Surface-modified Cu nanoparticles were synthesized by a two-step liquid reduction method.The anti-wear and friction-reducing behavior and the repair to the wear surface of steel balls were investigated by a four-ball machine

with Cu nanoparticles as 150N lubricating oil additive.The worn surface morphology and typical elemental distributions were analyzed by means of scanning electron microscope and energy dispersive spectrometry.The results showed that surface-modified Cu nanoparticles as lubricating oil additive could evidently improve the anti-wear and friction-reducing ability of 150N lubricating oil.Cu nanoparticles could fill-up worn surface and form a thin film on the surface.