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TLcryo2005 - Tribology Letters Vol 20 No 2 October 2005...

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Cryogenic friction behavior of PTFE based solid lubricant composites N.L. McCook, D.L. Burris, P.L. Dickrell and W.G. Sawyer* Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA Received 13 April 2005; accepted 24 July 2005 Solid lubricants used in aerospace applications must provide low friction and a predictable operation life over an extreme range of temperatures, environments and contact conditions. PTFE and PTFE composites have shown favorable tribological performance as solid lubricants. This study evaluates the effect of temperature on the friction coefficient of neat PTFE, a PTFE/ PEEK composite and an expanded PTFE (ePTFE)/epoxy coating. These experiments evaluate friction coefficient over a temperature span which, to the investigators’ knowledge, has not been previously examined. Results show a monotonic increase in friction coefficient as sample surface temperature was decreased from 317 to 173 K for all three samples. The frictional performance of these and other published solid lubricant polymers was modeled using an adjusted Arrhenius equation, which correlates the coefficient of friction of the polymer materials to their viscoelastic behavior. A model fit of all the polymer data from 173 to 450 K gives an activation energy of 3.7 kJ/mol. This value suggests that breaking of van der Waals bonds is the likely mechanism responsible for the frictional behavior over this temperature range. KEY WORDS: cryotribology; polytetrafluoroethylene; PTFE; solid lubrication; space lubrication 1. Introduction Solid lubricants are frequently used under opera- tional conditions where the use of fluid lubricants is precluded. Such conditions are typically considered extreme, often existing at opposite ends of the temper- ature spectrum either high ( T > 200 ° C) or low ( T < 0 ° C). Other extreme conditions include vacuum and harsh chemical environments. Aerospace applications include both temperature extremes and vacuum. The desire is to operate moving mechanical assemblies in such environments for extended operational cycles. This objective can be met using solid lubricants and solid lubricant coatings [1,2]. For moving mechanical assemblies that require lim- ited operational cycles, such as hinges and other single event assemblies, thin sacrificial solid lubricants such as burnished films of MoS 2 have been successfully used [3]. These thin films pose two main problems for the design engineer: (1) they wear during operation and thus have finite life, and (2) they are often poor performers in the Earth’s environment where they are operated prior to launch and deployment. Bulk polymeric composites containing Polytetrafluoroethylene (PTFE) offer an attractive option to the design engineers because of their predictable operational life [4,5], ability to operate from cryogenic temperature (4 K) to 500 K, and their relative insensitivity to oxygen and humidity [3].
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