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The tribological properties of MoS2 microsized spheres (MS-MoS2) with diameter of 0.5-3 p,m modified by self-prepared surfactant quaternary ammonium salt of 2-undecyll- dithioureido-ethyl-imidazoline (SUDEI) as an additive in base oil 500 SN were investigated and compared with those of commercial colloidal MoS2 (CC-MoS2) on a four-ball tester and an Optimol SRV oscillating friction and wear tester in a ball-on-disk contact configuration. The worn surfaces of the bottom fiat disk were examined with scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that the MoS2 microsized spheres product was a much better extreme-pressure additive and antiwear and friction-reduction additive in 500 SN than commercial colloidal MoS2 (CC-MoS2). Under the appropriate concentration of 0. 1% and 0.25% for MS-MoS2 and CC-MoS2 and the load of 400 N, the friction coefficient of MS-MoS2/oil and CC-MoS2/oil decreased about 25.0% and 12.5% and the wear volume loss decreased about 50.4% and 12.9% compared with the pure base stock. The boundary lubrication mechanism could be deduced as the effective chemical adsorption film formed by the long chain alkyl (C11H23) and active elements (S and N) in the surfactant SUDEI and tribochemical reaction film composed of the tribochemical reaction products. [ABSTRACT FROM AUTHOR]