**1. Introduction**

Lubrication is essential in applications where moving parts are involved. Aircraft propulsion systems involve large numbers of moving parts, many of which move at high speeds under severe temperatures and stresses. Turbine engine lubricants perform essential functions in reducing wear, reducing friction and dissipating heat from the engine. Modern engines are designed to operate at higher temperatures and shear rates, placing increased demands on the lubricants and additives. Typical turbine engine lubricants consist of a basestock which is a mixture of synthetic esters and a series of additives that modify the properties of the basestock. Additives are included to reduce oxidation of the basestock, reduce wear of the metal bearings or modify properties of the lubricant [1].

All lubricants, when subjected to high temperatures undergo degradation, which changes both the physical and chemical properties of the material. Physical property changes can include increases or decreases in viscosity, changes in boiling point or freezing point among others. Chemical properties that can change include corrosion of metals, formation of polymers and oxidation of the base stock. In addition to the basestock lubricants contain a range of additives that modify the properties of the basestock. Degradation of the additives reduces their effectiveness and can

result in the failure of the engine. In turbine engine applications, additive depletion is an important diagnostic of lubricant health.

In addition to lubricant degradation being important to engine health there are significant implications to human health. On the vast majority of commercial aircraft, the air used to pressurize the cabin is drawn from the engine just after the compressor section. Lubricant degradation products have been shown to pass from the engine into the cabin on seal failure with severe health effects. Of perhaps greater significance is the normal low level leakage of lubricants and degradation products into the cabin under normal flight conditions. It is known that all seals leak some and some of the leaked material can be transmitted into the passenger cabin as both vapors and nano-droplets. The chronic toxicity of these materials is of great concern [2].

In this chapter, the composition of typical turbine engine lubricants will be presented in Section 2. The decomposition mechanisms of the basestock are presented in Section 3, followed by the additive degradation mechanism in Section 4. Finally, in Section 5 synergistic and antisynergistic interactions of lubricants and additives are examined. Changes in bearing systems and the incorporation of ionic liquids and nanoparticles will be included and finally in Section 6, some of the consequences of lubricant degradation will be examined.
