Deformation of elastic coatings in adhesive contacts with spherical probes

This study addresses the problem of adhesive contacts between layered substrates and axisymmetric probes. A semi-analytical approach to adhesive contacts has been developed as an extension of a model recently published by Perriot and Barthel (2004 J. Mat. Res. 19 600-8) for the axisymmetric elastic indentation of non-adhesive, coated substrates. In addition to the load and penetration at equilibrium, the model allowed the derivation of the shape of the free surface in the contact zone. The validity of the approach was verified from experiments using contacts between acrylate films above their glass transition temperature (T-g) and spherical glass lenses. When the adhesive contacts were quenched below T-g, stable imprints were obtained which allowed determination of the surface deformations of the films. The latter were found consistent with the hypothesis of short range surface forces which were embedded in the contact model. Deviations from theory in the form of fingering instabilities at the periphery of the contact were observed when the confinement of the film was increased. A calculation of the stresses within the adhesive contacts indicated that these instabilities are probably driven by the release of lateral constraints within the confined films.