Cracklike Processes within Frictional Motion: Is Slow Frictional Sliding Really a Slow Process?
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ckground The dynamics of frictional motion are critical to fields ranging from nanomachines1 to the study of earthquakes.2–7 Frictional motion involves a huge range of time and length scales,8–11 coupling the elastic fields of two blocks under stress to the dynamics of the myriad interlocking microscopic contacts that form the inter-
face defining their plane of separation. Despite the immense practical and fundamental importance of friction, the basic physics of the dynamics of frictional motion along rough, spatially extended interfaces is far from complete. The vast majority of the research performed on the dynamics of dry friction
MRS BULLETIN • VOLUME 33 • DECEMBER 2008 • www.mrs.org/bulletin
has focused on the macroscopic (overall) motion of two nominally rigid bodies in frictional contact. In this view, the bodies in motion are considered rigid, and the relevant spatial degrees of freedom are the center-of-mass coordinates of each body. The sliding velocities considered in these studies are normally quite slow (micrometers to millimeters per second). Herein, we review recent work on these processes in which the spatial degrees of freedom along a large contact plane (consisting of 106–107 discrete microcontacts) separating the two bodies are taken into account. These results demonstrate that families of rapid cracklike fronts sweep through the interface, described by the contact plane, and provide the underlying mechanism that gives rise to the transition to macroscopic frictional motion. Although the sliding velocities are quite low, the velocities of these cracklike fronts can be very high (sometimes approaching sound speeds of the sliding materials), so that short (microsecond to millisecond) time scales must be considered. This approach is quite new. We first present a brief overview of a number of key ideas and concepts. We then go on to describe how the short-time behavior of the spatial degrees of freedom along the contact surface can be measured, and we conclude with a description of the experimental results that show how the dynamics of these fronts drive the transition to frictional motion.
Macroscopic Descriptions of Friction and the Importance of the Net Contact Area The dynamics of friction are generally considered to be slow processes, where the overall macroscopic motion of two bodies in frictional contact is mainly considered. In typical experiments where frictional properties are studied, both a normal force, FN, and a shear force, FS, are applied to two blocks in contact. The sliding bodies are considered as rigid blocks that are characterized by spatially uniform parameters such as their overall sliding velocity (V), normal and shear forces, and friction coefficients (µ). In the traditional Amontons–Coulomb description of friction, friction coefficients are defined as the ratio FS/FN at the onset of motion (static friction) or during sustained sliding (dynamic friction). Although this description provides the basis for our phenomenal understanding of frictional dynamics, over the past two
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