Ultrahigh molecular weight polyethylene (UHMWPE) has been widely studied and applied in water-lubricated bearings for shipping due to its excellent comprehensive performance. This study aimed to investigate the frictional noise properties of underwater contact by preparing UHMWPE with different molecular weights. The experiments were carried out using an RTEC tribo-tester, with friction and vibration tests conducted under varying speeds, loads, and materials. The results demonstrated that as the load and speed increase within a specific range, the lubricated status of the bearing is transformed into hydrodynamic lubrication, leading to a decline in the frictional coefficient. Furthermore, the vibration of UHMWPE, which is distinct from the frictional coefficient, intensifies with an increase in elastic deformation. Under high speed and heavy load, the vibrational accelerations and their major vibrational frequency increase with the molecular weight. However, the opposite is true under high speed and light load. The stick-slip phenomenon observed in the UHMWPE material is caused by friction, resulting in plastic deformation of the material surface and leaving ripples on the surface after the friction. These findings offer valuable insights for selecting water-lubricated bearing materials and decision-making for underwater vehicles.
UHMWPE, which is formed by the polymerization of a vinyl monomer, possesses a molecular weight of over 1,500,000 g mol−1. The preparation process, which includes the manipulation of temperature, pressure, and time, is a critical determinant of the tribological and physical-mechanical properties of UHMWPE. During the preparation process, when the temperature exceeds the melting point temperature, the initial chain segment diffuses toward the surrounding interface by overcoming the Van der Waals forces with the help of molecular thermal motion. The unentanglement occurs, and the overall migration of the molecule chain is caused by the linear motion of the initial segment, which is known as the "melt explosion" (Chen, 2021; Yang, 2014; Deplancke, 2013). The "melt explosion" phenomenon usually occurs at the initial melting stage of UHMWPE nascent powder. The entanglement network composed of the interlocking structure of UHMWPE particles becomes more complex with higher molecular weight, and the highly entangled non-crystalline zone results in a more intense "melt explosion" phenomenon during the melting process (Deplancke, 2015; Zhou, 2022). As a result of the "melt explosion", the molecular chains re-entangle and overlap, forming a certain volume of material through the accumulation of crystalline layers. The mechanical properties of UHMWPE are controlled by the thickness and distribution of these crystalline layers after recrystallization. Due to the presence of low-density crystals, UHMWPE exhibits better chain segment movement space and ductility. Conversely, a higher proportion of high-density crystals results in better impact performance as more external energy can be absorbed. Zhou (2022) conducted a study on the tribological behaviors of UHMWPE under different molding temperatures and found that a molding condition designed at 200°C and 20MPa for 30 minutes had a significant impact on reducing the friction coefficient of UHMWPE, leading to a low and stable tribological system.
