Incipient Plasticity During Indentation of a Well Characterized 3 Nanometer Radius Tip

We present the results of nanoindentation testing of a well characterized tip geometry with a spatial scale easily matched by existing atomistic simulation. The atomically defined tungsten asperity of 3 nm radius was fabricated and imaged by field ion microscopy and brought into contact with a Au(111) terrace in ultra-high vacuum conditions. The mechanical evolution of the asperity contact under cyclic indentation testing was monitored by a simultaneous load-displacement and electrical current-displacement measurement. Load displacement curves of the pristine surface showed multiple discrete plastic (``pop-in'') events during loading, with energies consistent with the nucleation of individual defects. During unloading, we observe reverse plasticity and complete self-healing of the induced defect. Both the qualitatative behaviour and measured energy dissipation values are in agreement with recent molecular dynamics simulations of incipient plasticity in metallic asperity contacts.