Adhesion–Delamination Mechanics of a Prestressed Circular Film Adhered onto a Rigid Substrate

A thin circular film clamped at the periphery is adhered to the planar surface of a rigid cylindrical punch. An external tensile load is applied to the punch, causing the film to delaminate from the substrate and the circular contact edge to contract. The film spontaneously separates from the punch, or pulls off, when the contact radius reduces to a range between 0.1758 and 0.3651 of the film radius, depending on the magnitude of the residual membrane stress. The mechanical delamination process is derived by a thermodynamic energy balance based on a coupled interfacial adhesion and residual membrane stress. The theoretical model has significant implications in nanoforce measurement, microelectromechanical systems (MEMS) comprising active moveable films, and biological cell adhesion.     

Adherence of an Axisymmetric Flat Punch on a Thin Flexible Membrane

An equilibrium theory of adhesion between an axisymmetric flat punch and a thin flexible membrane fixed at its circumference is derived. When the pulling force on the punch exceeds a “pull-off” threshold, a spontaneous delamination occurs at a finite radius leading to a complete separation between the adherends. Experiments on a simple materials system supported the analysis.     

Adherence of an Axisymmetric Flat Punch on a Thin Flexible Membrane

An equilibrium theory of adhesion between an axisymmetric flat punch and a thin flexible membrane fixed at its circumference is derived. When the pulling force on the punch exceeds a “pull-off” threshold, a spontaneous delamination occurs at a finite radius leading to a complete separation between the adherends. Experiments on a simple materials system supported the analysis.     

Adhesive Contact of a Compliant Sphere to an Elastic Coated Substrate: The Thin Film Limit

Experimental results for adhesive contacts on substrates coated with elastomeric thin films have recently been obtained by Tardivat and Léger [1 Tardivat , C. and Léger L. , J. Adhesion Sci. Technol. 15 , 1055 – 1078 ( 2001 ).[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]] by the so-called Johnson-Kendall-Roberts (JKR) test, which provides both adhesion energy and elastic modulus. These data show that on substrates coated with thin films the adhesion and effective elastic modulus of the sphere depend upon the film thickness. In keeping with the experimental conditions, we try to interpret these data using a simple model [4 Barthel , E. and Perriot , A. , J. Phys. D: Appl. Phys. 40, 1059–1067 (2007).[Crossref] , [Google Scholar]] in the thin film limit, i.e., when the film thickness is small compared with the contact radius. Although the film does impact the local crack tip stress field, we show that no effect on the macroscopic contact variables is expected for the adhesion to coated substrates in such confined geometries. The deviations from the experimental results are ascribed to the idealized contact boundary conditions assumed in the model.     

Delamination Mechanics of a Clamped Rectangular Membrane in the Presence of Long-Range Intersurface Forces: Transition from JKR to DMT Limits

A 1-dimensional rectangular freestanding membrane clamped at opposite ends adheres to the planar surface of a rectangular punch. A tensile load applied to the punch causes the membrane to deform and gradually delaminate from the substrate. At equilibrium, the applied load is balanced by the disjoining pressure at the membrane-punch interface with range, y, and magnitude, p. Applying the Dugdale-Barenblatt-Maugis cohesive zone approximation, the disjoining pressure is taken to be uniform and confined to a finite cohesive length at the contact edge. For a fixed adhesion energy, γ = p y, we investigate the following: (i) the Derjaguin-Muller-Toporov (DMT) limit where y → ∞ and p → 0, (ii) the Johnson-Kendall-Roberts (JKR) limit where y → 0 and p → ∞, and (iii) the general case for intermediate but finite y and p. Delamination continues until the contact area shrinks to a line prior to “pinch-off”. The results are compared with the 2-dimensional axisymmetric membrane counterpart.     

Microstructural,mechanical and thermal shock properties of triple-layer TBCs with different thicknesses of bond coat and ceramic top coat deposited onto polyimide matrix composite

In this study, a triple-layer thermal barrier coating (TBC) of Cu-6Sn/NiCrAlY/YSZ was deposited onto a carbon-fiber reinforced polyimide matrix composite. Effects of different thicknesses of YSZ ceramic top coat and NiCrAlY intermediate layer on microstructural, mechanical and thermal shock properties of the coated samples were examined. The results revealed that the TBC systems with up to 300 µm top coat thicknesses have clean and adhesive coating/substrate interfaces whereas cracks exist along coating/substrate interface of the TBC system with 400 µm thick YSZ. Tensile adhesion test (TAT) indicated that adhesion strength values of the coated samples are inversely proportional to the ceramic top coat thickness. Contrarily, thermal shock resistance of the coated samples enhanced with increase in thickness of the ceramic coating. Investigation of the TBCs with different thicknesses of NiCrAlY and 300 µm thick YSZ layers revealed that the TBC system with 100 µm thick NiCrAlY layer exhibited the best adhesion strength and thermal shock resistance. It was inferred that thermal mismatch stresses and oxidation of the bond coats were the main factors causing failure in the thermal shock test.     

热处理对Ni-P镀层内应力及结合强度的影响

采用薄片弯曲法和压痕法分别研究了热处理温度对化学镀镍层的内应力以及镀层与基体的结合强度的影响规律。结果表明 :镀层的内应力为拉应力 ;当热处理温度为 2 0 0℃时 ,内应力减小 ,镀层经过 30 0、4 0 0或 6 0 0℃热处理 ,内应力逐步增大。随热处理温度升高 ,镀层与基体的结合力增大     

Investigating the change in surface acoustic wave velocity due to the change in adhesion of a SU-8 thin film using a SU-8/AlN/Si SAW sensor

This study investigates the change in surface acoustic wave (SAW) phase velocity due to the change in adhesion of a SU-8 thin film using an Aluminum Nitride (AlN)/Silicon (Si) SAW sensor. The variation in the stress field at the SU-8/AlN interface at different levels of SU-8 adhesion is also investigated and correlated to the change in SAW phase velocity. A theoretical model is developed to determine the change in wave dispersion profile due to the change in adhesion of the SU-8 film on the surface of the AlN/Si sensor. The interface of the SU-8/AlN layers is represented by a layer of massless springs with interface stiffness K (N/m³). The results illustrate that as the adhesion of the SU-8 film weakens on the surface of the AlN/Si SAW sensor the velocity dispersion profiles fluctuate. The calculated stress field at the SU-8/AlN interface also fluctuates because the weakened interface cannot sustain the increased stress due to the confinement of the wave near the interface. Therefore, the mode of wave propagation varies between the perfect bond case where the SU-8 layer is perfectly bonded to the AlN/Si surface and when the SU-8 is completely de-bonded. Four SAW sensor designs operating in the frequency range of 84–208 MHz are developed to measure the shift in the center frequency values and the corresponding change in SAW phase velocities due to the change in adhesion of the SU-8 layer. The adhesion of the SU-8 film on the surface of the AlN/Si SAW sensor is changed by using different adhesion layers. The adhesion layers used are a gold film and an omnicoat coated gold film. Omnicoat is an adhesion promoter used to improve the adhesion of SU-8. The results illustrate that as the adhesion of the SU-8 film improves for the sensors with omnicoat the wave velocity shifts to a lower value. The wave velocity values are fitted with the wave dispersion profiles for different values of the interface spring stiffness K. The interface spring stiffness values for the SAW sensors with omnicoat coated gold and gold films are 8.1×10⁹ N/m³ and 7.95×10⁹ N/m³, respectively. The use of omnicoat improves the adhesion of the SU-8 film and corresponds to a higher interface spring stiffness value. The stress and displacement fields generated in the SU-8 layer for different values of the interface spring stiffness K are plotted. The results illustrate that as the adhesion of the SU-8 layer improves the stress transmitted to the SU-8 layer increases and the wave is more confined in the SU-8 layer, which justifies the drop in wave velocity since the Rayleigh wave velocity in SU-8 is much lower in comparison to AlN and Silicon; 1166, 5600, 5000 m/s, respectively.     

A Method for Calculation of Final Film Thickness in Free Coating of Viscoelastic Fluids onto a Vertical Surface

A method for the calculation of the final film thickness in free coating of a viscoelastic fluid onto a vertical surface withdrawn from its vessel is developed. The method is based on the definition of an objective function, the minimization of which guarantees that the kinematic as well as the dynamic conditions at the lower boundary of the dynamic meniscus region are simultaneously satisfied. A systematic approach is provided in order to localize the optimum value of the final film thickness within the optimization interval. It was observed that there was a clear relationship between the estimated final film thickness and the value of a parameter A. The closeness of this parameter to zero corresponds to the dynamic constraint being fulfilled at the lower boundary of the dynamic meniscus region. This relationship is used as an objective means of determining the direction to update the interval of optimization to obtain the final thickness of the film. The results of the proposed method are compared to the previous works on the free coating of viscoelastic fluids, which are based on a trial‐and‐error method. The performance of the previously applied rheological models to the formulation of the free coating process, mainly the modified Oldroyd models, is also compared by introducing the present method.     

Closed Form Nonlinear Analysis of the Peninsula Blister Test

Previous work by Liechti and Shirani [1 Liechti , K. M. and Shirani , A. , Int. J. Fracture 67 , 21 – 36 ( 1994 ). [CSA] [Crossref] , [Google Scholar]] has shown that, among the family of blister tests, the peninsula blister test is the best because of low plastic deformation. This work further examines the peninsula blister test, derives new solutions, and accounts for residual stresses. An approximate nonlinear analysis of the peninsula blister based on the minimum potential energy method was developed for extracting the toughness of thin films bonded to stiff substrates. This analysis, which is easily applied to specimens with finite dimensions, was validated against an exact analytical solution for plane strain and a three-dimensional finite element analysis. Experiments with a film adhesive, Hysol EA 9696®, were conducted. Bulge tests were used to obtain the elastic properties of and the residual stresses in the film. The fracture experiments were used to check the solutions and determine the toughness of the bond between the adhesive and aluminum. In both experiments, in addition to the usual measurements of volume and pressure, the deflection of the specimen was measured using shadow moiré. This allowed the residual stresses to be determined and their effects on both membrane deflection and energy release rate to be examined.     

Field Dependence of the Spin Relaxation Within a Film of Iron Oxide Nanocrystals Formed via Electrophoretic Deposition

The thermal relaxation of macrospins in a strongly interacting thin film of spinel-phase iron oxide nanocrystals (NCs) is probed by vibrating sample magnetometry (VSM). Thin films are fabricated by depositing FeO/Fe₃O₄ core–shell NCs by electrophoretic deposition (EPD), followed by sintering at 400°C. Sintering transforms the core–shell structure to a uniform spinel phase, which effectively increases the magnetic moment per NC. Atomic force microscopy (AFM) confirms a large packing density and a reduced inter-particle separation in comparison with colloidal assemblies. At an applied field of 25 Oe, the superparamagnetic blocking temperature is T _B^SP ≈ 348 K, which is much larger than the Néel-Brown approximation of T _B^SP ≈ 210 K. The enhanced value of T _B^SP is attributed to strong dipole–dipole interactions and local exchange coupling between NCs. The field dependence of the blocking temperature, T _B^SP(H), is characterized by a monotonically decreasing function, which is in agreement with recent theoretical models of interacting macrospins.     

Influence of the Film Former on Fibre-Matrix Adhesion and Mechanical Properties of Glass-Fibre Reinforced Thermoplastics

The film former is in mass percentage the main component in most glass fibre sizes. Its influence on the fibre-matrix adhesion and the compound properties is studied in two thermoplastic matrices, i.e. polypropylene (PP) and rubber-modified styrene-co-maleic anhydride. The composition of the size is varied by changing the chemical nature and molecular mass of the film former and by leaving out the silane. The size's morphology on the glass fibres is studied with scanning electron microscopy and X-ray photoelectron spectroscopy. The effect on fibre-matrix adhesion is inferred from 0° bending and interlaminar shear strength measurements of unidirectional, continuous fibre reinforced composites. The possible effect of the film former on matrix crystallinity in PP is studied with differential scanning calorimetry and polarized-light microscopy. The same sizes are also tested in injection moulded, short fibre reinforced compounds by measuring their mechanical properties. In addition, the diffusion of the size into the matrix is studied by confocal scanning laser microscopy. The main conclusion of this study is that the film former plays a crucial role in the level of fibre-matrix adhesion because it does not diffuse away from the fibre-matrix interface into the matrix. Its effect may be larger than that of the silane. In addition, most mechanical properties of short fibre reinforced compounds improve with increasing fibre-matrix adhesion.