**8. Conclusion**

The common mechanisms that degrade lubricant basestocks and additives have been discussed in the sections above. The degradation of the basestock is considered to be of greatest concern for the general health of the engine. The degradation of

**89**

**Author details**

lubricants a reality.

David W. Johnson

provided the original work is properly cited.

University of Dayton, Dayton, Ohio, USA

\*Address all correspondence to: djohnson1@udayton.edu

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

not depleted completely they will function in that capacity.

oxidation of sulfur and nitrogen compounds found in mineral oils.

the additives is a large part of how they work. Considerable effort has been put into finding additives that react appropriately, and are of limited volatility and thermally stable. They are included as part of the lubricant to degrade, and as long as they are

Through molecular design of the esters used in the basestock, the importance of some of the mechanisms have been reduced. Modern esters used in the basestock are based on polyols that do not base hydrogen atoms in the β position making β elimination impossible by this mechanism. Hydrolysis is of significant concern, since it both produces acids and alters the physical properties (viscosity, and pour point in particular) of the lubricant. Oxidation also has the potential to produce acids and change the physical properties of the lubricant. The addition of better and better antioxidants has reduced the importance of this mechanism. It also should be noted that the acids produced by either oxidation or hydrolysis are carboxylic acids which are much less corrosive than the mineral acids frequently formed by the

Lubricants are under development that will continue to increase the operating temperature without significant degradation of their properties. Molecular design has been used to slow the various basestock degradation mechanisms through the choice of the acids used to form polyol esters can block both oxidation and hydrolysis. The knowledge of these mechanisms has made preparation of high performance

*DOI: http://dx.doi.org/10.5772/intechopen.82398*

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

## *Turbine Engine Lubricant and Additive Degradation Mechanisms DOI: http://dx.doi.org/10.5772/intechopen.82398*

*Aerospace Engineering*

**6.3 Nanoparticle based lubricant additives**

in current use in aerospace liquid lubricants.

**7. Consequences of lubricant degradation**

important that all of these effects be minimized for safe air travel.

ments for aircraft using bleed air for cabin pressurization [58].

The common mechanisms that degrade lubricant basestocks and additives have been discussed in the sections above. The degradation of the basestock is considered to be of greatest concern for the general health of the engine. The degradation of

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].

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

Lubricant degradation has a significant effect on the properties of the lubricant which can have significant consequences in aerospace. Degradation results in an increase in the chemical reactivity of the oil through the formation of acid and bases, changes in viscosity and changes in thermal conductivity. All of these can result in reduced life of the engine and also decreased operational efficiency. It is

There is an additional safety concern associated with lubricants and their degradation products present in most commercial and military aircraft. Air used to pressurize the cabin is drawn from the engine through a bleed air nozzle. While under normal operation, the air is thought to be safe, seal leakage results in traces of lubricant directed into the cabin. In cases of seal failure, high concentrations of lubricants, additive and degradation products enter the cabin. Smoke events are caused by seal failures, as well as other causes. Fume events occur in 2.1 of every 10,000 flights [51] and oil fumes are noted in 1% of all flights. The health related concerns are indicated by the 30% of fume events where crew impairment has been recorded even though there is recognized under reporting of impairment [52]. Aerotoxic syndrome has been described as an occupational illness along with epidemiological evidence [53]. Possible toxicological mechanism leading to aerotoxic syndrome has been described by Howard et al. [54]. A possible cause for Aerotoxic syndrome is based on repeated low dose exposure to organo-phosphorus compounds derived from phosphate esters [55]. High doses of organophosphates are known to cause organophosphate induced peripheral neuropathy (OPIDN) [56], however the doses encountered here are much lower, suggesting other chronic mechanisms [57]. The toxicity evidence indicates the need for clean air require-

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**8. Conclusion**

the additives is a large part of how they work. Considerable effort has been put into finding additives that react appropriately, and are of limited volatility and thermally stable. They are included as part of the lubricant to degrade, and as long as they are not depleted completely they will function in that capacity.

Through molecular design of the esters used in the basestock, the importance of some of the mechanisms have been reduced. Modern esters used in the basestock are based on polyols that do not base hydrogen atoms in the β position making β elimination impossible by this mechanism. Hydrolysis is of significant concern, since it both produces acids and alters the physical properties (viscosity, and pour point in particular) of the lubricant. Oxidation also has the potential to produce acids and change the physical properties of the lubricant. The addition of better and better antioxidants has reduced the importance of this mechanism. It also should be noted that the acids produced by either oxidation or hydrolysis are carboxylic acids which are much less corrosive than the mineral acids frequently formed by the oxidation of sulfur and nitrogen compounds found in mineral oils.

Lubricants are under development that will continue to increase the operating temperature without significant degradation of their properties. Molecular design has been used to slow the various basestock degradation mechanisms through the choice of the acids used to form polyol esters can block both oxidation and hydrolysis. The knowledge of these mechanisms has made preparation of high performance lubricants a reality.
