Abstract
The development of advanced lubricant additives has been a critical enabler in paving the way for increasing energy efficiency and durability in many industries. However, the growth mechanisms of additive-induced protective tribofilms are not yet fully understood due to the complex chemomechanical interactions at the contact interface and limited spatial resolution of characterizing techniques. Here tribofilms formed by three antiwear additives are systematically studied, i.e. a phosphonium-phosphate ionic liquid (IL), a zinc dialkyldithiophosphate (ZDDP), and a combination of them, on a grey cast iron surface. All three additives provided excellent wear protection and the IL+ZDDP combination exhibited a synergetic effect with further reduced friction and wear. Atom probe tomography (APT) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) were used to delineate the sub-nm chemistry and bonding states of atoms for each tribofilm of interest. For the IL tribofilm, it appears amorphous with Fe, P and O rich. Wear debris particles with an iron rich core and an oxide shell were revealed in the tribofilm and a transitional oxide (Fe2O3) containing layer was discovered at the interface between the tribofilm and the cast iron substrate. For the ZDDP sample, except for some Fe3O4 particles, such an oxide-rich interlayer was not detected. The ZDDP+IL tribofilm shares some similarities from the IL and ZDDP tribofilms, but contains more Zn atoms compared with the ZDDP tribofilm, which may help explain the further improved wear protection. Tribofilm growth mechanisms for the IL and IL+ZDDP combination are then proposed based on the observations at a sub-nm resolution.
