Investigation of Surface Roughness and Load Effects on the Tribological Behavior of Polymer–Metal Friction Pairs Under Water Lubrication

Abstract Polymers are promising candidates for marine stern bearings, where friction and wear behavior critically affect durability and operational efficiency. This study systematically investigates the tribological performance of five materials—polyether ether ketone (PEEK), carbon fiber-reinforced PEEK (CF/PEEK), ultra-high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), and 45 steel—under varying surface roughness, load, and lubrication conditions. A key novelty lies in revealing the multiparameter interaction mechanisms among surface roughness, applied load, lubrication state, and polymer hardness, moving beyond single-factor studies in the existing literature. Results show that for harder polymer–metal pairs (e.g., CF/PEEK–45 steel), increased roughness and load promote transfer film formation and enhance wear resistance under water lubrication. Conversely, softer polymers (e.g., UHMWPE and PTFE) are more sensitive to surface conditions: high roughness leads to surface damage and disrupted transfer films, while smoother surfaces and moderate loads yield better performance. Under dry conditions, the absence of lubrication amplifies asperity interaction and debris accumulation, increasing friction and wear, especially in soft polymers. These findings provide an integrated understanding of how tribological behavior evolves with coupled parameters, offering practical guidance for material selection, surface finishing, and operational design of marine water-lubricated bearings.