Atomic-Scale Simulation of Adhesion Between Metallic Surfaces
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ATOMIC-SCALE SIMULATION OF ADHESION BETWEEN METALLIC SURFACES PAUL A. TAYLOR Sandia National Laboratories,
P.O. Box 5800, Albuquerque,
NM87185
ABSTRACT We have performed MD simulations of adhesive phenomena, on an atomic scale, between metals possessing both smooth and stepped-surfaces. Studies of adhesion between identical metals, consisting of either Au, Cu, or Ni, with (001) or (111) orientations, reveal the existence of adhesive avalanches as the bodies are brought to within a critical separation (-2 A). That is, as the surfaces approach one another, one or both surface layers becomes unstable, and abruptly moves towards the other. This signals a transition from an initial system with two distinct surfaces to one possessing no identifiable surfaces. The presence of adhesive avalanches will pose difficulties in determining adhesive forces and energies by means of atomic force microscopy at sub-nanometer separations of probe tip and sample surface. INTRODUCTION The study of adhesion between metals, semiconductors, glasses, etc., is becoming increasingly more urgent as technological advances rely upon the ability to fabricate devices on smaller length scales (e.g., nanostructures). Phenomena such as friction and wear involve the formation and destruction of adhesive interfaces. Unfortunately, since adhesion involves the formation of a buried interface, it is difficult to study experimentally. Current stateof-the-art experimental techniques to study adhesion have relied primarily on atomic force microscopy (AFM) [1]. Basically, AFM involves the movement of an atomic-size probe tip over a sample surface to determine a constant force contour, reflecting, in some sense, on the topology of the sample surface. The advantage of AFM over related techniques, such as scanning tunneling microscopy, is that the tip and sample need not be conductors, and thus, a wider range of applications is possible. AFM has more recently been applied to the study of adhesive forces between various materials from metals to insulators [2-5]. These works have employed AFM to basically measure the interfacial force between an AFM probe tip and sample surface as a function of interfacial separation. Integration of the experimentally measured force data from infinite separation could, in principle, allow the adhesive energy to be determined as a function of the separation. Thus, this technique would appear to be a useful quantitative tool for study and analysis of surface and interfacial energetics. To date, we have performed atomistic simulations of adhesive phenomena between smooth and stepped-surfaces of identical metals of either Au, Cu, or Ni with (001) or (111) orientations. In all cases, the presence of adhesive avalanches were predicted. First coined by Smith et al [6], an "adhesive avalanche" is predicted whenever strongly interacting surfaces approach one another, where one or both of the surface layers becomes unstable, signaling an abrupt transition from an initial system consisting of two distinct structures with interacting surfa
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