Effect of hydrogenation or not of DLC on its tribological properties in vapor and liquid methanol

Abstract

Green methanol as a fuel can effectively solve energy shortage and reduce air pollution. However, its lubrication failure caused in internal combustion engine systems is a shortcoming. In this study, we investigate the tribological properties of diamond-like carbon (DLC) and hydrogenated diamond-like carbon film (HDLC) in vapor methanol and liquid methanol against ceramic ball in view of different states of methanol working in different components. The results showed that the tribological performance of DLC in vapor methanol was better than that of HDLC, with Si₃N₄ vs. DLC having the best tribological performance (friction coefficient (COF) is 0.068 and wear rate is 0.33 × 10⁻⁸ mm³/N·m). While HDLC in liquid methanol has better tribological performance than DLC, in which Si₃N₄ vs. HDLC is the best (COF is 0.061 and wear rate is 0.89 × 10⁻⁸ mm³/N·m). In addition, it was found that no transfer film was formed on the wear scar in both states. SEM and EDS mapping characterization of the wear debris revealed that the tribological properties of the carbon films in vapor methanol were mainly dominated by the degree of graphitization of the wear debris, whereas in liquid methanol they were highly correlated with the oxygen content of the wear debris. This study can be instructive for the design of methanol internal combustion engine systems.