AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS1-TuM

Paper NS1-TuM9
The Formation and Evolution of Pileup in Nanoscale Contacts

Tuesday, October 26, 1999, 11:00 am, Room 612

Session: Nanomechanics
Presenter: K.F. Jarausch, North Carolina State University
Authors: K.F. Jarausch, North Carolina State University
J.D. Kiely, Sandia National Laboratories
J.E. Houston, Sandia National Laboratories
P.E. Russell, North Carolina State University
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The interfacial force microscope (IFM) was used to indent and image defect free Au <111> surfaces, providing atomic-scale experimental observations of the onset of pileup. The 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 of plasticity indicate that these slight load-relaxation events result in permanent deformations 0.25 to 4.0 nm deep with no evidence of pileup or orientation dependence. The second stage is marked by a series of dramatic load-relaxation events and permanent deformations 10-100+ nm deep. Images acquired following this second stage document 0.25 nm high pileup terraces which reflect the crystallography of the surface as well as the indenter geometry. Attempts to plastically displace the indenter 4-10 nm deep into the Au <111> surface were unsuccessful, demonstrating that the transition from stage one to stage two plasticity is associated with overcoming some sort of barrier. Stage one plasticity is consistent with previously reported models of dislocation nucleation. The dramatic load relaxations of stage two 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 insight into the nucleation, structure and evolution of the dislocations associated with the very early stages of plasticity. This work was supported by the United States Department of Energy under Sandia Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company.