Synthesis of N-doped carbon quantum dots as lubricant additive to enhance the tribological behavior of MoS₂ nanofluid

In this study, a novel lubricant additive nitrogen-doped carbon quantum dot (N-CQD) nanoparticle was prepared by the solvothermal method. The synthesized spherical N-CQD nanoparticles in the diameter of about 10 nm had a graphene oxide (GO)-like structure with various oxygen (O)- and nitrogen (N)-containing functional groups. Then N-CQDs were added to MoS₂ nanofluid, and the tribological properties for steel/steel friction pairs were evaluated using a pin-on-disk tribometer. Non-equilibrium molecular dynamics (NEMD) simulations for the friction system with MoS₂ or MoS₂ + N-CQD nanoparticles were also conducted. The results showed that friction processes with MoS₂ + N-CQD nanofluids were under the mixed lubrication regime. And MoS₂ nanofluid containing 0.4 wt% N-CQDs could achieve 30.4% and 31.0% reduction in the friction coefficient and wear rate, respectively, compared to those without N-CQDs. By analyzing the worn surface topography and chemical compositions, the excellent lubrication performance resulted from the formation of tribochemistry-induced tribofilm. The average thickness of tribofilm was about 13.9 nm, and it was composed of amorphous substances, ultrafine crystalline nanoparticles, and self-lubricating FeSO₄/Fe₂(SO₄)₃. NEMD simulation results indicated the interaction between S atoms in MoS₂ as well as these O- and N-containing functional groups in N-CQDs with steel surfaces enhanced the stability and strength of tribofilm. Thereby the metal surface was further protected from friction and wear.