and their controllable interaction force, among other features. While AFMs can effectively grasp, move, and position nanoscale objects in 2D environments through basic pull/push operations, they often lack the high precision required in many 3D pick and place applications, especially in non-vacuum environments. In this study, a novel method to resolve the adhesion problem between nanoscale objects and the AFM tip has been developed and tested. The method is based on the application of a high electrostatic voltage to the tip to produce the repulsive force required for the release of the nanoobject. The method is proposed for conductive nanoparticles and tips used in many nanomanipulation applications, and can be easily implemented on typical AFMs with minimal alterations. The applicability of the proposed method is investigated through a series of combined Molecular Dynamics/Finite Element simulations.

A first insight on the bio-functionalization mechanisms of TiO2 nanotubes with calcium, phosphorous and zinc by reverse polarization anodization

The decoration of titanium (Ti) implant surfaces with TiO2 nanotubes has emerged as a promising strategy to improve osseointegration and avoid infection. Nevertheless, it has been reported that nanotubular films are prone to peeling off from the Ti substrate due to the poor interfacial adhesion. The knowledge on the interfacial properties of such interface, although not well explored, is crucial for understanding the mechanisms behind the poor adhesion problem of these films and to further achieve an easy and effective solution to solve it.This paper is focused on the bio-functionalization of TiO2 nanotubular films with zinc (Zn) as an antimicrobial and bone healing agent, together with two major components of bone matrix, namely calcium (Ca) and phosphorous (P). The main aim is, for the first time, the thorough characterization of the interface between TiO2 nanotubes and the Ti substrate, along with the better understanding of the bio-functionalization mechanisms of TiO2 nanotubes and their influence on the interfacial features of the films.TiO2 nanotubes were successfully synthesized by two-step anodization and their bio-functionalization with Ca, P and Zn was achieved by reverse polarization anodization treatments. The in-depth characterization of the morphological and chemical features of TiO2 nanotubes was carried out along their length by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS), before and after bio-functionalization treatments. STEM images showed that the interface between conventional TiO2 nanotubes and Ti is non-continuous due to the existence of a hollow space. However, bio-functionalized TiO2 nanotubes evidenced an interface with different features, due to the formation of an interfacial oxide film as a consequence of anodization, with a thickness comprised between 230 and 250nm.The results presented in this work may inspire the emergence of novel surface treatment strategies seeking the long-term performance of metallic-modified osseointegrated implants.

Cylindrical lateral depth-sensing indentation of anisotropic elastic tissues: Effects of adhesion and incompressibility

ABSTRACTA two-dimensional frictionless adhesive contact problem for a parabolic indenter pressed against an orthotropic elastic layer resting on a smooth rigid substrate is studied in the framework of the Johnson–Kendall–Roberts (JKR) theory. In the case of a relatively small contact zone with respect to the layer thickness, the fourth-order asymptotic solution (up to terms of order ) is obtained, and the pull-off force is expanded in terms of the non-dimensional measure of the work of adhesion. In particular, a pinch/compression method for soft tissue is considered, and the testing methodology is suggested based on a least-squares best fit of the first-order asymptotic model to the depth-sensing indentation data for recovering two of the three independent elastic moduli which characterize an incompressible transversely isotropic material. The case of a weakly compressible material, which is important for biological tissues, is also discussed. The developed asymptotic model can be effectively used for small values of a certain dimensionless parameter, which is proportional to the work of adhesion and the indenter radius squared, on the one side, and inversely proportional to the effective elastic modulus and the elastic layer thickness cubed, on the other.

Back plate electroplating for high aspect ratio processes

PurposeIn this process the electrical contact is brought to the backside of a standard silicon wafer. The details of the entire process are disclosed, from the photolithography processes to the electrodepositing step, and a model for electrical contact was designed.Design/methodology/approachThe localized Cu growth of high aspect ratio (AR) microstructures was obtained through an SU-8 photolithography by exploiting the optimal adhesion on the silicon surface and the possibility of generating thick layers with a single spun processFindingsThe experimental results showed an unexpected behaviour that is theoretically explained in detail considering the energy band theory. The obtained geometries showed a remarkable 6:1 AR without any adhesion problem. The non-invasive front-side manipulation represents a noteworthy improvement and simplification for the design of a multi-step production process.Originality/valueAn alternative technological approach, called back plate electroplating, has been carried out to obtain Cu growth on the front side of a standard n-type Si wafer through a back side electrical contact. This technique was then applied to fabricate a master for hot-embossing in a LIGA (Lithographie, Galvanoformung, Abformung)-like process flow. For this purpose, an SU-8 thick mask on a standard n-doped wafer was used. Finally, by using this process, it was possible to obtain high AR Cu geometries, avoiding any complex designing and patterning of the contacts on the front side and thus ensuring good adhesion of the SU-8.

Reoperations and postoperative complications after osteosynthesis of phalangeal fractures: a retrospective cohort study

Objectives: The aim of the study was to describe the reoperation rates and postoperative complications associated with different methods of osteosynthesis in all extra-articular, closed fractures of the proximal and middle phalanges operated on in the Department of Hand Surgery at Södersjukhuset beween 2010–2014, and to describe the associated patient demographics.Patients and methods: This study included all the relevant operations, which comprised operations on 181 fractures in 159 patients (82 male, 77 female), median and mean age = 43 (range = 14–88 years). The clinical records and radiographs were examined retrospectively. A logistic regression analysis was performed on the reoperation data to determine which method of osteosynthesis was the most important descriptor for reoperation, and whether the fracture type was a significant confounder.Results: Forty-seven patients (26%) were reoperated on, mainly due to finger stiffness. The reoperation rates were 25% after K-wire, 15% after screws, and 42% after plate fixation. The unadjusted reoperation rate after plate fixation was significantly higher than for the other methods, but not after adjustment for fracture complexity. The proximal phalanx was affected in 88% of the fractures, and 77% were located in the fourth or fifth finger. Falls, animal-related, and sports injuries were the most frequent causes of injuries.Conclusion: Open reduction with plate fixation was associated with a higher reoperation rate, but this method was also used for the more complex fractures. Plate fixation for phalangeal fractures often entails a need for later tenolysis and plate removal. More aggressive mobilisation regimes might be indicated to prevent adhesion problems.

Effect of Multilayered Structure on Properties of Ti/TiN Coating

Metal nitride hard coatings, such as TiN and TiAlN are widely used to protect materials because of their higher hardness and wear properties. However, these coatings always contain a high degree of internal stress which could arouse adhesion problems. It is hard to synthesize monolayer TiN or TiAlN coatings thicker than 10 μm by PVD (physical vapor deposition). The multilayer composite structure, offering efficient means of controlling residual stress, is a successful way to synthesize thicker hard coatings. In the present study, a series of multilayer composite Ti/TiN coatings with different composite periods were synthesized by plasma enhanced ion plating and the effects of multilayered structure mechanical properties of the coating was studied. The result shows that the mechanical properties of the coating are strengthened with increasing the periods of composite Ti/TiN layer. The micro hardness (HV0.25) can reach to about 27 500 MPa and better toughness and higher thickness (≥50 μm) are obtained. The toughness of the coating is almost directly proportional to hardness. Meanwhile, the tribological performance of the coating is improved with a lower dry friction coefficient (about 0.35) and higher wear resistance by alternating 48 periods of Ti/TiN layer. However, the bonding strength of the coating is weakened with further increasing the period of Ti/TiN layer due to the weak interface of coating and substrate.

Measurement of nano particle adhesion by atomic force microscopy using probability theory based analysis

We present a method to detect adhesive forces of nano particles by analysis of the distribution of measured lateral forces. The measurement protocol is suitable for all types of atomic force microscopes with a lateral force channel. Lateral forces are measured, in constant normal force contact mode, by scanning of substrates decorated with nano beads. By using probability theory, geometry based measurement errors are compensated and the real adhesion force is determined within a given confidence interval. The theoretical model can be adapted for particles with arbitrary shape and distribution of adhesion forces. It is applied to the adhesion problem of spherical particles with a Gaussian distribution of adhesion forces. We analyze the measured force distribution qualitatively and quantitatively. The theory predicts a systematic underestimation of the mean value of any particle adhesion measurement done by lateral pushing. Real measurement data of 50 nm diameter silica nano beads on silicon substrate is used to test the theoretical model for plausibility by means of information theory.

Numerical method for the adhesive normal contact analysis based on a Dugdale approximation

Modeling adhesion between two contacting surfaces plays a vital role in nano-tribology. However, providing analytical models, although desirable, is mostly impossible, in particular for complex geometries. Therefore, much attention has to be paid to numerical modeling of this phenomenon. Based on the adhesive stress description of the Maugis-Dugdale model of adhesion, which is credible over a broad range of engineering applications, an extended Conjugate Gradient Method (CGM) has been developed for adhesive contact problems. To examine the accuracy of the proposed method, the common case of the adhesive contact of a rigid sphere on an elastic half-space is investigated. To further evaluate the accuracy of this method, the adhesive contact of a rigid sphere over a wavy elastic half-space is also studied for different combinations of the amplitude and wavelength. There is good agreement between the analytical solution and the values predicted by the proposed method in the force-approach curves. Moreover, the calculation of pull-off force at a bisinusoidal interface between two surfaces is carried out for various cases to study the effects of different influential parameters including work of adhesion, elastic modulus, radius curvature at a crest, and the wavelength ratio. A curve is fitted on the calculated pull-off force in order to express it as an analytical relation. Similar to the JKR and DMT expressions for the pull-off force of a rigid ball on an elastic half-plane, the fitted curve is not affected by the elastic modulus and is linearly dependent on the radius of curvature and the work of adhesion. In addition, a power law governs the relation between pull-off force and the wavelength ratio. In the end, it is shown that roughness can either increase or decrease the adhesive force at a rough interface depending on the degree of the roughness.