Mechanisms and Kinetics of Iion Beam-Induced Compositional Modifications
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MECHANISMS AND KINETICS OF ION BEAM-INDUCED COMPOSITIONAL MODIFICATIONS* N. Q. LAM Materials Science Division, Argonne National Laboratory, Argonne,
IL
60439
ABSTRACT Near-surface
compositional
modification
of
ion-bombarded
alloys
results from the dynamic interplay of several atomistic processes. In addition to displacement mixing leading to t randomization of atomic locations, which is dominant at relatively low temperatures, and preferential loss of alloying elements by sputtering, many thermally-activated processes, including radiation-enhanced diffusion, radiation-induced segregation and Gibbsian adsorption, also play important roles. The relative contributions of these processes to the evolution of the target composition profile depends on the target materials and irradiation variables. Although a good understanding of the individual processes has been achieved, information regarding their synergistic effects on alloy surface modification is still limited. In the present article, these processes will be characterized in simple physical terms, and the present understanding of their relative significance and contributions in changing the target composition during ion bombardment will be discussed in view of recent progress in theoretical modeling and experimental study. INTRODUCTION The field of ion beam-induced compositional modifications has created considerable scientific excitement in recent years. The main driving force behind the various detailed investigations of this phenomenon is the vital importance of surface modifications in many areas of materials science and technology, ranging from tribological applications (hardness, friction and wear, etc.) to chemical and physical effects (catalysis, corrosion, oxidation, adhesion and reflectance, etc.). The current interest in this field is also motivated by routine applications of ion beam etching, sputter cleaning, and especially, ion sputtering in conjunction with a number of chemical analysis techniques to depth-profile the compositions of multicomponent materials and by great concern about plasma contamination by sputtered particles from walls and limiters. All these applications involve the interactions of energetic ions with atoms of the solid, which give rise to atomic ejections and rearrangements, leading to surface erosion, microstructural changes and spatial redistribution of the target composition. These alterations can be beneficial in certain cases, but should be minimized or suppressed in other cases. A number of physical processes are known to contribute to the evolution of the composition and microstructure in the near-surface region of solids during ion bombardment. The relative importance of these processes in the development of the compositional profile of the target depends on the target materials variables and bombardment conditions. For example, during ion sputtering near room temperature, the surface compositions of numerous alloy systems are changed by preferential sputtering (PS) and displacement mixing (DM) [1-5]. As the tem
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