Tribological Behavior of Polymers Simulated by Molecular Dynamics
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Using molecular dynamics to simulate behavior of polymer surfaces during scratch testing, we report the first results of computer simulations of scratch behavior of noncrystals. A previously described procedure for creating realistic polymeric materials on the computer [W. Brostow, A.M. Cunha, and R. Simoes, Mater. Res. Innovat. 7, 19 (2003)] and used until now to simulate mechanical behavior of metals [S. Blonski, W. Brostow, and J. Kubat, Phys. Rev. B 49, 6494 (1994)] and one- and two-phase polymers [W. Brostow, A.M. Cunha, J. Quintanilla, and R. Simoes, Macromol. Theory Simul. 11, 308 (2002); W. Brostow, A.M. Cunha, and R. Simoes, Proc. Ann. Tech. Conf. Soc. Plastics Engrs. 60, 3105 (2002)] was applied. While experiments provide only the macroscopic penetration depth and the recovery (healing) depth, the simulations give the behavior of each macromolecular chain segment at each moment in time. We report results for one-phase polymers and also for systems with varying concentrations of a liquid crystalline (LC) second-phase that acts as a reinforcement. We relate the local structure to scratch resistance and recovery. The orientation of the chemical bonds is a major factor. The presence of a LC phase improves the tribological properties; however, the effect is not as significant as might have been expected.
I. INTRODUCTION
The tribological properties of materials are important for their service performance—as argued eloquently and in detail by Rabinowicz.1 However, his book deals almost exclusively with metals, and metal surfaces are not easily scratched anyway. A Swiss collective book on tribology2 has only one short chapter on polymers, mostly data tabulation of friction values. A book by Goldman on polymeric materials covers a large variety of deformation modes but says little about tribological properties.3 While Teflon is widely used because of its low friction, its scratch resistance is quite poor.4 Scratch resistance is particularly important; a scratch may acquire the role of a crack and become the origin of crack propagation.5,6 The polymer tribology we need to develop has to be based on chemical structures of the polymeric materials. Otherwise, all practical applications would be condemned to a trial-and-error procedure: mixing something with something else and “hoping for the best.” Such an approach represents a significant time and resource
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Address all correspondence to this author. e-mail: brostow@ unt.edu J. Mater. Res., Vol. 19, No. 3, Mar 2004
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investment—and for many cases will not result in any improvement over the initial properties of a material. One obtains two values from performing an experiment: the instantaneous penetration depth Rp and the residual (healing) depth Rh.4,7,8 However, testing does not provide any information about the evolution of that recovery from the instant the indenter hits the material through the moment when total recovery has occurred. Moreover, experiments do not provide connections between macroscopic t
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