(709h) Low Concentration of Ionic Liquids in Glycol Ether: Interfacial Layering Friction Behavior

The lubrication failure is the main cause that results in the breakdown of the triboelements, in many engineering applications ranging from aerospace to micro/nano electro-mechanical systems. This lubrication problem can even inflict fatal damage to the spacecrafts, and impede the human exploration of the universe. Ionic liquids (ILs) are considered to be lubricant and candidate to substitute traditional volatile organic solvents owing to unique and desirable physicochemical properties, such as low vapor pressure, high thermal stabilities, wide temperature range, high conductivity, excellent solubility and good tribological properties.

However, liquid lubrication is mainly in the hydrodynamic and elastohydrodynamic regimes, where the main cause of friction is hydrodynamic drag. This consumes 10-15% of the total energy and is proportional to the lubricant viscosity. One method to improve ILs lubricating efficiency is to use less viscous lubricants by employing ILs as additives in base oils. We therefore in this work, dissolved 25wt% halogen-free ILs in base oil, diethylene glycol dibutyl ether, and then evaluated the nanotribological performance for the IL-base oil mixtures at titamium interfaces.

A layering friction behavior of ionic liquid-glycol ether oil mixtures (1:3, wt/wt) at Ti interfaces were observed by atomic force microscopy (AFM), in which, the characteristic two regime friction behavior is maintained. The friction decreases with the increasing load at smaller loads (<15~20 nN), while the monotonic increase of friction with load was observed in the high load regime (>20 nN). The decreasing-then-increasing friction trend with the transition from negative to positive load dependence, was so-called layering friction behavior. Similar layering phenomena appeared in force-separation profiles in IL-base oil mixtures further supports the layering friction behavior. We mimic the tip approaching the surface at two regimes by varying slit pore widths between the tip and surface using molecular simulation. The number density profiles exhibit IL- and base oil-rich regimes between the tip and TiO₂ walls, indicative of the appearance of two significant IL-base oil layering structures. The distinguishable diffusion features in two regimes, i.e., larger and smaller slit pore widths, essentially confirm the robustness of the characteristic layering friction behavior, two regimes in force-separation profiles, and layered density distribution. This remarkable layering friction behavior may help to further provide implications for the development of lubrication strategies.