Dislocation-mediated Mechanisms of Mass Transport around Nanoindentations in Fcc Metals
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Dislocation-mediated Mechanisms of Mass Transport around Nanoindentations in Fcc Metals
Oscar Rodríguez de la Fuente, Esther Carrasco, Miguel A. González and Juan M. Rojo Departamento de Física de Materiales, Universidad Complutense, 28040 Madrid, SPAIN ABSTRACT
We present evidence for the operation on reconstructed Au(001) of a novel mechanism, involving dislocation motion, which is much more efficient than surface diffusion to redistribute mass around nanoindentations. Cross-slip of individual dislocations generated around the indentation point, with a screw component perpendicular to the surface, is shown to be responsible for the generation of multiple-storied, crystallographically-oriented terraces around the nanoindentation points. We also show that standard dislocation theory can be used to quantitatively describe the characteristics of the dislocations involved in the different processes around the nanoindentation.
INTRODUCTION
In recent years, nanoindentation has proved to be a technique capable of providing information about local mechanical properties of a surface [1] and, in certain occasions, even of identifying individual emission of dislocations near an indented surface [2]. Nevertheless, in connection with surface mechanical properties, there are a number of atomic processes −for example mass transport− still awaiting elucidation. In solid-state processes near surfaces, mass transport is usually assumed to be controlled by surface diffusion. In particular, diffusion is considered to be responsible for the mass redistribution resulting from the atoms dislodged around the contact region of a nanoindentation and, subsequently, redistributed in protruding mounds around the nanoindentation print. In this communication, we present evidence for the operation of a qualitatively different mechanism, involving dislocation generation and motion, which can be much more efficient than surface diffusion to redistribute mass around nanoindentations. Most of our study has been performed on reconstructed Au(001) surfaces but preliminary data on other surfaces suggest that this type of dislocation-mediated mass transportation is rather general. We also argue that standard dislocation theory in the continuum approximation adequately describes, even quantitatively, the characteristics of the dislocations involved in the different processes.
EXPERIMENTAL
The experiments were performed in an ultrahigh-vacuum chamber with a base pressure of 2×10-10 torr equipped with Auger electron spectroscopy (AES), low energy electron diffraction
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(LEED) and a homemade scanning-tunnel-microscope (STM). The clean Au(001) and Pt(001) samples were prepared by cycles of 600 eV Ar+ bombardment and high-temperature annealing. Nanoindentations were performed in the STM by letting tungsten tips progress towards the sample once the tunnelling current had been established. This procedure is done manually and with the feedback switched off. All the STM images shown in this paper were obtained in the constant current mode w
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