Abstract
The nanofrictional behavior of non-halogentated phosphonium-based ionic liquids (ILs) mixed with diethylene glycol dibutyl ether in the molar ratios of 1:10 and 1:70 was investigated on the titanium (Ti) substrate using atomic force microscopy (AFM). A significant reduction is observed in the friction coefficient μ for the IL-oil mixtures with a higher IL concentration (1:10, μ ∼ 0.05), compared to that for the lower concentration 1:70 (μ ∼ 0.1). AFM approaching force-distance curves and number density profiles for IL-oil mixtures with a higher concentration revealed that the IL preferred to accumulate at the surface forming IL-rich layered structures. The ordered IL-rich layers formed on the titanium surface facilitated the reduction of the nanoscale friction by preventing direct surface-to-surface contact. However, the ordered IL layers disappeared in the case of lower concentration, resulting in an incomplete boundary layers, because the ions were displaced by molecules of the oil during sliding and revealed to be less efficient in friction reduction.
Highlights
Friction is one of the major causes that results in energy and material losses in many technical applications, including nano-fluidic technology, micro/nano electromechanical systems, etc
The non-directly observed films of neat oil or Ionic liquids (ILs)-oil mixtures enabled the reduction of surface roughness, leading to less cracks/scratches and particles
It is found that the higher IL concentration leads to tightly packed boundary layers on the Ti substrate, while the lower ILs concentration results in incomplete boundary layers and the ions of ILs are displaced by the molecules of oil during sliding
Summary
Friction is one of the major causes that results in energy and material losses in many technical applications, including nano-fluidic technology, micro/nano electromechanical systems, etc. Implementing lubrication technologies, such as solid or liquid lubricants, enable the decrease of friction and wear between the sliding contacts, potentially reducing the energy losses by 18%–40% [1]. While conventional liquid lubricants have insurmountable shortcomings [3], e.g., volatility, degradability, and narrow liquid range Such oil lubricants adhere weakly to solid surfaces, and can be squeezed out of the contacts during sliding, resulting in direct surface-to-surface contacts, providing a high friction [4]. ILs offer an ability to resist ‘squeeze out’ because of their strong interactions with solid surfaces, including van der Waals and electrostatic forces, as well as hydrogen-bonding interactions
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