Superior extreme pressure properties of different layer LDH nanoplatelets used as boundary lubricants

[Display omitted] •Ultrathin layered double hydroxide (LDH) nanoplatelets were synthesized in an organic solution.•Ethylene glycol molecules were adsorbed on the surface of LDHs, and LDHs were modified into hydrophobic materials.•Ultrathin LDHs showed excellent extreme pressure property and anti-wear performance in boundary lubrication. Layered double hydroxides (LDHs) are a class of strongly adsorbed two-dimensional materials composed of divalent and trivalent metal cations. In this study, LDHs nanoplatelets with three different thickness were synthesized by changing the crystallization method during the hydrothermal reaction process. The three-dimensional size and layer number of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The synthesized ultrathin Co/Al-LDH (U-CA-LDH) nanoplatelets, only ca. 1 nm thick (single or double layer), were surface modified with ethylene glycol for good dispersion in an oil-based system. As a lubricant additive, the base oil with U-CA-LDH nanoplatelets can withstand extreme pressures of up 2000 N, almost three times the ultimate load-bearing capacity for commercial additives. In particular, through physical property and chemical composition analysis of tribofilms, U-CA-LDH nanoplatelets with a partial tetrahedral coordination form were revealed to exhibit better tribological properties than multilayer Mg/Al-LDH (M−MA−LDH) nanoplatelets with a stable octahedral coordination form. Incomplete coordinated nanoplatelets exhibited extremely high chemical activity in the high-temperature tribological region and promoted the formation of a denser protective tribofilm on the sliding surface. Owing to their superior tribological properties as oil-based additives, U-CA-LDH nanoplatelets hold great potential for practical application in the mechanical industry in the future.