After purified by mixture of sulfuric acid and nitric acid, the multi-walled carbon nanotubes(MWNTs) were modified with stearic acid(SA). The modified carbon nanotubes as lubricant additive were utilized to prepare lubricant, and the effects of carbon nanotubes on the tribological properties were investigated by using a pin-on-plate wear tester. The surface structure of MWNTs was examined by transmission electron microscopy, Raman spectroscopy and infrared spectroscopy. The results show that the surfaces of MWNTs are coated with a modified layer of SA. Furthermore, the modified MWNTs as lubricant additive can effectively improve the friction-reduction and anti-wear ability of lubricant. The friction coefficient of base lubricant decreases by about 10% and the wear loss of base lubricant decreases by 30%40% when the concentration of modified MWNTs in lubricant is 0.45%. In addition, the mass ratio of SA to MWNTs influences the friction-reduction and anti-wear ability of the modified MWNTs as lubricant additive. The optimum mass ratio of MWNTs to stearic acid is about 3∶81∶2.
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