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With laminated structure like graphite, hexagonal boron nitride nanoparticle (nano-hBN) performed excellent anti-wear and friction-reducing properties. In this paper, hBN nanofluids were prepared using sodium polyacrylate (PAAS) as dispersant in water. Four-ball tribometer and pin-on-disk tribometer were employed to evaluate the tribological performance and lubrication behavior of the above-prepared lubricating fluids during steel/steel friction. The worn surface of balls and disks was analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and x-ray photoelectron spectrometer (XPS). The minimum lubricating film thickness was calculated by Hamrock and Dowson equation. Results showed that 0.7 wt% was the optimum concentration of nano-hBN in water which significantly reduced the coefficient of friction (COF) and wear scar diameter (WSD) by about 29% and 15%, respectively. The lubricating film thickness was lower than both nanoparticles diameter and worn surface roughness. From the above it was concluded that the worn area could be divided into direct contact, particle lubricating and liquid lubricating area with rolling mehanism, interlayer sliding mechansim, polishing mechansim and mending mechansim of nanoparticles. A protective film was formed on friction surface by PAAS molecule, FeOOH and FeO due to the absorption effect and tribochemical reaction between the nanofluids and worn surface, which further reduced the friction and wear rate.

Nonequilibrium molecular dynamics (NEMD) simulations were performed to investigate the tribology behaviors of Al 2 O 3 and MoS 2 nanoparticles confined between iron (Fe) slabs. Results indicated that the combined use of these two nanoparticles yielded the lowest and most stable friction force, normal force, interface temperature and wear rate, which exhibited a significant synergistic lubrication effect. A novel parameter the rolling/sliding motion coefficient (K rs ) was proposed to evaluate the motion pattern of spherical Al 2 O 3 . There were 51% rolling ? 49% sliding motion when used alone and 91% rolling ? 9% sliding in the existence of MoS 2 . Similarly, about 72.3% of the friction was shared by interlayer sliding of MoS 2 monolayers in the presence of Al 2 O 3 , which was higher than used alone (54.8%). Then, the diffusion of atoms at the friction interface was explored to reveal the synergistic lubrication mechanism. The tribofilm formed by the diffusion of Fe and S atoms could protect the metal surfaces from further wear. The adsorption of S atoms to Al 2 O 3 nanoparticle could promote its rolling effect and prevent it from embedding into iron matrix. Besides, Al 2 O 3 could also facilitate the separation of MoS 2 monolayers to enhance their interlayer sliding effect.

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.