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634 Lubricant Additives: Chemistry and Applications
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25
Long-Term Additive Trends
in Aerospace Applications
Carl E. Snyder, Lois J. Gschwender, and Shashi K. Sharma
CONTENTS
25.1 Unique Requirements for Aerospace Applications ............................................................. 637
25.2 Why Are Additives Needed? .............................................................................................. 638
25.3 How Additives Work ........................................................................................................... 639
25.4 Additives for Space Applications ........................................................................................640
25.5 Additives for Hydraulic Fluids ............................................................................................642
25.6 Additives for Gas Turbine Engine Oils ...............................................................................642
25.7 Future Trends ......................................................................................................................643
References ......................................................................................................................................644
25.1 UNIQUE REQUIREMENTS FOR
AEROSPACE APPLICATIONS
Aerospace, especially military aerospace systems, requires faster, highly maneuverable, and higher-
temperature systems. The higher temperatures typically experienced in aerospace applications are
common for all lubricants, but we use hydraulic uids as an example. The high temperatures expe-
rienced in aerospace hydraulic systems are the result of three related factors. First, for military
aircraft, the operational requirements for the aircraft (e.g., high speed and high degree of maneuver-
ability) impose more severe demands on the hydraulic systems. Second, weight is always a critical
issue for aircraft that drives hydraulic system components that are small and operate under more
strenuous conditions (e.g., higher speed and higher loads) and that impose more severe operat-
ing conditions on the hydraulic uid (e.g., higher temperatures and higher shear rates). Third, the
high emphasis on weight savings for aircraft applications results in smaller uid volume systems.
All these factors result in higher temperatures being imposed on the hydraulic uids. Although
high maneuverability is usually not an issue for commercial aerospace applications, the other two
factors are equally important for commercial as well as military applications. In addition to the
higher temperatures imposed on aerospace hydraulic uids, they must also be capable of operating
at extremely low temperatures. Aircraft hydraulic and lubrication systems are required to operate
at temperatures as low as −54°C. Typical operational temperature ranges for the hydraulic uids
used in aerospace are shown in Figure 25.1. Similar factors make unique demands on other aero-
space lubricants such as gas turbine engine oils and greases. Aerospace applications include both
spacecraft and aircraft applications. The major emphasis of this chapter is aircraft applications.
Hydraulic uids and gas turbine engine oils are discussed in this chapter because they represent
the larger volume usage, but similar factors and issues exist for the other aerospace lubricants and
uids such as greases and coolants. Liquid and grease lubricants for space applications are also dis-
cussed because the space environment creates some unique and challenging problems for lubricant
additives. Often, the customary additives are not adequate for aerospace applications, and more
development is needed to make acceptable aerospace lubricants [1–8].
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638 Lubricant Additives: Chemistry and Applications
25.2 WHY ARE ADDITIVES NEEDED?
Additives are required for aerospace uids and lubricants to enhance or impart a required opera-
tional capability to a speci c uid or lubricant not provided by the base uid alone. When a lubricant
(lubricating oil, hydraulic uid, grease, etc.) is stressed in its operating environment, it can degrade
such that the performance of the system or components in which it is used is adversely affected.
To enhance the performance of a lubricant or to mitigate its degradation during use, additives are
required. Some of the additive types used in aerospace applications are listed in Table 25.1. Depend-
ing on the application, a formulation may contain one or more of these and other additives:
1. Boundary lubrication additives (antiwear, lubricity, extreme pressure)
2. Antioxidants
3. Anticorrosion additives
4. Metal deactivators
5. Viscosity index (VI) improvers
A lubricant has to be effective in both the uid- lm lubrication and the boundary lubrication regimes.
In uid- lm lubrication, a uid lm separates the interacting surfaces, whereas in boundary lubri-
cation, more direct contact occurs between the two surfaces. Fluid- lm lubrication can be further
divided into two broad categories—hydrodynamic lubrication and elastohydrodynamic lubrication.
In uid- lm lubrication, the physical properties of the lubricant such as viscosity, pressure viscos-
ity, and traction determine the performance of the lubricated contact. Although uid- lm lubrica-
tion is the desired mode of operation, the boundary lubrication regime cannot be avoided. Even in
uid- lm lubrication, boundary lubrication occurs during the start-up and shutdown and occasional
asperity interaction during operation. Therefore, the material (both the surface and the lubricant)
properties important for the boundary lubrication regime are equally important for the uid- lm
lubrication. Boundary lubrication additives are generally needed for reducing friction and wear of
the components [9].
Antioxidants are required to prolong the useful life of uids and lubricants. Most uids and
lubricants are hydrocarbon-based and are susceptible to oxidative degradation through a free-radical
mechanism. Antioxidants react with the initially produced free radicals to prevent the oxidation pro-
cess from proceeding from the slow initiation phase of the process to the rapid autocatalytic phase.
Corrosion inhibitors are usually synonymous with rust inhibitors for most lubricants. How-
ever, as we go to more exotic metals and with the emergence of halogenated lubricants, corrosion
−65 0 100 200 300 400 500 600
AS1241
MIL-PRF-5606
MIL-PRF-87257
MIL-PRF-
83282
MIL-PRF-
27601
Temperature (°F)
FIGURE 25.1 Temperature ranges for aerospace hydraulic uids.
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Long-Term Additive Trends in Aerospace Applications 639
can occur with nonferrous metals and by attack of very aggressive uid degradation products. For
example, degradation products from per uorinated polyalkylethers react with various metals to
cause what one could call corrosion [10,11]. A better de nition of corrosion inhibitors would be
additives that inhibit the reaction of moisture or uid degradation products with metals. They are
widely used for that function in many classes of uids and lubricants.
Metal deactivators are additives that passivate the surface of metals. The passivation can result
in either reducing the metal’s catalytic effect on lubricant degradation or reducing the effect of the
lubricant on the surface of the metal. They are widely used in hydraulic uids and lubricants.
VI improving additives are discussed in Section 25.5.
25.3 HOW ADDITIVES WORK
In general, certain performance-improving additives are optimized for speci c base oils, but in
some instances, the same additive can be effective in various classes of base uids. However, in
some cases, additives that have been effective at improving a speci c property or performance
parameter in one or several classes of base uids are not effective in others. For example, substituted
aryl phosphates have been very effective antiwear additives for steel-on-steel rubbing contacts in
a large number of chemical base stocks, for example, mineral oils, synthetic hydrocarbons, esters,
and even more exotic base uids such as the polyphenyl ethers and the silahydrocarbons. They are
not effective additives, however, for steel-on-steel contacts in polydimethyl siloxanes or chlorotri-
uoroethylene lubricants. The reason for speci city of some additives is beyond the scope of this
chapter, but it is important to recognize this phenomenon as we consider future trends for additive
applications. As we attempt to develop advanced lubricants using new and novel base oils, we must
recognize that new additive technology may be required. How different additives enhance the per-
formance of a lubricant is discussed throughout the rest of the chapter.
In boundary lubrication, the interacting surfaces react with the lubricant components to form
protective physisorbed, chemisorbed, or reaction lms [12]. When metal-to-metal contact occurs
between two surfaces in a lubricated contact, the asperities on the surfaces shear, thereby exposing
a fresh metal surface. The appropriate lubricant component reacts with the exposed fresh metal to
form protective surface lms. The surface lms thus formed are generally low-friction and wear-
resistant and protect the surfaces from early failure/wear. During operation, these surface lms
TABLE 25.1
Typical Additives Used in Aerospace Fluids and Lubricants
Hydraulic Fluid Space Lubricant
Fluid Gas Turbine Engine Oil Hydrocarbon Phosphate Ester Hydrocarbon PFPAE
Additive antioxidant Aromatic amines, for
example, PANA
a
Hindered phenol Proprietary Hindered phenol Experimental
Lubricity/antiwear Aryl phosphate Aryl phosphate Proprietary Aryl phosphate Experimental
Viscosity Polymethyl
Index N/A Methacrylate Proprietary N/A
Rust Metal sulfonate or metal
Inhibitor Organic acid Sulfonate Proprietary N/A NaNO
2
b
Antifoam Silicone Silicone Proprietary N/A N/A
Metal deactivator Benzotriazole Benzotriazole Proprietary N/A N/A
a
Phenylalphanaphthylamine.
b
Greases only.
Note: N/A, not typically used for this application.
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640 Lubricant Additives: Chemistry and Applications
can wear off and regenerate. Antiwear or lubricity additives are generally added to the base oil to
enhance the formation of the protective surface lms during boundary lubrication. Thus, in bound-
ary lubrication, the chemistry of the lubricant along with the material properties of the interacting
surfaces determines the performance of the lubricated contact.
Many researchers have expended signi cant effort at determining the detailed mechanism by
which speci c additives work. In some cases, for example, the free-radical interceptor mechanism
of most effective antioxidants, the mechanisms have been intensively studied, worked out, and
generally well understood [13]. In other cases, however, although extensive research has been con-
ducted, for example, the mechanism of action of tricresyl phosphate, there are a number of theories,
many of which are supported by chemical data, but no one theory is universally accepted.
When one reviews the published data and, in the case of many researchers, their unpublished
data, a few common facts can be found for many types of performance-improving additives. First,
most additives appear to be effective through a competitive reaction mechanism. In other words,
the additive is reacting with some undesirable species that occurs in the lubrication system that, if
unchecked, will lead to premature failure of the lubricant or component. In the case of an antioxi-
dant in a hydrocarbon lubricant, the additive is reacting with the initial free radicals formed early in
the oxidation process to retard or prevent their attack on the lubricant molecules in an autocatalytic
process that results in rapid deterioration of the lubricant. In the case of the antiwear additives in
hydrocarbon lubricants, the additive reacts with the newly formed high-energy, fresh metal sur-
faces formed by metal-to-metal contact, thereby passivating these surfaces by forming, in most
cases, lubricious reaction products that prevent the catastrophic wear of the component. The chem-
istry of that reaction product will vary depending on the chemistry of the additive and the surface.
The mechanism of wear protection, however, may vary with lubricants and surfaces of different
chemistries, especially as discussed per per uoropolyalkylethers (PFPAEs) in 25.4.
Another common factor with many is that the additives are generally unreactive and stable
during the normal functioning of the lubricant and only become effective and reactive on demand.
For example, antioxidants in hydrocarbons become effective and reactive only when free radicals
are formed in the lubricant due to oxidation of the lubricant molecules. Similarly, effective antiwear
additives in hydrocarbon lubricants generally do not react until the fresh metal surfaces are formed.
Whether this reaction is induced by the reactivity of the fresh metal surface or the momentary high
temperatures that occur during the metal-to-metal contact is an issue still under debate, but it is
generally accepted that the additive, until this reactivity is triggered by a speci c event, is inactive
and stable. Something occurs to lower the activation energy and allows the additive to be effective.
Other additives are less affected by events that trigger their action. For example, VI improvers
[14] and rust inhibitors are generally effective during the entire life of the lubricants and do not
require some speci c (unusual) event to trigger their actions. But as we consider future additive
requirements for future classes of base uids and surfaces requiring lubrication, new additive tech-
nology must be developed.
An additive may work by more than one mechanism. For example, many metal-alkylated sul-
fonate rust inhibitors form a physical barrier on metal surfaces. The polar portions of the molecules
are adsorbed on the metal, effectively blocking water from the corrosion-prone surfaces [8]. The
molecules also form micelles or groups of molecules loosely in rings with their polar portions
toward the center. These act as little water traps to surround and tie up water molecules and, again,
to prevent them from getting to the metal surfaces.
25.4 ADDITIVES FOR SPACE APPLICATIONS
Lubricants for outer space applications are particularly challenging. First, the volume of space lubri-
cants used is so small that major oil companies have little interest in investing in research, devel-
opment, and commercialization, even if the lubricants command a high price. Fortunately, several
companies deal with specialty lubricants and are willing to supply to this niche market.
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Long-Term Additive Trends in Aerospace Applications 641
The most important additive for a space lubricant is an antiwear additive [7]. Spacecraft mecha-
nisms generally operate on a very small quantity of lubricant. With lubricant depletion due pri-
marily to evaporation or lubricant creep, the lubricated components experience a higher degree of
boundary lubrication. Effective antiwear additives are required for prolonged life of these compo-
nents. As a general rule, per uoropolyalkylether (PFPAE) lubricants are not used in applications
where the bearing contact stress is >100,000 psi because they may undergo tribocorrosion-induced
failure. An antioxidant is often used to protect a spacecraft hydrocarbon oil and its antiwear additive
for the time the formulation is on Earth, waiting for launch, but is not essential in space, where little
oxygen is available.
The major technical challenges for spacecraft lubricant additives are low volatility and good
solubility. Spacecraft lubricants were originally mineral oils/greases, but these are gradually being
replaced with synthetics including hydrocarbon polyalphaole ns (PAOs) and multialkylated cyclo-
pentane (MAC; Pennzane
®
) and PFPAEs. Even lower volatility and improved low-temperature
viscosity silahydrocarbon uids are in development for spacecraft [15]. Soluble and effective addi-
tives are readily available for terrestrial hydrocarbon oils and, with some looking, commercially
available low-volatility additives may also be found. In greases, solubility is less important, but it is
generally believed that at least partially soluble additives are advantageous in grease performance
over nonsoluble additives.
For many years, there were no soluble additives for PFPAE lubricants. Conventional additives are
not soluble in PFPAE uids. Solubilizing substituents, similar to the backbone structure of PFPAE u-
ids, are required in the additive structure to make it soluble in PFPAE uids. However, PFPAE manu-
facturers have recently offered soluble additives for their uids. These additives are made for use in the
manufacturers’ own product lines and are not available for purchase to use in formulations in products
produced by other companies. The U.S. Air Force has conducted research and development of soluble
additives for PFPAE uids as part of a high-performance turbine program [16]. Additives are needed
for PFPAE uids because of their unique behavior especially in boundary lubrication in low-humidity
environments [17]. A phenomenon called tribocorrosion can occur, which is a combination of uid
decomposition and wear metal corrosion. This happens at much lower temperatures than are predicted
by static, nonwear stability tests. The additives act to interrupt the cycle of uid decomposition and
metal interaction, thereby minimizing the tribocorrosion and improving wear performance [18].
Besides antiwear additives, antirust additives are needed to prevent steel components from rust-
ing when used with PFPAE uids or greases. Conventional hydrocarbon lubricants tend to leave a
protective lm on the surface of steels, whereas PFPAE uids do not, thus leaving low-chrome steel
components prone to rusting after use with a PFPAE uid. This is due to the high solubility of oxy-
gen into the PFPAE uids, which allows oxygen to get to the iron surfaces, leading to rust.
Several important new classes of additives were developed in the Air Force program [16],
most notably a per uoroalkyldiphenylether (DPE), a per uoroalkyltriphenylether (TPE), and a tri
(per uoropolyalkyletherphenyl)phosphine (PH3) [19]. These provide improved antiwear and high-
temperature stability to the PFPAE uids by interacting with and neutralizing uid decomposition
products that, without the additives, attack metal surfaces and cause an autocatalytic decomposition
of the uid.
As the molecular weight of any lubricant increases, the solubility for polar additives decreases.
This is especially true for the very nonpolar PAOs. Long-chain alkyl groups on the additives improve
additive solubility but often reduce effectiveness. Additives, in general, are polar compounds, which
is what makes them effective. All of this makes for a very short list of effective, soluble, and low-
volatility additives for spacecraft.
The spacecraft lubricant additives do not need to be especially thermally stable as the space-
craft mechanisms do not operate at high temperatures. They may, however, operate at extreme cold
temperatures, and the additives must stay in solution in oils.
Test methods for spacecraft lubricants are not well standardized and often are peculiar to a
particular satellite company. Because there are few second chances in spacecraft lubricants once
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642 Lubricant Additives: Chemistry and Applications
in operation, usually a “life test” is conducted for years on Earth under high vacuum to simulate
actual operation.
25.5 ADDITIVES FOR HYDRAULIC FLUIDS
Hydraulic uids are operated in closed systems, although the uid reservoir may be open to
air. Hydraulic uid additives may include antioxidant, antiwear, rubber swell, VI improver, and
minor amounts of metal deactivator and dye. (This is a simpler list than that of engine oil, where
thermal-oxidative stability is critical.) The application temperature often dictates the additives used.
Hydraulic uids for military applications are de ned by the military speci cations including MIL-
PRF-5606H [20] (mineral oil–based uid), MIL-PRF-83282D [21], and MIL-PRF-87257 [22] (both
synthetic hydrogenated PAO-based uids). Hydraulic uids for commercial aircraft, which are
phosphate ester based, are de ned by SAE AS 1241 [23].
Antioxidants are usually ashless (containing no metals) and are believed to serve primarily to
protect the uid in storage rather than in an application. A test such as ASTM D4636, corrosiveness
and oxidation stability to aircraft turbine engine lubricants, and other highly re ned oils is often
used to evaluate the antioxidant effectiveness.
Arylphosphate antiwear additives in hydraulic uids are widely used and are very effective for
steel-on-steel rubbing surfaces. The ASTM D4172, wear preventive characteristics of lubricating
uids (four-ball wear method), is widely used to evaluate antiwear additives, but other methods are
also used.
Rubber swell additives are used with synthetics such as PAOs that, by themselves, provide little
swell or even shrinkage to nitrile seals, the most widely used seal material in military aircraft.
Nitrile seals were originally developed to swell an appropriate volume with naphthenic mineral oil.
When synthetic PAOs were introduced, they had to be adjusted to be a drain-and- ll replacement
for mineral oils, so rubber swell additives were added to the synthetic formulations. Rubber swell
additives are usually polar esters actually adsorbed into the elastomeric compounds, creating a posi-
tive seal to the hydraulic system. Seal tests may include swell per FED-STD-791 method 3603 and
other physical property determinations.
VI improvers are used mainly with naphthenic mineral and phosphate ester oils to increase the
VI (lower the amount of change in viscosity with temperature). They are long-chain polymers such
as polymethylmethacrylates, but other chemical classes are also used. In theory, they are coiled
at low temperature and strung out at higher temperature and therefore impede ow less at lower
temperatures but more at higher temperatures. The VI improvers do impede ow in bulk, such as
past seals, and provide additional uid- lm thickness in low-shear environments, but do little to
improve the uid- lm thickness in the elastohydrodynamic regime in bearings [14]. VI improvers
are broken into smaller pieces in high-shear and high-stress environments, present in both elastohy-
drodynamic and boundary lubrication regimes, and therefore become less effective. A shear stabil-
ity test, for example, as in the MIL-PRF-5606 hydraulic uid speci cation [20], is used to determine
a formulation’s resistance to shear under a speci c set of conditions, but this test does not duplicate
the severity of real lubricated contacts.
A minor amount (0.05%) of metal deactivator is needed with PAO-based hydraulic uids for
them to pass all the metal weight changes in ASTM D4636. A small amount of dye is used in
hydraulic uids for leak detection. Hydrocarbon hydraulic uids are usually dyed red, whereas
phosphate ester hydraulic uids are dyed purple.
25.6 ADDITIVES FOR GAS TURBINE ENGINE OILS
Aircraft engine oils must have a critical balance of performance-improving additives, especially if
used at high temperatures. Engine oils are often sprayed onto hot bearings with considerable air ow
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Long-Term Additive Trends in Aerospace Applications 643
present to help cool the bearing. This gives rise to hot spots and thin oil lms with lots of contact
with oxygen. Coking of the uid and additives, when they oxidize and turn to carbonaceous depos-
its, is detrimental to the cooling ef ciency of the engine. Also, coke can clog the breathing and oil
delivery tubes in an engine.
Engine oils are oxidatively very stable polyol ester–based with one or more antioxidant, and
antiwear, antifoam, and metal deactivator additives. All the additives must have a low propensity
to form coke. Often the optimized oil represents a compromise between low cooking and optimum
performance as provided by speci c additives. For example, different aryl phosphate antiwear addi-
tives exhibit various degrees of effectiveness, and it may not be possible to select the best antiwear
additive because its presence in the oil causes too much coke formation. As a rule, the coking ten-
dencies of formulations are greater with more effective aryl phosphates and with higher percentages
of additive.
An antifoam additive, a speci c molecular-weight-range dimethyl silicone, may be used. The
following tests are generally used to evaluate the effectiveness of the additives:
1. Oxidative stability: ASTM D4636
2. Antiwear performance: ASTM D1947, Ryder Gear Test and Modi ed ASTM D5182, FZG
Test
3. Anticoking performance: Fed-Std 791, Method 3450, Bearing Deposition
4. Foaming: Fed-Std 791, Method 3213, Foaming Characteristics of Aircraft Turbine Engine
Oils (Static Foam Test)
In addition to these tests, each new engine oil quali ed to a military speci cation must pass an
actual engine test as described in the speci cations.
25.7 FUTURE TRENDS
Three major factors will drive the need for new additive research and development for aerospace
applications. The rst, which has been discussed in detail in Section 25.1, is the need for addi-
tives effective over a wide temperature range, from as low as −54°C to >300°C. The second has
already been discussed to some extent, and that is the new class of the base oils developed to meet
new, more demanding requirements. The class discussed earlier, the PFPAEs, sometimes called
per uoroethers or per uoropolyethers, is one of the examples. Another class that demonstrated
unique requirements is the chloro uorocarbon oligomers that served as the base uid for a −54 to
175°C non ammable aerospace hydraulic uid. Although most additives typically used in hydro-
carbon-based lubricants are quite soluble in the chloro uorocarbon base stock, most of them have
no bene cial effect. The successful development of an effective formulation using that unique base
uid required years of research. The nal formulation that met the Military Speci cation MIL-
H-53119 [24] utilized a carefully balanced additive package composed of a lubricity additive and a
rust-inhibiting additive. Although these are two new and unique base uids with which the authors
have rsthand experience, other new base stocks could be developed for future applications that also
require new additive research and development.
The third factor that will prompt the development of new performance-improving additives for
future lubricating systems is the change in the chemistry of the surfaces with which the lubricants
will come into contact and be expected to lubricate and provide long life. The main type of additive
in uenced by this factor is the lubricity additive. Two examples are new metal alloys and surface
modi cations such as hard coatings. In both the cases, the metal atoms, which in the past have been
available to interact with the additive, are not available or are not readily available.
The chemical reactions that activate the additive are halted. However, the surface modi ca-
tions may provide a barrier for any undesirable interaction between the lubricant and the substrate
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