Study of Plastic Flow in Ultra--Small Au Contacts

Yielding properties of Au point contacts of nanometer-scale dimensions have been studied using a scanning tunneling microscope supplemented by a force sensor for measuring tip-sample forces. The contacts are made by indenting the tip typically 10~nm into the substrate, whereby an adhesion neck is formed. Three consecutive deformation phases of the neck can be identified during retraction of the tip: (1) build-up of tensile stress, (2) incomplete fracture, and (3) quasi-continuous plastic flow. Finally the neck breaks when a maximum of three to four atoms are left in the contact. In the plastic flow regime, the conductance and thus the contact area shrink exponentially with elongation of the neck. This provides strong evidence that plastic deformation occurs locally within 5 to 6 atomic layers. The latter is inferred from the decay constant. The applied stress during plastic flow is initially of the order of 10 GPa and gradually increases to ~20 GPa shortly before the neck breaks. From a fit to the data accounting for a surface force contribution, an intrinsic yield strength of the order of 6 GPa is obtained, which is more than one order of magnitude larger than the macroscopic yield strength of Au.