PEsimulation2008 - Effect of the sliding orientation on the...

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Unformatted text preview: Effect of the sliding orientation on the tribological properties of polyethylene in molecular dynamics simulations Seong Jun Heo, 1, a ! Inkook Jang, 1, b ! Peter R. Barry, 1 Simon R. Phillpot, 1 Scott S. Perry, 1 W. Gregory Sawyer, 1,2 and Susan B. Sinnott 1, c ! 1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400, USA 2 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611-6400, USA s Received 27 August 2007; accepted 28 January 2008; published online 16 April 2008 d The role of sliding orientation on the tribological properties of polyethylene s PE d is investigated by using classical molecular dynamics simulations. Cross-linked PE surfaces slide against one another in two different directions: one that is perpendicular to and one that is parallel to the aligned direction of the polymer chains. The results indicate that sliding in the parallel direction occurs with a lower friction coefficient than sliding in the perpendicular direction. In both cases, gross level stick-slip motion is observed to be associated with the sliding of a restrained, corrugated molecular interface. In addition, the simulations demonstrate the way in which the system stores more shear strain energy during sliding in the perpendicular direction. The tribological behavior of these PE surfaces is compared to the behavior of similarly modeled polytetrafluoroethylene surfaces; the differences and similarities between the two systems are discussed. © 2008 American Institute of Physics . f DOI: 10.1063/1.2900884 g I. INTRODUCTION Polyethylene s PE d is one of the most widely used poly- mers because of its versatility and manufacturability. Its me- chanical and tribological properties greatly vary depending on its degree of crystallinity, structure, and molecular weight. Ultrahigh molecular weight PE s UHMWPE d has a molecular weight of millions of grams per mole and a highly crystalline structure. It exhibits outstanding wear resistance and tough- ness and has favorable tribological properties. 1 Conse- quently, PE is increasingly used in applications wherein its tribological performance is critical, including thin coatings for silicon-based microelectromechanical systems 2 and arti- ficial joints. 3 – 5 It has been suggested that the friction properties of poly- mers are determined more by their molecular profiles than by their chemical compositions. For instance, Pooley and Tabor 6 observed that PE and polytetrafluoroethylene s PTFE d exhibit low friction properties, while polymers with bulky side groups, such as polypropylene and tetrafluoroethylene- hexafluoropropylene, have high friction coefficients. They at- tributed the low friction of PE and PTFE to the smooth mo- lecular profiles of these polymers. In addition, Schönherr and Vancso 7 found by using scanning electron microscopy that PE and PTFE surfaces show oriented fibrillar morphologies along the sliding direction. They also observed that the ori-along the sliding direction....
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PEsimulation2008 - Effect of the sliding orientation on the...

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