Poly(tannic acid)-functionalized onion-like carbon nanoparticles derived from candle soot serving as potent lubricant additives

Spheroidal onion-like carbon (OLC) nanoparticles exhibit heliocentric graphitic layers are known to possess excellent lubricating properties. Herein, tannic acid (TA) was assembled in-situ onto the onion-like carbon material with Tris (2-aminoethyl) amine (TAEA) to form poly(tannic acid)-functionalized onion-like carbon nanoparticles (P(TA-TAEA)@OLC) via Schiff-base reaction and/or Michael addition. Subsequently, the commercial lubricating additive dialkyl dithiophosphate (DDP) was introduced via Michael addition to bond with the poly(tannic acid) of P(TA-TAEA)@OLC nanoparticles in Tris aqueous solution, to obtain DDP-functionalized onion-like carbon (DDP@OLC) nanoparticles. The as-obtained DDP@OLC nanospheres in the role of lubricating additives demonstrated remarkable friction reduction and wear resistance. With addition of 1 wt % DDP@OLC nanoparticles, the coefficient of friction reduced from 0.17 to 0.10, while the corresponding wear volume loss decreased by 96 %, and the loading capacity reached 600 N from 100 N. The outstanding tribological performance of these as-obtained nanoparticles can be attributable to the formation of a graphitic layer exfoliated from OLC nanoparticles under a high load, and the protective film derived from tribochemical reactions via N, P and S elements of DDP@OLC. Meanwhile, the DDP@OLC nanoparticles can also act as spacers and ball bearings between sliding surfaces, facilitating the enhancement of anti-wear and friction reduction properties.