Penetration of lubricating water molecules at the frictional interface of UHMWPE: Insights from molecular dynamics simulations

The absorption of water at the polymer interfaces has been proved to be critical to the mechanical and tribological behaviors of the water lubrication system. In the present work, the diffusion and penetration of water molecules from the frictional interface into the ultra-high molecular weight polyethylene (UHMWPE) matrix, as well as their effects on the tribological behaviors, are investigated by molecular dynamics simulations. Under water lubricated conditions, the interfacial water molecules gradually diffuse into the UHMWPE substrate, which in turn preferentially aggregate into water clusters during friction. Moreover, the diffusion and the following aggregation of water could be strengthened by decreasing the sliding velocity. We also find the UHMWPE substrate undergoes a shear deformation during the frictional process, owing to the molecular attractions from the linearly moved Cu slabs. However, the extent of the deformation decreases as the interfacial contact between Cu and the UHMWPE substrate weakens. The shear deformation at the frictional interface changes the arrangement of polyethylene chains in UHMWPE, manifested as the increased crystallinity and disentanglement, which provides possible passageways for the diffusion of lubricated water molecules.