Titanium dioxide nanoparticles (TiO2) were synthesized and then dual-coated with silane coupling agent (KH-570) and OP-10 in sequence in order to be dispersed stably in water as lubricant additives. The tribological properties and the application performance in Q235 steel machining of the nanoparticles as water-based lubricant additives were investigated on an MSR-10D four-ball tribotester and on a bench drilling machine, respectively. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to analyze the worn surface. The results show that the surface-modified TiO2 nanoparticles can remarkably improve the load-carrying capacity, the friction reducing, and anti wear abilities of pure water. The wear scar diameter and the coefficient of friction of the water-based lubricating fluids with TiO2 nanoparticles decreased, and the thick deep furrows on the surface of wear scar also decreased obviously with the increase of TiO2 concentration. The power consumption in drilling process was lower and the cutting surface was smoother using the water-based lubricating fluids added TiO2 nanoparticles compared to the fluid without addition. The reason for nanoparticles improving tribological properties of water based lubricating fluid might be the formation of a dynamic deposition film during rubbing process according to analysis of the worn surface. 1. Introduction Environment protection and energy conservation are becoming very important issues in modern industrial production. The main cause of energy loss in a mechanical system is friction [1]. The conventional choice is use of a mineral oil-based lubricant to reduce wear and friction in mechanical systems [2]. However, due to the inherent toxicity and the nonbiodegradable nature of oil-based lubricants, developing environment-friendly water-based lubricating fluids is meaningful for resource conservation society [3, 4]. Nevertheless, the poor tribological properties of water-based lubricating fluids [5] make it unacceptable for most of tribological applications. In order to adjust the performance and to improve the properties of water based lubricants, high-quality additives are used [6]. Liu et al. synthesized poly(N-isopropylacrylamide) brush as water-soluble additive and found that the additive helped decrease the friction coefficient of the lubricant to 0.03 due to its physical adsorption of the polymer chains [7]. Zhang et al. investigated the friction and wear behaviours of a (Ca, Mg)-sialon/SAE 52100 steel pair under the lubrication of various polyols in water and found that the
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