Unraveling Tribochemistry and Self-Lubrication Mechanism of Polytetrafluoroethylene by Reactive Coarse-Grained Molecular Dynamics Simulations.

Abstract

Lubrication of polymeric materials generally involves processes of atomic-scale chemical bond forming/breaking at the interface and mesoscale chain reorientation, disentanglement, and so forth. However, it is difficult to describe the important aspects of tribochemical reactions by conventional coarse-grained molecular dynamics (CGMD) simulations. Here, reactive CGMD simulations were conducted based on the ReaxFF force field to study the tribochemical interactions between polytetrafluoroethylene (PTFE) and iron. The chemical bond forming/breaking between the molecular chain and countersurface was fitted through the bond dissociation energies of specific reaction sites from all-atom ReaxFF-MD simulations. This enabled a quantitative description of tribochemical reactions in a macromolecule system. First, the number of anchoring bonds between PTFE molecules and the countersurface showed a strong correlation with the friction coefficient. The shearing process induced breaking of the interfacial anchoring bonds as well as chain disentanglement in the matrix, which consequently led to ordering reorientation of molecular chains toward sliding direction and hence decrease of friction. Second, two competitive factors were clarified to affect polymer friction with varying temperatures. The decrease of interfacial anchoring reactivity and molecular chain mobility at low temperature prohibited reorientation of molecular chains and increased the friction coefficient. On the other hand, the hardening of PTFE and the reduction in effective contact area at low temperatures decreased the friction coefficient. This led to a turning point with a maximum friction coefficient around 100 K. These results shed light on the essential role of tribochemical reactions on polymer lubrication, especially under low temperatures, which provides design guidance of polymeric lubrication systems for engineering applications.