界面水分子扩散对超高分子量聚乙烯摩擦学性能影响的反应分子动力学模拟研究

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

采用基于反应力场(ReaxFF)的分子动力学模拟方法,研究了摩擦界面水分子向超高分子量聚乙烯(UHMWPE)基体扩散和渗透的基本过程.分子模拟结果表明:摩擦过程中,水分子稳定吸附在Fe板表面,并与聚乙烯链形成分子内摩擦,使聚乙烯分子产生剪切变形.当Fe板表面存在纳尺度外凸结构时,其在UHMWPE表面的耕犁作用更为显著,使摩擦界面的内摩擦力显著增加.当摩擦速度增加时,摩擦界面原子温度显著升高.在水润滑条件下,界面水分子逐渐扩散到UHMWPE基体中,引起相邻聚乙烯链之间的原子距离增加,这导致聚乙烯链之间的相互作用强度降低.此外,摩擦界面处还伴随着水分子中氢氧键断裂,并引起相应原子的电荷跃变.此时,水氧原子与Fe原子形成Fe―O化合物,且具有与Fe2O3相似的晶体结构.水分子扩散进入UHMWPE内时,还引起其周围聚乙烯分子的电荷发生改变,造成UHMWPE表层原子电荷分布不均匀.

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.