Practical Work of Adhesion of Polymer Coatings Studied by Laser Induced Delamination
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Practical Work of Adhesion of Polymer Coatings Studied by Laser Induced Delamination A. Fedorov, J. Th. M. De Hosson, R. van Tijum, W.-P. Vellinga Department of Applied Physics, Materials Science Centre and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ABSTRACT 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 µm 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/m2. INTRODUCTION Adhesion properties of polymer coatings on steel are of great interest in various industrial applications. However there are not so many experimental techniques present which are capable of providing quantitative characterization of the strength of the polymer-steel interface [1]. Especially problems arise if a coating has a good adhesion and during the testing procedure considerable plastic deformations are introduced [2]. In the present work polymer coatings on steel were studied with the Laser Induced Delamination technique [3-4]. 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 calculate the stresses in the film, blister pressure and practical work of adhesion. In order to validate the model the stress fields in the film were simulated with a finite element model (FEM) with a mixed mode cohesive zone. The comparison of two approaches is presented in this work. EXPERIMENTAL In the Laser Induced Delamination technique a coating under study is subjected to a series of laser pulses with stepwise increase of intensity. Every shot is carried out through a mask resulting in a formation of two parallel cylindrically shaped blisters as shown on the left in Figure 1. The strip of a film between the blisters is not exposed to the laser irradiation and is attached to the substrate in the beginning of each series. Upon increasing the laser pulse intensity, the pressure inside the blisters reaches the
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