W.P Vellinga

Reversible strain by physisorption in nanoporous gold

Reversible strain amplitudes up to 0.02% in response to a 15% change in relative humidity were detected in nanoporous gold. We show that the mechanism involved in dimensional changes during physisorption is associated with changes in the surface stress when molecules are adsorbed from the vapor phase onto the metal interface.

Adhesion along metal-polymer interfaces during plastic deformation

In this paper a numerical study is presented that concentrates on the influence of the interface roughness that develops during plastic deformation of a metal, on the work of adhesion and on the change of interface energy upon contact with a glassy polymer. The polymer coating is described with a constitutive law that mimics the behavior of Poly-Ethylene Terephthalate. It includes an elastic part, a yield stress, softening and hardening with increasing strains. For the interface between the metal and the polymer a mixed-mode (mode I and II) stress-separation law is applied that defines the interface energy and an interaction length scale. At the onset of deformation the surface of the substrate has a self-affine roughness characterized by the so-called Hurst exponent, a correlation length and an rms roughness amplitude, that evolves as a function of increasing strain. The findings are the following: the interface energy decreases until the strain at yield of the polymer coating. Interestingly, after yielding as the polymer starts to soften macroscopically, the decreasing average stress levels result in partial recovery of the interface energy at the interface. At higher strains, when macroscopic hardening develops the recovery of the interface stops and the interface energy decreases. The effect of coating thickness is discussed as well as the physical relevance of various model parameters.

Work of adhesion in laser-induced delamination along polymer-metal interfaces

Laser-induced delamination is a recent technique aimed at characterizing adhesive strength of thin polymer coatings on metal substrates. A laser pulse is used to create a blister that initiates further delamination of the film under pressure. To process the experimental data a simple elastic model was developed. The model predicts values for the blister height and pressure in fair agreement with the experimental findings. The critical stress state required for delamination and the work of adhesion are derived. To account for possible plastic deformation computer simulations using finite elements with a mixed mode cohesive zone were carried out. The polymer coating is described with a constitutive law that includes an elastic response, yielding and hardening with increasing strain. The stress fields calculated with finite element model are in agreement with those predicted by the elastic model and it suggests that the contribution of plastic deformation to the work of adhesion is rather limited. Comparativ...

Effects of tensile and compressive in-plane stress fields on adhesion in laser induced delamination experiments

In this work, the adhesion of a polymer coating on steel substrate subjected to uniaxial tensile plastic deformations was studied with the laser induced delamination technique. A decrease in the practical work of adhesion has been measured as the deformation of the substrate progressed. Moreover, it was observed that the deformation of the metal substrate introduced in-plane stresses in the polymer coating. Delamination was studied under compressive or tensile in-plane stresses. The model presented enabled to derive from the experimental data not only the practical work of adhesion but also the magnitude and the character (tensile or compressive) of the in-plane stress. The strain fields of the metal substrate during plastic deformation and after the elastic relaxation were monitored with a digital image correlation analysis system. The measured strain fields were confronted to the in-plane stresses obtained from the delamination experiments.

Direct measurement of intrinsic critical strain and internal strain in barrier films

Resistance measurements during uniaxial tensile deformation of very thin (10 nm) conducting oxide films deposited on 150 nm SiN films on polyethylene naphthalate are discussed. It is first shown that certain characteristics of resistance versus strain curves are representative for the fracture behavior of the SiN film and not for that of the thin conducting oxide film. Subsequently, it is shown that the hysteresis in curves of resistance as a function of strain offers a way to directly measure the intrinsic critical strain of the SiN film without the need to determine internal strains from independent (curvature) measurements that rely on knowledge of moduli and geometry. The method should be applicable, in general, to measure intrinsic critical strain and residual strains of thin brittle films on polymers. Advantages and limitations of the method are discussed.

Effect of relative humidity on crack propagation in barrier films for flexible electronics

A set of propagating cracks in a SiN barrier film on poly ethylene naphthalate (PEN) were subjected to differing levels of relative humidity. It was observed that the propagation speed of the cracks increased for increasing levels of relative humidity. This was shown using two independent, simultaneous techniques. One of the techniques (a resistance measurement) gives a qualitative measure of the averaged crack tip speed and the other (a microscopic technique) a quantitative measure. An attempt is made to quantify the resistance measurements in terms of crack tip speed. The effects that humidity may have on the crack driving force through differences in hygroscopic expansion are discussed, using independent determination of the diffusion constant of water into PEN. It is concluded that hygroscopic expansion alone cannot account for the observations.

In situ observations of crack propagation mechanisms along interfaces between confined polymer layers and glass

This paper concentrates on microscopic observations of the propagation of cracks along polymer-glass interfaces and crack propagation mechanisms. The experimental set-up consists of an asymmetric double cantilever beam in an optical microscope. Image processing techniques used to isolate the crack fronts are presented. The fronts propagate inhomogeneously in space and time, i.e., in bursts that spread laterally along the front over a certain distance. It is interesting to note that two different cases are detected; one in which crack propagation is dominated by initiation of instabilities on the front, and another one in which it is dominated by propagation of existing instabilities.

Local delamination on heavily deformed polymer-metal interfaces: evidence from microscopy

In this work the microstructure of interfaces present in heavily bi-axially deformed polymer-coated metal is studied. Cross sections of deformed polymer-coated steel are prepared using several polishing strategies, including the use of focused ion beam, and are imaged using optical microscopy and scanning electron microscopy. We find that the interfaces show significant details right down to the smallest scale observable with the preparation techniques used of about ~10xa0nm. Local delamination events at these deformed interfaces are observed and are found to be preferentially associated with overhanging parts on the interface. Overhanging parts are frequently observed, but only below a certain length-scale on the interfaces that are otherwise found to be self-affine up to a certain correlation length. The smallest detail includes the tail of the size distribution of the overhanging features. Together this suggests that the physical mechanisms determining the formation of critical features for adhesion operate at sub-grain level as well as at grain level.

In-Situ Observations on Crack Propagation Along Polymer/Glass Interfaces.

The propagation of crack fronts along a PET-glass interface is illustrated. The experimental set-up consists of an Asymmetric Double Cantilever Beam in an optical microscope. Image processing techniques used to isolate the crack fronts are discussed in some detail. The fronts are found to propagate inhomogeneously in space and time, in forward bursts that spread laterally along the front for some distance. In some cases the forward movement of a crack can be almost entirely due to the lateral movement of forward steps (analogous to kinks) along the crack front.

Practical work of adhesion of polymer coatings studied by laser induced delamination

Laser Induced Delamination is a novel technique aimed at measuring the practical work of adhesion of thin polymer coatings on metal substrates. In this technique a laser pulse is used to create initial blisters which initiate further delamination of the film under the blister pressure. A simple elastic model is developed to describe the formation of the blisters. The model predicts the values for the blister height and pressure, which are in fair agreement with the experimental results. In order to account for possible plastic deformations, simulations using a finite element model with a mixed mode cohesive zone were carried out. The calculated stress fields are in agreement with those predicted by the elastic model suggesting that the contribution of plastic deformation to the measured work of fracture is rather limited. Measurements are carried out on a number of samples, presenting ECCS steel substrate covered with 35 mu m thick polyethylene terephthalate (PET) film. The tensile stresses created in the film at the interface required for delamination are estimated at 7-8 MPa, which corresponds to the practical work of adhesion G = (0.6 +/- 0.1) J/m(2).