Analysis of the Elastic Modulus of a Thin Polymer Film
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Analysis of the elastic modulus of a thin polymer film S. Roche, S. Bec, J.L. Loubet* Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513, 36, avenue Guy de Collongue, 69134 Ecully, FRANCE ABSTRACT This paper discusses the interaction between mechanical properties of a thin layer and those of its substrate. The underlined questions are: how the substrate’s mechanical properties affect those of the film; how the layer's thickness affects its mechanical properties. Nanoindentation tests were performed on polymer films of different thicknesses deposited onto a silica substrate. The mechanical response of such a system is known to be a composite response (film+substrate). Our aim is to understand the elastic behavior of the film inside this structure. Model was used to estimate the elastic modulus of the film from the global measured value. For thin and "compliant" polymer layers deposited on "hard" substrates, an increase of the film elastic modulus was observed along indentation. We attribute this increase to the "anvil effect": the bulk elastic modulus increases with the hydrostatic pressure which results from the compression of the film. In conclusion, one first interpretation is proposed to understand specific behavior of thin soft polymer layers on rigid substrates. INTRODUCTION Thin polymer coatings are increasingly used in multiple applications, such as automotive clearcoats or optical disks topcoats. The determination of their mechanical properties is a crucial point. One convenient way to measure them is to use the technique of instrumented nanoindentation coupled with a model to take into account the influence of the substrate's mechanical properties. The study dealt with different polymer coatings formulations of different thicknesses deposited onto different substrates. This article presents only the results obtained for one formulation on one sodocalcic glass as substrate. It focuses on the effect observed for thin polymer films on hard substrate: the "anvil effect". One first interpretation of the phenomenon and its consequences are presented. EXPERIMENTAL Samples The studied material was a U.V. photoreticulated polymer film deposited onto sodocalcic glass as substrate. The polymer film is composed mainly of two acrylate polymers (aliphatic urethane acrylate and tetrahydrofurfuryl acrylate). An adhesion promoter (monofunctional acidester) and two photo-initiators are added. Before deposition, the substrate was cleaned with acetone. Two film's thicknesses were studied 18.4 µm and 7.5 µm.
*[email protected] Downloaded from https://www.cambridge.org/core. UCSD University of California San Diego, on 30 May 2020 at 06:58:49, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-778-U4.8
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Experimental techniques Nanoindentation tests Nanoindentation tests were performed with a MTS XP Nano Indenter® using the continuous stiffness measurement method [1-3] to determine the film +
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