Sulfur-Doped Alkylated Graphene Oxide as High-Performance Lubricant Additive

Sulfur-doped graphene oxide (SA-GO) prepared by sulfuration and alkylation of graphene oxide is applied as an efficient green anti-wear additive for harsh operation conditions of engines. X-ray photoelectron spectroscopy analysis reveals the sulfur content of octadecylamine-modified SA-GO (sulfuration follows alkylation) is increased by 79 times compared with the reverse process that alkylation follows sulfuration, suggesting the preparation route is a key factor of the sulfuration process. The higher sulfur content and –C–S–C– sulfur bonding constitution result better lubrication effect, while the investigation of chain length of alkylation modification and concentration of the alkylated sulfur-doped graphene oxide indicates the octylamine-modified SA-GO shows smaller diameter of wear scar within the concentration range between 1 × 10−4 and 2.5 × 10−4 wt%. The decrement percent of wear scar diameter is 43.2% in 928 lubrication oil and 17.2% in PAO4 oil while the SA-GO modified by octylamine is applied with the concentrations 2.5 × 10−4 wt% in PAO4 and 1 × 10−4 wt% in 928 oil, respectively. The sulfur content in oil samples is only 0.006~0.001 wt%, which is much lower than the sulfur content standard recommended by ILSAC that is 0.5 wt%. The research work indicates the SA-GO additive is more feasible for the pollution treatment which focuses the substantial reduction of sulfur content in lubrication oil on the premise of improving lubrication capability.