Considerable effort is aimed at using the Scanning Force Microscope (SFM) to measure the mechanical properties of surfaces with nanometer-scale resolution. The properties of interest include the Young and shear moduli, shear strength, and work of adhesion. The most widely used approach is to extract these properties from the SFM data by simply scaling the results of macroscopic continuum contact mechanics theory to the dimensions and forces of an SFM contact. This talk will focus on two aspects of this scaling problem@footnote 1@: (1) the mechanism for failure of a contact under shear and (2) the effect of creep on contacts to viscoelastic materials. In a contact subjected to a shear strain, contact mechanics predicts that a crack propagates at the interface and causes a non-linear increase in shear force until the interface ruptures and sliding begins. This behavior, called microslip, is observed for macroscopic contacts but not for SFM contacts, which suggests that the contact mechanics picture must be modified for nanometer-scale contacts. In contacts to viscoelastic materials, creep can significantly modify the formation and rupture of a contact compared to contacts to elastic materials. The most important effect is that the maximum contact area depends on the loading history and, unlike elastic materials, can reach its maximum value well after the maximum load is applied. The status of theoretical models for the analysis of contacts to linear viscoelastic solids including the effects of adhesion will be described.
@FootnoteText@
@footnote 1@K.J. Wahl, S.V Stepnowski, W.N. Unertl, Tribology Lett. (in press 1998).