Ammonia-induced tribofilm instability and friction-wear decoupling in zinc phosphate coated piston rings

Ammonia is a promising carbon-free fuel for the development of next-generation zero/low carbon emission internal combustion engines. However, ammonia’s tribological effects on conventional engine components remain unclear. This study investigates the tribological response of a cast-iron zinc phosphate coated piston ring under ammonia-affected lubrication. Engine oil (SAE 10W-40) was collected after 20 h of operation in an ammonia-fueled engine and compared with the same type of fresh oil. Reciprocating sliding tests were conducted using non-conformal contact between a curved zinc phosphate coated cast iron piston ring segment and a flat FC250 cast iron liner under boundary lubrication at 160°C, 10 N load, and sliding speeds of 100-3000 mm/min. During the early sliding stage (0-60 m), the ammonia-affected oil produced consistently higher coefficients of friction than the fresh oil across all sliding speeds (0.135-0.144 versus 0.121-0.128). During the late stage (100-175 m), the differences diminished and were no longer statistically significant. Despite this convergence in friction, the ammonia-affected oil produced greater piston ring wear, with wear volumes approximately 39-87% higher than those obtained with fresh oil. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis of the wear track of piston ring specimens revealed increased Ca, P, and S, slightly lower Zn, and higher Fe exposure. These results suggest that ammonia-affected lubrication alters the formation and evolution of additive-derived surface films on the zinc phosphate coated piston ring, resulting in spatially non-uniform surface protection and increased wear despite broadly comparable late-stage friction. The proposed wear mechanism is inferred from tribological observations and SEM-EDS elemental analysis rather than direct chemical identification. Furthermore, the findings are based on ammonia-affected engine oil obtained by a single engine durability test; therefore, additional studies covering a broader range of operating conditions are needed to establish the generality of these observations.