AVS 49th International Symposium
    Nanometer Structures Monday Sessions
       Session NS-MoM

Paper NS-MoM7
Creep Compliance and Stress Intensity in Small Viscoelastic Contacts

Monday, November 4, 2002, 10:20 am, Room C-207

Session: Nanomechanics
Presenter: W.N. Unertl, University of Maine
Authors: W.N. Unertl, University of Maine
M. Giri, Hewlett-Packard - Corvallis
Correspondent: Click to Email
Adhesive contacts to viscoelastic materials with dimensions smaller than a few microns are difficult to analyze. This is due, in part, to the inability to measure the contact size directly. One consequence is the lack of a quantitative method to measure time-dependent mechanical properties. We demonstrate a method to overcome these difficulties. First, we extend a theory of viscoelastic contact@footnote 1@ to show how the contact radius, the stress intensity at the contact edge, and the creep compliance function can be extracted directly from load vs. deformation data. Then, we apply this analysis to load controlled indentation data for a paraboloidal diamond probe on a styrene-butyl acrylate substrate with 27 C glass transition temperature. The probe is brought into contact, the load is increased linearly to a predetermined maximum, and then decreased until the contact ruptures. Loads up to 3 mN result in deformations up to 2 mm in depth depending on the loading rate and contact time. Viscoelastic effects, indicated by the occurrence of maximum penetration after maximum load, were largest for contact times near 20 s. Calculated contact radii are up to 6 mm. The creep compliance for this material is described by a power law in time with exponent near 0.8. In contrast to predictions of simple fracture mechanics models, the stress intensity is not a unique function of the speed of the contact edge. This suggests either an interaction potential between the probe and polymer that is rate dependent or a polymer response that is non-linear under the conditions that occur at the contact periphery in these experiments. These results bring into question all previously reported nanoscale measurements of the mechanical properties of viscoelastic materials. @FootnoteText@ @footnote 1@C.Y Hui, J.M. Baney, and E.J. Kramer, Langmuir 14, 6570 (1998).