# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 5 Some Applications
### Titanium  
Titanium is an attractive material with wide applications in industry due to a high strength and excellent resistance to corrosion and wear. In fact, an oxidized titania layer is generally present on the titanium surface, preventing material wear [112]. However, a critical problem of titanium use is its poor lubrication capacity with classical lubricants. Considering the high affinity of ILs for solid surfaces and their robust boundary layers, [PERSON] et al. investigated mixtures of IL trihexyl- (tetradecyl)phosphonium bis(2,4,4-trimethyl)phosphine with hexadecane at different concentrations [113]. As already discussed, sometimes the costs of production of ILs can be a limitation in using ILs as lubricants, and for this reason, ILs as an additive can be often a valid solution for industrial applications. The researchers showed that pure IL and some IL/hexadecane mixtures at different loads remarkably reduced the friction coefficient compared to pure hexadecane and to its mixture with a traditional antiwear additive (zinc-dialkyl-dithophosphate; ZDDP) (Table 5).  
In 2019, four quaternary phosphonium ILs with variable structures (Figure 17) were selected as a titanium lubricant, considering that quaternary phosphonium-based ILs can easily be dissolved in apolar hydrocarbon oils, including mineral oil, and are generally very stable [114].  
Consequently, they show optimal tribological characteristics both as pure components and as diluted in hydrocarbon oils. In particular, the friction reduction seems to be strongly dependent on IL viscosity because the less viscous P\({}_{6,6,6,14}\) TFSI was, the more effective IL was for macrotribology performance in a mixed lubrication regime (Table 6). On the contrary, in nanotribology tests, only the structure of the ILs was really important; in fact, the cation of P\({}_{8,8,8,6(2)}\) BEHP adsorbed better to the titanium surface, forming a resistant film that minimized the energy dissipation. On the other hand, this different behaviour is not surprising, because both the variable alkyl chains of cations and the chemical structure of the bis-trifluoromethylsulfonyl)imide (NTF2) anion, which is different than other ILs characterized by phosphate and phosphonate groups, can dramatically affect both their chemical-physical properties and their chemical interactions with the sliding surface.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**IL** & **Friction Coefficient** \\ \hline P\({}_{8,8,8,6(2)}\) BEHP & 0.08 \\ P\({}_{6,6,6,6,14}\) BEHP & 0.33 \\ P\({}_{6,6,6,6,14}\)(C\({}_{8}\))\({}_{2}\)PO\({}_{2}\) & 0.16 \\ P\({}_{6,6,6,6,14}\) TFSI & 0.17 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Friction coefficients of some ILs used as lubricants.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Lubricant**} & \multicolumn{3}{c}{**Friction Coefficient**} \\ \cline{2-3}  & **2N** & **10N** \\ \hline Hexadecane (HD) & 0.45 \(\pm\) 0.05 & 0.46 \(\pm\) 0.05 \\ HD + 0.01\% IL & 0.46\(\pm\) 0.04 & 0.47 \(\pm\) 0.04 \\ HD + 0.1\% IL & 0.47\(\pm\) 0.05 & 0.48 \(\pm\) 0.04 \\ HD + 1\% IL & 0.45 \(\pm\) 0.05 & 0.46 \(\pm\) 0.05 \\ HD + 2\% IL & 0.11 \(\pm\) 0.02 & 0.45\(\pm\) 0.06 \\ HD + 10\% IL & 0.12 \(\pm\) 0.02 & 0.47\(\pm\) 0.05 \\ Pure IL & 0.09 \(\pm\) 0.01 & 0.09\(\pm\) 0.01 \\
2\% ZDDP & 0.42 \(\pm\) 0.06 & 0.45\(\pm\) 0.04 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Lubrication activity at different load of hexadecane and its mixture with IL and ZDDP.