Thermally Activated Friction

Professor W. Gregory Sawyer
Mechanical & Aerospace Engineering
University of Florida

There are a number of applications where operation in a temperature range from 200 to 400 K or larger is required for device success.  These extreme conditions are often the motivation for variable temperature studies in tribology, but there is a paucity of relevant tribology data available for temperatures below 273 K.  In the range from 300 K to 400 K the friction coefficient of various solid lubricants is shown to increase with decreasing temperature.  It is well known that many solid lubricant films transfer and adhere strongly to the counterface, and a modern hypothesis is that both friction and wear of these films are dominated by the interactions of interfacial sliding at weak self-mated interfaces.  Recent work by our group found that friction of polytetrafluoroethylene matrix composites continued to increase in the cryogenic regime down to 200 K, and the notion of a thermally activated friction coefficient was proposed (analysis of an activation energy gave Ea=3.7 kJ/mol).  A recent molecular scale study of graphite used an atomic force microscope to collect friction data on molecularly smooth terraces of over a temperature range from 140 K to 750 K at a vacuum level of 2x10-10 torr.  The friction coefficient again increased with decreasing temperature, and the data collected followed an Arrhenius dependence with an activation energy of Ea = 9.6 kJ/mol.  These molecular scale experimentsaddressed many of the uncertainties raised in the macroscopic experiments conducted by our group; namely, the sliding interface was well characterized, interfacial sliding was confirmed, and the experiments were run in ultra-high-vacuum at temperatures well above the temperature for equilibrium ice formation on the surfaces (frost-point).

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W. Gregory Sawyer is an Associate Professor in the Department of Mechanical and Aerospace Engineering at the University of Florida.  His research interests are in tribology, which is the study of friction, wear, and lubrication.  The focus of our research group is in the area of solid lubrication specifically targeting operation in extreme environments where the use of fluid lubricants is precluded.  Dr. Sawyer has published over 60 journal papers and together with his students has given over 60 presentations at international conferences (9 invited).  He was the inaugural recipient of the ASME Marshall B. Peterson Award in 1998 and received the ASME Burt L. Newkirk Award in 2004.

 

 Thursday, November 15th
337 Towne Bldg.
2:00 – 3:00 p.m.