Ion-Induced Surface Modification of Alloys
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ION-INDUCED SURFACE MODIFICATION OF ALLOYS* H. WIEDERSICH Argonne National Laboratory,
9700 South Cass Avenue,
Argonne,
IL 60439
ABSTRACT In addition to the accumulation of the implanted species, a considerable number of processes can affect the composition of an alloy in the surface region during ion bombardment. Collisions of energetic ions with atoms of the alloy induce local rearrangement of atoms by displacements, replacement sequences and by spontaneous migration and recombination of defects within cascades. Point defects form clusters, voids, dislocation loops and networks. Preferential sputtering of elements changes the composition of the surface. At temperatures sufficient for thermal migration of point defects, radiation-enhanced diffusion promotes alloy component redistribution within and beyond the damage layer. Fluxes of interstitials and vacancies toward the surface and into the interior of the target induces fluxes of alloying elements leading to depthdependent compositional changes. Moreover, Gibbsian surface segregation may affect the preferential loss of alloy components by sputtering when the kinetics of equilibration of the surface composition becomes competitive with the sputtering rate. Temperature, time, current density and ion energy can be used to influence the individual processes contributing to compositional changes and, thus, produce a rich variety of composition profiles near surfaces.
INTRODUCTION Ion beam modification of alloy surfaces has become an active field of study during the past few years. The field has roots in several areas: ion implantation in semiconductors, radiation effects on the structure and properties of materials, effects of ion sputtering, and recognition of the potential to improve surface properties by ion beams. Recent conferences and books addressing these aspects of ion-solid interactions are cited in references [1-7]. The motivation for a large fraction of the work on alloys is the frequently beneficial effect of ion bombardment on technologically important properties and processes such as hardness, friction, wear, corrosion, catalysis, adhesion, and reflectance. Underlying these effects are ion-induced changes in microstructure, i.e., composition, phase distribution, crystal structure and defect microstructure, in the surface and the near surface regions of the material. It has become evident that a considerable number of distinct processes contribute to the evolution of the microstructure during ion bombardment, see e.g. [8]. Energetic ions cause rearrangement of atoms of the solid in displacement cascades, remove near surface atoms by sputtering and become incorporated into the material at the end of range. Disordering of atoms may transform the crystalline structure into an amorphous phase as is frequently observed in covalently bonded elements and compounds. Whereas most pure metals appear to remain crystalline during ion implantation even at low temperatures, alloys above a critical concentration are observed to become
*Work supported by the U.S.
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