Effect of Mass Fraction on Rheological Behaviors and Tribological Properties of Nano-Silica Greases

As a kind of inorganic nanomaterial, nano-silica has been proved to display excellent friction-reduction and antiwear properties, which also has been widely studied as a lubrication additive. However, there are few researches about the thickening mechanism and the properties of as-prepared greases when nano-silica is used as a kind of inorganic thickeners. In this study, five greases containing 12%, 14%, 16%, 18%, and 20% nano-silica were prepared to study the influence of the mass fraction on their physicochemical properties, low temperature fluidities, rheological behaviors and tribological performances. These results showed that their cone penetrations and oil separations gradually reduced with the mass fraction increasing of nano-silica, which indicated that the colloid stabilities of these as-prepared greases were constantly improved. In addition, it was found that the dropping point of all nano-silica greases was higher than 330 ℃ in the test process, which was consistent with the records of relevant researches, but the specific reasons were not clear. SEM analysis revealed that the microstructure of nano-silica greases presented the chain of aggregate structure, and the size of monomer reached 100 nm. This indicated that nano-silica thickened base oils and formed the colloid structure by the hydroxyl bonding and intermolecular forces. Their rheological results showed that the structural strength and shear resistance of the greases were gradually enhanced with the increasing of nano-silica content, and their thixotropy were also improved. When the mass fraction reached 16%, their strain at yield points and flowing points did not change significantly indicating the system had already formed a stable skeleton structure. At the same time, the corresponding greases appeared a "platform" with the stable viscosities between 0 and 20 ℃ according to the viscosity-temperature curve, which may be related to the deformation of the structure and the Brownian movement of nanoparticles. Based on the above analysis, a speculative description and schematic diagram of the nano-silica greases’ microstructure and their evolution under different shear stress has been expressed. When the shear stress was weak, the skeleton structure of greases might maintain stable through deformation, such as stretch or spin. When the shear stress became strong, the original skeleton structures would be destroyed but they could still be reconstituted by forming the new one to reach the new stable state. Furthermore, the tribological performance showed that the greases with 20% of mass fraction had the friction-reduced effect only at higher temperature and heavy load. The friction-reduced property of the other greases with different mass fractions showed a correlation with the temperature. Their friction coefficients increased with the increasing of the mass fraction at 50 ℃. However, they decreased with the increasing of the mass fractions at 100 ℃. When the temperature was 150 ℃, all greases showed the similar friction-reduced performances. Moreover, these as-prepared greases had the similar low temperature fluidity compared with the conventional soap-base greases. The lower the temperature was, the higher the flow pressure was. At the same temperature, their flow pressure values were increased with the increasing of the mass fraction of nano-silica.