macroActivatedTL2007 - ORIGINAL PAPER Macroscopic Evidence...

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Unformatted text preview: ORIGINAL PAPER Macroscopic Evidence of Thermally Activated Friction with Polytetrafluoroethylene David L. Burris Scott S. Perry W. Gregory Sawyer Received: 22 February 2007 / Accepted: 18 May 2007 / Published online: 7 June 2007 Springer Science+Business Media, LLC 2007 Abstract Under macroscopic pin-on-disk testing the slid- ing friction coefficient of Polytetrafluoroethylene (PTFE) was investigated over a temperature range of approximately 200400 K. This study examines the nature of the temper- ature dependence by testing PTFE pins at varying tempera- ture and humidity on a linear reciprocating pin-on-disk tribometer. The friction coefficient increased monotonically with decreasing temperature from l = 0.075to l = 0.210 in a manner consistent with thermal activation; it deviated from this trend only during phase and glass transitions in the PTFE and temperatures below the frost-points for the respective environments. Keywords Cryogenic tribology Polytetrafluoroethylene PTFE Solid lubrication Introduction There are a number of applications where operation in a temperature range from 200 to 400 K or larger is required for device success [ 1 3 ]. These extreme conditions are often the motivation for variable temperature studies in tribology [ 4 7 ], but there is a paucity of relevant tribology data available for temperatures below 273 K. In the range from 300 to 400 K the friction coefficient of PTFE increases with decreased temperature and increased speed. This behavior has been interpreted as evidence for a vis- coelastic friction dependence of PTFE since McLaren and Tabor [ 8 ] first proposed the hypothesis in 1963. It is well- known that PTFE films transfer and adhere strongly to the counterface, and a modern hypothesis is that both friction and wear of PTFE are dominated by the interactions of PTFE chains sliding past one another at weak self-mated interfaces, often within transfer films [ 9 15 ]. A recent paper by McCook et al. [ 4 ] found that friction of PTFE matrix composites continued to increase in the cryogenic regime down to 200 K, and the notion of a thermally activated friction coefficient for PTFE was proposed (analysis of an activation energy gave E a = 3.7 kJ/mol). A recent molecular scale study of graphite by Zhao et al. [ 16 ] provided evidence that the temperature dependent friction found for PTFE may be a general result of inter- facial sliding. In these studies, an atomic force microscope was used to collect friction data on molecularly smooth terraces of graphite over a temperature range from 140 to 750 K at a vacuum level of 2 10 10 torr. The friction coefficient increased with decreasing temperature, and the data collected followed an Arrhenius dependence with an activation energy of E a = 9.6 kJ/mol. The molecular scale experiments by Zhao et al. [ 16 ] addressed many of the uncertainties raised in the macroscopic experiments con- ducted by McCook et al. [ 4 ]; namely, the sliding interface...
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macroActivatedTL2007 - ORIGINAL PAPER Macroscopic Evidence...

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