**6. Lubricant additives**

Additives are a part of all lubricant systems because they can impart properties to the overall lubricant that the basestock does not possess. They can also allow longer lubricant lifetimes by eliminating certain modes of basestock decomposition. For example, when a lubricant is used, it is exposed to temperatures much higher than the bulk oil temperature for short periods of time, in the presence of oxygen. Additives can reduce the decomposition of the basestock by scavenging free radicals formed in the initial stages of the reaction. Aircraft lubricants typically contain the following groups of additives:


The choices of additives are frequently limited based on the thermal stability of the addi‐ tive, needs for long lifetimes, and tendencies of the additives to form deposits [12]. The mechanisms of action associated with each of the types of additive are described in the following sections.

#### **6.1. Antioxidants**

One of the primary decomposition reactions of lubricants is oxidation primarily by atmos‐ pheric oxygen. In lubricants derived from crude oil, there are normally enough naturally occurring sulfur compounds to inhibit oxidation for a period of time. Synthetic lubricants and some highly refined natural lubricants, however, must have additives to reduce or eliminate oxidation in the presence of oxygen. Oxidation of lubricants typically leads to an increase in viscosity and the formation of sludge and is the primary limit to the maximum bulk oil temperature.

Most classes of natural and synthetic lubricants are oxidatively degraded via a radical chain mechanism. In this mechanism, molecular oxygen attracts a hydrogen atom creating a free radical. The radical formed reacts with other lubricant molecules attracting hydrogen atoms or other groups, generally increasing the size of the molecule. The chain continues until it encounters another radical, resulting in chain termination.

Antioxidant additives can act in two different ways. First, they can react with oxygen to form a stable species. Second, the antioxidant can react with radicals formed, acting as a chain termination step. Among the most common antioxidants are hindered phenols and aromatic amines. The structure of these compounds is shown in **Figure 9**.

**Figure 9.** Structures of some common antioxidants.

**6. Lubricant additives**

46 Recent Progress in Some Aircraft Technologies

**•** Antioxidants

**•** Anti-corrosion additives

**•** Anti-foaming additives

**•** Viscosity index improvers

**•** Metal deactivators

following sections.

**6.1. Antioxidants**

temperature.

typically contain the following groups of additives:

Additives are a part of all lubricant systems because they can impart properties to the overall lubricant that the basestock does not possess. They can also allow longer lubricant lifetimes by eliminating certain modes of basestock decomposition. For example, when a lubricant is used, it is exposed to temperatures much higher than the bulk oil temperature for short periods of time, in the presence of oxygen. Additives can reduce the decomposition of the basestock by scavenging free radicals formed in the initial stages of the reaction. Aircraft lubricants

The choices of additives are frequently limited based on the thermal stability of the addi‐ tive, needs for long lifetimes, and tendencies of the additives to form deposits [12]. The mechanisms of action associated with each of the types of additive are described in the

One of the primary decomposition reactions of lubricants is oxidation primarily by atmos‐ pheric oxygen. In lubricants derived from crude oil, there are normally enough naturally occurring sulfur compounds to inhibit oxidation for a period of time. Synthetic lubricants and some highly refined natural lubricants, however, must have additives to reduce or eliminate oxidation in the presence of oxygen. Oxidation of lubricants typically leads to an increase in viscosity and the formation of sludge and is the primary limit to the maximum bulk oil

Most classes of natural and synthetic lubricants are oxidatively degraded via a radical chain mechanism. In this mechanism, molecular oxygen attracts a hydrogen atom creating a free radical. The radical formed reacts with other lubricant molecules attracting hydrogen atoms or other groups, generally increasing the size of the molecule. The chain continues until it

Antioxidant additives can act in two different ways. First, they can react with oxygen to form a stable species. Second, the antioxidant can react with radicals formed, acting as a chain termination step. Among the most common antioxidants are hindered phenols and aromatic

encounters another radical, resulting in chain termination.

amines. The structure of these compounds is shown in **Figure 9**.

**•** Boundary lubrication additives (anti-wear, lubricity, and extreme pressure)

As higher-temperature lubricants are being developed, these antioxidants are not sufficiently stable, which has led to the introduction of some high molecular weight polymeric antioxi‐ dants. Although the structures of these systems are proprietary, they solve similar problems and may involve similar chemistry.

All of the antioxidants above react to quench radical chain reactions. Because the amine antioxidants are common in aircraft lubricants, their mechanism will be discussed in more detail. It is commonly thought that the hindered amines are rapidly oxidized to the aminoxy radical. This radical reacts with alkyl radicals, trapping them to form N-alkoxy amines, which react with peroxyl radicals to give products and regenerate the aminoxy radical [13]. The oxidation of BHT has also been thoroughly studied. The initial oxidation by a radical removes a hydrogen atom forming a phenoxy radical that then reacts to form a quinoid, which is relatively stable.

## **6.2. Anti-corrosion additives**

A corrosion inhibitor is a compound that decreases the rate of corrosion of a material, typically a metal or metal alloy. Corrosion inhibitors are of greater importance in the MIL-PRF-23699 CI lubricants primarily because of their intended use in more corrosive environments. Typically, with iron-based alloys (steel), corrosion occurs due to the presence of oxygen in contact with the steel, which results in the formation of rust. Corrosion inhibitors act by forming a passive layer on the surface of the alloy that protects the surface from further oxidation. Metal surfaces are typically covered with a layer of oxides and hydroxides formed when the metal is placed in contact with air. In some metals, such as aluminum, the oxide layer protects the surface from further oxidation (passivation). In iron-based alloys, however, the oxide layer is porous and further protection is needed.

Typical rust inhibitors include metal sulfonates and metal carboxylates. These additives act because they have an ionic head group that can bind to the metal oxide surface and a nonpolar tail that forms a protective coating over the metal. These additives, however, are not ash less and may not qualify for the MIL-PRF-23699 CI standard. A product that avoids this problem consists of a mixture of an amine, carboxylic acid esters, and a phosphate ester. The mixture itself is proprietary; however, the MDS does at least give an approximate composition.

## **6.3. Anti-foaming additives**

Foam is formed when air is trapped in a liquid forming bubbles. It is frequently observed when a stream of liquid re-enters the bulk liquid, such as when fuel is added to a fuel tank. Foams can result in reduced capacity and reduced pumping efficiency and prevent lubricants from effectively flowing. In aircraft applications, lubricants should form minimal amounts of foam and the foam formed should rapidly collapse. Typically, lubricant foaming is caused by contamination with high molecular weight silicone greases, which form foams that do not easily collapse. Foams collapse when the air bubbles merge forming larger bubbles that rise and the bubbles pop releasing the air at the surface [14].

Anti-foaming additives are all surface active agents that are insoluble in the lubricant. They typically are of low viscosity, which allows them to spread over the surface of the oil. Many also contain some solid particles dispersed in the additive. When a foam forms, the bubbles have structures similar to lipid bilayers called lamella. The anti-foaming additive locates itself at the air-liquid interface, maximizing the surface area of the foam bubbles. Smaller bubbles merge and the larger bubbles move toward the surface, causing the foam to collapse.

Aircraft lubricant foam control additives are typically moderate molecular weight silicone oils. It have been observed that silicone oils can act in many ways, causing foam in some cases and eliminating the foam in others depending on molecular weights and structures. The current anti-foaming additives have been shown to successfully eliminate foaming in the lubrication system.

### **6.4. Boundary lubrication additives**

In aircraft applications, a lubricant must be effective in different lubrication regimes, including the fluid film lubrication regimes (hydrodynamic lubrication and elasto-hydrodynamic lubrication) and boundary lubrication. Hydrodynamic lubrication, where the two bearing surfaces are separated by a complete film of the lubricant, is generally desirable. As the shear stress on the bearing increases, the lubricant properties are not adequate and the film is no longer of sufficient thickness. Elasto-hydrodynamic lubrication occurs between rolling bodies [15]. These systems operate under stresses where a fluid film would not be expected to be maintained. The high pressure, however, causes a temporary increase in the viscosity of the lubricant in the contact area and a fluid film is maintained. Boundary lubrication involves direct contact between the surfaces of the bearings. Boundary lubrication is generally unde‐ sirable but is also unavoidable especially during start-up and shutdown. Boundary lubrication additives are required to reduce friction between and wear of the engine components until lubricant flow is adequate to maintain fluid film lubrication.

A number of boundary lubrication additives have been developed for various purposes and the vast majority are based on phosphorus or sulfur compounds. For aircraft applications, sulfur compounds are generally less desirable because of the lack of stability and the formation of strong acids on oxidation. Phosphorus compounds can include phosphate esters, thiophos‐ phate esters, and metal thiophosphates. Metal thiophosphates and thiophosphate esters contain sulfur, and the metal dithiophosphates are more likely to form metal containing ash on decomposition. For aircraft applications, phosphate esters and, in particular, triaryl phosphates are the additives of choice.

Triaryl phosphates are characterized by three aromatic rings attached to the phosphorus through oxygen linkages. The most common commercial additives are tricresyl phosphate and butylated triphenyl phosphate. The structures of these compounds are shown in **Figure 10**.

**Figure 10.** Structures of some common tri-aryl phosphate esters.

**6.3. Anti-foaming additives**

48 Recent Progress in Some Aircraft Technologies

system.

**6.4. Boundary lubrication additives**

and the bubbles pop releasing the air at the surface [14].

lubricant flow is adequate to maintain fluid film lubrication.

Foam is formed when air is trapped in a liquid forming bubbles. It is frequently observed when a stream of liquid re-enters the bulk liquid, such as when fuel is added to a fuel tank. Foams can result in reduced capacity and reduced pumping efficiency and prevent lubricants from effectively flowing. In aircraft applications, lubricants should form minimal amounts of foam and the foam formed should rapidly collapse. Typically, lubricant foaming is caused by contamination with high molecular weight silicone greases, which form foams that do not easily collapse. Foams collapse when the air bubbles merge forming larger bubbles that rise

Anti-foaming additives are all surface active agents that are insoluble in the lubricant. They typically are of low viscosity, which allows them to spread over the surface of the oil. Many also contain some solid particles dispersed in the additive. When a foam forms, the bubbles have structures similar to lipid bilayers called lamella. The anti-foaming additive locates itself at the air-liquid interface, maximizing the surface area of the foam bubbles. Smaller bubbles

Aircraft lubricant foam control additives are typically moderate molecular weight silicone oils. It have been observed that silicone oils can act in many ways, causing foam in some cases and eliminating the foam in others depending on molecular weights and structures. The current anti-foaming additives have been shown to successfully eliminate foaming in the lubrication

In aircraft applications, a lubricant must be effective in different lubrication regimes, including the fluid film lubrication regimes (hydrodynamic lubrication and elasto-hydrodynamic lubrication) and boundary lubrication. Hydrodynamic lubrication, where the two bearing surfaces are separated by a complete film of the lubricant, is generally desirable. As the shear stress on the bearing increases, the lubricant properties are not adequate and the film is no longer of sufficient thickness. Elasto-hydrodynamic lubrication occurs between rolling bodies [15]. These systems operate under stresses where a fluid film would not be expected to be maintained. The high pressure, however, causes a temporary increase in the viscosity of the lubricant in the contact area and a fluid film is maintained. Boundary lubrication involves direct contact between the surfaces of the bearings. Boundary lubrication is generally unde‐ sirable but is also unavoidable especially during start-up and shutdown. Boundary lubrication additives are required to reduce friction between and wear of the engine components until

A number of boundary lubrication additives have been developed for various purposes and the vast majority are based on phosphorus or sulfur compounds. For aircraft applications, sulfur compounds are generally less desirable because of the lack of stability and the formation of strong acids on oxidation. Phosphorus compounds can include phosphate esters, thiophos‐ phate esters, and metal thiophosphates. Metal thiophosphates and thiophosphate esters contain sulfur, and the metal dithiophosphates are more likely to form metal containing ash

merge and the larger bubbles move toward the surface, causing the foam to collapse.

Phosphate esters are known to react at the surface of metals to form a coating that is both durable and lubricious. Phosphate esters have been shown to react with oxides or hydroxides on the metal surface to form a multilayer coating that is chemically bound to the surface. As the coating wears away, iron diffuses to the surface and more of the coating forms. The mechanism for coating formation begins with the oxide surface of the iron. A surface oxide displaces one of the alkyl groups from the phosphate ester giving a bound phosphate di-ester. Further reaction occurs to form iron polyphosphate, which has, at the surface, bound aryl groups (**Figure 11**) [16].

If a metal bearing is operated under conditions where little oxygen is present, there still appears to be a reaction to give a similar film. The principle difference in the film formation mechanism is to break a C-O bond to the aromatic ring, leaving a bound metal phosphate. The product of this reaction has been shown to add to another aromatic ring forming higher molecular weight phosphate esters. Under these circumstances, a coating of similar structure is still formed [17].

Advanced bearings under development have a very different surface chemistry. These bearings will be made from carburized or nitrided stainless steels. After machining of the bearing, it is heat treated in the presence of a carbon or nitrogen source. The resulting surfaces have little oxide or hydroxide present but a surface dominated by carbides or nitrides. It is unknown if phosphate ester additives will form a lubricious film on these surfaces. The nonheat-treated steels have been shown to react; however, heat-treated steals are currently under study [18]. The interaction of antioxidants, esters, and phosphate esters in the presence of various carbides has been shown to increase reactivity [19].

**Figure 11.** Structure of surface films formed by the reaction of phosphate esters with a steel bearing surface.

#### **6.5. Metal atom deactivators**

Metal ion deactivators are incorporated into lubricants to react with metal ions typically formed from the action of naturally occurring acids on the metallic parts of the lubrication system. The metal ions are of concern because of their catalytic effects, principally the tendency of copper ions to form a gummy copper mercaptide gel.

In aviation lubricants, the primary metal ion deactivator is benzotriazole. It is thought that benzotriazole reacts with metal atoms at the surface and in solution. On the surface, benzo‐ triazole forms a passive layer that prevents further reaction at the metal surface. Dissolved metal ions react with the benzotriazole to form a complex, which reduces the reactivity of the soluble metal ion. An added benefit of benzotriazole as an additive is that it also acts as an antioxidant [20].
