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204 Lubricant Additives: Chemistry and Applications
2. Partial esters, speci cally glycerol mono-oleate (GMO) (Figure 7.6). GMO is a reaction
product of glycerin (natural alcohol with three hydroxyl groups) and olein (as mentioned
previously). Investigations have shown that the alpha version (terminal hydroxyl groups
esteri ed) is the active component rather than the beta one (middle hydroxyl group esteri-
ed). Special production techniques are required to manufacture high-alpha-containing
products.
The mode of action of both product groups is based on the formation of adsorbed layers that can easily
be sheared off, leading to reduced friction. It is expected that further research will result in new and
improved types to cope with more severe requirements with regard to friction retention over time.
Within the group of metallo-organic compounds, molybdenum dithiocarbamate [Mo(dtc)
2
]
seems almost exclusively to be used to obtain friction reduction (Figure 7.7). Research [7,20] has
shown that the friction-reducing activity of Mo(dtc)
2
is based on the exchange of functional groups
with zinc dialkyldithiophosphates [Zn(dtp)
2
] (Figure 7.8).
It was found that oxidation affects these exchange reactions signi cantly and that the most
effective friction reduction is achieved at the later stages of oxidation when the concentrations of
the single-exchange product [Mo(dtc)(dtp)] and the double-exchange product [Mo(dtp)
2
] are high.
When both products are nearly consumed by oxidation, friction reduction ceases.
7.8 FRICTION MODIFIER PERFORMANCE
Literature suggests that FMs act both in the BL and ML regimes [7,16,21]. Their mode of action
should depend on FM chemistry and prevailing engine conditions. It is further suggested that
organic FMs are most active in the mixed regime, whereas metallic types are predominantly active
NH
2
O
O
O
OH
OH
Oleylamide Glycerol mono-oleate (GMO)
FIGURE 7.6 Organic FMs—structural drawings.
R
N
C
R
S
S
Mo
S
S
Mo
S
S
C N
R
R
OO
FIGURE 7.7 Molybdenum dithiocarbamate—
structural drawing.
FIGURE 7.8 Molybdenum dithiocarbamate—
exchange of functional groups.
Mo(dtc)
2
Mo(dtp)
2
Mo(dtc)(dtp)
+
+
+
Zn (dtp)
2
Zn (dtp)(dtc)
Zn(dtc)
2
A
B
C
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Organic Friction Modifi ers 205
in the BL regime. Recent investigations by the authors, carried out with a pin-on-ring tribometer,
showed that it is likely that organic FMs act predominantly in the BL regime as well.
Tests were carried out with CEC reference oil RL 179/2, which is applied in the CEC L-54-T-96
fuel economy test. RL 179/2 is a formulated 5W/30 engine oil that does not contain any FM and that
has a proven fuel economy bene t CEC round-robin tests.
Frictional behavior was investigated by establishing stabilized Stribeck curves. By determin-
ing these, both boundary and mixed friction can be investigated. Stabilized Stribeck curves are
obtained by measuring the coef cient of friction over a speed range from ∼0.0025 to 2 m/s at
appropriate steps. A number of runs are carried out until two consecutive runs give a good match.
Usually, after four runs, the curve has stabilized, meaning that process roughness has stabilized to
a large extent.
Performance criteria in considering the results are frictional level in the BL and ML regimes in
combination with specimen wear. The reason for looking at wear is that this parameter corresponds
with contact pressure, which in turn in uences the ML/EHL transition.
The relationship between wear and contact pressure is given by the expression
F
A
p
n
=
(7.3)
where
F
n
= normal force (load)
A = wear scar
p = contact pressure
Consequently, a high-wear scar leads to a lower contact pressure, and a lower contact pressure does
shift the ML/EHL transition in the Stribeck curve to the left (see Figure 7.9).
7.8.1 STRIBECK CURVE DETERMINATIONS
Stabilized Stribeck curves have been determined with a pin-on-ring tribometer at which the ring
was a 100Cr6 stainless steel ring with a 730 mm diameter. These rings are high-quality materials
used in standard bearings and therefore easily available. The pin used was a cylinder from the same
material with an 8 mm diameter, also used in bearings. To get proper line contact, the cylinders
have been provided with exible ends to allow full alignment with the ring.
FIGURE 7.9 In uence of wear/contact pressure on ML/EHL transition.
Boundary lubrication
Mixed lubrication
Elasto-hydrodynamic
lubrication
Lubricant parameter
(viscosity × speed/load)
Contact pressure
(wear)
High Low
Friction coefficient f
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206 Lubricant Additives: Chemistry and Applications
Ring roughness, R
a
, was ∼0.15 µm, although the cylinder was very smooth. Hence, the rough-
ness of the ring determined the shape of the Stribeck curve, speci cally the BL/ML transition. The
load (normal force F
n
= 100 N) was chosen such that heat development in the contact zone was
negligible, so that the viscosity was constant. Hence, it was possible to determine the Stribeck curve
only as a function of speed. The temperature chosen was 40˚C.
The following graph (Figure 7.10) shows comparative data for RL 179/2 as well as this oil with
addition of 0.5% GMO and addition of 0.5% of organic FMs A and B. (A and B are products with
both free and esteri ed hydroxyl groups.)
All the FMs studied here show a signi cant reduction of the friction coef cient in the BL
regime. Organic FMs A and B show a reduction in the mixed regime as well. On rst sight this
looks favorable.
The next graph (Figure 7.11) shows the wear, taken at similar sliding distances.
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.010
0.100
1.000 10.000
Contact speed v (m/s)
RL 179/2 (5W/30)
+ 0.5% GMO
+ 0.5% OFM A
+ 0.5% OFM B
Friction coefficient f (−)
FIGURE 7.10 Stribeck curves of CEC RL 179/2 plus organic FMs.
9.000
8.000
7.000
6.000
5.000
4.000
3.000
2.000
1.000
0.000
Wear area
RL179/2 + 0.5% GMO + 0.5% OFM A + 0.5% OFM B
Wear scar (mm
2
)
FIGURE 7.11 Wear scars of CEC RL 179/2 plus organic FMs.
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Organic Friction Modifi ers 207
The wear of the oil containing A and B is twice as high as those of the reference oil and that
oil with addition of 0.5% GMO. Consequently, the contact pressure p is twice as low as those of the
others and is what makes the ML/EHL transition shift to the left.
Thus it seems that organic FMs are predominantly active in the BL regime and that the shifts
observed in the mixed regime are likely to be caused by other phenomena that must not be ignored.
7.8.2 FRICTION AS A FUNCTION OF TEMPERATURE
Another aspect of FM performance is friction as a function of temperature. Temperature plays an
important role with regard to adsorption/desorption phenomena as for the formation of adsorbed
layers as well as regarding those of reacted layers.
The graph in Figure 7.12 shows the frictional behavior of some organic FMs as a function of
temperature, using the pin-on-ring tribometer as before with the same specimens and con guration.
Again, CEC reference oil RL 179/2 was used, and the speed chosen (0.03 m/s) assured operation
well within the BL regime.
All the organic FMs studied show a signi cant friction reduction over the temperature range
tested. GMO and oleylamide perform best, and the optimum adsorption seems to be obtained at
∼70°C. At higher temperatures, desorption may start to occur as well as some kind of competition
with other surface-active additives, leading to a higher coef cient of friction. Oleylamide, however,
continues to show high friction-reducing properties at elevated temperatures.
Figure 7.13 shows a comparison between organic FM GMO and metallic-type FM molybde-
num dithiocarbamate. Two sources of the latter were used at a concentration equivalent to 0.07%
molybdenum.
GMO and the molybdenum dithiocarbamates show a marked performance difference. Although
GMO is active over a wide temperature range, the molybdenum dithiocarbamates start to reduce
friction at temperatures of 120°C and above only. This has to be considered as an induction period
that can be explained by the necessary exchange of ligands between molybdenum dithiocarbamate
and zinc dialkyldithiophosphate (see Section 7.7). Once molybdenum dithiocarbamate has “lighted
off,” a fast drop of friction is noticed. At the end of the test cycle 140°C, the system has not stabi-
lized and the friction coef cient might decrease further.
FIGURE 7.12 Friction coef cient versus temperature—CEC RL 179/2 plus organic FMs.
40
50
60 70
80
90
100 110 120
130
140
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
0.14
Temperature (°C)
Friction coefficient f (−)
RL 179/2 + 0.5% GMO + 0.5% OFM A + 0.5% Oleylamide
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208 Lubricant Additives: Chemistry and Applications
The difference in friction-modifying characteristics between organic FMs and molybdenum
dithiocarbamate suggests that it might be bene cial to use a combination of these materials.
7.9 CONSEQUENCES OF NEW ENGINE OIL
SPECIFICATIONS AND OUTLOOK
Although initial fuel economy requirements were focused on fresh oil only, new engine oil speci -
cations will address fuel economy longevity as well. A good example is Sequence VIB, which has
been developed for the ILSAC GF-3 speci cation.
Sequence VIB includes aging stages of 16 and 80 h to determine fuel economy as well as fuel econ-
omy longevity. These aging stages are equivalent to 4000–6000 mi of mileage accumulation required
before the EPA metro/high-highway fuel economy test. That test is used in determining CAFE.
To obtain engine oil formations that are optimized with regard to fuel economy longevity,
high requirements are demanded for base oil selection and additive system design [3,7,22]. These
requirements are
To minimize the increase of viscosity thereby maintaining a low electrohydrodynamic fric-
tion coef cient
To maintain low boundary/mixed friction
A minimum increase of viscosity can be obtained by base uid selection (in terms of volatility,
oxidation stability, and antioxidant susceptibility) and selection of antioxidants and their treat level.
The market is already anticipating requirements by increasing the production capacity of groups II
(HIVI) and III (VHVI) base uids and by increased interest in groups IV (PAOs) and V (a.o. esters)
base uids.
To achieve low friction under BL and ML conditions, the use of effective friction-reducing addi-
tives is needed. To maintain low boundary and mixed friction over time, it is necessary to prevent
consumption of these additives by processes such as oxidation and thermal breakdown. Therefore,
selecting suitable antioxidant systems for molybdenum compounds and organic FMs and develop-
ing organic FMs with highest thermal/oxidative stability will be key for high fuel economy longev-
ity and a successful application in engine oil formulations.
•
•
FIGURE 7.13 Friction coef cient versus temperature—CEC RL 179/2 plus molybdenum dithiocarbamates.
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
40 50 60 70 80 90 100 110 120 130 140
Temperature (°C)
Friction coefficient f (−)
RL 179/2
+ 0.5% GMO
+ 0.07% Mo (A)
+ 0.07% Mo (B)
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Organic Friction Modifi ers 209
Further studies on the mechanisms of FM action, for example, through molecular modeling
techniques, could also speed up the development of optimized additives and additive systems. Apart
from frictional properties, other important tribological parameters such as wear rate and surface-
metal geometry should be investigated as well. In most papers studied, this seems to be ignored,
although all three parameters should be considered in relation to one another.
7.10 BENCH TESTS TO INVESTIGATE FRICTION-REDUCING COMPOUNDS
Several bench tests can be thought of to investigate the frictional properties of base uids and for-
mulated products. In recent literature [8,23,24], the following test equipment has been used:
1. The high-frequency reciprocating rig (HFRR) to measure boundary friction. Although
originally developed to measure diesel fuel lubricity, the equipment can be successfully
applied to measure lubricant properties as well.
Frequency 10–200 Hz
Stroke length 20–2000 µm
Load 0–1000 g
Ambient temperature 200°C
A 6 mm diameter ball is the upper specimen and a 3 mm thick smooth disk with a
10 mm diameter is the lower specimen. HFRR speci cations are the test conditions that
the authors applied to screen FMs; these include a 40 Hz frequency, a stroke of 1000 µm,
and a 400 g load.
2. A mini traction machine (MTM) to measure mixed and (E)HD friction, for example, by
the determination of Stribeck curves. The MTM rig is capable of measuring at either con-
stant or varying slide/roll ratios if required.
Speed range Up to 5 m/s
Slide/roll ratio 0–200% (Full rolling to full sliding)
Load 0–75 N
Ambient temperature 150°C
Standard specimens are a 19.05 mm diameter ball as upper specimen and a 50 mm
diameter disk as lower specimen. Both are manufactured from AISI 52100 bearing steel.
The standard disk is smooth, which allows measurement of mixed- lm and full- lm fric-
tion. Alternatively, rough disks are available for measurements in the BL regime. MTM
speci cations are the test conditions that the authors applied to test FMs; these comprise a
speed range of 0.001–4 m/s, a 30 N load, and a 200% slide/roll ratio.
3. An optical rig provided with a disk coated with a spacer layer to measure EHD lm thick-
ness. Such a rig enables lm-thickness measurements down to <5 nm with a precision
between 1 and 2 nm.
Some other literature refers to the low-velocity friction apparatus (LVFA). Alternative
reciprocating rigs may be suitable as well.
REFERENCES
1. Wilk, M.A., W.D. Abraham, B.R. Dohner. An investigation into the effect of zinc dithiophoshpate on
ASTM sequence VIA fuel economy. SAE Paper 961,914, 1996.
2. Houben, M. Friction analysis of modern gasoline engines and new test methods to determine lubricant
effects, 10th International Colloquium, Esslingen, 1996.
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210 Lubricant Additives: Chemistry and Applications
3. Korcek, S. Fuel ef ciency of engine oils—current issues. 53rd Annual STLE Meeting, Detroit, 1998.
4. LaFountain, A., G.J. Johnston, H.A. Spikes. Elastohydrodynamic friction behavior of polyalphaole n
blends. Tribololgy Series 34:465–475, 1998.
5. Spikes, H.A. Boundary lubrication and boundary lms. Tribology Series 25:331–346, 1993.
6. Spikes, H.A. Mixed lubrication—an overview. Lubrication Science 9(3):221–253, 1997.
7. Korcek, S. et al. Retention of fuel ef ciency of engine oils. 11th International Colloquium, Esslingen,
1998.
8. Sorab, J., S. Korcek, C. Bovington. Friction reduction in lubricated components through engine oil for-
mulation. SAE Paper 982,640, 1998.
9. Goodwin, M.C., M.L. Haviland. Fuel economy improvements in EPA and road tests with evine oil and
rear axle lubricant viscosity reduction. SAE Paper 780,596, 1978.
10. Waddey, W.E. et al. Improved fuel economy via engine oils. SAE Paper 780,599, 1978.
11. Clevenger, J.E., D.C. Carlson, W.M. Keiser. The effects of engine oil viscosity and composition on fuel
ef ciency. SAE Paper 841,389, 1984.
12. Dobson, G.R., W.C. Pike. Predicting viscosity related performance of engine oils. Erd-1 und Kohle-
Erdgas 36(5):218–224, 1982.
13. Battersby, J., J.E. Hillier. The prediction of lubricant-related fuel economy characteristics of gasoline
engines by laboratory bench tests. Proceedings of International Colloquium, Technische Akademie
Esslingen, 1986.
14. Grif ths, D.W., D.J. Smith. The importance of friction modi ers in the formulation of fuel ef cient
engine oils. SAE Paper 852,112, 1985.
15. Taylor, R.I. Engine friction lubricant sensitivities: A comparison of modern diesel and gasoline engines.
11th International Colloquim, Esslingen, 1998.
16. Crawford, J., A. Psaila. In R.M. Mortier and S.T. Orszulik, eds. Chemistry and Technology of Lubri-
cants, Miscellaneous Additives. London, 1992, pp. 160–165.
17. Allen, C.M., E. Drauglis. Boundary lubrication: Monolayer or multilayer. Wear 14:363–384, 1969.
18. Akhmatov, A.S. Molecular physics of boundary lubrication. Gos. Izd. Frz.-Mat. Lit., Moscow, p. 297,
1969.
19. Beltzer, M., S. Jahanmir. Effect of additive molecular structure on friction, Lubrication Science 1–1:
3–26, 1998.
20. Arai, K. et al. Lubricant technology to enhance the durability of low friction performance of gasoline
engine oils. SAE Paper 952,533, 1995.
21. Christakudis, D. Friction modi ers and their testing, additives for lubricants. Kontakt Stud 433:134–162,
1994.
22. Effects of aging on fuel ef cient engine oils. Automotive Engineering (Feb):1996.
23. Moore, A.J. Fuel ef ciency screening tests of automotive engine oils. SAE Paper 932,689, 1993.
24. Bovington, C., H.A. Spikes. Prediction of the in uence of lubricant formulations on fuel economy from
laboratory bench tests. Proceedings of International Tribology Conference, Yokahama, 1995.
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Part 3
Antiwear Additives and
Extreme-Pressure Additives
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213
8
Ashless Antiwear and
Extreme-Pressure Additives
Liehpao Oscar Farng
CONTENTS
8.1 Introduction ........................................................................................................................... 214
8.2 Chemistry, Properties, and Performance (Classi ed by Elements) ...................................... 215
8.2.1 Sulfur Additives ........................................................................................................ 215
8.2.1.1 Sulfurized Ole ns ....................................................................................... 216
8.2.1.2 Sulfurized Esters and Sulfurized Oils ........................................................ 219
8.2.1.3 Other Sulfur Additives ................................................................................220
8.2.2 Phosphorus Additives ................................................................................................220
8.2.2.1 Phosphate Esters ......................................................................................... 221
8.2.2.2 Phosphites ...................................................................................................222
8.2.2.3 Dialkyl Alkyl Phosphonates .......................................................................224
8.2.2.4 Acid Phosphates .........................................................................................225
8.2.3 Sulfur–Phosphorus Additives ...................................................................................225
8.2.3.1 Ashless Dithiophosphates ...........................................................................225
8.2.3.2 Ashless Phosphorothioates and Thiophosphates ........................................226
8.2.4 Sulfur–Nitrogen Additives ........................................................................................228
8.2.4.1 Dithiocarbamates ........................................................................................228
8.2.4.2 Dimercaptothiadiazole and Mercaptobenzothiazole Additives..................229
8.2.4.3 Other Sulfur–Nitrogen Additives ...............................................................230
8.2.5 Phosphorus–Nitrogen Additives ............................................................................... 231
8.2.5.1 Amine Phosphates ...................................................................................... 231
8.2.5.2 Amine Thiophosphates and Dithiophosphates ...........................................232
8.2.5.3 Other Phosphorus–Nitrogen Additives .......................................................232
8.2.6 Nitrogen Additives ....................................................................................................232
8.2.7 Additives with Multiple Elements .............................................................................234
8.2.8 Halogen Additives ..................................................................................................... 235
8.2.9 Nontraditional Antiwear/Extreme-Pressure Additives .............................................236
8.3 Manufacture, Marketing, and Economics.............................................................................237
8.4 Evaluation Equipment/Speci cation .....................................................................................238
8.4.1 Lubricant Speci cations ............................................................................................238
8.4.2 Additive Speci cations ..............................................................................................239
8.4.3 Test Methods and Equipment ....................................................................................239
8.5 Outlook .................................................................................................................................243
Acknowledgment ...........................................................................................................................245
References ......................................................................................................................................246
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