Creep effects in nanometer-scale contacts to viscoelastic materials: A status report

Effects of creep on the behavior of nanometer-scale contacts to viscoelasti c materials are described from the viewpoint of the contact mechanics theor y developed by Ting. The two most important effects are: (1) The time at wh ich maximum contact area and maximum deformation occur can be delayed subst antially from the time of maximum applied load. (2) The deformation at sepa ration is related to the loss tangent. These long-range effects due to cree p are distinct from the much shorter-range crack tip effects induced by adh esion at the periphery of the contact and associated with the names Barquin s and Maugis. Consideration of relevant time scales reveals that creep effe cts are expected to dominate in SFM-scale contacts for a wide range of comp liant viscoelastic materials. Guidelines For selection of optimal experimen tal parameters for nanometer-scale studies are presented. The need For a co mprehensive theory is emphasized.