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UDK 620.179.112 Xiong Liping,
Shool of Basic Science, Eastchina Jiaotong University, Nanchang 330013, PR China
He Zhongyi,
School of chemistry and chemical engineering, Shanghai Jiao Tong University,
Shanghai, 200240, China Xu Huan,
Shool of Basic Science, Eastchina Jiaotong University, Nanchang 330013, PR China
Qiu Jianwei,
PetroChina Lanzhou Lubricating Oil R & D Institute, Lanzhou 730060, PR China
Fu Xisheng,
PetroChina Lanzhou Lubricating OilR & D Institute, Lanzhou 730060, PR China
TRIBOLOGICAL STUDY OF XANTHATE-CONTAINING ACETIC ESTER AS ADDITIVES IN HYDROGENATED OIL
Abstract. A novel ester derivative, hexadecyl xanthate acetic hexadecyl xanthate ester(HXAE) was synthesized and it's tribological behaviors as additives in hydrogenated oil, were evaluated using a four-ball tester. Results show that the compound possesses good antiwear performance, extreme pressure capacity, and good friction-reducing property. The action mechanism was estimated through analysis of the worn surface with X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscope (SEM). The results of XPS and SEM analyses illustrate that the prepared compound as an additive in hydrogenated oil forms a protective film containing ferric sulfide and ferric sulfate compounds on the rubbed surface.
Keywords: acetic ester derivative; additive; hydrogenated oil; tribological mechanism.
1. Introduction
It is well-known that additive[1] is a essential component for lubricant, and it play improtant role for ensuring lubricant capability and satisfing specifically requestment.
Sulfuration alkene is a very important extre-ment pressure additive, and it can offer effective protect for qear and axle, in order to avoid tiredness, sinter and worn of machine, it had been used in gear oil. When Sulfuration alkene was mixed with P-containing additive(such as dibuthylphosphate), it will easy produce mercaptan smell[2], and the mercaptan is damage for human and environment, so that confined it's business application in some degree.
Xanthate group contains sulphur element, many study results shown that sulphur element[3] can improve lubricating oil extreme pressure capability. The C-S bind energy of xanthate is more than that of sulfu-
ration alkene[1], and that made it has higher temperature stability and lower corrosivity. Even it possesses friction-reducing and antioxidation multifunction capability, and substituting sulfuration alkene aim will be realize by using S-containing compound. It will increase the extreme presuure and antiwear property by introducing the xanthate group into ester compound. The anioxidation of base oil is decreased after hydrogrnated, the xanthate can improve base oil anti-oxidation capability.
In the paper, we use xanthate as raw material, to synthesis a kind of xanthate-containing ester derivative, which accord with biodegradable requestment of lubricating oil additive. The tribological behaviors of synthetic ester derivative as additive in hydrogenated oil were evaluated with a four-ball machine. The tri-bological mechanism was discussed by analysis of solid film structure of rubbed surface using XPS and SEM.
2. Experimental details
2.1. Lubricating oil and additives
A commercial hydrogenated oil product 5Cst, which v100°c is 5.539 mm2s-1, flashpoint is 238°C, viscosity coefficient is 110, made by Daqing Refinery Factory of China, was used as the lubricating oil without any further treatment.
The aimed compound(HXAE) was synthesized according to the pathway outlined in Scheme 1.
These products were characterized by IR and elementals analysis. The elemental analysis results was that C was 63.88(63.33), H was 10.09(10.00), S was 17.19(17.78), value in parentheses were calculated, and that are in good agreement with the required values within the limits and experimental error of lubricating oil additives.
Современные технологии. Механика и машиностроение
S ClCH2COOCH2CH2Cl S S
KS -C -OCi6H33 -► CieHss^ C - S - CH2COOCH2CH2 - S - C - OC16H33
Scheme 1. Reaction pathway of novel compounds
2.2. Specimens and testing apparatus
The wear properties of the ester derivative in hydrogenated oil were evaluated with a four-ball machine at a rotating speed 1450 rpm, test duration of 30 min, and room temperature. The balls used in the tests were made of GCr15 bearing steel (AISI52100) with an HRC of 59-61. The load-carrying capacity of the additive was obtained according to GB3142-82, similar to ASTM D-2783. An optical microscope was used to determine the wear scar diameters of the three lower balls with an accurate reading to 0.01 mm. Then, the average of the three wear scar diameters was calculated and cited as the wear scar diameter reported in this paper. The friction coefficients were recorded automatically with a self-recording apparatus with the four-ball tester. Before each test, the specimens were cleaned in petroleum ether, then dried.
2.3. Worn surface analysis
X-ray photoelectron spectroscopy (XPS) was conducted with a PHI-5702 X-ray photoelectron spectrometer. The upper ball used for XPS analysis was washed ultrasonically with petroleum ether and dried after testing at additive concentration of 2.0 wt.% under load of 392 N for test duration of 30 min. The MgKa radiation was used as the excitation source at pass energy of 29.35eV, and the binding energy of C1s (284.6eV) was used as the reference. The wear scar morphology was visualized with JEM-1200EX Scanning electron microscopy at voltage 20 kV, to study the rubbed surface morphology.
3. Results and discussion
3.1. The maximum non-seizure load (Pb value)
The maximum non-seizure load (PB value) of base oil (5Cst), and different concentration addi-tives/5Cst were shown on table 1.
Tablel
The maximum non-seizure load (PB value) of different
concentration additive
Additives 5Cst 1.0% 2.0% 3.0%
Pb value (N) 686 588.4 646.8 706.1
The results show that the PB values of the compound are much higher than that of base oil, and the PB value increases with the additive concentration increasing. This indicated that the synthesized compound has excellent load-carrying capacity.
3.2. Friction-reducing Performance
The friction coefficient of synthesized compound in seven different concentrations under 392N and different applied loads at the additives concentrations 2.0wt% are shown in Fig1.
The friction coefficient decreased with the applied load increasing. It means that the xanthate- containing ester additives possess friction-reducing behavior at long range applied load.
The friction coefficient of the 5Cst was 0.113 under the applied load 392N, but it was reduced 13.3% to 0.107 by the addition of 2.0wt% additive. With the higher of additive concentration, the friction coefficient increased. The decreasing of friction coefficient can be attributed to the formation of adsorption film and/or reaction film by the additive on the rubbing surface[4]. The more novel additive is added, the more molecular layers within the adsorption film and more reaction products are generated to prevent the asperities on the rubbing surfaces from direct contact, and the lower the friction coefficients become. When the concentration arrives at some degree, the adsorption process tends to be saturated, it will not add the adsorption of additive.
0.14
0.13-
c 0.11-o
£ 0.100.09-
98N 196N 392N 490N
Applied load (N)
0.115
0.110-
с ID
I 0.105-
0.095
0 12 3
Additives Concentration (wt%)
Fig. 1. The Friction coefficient of various applied load and additive concentration
ИРКУТСКИМ государственный университет путей сообщения
3.3. Antiwearperformance Fig.2 gives the wear scar diameter (WSD) as the function of the additive concentration at the applied load at 392N and applied load at the additives concentration 2.0% in base oil.
196N 392N
Applied load (N)
friction process, generating chemisorption[5]. And it is generally accepted that the tribological behaviors of additive are closely related to the performance of the protective film formed by physisorption, chemisorp-tion and tribochemical reaction during the process.
0 12 3
Additives concentration (wt%)
Fig. 2. Variations in the wear scar diameter with concentration and applied load (N)
The results indicate that the additive exhibit good antiwear properties in a wide range of applied load. The wear scar diameter increases with the applied load from 98N to 490N.
It can be seen that the addition additive in base stock significantly reduce the wear scar diameter, this results indicate that the novel xanthate-containing ester additive has excellent antiwear property. The lower WSD is obtained along with additive concentration increasing. With the increasing of additive concentration, the S content are increased, the antiwear capability is increased. It is due to the protective film formed by the additives and its decomposers on the sliding surface under the boundary tribological conditions.
3.4. A discussion of tribology mechanism of novel additive
The enlarged SEM photographs is shown in
Fig.3.
It indicate that severe scuffing occurs with lubrication of 5Cst alone, taking on grain abrasion characteristic, while only slight frictional tracks appear with lubrication of HXAE, and there has appeared some layer structure matter, which assuming the characteristics of corrosive worn. It maybe the novel S element had reacted with the metal surface during the
Fig. 3. SEM morphologies of worn surface lubricated with
5Cst(left) and 2.0wt% HXAE (right) under392N
In order to explore the lubricating mechanism of the additive in hydrogenated oil, XPS analysis of the worn surface was carried out, and the analysis results are shown in Fig. 4.
The spectrum[6] of S2p of HXAE illustrates the existence of peak at 168.8eV, 170.0eV and 171.2eV which corresponds to ferric sulfide, ferric sulfate and organicsulfur compound on the worn scar, showing the tribochemical reaction that occurred between the additive with the metal surface during the sliding processes. The Fe2p peak appearing at binding energy 711.2 eV and 724.7eV, corresponds to iron oxide and/or sulfide, indicating that the lubricated steel surface is liable to oxidize or sulfurize in the friction process. The Ois peak corresponding to iron oxide appears at 533.4 eV, and means that it had occurred tribochemical reaction between the additive and steel ball surface during the lubricating process.
Surface analysis results demonstrate that the synthesized additive molecules maybe decomposed [7] to produce (RO)2CS(S)H (or other SH compound), so a stable lubricating film can be formed on the rubbed surface. This lubricating film is complex and consists of reaction layer and adsorption layer. The reaction layer originates from the tribochemical reaction of S element contained in the xanthate group, which can easily interact with the freshly metal surface to form extreme pressure and antiwear surface film[8] which containing FeSO4, and / or FeS. Products of tribochemical reactions between additives and metal surface can be transformed to an adherent antiwear surface film, which can prevent the direct contact of metal and metal, to reduce the metal stock abrasion.With such stable reaction and adsorption layers, the novel
98N
490N
Современные технологии. Механика и машиностроение
additive can effectively decrease the friction and wear, and possesses excellent tribological performances.
9000
8000
7000
6000
w 5000 -О
4000 3000 2000 1000
296 294 292 290 288 286 284 282 280 278 276 Binding energy(eV)
178 176 174 172 170 168 166 164 162 160 158 Binding energy(eV)
Binding energy(eV)
7000 6500 6000 5500 ÇO 5000-
o
4500 4000 3500 3000
544 542 540 538 536 534 532 530 528 526 Binding energy(eV)
Fig 4 The XPS spectra of C1s, S2p, Fe2p, O1s (2.0wt% HXAE /5Cst)
4. Conclusions
From the above results, the following conclusions can be drawn:
1. The synthesized ester derivative as additives in 5cst base oil show excellent load-carrying capacity and improve the antiwear and friction-reducing behavior at appropriate concentrations.
2. The friction-reducing and antiwear behavior of the additive are sensitive to weight concentration and applied load.
3. Through the SEM and XPS analysis results, the synthetic additive function to reduce friction and wear of steel-steel sliding system by chemical adsorption on and tribochemical reaction with the steel surface. The protective film formed during sliding processes contributed to the increase in the wear resistance and friction reduction
Acknowledgements
The work reported here were supported financially by Jiangxi Natural Science Foundation of China (Grant No 2007GZH0838) and Master Innovation Foundation of East China Jiaotong University(Grant No YC08C001), Science Foundation of East China Jiaotong University(Grant No 04ZKJC11, 05ZKJC15) and Doctor Foundation of East China Jiaotong University.
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