In mechanical systems, high-efficiency and environmentally friendly lubricant oil is demanded for extension of the equipment service life and energy saving demand highly efficient and environmentally friendly lubricating oil. Over last few decades, more attention have been paid to the addition of carbon nanoparticles, such as graphite, graphene and carbon nanotubes etc., in order to reduce the friction and wear in lubricating oil. At present, nanodiamond particles (NDPs) have been considered to be one of the most promising candidates for enhancing the friction reduction and anti-wear properties of lubricating oil effectively and economically due to its excellent properties of hardness, chemical stability, thermal stability and high thermal conductivity. Although NDPs as lubricant additives in lubricating oil have been reported, its dispersity in lubricating oil has not been well investigated. NDPs are easy to agglomerate in many media and have poor dispersive capacity in lubricating oil. In this work, in order to improve the dispersity of NDPs in anti-wear hydraulic oil (AHO), acidified and heat-treated NDPs (T-NDPs) were prepared and mixed with a variety of additives [oleylamine, polyisobutylene succinimide (T154) and high alkaline calcium sulfonate (T106)], then the mixture was added into AHO to prepare AHO containing T-NDPs. The materials and dispersity were characterized by FESEM, a Zeta potentiostat, FT-IR and static sedimentation experiment. The tribological property of AHO with T-NDPs and worn surfaces were investigated by the four-ball tribometer, 3D laser scanning microscopy and SEM/EDS. The results showed that the average diameter of NDPs decreased from 270.2 nm to 153.5 nm after acidified and heat-treatment. The average diameter of T-NDPs after adsorption additives was 101.5 nm. Additives could increase oil solubility of T-NDPs and imped their aggregation. Hence, AHO with T-NDPs possessed good dispersion stability. Particularly, the friction coefficient and wear scar diameter of AHO containing 0.04wt% of T-NDPs were reduced 13.2% and 17.8% compared with AHO without T-NDPs. We can analysed the friction reduction and anti-wear mechanism of T-NDPs as lubricant additives from two aspects, one is that the T-NDPs could support the load and act as roll bearing between the friction interfaces, and the other one is that the T-NDPs participated in the lubricating film formation between the friction surfaces.
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