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194 Lubricant Additives: Chemistry and Applications
Another advantage of using boron nitride is the heat conductivity property of the material. For
applications that would require rapid heat dissipation, boron nitride serves quite well and is superior
to graphite in that regard. Thermal conductivity values of boron nitride powder will vary depending
on its quality. But, boron nitride in any of its grades is invariably more thermally conductive than
graphite or MoS
2
. Applications such as high-performance cutting oils are claimed to deliver bene ts
of enhanced lubrication and heat withdrawal when nely dispersed submicron particles of boron
nitride are incorporated into the uid [17].
REFERENCES
1. Jacobs, N.L. U.S. Patent 5,180,509. 1/19/93.
2. Ludema, K.C. Friction, Wear, Lubrication, A Textbook in Tribology. Boca Raton, FL: CRC Press, 1996,
p. 123.
3. Acheson Colloids company, J. Brian Peace Lecture.
4. Clauss, F.J. Solid Lubricants and Self-Lubricating Solids. New York: Academic Press, 1972, p. 45.
5. Savage, R.H. Graphite lubrication. J Appl Phys 19:1, 1948.
6. Barry, H.F. Factors relating to the performance of MoS
2
as a lubricant. J Am Soc Lubr Eng 33(9):475–480,
1977.
7. Kohli, A.K., B. Prakash. Contact pressure dependency in frictional behavior of burnished molybdenum
disulphide coatings. Tribology Trans 44(1), 2001.
8. Du Pont Te on
®
Fluoroadditives brochure.
9. Bowden, F.P., D. Tabor. The Friction and Lubrication of Solids. New York: Oxford University Press,
1986, p. 165.
10. Kaur, R.G., C.F. Higgs, H. Hesmat. Pin-on-disc tests of pelletized molybdenum disul de. Tribology
Trans 44:79–87, 2001.
11. Pacholke, P.J., K.M. Marshek. Improved worm gear performance with colloidal molybdenum disul de
containing lubricants. ASLE paper presented at the 41st Annual Meeting in Toronto, Ontario, Canada,
May 12–15, 1986.
12. Haviland, M.L., M.C. Goodwin. Fuel economy improvements with friction-modi ed engine oils in
Environmental Protection Agency and road tests.
Society
of Automotive Engineers Technical Paper
790,945, Oct. 1979.
13. Haviland, M.L., J.L. Linden. Taxicab fuel economy and engine and rear axle durability with low
viscosity and friction modi ed lubricants. Society of Automotive Engineers Technical Paper 821,227
Oct. 1982.
14. Broman, V.E. et al. Testing of friction modi ed crankcase oils for improved fuel economy. Society of
Automotive Engineers Technical Paper 780,597, June 1978.
15. Peters, J.A. U.S. Patent, 5,702,769. 12/30/97.
16. Acheson Colloids test data.
17. Watari, K., H.J. Huang, M. Turiyama, A. Osuka, O. Yamamoto. U.S. Patent 5,985,802. 11/16/99.
18. ZYP Coatings technical data sheet, Boron Nitride Powders for Research and Industry.
19. Booser, R.E. Theory and Practice of Tribology, Vol. II. Theory and Design. Boca Raton, FL: CRC
Press, 1983, p. 276.
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195
7
Organic Friction Modifi ers
Dick Kenbeck and Thomas F. Bunemann
CONTENTS
7.1 Introduction ........................................................................................................................... 195
7.2 Friction and Lubrication Regimes ........................................................................................196
7.2.1 Friction Reduction through the Lubricant ................................................................. 197
7.3 Friction Modi ers versus Antiwear/Extreme-Pressure Additives ........................................ 199
7.4 Chemistry of Organic Friction Modi ers .............................................................................200
7.4.1 Friction Modi er Mechanisms ..................................................................................200
7.4.1.1 Formation of Reacted Layers ......................................................................200
7.4.1.2 Formation of Absorbed Layers ................................................................... 201
7.4.1.3 Formation of In Situ Polymers ....................................................................202
7.5 Chemistry of Other Friction Modi ers .................................................................................202
7.5.1 Metallo-Organic Compounds ....................................................................................202
7.5.2 Mechanical Types ......................................................................................................203
7.6 Factors In uencing Friction-Reduction Properties ...............................................................203
7.7 Friction Modi ers: Current Practice .....................................................................................203
7.8 Friction Modi er Performance .............................................................................................204
7.8.1 Stribeck Curve Determinations .................................................................................205
7.8.2 Friction as a Function of Temperature ......................................................................207
7.9 Consequences of New Engine Oil Speci cations and Outlook ............................................208
7.10 Bench Tests to Investigate Friction-Reducing Compounds ..................................................209
References ......................................................................................................................................209
7.1 INTRODUCTION
Friction modi ers (FMs) or friction reducers have been applied for several years. Originally, the applica-
tion was for limited slip gear oils, automatic transmission uids, slideway lubricants, and multipurpose
tractor uids. Such products made use of friction modi cation to meet requirements for smooth transi-
tion from static to dynamic condition as well as reduced noise, frictional heat, and startup torque.
Since fuel economy became an international issue, initially to reduce crude oil consumption,
FMs have been introduced into automotive crankcase lubricants, as well, to improve fuel ef ciency
through the lubricant. In the United States, additional pressure is imposed on original equipment
manufacturers (OEMs) by the corporate average fuel economy (CAFE) regulation.
Following the introduction of vehicle exhaust emission regulations in various regions around
the world, emphasis on friction reduction further increased. This can be well understood if it is
realized that 20–25% of the energy generated in an engine by burning fuel is lost through friction
[1]. The biggest part is lost by friction on the piston liner/piston ring interface and a smaller part by
bearing and valve train friction. It is predicted that in future engines the contribution of the piston
group to engine friction will increase up to 50% [2].
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196 Lubricant Additives: Chemistry and Applications
Reduction of fuel consumption and emissions can be achieved through [3] engine design changes
and modi cations, such as
Application of roller followers
Use of coatings
Surface modi cations
Material selection
Fuel quality
The engine lubricant
All these aspects are looked at and applied in the automotive industry. This chapter concentrates on
the engine lubricant.
The need to measure fuel savings has led to the development of American Petroleum Institute
(API) test sequences such as VI and VIA in the United States. Sequence VIB will be used for
International Lubricant Standards Approval Committee GF-3. In Europe, a fuel economy test has
been developed by Conseil Européen de Co-ordination pour le Dévelopments des Essais de Per-
formance des Lubrifiants et der Combustible pour Moteurs (CEC) (test number CEC L-54-T-96)
for the Association des Constructeurs Européens d’automobiles A1 and B1 speci cations using the
DBM 111 engine. Both tests require that the candidate lubricant shows decreased fuel consumption
relative to reference oil.
7.2 FRICTION AND LUBRICATION REGIMES
Friction is de ned as the resistance a body meets while moving over another body in respect of
transmitting motion. The friction coef cient is de ned as
F
F
W
n
(7.1)
where F
W
is the frictional force and F
n
the normal force or load.
For a lubricated surface, the coef cient of friction is determined by the lubrication regime. In
simple terms, the following three lubricant regimes can be distinguished:
1. Elasto-hydrodynamic lubrication (EHL) regime characterized by a (relatively) thick lubri-
cant lm [4]. The mating surfaces are far enough from one another to prevent metal-to-metal
contact. The load on the system is completely carried by the lubricant lm, and the viscosity
of the lubricant determines the friction coef cient. Viscosity depends on temperature and
pressure/viscosity coef cient.
2. Boundary lubrication (BL) regime characterized by a thin lubricant lm [5]. Under high
loads, high temperature, or with low viscosity oils, most of the lubricating lm is squeezed
out between the metal surfaces, and metal-to-metal contact occurs. The load is entirely car-
ried by the metal asperities. A thin layer of absorbed or otherwise deposited molecules is
necessary to prevent the two surfaces and their asperities from plowing into one another.
3. Mixed lubrication (ML) regime characterized by a lubricant lm of intermediate thickness
[6]. The two metal surfaces have come closer compared to hydrodynamic lubrication, and
metal-to-metal contact occasionally occurs. The load is carried by both the lubricant and
the asperities.
These regimes are related to the friction coef cient f by a lubricant parameter de ned as
su
F
su
p
or
(7.2)
•
•
•
•
•
•
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Organic Friction Modifi ers 197
where
s = system speed
u = lubricant dynamic viscosity
F = load (F
n
)
p = contact pressure
The so-called Stribeck curve gives the relationship between f and these lubricant parameters. The
shape of the Stribeck curve and the transitions from BL to ML and ML to EHL depend on a number
of parameters such as material roughness (microgeometry), contact pressure, and lubricant viscos-
ity. High contact pressure such as that present at point contacts leads to a different Stribeck curve as
at line contact (lower contact pressure) (see Figures 7.1 and 7.2).
7.2.1 FRICTION REDUCTION THROUGH THE LUBRICANT
Engine friction originates from several components, that operate at different conditions of load, speed,
and temperature. Hence, these components may experience various combinations of EHL, ML, and BL
during engine operation. For each of these regimes, a number of factors govern engine friction.
FIGURE 7.1 Stribeck curve at high contact pressure.
Boundary lubrication
BL
ML
Mixed lubrication
Elasto-hydrodynamic lubrication
EHL
Lubricant parameter
(viscosity × speed/load)
Friction coefficient f
FIGURE 7.2 Stribeck curve at low contact pressure.
Boundary lubrication
BL
ML
Mixed lubrication
Elasto-hydrodynamic lubrication
EHL
Lubricant parameter
(viscosity × speed/load)
Friction coefficient f
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198 Lubricant Additives: Chemistry and Applications
Basically, two options to reduce friction and improve fuel ef ciency come forward [7,8].
1. Use of low-viscosity engine oils (SAE 0W/5W-20/30) when uid lubrication (EHL) is the
governing factor [9–11]. Fluid lubrication is especially prevalent in the bearings. The grad-
ual reduction of engine oil viscosity over the years has already brought signi cant fuel
savings (see Figure 7.3).
In the preceding case, oil selection is crucial. In terms of frictional characteristics,
one must emphasize low kinematic viscosity, high viscosity index, low “high-temperature,
high-shear” (HTHS) viscosity, and a low-pressure/viscosity coef cient [12,13]. However, it
has to be realized that other base uid properties, such as volatility and thermal/oxidation
stability, must not be ignored.
2. Addition of friction-reducing agents when BL and ML are the governing factors [14].
These are prevalent in the valve train and the piston group.
In the preceding case, additive system design is the crucial factor. One must empha-
size selecting proper FMs and controlling additive–additive and additive–base uid
interactions.
To assess possible fuel economy improvements in the engine sequences prescribed, an overview of
the lubrication regimes existing in various test engines is provided. The data those used for current
and previous ILSAC speci cations are given in Table 7.1.
FIGURE 7.3 Relationship between SAE viscosity grades and fuel savings based on eet car trials.
10W-40
15W-50
5W-30
15W-40
20W-50
8
6
4
2
0
6
810
12
14 16
High shear viscosity at 100°C, 4
.
10
5
s
-1
(mm/s)
Relative fuel savings (%)
TABLE 7.1
Lubrication Regimes in API Sequences VI and VIA
API Sequence VI (%) API Sequence VIA (%)
Boundary lubrication 37 24
Mixed lubrication 15 4
Elasto-hydrodynamic
lubrication
48 72
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Organic Friction Modifi ers 199
In Sequence VIA, which is prescribed for ILSAC GF-2, EHL is dominating, leading to a sub-
stantial effect of engine oil viscosity on fuel economy. Effects of FMs will be small due to the low
presence of BL and ML conditions, which is due to the application of roller followers. Hence, the
Sequence VIA test engine is often indicated as “a very expensive viscometer.”
This characterization of the Sequence VIA engine will be addressed by Sequence VIB, to be
used for ILSAC GF-3, for which an engine, a bucket tappet sliding valve train will be used, leading
to an increase of the BL and ML regimes [3].
In Europe, the M111 engine is used for the CEC L-54-T-96 fuel economy test, which is pre-
scribed in the ACEA A1 and B1 engine oil speci cations. Similar data as aforementioned are not
available to the authors, but data given in a Shell paper [15] indicate the frictional loss occurring in
this engine, which can be translated to lubrication regimes (Table 7.2).
On the basis of the relatively high amount of frictional loss in the valve train and piston
assembly, the M111 engine should be sensitive to FMs. This is due to the use of four valves per
cylinder to improve combustion ef ciency and so to obtain more power from a given amount
of fuel.
However, compared to other engine designs, the frictional loss in the M111E valve train will be
higher. Provided that the higher-power output obtained from the four-valve assembly is signi cantly
higher than that is lost by higher valve train friction, this approach is favorable with regard to fuel
economy.
7.3 FRICTION MODIFIERS VERSUS ANTIWEAR/
EXTREME-PRESSURE ADDITIVES
A point of debate is often about the difference between FMs and antiwear/extreme-pressure (AW/EP)
additives, especially when it is about FMs active at BL conditions. For a good understanding, this
should be clari ed; therefore, this section deals with the principal difference between these two addi-
tive categories [16].
AW/EP additives are types of compounds that provide good BL. Such materials have the capac-
ity to build strong BL layers under severe load conditions. Hence, AW/EP additives protect closely
approaching metal surfaces from asperities damaging the opposite surface. On the contrary, most
AW additives have little friction-modifying properties.
The crucial differences between AW/EP and FM lms are their mechanical properties. AW/EP
lms are semiplastic deposits that are dif cult to shear off. Thus, under shearing conditions, their
coef cient of friction is generally moderate to high. Conversely, FM lubricant lms are built up of
orderly and closely packed arrays of multimolecular layers, loosely adhering to one another and
with the polar head anchored on the metal surface. The outer layers of the lm can be easily sheared
off, allowing for a low coef cient of friction.
TABLE 7.2
Lubrication Regimes in the DB M111E Engine
Frictional Loss (%) Main Lubrication Regime
Valve train 25 Boundary lubrication
Piston assembly 40 Mixed lubrication
Bearings 35 Elasto-hydrodynamic
lubrication
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200 Lubricant Additives: Chemistry and Applications
The difference between the two types of lms and other lubrication modes is best illustrated by
the data presented in Table 7.3.
7.4 CHEMISTRY OF ORGANIC FRICTION MODIFIERS
Organic FMs are generally long, slim molecules with a straight hydrocarbon chain consisting of at
least 10 carbon atoms and a polar group at one end. The polar group is one of the governing factors
in the effectiveness of the molecule as an FM. Chemically, organic FMs can be found within the
following categories [16]:
Carboxylic acids or their derivatives, for example, stearic acid and partial esters
Amides, imides, amines, and their derivatives, for example, oleylamide
Phosphoric or phosphonic acid derivatives
Organic polymers, for example, methacrylates
Another classi cation can be given by mode of action and FM type (Table 7.4).
Owing to the different mode of actions, the mechanism of friction reduction varies for each
category.
The next section further deals with details about their mode of action, and another section deals
with the current chemistry used as well as speci c products.
7.4.1 FRICTION MODIFIER MECHANISMS
7.4.1.1 Formation of Reacted Layers
Similar to AW additives, protective layers are formed by chemical reaction of the additive with the
metal surface. However, the principal difference is that the reaction has to occur under the relatively
•
•
•
•
TABLE 7.3
Lubrication Modes versus Friction Coeffi cient
Lubrication Mode Friction Coeffi cient Comparison
Nonlubricated surface 0.5–7 Dragging an irregular rock over rocky ground
AW/EP lms 0.12–0.18 Dragging a at stone over a at rock
Friction-modi ed lms 0.06–0.08 Ice skating
EHL 0.001–0.01 Hydroplaning
TABLE 7.4
FM Type and Mode of Action
Mode of Action/Type of FM Products
Formation of reacted layers Saturated fatty acids, phosphoric and thiophosphoric
acids, sulfur-containing fatty acids
Formation of absorbed layers Long-chain carboxylic acids, esters, ethers, amines,
amides, imides
Formation of polymers Partial complex esters, methacrylates, unsaturated fatty
acids, sulfurized ole ns
Mechanical types Organic polymers
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Organic Friction Modifi ers 201
mild conditions (temperature and load) of the ML regime. These conditions require a fairly high
level of chemical activity as re ected by the phosphorus and sulfur chemistry applied.
An exception to this is stearic acid. Theoretically, the friction-reducing effect of stearic acid
should decrease with increasing temperature due to desorption of the molecule from the metal sur-
face. However, stearic acid experimentally shows a remarkable drop of friction with increasing tem-
perature, which can only be explained by the formation of chemically reacted protective layers.
7.4.1.2 Formation of Absorbed Layers
The formation of absorbed layers occurs due to the polar nature of the molecules. FMs dissolved in
oil are attracted to metal surfaces by strong absorption forces, which can be as high as 13 kcal/mol.
The polar head is anchored to the metal surface, and the hydrocarbon tail is left solubilized in the
oil, perpendicular to the metal surface (see Figure 7.4). Next the following steps occur:
1. Other FM molecules have their polar heads attracted to one another by hydrogen bonding
and Debye orientation forces, resulting in dimer clusters. Forces are ∼15 kcal/mol.
2. Van der Waals forces cause the molecules to align themselves such that they form multi-
molecular clusters that are parallel to one another.
3. The orienting eld of the absorbed layer induces further clusters to position themselves
with their methyl groups stacking onto the methyl groups of the tails of the absorbed
monolayer [17,18].
As a result, all molecules line up, straight, perpendicular to the metal surface, leading to a multi-
layer matrix of FM molecules (see Figure 7.5).
The FM layers are dif cult to compress but very easy to shear at the hydrocarbon tail interfaces,
explaining the friction-reducing properties of FMs. Owing to the strong orienting forces, mentioned
earlier, sheared-off layers are quite easily rebuilt to their original state.
The thickness and effectiveness of the absorbed FM lms depend on several parameters, four
of which are explained here.
1. Polar group. Polarity itself is not necessarily suf cient for adsorption; the polar group
must also have hydrogen-bonding capability. Molecules with highly polar functional
groups that are not capable of forming hydrogen bonds, such as nitroparaf ns, do not adsorb.
Van der Waals
forces
Van der Waals
forces
Adhesive
hydrogen bonding
Long, nonpolar chains
Polar heads
Dipole−dipole
interactions
Oxidized and hydroxylated metal surface
FIGURE 7.4 Organic FMs—formation of adsorbed layers.
FIGURE 7.5 Multilayer
matrix of FM molecules.
Metal surface
/
///////////////////
H H H H H = Polar head
T T T T T T = Hydrocarbon tail
T T T T T T
H H H H H
H H H H H
T T T T T T
OIL OIL OIL
OIL OIL OIL
OIL OIL OIL
T T T T T T
H H H H H
H H H H H
T T T T T T
T T T T T T
H H H H H
/
///////////////////
Metal surface
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202 Lubricant Additives: Chemistry and Applications
Hence, these do not function as friction-reducing additives. However, polarity plays a
major role among the various lateral surface interactions through strong electrostatic
dipole–dipole interactions. These may be either repulsive or attractive, depending on the
orientation of the adsorbed dipoles with respect to the surface [19].
2. Chain length. Longer chains increase thickness of the absorbed lm, and the interactions
between the hydrocarbon chains increase as well [18].
3. Molecular con guration. Slim molecules allow for closer packing as well as increased
interaction between adjacent chains, leading to stronger lms. Therefore, straight chains
may be preferred.
4. Temperature. Temperature in uences FM lm thickness and tenacity. Adsorption of friction-
reducing compounds to the metal surface does occur at relatively low temperatures. AW
additives form protective layers by chemical reactions for which higher temperatures are
needed.
If the temperature is too high, enough energy might be provided to desorb the friction-
reducing molecules from the metal surface.
7.4.1.3 Formation of In Situ Polymers
The formation of low-friction-type polymer lms can be considered a special case. Instead of the
usual solid lms, uid lms are formed under in uence of contact temperature ( ash temperature)
and load. Another difference is that the polymers are developed at the interface between metal
asperities without reacting with the metal surface.
The requirements of such polymers are
1. Polymers must have relatively low reactivity. Polymerization must be generated by fric-
tional energy.
2. The polymers formed must be mechanically and thermally stable and should not be soluble
in the lubricant.
3. The polymers must develop a strong bond to the metal surface either by absorption or by
chemical bonding.
4. The formation and regeneration of lms must be fast to prevent competitive adsorption by
other additives.
Examples of polymer-forming FMs are
Partial complex esters, for example, a sebacic acid/ethylene glycol partial ester methacrylates
Oleic acid (olein), which may be explained through thermal polymerization (formation of
dimers and higher oligomers)
7.5 CHEMISTRY OF OTHER FRICTION MODIFIERS
Within this group, the following categories can be distinguished by chemical type:
1. Metallo-organic compounds
2. Oil-insoluble materials
Classi cation by type appears in Table 7.5.
7.5.1 METALLO-ORGANIC COMPOUNDS
Molybdenum dithiophosphate, molybdenum dithiocarbamate, and molybdenum dithiolate as well
as copper-oleate, copper-salicylate, and copper-dialkyldithiophosphate are examples of friction-
reducing metallo-organic compounds.
•
•
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Organic Friction Modifi ers 203
The mechanisms of operation of this class of products are not fully understood, but the follow-
ing hypotheses are presented:
Diffusion of molybdenum into the asperities
Formation of polymer-type lms
In situ formation of molybdenum disul de (most accepted hypothesis)
Selective transfer of metal (copper) leading to the formation of thin, easy-to-shear metal lms
7.5.2 MECHANICAL TYPES
In this group, the classical types such as graphite and molybdenum disul de as well as some more
recent FMs such as te on (polytetra uoroethylene, PTFE), polyamides, uoridized graphite, and
borates can be found. The friction-reducing mechanisms can be explained by
The strati ed structure and formation of easy-to-shear layers
The formation of elastic or plastic layers on the metal surface
7.6 FACTORS INFLUENCING FRICTION-REDUCTION PROPERTIES
This section lists the main factors that impact friction-reducing properties.
1. Competing additives. Other polar additives with af nity to metal surfaces such as AW/EP
and anticorrosion additives as well as detergents and dispersants may compete with FMs.
This emphasizes that lubricant formulations have to be balanced carefully to achieve opti-
mal performance.
2. Contaminants. Short-chain acids, which are formed by oxidative degradation of the
lubricants, may compete at the metal surface, resulting in a loss of friction-modifying
properties.
3. Metallurgy. The type of steel alloy used will affect the adsorption of FMs.
4. Concentration. Increase of FM concentration results in an increase of friction reduction up
to a point above which improvements are marginal. Generally, the friction-reducing effect
is most (cost-)effective at concentrations of ∼0.25 to 1% for organic FMs and 0.05–0.07%
for molybdenum dithiocarbamates.
7.7 FRICTION MODIFIERS: CURRENT PRACTICE
The most frequently used organic FMs include
1. Long-chain fatty amides, speci cally oleylamide (Figure 7.6). This is a reaction product
of olein (main component oleic acid, a straight-chain unsaturated C18 carboxylic acid) and
ammonia (NH
3
).
•
•
•
•
•
•
TABLE 7.5
Classifi cation of Other FMs
Types of FMs Products
Metallo-organic compounds Molybdenum and copper compounds
Mechanical types Molybdenum disul de, graphite, te on (PTFE)
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