Comprehensive Investigation of Various Organophosphate Intercalated CoAl-LDHs As Additives in Polyalphaolefin

This study focuses on the synthesis and investigation of four organophosphate-intercalated CoAl-LDHs as additives for oil lubricants. Phosphate esters have been well-known for their excellent antiwear properties in automobile applications. On the other hand, layered double hydroxide (LDH) has a unique lamellar structure that can contain anionic species that can be released under certain conditions. The successful intercalations of various organophosphates, including methyl phosphate (C1P), isopropyl phosphate (C3P), phenyl phosphate (C6P), and dodecyl phosphate (C12P), into CoAl-LDH were achieved. Ultimately, the C6P-LDH demonstrated exceptional antiwear performance, reducing friction and wear loss by 25% and 69%, respectively, compared to PAO-4 base oil. Interestingly, the C6P tribofilm shows two distinct regions with different colors under an optical microscope. Thorough characterizations of the two regions of interest were conducted via Raman spectroscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and cryo-electron energy loss spectroscopy techniques and demonstrated that the blue tribofilm pads consisted of mixed metallic oxides and phosphates, while the gray pads were rich in metallic cobalt, which was unexpected for tribofilm formed in situ during lubrication under ambient atmosphere. The thick oxide-phosphate layer is crucial for preventing wear loss. Additionally, a thin iron oxide layer at the steel interface strengthens the tribofilm adhesion to the steel base. The tribofilm formation mechanism was discussed and proposed. The influence of LDH dispersibility on the tribological behaviors was also discussed.