activatedTL2007 - Tribol Lett (2007) 27:113117 DOI...

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ORIGINAL PAPER Thermally Activated Friction Xueying Zhao Æ Matt Hamilton Æ W. Gregory Sawyer Æ Scott S. Perry Received: 22 November 2006 / Accepted: 27 March 2007 / Published online: 4 May 2007 Ó Springer Science+Business Media, LLC 2007 Abstract The temperature dependence of the kinetic friction between a highly oriented pyrolytic graphite sur- face and a silicon nitride probe tip has been evaluated through atomic force microscopy measurements performed under an ultra-high vacuum environment over the tem- perature range 140–750 K. As temperature increases from 140 to 400 K a sharp decrease in friction is observed. A relatively weaker dependence on temperature is observed in the friction measured between 400 and 750 K. Collec- tively, these results obtained from a fundamental interface are consistent with an activated mechanism of energy dissipation during sliding. Keywords Friction mechanisms ± Graphite ± AFM Introduction The relative motion of two bodies in contact involves a number of forces acting across the interface and in±uenc- ing the nature of sliding and wear. Of these, frictional forces play a central role and are known to depend on many factors including composition, topography, third bodies (adsorbates, lubricants), velocity, and temperature. Of particular interest here, large variations in friction are routinely experienced in practical applications as opera- tion conditions diverge from room temperature in both cryogenic and elevated temperature directions. Previously, macroscopic testing of solid lubricants performed at tem- peratures below ambient have been frequently conducted in submerged environments; the state-of-the-art is less than optimal [ 1 , 2 ]. Attempts to operate pin-on-disk tribometers under cryogenic gas boil off have provided friction coef- ²cient data down to ~ 77 K under He [ 3 , 4 ] and N 2 [ 2 ] gas streams. Contamination with water has been a particular concern for these studies; 1 ppb of H 2 0 in the gas stream is the equilibrium vapor pressure above ice at approximately 160 K. Over the temperature range from 150 to 450 K macroscopic pin-on-disk tests with polytetra±uoroethylene (PTFE) ²lms have shown a friction coef²cient response that was characteristic of a thermally activated Arrhenius behavior [ 2 ]. In separate experiments, employing a cryostat with a tilting slider-block tribometer using steel pins on PTFE, steel pins on sapphire, and sapphire on sapphire, no discernable temperature dependence from 4 to 300 K was observed, although wear was apparent in these systems [ 5 ]. Fundamental studies of the origin of frictional forces and their dependence on environmental conditions must seek to account for the simultaneous contribution of indi- vidual in±uences (composition, velocity, temperature, etc.) or seek to eliminate their contribution. Such studies will be facilitated through the assessment of systems in which interfacial sliding occurs between clean, model surfaces, and for contacts of known area and location. The discus- sions of friction coef²cients based on a thermally activated
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activatedTL2007 - Tribol Lett (2007) 27:113117 DOI...

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