Scanning Probe Microscopy Measurements of Friction
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Scanning Probe
Microscopy Measurements of Friction Scott S. Perry
Abstract This article describes the details of scanning probe microscopy measurements of interfacial friction from an experimental perspective. In such studies, the probe tip is taken as a model of a single asperity within a tribological contact, and interfacial forces are measured as a function of the sliding contact of the probe tip with the surface. With appropriate detection schemes, friction and load forces can be monitored simultaneously and used together to describe the frictional properties of the microscopic contact. This article provides a detailed description of the procedures and protocols of friction measurements performed with scanning probe microscopy, the relevant properties of probe tips, and the influence of environment on microscopic friction measurements. In addition, the article provides a brief overview of several categories of friction studies performed with scanning probe microscopy, highlighting the type of materials characterized in these studies as well as the importance and impact of the microscopic measurements. Keywords: atomic force microscopy, interfacial friction, scanning probe microscopy, tribology.
Background and Perspective Tribology is one of the many fields in which the use of scanning probe microscopy (SPM) has provided new insights at length scales not previously accessible. Tribology is the study of contacting surfaces in relative motion, and interfacial friction is of primary concern; wear, adhesion, and lubrication are also under investigation. Over the past 17 years, SPM measurements have assisted in the rejuvenation of a scientific interest in tribology by presenting a number of advantages over previous approaches. Historically, macroscopic tribological measurements of friction have involved the contact of engineering surfaces possessing surface roughness on the nanometer to micrometer scale. This roughness results in the formation of multi-asperity contacts and largely prevents the fundamental study of forces, as interfacial pressures vary throughout the contact region. Tribological studies performed with SPM offer the opportunity to study singleasperity contacts by simulating the contact
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region through the interaction of the probe tip with a surface of interest. Furthermore, the mechanical nature of the SPM detection scheme (cantilever deflection) is well suited to the measurement of the load and friction forces present within the interfacial contact. The precision of SPM also allows these forces to be controlled or monitored in such a way as to prevent undesired wear of the interface, thus allowing studies of friction in the absence of wear. Despite these advantages, fundamental SPM measurements of friction are not without limitations. These include uncertainty with respect to the exact area of contact, the general inability to analyze the composition of the probe tip, and the potential for changes in the tip composition or geometry (due to wear) during the course of measurements. Nonetheless,
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