This study addresses the problem of the measurement of the mechanical properties of thin films using contact mechanics methods. In a first stage, an analytical contact model recently developed by Perriot and Barthel [A. Perriot and Barthel, E. J. Mat. Res, 2004. 19(2): 600-608] is used to derive a first order approximation within the limits of confined geometries. Together with indentation experiments using polymer films on elastic substrates, this approach demonstrates the essentially oedometric nature of the coating's response, provided it is not to close to incompressibility. In a second stage, an extension of this approximate description to lateral contact loading allowed to relate the contact stiffness to the shear modulus of the film. This approach was successfully applied to the determination of the viscoelastic modulus of an acrylate polymer film in the glass transition zone of the film, with an emphasis on its sensitivity to hydrostatic pressure. This study suggests that lateral contact experiments are more appropriate than indentation ones for the measurement of film properties close to incompressibility.
This chapter addresses the problem of the mechanical properties of thin polymer films geometrically confined within contacts between elastic substrates. Analytical contact mechanics solutions for coated substrates are used to derive, within the limits of confined contacts situations, an approximate oedometric solution for the indentation of a thin film lying on a more rigid substrate. From a discussion of this approximate model, it is shown that lateral contact methods are an interesting alternative to indentation experiments for the measurement of the visco-elastic properties of polymer films in their glass transition range or rubbery state. As an example, the hydrostatic pressure dependence of the viscoelastic properties of confined polymer films is analyzed in the light of lateral contact stiffness measurements. The effects of hydrostatic pressure on the onset of plastic flow within confined polymer coatings are also discussed.
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