XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
HARD-CORE REVERSE MICELLES IN TRIBOFILM
FORMATION AND SOLUBILIZATION PROCESSES
IN ENGINE OIL
Zenon PAWLAK1) and Barney E. KLAMECKI2)
1)
University of Technology and Agriculture,
Faculty of Technology and Chemical Engineering,
85-326 Bydgoszcz, Seminaryjna 3;
and Utah Department of Health, Environmental Chemistry,
Salt Lake City. 46 Medical Drive, UT 84113, USA
2)
University of Minnesota, Department of Mechanical Engineering,
111 Church Street SE, Minneapolis, MN 55455-0111, USA.
ABSTRACT
The tribofilm formation requires that tribochemical reaction occur
between the solubilized zinc dialkyldithiophosphate (ZDDP) by soft-core
reverse micelles (RMs) and the metallic surfaces under boundary lubrication.
In the case of hard-core reverse micelles, the colloidal core (CaCO3)x or (CuO)x
is directly introduced in the sliding contact, and undergoes physical or
tribochemical changes during the film build-up. Hard-core RMs in hydrocarbon
formulation is one or two orders of magnitude larger than that of the soft-core
RMs. Discussion of hard-core reverse micelles formation, solubilization, acidbase interaction and related phenomena are presented in this paper.
INTRODUCTION
Interactions between additives are of practical importance in
hydrocarbon and synthetic oil formulation. Engine oil additives are dissolved or
dispersed in an oil formulation, some occur in a micellar form as hard-core
reverse micelles, RM's, or micellar aggregates [1,2,3].
Considering tribochemical mechanism for the action of the hard-core,
RM's are under intensive investigation. The (Cumetallic) tribofilm formation
requires the tribochemical reaction to occur between the (CuO)x(oleic acid)n, RMs
and the metallic surfaces.
(CuO)x (oleic acid)n(RMs) + Triboelectrons (e- tribo) → (Cumetallic)x Tribofilm
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In the case of the calcium hard-core reverse micelles, the colloidal
(CaCO3)x or (CaBorate)x is directly introduced in the sliding contact and
undergoes physicochemical changes during the film build-up. Consequently, no
tribochemical reaction with the hard-core RMs take place [3].
(CaCO3)x(sulfonate)n (RMs) + Friction → (CaCO3)x –calcite surface layer
(2)
To improve antiwear and extreme-pressure performance of micellar
carbonate (borate), a chemical modification of these colloidal species with zinc
dialkyldithiophosphate (ZDDP), and with sulfurized carboxylic acids was
performed [5,7]. In case of modified hard-core RMs better performance is
observed in comparison to Ca borate, RMs.
(CaBorate)x(sulfonate)n-ZDDP (RMs) + Friction →
→ Ca(PO3)2 (Calcium phosphate, tribofilm)
(3)
This suggests that under rubbing conditions, the borate-sulfonate micelles
interact more effectively than ZDDP on the surface. ZDDP and the boratesulfonate micelles interact with each other under rubbing conditions and
form calcium phosphate [7].
RESULTS AND DISCUSSION
Hard-core Reverse Micelles Formation. Hard-core reversed micelles
are composed of detergent molecules, e.g. sulfonate strongly bonded to the
calcium carbonate core (CaCO3)x(sulfonate)n or oleic acid to the copper oxide core
(CuO)x(oleic acid)n (Fig.1). The sizes of the carbonate-sulfonate RMs and
surfactant layer thickness were investigated using small angle neutron
scattering method (SAXS) [8,9,10]. Typical hard-core RMs sizes (Å): (a)
calcium sulfonate surfactant layer 21.5, (CaCO3 )x hard-core diameter 39, total
diameter 82 and (b) calcium phenolate surfactant layer 15, (CaCO3)x core 29
and total diameter 59.
In industrial formulations a variety of surfactants is used to coat calcium,
barium and magnesium carbonate, or borate particles. Colloidal dispersions of
inorganic carbonates can fulfill most requirements as effective acid-neutralizing
additives with the following features: strong base, stable after reaction with
acid, not harmful neutralization product, clear and stable oil solution. Calcium
may be preferred for specialized diesel engine applications and magnesium for
rust control in gasoline car engines [2].
Tribochemical Nature of Antiwear Films. A "Tribological Tree"
shown in Fig. 2 will summarize our knowledge of some most important
processes proceeding in the bulk oil and the effects of those processes on the
mechanically activated surfaces. The "Tribological Tree" consists of additive
mixtures which interact in a variety of ways, both in the bulk oil and on
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surfaces. The most important factor governing the tribochemical reactions
under boundary friction is due to the action of RMs [2].
Fig. 1 . Schematic represenation of normal micele (M) in water, soft-core reverse
micelle (RM), and hard-core reverse micelle (RM) in hydeocarbon formulation
(
O) – detergent molecule.
Fig. 2. Tribological „Tree”
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XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
The core RMs of calcium carbonate (or borate) core surrounded by
calcium benzensulfonate, phenolate or salicylate molecules [3] have been
recognized as an efficient multifunction class of antiwear/anticorrosive
additives. The analytical results obtained on the wear particles allow to
conclude that during friction micelles undergo chemical and structural changes.
Hard-core RMs under the wear conditions lead to the loss of the organic shell.
Simultaneously the metastable amorphous calcium carbonate undergoes
crystallization into the calcite structure. The following tribochemical
mechanism for the film build-up can be proposed [3] to confirm the good
antiwear properties of the micelles:
ƒ the feeding of the rubbing surface is achieved by adsorption of the
hard-core RMs onto the friction surfaces,
ƒ the micelles lose their organic shell and form a polycrystalline film
adherent to the metallic surfaces,
ƒ tribological film delamination leads to protection of the rubbing
surfaces.
The combination of zinc dialkyldithiophosphate (ZDDP) and hard-core RMs
leads to a synergistic effect of metallic detergents on the degradation of ZDDP.
These phenomena are observed in many tests and can be explained in terms of:
ƒ the acid neutralization property of hard-core RMs leads to the
prevention of decomposition of ZDDP (in the valve train wear test and
the thin film oxygen uptake test),
ƒ the competitive solubilizaton reduces the effective concentration of
ZDDP on the metal surface (in the four-ball test),
ƒ mixed borophosphate tribofilms are formed on the metal surface (in
the Falex wear test).
Calcium borate hard-core micelles have recently been recognized as an
efficient multifunction class of anticorrosive-antiwear additives. The additive
acts as an antiwear agent by the formation of a calcium borate glass tribofilm
material. The boron behaves as a glass former like phosphorus does with the
ZDDP additive. The combination of ZDDP and calcium borate-salicylate
micelles is expected to be synergistic, due to the formation of a mixed
phosphate-borate glass tribofilm. Micellar borate-salicylate (BS) and ZDDP
produce long chain oxide glasses as antiwear tribofilm. The friction coefficient
in the steady-state conditions is 0.12 for ZDDP, 0.09 for (BS) and 0.11 for
ZDDP/BS. There is an evidence for P-O-B bonding in this glass, indicating an
atomic scale mixing of the two additives in the tribocontact. Iron is not seen in
the tribofilm composition [3,5,11,12].
Hard-core RMs (CuO)x-(oleic acid)n. The formation of a copper
tribofilm takes place during the friction process and disintegration of the
RMs takes place in a tribochemical reaction. The colloidal particle diameters
are about 10 to 40 nm and the particles are easily dispersible in oils, fuel, and
water forming a stable micelle solution [13]. Transfer and adhesion of the
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micellar particles accelerate surface modification forming of a fine copper
tribofilm. Boundary friction and the film decrease wear and the friction
coefficient induces the formation of a thin copper tribofilm. Since tribofilm
formation takes place during the friction process, disintegration of the RMs
takes place in a tribochemical reaction:
Cu+2 + 2e-tribo → Cu0
(4)
A thin copper tribofilm, which exhibits considerable compressive
strength and a low tangential shear strength is formed on the tool surfaces. If
hard-core RMs are delivered to the friction zone during cutting, the tribofilm
formed will be present on the tool over the whole period of operation. The
tribofilm reduces the coefficient of friction, the extent of direct contact
between the cutting tool and the workpiece, the temperature in the cutting
zone and hence tool wear. In water-based fluids a different tribochemical
mechanism is proposed for the action of hard-core reversed micelles (see Fig.1)
of (CaCO3)x-(sulfonate)n. During the rubbing process, the micelles lose their
organic shell and their mineral cores crystallize and a polycrystalline film
adherent to the metallic surface is formed [2, 3].
The tribochemical reactions during the friction process of hard-core
reverse micelles copper oxide, (CuO)x-(oleic acid)n and oil-based soft-core
(RMs) (see Fig.1) are very similar. Tribofilm formation requires a
tribochemical reaction to occur between the micellar additive and metallic
surface during the friction processes. It is known that under boundary
friction conditions and/or surface damage caused by fatigue processes,
triboemisions can lead to the release of triboelectrons (see reaction 4),
charged particles and photons. When the friction condition become severe,
for example in heavy load, high speed machining or drilling, RMs
decompose and the useful atoms or compounds produced, eg., Cumetallic or
polyphosphate, and the original structure of the rubbing surface is replaced
by a tribofilm (see reaction 1) [2, 3].
Micellar Solubilization in Lubrication. Several mechanisms of action
of RMs are generally recognized: solubilization of organic acids, solubilization
of guest molecules, e.g., ZDDP, steric and electrostatic stabilization of
contaminants. The solubilized compounds go into micelles interior cavity
which consist of ionic or polar groups. The RMs interface solubilize or
deactivate polar compounds, such as water, catalytic metal ions, organic acids
and sludge, Fig. 3 [2].
Processes of solubilization and interaction of water, polar molecules and
acidic materials with reverse micelles has been expressed in terms of the
adsorption of the substrate into the micellar surface. The acid deactivation
mechanism in hydrocarbon media is supplementary to the neutralizing action of
detergents. Traces of strong sulfur, nitrogen or halogen acids are scavenged by
colloidal (CaCO3)x and by neutral detergents. The strong acid reacting with
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carboxylate, sulfonate or carbonate micelles to form mineral salts and long
chain acids which will be retained in the micelle [9,10,11].
HA
ZDDP
(H2O)n
HA
HS HS HS
HS
HS
ZDDP
ZDDP
ZDDP
HA
HA
HS CaCO3 HS
HS
HS
HS HS HS
ZDDP
ZDDP
Soft-core reverse micelle
metal alkyl phenate
sludge particle
HA organic acid
ZDDP zinc dialkyldithiophosphate
Hard-core reverse micelle
metal alkyl benzenesulfonate
CaCO3
colloidal core
HS sulfur acid, nitrogen acid
Fig. 3. Solubilization of oxidation products, e.g., organic acids, ZDDP and inorganic
acids (HS) by hard-core reverse micelles in oil formulation
Reverse micelles manage the prevention of agglomeration and
electrostatic stabilization mechanisms. The steric stabilization mechanism
provides a physical barrier to attraction between soot particles (P) and
surfactant molecules by RMs formation. This effect contributes to the internal
cleanliness of engine by minimizing the agglomeration and subsequent
deposition of sludge particles.
REFERENCES
[1]
[2]
[3]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
H.A. Spikes, Lubrication Science, 1989, 2, 1.
Z. Pawlak, Tribologia, 2003, 2, 65-80.
J.L. Mansot, M. Hallouis, J.M. Martin, Colloids Surfaces, 1993, 75, 25.
K. Inoue, Lubr. Eng., 1993, 49, 263.
B.L. Papke, L.D. Rubin, SAE Paper , No. 922281(1992).
K. Varlot, M. Kasrai, M.M. Bancroft, E.S. Yamaguchi, P.R. Rayson, J. Igarashi, Wear,
2001, 249, 1029.
J.F. Marsh, Chem. Ind., 1987, 20 July, 470.
S. Giasson, D. Espinat, T. Palermo, R. Ober, M. Passah, M.F. Morizur, J. Colloid. Inter.
Sci., 1992, 153, 355.
I. Markovic, R.H. Ottevill, D.J. Cebula, I. Field, J.F. Marsh, Coll. Polym. Sci., 1984, 262,
648.
K. Varlot, J.M. Martin, B. Vacher, K. Inoue, in: Lubrication at the Frontier, D. Dowson et.
al., (Eds.), Elsevier, 1999, 433-438.
S. Shirahama, M. Hirata, Lubr. Sci., 1989, 1, 365.
G.P. Shpenkov, Friction Surface Phenomena, Elsevier, 1993.
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