**3.4 Role of bearing materials as catalysts**

Lubricant basestocks, in addition to being subjected to high temperatures and pressures, are also in contact with bearing surfaces which contain a combination of metals, metal oxides and surface carbides. Under normal circumstances, ferrous

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*Turbine Engine Lubricant and Additive Degradation Mechanisms*

metal are known to increase the rate of thermal degradation of polyol ester based lubricants, especially at temperatures above 220°C. The mechanism for this reaction, however is not completely understood [15]. The incorporation of phosphate esters is known to reduce the catalytic effect of ferrous metals, probably due to the formation of a phosphate film on the surface of the metal (see Section 4.1.2) [16].

Lubricant additives are in many ways designed to degrade as they serve their purpose in the formulated lubricant. As the lubricant is lost in service primarily due to leakage, new lubricant is added which act to replenish the additives used. Lubricant loss is typically estimated at as much as one quart per hour depending on the engine [17]. It is possible to use the amount of remaining additives to determine the need for engine service or lubricant replacement. One example of an instrument for the analysis of remaining antioxidant as an engine diagnostic is RULER [18].

Phosphate esters are normally required as an extreme pressure or anti-wear additive. The phosphate esters react with the metal surface to form a lubricious polymeric coating. The coating protects the bearing under conditions of start-up, inadequate flow or extreme shear, where the coating wears away, but is continuously reformed from unreacted phosphate ester in the lubricant. The mechanism of

Hydrolysis of phosphate esters is the reaction of the triester with water to form a diester and an aromatic alcohol. The diester can further react under the same conditions to form the monoester and eventually phosphoric acid. Two classes of mechanisms have been proposed for the hydrolysis in aqueous solu-

mechanisms then proceed through a penta coordinate phosphorous intermediate [20]. The likely mechanism in the non-polar lubricant medium where the attacking species is a water molecule is most likely through the associative mechanism,

is based on the addition of water to the phosphorus atom, followed by loss of a proton and elimination of the alcohol (phenol) [21]. The mechanism is shown

<sup>−</sup> anion and associative

<sup>−</sup> ion. The mechanism of the reaction

action of the additive causes its degradation over time [19].

tion, dissociative mechanisms that proceed through a PO3

which does not require formation of a PO3

schematically in **Figure 7**.

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

**4. Lubricant additive degradation**

*Mechanism for the β elimination reaction.*

**4.1 Phosphate esters**

**Figure 6.**

*4.1.1 Hydrolysis*

**Figure 5.** *A part of the mechanism for the oxidation of neopentyl polyols.*

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

**Figure 6.** *Mechanism for the β elimination reaction.*

*Aerospace Engineering*

esters is shown in **Figure 5**.

**3.3 Elimination reactions**

**3.4 Role of bearing materials as catalysts**

*A part of the mechanism for the oxidation of neopentyl polyols.*

[13, 14].

bond between the first carbon of the alcohol and the ester oxygen, followed by further oxidation at that carbon to form the organic acid [12]. After the initial attack, the reaction can progress to form anhydrides which continue to react to form aldehydes, acids and eventually high molecular weight compounds which can form sludge in the engine. The mechanism of the initial stages of the oxidation of the

Ester based lubricants have been observed to decompose One possible reaction of esters is an elimination reaction in which an alkene and a carboxylic acid are the products. The mechanism for this reaction involves the loss of a proton on the β carbon atom leading to the formation of a double bond and the elimination of the carboxylate

The use of alcohols without hydrogen atoms at the β carbon atom eliminates this mechanism, but under operating conditions of turbine engines, high temperature and metal catalyzed elimination reactions are possible. For this reason, modern ester based lubricants are based on neopentyl polyols, where elimination is blocked due to the lack of hydrogen atoms at the β position. Significant work has been conducted on optimizing the properties of the lubricant for use in turbine engines

Lubricant basestocks, in addition to being subjected to high temperatures and pressures, are also in contact with bearing surfaces which contain a combination of metals, metal oxides and surface carbides. Under normal circumstances, ferrous

anion. The mechanism for the β elimination reaction is shown in **Figure 6**.

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**Figure 5.**

metal are known to increase the rate of thermal degradation of polyol ester based lubricants, especially at temperatures above 220°C. The mechanism for this reaction, however is not completely understood [15]. The incorporation of phosphate esters is known to reduce the catalytic effect of ferrous metals, probably due to the formation of a phosphate film on the surface of the metal (see Section 4.1.2) [16].
