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ИРКУТСКИМ государственный университет путей сообщения
Liu Hong, Xiong Liping, Zhou Xiang, He Zhongyi, Zhang Shaoming
УДК 621.89
TRIBOCHEMICAL STUDY OF TWO TRIAZINE DERIVATIVES AS ADDITIVES IN RAPESEED OIL
1. Introduction.
Rapeseed oil(RSO) is traditional environmental friendly lubricants and posses good lubricating property, high-biodegradability and well-recycled performance. And it's extreme pressure and tribological behaviors can be increasing by adding lubricants. The N-containing heterocyclic compounds, such as benzotriazole, imidazole, triazine, etal, have been reported as excellent multifunctional lubricating oil and grease additives.
In the lubrication area, it is well-known that zinc diallkyldithiophosphates (ZDDP) are very commonly used excellent multifunctional lubricating additives, but use of these additives has been brought into question on account of problems of toxicity, waste disposal, filter clogging, pollution, etc . The recent AW and EP additives technology for both automotives and industrial applications is based on ashless sulphur-phosphorus chemistry. It will increase the extreme presuure and antiwear property by introducing the thiophosphate group into N-containing heterocyclic compounds. Amino takes on some extent alkalinity, it can control the corrosion wear by S and P in tribological process.
In the paper, we synthesized two novel ashless S-P-N type lubricating oil additives, 2,4-bis-dibuthylamino-6-(0,0'-dibuthyldithioph osphate)-s-1,3,5-triazine (code as BuBT), 2,4-bis-morpholinyl-6-(0,0'-dibuthyldithiophos phate)-s-1,3,5-triazine (code as MoBT). The
Scheme 1. Reaction pathway of novel compounds.
tribological behavior of synthetic triazine derivatives and a commercial ZDDP as additives in rapeseed oil were evaluated with a four-ball machine. The tribological mechanism was discussed by analysis of Solid film structure of rubbed surface using Xray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM).
2. Experimental details.
2.1 Lubricating oil and additives.
A commercial RSO product, made by Xi'an Jiali Oil and Grease Factory of China, was used as the lubricating oil without any further treatment.
The triazine derivatives were synthesized according to the pathway outlined in Scheme 1.
These products were characterized by IR, 'HNMR and elementals analysis. The results of elemental analysis listed in table 1 are in good agreement with the required values within the limits and experimental error of lubricating oil additives.
2.2 Specimens and testing apparatus.
The wear properties of these novel triazine derivatives in rapeseed oil were evaluated with a four-ball machine at a rotating speed 1450 rpm, test duration of 30 min, loads of 98, 196, 294, 392, 490, 588 N, and room temperature about 20oC. 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.
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Table 1
The elemental analysis result of the synthesized compounds.
Compounds Elemental analysis [Found / Calcd]
C H N S P
MoBT 46.88/46.44 7.09/6.92 13.09/14.26 13.88/13.03 6.65/6.31
BuBT 55.97/56.35 9.61/9.39 11.89/12.17 11.59/11.13 5.71/5.57
Table 2
The maximum non-seizure load (PB value) of the RSO and RSO containing 1.0wt% additives.
Additives BuBT MoBT ZDDP RSO
PB value(N) 1490 1127 980 686
For comparison, the lubricating performance of a commercial ZDDP, which was produced by Lanzhou refinery, was evaluated at the same time. 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 1.0 wt.% under load of 294 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 and the S, P and N elements distribution in the rubbed surface film.
3. Results and discussion.
3.1. The maximum non-seizure load (PB value).
The maximum non-seizure load (PB value) of the base oil (RSO), and 1.0 wt% additives/RSO were shown on table2. The results show that the PB values of the two compounds are much higher than that of base oil and ZDDP. This indicated that these synthesized compounds have excellent load-carrying capacity. The PB value of BuBT is higher than that of MoBT, and it indicates that the containing alkylamine triazine derivative possesses excellent load-carrying capacities than
that of containing cycloamine triazine derivative in RSO at same weight concentration.
3.2 Friction-reducing Performance.
The friction coefficient of synthesized compounds and ZDDP in seven different concentrations and different applied load at the additives concentrations 3.0 wt% are shown in Fig1. The friction coefficient decreased with the applied load increasing, and BuBT is lower than that of ZDDP and that of MoBT is higher than that of ZDDP at same condition. It means that the alkylamino-containing triazine additives possess excellent friction-reducing behavior than cycloamino-containing triazine additive at long range applied load.
The friction coefficient of the RSO was 0.12 under the applied load 392N, but it was reduced 24.1 to 0.091of BuBT and 11.7% to 0.106 of MoBT by the addition of 0.5 additive to the base stock. As the adding BuBT amount reached 2.0%, a 39.1% reduction of friction coefficient was observed, and 29.6% for ZDDP. With the higher of additives concentration, the friction coefficient increased when additive concentration is more than 2.0 wt%. The decrease of friction coefficient can be attributed to the formation of adsorption film and/or reaction film by the additive on the rubbing surface'51. 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. According to
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Fig. 1. The Friction coefficient of various applied load and additive concentration.
the lone electron part of amino, morpholine and triazine of MoBT can form a big p bind, it effect the additive adsorption to the metal surface. The alkylamino is easy contact to metal surface than cycloamino, and the reaction film tends to completely separate the asperities, therefore, the reduction tendency of friction coefficients decrease.
3.3 Antiwear performance.
Fig.2 gives the wear scar diameter (WSD) as the function of the additives concentration at the applied load at 392N and applied load at the additives concentration 3.0% in rapeseed oil. The results indicate that the additives exhibit good antiwear properties in a wide range of applied load. The wear scar diameter increases slightly at an applied load from 196N to 588N, it is similar to that of ZDDP. The MoBT possesses excellent antiwear behavior than others additives at lower applied load.
It can be seen that the addition additives in base stock significantly reduce the wear scar diameter, this results indicates that the novel S, P-containing triazine additives have excellent antiwear properties. The low WSD is reached at an additives concentration 2.0 wt%. When the additives concentration are more than 3.0wt%, the wear scar diameter heighten with the increasing of additives concentration. With the increasing of additives concentration, the S and P content are increased, the corrosive worn are increased. For BuBT, the performance is most outstanding in these additives. However the antiwear property of ZDDP is slightly better than that of those synthesized additives at lower concentration. 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 additives.
The enlarged SEM photographs is shown in Fig.3. It indicate that severe scuffing occurs with lubrication of rapeseed oil alone, taking on grain abrasion characteristic, for MoBT, it takes on grain abrasion pattern, but it is slighter than that of RSO, while only slight frictional tracks appear with lubrication of BuBT, assuming the characteristics of corrosive worn. It maybe the novel S and P elements had reacted with the metal surface during the friction process, generating chemisorption. And it is generally accepted that the tribological behaviors of the additives are closely related to the performance of the protective film formed by physisorption, chemisorption and tribochemical reaction during the process.
In order to explore the lubricating mechanism of these additives in RSO, XPS analysis of the worn surface was carried out, and the analysis results are shown in Fig.4.
For the N element of MoBT, the binding energy of N1s is 400.5eV, existing in a organic nitrogen chemical state, which means that there is a just absorbed organic nitrogen on the steel ball surface and does not take part in tribochemical reactions.
The spectrum of S2p of MoBT illustrates the existence of peak at 168.8eV, which corresponds to sulfate 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 710.6 eV corresponds to iron oxide and/or
МЕХАНИКА. ТРАНСПОРТ. МАШИНОСТРОЕНИЕ
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Fig .2. Variations in the wear scar diameter with concentration and applied load (N).
Fig. 3. The SEM morphologies of worn surface lubricated with RSO and 1.0wt% additive under294N.
Fig. 4. The XPS spectra of N1s, S , P , O1s (1.0wt% MoBT/RSO).
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sulfide, indicating that the lubricated steel surface is liable to oxidize or sulfurize in the friction process. The O1s peak corresponding to iron oxide appears at 530.2 eV. The binding energies of P2p is 133.9eV, which correspond to
PO„
and means that it had occurred
tribochemical reaction between the additives and steel ball surface during the lubricating process.
Surface analysis results demonstrate that the synthesized additives molecules maybe decomposed to produce (RO)2PS(S)H (or other SH compound), and amino-containing triazine group, 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, P elements contained in the dibutylthiophosphate 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) and PO43J. 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 additive can effectively decrease the friction and wear, and possesses excellent tribological performances.
4. Conclusions.
From the above results, the following conclusions can be drawn:
1. Three synthesized triazine derivatives as additives in RSO show excellent load-carrying capacity and improve the antiwear and friction-reducing behavior at appropriate concentrations. The BuBT has excellent friction-reducing performance and load-carrying capacity than that of ZDDP and MoBT. The MoBT has better antiwear property than that of BuBT.
2. The friction-reducing and antiwear behavior of these additives are sensitive to weight concentration and applied load.
3. Through the SEM and XPS analysis results, the synthetic additives 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 Jiangxi Education Department Foundation of China (Grant No 2007184) and Science Foundation of East China Jiaotong University(Grant No 04ZKJC11, 05ZKJC15) and Doctor Foundation of East China Jiaotong University.
BIBLIOGRAPHY
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3. He,Zhongyi,Rao,Wenqi, Ren,Tianhui, Liu,Weimin, Xue,Qunji, The Tribochemical Study of Some N-Containing Heterocyclic Compounds as Lubricating Oil Additives, Tribology Letters. 2002;13:87-93.
