Publications
The Onset of Pileup in Nanometer-Scale Contacts
The interfacial force microscope (IFM) was used to indent and image defect free Au(111) surfaces, providing atomic-scale observations of the onset of pileup and the excursion of material above the initial surface plane. Images and load-displacement measurements demonstrate that elastic accommodation of an indenter is followed by two stages of plasticity. The initial stage is identified by slight deviations of the load-displacement relationship from the predicted elastic response. Images acquired after indentations showing only this first stage indicate that these slight load relaxation events result in residual indentations 0.5 to 4 nm deep with no evidence of pileup or surface orientation dependence. The second stage of plasticity is marked by a series of dramatic load relaxation events and residual indentations tens of nanometers deep. Images acquired following this second stage document 0.25 nm pileup terraces which reflect the crystallography of the surface as well as the indenter geometry. Attempts to plastically displace the indenter 4-10 nanometers deep into the Au(111) surface were unsuccessful, demonstrating that the transition from stage I to stage H plasticity is associated with overcoming some sort of barrier. Stage I is consistent with previously reported models of dislocation nucleation. The dramatic load relaxations of stage II plasticity, and the pileup of material above the surface, require cross-slip and appear to reflect a dynamic process leading to dislocation intersection with the surface. The IFM measurements reported here offer new insights into the mechanisms underlying the very early stages of plasticity and the formation of pileup.