# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 3 Mechanism of Action of ILs as Lubricants
### Boundary lubrication (BL)  
Boundary lubrication occurs when the sliding surfaces are so close that the superficial interactions between films of lubricants and the asperities of the materials dominate the contact (see Figure 5c).  
This lubrication offers two different typologies of protective films: (1) _adsorbed layers_, which have a low shear rate promoting a better sliding between surfaces and (2) _protective layers_ that derive from chemical reactions of ILs with the surfaces. In more detail, for the  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{3}{c}{**Kinematic Viscosity**} \\  & **Fluid** & \multicolumn{2}{c}{**(mm2/s)**} & **Viscosity** \\ \cline{3-5}  & **40 °C** & **100 °C** & **Index** \\ \hline \hline (PP13)[TFSI] & & 52.5 & 10.0 & 181 \\ \hline \hline [P(h3)t][TFSI] & & 118 & 15.4 & 137 \\ \hline \hline [BMIM][PF4] & & 44.5 & 9.01 & 189 \\ \hline \hline [BMIM][PF6] & & 81.0 & 13.6 & 172 \\ \hline \hline [BMIM][TFSI] & & 29.0 & 6.88 & 211 \\ \hline \hline [BMIM][II] & & 203 & 21.0 & 122 \\ \hline \hline [BMIM][TCC] & & 14.6 & 3.7 & 146 \\ \hline \hline [EMIM][DCN] & & 8.6 & 3.01 & 252 \\ \hline \hline \end{tabular} \begin{tabular}{c c c c} \hline  & Synthetic oil (PAO4) & 16.8 & 3.9 & 129 \\ \hline \hline \end{tabular} \
\begin{tabular}{c c c c} \hline  & Synthetic oil (PAO6) & 30.5 & 5.79 & 135 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c} \hline  & Mineral base oil & 30.1 & 5.28 & 106 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Viscosity index of some ionic liquids and base oils at different temperatures.  
second mechanism, the tribochemical reactions may be promoted by temperature and pressure contact. In fact, the friction between two surfaces produces the conversion of mechanical energy into heat with the generation of local high temperatures. This allows the emission of electrons and of other particles (i.e., ions, lattice components, etc.) [65]. The reaction of emitted electrons with lubricants adsorbed on a surface may produce anions and radicals that are really responsible for its lubrication. A general study of [PERSON] et al. about the anionic-radical lubrications due to the action of low-energy electrons indicates that the negative ions produced by the exoelectron emission are chemisorbed on the positively charged friction microareas of metallic surfaces (or ceramics), furnishing an organometallic (or inorganic) protecting tribofilm for them (Figure 6, left panel) [66].  
In addition, the local high temperature of friction destructs the created lubricant layer and generates activated surface spots, forming a new protective film (Figure 6, right panel).  
In this framework, it is clear that the physics of the IL no longer plays a pivotal role, and the chemical/reactive aspect of the molecules involved needs to be taken into account. It is interesting to notice that, in this case, the assumption that the lubricating fluid needs to be an inert material is no longer valid, a consideration that needs to be pointed out for a panoramic vision of the overall phenomenon of lubrication.