Asymmetry in Multilayer Adhesion: the Zinc Oxide / Silver Interface
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Asymmetry in Multilayer Adhesion: the Zinc Oxide / Silver Interface E. Barthel1, P. Nael1, O. Kerjan1 and N. Nadaud2 1 Unité Mixte CNRS/Saint-Gobain « Surface du Verre et Interface », Saint-Gobain Recherche, F-93303, Aubervilliers, Cedex, France. 2 Saint-Gobain Recherche, F-93303, Aubervilliers, Cedex, France. ABSTRACT We have developed a wedge-loaded double-cantilever beam adhesion measurement set-up for thin films deposited on glass by magnetron sputtering. Results on the Glass/ZnO/Ag/ZnO multilayer evidence that the upper Ag/ZnO interface is tougher than the lower ZnO/Ag interface. This asymmetry in the adhesion energy between identical materials is not expected from first principles. It results from different non-equilibrium interfacial structures. INTRODUCTION Thin film multilayers deposited on glass are widely used for flat optical, photoelectric and electrochromic devices. Typical applications are Infra-Red filters against heat-generating solar radiations (solar control), solar cells for electrical power generation or voltage-controlled devices for tunable absorption in the visible light frequency range. In many applications, adhesion is a crucial issue. This is the case for instance when mechanical strength is required for further processing or for integration in complex systems. Scratch resistance is also a general concern either during process or during service. Due to the many features specific to films such as stoechiometry and residual stresses, it is necessary to measure the adhesion energy directly on the thin films themselves. For that purpose, we have developed a set-up to measure the adhesion energy of multilayer films in the tens of nanometer thickness range deposited by RF sputtering on thick glass substrates. In the present paper, sublayer/Ag/ZnO stacks are investigated, where the typical sublayer is ZnO, which has also been substituted by SnO2 and Si3N4. EXPERIMENTAL DETAILS Energy measurements - The Double Cantilever Beam test The literature on thin film adhesion energy measurements is wide [1,2] because many measurement set-ups are specific to a given film-substrate combination. The applicability of a given method will depend, among other parameters, upon the thickness of the layer, the respective mechanical properties of the layer and the substrate, the relevant crack velocity. For thin films, a number of experimental set-ups, which are easy to implement, such as the scratch test or the pull-out test, return qualitative rather than quantitative results. For more quantitative measurements, it is necessary to apply the mechanical stress with the help of a backing. Here, the backing is made out of glass: in our cleavage set-up, two glass plates, one of them bearing the multilayer, are glued together (Fig. 1). Cleavage of such a sandwich is a complex physico-mechanical process but provided rupture occurs within the multilayer, the cleavage energy, or joint toughness, will reflect the adhesion energy at the interface of rupture in some way.
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Figure 1. Typical sandwich specimen
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