Achieving stable and long-lasting low friction and extended lifespan for polymeric components in underwater mechanical systems remains challenging. Inspired by the low friction and efficient lubrication provided by the mucus secreted on fish skin, this study fabricated the bioinspired functionalized ionic liquid/carnauba wax (IL-SiO₂/CW) reinforced polyurethane composites (PUCW-ILs) and investigated its properties. The results demonstrate that PUCW-1.5ILs, with a 1.5 wt.% addition ratio, achieved stable and persistent low friction and wear performance: an average coefficient of friction as low as 0.07 and an average wear loss of 0.006 g at 0.5 MPa (decreased 81% and 84.2% compared to PU respectively). This superior performance stems from the enhanced shear resistance of the matrix by IL-SiO₂, the formation of stable lubricating and transfer films at the interface by both CW and IL-SiO₂ and the hydration effect of IL-SiO₂.

Boundary lubrication under harsh working conditions results in severe wear of water-lubricated bearing materials, e.g., a tail bearing in a ship. Inspired by cartilage lubrication, we prepare a smart hydrogel with balanced hydration and load-bearing properties through the construction of polyvinyl alcohol (PVA)-chitosan/sodium alginate (CS/SA) double networks and the introduction of aramid nanofibers. The hydrogels are blended with ultrahigh molecular weight polyethylene (UHMWPE) particles into new bionic biphasic hydrogel-containing composites. Thorough assessments (chemical, thermal, surface, and bulk mechanical properties) of the hydrogels and composites revealed that the high hydrophilicity of the hydrogel particles encapsulated in bulk UHMWPE facilitates water absorption, leading to improved friction performance under the boundary lubrication mode, e.g., at startup. The stripped hydrogel pits and induced microtextures between the friction interfaces as the hydration layer play a role in separating the friction interface, effectively reducing the friction contact. Under a 40 N load, the friction coefficient and wear rate of one composite are 28.7% and 14% lower than those of the plain UHMWPE composite, respectively. After soaking in seawater for 28 days and holding at 50 °C for 1 h, the mechanical properties of the composite material are still better than those of plain UHMWPE. Taken together, the smart biphasic hydrogel-containing composites were able to improve the lubrication state according to the operating conditions.

The running-in of cylinder liner-piston rings (CLPRs) is the most important process that must be performed before a marine diesel engine can be operated. The quality of running-in directly affects the reliability of a CLPR. The surface texture of a CLPR has been proven to significantly affect its lubrication performance. In this study, the tribological behavior of a CLPR during running-in is investigated. Three types of surface textures are generated on the CLPR via laser processing: dimple texture on piston rings, groove texture on cylinder liners, and co-texture on both sides. Subsequently, a series of tests are performed on a slice tester. A load of 300 N (1.64 MPa) is applied, and two speeds (50 and 100 rpm) are adopted. The CLPR running-in quality is characterized based on three parameters, i.e., the friction coefficient, contact resistance, and wear topography. Experimental results show that, compared with a non-textured surface, the three types of surface textures mentioned above improved the friction performance during running-in. The lubricant supply capacity of the dimple texture on the piston ring, as a mobile oil reservoir, is stronger than that of the groove texture on the cylinder liner serving as a static oil reservoir. By contrast, the wear resistance of the dimple texture, as a movable debris trap on the piston ring, is weaker than that of the groove texture on the cylinder liner, which serves as a static debris trap. It is demonstrated that the co-texture combines the advantages of dimples and groove textures. Compared with non-textured surfaces, the friction coefficient decreased the most at 100 rpm (44.5%), and the contact resistance improved the most at 50 rpm (352.9%). The coupling effect provides the surface with improved running-in quality by optimizing the tribological performance, particularly at the dead center. This study provides guidance for the tribological design and manufacturing of CLPR in marine diesel engines.