Degradation of hydrocarbon oil at sliding iron interfaces

The development of lubricant materials capable of reducing energy dissipation and wear due to friction is a critical issue in industrial applications. Hydrocarbon molecules are highly promising, but they also undergo degradation under extreme conditions. In this study, we investigate the in-situ formation of a carbonaceous tribofilm from dodeca-5,7-diene on iron surfaces through large-scale molecular simulations using machine learning potentials. Our results reveal that the process is initially driven by hydrogen abstraction, primarily enhanced by high pressure or elevated temperatures. In contrast, sliding alone does not initiate dehydrogenation but promotes molecular mixing, accelerating the process when favorable conditions are met. As hydrogen atoms are released from hydrocarbons, the number of carbon-carbon double bonds increases, leading to the formation of progressively larger carbon clusters. Under extreme conditions, these clusters transition into an abrasive tribofilm that induces wear on the iron surface. Ab initio calculations confirm that the most favorable hydrogen abstraction sites are carbon atoms adjacent to a double bond, as radical formation is stabilized by electron delocalization. Moreover, the presence of double bonds is crucial for the formation of a single-layer tribofilm, as interactions between pi-electrons in carbon-carbon double bonds and the d-orbitals of the iron surface enhance stability and facilitate hydrogen dissociation. These findings provide insights into tribofilm formation and oil degradation, emphasizing the critical role of molecular structure in lubricant efficiency. Ultimately, as hydrocarbons undergo dehydrogenation and polymerization, the formation of a carbonaceous film alters the tribological behavior of the system, increasing friction and wear.