Dynamic contacts on viscoelastic films: Work of adhesion

Dynamic mechanical contacts with nanometer to millimeter dimensions are imp ortant in scanned probe microscopy, contact mechanics analysis, ultralow lo ad indentation, microelectromechanical systems, compact disks, biological s ystems, pressure sensitive adhesives and so forth. The response of these co ntacts is poorly understood if they involve adhesive viscoelastic materials . We have used indentation to study contacts to styrene-butadiene latex fil ms with a range of glass transition temperatures. Contact times were in the range 0.01-200 s and loads were in the micronewton to millinewton range. D iamond probes with Berkovich and spherical end shapes were used. Load versu s displacement data showed substantial adhesion hysteresis between the load ing and unloading portions. The hysteresis is at least partially due to cre ep as indicated by the continued increase in penetration after the start of unloading. We show that an extended Johnson-Kendall-Roberts (JKR) model du e to Johnson provides a robust way to extract works of adhesion from data o btained at low loading. The range of the interaction potential between the probe and substrate is also obtained from the fits. Because this model negl ects long-range creep effects, it breaks down at high loading rates.