Deformation mechanisms of polytetrafluoroethylene at the nano- and microscales

Polytetrafluoroethylene (PTFE) has not only a low coefficient of friction but also a high wear rate. Numerous studies investigating the possibility of reducing wear and the mechanisms of deformation have been conducted using experimental and computational studies. In this paper the deformation and mechanical failure of single chains and bulk PTFE are investigated using molecular dynamics (MD). Due to the length scale limitations of the model that MD simulations can investigate, a coarse-grained model is developed and PTFE particles of varying densities are generated to investigate their mechanical properties at the microscale. The coarse-grained potential parameters and coarse-grained model of PTFE are derived from first principles based ReaxFF simulations and are then used to investigate the microscale mechanical properties of PTFE. The MD study indicates that temperature has a pronounced effect on the maximum strength of a single chain, the elastic modulus is dependent on chain length, and chains shorter than 100 angstrom have an increase in maximum strength. During indentation and scratch the frictional coefficient of bulk PTFE is dependent on the direction of the scratch on the PTFE substrate. The coarse-grained PTFE simulations show that indentation force is dependent on the density of PTFE, and the smoother the surface of the particle the lower the coefficient of friction. The coarse-grained model that connects the atomic and macroscales of PTFE will lead to a direct comparison with experimental PTFE thin films.