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Tribological properties of 4,5-di(cetyl thio)-1,3-dithiole-2-thione as additive in liquid paraffin
WEAR ELS EVI E R
wear 210 (1997) 273-277
Tribological properties of 4,5-di (cetyl thio) - 1,3-dithiole-2-thione as additive in liquid paraffin Zefu Zhang *, Weimin Liu, Qunji Xue Laboratory of Solid Lubrication. LanS+ou Institute vf Chemical Physics, Chinese Academy o.fSciences, Lanzhou, 730000. People's Republicof China
Received I I December 1996;accepted t I March 1997
A~tract
The synthesis of a new type of oil additive, 4,5-di( cetyl thio )- 1,3-dithiole-2-thione, is described. The thermal stability of the compound was investigated by thermal gravity analysis (TGA). The tribological properties of the compound as additive in liquid paraffin were also evaluated using a four ball tester. The results show that the novel compound has good tribological properties in liquid paraffin. The lubricating mechanism was studied by electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS), © 1997 Elsevier Science S.A. Ke.x~vords: Oil additive; Tribological properties: S-containing heterocyclic compound: Thermogravimetric analysis: Electron probe microanalysis; X-ray photoelectron spectroscopy
1. Introduction:
Organosulfur compounds as additives in lubricating oil or grease have been known for about 50 years. They are one of the most popular and best performing types of antiwear and extreme pressure additive in lubricating oil [ I-3 ]. It is generally accepted that the antiwear and extreme pressure properties of organosulfur oil additives are related to the C-S bond energy of monosuifides [ 4]. in the last decades, a large number of research programs on heterocyclic compounds as oil additives have been carried out. Compounds having compact and stable structures such as substituted 1,2,4-tdazoles, 1,2,3-thiadiazoles, 2-mercaptobenzothiazole and benzotriazole have been proved to possess anti-corrosion, anti-rust, copper deactivating and antiwear properties [5-10]. These oi! addS'ires are usually multifunctional and cause less pollution tlian others, and some of them possess very good antiwear, extreme pressure and friction reduction properties. When a chlorine or sulfur atom is introduced into the molecular formula of the additives, a significant increase in antiwear and extreme pressure efficiency is obtained [ 11-14]. However, there no heterocyclic thione derivatives are used as additives in lubricating oil. In addition +,obeing heterocyclic compounds, they have several sulfur atoms in the molecular structure. So we thought that they may be potential oil additives. * Corresponding author. 0043-1648/97/$17.00 © 1997 ElsevierScience S.A. All rights re,fred PIi S0043-1648 ( 97 )0005 3-7
In this paper, the synthesis of an S-containing heterocyclic compound, 4,5-di(cetyl thio)-l,3-dithiole-2-thione, is described. The thermal stability and tribological properties of this compound as additive in liquid paraffin were evaluated. The lubricating mechanism of the additive in liquid paraffin was also investigated.
2. Experimental details 2,1. Synthesis o f 4,5-di(cetyl thio)-l,3-dithiole-2-thione
With the protection of nitrogen, dimethylformarnide (60 ml ) was slowly dropped into the mixture solution of carbon disulfide (50 mi) and potassium (10 g), and stirred. As dimethylformamide was dropped, the color of the solution changed to red. The solution was refluxed until the reaction was complete. The solution was diluted by adding anhydrous methanol, filtered. Cetyl bromide (0.04 reel) was dropped into the above solution (0.02 reel) with stirring, placed overnight. There was a lot of yellow solid produced, which was filtered. The product was recrystallized. The product was dried by pumping off. The results of elemental analysis listed in Table ! are in good agreement with the required values. In the IR analysis spectrum (see Fig. l ), the peaks at 2956, 2848 and 1470 c m - t show that CH3 and CH 2 are present in
Z Zhang et aLI Wear210 (1997) 273-277
274
Table 2 Typical propertiesof liquid paraffin
Table I The results of elemental analysis Element
Found (%)
Required (%)
C H
65.08 10.27
65.02 10.22
120
0.8456 21.49 4A2 i 17 I > 300
fourball machine under the following conditions: rotating rate, 1480 rev min-*: test duration, 30 min; load, 196 N, 294 N, 392 N, 490 N; temperature, 20 °C. The balls (diameter 12.7 mm) used in the test were made of GCrl5 bearing steel
.
-40
Density (gcm -3) Viscosity (ram2 s-') 40 °C 100 *C Viscosity index S content (ppm) Boiling point (°C)
2'soo iooo ;soo ;ooo Wavenember (¢m.1)
Fig. 1. Diagramshowing the IR analysisof the compound. the molecular structure; the peak at 1059 c m - ~ proves that C - S is present. The peak at 888 cm-J proves that C-S is present. Thus, the molecular structure of this compound is determined as follows:
(0.95%-1.05% C, 0.15%-O.35% Si, 0.24%-0.40% Mn, 0.027% P, <0.020% S, 1,30%-1.67% Cr, <0.30% Ni and <0.025% Cu) with a HRc of 59-61. The load-carrying capacity of the compound as additive in liquid paraffin was obtained by GB 3142-82, similar to ASTM D2783. The chemical characteristics of the liquid paraffin used as base stock are listed in Table 2.
2.4. Surface film analysis
The elemental distribution of the boundary film and the morphology of the worn surface were determined using an EPM-810 Q model electron probe microanalyzer (EPMA). 2.2. Thermal stability
The thermal stability of this compound was evaluated by thermal gravity analysis (TGA) using a Perkin-Elmer model 7 thermal analyzer, The heating rate was 20 °C min-i in nitrogen.
X-ray photoelectron spectroscopy (XPS) was conducted using the PHI-5702 multi-technique system, passed energy 29.35 eV, Mg Ka radiation. The binding energy ofCl~ (284.4 eV) was used as standard value.
3. Results and discussion
2.3. Tribological properties of the compound as additive in liquid par~ffin
3. !. Thermal stabili~ analysis
The antiwear and friction reduction properties of the compound as additive in liquid paraffin were evaluated with a
The results of thermal stability analysis of the compound are shown in Fig. 2. From Fig. 2, it can be seen that the
i|1
•
~Onsa 325.97"C
80.0 -20.0
,-,o.o t
,-6O.0
20.0
0.0
-',
too.o
2o .o 3 o.o Temperamt'e(°C)
''-
•
4oo.o
Fig. 2. Diagramshowingthe thermal analysisof the compound in nitrogen.
o~
7_. Zhang et al. / Wear210 (1997) 273-277
v'aue(N)
¢IDO
4(X)
300
j
_
-
i
!
l
0
I
0.1
I
[
(15
(225
1.0
;~k~tive oormnlraion(wt°,~) Fig. 3. The relationship bczw¢cn PB and the additi,,c concen(ration.
compound has a higher decomposition temperature ( 326 °C) in nitrogen.
3.2. Maximum non-seizure load (P8 value) The Pa values of liquid paraffin containing the compound are showed in Fig. 3. As a comparison, the data for the base stock are also given. It can be seen that the compound as additive in liquid paraffin can improve the Pa value of liquid paraffin considerably. Even when 0.1 wt.% compound was added to liquid paraffin, the Pn value of liquid paraffin was improved to 58%. With increasing concentration of additive, the Pa value of liquid paraffin improved. When the concentration of the compound in liquid paraffin was I wt.%, the PB value of liquid paraffin was improved to 79%. So it can be thought that this compound is a good extreme pressure additive in liquid paraffin.
3.3. Antiwear and friction reduction properties The relationship between the concentration of the compound in liquid paraffin and the wear scar diameter is shown in Fig. 4. It can be seen that this compound in liquid paraffin can improve the anfiwear properties of liquid paraffin considerably. When 0.25 wt.% of the compound was added into liquid paraffin, the best antiwear properties were obtained. The reason that 0,25% is the best concentration may be that this compound contains a higher sulfur content. When the concentration of additive increases, corrosive wear occurs. Wear scatof dkrneter(mm)
The wear scar diameter as a function of load is given in Fig. 5. It can be seen that the compound in liquid paraffin has good antiwear properties untlcr the different loads. Even under the load of 490 N, the compound as additive in liquid paraffin still has good antiwear properties. The relationship between the friction coefficient of the compound as additive in liquid paraffin and time is shown in Fig. 6. It can be seen that the friction coefficient of tic additive in liquid paraffin was higher than that of the base stock at the beginning. However, as the time continued, Om friction coefficient of 0.25 wt.% compound in liquid paraffin became lower than that of the base stock; the friction coefficient of 0.1 wt.% compound in liquid paraffin was equal to that of the base stock, the friction coefficient of 0.5 wt.% and 1 wt.% compound in liquid paraffin was higher than that of the base stock. So it can be seen that the optimal concentration of tbe compound as additive in liquid paraffin is 0.25 wt.%, when the lowest friction coefficient of liquid paraffin is obtained. From the above discussion, it can be seen that the compound as additive in liquid paraffin has good antiwear properties. Meanwhile, when the optimal concentration of the compouno as additive in liquid paraffin is 0.25 wt.%, the best antiwear and friction reduction properties were obtained.
3.4. Surface film analyses It is inferred that the compound as additive in liquid paraffin is effective at improving the carrying capacity and reducing the wear of liquid paraffin. Hence, the next step was to determine the mechanism of the compound as additive in liquid paraffin using EPMA a,'~d,"PS. At first, the morphology of the worn surface and the elemental distribution of the boundary film were determined using an EPM-810 Q model electron probe microanalyzer. Wear scar of diameteffmm) 0.8
~1111~
0.7
0.6 0.5 0.4 ' t96
'
'' ' 392 490 Load(N) Fig. 5. The wear scar diamclerasa function of load (additive concentration 294
0.25 wt.%, 30 min).
0.8
Frictioncoeff¢ient 0.1 ~ . . . 0.1 ~
0.7
0.6
.
.
.
.
.
.
.
BmmMoq:k -40.1 ~ O.25
O. O.O9
0.5 0.4
275
0
'
'
'
-
'
'
0.1 0.25 0.5 1.0 1,5 Concentration of add~iOt,)
Fig. 4. The wear scar diameter as a function of additive concentration ( 30 rain. 294 N).
0.06
0.5
0.07
-41-1.0
O.O6 0
O.S
S
~
~
2O
2S
3O
--e--
~hm(nVn.)
Fig. 6. The friction coefficient as a funclion of dine (30 rain, 294 N).
276
Z Zhang et al. / Wear 210 (1997) 273-277
Table 4 S=pbinding energy (eV) of reference sulfur compounds
Fig. Z The ~ o~a were scar whea ~ with (a) brae stock sad (b) 0.25 wt.% additive in brae stock at a load of 294 N and testing time of 30 min.
The results of analyses are shown in Figs. 7 and 8 respectively. From Fig. 7, it can be seen that the morphology of the wear scar of base oil is larger than that of the additive under the same magnification. This means that the antiwear property of the additive is better than that of base oil. This result is in agreement with the results of the above wear test. Fig. 8 shows the sulfur element distributed in the surface of the wear scar. This indicates that there was a transfer film containing sulfur on the worn surface after the friction process. XPS spectra were used to determine the chemical state of the sulfur element on the worn surface. The results are listed in Table 3. For comparison, the S2p spectrum of the compound is also indicated. The standard values of Sap are summarized in Table 4 [ 15,16]. From Table 3, it can be seen that the binding energy of S2p of the compound changed from 163.62 eV to 161.25 and ~.
~
-=
.
.
.
.
.
.
.
.
Compound
Binding energy (eV)
Ferrous sulfide (FeS) Iron disulfide (FeS2) Elemental sulfur (Ss) Dibenzyl disulfide Ferrous sulfate (FeSO4)
160.8 161.6 162.4 162.8 168.3
168.25 eV after friction. This indicated that the tribochemical reaction happened during the sliding process. According to Table 4, the binding energies of S2p at 161.25 eV and 168.25 eV indicate sulfide and ferrous sulfate respectively. According to the above discussion, it can be thought that the compound as additive in liquid paraffin underwent tribochemical reaction with the metal surface during the sliding process. The tribochemical reaction film containing sulfide and ferrous sulfate on the worn surface may be the main factor why the compound as additive in liquid paraffin has good antiwear properties and load-carrying capacity. 4. Cenclustons Based on the results reported above, the following conclusions can be drawn. 1. The 4,5-di(cetyl thio)-1,3-dithiole-2-thione has good thermal stability in nitrogen. 2. The 4,5-di(cetyl thio)- 1,3-dithiole-2-thione as additive in liquid paraffin is effective at reducing the wear and increasing the load-carrying capacity of the base stock. It is a new type of antiwear and extreme pressure additive in lubricating oil. At the optimal concentration of 0.25 wt.%, the best antiwear and friction reduction properties were obtained. 3. The tribocbemical reaction film on the worn surface conraining sulfide and ferrous sulfate may be the main factor why the compound has good tribological properties. Acknowledgements
'~<
~=
~ :~
~
~
~
:
~
~~ ~ ~
~~
The authors would like to acknowledge the financial support of the National Nature Science Foundation, and to thank Senior Engineers Shangkui Qi and Pinyu Zhang for their assistance in surface analyses and thermal analysis.
Fig. 8. X-ray images of sulfur in the worn surface.
References Table 3 The results of XPS spectra
Pure compound Wear scar
S2p (eV)
C,, (eV)
163.62 161.25. 168.25
284.6 284.6
Four-ball tester, additive concentration 0.25 wt.%, 294 N, 30 rain.
[ I ] W. Davey, J. Inst. Petrol., 31 (1945) 154. [2] W. Davey, J. Insl. PetroL, 32 (1946) 575. [31 W. Davey, Wear. ! (1957) 291. [4] Q.-J. Xue and W.oM. Liu, Tribochemistry and the development of AW and EP oil additives--a review, Lubr. Sci., 7 (1994) 81. [51 T. Singh and C.V. Chandrasekharam, The effect of nitrogen and sulphur compounds on extreme pressure lubrication, Tribol. Int., 26 (1993) 245.
Z. Zhang et al. / Wear 2tO (1997J 273-277 [6] T. Singh, A. Bhattacharya and V.K. Verma, EP activity assessment of certain nitrogen and sulfur heterocyclic compounds as potential additives in four ball tester, Lubr. Eng,, 46 (1989) 681. [7] T. Ren, Q. Xue and H. Wang, A study of S-( ! H-benzotriazole-yl)methyl N,N-dialkyldithiocad~amates as novel multifunctional oil additives, Wear, 172 (1994) 59. 18l T. Ren, Q. Xue and H. Wang, A study ofll~ tribological properties of S-(I H-bcnzolriazole-yl) methyl N,N-dialkyldithiophosphate as additive in liquid paraffin, Wear, 173 (1994) 167. [91 O. Okordudu, Otganophosphorous derivatives of bcnzodiazole and tl¢ir use as load carrying additives, US Patent, 3986967, 1976. [ ! 0 ] Y. Wan and Q.-J. Xue, Antiwear and extren~ pressure characristics of 2-mercapto-benzo~hiazole derivative as the potential lubricating oil additive, Wear, 192 (1996) 74. [ I I ] A. Bhattacharya, TriboL Int., 27 (1990) 361. [ 12l A. BhaUachmya, Wear, 136 (1990) 345. [ 13] T. Singh, Tribol. Int., 41 (1990) 23. [14] W.-M. Liu and Q.-J. Xue, Pl'oc. China Int. Synlp. of Youth Tribologists, Lanzhou, China, 1992, p. 317. [ 15] C.D. Wagner, Handbook of X-ray Phowelecrroscopy. Perkin-EImer. Physical Electronic Division, MI, 1979. [ 16] E.A. Baldwin, Relationship between surface composition and wear: an X-ray photoelectron spectroscopic study of surface tested with organic sulfur compounds, ASLE Trans.. 19 (1976) 335.
Biographies Zefu Zhang received his BS in applied chemistry from the Department of Chemistry, Lanzhou University in 1991. He joined the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1991.
277
He has been a PhD candidate in the Lanzhou Institute of Chemical Physics, Chinese Academy of Science since 1993. His research interests include interactions of lubricant additives, lubricating grease and triboc,h~mislry. Weimin Liu received his BS in Chemistry at Shandong Normal University in 1984 and PhD in Tribology at Lanzhou Institute of Chemical Physics in !990. He joined the Laboratory of Solid Lubrication, l.anzhou Institute of Chemical Physics, Chinese Academy of Science in 1990, and from 1993 to 1994 he spent one year as a visiting scientist atthe Department of Chemical Engineering, Pennsylvania State University. His research interests include lubricant additive interactions, lubrication of ceramics, solid lubrication, tribochemistry and fuel additives. Currently, he is a professor and deputy director of the Laboratory of Solid Lubrication. Qunji Xue graduated from the Department of Chemistry, Shandong University of China and received his MSc &gree at Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1967. From 1980 to 1982, he worked at the University of Michigan. Since 1965, he has worked in the field of tribology with special emphasis on lubricating materials and has published more than 160 research papers in national and international journals. Currently, he is a professor and head of the Laboratory of Solid Lubrication, also deputy director of the Lanzhou Institute of Chemical Physics, Chinese Academy of Science.
wear 210 (1997) 273-277
Tribological properties of 4,5-di (cetyl thio) - 1,3-dithiole-2-thione as additive in liquid paraffin Zefu Zhang *, Weimin Liu, Qunji Xue Laboratory of Solid Lubrication. LanS+ou Institute vf Chemical Physics, Chinese Academy o.fSciences, Lanzhou, 730000. People's Republicof China
Received I I December 1996;accepted t I March 1997
A~tract
The synthesis of a new type of oil additive, 4,5-di( cetyl thio )- 1,3-dithiole-2-thione, is described. The thermal stability of the compound was investigated by thermal gravity analysis (TGA). The tribological properties of the compound as additive in liquid paraffin were also evaluated using a four ball tester. The results show that the novel compound has good tribological properties in liquid paraffin. The lubricating mechanism was studied by electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS), © 1997 Elsevier Science S.A. Ke.x~vords: Oil additive; Tribological properties: S-containing heterocyclic compound: Thermogravimetric analysis: Electron probe microanalysis; X-ray photoelectron spectroscopy
1. Introduction:
Organosulfur compounds as additives in lubricating oil or grease have been known for about 50 years. They are one of the most popular and best performing types of antiwear and extreme pressure additive in lubricating oil [ I-3 ]. It is generally accepted that the antiwear and extreme pressure properties of organosulfur oil additives are related to the C-S bond energy of monosuifides [ 4]. in the last decades, a large number of research programs on heterocyclic compounds as oil additives have been carried out. Compounds having compact and stable structures such as substituted 1,2,4-tdazoles, 1,2,3-thiadiazoles, 2-mercaptobenzothiazole and benzotriazole have been proved to possess anti-corrosion, anti-rust, copper deactivating and antiwear properties [5-10]. These oi! addS'ires are usually multifunctional and cause less pollution tlian others, and some of them possess very good antiwear, extreme pressure and friction reduction properties. When a chlorine or sulfur atom is introduced into the molecular formula of the additives, a significant increase in antiwear and extreme pressure efficiency is obtained [ 11-14]. However, there no heterocyclic thione derivatives are used as additives in lubricating oil. In addition +,obeing heterocyclic compounds, they have several sulfur atoms in the molecular structure. So we thought that they may be potential oil additives. * Corresponding author. 0043-1648/97/$17.00 © 1997 ElsevierScience S.A. All rights re,fred PIi S0043-1648 ( 97 )0005 3-7
In this paper, the synthesis of an S-containing heterocyclic compound, 4,5-di(cetyl thio)-l,3-dithiole-2-thione, is described. The thermal stability and tribological properties of this compound as additive in liquid paraffin were evaluated. The lubricating mechanism of the additive in liquid paraffin was also investigated.
2. Experimental details 2,1. Synthesis o f 4,5-di(cetyl thio)-l,3-dithiole-2-thione
With the protection of nitrogen, dimethylformarnide (60 ml ) was slowly dropped into the mixture solution of carbon disulfide (50 mi) and potassium (10 g), and stirred. As dimethylformamide was dropped, the color of the solution changed to red. The solution was refluxed until the reaction was complete. The solution was diluted by adding anhydrous methanol, filtered. Cetyl bromide (0.04 reel) was dropped into the above solution (0.02 reel) with stirring, placed overnight. There was a lot of yellow solid produced, which was filtered. The product was recrystallized. The product was dried by pumping off. The results of elemental analysis listed in Table ! are in good agreement with the required values. In the IR analysis spectrum (see Fig. l ), the peaks at 2956, 2848 and 1470 c m - t show that CH3 and CH 2 are present in
Z Zhang et aLI Wear210 (1997) 273-277
274
Table 2 Typical propertiesof liquid paraffin
Table I The results of elemental analysis Element
Found (%)
Required (%)
C H
65.08 10.27
65.02 10.22
120
0.8456 21.49 4A2 i 17 I > 300
fourball machine under the following conditions: rotating rate, 1480 rev min-*: test duration, 30 min; load, 196 N, 294 N, 392 N, 490 N; temperature, 20 °C. The balls (diameter 12.7 mm) used in the test were made of GCrl5 bearing steel
.
-40
Density (gcm -3) Viscosity (ram2 s-') 40 °C 100 *C Viscosity index S content (ppm) Boiling point (°C)
2'soo iooo ;soo ;ooo Wavenember (¢m.1)
Fig. 1. Diagramshowing the IR analysisof the compound. the molecular structure; the peak at 1059 c m - ~ proves that C - S is present. The peak at 888 cm-J proves that C-S is present. Thus, the molecular structure of this compound is determined as follows:
(0.95%-1.05% C, 0.15%-O.35% Si, 0.24%-0.40% Mn, 0.027% P, <0.020% S, 1,30%-1.67% Cr, <0.30% Ni and <0.025% Cu) with a HRc of 59-61. The load-carrying capacity of the compound as additive in liquid paraffin was obtained by GB 3142-82, similar to ASTM D2783. The chemical characteristics of the liquid paraffin used as base stock are listed in Table 2.
2.4. Surface film analysis
The elemental distribution of the boundary film and the morphology of the worn surface were determined using an EPM-810 Q model electron probe microanalyzer (EPMA). 2.2. Thermal stability
The thermal stability of this compound was evaluated by thermal gravity analysis (TGA) using a Perkin-Elmer model 7 thermal analyzer, The heating rate was 20 °C min-i in nitrogen.
X-ray photoelectron spectroscopy (XPS) was conducted using the PHI-5702 multi-technique system, passed energy 29.35 eV, Mg Ka radiation. The binding energy ofCl~ (284.4 eV) was used as standard value.
3. Results and discussion
2.3. Tribological properties of the compound as additive in liquid par~ffin
3. !. Thermal stabili~ analysis
The antiwear and friction reduction properties of the compound as additive in liquid paraffin were evaluated with a
The results of thermal stability analysis of the compound are shown in Fig. 2. From Fig. 2, it can be seen that the
i|1
•
~Onsa 325.97"C
80.0 -20.0
,-,o.o t
,-6O.0
20.0
0.0
-',
too.o
2o .o 3 o.o Temperamt'e(°C)
''-
•
4oo.o
Fig. 2. Diagramshowingthe thermal analysisof the compound in nitrogen.
o~
7_. Zhang et al. / Wear210 (1997) 273-277
v'aue(N)
¢IDO
4(X)
300
j
_
-
i
!
l
0
I
0.1
I
[
(15
(225
1.0
;~k~tive oormnlraion(wt°,~) Fig. 3. The relationship bczw¢cn PB and the additi,,c concen(ration.
compound has a higher decomposition temperature ( 326 °C) in nitrogen.
3.2. Maximum non-seizure load (P8 value) The Pa values of liquid paraffin containing the compound are showed in Fig. 3. As a comparison, the data for the base stock are also given. It can be seen that the compound as additive in liquid paraffin can improve the Pa value of liquid paraffin considerably. Even when 0.1 wt.% compound was added to liquid paraffin, the Pn value of liquid paraffin was improved to 58%. With increasing concentration of additive, the Pa value of liquid paraffin improved. When the concentration of the compound in liquid paraffin was I wt.%, the PB value of liquid paraffin was improved to 79%. So it can be thought that this compound is a good extreme pressure additive in liquid paraffin.
3.3. Antiwear and friction reduction properties The relationship between the concentration of the compound in liquid paraffin and the wear scar diameter is shown in Fig. 4. It can be seen that this compound in liquid paraffin can improve the anfiwear properties of liquid paraffin considerably. When 0.25 wt.% of the compound was added into liquid paraffin, the best antiwear properties were obtained. The reason that 0,25% is the best concentration may be that this compound contains a higher sulfur content. When the concentration of additive increases, corrosive wear occurs. Wear scatof dkrneter(mm)
The wear scar diameter as a function of load is given in Fig. 5. It can be seen that the compound in liquid paraffin has good antiwear properties untlcr the different loads. Even under the load of 490 N, the compound as additive in liquid paraffin still has good antiwear properties. The relationship between the friction coefficient of the compound as additive in liquid paraffin and time is shown in Fig. 6. It can be seen that the friction coefficient of tic additive in liquid paraffin was higher than that of the base stock at the beginning. However, as the time continued, Om friction coefficient of 0.25 wt.% compound in liquid paraffin became lower than that of the base stock; the friction coefficient of 0.1 wt.% compound in liquid paraffin was equal to that of the base stock, the friction coefficient of 0.5 wt.% and 1 wt.% compound in liquid paraffin was higher than that of the base stock. So it can be seen that the optimal concentration of tbe compound as additive in liquid paraffin is 0.25 wt.%, when the lowest friction coefficient of liquid paraffin is obtained. From the above discussion, it can be seen that the compound as additive in liquid paraffin has good antiwear properties. Meanwhile, when the optimal concentration of the compouno as additive in liquid paraffin is 0.25 wt.%, the best antiwear and friction reduction properties were obtained.
3.4. Surface film analyses It is inferred that the compound as additive in liquid paraffin is effective at improving the carrying capacity and reducing the wear of liquid paraffin. Hence, the next step was to determine the mechanism of the compound as additive in liquid paraffin using EPMA a,'~d,"PS. At first, the morphology of the worn surface and the elemental distribution of the boundary film were determined using an EPM-810 Q model electron probe microanalyzer. Wear scar of diameteffmm) 0.8
~1111~
0.7
0.6 0.5 0.4 ' t96
'
'' ' 392 490 Load(N) Fig. 5. The wear scar diamclerasa function of load (additive concentration 294
0.25 wt.%, 30 min).
0.8
Frictioncoeff¢ient 0.1 ~ . . . 0.1 ~
0.7
0.6
.
.
.
.
.
.
.
BmmMoq:k -40.1 ~ O.25
O. O.O9
0.5 0.4
275
0
'
'
'
-
'
'
0.1 0.25 0.5 1.0 1,5 Concentration of add~iOt,)
Fig. 4. The wear scar diameter as a function of additive concentration ( 30 rain. 294 N).
0.06
0.5
0.07
-41-1.0
O.O6 0
O.S
S
~
~
2O
2S
3O
--e--
~hm(nVn.)
Fig. 6. The friction coefficient as a funclion of dine (30 rain, 294 N).
276
Z Zhang et al. / Wear 210 (1997) 273-277
Table 4 S=pbinding energy (eV) of reference sulfur compounds
Fig. Z The ~ o~a were scar whea ~ with (a) brae stock sad (b) 0.25 wt.% additive in brae stock at a load of 294 N and testing time of 30 min.
The results of analyses are shown in Figs. 7 and 8 respectively. From Fig. 7, it can be seen that the morphology of the wear scar of base oil is larger than that of the additive under the same magnification. This means that the antiwear property of the additive is better than that of base oil. This result is in agreement with the results of the above wear test. Fig. 8 shows the sulfur element distributed in the surface of the wear scar. This indicates that there was a transfer film containing sulfur on the worn surface after the friction process. XPS spectra were used to determine the chemical state of the sulfur element on the worn surface. The results are listed in Table 3. For comparison, the S2p spectrum of the compound is also indicated. The standard values of Sap are summarized in Table 4 [ 15,16]. From Table 3, it can be seen that the binding energy of S2p of the compound changed from 163.62 eV to 161.25 and ~.
~
-=
.
.
.
.
.
.
.
.
Compound
Binding energy (eV)
Ferrous sulfide (FeS) Iron disulfide (FeS2) Elemental sulfur (Ss) Dibenzyl disulfide Ferrous sulfate (FeSO4)
160.8 161.6 162.4 162.8 168.3
168.25 eV after friction. This indicated that the tribochemical reaction happened during the sliding process. According to Table 4, the binding energies of S2p at 161.25 eV and 168.25 eV indicate sulfide and ferrous sulfate respectively. According to the above discussion, it can be thought that the compound as additive in liquid paraffin underwent tribochemical reaction with the metal surface during the sliding process. The tribochemical reaction film containing sulfide and ferrous sulfate on the worn surface may be the main factor why the compound as additive in liquid paraffin has good antiwear properties and load-carrying capacity. 4. Cenclustons Based on the results reported above, the following conclusions can be drawn. 1. The 4,5-di(cetyl thio)-1,3-dithiole-2-thione has good thermal stability in nitrogen. 2. The 4,5-di(cetyl thio)- 1,3-dithiole-2-thione as additive in liquid paraffin is effective at reducing the wear and increasing the load-carrying capacity of the base stock. It is a new type of antiwear and extreme pressure additive in lubricating oil. At the optimal concentration of 0.25 wt.%, the best antiwear and friction reduction properties were obtained. 3. The tribocbemical reaction film on the worn surface conraining sulfide and ferrous sulfate may be the main factor why the compound has good tribological properties. Acknowledgements
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The authors would like to acknowledge the financial support of the National Nature Science Foundation, and to thank Senior Engineers Shangkui Qi and Pinyu Zhang for their assistance in surface analyses and thermal analysis.
Fig. 8. X-ray images of sulfur in the worn surface.
References Table 3 The results of XPS spectra
Pure compound Wear scar
S2p (eV)
C,, (eV)
163.62 161.25. 168.25
284.6 284.6
Four-ball tester, additive concentration 0.25 wt.%, 294 N, 30 rain.
[ I ] W. Davey, J. Inst. Petrol., 31 (1945) 154. [2] W. Davey, J. Insl. PetroL, 32 (1946) 575. [31 W. Davey, Wear. ! (1957) 291. [4] Q.-J. Xue and W.oM. Liu, Tribochemistry and the development of AW and EP oil additives--a review, Lubr. Sci., 7 (1994) 81. [51 T. Singh and C.V. Chandrasekharam, The effect of nitrogen and sulphur compounds on extreme pressure lubrication, Tribol. Int., 26 (1993) 245.
Z. Zhang et al. / Wear 2tO (1997J 273-277 [6] T. Singh, A. Bhattacharya and V.K. Verma, EP activity assessment of certain nitrogen and sulfur heterocyclic compounds as potential additives in four ball tester, Lubr. Eng,, 46 (1989) 681. [7] T. Ren, Q. Xue and H. Wang, A study of S-( ! H-benzotriazole-yl)methyl N,N-dialkyldithiocad~amates as novel multifunctional oil additives, Wear, 172 (1994) 59. 18l T. Ren, Q. Xue and H. Wang, A study ofll~ tribological properties of S-(I H-bcnzolriazole-yl) methyl N,N-dialkyldithiophosphate as additive in liquid paraffin, Wear, 173 (1994) 167. [91 O. Okordudu, Otganophosphorous derivatives of bcnzodiazole and tl¢ir use as load carrying additives, US Patent, 3986967, 1976. [ ! 0 ] Y. Wan and Q.-J. Xue, Antiwear and extren~ pressure characristics of 2-mercapto-benzo~hiazole derivative as the potential lubricating oil additive, Wear, 192 (1996) 74. [ I I ] A. Bhattacharya, TriboL Int., 27 (1990) 361. [ 12l A. BhaUachmya, Wear, 136 (1990) 345. [ 13] T. Singh, Tribol. Int., 41 (1990) 23. [14] W.-M. Liu and Q.-J. Xue, Pl'oc. China Int. Synlp. of Youth Tribologists, Lanzhou, China, 1992, p. 317. [ 15] C.D. Wagner, Handbook of X-ray Phowelecrroscopy. Perkin-EImer. Physical Electronic Division, MI, 1979. [ 16] E.A. Baldwin, Relationship between surface composition and wear: an X-ray photoelectron spectroscopic study of surface tested with organic sulfur compounds, ASLE Trans.. 19 (1976) 335.
Biographies Zefu Zhang received his BS in applied chemistry from the Department of Chemistry, Lanzhou University in 1991. He joined the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1991.
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He has been a PhD candidate in the Lanzhou Institute of Chemical Physics, Chinese Academy of Science since 1993. His research interests include interactions of lubricant additives, lubricating grease and triboc,h~mislry. Weimin Liu received his BS in Chemistry at Shandong Normal University in 1984 and PhD in Tribology at Lanzhou Institute of Chemical Physics in !990. He joined the Laboratory of Solid Lubrication, l.anzhou Institute of Chemical Physics, Chinese Academy of Science in 1990, and from 1993 to 1994 he spent one year as a visiting scientist atthe Department of Chemical Engineering, Pennsylvania State University. His research interests include lubricant additive interactions, lubrication of ceramics, solid lubrication, tribochemistry and fuel additives. Currently, he is a professor and deputy director of the Laboratory of Solid Lubrication. Qunji Xue graduated from the Department of Chemistry, Shandong University of China and received his MSc &gree at Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1967. From 1980 to 1982, he worked at the University of Michigan. Since 1965, he has worked in the field of tribology with special emphasis on lubricating materials and has published more than 160 research papers in national and international journals. Currently, he is a professor and head of the Laboratory of Solid Lubrication, also deputy director of the Lanzhou Institute of Chemical Physics, Chinese Academy of Science.