Mechanics of nanometer scale indentation of a metal surface

We investigate the mechanics of nanometer scale indentation into an atomically flat metallic surface. It is shown that continuum elastic contact mechanical models are able to describe the interaction well with the inclusion of a simple adhesion potential. This is due in part to the lack of any mechanical instability throughout the approach of indenter to sample. This absence arises from the unexpected long range and hence mild force gradients of the observed surface force. The plastic response of the system is found to lie in a new regime beyond that reported in other submicron scale mechanical studies. Topological hardening to the theoretical crystal strength due to the exclusion of dislocations in a localized stress field has previously been reported. Here we report a general hardening beyond the dislocation nucleation limit which we attribute to extreme stress localization, below the natural dislocation width length scale. Occasional early onset of non-elastic response is observed, but found to be inconsistent with simple dislocation creation on energy grounds. Experiments were performed in ultrahigh vacuum at liquid nitrogen temperatures. Indentation of sharp single crystal W(111) tips with atomic structure characterized in situ by field ion microscopy was performed on atomically flat terraces of Au(111) oriented thin films fixed to a cantilevered mount. Cantilever deflections were measured by a sensitive differential interferometer to determine forces