Tribological Behavior of Very Thin Confined Films
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Tribological Behavior of Very Thin Confined Films ¨ M. H. Muser Institut f¨ur Physik, WA 331 Johannes Gutenberg-Universit¨at Mainz 55099 Mainz, Germany ABSTRACT The tribological properties of two smooth surfaces in the presence of a thin confined film are investigated with a generic model for the interaction between two surfaces and with computer simulations. It is shown that at large normal contact pressures, an ultra thin film automatically leads to static friction between two flat surfaces - even if the surfaces are incommensurate. Commensurability is nevertheless the key quantity to understand the tribological behavior of the contact. Qualitative differences between commensurate and incommensurate contacts remain even in the presence of a thin film. The differences mainly concern the thermal diffusion of the contact and the transition between smooth sliding and stick-slip. INTRODUCTION Understanding the dynamics of a system under shear containing a confined fluid is intimately connected with understanding the effects that are invoked through the corrugation of the confining walls. In the case of bare, flat surfaces, not only the degree of corrugation is relevant, but more importantly, the correlation of the corrugation in the upper wall and in the lower wall, i.e., commensurate systems exhibit wearless static friction while incommensurate systems do not [1-3]. This situation is dramatically changed if small amounts of “fluid” are injected into the interface and the fluid atoms do not form covalent bonds with neither surface [4]. Independent of the type of commensurability, static friction can be expected to occur. Here we will discuss why such mobile atoms in the interface lead to friction and address the question whether the differences between commensurate and incommensurate systems are remedied in the presence of a fluid layer. The studies discussed here address fundamental issues rather than questions of direct, practical use. How does a tiny amount of fluid/contamination between two perfectly flat, crystalline surfaces alter the tribological behavior of the contact and what are the implications of commensurability in the presence of a thin film? Experimentally, it might be impossible to study these effects satisfactoryly, because clean surfaces are hard to obtain even in UHV, e.g., the contaminant may reside within the bulk and diffuse to the surfaces only after sliding has been initiated. THEORY AND COMPUTER SIMULATION MODEL A simple, idealized model to treat interactions between two flat walls with only atomistic roughness allows a large number of predictions [5]. The main feature of the model is that two surfaces pay a local energy penalty that increases exponentially fast as the distance between the surfaces is decreased or the overlap is increased. The consequences of this model are among others: (i) Commensurate systems show a static friction coefficient s that is independent of the area of contact p Ac , (ii) for amorphous crystalline contacts s / 1= Ac is found, and (iii) for incommensurate
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