**5. Synergistic reactions between lubricants, additives and bearing materials**

The reactions of the individual components are not always sufficient to predict the chemistry of a formulated lubricant. Some reactions are inhibited by the

**87**

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

instead of the individual reactivities of the various components.

additives, but may be accelerated by combinations of additives and surface chemistries. Rolling contact fatigue testing with M-50 bearings, for example indicated that PANA and DODPA added to a lubricant along with tricresyl phosphate resulted in an increase in wear over systems where the PANA and DODPA were absent [35]. An explanation might include the antioxidants reduce the oxidation of the metal surface which interferes with the binding of the phosphate ester to the surface [36]. Another system where results are unpredictable is when advanced bearing materials are used with polyolesters and phosphate esters. These observations demonstrate the importance of considering all of the components in the lubrication system

**6. Incorporation of advanced bearing steels, ionic liquid additives and** 

The need for more efficient and more powerful jet engines for military and commercial applications has caused a need for lighter, more durable bearing materials. Harder metal alloys and ceramic bearings are approaches to serve these needs. Changes in bearing materials, however may not be completely compatible with current lubri-

Many advanced bearing materials are made from carburized stainless steels. The materials begin with a stainless steel which can be formed into the desired shape. The part is then heat treated in the presence of a carbon source resulting in the formation of surface carbides [37]. The surface carbides increase the hardness of the surface significantly. Phosphate esters have been shown to interact with the stainless steels in the absence of carburization [38], but in the presence of all three components, metal carbides, phosphate esters and polyol esters the decomposition is much more rapid [39]. When carburized bearings were tested with polyol ester based lubricants formulated with phosphate esters, an increase in fatigue life and

Ceramic bearings have good potential for high temperature use in turbine engines. Unlubricated ceramic bearings performed poorly, however when an appropriate lubricant was added they performed better [41]. Typical lubricant additives, however did not perform well under conditions typically seen in steel bearings. At very high temperatures, a film was formed but it did not decrease friction or increase bearing life [42]. To form a lubricious anti-wear coating, the ceramics were pretreated to introduce a thin film of iron which allowed the phosphates to from an

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

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

**nanopatritle based additives**

cant basestocks and additive packages.

wear performance was observed [40].

anti-wear coating [43].

**6.2 Ionic liquid additives**

**6.1 Advanced bearing steels and ceramic bearings**

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

*Aerospace Engineering*

Aerospace lubricants typically rely on the hindered aryl amines N-phenyl-1 naphthylamine (PANA) and p-dioctyldiphenyl amine (DODPA) (structures shown in **Figure 3**) as antioxidants because they have the potential to react with a greater number of hydroperoxy radicals [31]. There are two very common mechanisms in which aryl amines act as antioxidants, a low temperature (<120°C) and a high temperature mechanism (>120°C). A common feature of the mechanisms is the reaction of the amine to form radicals. These reactions form aminoxy radicals to form N-alkoxyamines which appear to be the actual antioxidant species [32]. The high temperature mechanism through which aryl amines act as antioxidants is

*High temperature mechanism for the antioxidant activity of alkylated diphenyl amine antioxidants.*

Other mechanisms that have been reported examined the possibility that the diphenyl amine radical formed in the first step in **Figure 14** could disproportionate and then react with itself to form more complex species that eventually lead to poly conjugated systems upon reaction with additional hydroperoxy radicals. The reaction of N-phenyl-1-naphthylamine proceeds somewhat differently due to the susceptibility of the α hydrogen of the naphthyl ring to radical attack leading to the formation of dimers and higher polymers as in **Figure 15** [33] or the formation

**5. Synergistic reactions between lubricants, additives and bearing** 

The reactions of the individual components are not always sufficient to predict the chemistry of a formulated lubricant. Some reactions are inhibited by the

**86**

shown in **Figure 14**.

**Figure 15.**

**Figure 14.**

**materials**

of quinone imines and naphthoquinones [34].

*Products of the reaction of PANA as an antioxidant in lubricants.*

additives, but may be accelerated by combinations of additives and surface chemistries. Rolling contact fatigue testing with M-50 bearings, for example indicated that PANA and DODPA added to a lubricant along with tricresyl phosphate resulted in an increase in wear over systems where the PANA and DODPA were absent [35]. An explanation might include the antioxidants reduce the oxidation of the metal surface which interferes with the binding of the phosphate ester to the surface [36]. Another system where results are unpredictable is when advanced bearing materials are used with polyolesters and phosphate esters. These observations demonstrate the importance of considering all of the components in the lubrication system instead of the individual reactivities of the various components.
