Reactive Molecular Dynamics Simulations on the Influence of Interfacial Water Diffusion on the Tribological Properties of Ultra-high Molecular Weight Polyethylene

T Zheng and JX Gu and QH Cheng and HC Zhang, ACTA POLYMERICA SINICA, 55, 222-234 (2024).

DOI: 10.11777/j.issn1000-3304.2023.23202

The molecular dynamics simulation method based on the reactive force field (ReaxFF) was applied to investigate the basic process of diffusion of water molecules at the friction interface into ultra-high molecular weight polyethylene (UHMWPE) substrate. The molecular simulation results show that water molecules are stably adsorbed on the surface of the Fe plate during the friction process, and the internal friction between the Fe slab/water molecules and the UHMWPE substrate drives the shear and deformation process of polyethylene chains. Moreover, in the convex model with nano-scale convex ridges, the plowing effect on the UHMWPE becomes more obvious, which significantly increases the internal frictional force. However, if increasing the friction velocity, the atomic temperature at the frictional interface increases significantly. Under water-lubricating conditions, interfacial water molecules gradually diffuse into the UHMWPE substrate, which in turn increases the average atomic distance between adjacent polyethylene chains. As a result, interaction strength between polyethylene chains decreased. In addition, the rupture of the hydrogen-oxygen bonds in water molecules occurs during the frictional process, which also results in charge transfer at the frictional interface. Meanwhile, the water oxygen atoms after water dissociation and surface Fe atoms formed Fe-O compounds, which have a similar crystal structure to Fe2O3. The diffusion of water molecules into the UHMWPE substrate also causes rapid changes in the atomic charges of surrounding polyethylene chains, resulting in uneven distribution of atomic charges on the surface of UHMWPE.

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