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Tribological properties and lubrication mechanism of molybdenum dialkyldithiocarbamate and calcium compounds in greases
Wear, 130 (1989)
233
- 247
233
TRIBOLOGICAL PROPERTIES AND LUBRICATION MECHANISM MOLYBDENUM DIALKYLDITHIOCARBAMATE AND CALCIUM COMPOUNDS IN GREASES* EN-CA1
ZHENG
and XIANG-LIN
OF
QIAN
Research Institute of Tribology, East China University 130 Meilong Road, Shanghai 20023 7 (China)
of Chemical
Technology,
Summary Different greases mixed with molybdenum dialkyldithiocarbamate (MoDTC) alone, and with calcium compounds, were tested in four-ball and Amsler-type testers. After mixing MoDTC alone with lithium or calcium base grease, the friction and wear are reduced, and the ultimate non-scuffing load Pb is increased. MoDTC and calcium petroleum sulphonate or calcium acetate are synergic on reducing wear in lithium base grease but not in calcium base grease. The effectiveness in raising the Pb value decreases in the order CaCOs > calcium acetate > calcium petroleum sulphonate. Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses show that the surface film consists mainly of MO&, Moos, CaC03 and CaS04. A model for the surface film is put forward accordingly.
1. Introduction The idea of using MO-S complexes as oil-soluble additives was derived from the fact that MO& is an excellent lubricant. MO-S complexes have been attracting interest in their use as additives for about 30 years [l]. Renewed interest in MO-S complexes was stimulated by the desire to reduce gasoline consumption in motor vehicles at the end of the 1970s. A fuel saving of 3% 5% was reported [ 2 - 51. Up to now, two kinds of MO-S organic compounds, molybdenum dialkyldithiophosphate (MoDTP) and molybdenum dialkyldithiocarbamate (MoDTC), have been of interest. Their structural formulae are as follows: RO RO’
'P'
June
/s ‘MO,/x,/MO,,s+,P,
‘S’;
S
v
*Paper presented 26 - 29,1988.
0043-1648/89/$3.50
S
at
,OR OR
the
Nordic
Symposium
@ Elsevier
on
Trihology,
Sequoia/Printed
Trondheim,
Norway,
in The Netherlands
where R is an alkyl group, e.g. ethyl, isopropyl, n-butyl, isobutyl or 2-octyl; X, Y and 2 are sulphur or oxygen [ 1,6, 71. Zinc dialkyldithiophosphate (ZDTP) has been used as an antiwear and antioxidant additive in engine oils for a number of years. However, phosphorus in ZDTP may poison platinum catalysts that are used to reduce the harmful effects of exhaust gases. For this reason, the ZDTP content in engine oil should be controlled. Similar problems occur with MoDTP. However, MoDTC does not contain any phosphorus. Besides, preliminary laboratory and bench tests have shown that MoDTC possesses excellent frictionreducing, antiwear, antifatigue, antioxidation and extreme pressure properties [6 - 131. Fortunately, the problem of solubility of MoDTC in oil had been solved satisfactorily [7 ] . When MoDTC, calcium petroleum sulphonate and auxiliary solvent are mixed in the proportions 1:1:3, the solubility of MoDTC in mineral oil is improved, so that the concentration of molybdenum in oil can amount to as much as 1600 ppm; while the optimum quantity of molybdenum in engine oils is about 340 ppm. In order to reduce metal corrosion, Sakurai 2141 suggested that MoDTC needs to be refined to remove reactive sulphur or some of the chelate should be used to form a protective film on the metal surface. Thus MoDTC is a quite promising replacement for ZDTP. There is much evidence to confirm that MoSz is formed after the decomposition of MoDTC [6]. Thermogravimetric analysis indicates that MoDTC decomposes in two distinct stages in nitrogen or air. The differences in the first decomposition temperature for various MoDTC compounds may be ascribed to decomposition With regard to MoS, lubricant films, crystals on the film surface have their (001) MO& planes parallel to the sliding surface which leads to a minimum in friction [ 151. Under extremely severe conditions, some of the MoS, reacts chemically with iron so that molybdenum and FeS are formed 116,171. MoDTC, calcium petroleum sulphonate and polyisobutenyl succinimide have a good compatibility photoelectron of lubricated surfaces. In order that MoDTC can be used reliably as an additive in engine oils etc., the properties and the compatibility investigated. 2.
details
For the purpose of comparison, calcium petroleum sulphonate, CaCO, or calcium acetate was added together with MoDTC in number 3 lithium or calcium base grease.
235
Electric
Frame
reslstonce
foundotlon
(a)
(b)
Fig. 1. Set-up for determining friction coefficient tion measurement. (b) Connection of instruments.
Upper block
30:x
in four-ball
:
of fric-
v45
steel. HRC35-37
steel, HRC42-47
(a)
Fig. 2. Specimens of specimens.
(a) Sketch
3.5
“if2
45
machine.
@I
in MM-200
testing
machine.
Sketch
of frictional
couple
and dimensions
2.1. Assessment of the tribological properties The ultimate non-scuffing load Pb was determined in an MS-800 fourball machine according to GB3142-82 [18]. Wear tests were carried out under a load of 98 N for 1 h. The friction coefficient was recorded continuously by means of electric resistance strain foils, see Fig. 1. The size of the scar track was measured after 3 h running under 147 N in an Amsler-type MM-200 testing machine (Fig. 2) while the average friction coefficient was determined from the frictional work recorded. The upper block is fixed whereas the lower specimen rotates at a speed of 200 rev min-' . 2.2. Elemental and chemical analyses Elemental and chemical analyses were accomplished using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) using a PHI.550 XPS/AES. The samples analysed were taken from upper blocks that had undergone 3 h running under a load of 147 N in the MM-200 testing machine. Surfaces of the samples were bombarded by an argon ion stream of short duration to remove contaminants. They were also bombarded for depth profiling. The stripping rate is about 15 A min-’ . In order to avoid confusion concerning the source of calcium the samples lubricated with
236
calcium base grease were not analysed because AES and XPS are unable to distinguish between calcium derived from additives and that derived from base grease.
3. Experimental
results and discussion
3.1. Tribological
properties 3.1 .l. Ultimate non-scuffing load P,, The P,, value increases continuously
with increasing concentration of MoDTC up to 4 wt.% MoDTC in grease (Fig. 3). The P,, value of the lithium or calcium base grease, containing 4 wt.% MoDTC, is 79% or 62% higher than that of the base grease. The concentration of molybdenum in lithium base grease containing 4 wt.% MoDTC is just one half of that containing 1.6 wt.% MO&, but the P,, value of the former is 25% higher than that of the latter. This means that MoDTC can increase notably the film strength at a lower concentration of molybdenum than MO& does. The wear scar diameter-load curves (Fig. 4) show that the scar diameter decreases more
SK-
0
I
2
3
4
Concontrotion
5
Fig. 3. Pb value us. concentration grease.
Q.2%k----400500
6
of Mo DTC
7
0
9
(wt.%)
of MoDTC:
x , calcium base grease; l , lithium base
L
7cO9co Load(N)
(a)
(b)
Fig. 4. Wear scar diameter-load curves in four-ball machine. (a) Lithium base grease. (b) Calcium base grease. l, no additive;A, 4 wt.% MoDTC; x wt.% MoDTC.
,8
237
significantly under high load, which indicates that MoDTC is more suitable for high load conditions. The effectiveness in raising the Pi, value decreases in the order CaCOs > calcium acetate > calcium petroleum sulfonate, see Table 1. The P,, value of the lithium or calcium base grease containing 4 wt.% MoDTC and 2 wt.% CaC03 is 140% or 98% higher than that of the base grease, and is 35% or 24% higher than that containing 4 wt.% MoDTC alone. The calcium salts of the fatty acid in the calcium base grease and MoDTC are not synergic on the P,, value because of the fact that the effect of MoDTC on the P,, value in lithium base grease is more pronounced than that in calcium base grease (see Fig. 3 and Table 1). MoDTC and MO& in lithium base grease are also not synergic. 3.1.2. Wear After mixing 4 wt.% MoDTC with lithium or calcium base grease, the scar diameter decreases from 0.71 to 0.66 mm or from 0.37 to 0.31 mm. MoDTC and calcium petroleum sulphonate or calcium acetate are synergic in reducing wear in lithium base grease, but not in calcium base grease, as shown in Tables 2 and 3. 3.1.3. Coefficient of friction The coefficient of friction decreases continuously with increasing MoDTC concentration up to 4 wt.% (Fig. 5). After mixing 4 wt.% MoDTC with lithium or calcium base grease, the friction coefficient decreases from 0.078 to 0.066 or from 0.075 to 0.067 in the four-ball machine. There is no difference between the friction coefficient of grease containing MoDTC alone and that containing MoDTC and calcium in the four-ball machine, both are in the range 0.06 - 0.07. In the MM-200 testing machine, however, the friction coefficient of grease containing MoDTC alone is lower than that containing MoDTC and calcium (Table 4). The different behaviour I
c
0.10
: 0 ._
I
0.09
5,o 0.00
t
3,
._ E J
0.07
It
t
-b
-__&----______&=
11::
0123456769 Concrntrotion
of MoDTC (wt. %)
Fig. 5. Friction coefficient us. concentration base grease; l , lithium base grease.
of MoDTC
in four-ball
machine:
x , calcium
0.37
Calcium base grease
Lithium base grease
Base grease
0.75
4 wt.% MoDTC
0.69
4 wt.% MoDTC 2 wt.90 Calcium petroleum sulphonate
Scar width under 147 N for 3 h with different
0.31
0.66
4 wt.% MoDTC
additives
(mm)
(mm)
0.85
4 wt.% MoDTC 2 wt.% CaCO3
additives
0.31
0.43
4 wt.% MoDTC 2 wt.% Calcium petroleum sulphonate
under 98 N for 1 h with different
Scar widths in MM-200 testing machine
TABLE 3
0.71
No additive
Scar diameter
Lithium base grease
grease
Base
Scar diameters in four-ball machine
TABLE 2
1.06
1.8 wt.% MoSz 2 wt.90 Calcium petroleum sulphonate
0.30
0.61
4 wt.% MoDTC 2 wt.% Calcium acetate
4.50
2 wt.% Calcium petroleum sulphonate
0.33
0.67
4 wt.% MoDTC 2 wt.% CaCO3
co
M
240
of MoDTC in the two testing machines is ascribed to the operating conditions and the method of measuring the friction coefficient. It should be noted that the behaviour of MoDTC in oils is sometimes different from that in grease. Tests with oils containing MoDTC in the four-ball machine revealed that the friction coefficient and scar diameter decrease in the initial stage and then increase again with increasing MoDTC concentration; the optimum molybdenum concentration is about 340 ppm [7]. Besides, in engine oils, MoDTC and calcium petroleum sulphonate are not synergic in reducing wear. In this study, however, even if the molybdenum concentration is as high as 10 600 ppm in grease, the P,, value does not decrease and the friction coefficient does not increase with increasing MoDTC concentration. 3.2. Compositions of the surface film The elements molybdenum, sulphur, calcium, carbon, nitrogen and oxygen were detected using AES on steel surfaces lubricated with lithium base grease containing MoDTC and calcium compounds, see Fig. 6. Values of the elemental binding energy for different compounds are summarized in Table 5. As long as MoDTC is added in grease, MoS2, MOO, and Fe& can always be found on the worn surfaces. Provided calcium petroleum sulphonate is mixed in grease, CaCOs and CaS04 will be detected. CaC03 and CaS04 are derived from either tribochemical reaction products or original impurities in calcium petroleum sulphonate. From the results of AES and XPS analyses, it can be concluded that MO& and MOO, occurring in the surface film accounts for the low friction, wear-reducing and extreme pressure properties of MoDTC, while the synergism of MoDTC and calcium petroleum sulphonate in reducing wear relates closely to CaCO, and &SO4 found in the surface film.
\ b
Fig. 6. AES spectra of wear scar. In lithium base grease, N, 3 h: a, 4 wt.% MoDTC + 2 wt.% CaCOs; b, 4 wt.% leum sulphonate; c, 4 wt.% MoDTC.
on MM-200 testing machine, 147 MoDTC + 2 wt.% calcium petro-
Lithium grease
Base grease
Friction
base
coefficients
TABLE 4
0.05
4 wt.% MoDTC
0.09
4 wt.% MoDTC 2.wt.% Calcium petroleum sulphonate 0.09
4 wt.% MoDTC 2 wt.% CaCO3
under 147 N for 3 h with different additives
machine
Average friction coefficient
in MM-200 testing
0.10
1.8 wt.% MO& 2 wt.% Calcium petroleum sulphonate
0.12
2 wt.% Calcium petroleum sulphonate
242 TABLE
5
Binding
energy of elements
Elements in compounds
and corresponding
Standard values binding energy (ev)
compounds
in the surface
Values of binding energy with different 4 wt.% MoDTC Peaks in XPS spectra
Corrected
284.6
284.7
Fe Fe& Fe0 FeOOH
706.8 706.4 709.2, 710.8.
MO& Moos
228.9
Ca
CaCOs CaC04
346.8 347.4
S
MO&
162.2
C Fe
MO
232.5,
3.3. Effects
film
additives
in
4 wt.% MoDTC 2 wt.% CaCOs Peaks in XPS spectra
Corrected
284.6
284.7
284.6
706.8 706.6
706.7 706.5
710.2 711.5
707.0 706.4 708.8
706.9 706.3 708.7
711.6
711.5
228.6 232.0
228.5
232.6
228.5 232.6
228.4 232.5
347.0
346.9
of calcium
231.9
162.3
compounds
162.2
on elements
in the surface
film
The relative percentage of molybdenum, sulphur or calcium in the surface film with lithium base grease containing 4 wt.% MoDTC and 2 wt.% calcium petroleum sulphonate is higher than that containing either of the TABLE
6
Concentrations
of elements
in surface
Maximum relative percentage 0 f element (%)
Additives
MO 4 wt.% MoDTC 2 wt.% Calcium sulphonate 4 wt.% MoDTC 2 wt.% Calcium sulphonate 4 wt.% MoDTC 2 wt.% CaCOs aIn lithium
filma
3.6
MO
S
Ca
C
0
12.0
30.0
3.5
32.0
23.0
6.6
24.0
Co
S 5.4
petroleum
petroleum
Average relative percentage of element across film thickness (%)
3.0
2.8
6.2
5.0
29.5
14.4
3.6
23.0
7.7
3.7
4.0
6.7
2.8
1.7
1.7
testing
machine,
base grease on MM-200
147 N, 3 h
243
lithium
base grease (eV)
4 wt.% MoDTC 2 wt.% Calcium petroleum sulphonate
1.8 wt.% MoS? 2 wt.90 Calcium petroleum sulphonate
Corrected
Peaks in XPS spectra
Corrected
Peaks in XPS spectra
Corrected
Peaks in XPS spectra 285.0
284.6
285.0
284.6
284.7
284.6
707.0
706.9
706.2
705.8
709.9 711.2
709.5 710.8
708.6
708.5
232.6
347.2 347.8
347.1 347.7
709.4
709.0
232.2
228.9 232.8
228.5 232.4
347.1 347.8
346.7 347.4
346.8 348.2
346.4 347.8
162.8
162.4
162.0
161.6
2 wt.90 Calcium petroleum sulphonate
additives alone, as shown in Table 6. This means that MoDTC and calcium petroleum sulphonate have a joint effect on the concentration of molybdenum, sulphur and calcium in the surface film. The mechanism is not clear. Figure 7 shows element depth profiles. The highest relative percentage of calcium is found on the surface when MoDTC and CaC03 are added in grease but beneath the surface when MoDTC and calcium petroleum sulphonate are added. Furthermore, the relative percentage of molybdenum, sulphur or calcium in the surface film, lubricated with lithium base grease containing 4 wt.% and 2 wt.% CaC03, is lower than that containing 4 wt.% MoDTC and 2 wt.% calcium petroleum sulphonate. The reason is that
Sputtcrmg rime
(a)
(ml”)
(b)
Fig. 7. Elemental depth profile of wear scar. In MM-200 testing Lithium base grease, 4 wt.% MoDTC, 2 wt.% calcium petroleum base grease, 4 wt.% MoDTC, 2 wt.% CaC03.
machine, 147 N, 3 h. (a) sulphonate. (b) Lithium
244
CaC03 as an additive, may be brought to the friction surface by the base grease but it does not penetrate the base metal; while some of the calcium atoms of calcium petroleum sulphonate diffuse into the base metal by a tribochemical reaction. Oxygen in the air and the hydrocarbons in greases have undergone tribochemical reactions during sliding because great amounts of oxygen and carbon are found in the surface films (see Table 6). When MoDTC and calcium petroleum sulphonate are added together, the percentage of carbon in the surface film decreases observably, demonstrating reduced oxidation of hydrocarbons, in comparison with that when either of the additives is added alone.
4. A model of the surface film Lubricated with a lithium base grease containing MoDTC and calcium petroleum sulphonate, the surface film consists mainly of MO&, Fe&, Moos, CaCOs, CaS04, Fe0 and FeOOH. The surface film is 250 - 350 A thick. The texture of the surface film will be discussed below. Elements are distributed non-homogeneously across the film thickness, as shown in Fig. 7(a) and Table 7. The concentration of carbon or oxygen decreases rapidly across the film thickness, iron increases slowly and then reaches a stable value, while molybdenum, sulphur and calcium increase in the initial stage, then decrease.
TABLE Elemental
7 concentrations
Relative depth in to surface film
in different
depths across
film thicknessa
of elements
Relative percentage
(%)
MO
Ca
S
Fe
4.5 5.0 3.7 3.2 3.1 3.1
10.6 14.4 12.2 9.8 7.4 6.5
15.1 18.0 25.9 29.4 29.5 27.5
12.8 19.9 24.3 28.4 34.2 37.6
1;
2:9
2:9
17.1
54.1
2h
2:1
2:1
2:1
82:l
a 4 wt.% MoDTC and 2 wt.% calcium 200 testing machine, 147 N, 3 h.
petroleum
sulphonate
in lithium
base grease on MM-
245 TABLE Elemental
8 concentrations
Elements
MO Ca Fe S C 0 N
in different
locations
Relative percentage
along worn surfacea
of element
(%)
1
2
3
4
5
4.1 6.7 28.9 15.8 16.3 26.3 1.9
5.2 10.4 17.5 13.0 21.9 30.9 1.1
5.3 10.9 16.6 25.7 19.6 20.0 1.4
5.9 14.2 21.7 15.8 11.1 29.8 1.5
10.1 16.2 16.3 18.3 9.8 27.0 2.3
a4 wt.% MoDTC and 4 wt.% calcium MM-200 testing machine, 147 N, 3 h.
petroleum
sulphonate
in lithium
base grease,
on
Table 8 illustrates that elements are distributed non-homogeneously along the friction surface. Five equally spaced points were chosen for analysis along the worn surface. The concentration of molybdenum varies between 4.1% and lO.l%, calcium between 6.7% and 16.2% and sulphur between 13.0% and 25.7%. In the order point 1 to point 5, the concentration of calcium in the surface film increases together with that of molybdenum, but not proportionally. To sum up, a model of the surface film is put forward for lubrication with a lithium base grease containing MoDTC and calcium petroleum sulphonate, as shown in Fig. 8. On the condition that MoDTC is added alone in the grease, the MO& film formed by the tribochemical reaction can reduce friction and wear and also increase the load-carrying capacity. Provided that MoDTC and calcium petroleum sulphonate are mixed together in the grease, the MO& film on the asperities of the surface is replaced by a hard solid layer which consists mainly of Moos, CaCOs and CaS04, because MO&, which has an isotropic structure of layers, is easier to peel off than the other materials. The hard solid layer increases the wear resistance and reduces the friction coefficient. The high load-carrying capacity relates to the fact that CaCOs, which has a
Fig. 8. A model nate: z, MoSs;O,
of the surface film formed Moos;*, CaCOs or CaS04;
by MoDTC and calcium ///, base metal.
petroleum
sulpho-
246
high melting point and possesses high chemical stability, can raise the Pb value of the grease (see Table 1). As stated in Section 1, MoS, is formed by decomposition of MoDTC. However, how CaCOs and CaS04 are formed and how Fe& affects the tribological properties of the surface film remain to be answered. 5. Conclusions (1) In greases, MoDTC can significantly reduce friction and wear and also increase load-carrying capacity. MoDTC has an optimum concentration in grease. (2) If MoDTC and calcium compounds are added together in grease, wear and the ultimate non-scuffing load Pb depend on the kind of calcium compound. Calcium petroleum sulphonate and CaCOs are most effective respectively in improving antiwear and raising the Pb value. (3) The combined effects of MoDTC and calcium compounds are not consistent in reducing the friction coefficient obtained from different types of testing machine. (4) MO&, Moos, Fe&, CaC03 and CaS04 account for the synergism of MoDTC and calcium compounds on lubricating properties. (5) In the surface film formed by MoDTC and calcium petroleum sulphonate, elements are distributed non-homogeneously along the friction surface or across the film thickness. The concentration of calcium increases or decreases simultaneously with that of molybdenum but not proportionally. (6) MoDTC is a non-phosphorated, oil-soluble and multi-effective additive and has good compatibility with calcium detergents or dispersants in greases. Acknowledgments The authors wish to thank Professors Ru-lin Wang and Dian-lun Yang for their encouragement and instructive suggestions during the study. References 1 P.
C.
H. Mitchell, Oil-soluble MO-S 281 - 300. 2 E. R. Braithwaite and A. B. Greene, denum compounds in motor vehicles, 3 H. Hamaguchi, Y. Maeda and T. passenger cars, SAE Preprint n810316
compounds
as lubricant
additives,
Wear, 100
(1984)
A critical analysis of the performance of molybWear, 46 (1978) 405 432. Maeda, Fuel efficient motor oil for Japanese
for Meeting,
4 U.S. Patent 2,954,339. 5 U.S. Patent 4,149,965. 6 T. Sakurai, H. Okabe and H. Isoyama, carbamates)
dimolybdenum
Pet. Inst., 13 (1971)
(V) and their
243 - 249.
February
23 - 27, 1981.
The synthesis of di+-thio-bis(dialkyldithioeffects on boundary lubrication, Bull. Jpn.
247 7 J. Y. Dong, Study of an oil additive -molybdenum dialkyldithiocarbamate, Doctoral Thesis, East China University of Chemical Technology, Shanghai, December 1985 (in Chinese). 8 P. Y. Zheng and X. A. Han, Tribological properties of organic molybdenum compounds, Pet. Refining, 29 (12) (1985) 44 - 52 (in Chinese). 9 H. Isoyama and T. Sakurai, Inhibition of autoxidation by di+-dithio-bis(dialkyldithiocarbamate) dimolybdenum, Bull. Jpn. Pet. Inst., 16 (1974) 112 - 117. 10 T. Sakurai, Mechanism of organomolybdenum compounds as lubricant additives, Lubrication, 28(5) (1983) 338 - 342 (in Japanese). 11 P. Y. Zheng, X. A. Han and R. L. Wang, The mechanism of friction reduction of sulfurized oxymolybdenum di-(2-ethylhexyl)phosphorodithioate under boundary lubrication, ASLE Preprint 86-TC-4E-I for the ASME-ASLE Tribology Conf., October 1986. 12 Y. G. Hu, Combined effects of molybdenum dialkyldithiophosphate and engine oil additives on friction and wear, Master Thesis, Research Institute of Petroleum Processing, January 1986 (in Chinese). 13 K. Fujita, A. Yoshida and K. Matsuo, Effects of molybdenum disulphide and an organic molybdenum compound in gear oil on rolling contact fatigue, Wear, 95 (1984) 271 - 286. 14 T. Sakurai, Lecture Notes at East China University of Chemical Technology, May 1986. 15 A. I. Brudnyi and A. F. Karmadonov, Structure of molybdenum disulphide lubricant film, Wear, 33 (1975) 243 - 249. 16 J. Gansheimer and R. Holinski, Molybdenum disulfide in oils and greases under boundary conditions, Trans. ASME, Ser. F, 95 (1973) 242 - 248. 17 J. M. Chen, J. 2. Zhao, H. X. Dang and S. K. Rong, A study of X-ray diffraction MO& transfer films, J. Solid Lubr., 6(l) (1986) 22 (in Chinese). 18 The People’s Republic of China Standard GB3142-82, Lubricants determination of load-carrying capacity (four ball machine).
233
- 247
233
TRIBOLOGICAL PROPERTIES AND LUBRICATION MECHANISM MOLYBDENUM DIALKYLDITHIOCARBAMATE AND CALCIUM COMPOUNDS IN GREASES* EN-CA1
ZHENG
and XIANG-LIN
OF
QIAN
Research Institute of Tribology, East China University 130 Meilong Road, Shanghai 20023 7 (China)
of Chemical
Technology,
Summary Different greases mixed with molybdenum dialkyldithiocarbamate (MoDTC) alone, and with calcium compounds, were tested in four-ball and Amsler-type testers. After mixing MoDTC alone with lithium or calcium base grease, the friction and wear are reduced, and the ultimate non-scuffing load Pb is increased. MoDTC and calcium petroleum sulphonate or calcium acetate are synergic on reducing wear in lithium base grease but not in calcium base grease. The effectiveness in raising the Pb value decreases in the order CaCOs > calcium acetate > calcium petroleum sulphonate. Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses show that the surface film consists mainly of MO&, Moos, CaC03 and CaS04. A model for the surface film is put forward accordingly.
1. Introduction The idea of using MO-S complexes as oil-soluble additives was derived from the fact that MO& is an excellent lubricant. MO-S complexes have been attracting interest in their use as additives for about 30 years [l]. Renewed interest in MO-S complexes was stimulated by the desire to reduce gasoline consumption in motor vehicles at the end of the 1970s. A fuel saving of 3% 5% was reported [ 2 - 51. Up to now, two kinds of MO-S organic compounds, molybdenum dialkyldithiophosphate (MoDTP) and molybdenum dialkyldithiocarbamate (MoDTC), have been of interest. Their structural formulae are as follows: RO RO’
'P'
June
/s ‘MO,/x,/MO,,s+,P,
‘S’;
S
v
*Paper presented 26 - 29,1988.
0043-1648/89/$3.50
S
at
,OR OR
the
Nordic
Symposium
@ Elsevier
on
Trihology,
Sequoia/Printed
Trondheim,
Norway,
in The Netherlands
where R is an alkyl group, e.g. ethyl, isopropyl, n-butyl, isobutyl or 2-octyl; X, Y and 2 are sulphur or oxygen [ 1,6, 71. Zinc dialkyldithiophosphate (ZDTP) has been used as an antiwear and antioxidant additive in engine oils for a number of years. However, phosphorus in ZDTP may poison platinum catalysts that are used to reduce the harmful effects of exhaust gases. For this reason, the ZDTP content in engine oil should be controlled. Similar problems occur with MoDTP. However, MoDTC does not contain any phosphorus. Besides, preliminary laboratory and bench tests have shown that MoDTC possesses excellent frictionreducing, antiwear, antifatigue, antioxidation and extreme pressure properties [6 - 131. Fortunately, the problem of solubility of MoDTC in oil had been solved satisfactorily [7 ] . When MoDTC, calcium petroleum sulphonate and auxiliary solvent are mixed in the proportions 1:1:3, the solubility of MoDTC in mineral oil is improved, so that the concentration of molybdenum in oil can amount to as much as 1600 ppm; while the optimum quantity of molybdenum in engine oils is about 340 ppm. In order to reduce metal corrosion, Sakurai 2141 suggested that MoDTC needs to be refined to remove reactive sulphur or some of the chelate should be used to form a protective film on the metal surface. Thus MoDTC is a quite promising replacement for ZDTP. There is much evidence to confirm that MoSz is formed after the decomposition of MoDTC [6]. Thermogravimetric analysis indicates that MoDTC decomposes in two distinct stages in nitrogen or air. The differences in the first decomposition temperature for various MoDTC compounds may be ascribed to decomposition With regard to MoS, lubricant films, crystals on the film surface have their (001) MO& planes parallel to the sliding surface which leads to a minimum in friction [ 151. Under extremely severe conditions, some of the MoS, reacts chemically with iron so that molybdenum and FeS are formed 116,171. MoDTC, calcium petroleum sulphonate and polyisobutenyl succinimide have a good compatibility photoelectron of lubricated surfaces. In order that MoDTC can be used reliably as an additive in engine oils etc., the properties and the compatibility investigated. 2.
details
For the purpose of comparison, calcium petroleum sulphonate, CaCO, or calcium acetate was added together with MoDTC in number 3 lithium or calcium base grease.
235
Electric
Frame
reslstonce
foundotlon
(a)
(b)
Fig. 1. Set-up for determining friction coefficient tion measurement. (b) Connection of instruments.
Upper block
30:x
in four-ball
:
of fric-
v45
steel. HRC35-37
steel, HRC42-47
(a)
Fig. 2. Specimens of specimens.
(a) Sketch
3.5
“if2
45
machine.
@I
in MM-200
testing
machine.
Sketch
of frictional
couple
and dimensions
2.1. Assessment of the tribological properties The ultimate non-scuffing load Pb was determined in an MS-800 fourball machine according to GB3142-82 [18]. Wear tests were carried out under a load of 98 N for 1 h. The friction coefficient was recorded continuously by means of electric resistance strain foils, see Fig. 1. The size of the scar track was measured after 3 h running under 147 N in an Amsler-type MM-200 testing machine (Fig. 2) while the average friction coefficient was determined from the frictional work recorded. The upper block is fixed whereas the lower specimen rotates at a speed of 200 rev min-' . 2.2. Elemental and chemical analyses Elemental and chemical analyses were accomplished using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) using a PHI.550 XPS/AES. The samples analysed were taken from upper blocks that had undergone 3 h running under a load of 147 N in the MM-200 testing machine. Surfaces of the samples were bombarded by an argon ion stream of short duration to remove contaminants. They were also bombarded for depth profiling. The stripping rate is about 15 A min-’ . In order to avoid confusion concerning the source of calcium the samples lubricated with
236
calcium base grease were not analysed because AES and XPS are unable to distinguish between calcium derived from additives and that derived from base grease.
3. Experimental
results and discussion
3.1. Tribological
properties 3.1 .l. Ultimate non-scuffing load P,, The P,, value increases continuously
with increasing concentration of MoDTC up to 4 wt.% MoDTC in grease (Fig. 3). The P,, value of the lithium or calcium base grease, containing 4 wt.% MoDTC, is 79% or 62% higher than that of the base grease. The concentration of molybdenum in lithium base grease containing 4 wt.% MoDTC is just one half of that containing 1.6 wt.% MO&, but the P,, value of the former is 25% higher than that of the latter. This means that MoDTC can increase notably the film strength at a lower concentration of molybdenum than MO& does. The wear scar diameter-load curves (Fig. 4) show that the scar diameter decreases more
SK-
0
I
2
3
4
Concontrotion
5
Fig. 3. Pb value us. concentration grease.
Q.2%k----400500
6
of Mo DTC
7
0
9
(wt.%)
of MoDTC:
x , calcium base grease; l , lithium base
L
7cO9co Load(N)
(a)
(b)
Fig. 4. Wear scar diameter-load curves in four-ball machine. (a) Lithium base grease. (b) Calcium base grease. l, no additive;A, 4 wt.% MoDTC; x wt.% MoDTC.
,8
237
significantly under high load, which indicates that MoDTC is more suitable for high load conditions. The effectiveness in raising the Pi, value decreases in the order CaCOs > calcium acetate > calcium petroleum sulfonate, see Table 1. The P,, value of the lithium or calcium base grease containing 4 wt.% MoDTC and 2 wt.% CaC03 is 140% or 98% higher than that of the base grease, and is 35% or 24% higher than that containing 4 wt.% MoDTC alone. The calcium salts of the fatty acid in the calcium base grease and MoDTC are not synergic on the P,, value because of the fact that the effect of MoDTC on the P,, value in lithium base grease is more pronounced than that in calcium base grease (see Fig. 3 and Table 1). MoDTC and MO& in lithium base grease are also not synergic. 3.1.2. Wear After mixing 4 wt.% MoDTC with lithium or calcium base grease, the scar diameter decreases from 0.71 to 0.66 mm or from 0.37 to 0.31 mm. MoDTC and calcium petroleum sulphonate or calcium acetate are synergic in reducing wear in lithium base grease, but not in calcium base grease, as shown in Tables 2 and 3. 3.1.3. Coefficient of friction The coefficient of friction decreases continuously with increasing MoDTC concentration up to 4 wt.% (Fig. 5). After mixing 4 wt.% MoDTC with lithium or calcium base grease, the friction coefficient decreases from 0.078 to 0.066 or from 0.075 to 0.067 in the four-ball machine. There is no difference between the friction coefficient of grease containing MoDTC alone and that containing MoDTC and calcium in the four-ball machine, both are in the range 0.06 - 0.07. In the MM-200 testing machine, however, the friction coefficient of grease containing MoDTC alone is lower than that containing MoDTC and calcium (Table 4). The different behaviour I
c
0.10
: 0 ._
I
0.09
5,o 0.00
t
3,
._ E J
0.07
It
t
-b
-__&----______&=
11::
0123456769 Concrntrotion
of MoDTC (wt. %)
Fig. 5. Friction coefficient us. concentration base grease; l , lithium base grease.
of MoDTC
in four-ball
machine:
x , calcium
0.37
Calcium base grease
Lithium base grease
Base grease
0.75
4 wt.% MoDTC
0.69
4 wt.% MoDTC 2 wt.90 Calcium petroleum sulphonate
Scar width under 147 N for 3 h with different
0.31
0.66
4 wt.% MoDTC
additives
(mm)
(mm)
0.85
4 wt.% MoDTC 2 wt.% CaCO3
additives
0.31
0.43
4 wt.% MoDTC 2 wt.% Calcium petroleum sulphonate
under 98 N for 1 h with different
Scar widths in MM-200 testing machine
TABLE 3
0.71
No additive
Scar diameter
Lithium base grease
grease
Base
Scar diameters in four-ball machine
TABLE 2
1.06
1.8 wt.% MoSz 2 wt.90 Calcium petroleum sulphonate
0.30
0.61
4 wt.% MoDTC 2 wt.% Calcium acetate
4.50
2 wt.% Calcium petroleum sulphonate
0.33
0.67
4 wt.% MoDTC 2 wt.% CaCO3
co
M
240
of MoDTC in the two testing machines is ascribed to the operating conditions and the method of measuring the friction coefficient. It should be noted that the behaviour of MoDTC in oils is sometimes different from that in grease. Tests with oils containing MoDTC in the four-ball machine revealed that the friction coefficient and scar diameter decrease in the initial stage and then increase again with increasing MoDTC concentration; the optimum molybdenum concentration is about 340 ppm [7]. Besides, in engine oils, MoDTC and calcium petroleum sulphonate are not synergic in reducing wear. In this study, however, even if the molybdenum concentration is as high as 10 600 ppm in grease, the P,, value does not decrease and the friction coefficient does not increase with increasing MoDTC concentration. 3.2. Compositions of the surface film The elements molybdenum, sulphur, calcium, carbon, nitrogen and oxygen were detected using AES on steel surfaces lubricated with lithium base grease containing MoDTC and calcium compounds, see Fig. 6. Values of the elemental binding energy for different compounds are summarized in Table 5. As long as MoDTC is added in grease, MoS2, MOO, and Fe& can always be found on the worn surfaces. Provided calcium petroleum sulphonate is mixed in grease, CaCOs and CaS04 will be detected. CaC03 and CaS04 are derived from either tribochemical reaction products or original impurities in calcium petroleum sulphonate. From the results of AES and XPS analyses, it can be concluded that MO& and MOO, occurring in the surface film accounts for the low friction, wear-reducing and extreme pressure properties of MoDTC, while the synergism of MoDTC and calcium petroleum sulphonate in reducing wear relates closely to CaCO, and &SO4 found in the surface film.
\ b
Fig. 6. AES spectra of wear scar. In lithium base grease, N, 3 h: a, 4 wt.% MoDTC + 2 wt.% CaCOs; b, 4 wt.% leum sulphonate; c, 4 wt.% MoDTC.
on MM-200 testing machine, 147 MoDTC + 2 wt.% calcium petro-
Lithium grease
Base grease
Friction
base
coefficients
TABLE 4
0.05
4 wt.% MoDTC
0.09
4 wt.% MoDTC 2.wt.% Calcium petroleum sulphonate 0.09
4 wt.% MoDTC 2 wt.% CaCO3
under 147 N for 3 h with different additives
machine
Average friction coefficient
in MM-200 testing
0.10
1.8 wt.% MO& 2 wt.% Calcium petroleum sulphonate
0.12
2 wt.% Calcium petroleum sulphonate
242 TABLE
5
Binding
energy of elements
Elements in compounds
and corresponding
Standard values binding energy (ev)
compounds
in the surface
Values of binding energy with different 4 wt.% MoDTC Peaks in XPS spectra
Corrected
284.6
284.7
Fe Fe& Fe0 FeOOH
706.8 706.4 709.2, 710.8.
MO& Moos
228.9
Ca
CaCOs CaC04
346.8 347.4
S
MO&
162.2
C Fe
MO
232.5,
3.3. Effects
film
additives
in
4 wt.% MoDTC 2 wt.% CaCOs Peaks in XPS spectra
Corrected
284.6
284.7
284.6
706.8 706.6
706.7 706.5
710.2 711.5
707.0 706.4 708.8
706.9 706.3 708.7
711.6
711.5
228.6 232.0
228.5
232.6
228.5 232.6
228.4 232.5
347.0
346.9
of calcium
231.9
162.3
compounds
162.2
on elements
in the surface
film
The relative percentage of molybdenum, sulphur or calcium in the surface film with lithium base grease containing 4 wt.% MoDTC and 2 wt.% calcium petroleum sulphonate is higher than that containing either of the TABLE
6
Concentrations
of elements
in surface
Maximum relative percentage 0 f element (%)
Additives
MO 4 wt.% MoDTC 2 wt.% Calcium sulphonate 4 wt.% MoDTC 2 wt.% Calcium sulphonate 4 wt.% MoDTC 2 wt.% CaCOs aIn lithium
filma
3.6
MO
S
Ca
C
0
12.0
30.0
3.5
32.0
23.0
6.6
24.0
Co
S 5.4
petroleum
petroleum
Average relative percentage of element across film thickness (%)
3.0
2.8
6.2
5.0
29.5
14.4
3.6
23.0
7.7
3.7
4.0
6.7
2.8
1.7
1.7
testing
machine,
base grease on MM-200
147 N, 3 h
243
lithium
base grease (eV)
4 wt.% MoDTC 2 wt.% Calcium petroleum sulphonate
1.8 wt.% MoS? 2 wt.90 Calcium petroleum sulphonate
Corrected
Peaks in XPS spectra
Corrected
Peaks in XPS spectra
Corrected
Peaks in XPS spectra 285.0
284.6
285.0
284.6
284.7
284.6
707.0
706.9
706.2
705.8
709.9 711.2
709.5 710.8
708.6
708.5
232.6
347.2 347.8
347.1 347.7
709.4
709.0
232.2
228.9 232.8
228.5 232.4
347.1 347.8
346.7 347.4
346.8 348.2
346.4 347.8
162.8
162.4
162.0
161.6
2 wt.90 Calcium petroleum sulphonate
additives alone, as shown in Table 6. This means that MoDTC and calcium petroleum sulphonate have a joint effect on the concentration of molybdenum, sulphur and calcium in the surface film. The mechanism is not clear. Figure 7 shows element depth profiles. The highest relative percentage of calcium is found on the surface when MoDTC and CaC03 are added in grease but beneath the surface when MoDTC and calcium petroleum sulphonate are added. Furthermore, the relative percentage of molybdenum, sulphur or calcium in the surface film, lubricated with lithium base grease containing 4 wt.% and 2 wt.% CaC03, is lower than that containing 4 wt.% MoDTC and 2 wt.% calcium petroleum sulphonate. The reason is that
Sputtcrmg rime
(a)
(ml”)
(b)
Fig. 7. Elemental depth profile of wear scar. In MM-200 testing Lithium base grease, 4 wt.% MoDTC, 2 wt.% calcium petroleum base grease, 4 wt.% MoDTC, 2 wt.% CaC03.
machine, 147 N, 3 h. (a) sulphonate. (b) Lithium
244
CaC03 as an additive, may be brought to the friction surface by the base grease but it does not penetrate the base metal; while some of the calcium atoms of calcium petroleum sulphonate diffuse into the base metal by a tribochemical reaction. Oxygen in the air and the hydrocarbons in greases have undergone tribochemical reactions during sliding because great amounts of oxygen and carbon are found in the surface films (see Table 6). When MoDTC and calcium petroleum sulphonate are added together, the percentage of carbon in the surface film decreases observably, demonstrating reduced oxidation of hydrocarbons, in comparison with that when either of the additives is added alone.
4. A model of the surface film Lubricated with a lithium base grease containing MoDTC and calcium petroleum sulphonate, the surface film consists mainly of MO&, Fe&, Moos, CaCOs, CaS04, Fe0 and FeOOH. The surface film is 250 - 350 A thick. The texture of the surface film will be discussed below. Elements are distributed non-homogeneously across the film thickness, as shown in Fig. 7(a) and Table 7. The concentration of carbon or oxygen decreases rapidly across the film thickness, iron increases slowly and then reaches a stable value, while molybdenum, sulphur and calcium increase in the initial stage, then decrease.
TABLE Elemental
7 concentrations
Relative depth in to surface film
in different
depths across
film thicknessa
of elements
Relative percentage
(%)
MO
Ca
S
Fe
4.5 5.0 3.7 3.2 3.1 3.1
10.6 14.4 12.2 9.8 7.4 6.5
15.1 18.0 25.9 29.4 29.5 27.5
12.8 19.9 24.3 28.4 34.2 37.6
1;
2:9
2:9
17.1
54.1
2h
2:1
2:1
2:1
82:l
a 4 wt.% MoDTC and 2 wt.% calcium 200 testing machine, 147 N, 3 h.
petroleum
sulphonate
in lithium
base grease on MM-
245 TABLE Elemental
8 concentrations
Elements
MO Ca Fe S C 0 N
in different
locations
Relative percentage
along worn surfacea
of element
(%)
1
2
3
4
5
4.1 6.7 28.9 15.8 16.3 26.3 1.9
5.2 10.4 17.5 13.0 21.9 30.9 1.1
5.3 10.9 16.6 25.7 19.6 20.0 1.4
5.9 14.2 21.7 15.8 11.1 29.8 1.5
10.1 16.2 16.3 18.3 9.8 27.0 2.3
a4 wt.% MoDTC and 4 wt.% calcium MM-200 testing machine, 147 N, 3 h.
petroleum
sulphonate
in lithium
base grease,
on
Table 8 illustrates that elements are distributed non-homogeneously along the friction surface. Five equally spaced points were chosen for analysis along the worn surface. The concentration of molybdenum varies between 4.1% and lO.l%, calcium between 6.7% and 16.2% and sulphur between 13.0% and 25.7%. In the order point 1 to point 5, the concentration of calcium in the surface film increases together with that of molybdenum, but not proportionally. To sum up, a model of the surface film is put forward for lubrication with a lithium base grease containing MoDTC and calcium petroleum sulphonate, as shown in Fig. 8. On the condition that MoDTC is added alone in the grease, the MO& film formed by the tribochemical reaction can reduce friction and wear and also increase the load-carrying capacity. Provided that MoDTC and calcium petroleum sulphonate are mixed together in the grease, the MO& film on the asperities of the surface is replaced by a hard solid layer which consists mainly of Moos, CaCOs and CaS04, because MO&, which has an isotropic structure of layers, is easier to peel off than the other materials. The hard solid layer increases the wear resistance and reduces the friction coefficient. The high load-carrying capacity relates to the fact that CaCOs, which has a
Fig. 8. A model nate: z, MoSs;O,
of the surface film formed Moos;*, CaCOs or CaS04;
by MoDTC and calcium ///, base metal.
petroleum
sulpho-
246
high melting point and possesses high chemical stability, can raise the Pb value of the grease (see Table 1). As stated in Section 1, MoS, is formed by decomposition of MoDTC. However, how CaCOs and CaS04 are formed and how Fe& affects the tribological properties of the surface film remain to be answered. 5. Conclusions (1) In greases, MoDTC can significantly reduce friction and wear and also increase load-carrying capacity. MoDTC has an optimum concentration in grease. (2) If MoDTC and calcium compounds are added together in grease, wear and the ultimate non-scuffing load Pb depend on the kind of calcium compound. Calcium petroleum sulphonate and CaCOs are most effective respectively in improving antiwear and raising the Pb value. (3) The combined effects of MoDTC and calcium compounds are not consistent in reducing the friction coefficient obtained from different types of testing machine. (4) MO&, Moos, Fe&, CaC03 and CaS04 account for the synergism of MoDTC and calcium compounds on lubricating properties. (5) In the surface film formed by MoDTC and calcium petroleum sulphonate, elements are distributed non-homogeneously along the friction surface or across the film thickness. The concentration of calcium increases or decreases simultaneously with that of molybdenum but not proportionally. (6) MoDTC is a non-phosphorated, oil-soluble and multi-effective additive and has good compatibility with calcium detergents or dispersants in greases. Acknowledgments The authors wish to thank Professors Ru-lin Wang and Dian-lun Yang for their encouragement and instructive suggestions during the study. References 1 P.
C.
H. Mitchell, Oil-soluble MO-S 281 - 300. 2 E. R. Braithwaite and A. B. Greene, denum compounds in motor vehicles, 3 H. Hamaguchi, Y. Maeda and T. passenger cars, SAE Preprint n810316
compounds
as lubricant
additives,
Wear, 100
(1984)
A critical analysis of the performance of molybWear, 46 (1978) 405 432. Maeda, Fuel efficient motor oil for Japanese
for Meeting,
4 U.S. Patent 2,954,339. 5 U.S. Patent 4,149,965. 6 T. Sakurai, H. Okabe and H. Isoyama, carbamates)
dimolybdenum
Pet. Inst., 13 (1971)
(V) and their
243 - 249.
February
23 - 27, 1981.
The synthesis of di+-thio-bis(dialkyldithioeffects on boundary lubrication, Bull. Jpn.
247 7 J. Y. Dong, Study of an oil additive -molybdenum dialkyldithiocarbamate, Doctoral Thesis, East China University of Chemical Technology, Shanghai, December 1985 (in Chinese). 8 P. Y. Zheng and X. A. Han, Tribological properties of organic molybdenum compounds, Pet. Refining, 29 (12) (1985) 44 - 52 (in Chinese). 9 H. Isoyama and T. Sakurai, Inhibition of autoxidation by di+-dithio-bis(dialkyldithiocarbamate) dimolybdenum, Bull. Jpn. Pet. Inst., 16 (1974) 112 - 117. 10 T. Sakurai, Mechanism of organomolybdenum compounds as lubricant additives, Lubrication, 28(5) (1983) 338 - 342 (in Japanese). 11 P. Y. Zheng, X. A. Han and R. L. Wang, The mechanism of friction reduction of sulfurized oxymolybdenum di-(2-ethylhexyl)phosphorodithioate under boundary lubrication, ASLE Preprint 86-TC-4E-I for the ASME-ASLE Tribology Conf., October 1986. 12 Y. G. Hu, Combined effects of molybdenum dialkyldithiophosphate and engine oil additives on friction and wear, Master Thesis, Research Institute of Petroleum Processing, January 1986 (in Chinese). 13 K. Fujita, A. Yoshida and K. Matsuo, Effects of molybdenum disulphide and an organic molybdenum compound in gear oil on rolling contact fatigue, Wear, 95 (1984) 271 - 286. 14 T. Sakurai, Lecture Notes at East China University of Chemical Technology, May 1986. 15 A. I. Brudnyi and A. F. Karmadonov, Structure of molybdenum disulphide lubricant film, Wear, 33 (1975) 243 - 249. 16 J. Gansheimer and R. Holinski, Molybdenum disulfide in oils and greases under boundary conditions, Trans. ASME, Ser. F, 95 (1973) 242 - 248. 17 J. M. Chen, J. 2. Zhao, H. X. Dang and S. K. Rong, A study of X-ray diffraction MO& transfer films, J. Solid Lubr., 6(l) (1986) 22 (in Chinese). 18 The People’s Republic of China Standard GB3142-82, Lubricants determination of load-carrying capacity (four ball machine).