Imperfect Adhesion Research Articles

Analysis of the cause of imperfect adhesion of cataphoretic coating using SEM and EDS method

Analysis of the cause of imperfect adhesion of cataphoretic coating using SEM and EDS method

Titanium Boston keratoprosthesis with corneal cell adhesive and bactericidal dual coating

The Boston keratoprosthesis (BKPro) is a medical device used to restore vision in complicated cases of corneal blindness. This device is composed by a front plate of polymethylmethacrylate (PMMA) and a backplate usually made of titanium (Ti). Ti is an excellent biomaterial with numerous applications, although there are not many studies that address its interaction with ocular cells. In this regard, despite the good retention rates of the BKPro, two main complications compromise patients' vision and the viability of the prosthesis: imperfect adhesion of the corneal tissue to the upside of the backplate and infections. Thus, in this work, two topographies (smooth and rough) were generated on Ti samples and tested with or without functionalization with a dual peptide platform. This molecule consists of a branched structure that links two peptide moieties to address the main complications associated with BKPro: the well-known RGD peptide in its cyclic version (cRGD) as cell pro-adherent motif and the first 11 residues of lactoferrin (LF1–11) as antibacterial motif. Samples were physicochemically characterized, and their biological response was evaluated in vitro with human corneal keratocytes (HCKs) and against the gram-negative bacterial strain Pseudomonas aeruginosa. The physicochemical characterization allowed to verify the functionalization in a qualitative and quantitative manner. A higher amount of peptide was anchored to the rough surfaces. The studies performed using HCKs showed increased long-term proliferation on the functionalized samples. Gene expression was affected by topography and peptide functionalization. Roughness promoted α-smooth muscle actin (α-SMA) overexpression, and the coating notably increased the expression of extracellular matrix components (ECM). Such changes may favour the development of unwanted fibrosis, and thus, corneal haze. In contrast, the combination of the coating with a rough topography decreased the expression of α-SMA and ECM components, which would be desirable for the long-term success of the prosthesis. Regarding the antibacterial activity, the functionalized smooth and rough surfaces promoted the death of bacteria, as well as a perturbation in their wall definition and cellular morphology. Bacterial killing values were 58 % for smooth functionalised and 68 % for rough functionalised samples. In summary, this study suggests that the use of the dual peptide platform with cRGD and LF1-11 could be a good strategy to improve the in vitro and in vivo performance of the rough topography used in the commercial BKPro.

Exploiting interface patterning for adhesion control

Surface force-mediated adhesion, e.g. via van der Waals forces, is critical for direct bonding of bulk solids in the absence of chemical bonding or an adhesive layer. However, no two surfaces are ideally flat or perfectly conformal due to surface roughness or intentional patterning. When adhered, non-zero surface tractions arise wherever the local separation of the interface differs from its intrinsic equilibrium separation, inducing regions of tensile and compressive stresses. A fundamental understanding of such imperfect adhesion is important and unlocks opportunities to control interfacial strength and toughness in various applications including MEMS/NEMS, micro-transfer printing, and processes to manufacture advanced 3D integrated-circuits. This paper investigates the fundamentals of direct adhesion of non-conformal surfaces by examining the interplay of the (i) intrinsic traction-separation relation (TSR), (ii) interface topography, and (iii) elastic properties of the adhered bodies. The TSR adopted accounts for strong repulsion when the interface separation is less than the equilibrium separation, and only the normal surface tractions are considered. The effective TSR properties, including the overall adhesion strength and work of separation, are determined from numerical calculations. Simple closed-form solutions are obtained for rigid bulk solids. A finite element model utilizing cohesive elements in a periodic cell is constructed to study cases with non-uniform deformation in the elastic solids.

Effective elastic properties of layered composites under imperfect adhesion

Effective elastic properties of layered composites under imperfect adhesion

Constructing Yield Loci for Rigid-Plastic Reinforced Plates Considering the 2D Stress State in Fibers

Models of unidirectionally and orthogonally fiber-reinforced hybrid media of a regular periodic structure allowing one to determine the yield loci for these composites are constructed. Phase materials of the compositions are isotropic, and their mechanical behavior is described by the associated flow rule for rigid-plastic bodies with the Tresca yield condition. A plane stress state is assumed in the composition and all its components, and fibers are oriented along the directions of principal stresses in the composite body. The cases of rectangular and arbitrary cross sections of reinforcing fibers are considered. Analytical expressions for equations describing the regimes of yield conditions are obtained. Different passages to the limit of a homogeneous medium are investigated. Yield loci calculated by the models proposed and the widely used structural model with a one-dimensional stress state in fibers are compared. Yield loci in the case of imperfect adhesion between the reinforcement and binder material are constructed.

Эффективные упругие свойства стохастических однонаправленных волокнистых композитов при несовершенной адгезии

Эффективные упругие свойства стохастических однонаправленных волокнистых композитов при несовершенной адгезии

Effective elastic properties of layered composites considering non-uniform imperfect adhesion

• Analytical expressions for elastic interlayer properties of layered composites. • Two-layer model considered with non-uniform imperfect adhesion. • The three-layer model provides an excellent approximation of the two-layer model. • Effective properties for layered composites by numerical/analytical models. In this study, the effective elastic properties of layered composites are predicted by considering the influence of non-uniform imperfect adhesion. In practical terms, the influence of non-uniform imperfection on the effective properties of layers is evaluated when debonding or delamination occurs between adjacent layers. Thus, the effective properties of two- and three-layer composites with imperfect and perfect interfaces are calculated using the finite element method and an analytical technique based on the two-scale asymptotic homogenization method, respectively. In the analyses, it was used a two-layer model with imperfect interface conditions and a three-layer model with perfect conditions to derive analytical expressions of the interlayer properties, which are equivalent to the effect of the imperfection (e.g., debonding). Our numerical results demonstrate that the three-layer model provides an excellent approximation of the two-layer one and it illustrates how the effective properties of the layered composites made from titanium Ti6Al4V and ceramic AD-96 are affected by non-uniform imperfect adhesion.

Эффективные упругие свойства зернистых стохастических композитов при несовершенной адгезии

Эффективные упругие свойства зернистых стохастических композитов при несовершенной адгезии

Analysis of mechanical and electrical imperfect contacts in piezoelectric composites

The problem of a heterogeneous piezoelectric composite with mechanical and electrical imperfect contact conditions is studied by means of the asymptotic homogenization method. As an illustrative example, in order to provide a benchmark for the numerical validation of theoretical models, a one-dimensional two-phase periodic laminate piezoelectric composite is considered with imperfect adhesion. Numerical examples show the effect of interface imperfections on the effective properties. Reductions in the effective Young’s modulus, the piezoelectric coupling and the dielectric permittivity under the effect of the imperfect contact conditions are reported herein.

Interface Adhesion and Structural Characterization of Rolled-up GaAs/In0.2Ga0.8As Multilayer Tubes by Coherent Phonon Spectroscopy

We present a detailed experimental and theoretical study of the acoustic phonon modes in rolled-up multilayers with thickness of the layers in the nanometre and diameters in the micrometre range. We compare our results to planar, unrolled multilayers grown by molecular beam epitaxy. For the planar multilayers the experimentally obtained acoustic modes exhibit properties of a superlattice and match well to calculations obtained by the Rytov model. The rolled-up superlattice tubes show intriguing differences compared to the planar structures which can be attributed to the imperfect adhesion between individual tube windings. A transfer matrix method including a massless spring accounting for the imperfect adhesion between the layers yields good agreement between experiment and calculations for up to five windings. Areas with sufficient mechanical coupling between all windings can be distinguished by their acoustic mode spectrum from areas where individual windings are only partially in contact. This allows the spatially resolved characterization of individual tubes with micrometre spatial resolution where areas with varying interface adhesion can be identified.

Analysis of fibrous elastic composites with nonuniform imperfect adhesion

In most composites, the fiber–matrix adhesion is imperfect; the continuity conditions for stresses and displacements are not satisfied. In this contribution, effective elastic moduli are obtained by means of the asymptotic homogenization method (AHM), for three-phase fibrous composites (matrix/mesophase/fiber) with parallelogram periodic cell. Interaction between fiber and matrix is considered, and this is called the mesophase model where the nonuniform mesophase is studied. Besides, there is another type of matrix–fiber contact which is called nonuniform spring imperfect contact. In this case, the contrast or jump of the displacements in the boundary of each phase is proportional to the corresponding component of the tension in the interface in terms of a parameter given by a certain function that depends on the position. The constituents of the composites exhibit transversely isotropic properties. A doubly periodic parallelogram array of cylindrical inclusions under longitudinal shear is considered. The three-phase model is validated by the Finite Element Method and the AHM both approaches applied to two-phase composites with nonuniform spring imperfect contact. Comparisons with theoretical and experimental results verified that the present model is efficient for the analysis of composites with presence of nonuniform imperfect interface and parallelogram cell. The effect of the nonuniform imperfection on the shear effective property is observed. The present method can provide benchmark results for other numerical and approximate methods.

Carbon nanotubes as non-contact optical strain sensors in smart skins

Progress is reported in the development of a novel non-contact strain measurement technology in which the sensors are single-walled carbon nanotubes. This approach exploits the characteristic short-wave infrared fluorescence signatures of semiconducting single-walled carbon nanotubes and the systematic shifts of their fluorescence wavelengths when the nanotubes are axially strained. A strain-sensing smart skin is prepared by coating the surface to be monitored with a thin film of a composite containing well-dispersed single-walled carbon nanotubes embedded in a urethane varnish host. Strain in the surface transfers through the host polymer to the embedded nanotubes. The nanotube strains are then quantitatively monitored by exciting the region of interest with a visible laser beam and capturing and analyzing the resulting single-walled carbon nanotube fluorescence spectrum. High-quality spectra are shown for strain-sensing smart skin films in which the nanotubes were pre-processed by selective extraction with the organic polymer poly(9,9-dioctylfluorenyl-2,7-diyl) and purification by centrifugation. The design of a compact, field-portable optical system for exciting, collecting, and interpreting strain-sensing smart skin spectral data is also described. This system is used to validate the linear dependence of peak shifts with strain in test specimens subjected to multiple tension cycles inducing strains from 0 to 5000 µε. Adequate linearity is found up to 1500 µε; however, minor hysteresis is observed above this level. The gage length for these measurements can be as small as several micrometers, and the strain resolution is currently between 10 and 100 µε (depending on strain magnitude). Cyclic reversible strain measurements are demonstrated, with minor hysteresis being attributed to imperfect adhesion between nanotube sidewalls and the surrounding polymer host.

Anti-corrosion behavior of layer by layer coatings of cross-linked chitosan and poly(vinyl butyral) on carbon steel

Anti-corrosion coatings of chitosan were fabricated by layer-by-layer (lbl) addition of chitosan (Ch) and poly vinyl butyral (PVB) on mild carbon steel substrate. Corrosion inhibition of chitosan in lbl coatings was analyzed by electrochemical, spectroscopic and morphological measurements. Sandwiching of chitosan between two hydrophobic PVB layers enhanced its bonding strength and enabled the amine and hydroxyl groups of chitosan to chelate with iron ions that led to the formation of a chitosan stabilized iron oxide passive layer on the metal surface. Effect of chitosan crosslinking on the stabilization process of passive oxide layer as well as the coating stability was studied by incorporating glutaraldehyde in the chitosan layer. The results obtained from electrochemical impedance spectroscopy and Tafel plots clearly showed a superior corrosion protection of lbl coatings generated by 1 % glutaraldehyde incorporation in the middle chitosan layer. SEM and Raman spectra confirmed the formation of passive oxide layer on the metal surface stabilized by chitosan polymer backbone. Improvement in the anti-corrosion properties of PVB_Ch/1 %Glu_PVB coatings were not observed after the incorporation of fillers like graphene and vermiculite in the chitosan layer probably due to the imperfect adhesion of the fillers with the polymeric matrix.

A Predictive Model for Tensile Strength Estimation of Syntactic Foams

A predictive model is developed to characterize the strength of syntactic foams by analyzing the state of stress around the inclusions with imperfect adhesion with the matrix. Using a three-phase model with imperfect interface between the inclusion and the matrix, the state of stress around the particle is determined. Employing the concept of stress concentration factor, the failure strength of syntactic foam is computed by considering the increase in stress concentration with volume fraction and the degree of interface bonding degradation. The interface effects are represented using a process parameter, and a method to estimate the same from the elastic behavior of the composites is demonstrated. The theoretical predictions are compared with the test data available in the literature.

Computation of tensile strength of syntactic foams

An analytical model is developed to characterise the strength of syntactic foams (hollow spherical inclusions typically embedded in a polymeric matrix) by analysing the state of stress around the inclusions with imperfect adhesion with the matrix. Using a three-phase model with imperfect interface between the inclusion and the matrix, the state of stress around the particle is determined. Employing the concept of stress concentration factor, a predictive model for computing the failure strength of syntactic foam is presented by considering the increase in stress concentration with volume fraction and the degree of interface bonding degradation. The theoretical predictions are compared with the test data available in the literature for hollow glass beads dispersed in polymeric matrices. The predicted strength values correlate well with the test data.

STRAIN-INDUCED CRYSTALLIZATION AND MECHANICAL PROPERTIES OF NBR COMPOSITES WITH CARBON NANOTUBE AND CARBON BLACK

Abstract The mechanical properties and strain-induced crystallization (SIC) of elastomeric composites were investigated as functions of the extension ratio (λ), multiwalled carbon nanotube (CNT) content, and carbon black (CB) content. The tensile strength and modulus gradually increase with increasing CNT content when compared with the matrix and the filled rubbers with same amount of CB. Both properties of rubber with CB and CNT show the magnitude of each CNT and CB component following the Pythagorean Theorem. The ratio of tensile modulus is much higher than that of tensile strength because of the CNT shape/orientation and an imperfect adhesion between CNT and rubber. The tensile strength and modulus of the composite with a CNT content of 9 phr increases up to 31% and 91%, respectively, compared with the matrix. Differential scanning calorimetry (DSC) analysis reveals that the degree of SIC increases with an increase in CNT content. Mechanical properties have a linear relation with the latent heat of crystallization (LHc), depending on the CNT content. As the extension ratio increases, the glass-transition temperature (Tg) of the composite increases for CB- and CNT-reinforced cases. However, the LHc has a maximum of λ = 1.5 for the CNT-reinforced case, which relates to a CNT shape and an imperfect adhesion with rubber. Based on these results, the reinforcing mechanisms of CNT and CB are discussed.

Effects of imperfect adhesion on thermal micro-residual stresses in polymer matrix composites

This paper investigates the influences of imperfect bonding between the fiber and matrix on thermal micro-residual stress fields in polymer matrix composites. For this purpose, a representative volume element consisting of a three-phase composite material subjected to a uniform temperature change is considered. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. Besides, a finite element model is developed and the results are compared with the analytical solution. Both the energy method and finite element analysis show similar trend for thermal stress distribution along the fiber length, while due to the stress singularity, the interfacial shear stress from the finite element solution cannot satisfy the stress-free condition at the fiber end. The analysis shows that the magnitude of thermal stresses and their distribution mainly depend on the bonding efficiency parameter. An increase in thermal and elastic properties bonding efficiencies leads to a considerable decrease in composite axial and shear residual stresses, while the Poisson's ratio bonding efficiency does not affect the thermal stress fields. The interfacial radial residual stress distribution is approximately independent of the bonding conditions. Inefficient bonding may result in higher residual stresses in comparison with the perfect bonding condition. It means that in cases of low bonding efficiency conditions, the ability of composites to sustain and transmit load decreases drastically. Thermal stress concentration occurs at the vicinity of the fiber ends, although peak values depend on the bonding efficiency value.

Thermal expansivities in fibrous composites incorporating hybrid interphase regions

The aim of this paper is the prediction of coefficients of thermal expansion in unidirectional fiber-reinforced composites. The representative volume element is a three phase composite structure subjected to a uniform temperature change. The advanced hybrid interphase concept is introduced, in which the interphase thickness depends on the property under consideration. This model involves also imperfect adhesion by immediate softening of material properties. Equations for the prediction of coefficients of thermal expansion are presented. Results are illustrated and discussed in terms of fiber volume fraction and adhesion coefficient. To validate the accuracy of these results finite element analysis has also been utilized. Predictions of coefficients of thermal expansion are in good agreement with experimental, finite element analysis and previous published results. The coefficients of thermal expansion of the considered polymer matrix composites are affected significantly by the parameters characterizing interphase.

Stress analysis of short fiber-reinforced polymers incorporating a hybrid interphase region

The finite element method is used to investigate the load-carrying characteristics and the stress profiles in unidirectional short fiber polymeric composites. The micromechanics composite model incorporates a hybrid interphase region surrounding the fiber. The new interphase concept involves different domains of interaction between different material properties depending on the corresponding properties of primary constituents, volumetric composition and macroscopic characterization of the composite. Thus, the hybrid interphase is not defined within a unique thickness, its outer radius being a function of the material property under consideration. The model describes also imperfect adhesion conditions by immediate softening of material properties. Numerical results are illustrated and discussed for a variety of the involved parameters. These results referring mainly to hoop and radial stresses along the fiber length confirm the intense variation of stress in this area. Parametric studies have been conducted accounting for different degrees of adhesion and shapes of the fiber tip and where it is possible comparisons with existing theories and experimental results taken from literature are illustrated.

Strength Modeling of Two-component Hybrid Fiber Composites in case of Simultaneous Fiber Failures

The strength of unidirectional short fiber hybrid structures and composites is strongly influenced by the length of the constituent and reinforcing elementary fibers. Based on our earlier fiber bundle-theoretical results, we have developed a simple statistical model and determined the strength of hybrid structures for elementary fibers, which simultaneously fail or debond from the environment in a brittle manner, and the dependence of the strength on the length of the elementary fibers. In the case of constant elementary fiber length, a simple analytical relation is obtained between the average tensile strength and the length of the elementary fiber. Based on these results, formulae have been developed to estimate the strength of unidirectional short hybrid fiber reinforced composites as a function of fiber length and fiber content. Practical applicability of the results has been demonstrated for basalt fiber hybrid composites taking into account fiber fragmentation, imperfect interfacial adhesion, and fiber orientation by using suitable correction factors.