The Evolution Process and Mechanical Characteristics of the Metamorphic Layer on the Wear Surface of UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) is the main liner material for artificial joints, and the tribological performances including material wear and abrasive chip formation, are the critical issues affecting its service life. The friction-induced shear deformation at the microscale is a precursor to the wear of UHMWPE, and the formation and evolution of surface plastic deformation layers depend on their microstructure. The evolution of the metamorphic layer in the wear surface of UHMWPE, as well as the corresponding mechanical properties, were investigated by characterizing microscopic characteristics and microstructures. The experimental results suggested that delamination could be easily observed on the surface of UHMWPE under dry friction conditions, while delamination and adhesion were not common under water lubrication conditions. However, stable and continuous transfer films could be observed on GCr15 steel balls, which were not obvious under dry friction due to the higher friction temperature. Through indentation and scratch tests, we found that the surface hardness of the wear surface increased with the increase of friction speed and pressure, while the scratch resistance along the sliding direction decreased. The Raman spectroscopy analysis result showed that the crystallinity of polyethylene in the wear surface increases as the friction speed increases, indicating that the polyethylene molecular chains formed a long-range ordered distribution state. This implies that the plastic flow on the surface of UHMWPE could result in the rearrangement of polyethylene molecules at the micro-scale, i.e., the change of microstructures, and reduce the mechanical performance in the wear area. In addition, the molecular dynamics simulation method was employed to investigate the indention and scratch molecular process of a diamond indenter on amorphous UHMWPE substrates. The simulation results suggested that both the indention depth and the scratch resistance decreased as the crystallinity of the amorphous UHMWPE substrates increases. Under a higher crystallinity of UHMWPE, polyethylene molecules were more likely to undergo plastic flow under the extrusion of the diamond indenter. On the contrary, with a lower crystallinity, polyethylene chains had entangled distribution, which exerted a higher resistance to their plastic flow, and increased scratch resistance in the direction of friction. Therefore, a metamorphic layer, where the microstructure and mechanical properties differ from the bulk UHMWPE, was formed on the wear surface. As the friction condition intensifies or time prolongs, the polyethylene molecular chains in wear zones or those connected to the substrate could break, causing fatigue fracture or formation of delamination wear on the surface of UHMWPE. At the same time, the wear debris could adhere to the counterpart by forming transfer films, especially under water lubrication conditions. The investigation of the evolution and mechanical characteristics of the metamorphic layer on the wear surface of UHMWPE may help the evaluation of the performance of artificial joints.