*4.1.2 Polymerization at metal surface*

Phosphate esters are used as anti-wear of extreme pressure lubricants and work by reactions with the bearing surface to form a polymeric coating that is durable and lubricious [22]. The reaction normally occurs at the oxidized metal surface and results in the formation of an initial layer of graphite, followed by a layer of an iron rich, iron polyphosphate [23]. After the initial coating is formed the film can increase in thickness as iron diffuses to the surface [24]. The coating continuously wears away during use and is reformed as iron diffuses through the coating. The nature of the polymeric lubricous film is shown in **Figure 8**.

The mechanism for the formation of a polyphosphate polymer begins with the bonding of the phosphate ester (typically tricresylphosphate) to the oxidized iron surface, displacing cresol. The initial steps of the mechanism that leads to the formation of a coating is shown in **Figure 9**.

The bound phosphate reacts further with other bound phosphate esters displacing additional cresol leading to the formation of a polymeric coating strongly bound to the metal surface. Typically, on the surface of the metal some of the partially reacted phosphate remains. X-ray photoelectron spectroscopy results show a surface composition corresponding to approximately one cresol remaining per phosphorus atom on the surface as is shown in **Figure 8**.

Under extreme pressure conditions, the outer layers are removed from the surface and are lost as polymeric phosphorus containing nanoparticles which are not reconverted to the triaryl phosphate in the lubricant. It should be noted that this mechanism explains how the phosphate esters act as an anti-wear additive but it also

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settle in the oil sump.

**Figure 8.**

**Figure 9.**

*4.1.3 Trans esterification lubricant esters*

*Mechanism of phosphate film formation and structure of phosphate film.*

reaction with trimethylolpropane [25, 26].

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

leads to the degradation of the phosphate esters. The formation of nanoparticles through the wear of the coating formed at the bearing surface leads to a darkening of the oil color, but many of these particles are remove by filtration or eventually

The last of the reactions of phosphate esters is the reaction between phosphate esters and lubricant esters to form aryl esters and alkyl phosphate esters. This is a reaction that can occur in either a single step or could initially for the acid which can further react to form another ester. The single step process is shown in **Figure 10**. This reaction can be of particular concern since the alkyl phosphate formed can undergo transesterification intra molecularly to form the product shown in **Figure 11** which is structurally similar to the known neurotoxin which would be formed by a similar

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

*Schematic representation of the iron phosphate film.*

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

#### **Figure 8.**

*Aerospace Engineering*

Hydrolysis is an important degradation mechanism because it forms a range of phosphate partial esters, some of which do not form a lubricous coating on the

Phosphate esters are used as anti-wear of extreme pressure lubricants and work by reactions with the bearing surface to form a polymeric coating that is durable and lubricious [22]. The reaction normally occurs at the oxidized metal surface and results in the formation of an initial layer of graphite, followed by a layer of an iron rich, iron polyphosphate [23]. After the initial coating is formed the film can increase in thickness as iron diffuses to the surface [24]. The coating continuously wears away during use and is reformed as iron diffuses through the coating. The

The mechanism for the formation of a polyphosphate polymer begins with the bonding of the phosphate ester (typically tricresylphosphate) to the oxidized iron surface, displacing cresol. The initial steps of the mechanism that leads to the

Under extreme pressure conditions, the outer layers are removed from the surface and are lost as polymeric phosphorus containing nanoparticles which are not reconverted to the triaryl phosphate in the lubricant. It should be noted that this mechanism explains how the phosphate esters act as an anti-wear additive but it also

The bound phosphate reacts further with other bound phosphate esters displacing additional cresol leading to the formation of a polymeric coating strongly bound to the metal surface. Typically, on the surface of the metal some of the partially reacted phosphate remains. X-ray photoelectron spectroscopy results show a surface composition corresponding to approximately one cresol remaining per phosphorus

bearing and contribute to the acids contained in the lubricant.

*Mechanism for the hydrolysis of phosphate esters in polyol ester-based lubricants.*

nature of the polymeric lubricous film is shown in **Figure 8**.

formation of a coating is shown in **Figure 9**.

atom on the surface as is shown in **Figure 8**.

*4.1.2 Polymerization at metal surface*

**Figure 7.**

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*Schematic representation of the iron phosphate film.*

#### **Figure 9.**

*Mechanism of phosphate film formation and structure of phosphate film.*

leads to the degradation of the phosphate esters. The formation of nanoparticles through the wear of the coating formed at the bearing surface leads to a darkening of the oil color, but many of these particles are remove by filtration or eventually settle in the oil sump.

#### *4.1.3 Trans esterification lubricant esters*

The last of the reactions of phosphate esters is the reaction between phosphate esters and lubricant esters to form aryl esters and alkyl phosphate esters. This is a reaction that can occur in either a single step or could initially for the acid which can further react to form another ester. The single step process is shown in **Figure 10**.

This reaction can be of particular concern since the alkyl phosphate formed can undergo transesterification intra molecularly to form the product shown in **Figure 11** which is structurally similar to the known neurotoxin which would be formed by a similar reaction with trimethylolpropane [25, 26].

**Figure 10.**

*Transesterification of a phosphate ester with a lubricants ester to form an alkyl phosphate and an aryl ester.*

#### **Figure 11.**

*Final product of the transesterification of pentaerythritol ester (A) and the known neurotoxin formed from trimethylol propane (B).*

The structure shown in **Figure 11(A)** assumes the final acid group has been hydrolyzed. Either this compound or the corresponding ester might be assumed to have a toxicity comparable or greater than the compound shown in **Figure 11(B)**.

#### *4.1.4 Addition to pendant groups*

A final reaction that occurs with phosphate ester additives is addition reactions on the pendant aromatic rings. In this reaction, the carbon–oxygen bond in a phosphate ester is broken at the metal surface. The leaving group remains at the metal surface until it is added to another molecule of phosphate ester [27]. The mechanism for the formation of addition products is shown in **Figure 12**.

These addition reaction result in higher molecular weight species that might in part be responsible for the formation of the layer of carbon, initially described as a carbide layer [28], but later determined to be either amorphous carbon or low order graphite [29], immediately adjacent to the iron surface. This layer is consistently observed in Auger spectroscopy as is shown in **Figure 13**.

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**4.2 Antioxidants**

*(sputter rate 1.5 nm/min).*

**Figure 13.**

**Figure 12.**

*phosphate.*

Synthetic lubricants are oxidative degraded via a radical chain mechanism at high temperatures. Molecular oxygen abstracts a hydrogen atom forming a free radical. The radical reacts with the basestock abstracting hydrogen atoms or other groups, adding that fragment and creating a new radical and in general increasing the size of the molecule. The chain mechanism continues until the growing chain encounters another radical, resulting in chain termination. Antioxidants are typically added to the lubricant formulation to reduce the rate of lubricant decomposition by reacting with radicals formed in the initiation step of lubricant oxidation. Anti-oxidant additives can act in two different ways. First, they can react with oxygen to form a stable species reducing the possibility of the chain initiation step in the mechanism. Second, the antioxidant can react with radicals formed, forming a more stable species and acting as a chain termination step [30]. Among the most common types of

*Auger depth profile of a film formed by the deposition of BTPP onto an iron foil at 425°C under nitrogen* 

*Reaction of phosphate esters with reduced metal surfaces showing the addition of a tolyl group to triphenyl* 

antioxidants used in lubricants are hindered phenols and aromatic amines.

*Turbine Engine Lubricant and Additive Degradation Mechanisms*

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

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

**Figure 12.**

*Aerospace Engineering*

**Figure 10.**

**Figure 11.**

*trimethylol propane (B).*

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*4.1.4 Addition to pendant groups*

The structure shown in **Figure 11(A)** assumes the final acid group has been hydrolyzed. Either this compound or the corresponding ester might be assumed to have a toxicity comparable or greater than the compound shown in **Figure 11(B)**.

*Final product of the transesterification of pentaerythritol ester (A) and the known neurotoxin formed from* 

*Transesterification of a phosphate ester with a lubricants ester to form an alkyl phosphate and an aryl ester.*

A final reaction that occurs with phosphate ester additives is addition reactions on the pendant aromatic rings. In this reaction, the carbon–oxygen bond in a phosphate ester is broken at the metal surface. The leaving group remains at the metal surface until it is added to another molecule of phosphate ester [27]. The mechanism for the formation of addition products is shown in **Figure 12**.

These addition reaction result in higher molecular weight species that might in part be responsible for the formation of the layer of carbon, initially described as a carbide layer [28], but later determined to be either amorphous carbon or low order graphite [29], immediately adjacent to the iron surface. This layer is consistently

observed in Auger spectroscopy as is shown in **Figure 13**.

*Reaction of phosphate esters with reduced metal surfaces showing the addition of a tolyl group to triphenyl phosphate.*

#### **Figure 13.**

*Auger depth profile of a film formed by the deposition of BTPP onto an iron foil at 425°C under nitrogen (sputter rate 1.5 nm/min).*

#### **4.2 Antioxidants**

Synthetic lubricants are oxidative degraded via a radical chain mechanism at high temperatures. Molecular oxygen abstracts a hydrogen atom forming a free radical. The radical reacts with the basestock abstracting hydrogen atoms or other groups, adding that fragment and creating a new radical and in general increasing the size of the molecule. The chain mechanism continues until the growing chain encounters another radical, resulting in chain termination. Antioxidants are typically added to the lubricant formulation to reduce the rate of lubricant decomposition by reacting with radicals formed in the initiation step of lubricant oxidation.

Anti-oxidant additives can act in two different ways. First, they can react with oxygen to form a stable species reducing the possibility of the chain initiation step in the mechanism. Second, the antioxidant can react with radicals formed, forming a more stable species and acting as a chain termination step [30]. Among the most common types of antioxidants used in lubricants are hindered phenols and aromatic amines.

**Figure 14.** *High temperature mechanism for the antioxidant activity of alkylated diphenyl amine antioxidants.*

**Figure 15.** *Products of the reaction of PANA as an antioxidant in lubricants.*

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 shown in **Figure 14**.

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 of quinone imines and naphthoquinones [34].
