Friction and wear of Langmuir-Blodgett films observed by friction force microscopy

Abstract

Lubrication is documented on a microscopic scale with a friction force microscope. A reduction in friction is observed for Langmuir-BIodgett film-covered surfaces, compared to the bare substrates. Film defects not detected in the topographic mode are clearly recorded in the friction force mode. With applied forces over 10 nN, the initial stages of wear are observed. Small islands of bilayer height are moved in their entirety, conserving the normal orientation of the aliphatic chains. This collective motion of molecules allows the shear strength of the films to be determined. The observed ability of the molecules to remain in the ordered state illustrates one of the fundamental origins of boundary lubrication.PACS numbers: 62.20.-x In tribology Langmuir-BIodgett films are used as model systems for boundary lubrication. Friction on metals is reduced by a factor of 10 and wear by a factor of 10000 or more [1,2]. Scanning probe microscopes, such as the atomic force microscope (AFM) [3,4] and the friction force microscope (FFM) [5], offer the opportunity to examine phenomena such as friction and wear on a local scale.The first observation of friction with a force microscope [6] showed that frictional forces between a tungsten tip and graphite vary with the atomic periodicity of the underlying graphite surface. Frictional forces were found to increase linearly with increasing load and a friction coeflficient of 0.01 was determined. The contact region was interpreted to be several thousands of atoms, producing atomic scale features from a certain degree of commensurability between the tip and sample. Recently, the anisotropy of frictional forces between muscovite mica sheets was investigated by FFM and surface force apparatus (SFA) [7]. This anisotropy was also attributed to the commensurability between the two contacting surfaces. In this study a frictionless state was postulated for the limit of complete incommensurability. In order to understand the origins of friction and wear on the nanometer scale, several models based upon ab initio calculations [8] and molecular dynamics calculations [9] have been introduced. These initial computational approaches have produced values for friction in agreement with the above FFM measurements on graphite [8]. They have also modeled the phenomenon of plastic deformation of surfaces under the AFM probe [9]. However, the particular case of boundary lubrication as probed by the AFM and FFM has yet to be described on a fundamental theoretical basis.AFM measurements on Langmuir-BIodgett (LB) films have shown that molecular resolution can be achieved [10,11]. From both force-distance curves [12] and the atomic scale imaging, it is concluded that the contact region is of the order of several square nanometers. The subject of this paper is to study lubrication on a microscopic scale and to examine the initial stages of wear on Langmuir-BIodgett films as they arise in this submicron regime.The friction studies are carried out in the following manner: One-and two-bilayer films of Cd arac...

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