**6.2 Ionic liquid additives**

Ionic liquids have been considered as potential replacements for both basestocks and additives. As a potential replacement for the basestock, increased costs make them inappropriate for use in turbine engines [44]. A number of ionic liquids are under investigation for use as anti-wear of extreme pressure additives. These additives contain phosphorus in either the cation, as a phosphonium ion or the anion as a tri-alkyl phosphate. Ionic liquids that incorporate the phosphorus in the phosphate anion have been shown to be the most effective [45]. Ionic liquids containing tri-alkyl phosphates interact strongly with metal surfaces through mechanisms also seen in the tri-aryl phosphates [46] discussed in Section 3.1. Ionic liquids with phosphonium cations with a non-phosphate anion have shown superior performance under high load [47]. Ionic liquids have the advantage of reduced volatility, which is important in some applications. Ionic liquid based anti-wear additives show some of the same interferences with antioxidants that are observed with triaryl phosphates, performing better in oils where the additives have been depleted [48].

## **6.3 Nanoparticle based lubricant additives**

Nanomaterials and nanoparticles have been studied for use as additives in liquid lubricants. Some of the initial problems that have been discovered are the dispersion of the nanoparticles and the stability of the dispersion. Capping metal nanoparticles with a monolayer of non-polar organic molecules have resulted in nanoparticles that are oil soluble [49]. A wide range of nanoparticles have been studied and several have shown promise for use in liquid lubricants. Chemical composition was found to be important in anti-wear performance, where morphology and size of the particles were more important in friction reduction. Nanoparticles with layered structures were among the better morphologies [50]. Nanomaterials as lubricant additives appear to have a bright future in lubrication, although none are in current use in aerospace liquid lubricants.
