Tribological performance degradation of cylinder liner-piston rings in marine ammonia engines induced by ammonia contamination of the lubricating oil

In pursuit of decarbonization, ammonia has attracted extensive attention as a zero-carbon fuel for marine propulsion. However, unburned ammonia and associated byproducts will alter the lubrication environment of the cylinder liner–piston ring (CL-PR) friction pair and induce abnormal damage, while the underlying mechanisms remain unclear. Hence, reciprocating friction tests were conducted under simulated service environments in ammonia marine engines using lubricating oils with varying ammonia contamination forms and concentrations. Based on the integrated evaluation of friction behavior, wear characteristics, lubrication condition, vibration response and mechanical properties, the results indicate that ammonia contamination universally degrades tribological performance, with ammonia solution exerting a more detrimental effect than ammonia gas, and the severity increasing with ammonia concentration. The introduction of ammonia gas primarily weakens oil film integrity and additive anti-wear performance, thereby intensifying mechanical wear. In contrast, the introduction of ammonia solution deteriorates the anti-wear effectiveness of additives, promotes localized starvation and interfacial corrosion, and induces the formation of extensive corrosion products. These effects collectively result in surface softening, stress concentration, and aggravated spalling, leading to a drop of over 40% in elastic modulus and hardness, with an increase of up to 2.7-fold in wear rate compared to fresh oil, ultimately driving the interfacial damage toward a corrosive-wear coupling effect associated with low-strength corrosion layers. These findings reveal the influence of ammonia contamination on the evolution of CL-PR tribological performance, providing a theoretical basis for optimizing lubricant formulations and improving anti-wear design of tribological components in marine ammonia engines.