Damaging of a Soft Polymeric Substrate by Crack Propagation Through Its Hard Coating
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DAMAGING OF A SOFT POLYMERIC SUBSTRATE BY CRACK PROPAGATION THROUGH ITS HARD COATING M. George, J. Colin, C. Coupeau, J.W. Grilhé Dpt of materials sciences, LMP-SP2MI, University of Poitiers, France. 1. Introduction. With the increasing number of applications (optical, magnetic or electronic devices, biomaterials) for covered materials, understanding and controlling the damaging and the rupture of thin films and coating are now of importance for scientific and technologic improvement. Several authors [1-7] have applied mechanics of rupture to bi-materials to model the propagation of cracks in thin films. From finite element modelisation (FEM), they have discussed about the possible propagation in the film, at the interface and in the substrate. However, physical experimental data are lacking to understand the cracking mechanisms in proportion of the thin film size. Most observations have been made by optical microscopy which allows a global approach of the cracking (critical stresses determination for instance). More recently in situ scanning electron microscopy observations of thin films under tensile stresses have been performed [8,9]. With this technique, more details on propagation of cracks at the surface can be obtained. The interactions between parallel cracks propagating in the films have been thus evidenced and a saturation density of cracks characterized. Neither optical nor electronic microscopy can yet deal with the tri-dimensional aspect of the cracking phenomena. To precise the mechanisms of damaging in thin films, a tensile test coupled with an atomic force microscope (AFM) have thus been developed at the LMP (Poitiers, France). In this paper, the first results obtained on the damaging of a polymide substrate coated with a nickel thin film are given and discussed with the help of FEM. 2. Apparatus and experimental procedure The observations are made on “dog-bone” tensile specimens of polyimide on which nickel films have been deposited at room temperature by ion beam sputtering. The dimensions of the useful part are 7.8×3×0.125 mm3. Deposits have been realized with a starting pressure of 2.10-8 torr; a maintained pressure of 10-4 torr is applied during the deposition process. The deposition has been assisted by an Ar beam during the first 75 deposited nanometers to improve the adhesion of the films on their substrates [10]. The films thickness has been controlled by exsitu measurement on steps realized on silicon during the deposition. The measured thickness has been found to be in agreement with the expected value of 100 nm. The films sustain biaxial compressive stresses that have been measured by curvature method [11] to be around 190 Mpa. Tensile tests have been performed with an apparatus “Microtest” developed by Deben (UK) which is particularly suited to in situ observations. Samples are mounted horizontally and clamped to a pair of jaws. Moreover, a dual threaded screw drives the jaws symmetrically in opposite directions, with an adjustable speed, keeping the sample centered in the field
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