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Tribochemistry of mono molecular additive films on metal surfaces, investigated by XPS and HFRR
Life Cycle Tribology D. Dowson et al. (Editors) © 2005 Elsevier B.V. All rights reserved
269
Tribochemistry of mono molecular additive films on metal surfaces, investigated by XPS and HFRR R. Kolma*, I.C. Gebeshubera-b, E. Keneseya, A. Eckera, A. Pauschitza, W.S.M. Wernerb, and H. Storib a
Austrian Center of Competence for Tribology AC2T research GmbH., Viktor Kaplan-Strafie 2, 2700 Wiener Neustadt, Austria b
Institut fur Allgemeine Physik, Technische Universitat Wien, Wiedner Hauptstrafie 8-10/E134, 1040 Vienna, Austria In order to see how oxygen and nitrogen containing substances influence the wear relating behaviour of diesel fuels various hydroxychinolines were investigated by means of a high frequency reciprocating rig (HFRR) tribometer. Attention was especially given to the connection between the position of the hydroxyl group at the molecule and the resulting influence on the "lubricity" properties of low sulfur diesel fuel. Monomolecular lubricant films were deposited from the liquid phase onto ultra thin copper films sputtered onto silicon wafers. This substrate serves as a model system for bronze materials. In order to compare the results of X-ray photoelectron spectroscopy (also called electron spectroscopy for chemical analysis, ESCA) measurements with tribological experiments performed with the high frequency reciprocating rig method additional substrates made of 100Cr6 steel were used. X-ray photoelectron spectroscopy investigations were performed using a VG ESCALAB Mk III equipped with a special preparation chamber, permitting the transfer of samples from a fluid cell to the analysis chamber under Helium protective gas. The structure of the molecular film is elucidated using angular resolved X-ray photoelectron spectroscopy. Preliminary X-ray photoelectron spectroscopy investigations of ultra thin layers of 8-Hydroxychinoline on copper samples were performed. Angular resolved measurements demonstrated that, without tribological stress, full coverage of the surface with 8-hydroxyquinoline is not possible. Those results however still need corroboration by atomic force microscopy investigations.
1. INTRODUCTION Additives are of major interest in lubrication technology. They influence the properties of base oils in several aspects. Anti-wear and extreme pressure additives reduce abrasion and wear in boundary and mixed lubrication. Molecules of these additives, which are physisorbed or chemisorbed onto the surface, form films on the surface of the solids. Though some mechanisms of action are understood others still remain unexplained. One of the unexplained mechanisms are different lubrication properties of isomers of hydroxyquinoline. Whereas 2- and 8-hydroxyquinoline exhibit good lubrication properties other isomers do not. In order to acquire knowledge of the behavior of these additives at the interface between * e-mail: [email protected]
solids in contact, a fundamental understanding of the interaction of additive molecules with the surface is necessary. Since little is known in this field of research, exploratory results, as they are presented in this paper, are justified. It is known that isomers of hydroxyquinoline act as lubricants, how and why remains, however, to be investigated. Since the hydroxyquinoline layers are very thin, possible experimental techniques for investigation of their friction reducing properties are limited. The high frequency reciprocating rig technique (HFRR) can provide macrotribological information, atomic force microscopy (AFM) can investigate the tribology on the micro- and nanoscale, and X-ray photoelectron spectroscopy (XPS) can provide information about the structural chemistry. Presently, there is a need to understand the
270
Upper specimen
Fretting flexure lock
Lower specimen
LVDT and flexure housing / /
Electromagnetic vibrator
Counterweight
/
o \
Heater block Main RTD location hole (in far side of block)
Force transducer
Figure 1. Scheme of our High Frequency Reciprocating Rig device. The fretting flexure lock solely enables oscillation of the upper specimen, preventing all other movements. The linear and vertical displacement transducer (LVDT) is mounted in the flexure housing. Binding energy [eV] Cu 2p 3 / 2 2-Hydroxychinoline 933.3 4-Hydroxychinoline 933.2 8-Hydroxychinoline 933.0 Table 1 Binding energies of copper samples for
C Is C Is 286.8 285.4 287.1 285.4 X 285.2
N Is 399.3 399.2 399.0
O Is 533.3 533.2 533.1
O Is 530.8 530.8 530.1
several hydroxychinolines investigated by XPS.
underlying mechanisms of lubricity, as common lubricity promoting substances are removed due to environmental concerns. A prime example is modern, low sulphur diesel fuel. Natural components with 'lubricity' promoting properties are removed from diesel as a result of the severe hydrotreating process. Therefore it became necessary to add 'lubricity' enhancing substances to diesel with very low sulfur-contents. Many substances were tested for their applicability as anti wear additives. Oxygen and nitrogen containing substances show a good suitability as 'lubricity' enhancers. In 1986, Wei and Spikes reported about the
'lubricity' enhancing effect of various nitrogen containing heteroaromatics. Their studies also revealed that 8-Hydroxychinoline already has a notably wear reducing potential at a concentration of 100 ppm. [1] In 1992, Wei and Song investigated the wear behaviour of steel lubricated by some oxygencontaining derivatives. The antiwear activity shows its highest potential with substances containing a hydroxyl group. These results imply that aromatics with an aldo group have a slightly worse 'lubricity' behaviour compared to hydroxyl containing substances. The worst 'lubricity' results were obtained with carbonyl and
271 2 i
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• c 1s !
•
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Figure 2. Intensities of various 2-Hydroxychinoline XPS peaks relative to the Cu 2p 3 / 2 peak intensity in the same spectrum, depending on the takeoff angle. In the case of C Is and 0 Is two different chemical binding states were evaluated. alkyloxyl substituents hanging at the ring system. In the case of 8-Hydroxychinoline, these authors experienced an enhancement of the 'lubricity' down to a lower concentration of 125ppm.[2] The results section describes similar results for various hydroxychinoline derivates. In 1998, Wei and coworkers reported 'lubricity ' experiments carried out by means of a four ball wear tester and a HFRR device. The substance class of heteroaromatics with a hydroxyl group represented in this study are 2Hydroxy-benzothiazole, 4-Hydroxybenzene and 8-Hydroxychinoline. The evaluation of the results indicates that the molecules with a hydroxyl group feature a significant improved 'lubricity' behavior and consequently far higher
wear protecting properties than the parent compound without a hydroxyl group. Subsequently, in order to understand the mechanisms and reactions which lead to this improved abrasion character the formed films were analyzed with XPS, Auger electron spectroscopy, ECR and optical microscopy. Wei et al. came to the conclusion that the hydroxyl group enhances the polymerforming tendencies of the parent compound. It also seems that the compounds substituted with a hydroxyl group raise the critical temperature which characterizes the desorption of additives from rubbing surfaces. [3] HFRR is the method of choice to investigate lubricity on a macroscopic scale and is widely used for the engineering of fuels, lubricants and en-
272
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ID C 4>
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Figure 3. Intensities of various 8-Hydroxychinoline XPS peaks relative to the Cu 2p3//2 peak intensity in the same spectrum, depending on the takeoff angle. In the case of O Is two different chemical binding states were evaluated. gine parts. In addition, XPS is used to obtain insight into the molecular mechanisms leading to the macroscopic lubricity.
2. MATERIALS AND METHODS Due to technical limitations, different types of samples were used for different experimental methods. While for HFRR the usual steel samples have been used, ultra flat copper samples as well as steel samples were used for XPS studies.
2.1. X-ray (XPS)
photoelectron
spectroscopy
XPS is a surface analytical method. In ultra high vacuum, the sample is irradiated with monochromatic X-rays. This causes the emission
of photoelectrons from the sample. These photoelectrons are separated by means of an electrostatic analyzer and detected. The kinetic energy of the photoelectrons depends on the atomic number and the chemical surroundings of an element. The shift in energy caused by neighboring atoms is called chemical shift and allows assignment of binding partners. Therefore XPS is most suited for compound specific analysis. Furthermore, XPS is highly surface sensitive due to the short decay length (attenuation length) of the signal, which usually is in the range of a few nanometers, and the exponential decay of the signal with sample depth. In angle resolved measurements (ARXPS), the angle between the surface normal of the sample and the analyzer (the emission angle) is varied,
273 Binding energy [eV] Fe 2p 3 / 2 Fe 2p 3 / 2 Fe 2p 3 / 2 Fe 2p 3 / 2 C Is C Is X 711.3 707.3 X X 100Cr6 steel 708.9 707.2 2-Hydroxychinoline 710.8 708.8 287.1 X 713.8 4-Hydroxychinoline 714.0 710.9 708.9 707.3 288.7 286.5 X X 8-Hydroxychinoline 713.7 710.8 708.8 707.2 N Is N Is O Is O Is Binding energy [eV] C Is X 100Cr6 steel 283.5 397.7 531.8 530.2 283.4 399.8 530.2 2-Hydroxychinoline X 531.8 401 4-Hydroxychinoline 283.4 398.7 531.9 530.2 X 8-Hydroxychinoline 283.3 400.1 531.7 530.2 Table 2 Binding energies of steel samples for several hydroxychinolines investigated by XPS. 0.8346g/cm3 0.8146g/cm3 Density at 15°C Density at 40°C 2 2 Viscosity at 40° C 4.7545mm /s Viscosity at 100° C 1.6851mm /s Table 3 Characteristic data of the base fluid used in the experiments therebye providing information about the depth distribution of the different compounds of a sample in their different chemical states. Taking all these into account, XPS is the method of choice to obtain information about organic molecules adsorbed onto solid surfaces, as is the case with additives interacting with tribological surfaces [4]. Copper samples were prepared by cutting about Icm2-sized slides from a silicon wafer and coating with 500nm of copper by sputtering. The surfaces of these slides were investigated with an atomic force microscope (MFP-3D, Asylum Research, Santa Barbara, CA). Then the slides were transferred into the preparation chamber (5 • W~ smbar) of the XPS. There, carbonaceous remnants as well as adsorbed gaseous contaminations were removed by argon ion sputtering. The purity of the samples were checked by XPS analysis prior to preparation of the additive films. 100Cr6 steel samples were prepared from sheets of 8xl2mm 2 , which were cut from a roll taken from a roller bearing. These sheets were ground with silicon carbide abrasive papers with grid sizes from 150 down to 4000 on a Struers LaboPol4 grinding machine. Afterwards, the sheets were polished on the same machine with a MD-Dac disk treated with DP-Spray P containing poly-
C Is 284.8 285.4 285.1 285.6
Density at 100°C
0.7746g/cm3
crystalline diamonds with a grain size of 1/im. After rinsing with deionized water, the sheets were transferred into the preparation chamber ((5 • \Q-&mbar)) of the XPS. There, argon ion sputtering was performed for half an hour with an acceleration voltage of 4kV and the surface was checked by XPS measurements. The additive films were prepared using 5 /zmol/ml solutions of 2-, 4- and 8-hydroxyquinoline in toluene (substances from Sigma-Aldrich). The solutions were transferred into the adsorption device, which is an extension of the XPS and is directly connected to the spectrometer in order to exclude atmospheric contaminations during the preparation and transportation of the samples. The adsorption setup used for all samples consists of two supply reservoirs. The first supply reservoir was used as storage for toluene cleaning the device, the second supply reservoir contained the solution of one of the hydroxyquinoline isomers, a pressure line, which was filled with He 6.0, to provide the pressure to transport the liquids into the adsorption vessel, and an adsorption vessel, where the samples were prepared. The whole device was permanently under a slight overpressure of He in order to keep it from being polluted. Furthermore, the Helium is used to degas the solutions.
274 274
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ity [cout
••«•
4) C
•*
100000 80000 60000 40000 -
w 20000 -
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o _* 1000
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200
binding energy [eV]
Figure 4. XPS survey spectrum of clean 100Cr6 steel recorded using Mg Ka radiation. The clean copper sheets were transferred from the preparation chamber of the spectrometer directly into the adsorption vessel. There, they were dipped at room temperature for 50 minutes into 5 /imol/ml solutions of the adsorbates. Afterwards, the samples were blown dry with Helium and directly, without exposing them to environmental conditions, transferred into the preparation chamber of the spectrometer. All spectra of the copper samples were recorded using Al KQ radiation with a power of 600W. No monochromator was used. 100Cr6 steel samples were measured using non monochromatized Mg Ka radiation. The source was operated at a power of 300W. The measurements were performed using constant analyzer energy mode and choosing a pass energy of 50eV. The step width was adjusted to leV for survey spectra and to
O.leV for the various compound regions. An angular aperture of the analyzer of 10° was used for ARXPS. ARXPS spectra were recorded at emission angles of 15°, 28°, 47.6°, 60.6°, 70° and 75°. The emission angles were varied by rotating the sample, which was mounted on a sample holder with a 45° slope, around an axis with a fixed angle to the analyzer. To enable comparison of the angular intensity distribution, the intensity ratios were calculated in order to exclude geometric artefacts from the measurements.
2.2. High Frequency Reciprocating Rig (HFRR) The HFRR apparatus is based on a steel ball which is loaded against a stationary steel plate. The plate, which is embedded into the lower specimen holder, is fully immersed into the fuel to be tested. During the measuring process the ball is
275 330000 -
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-100Cr6 2-HCH 4-HCH -8-HCH
u (A
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80000 720
715
710
705
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binding energy [eV]
Figure 5. Fe 2p 3 / 2 region of the photoelectron spectrum of layers of 2-, 4-, 8-Hydroxychinoline on 100Cr6 steel. For comparison a spectrum of clean 100Cr6 steel is also shown. oscillating with a certain frequency and is in constant contact with the steel plate (see Figure 1). A load of 200g is applied to the upper specimen holder, where the steel ball is embedded. The 200g load used for the HFFR device is given by the norm ISO 12156-1. This norm was introduced for testing the 'lubricity' behaviour of diesel fuel. The test results obtained with these parameters correlate to many fuel/hardware combinations and thereby provide good predictions of the lubricating quality of the fuel. The course of the measurement can be observed online by means of a computer program. This software enables the operator to follow the course of the film formation as well as the changes of the friction coefficient over the testing time. The formation of a lubricating film is indicated by the
electric potential existing between the steel ball and the plate. The built up lubricating film acts as an insulator and separates the test specimen, steel ball and the plate by a thin layer. After the test the abraded area on the ball is evaluated by means of an optical microscope. The observed dimensions are converted into a defined wear scar value by a formula. The WS 1.4 is a standardized measure for the 'lubricity' of diesel fuel systems. Additionally to the wear scar dimensions, the temperature and humidity at the beginning and the end of each testing procedure are also included into the evaluation of the Wear Scar 1.4. This measuring apparatus has many advantages compared to other 'lubricity' testing devices. For example, the low sample amount needed is very favourable if only small sample vol-
276
100Cr6 15° — 100Cr6 75° 2-HCH 15° - 2-HCH 75°
o
716
714
712
710
708
706
704
702
binding energy [eV]
Figure 6. Fe 2p 3//2 region of the photoelectron spectrum of layers of 2-Hydroxychinoline on 100Cr6 steel. The spectra are shown for two different electron takeoff angles. For comparison the spectra of clean 100Cr6 steel are also shown. Especially for the coated surface the different peak shapes for the nearly vertical (75°) and the grazing (15°) takeoffs are visible. umes are available. Hydroxychinolines were dissolved or dispersed into a low sulphur diesel with a sulphur content of less than lOppm. The test solutions with a concentration of 125ppm to 500ppm were consequently tested by a High Frequency Reciprocating Rig (HFRR) following the norm ISO 12156-1. 3. RESULTS 3.1. X P S Copper samples: The adsorption of the Hydroxychinolines onto the sputter coated samples turned out to take longer than expected. We had to put the copper sheets for 50 minutes into the solution of the hydroxychinolines in order to ad-
sorb a measurable amount of the hydroxychinolines onto the atomically flat surfaces. The survey spectra of all samples showed only photoelectron peaks of copper, oxygen, nitrogen and carbon as expected. Peak fitting was done with SPECTRA Presenter, a software provided by Ron Unwin. After Shirley background subtraction a combination of Gaussianand Lorentzian- peaks was fitted to the spectra. The positions resulting for the various compounds of the hydroxychinolines are listed in Table 1. The accuracy of the peak positions is about O.leV. The postion of the Cu 2p 3 / 2 photoelectron peak is nearly constant within few tenth of an eV and is little lower then that found for
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Figure 7. Intensity ratios of various 2-Hydroxychinoline XPS peaks relative to the 100Cr6 peak intensity in the same spectrum, depending on the takeoff angle. In the case of 0 Is two different chemical binding states were evaluated. copper 8-Hydroxychinoline additives after friction [5]. The lowest binding energy was found for 8-Hydroxychinoline and the highest for 2Hy droxychinoline. Alike behaved the binding energies for the N Is photoelectrons. The position of the N Is peak of 8-Hydroxychinoline is like the one of the Cu 2p 3 / 2 peak 0.6eV lower than that found by Yu and co-workers after friction [5] but is in good agreement with the binding energy measured for N Is in 8-Hydroxychinoline by Yoshida [6]. The 0 Is photoelectron peak was composed of two peaks. The one at a binding energy above 533eV was equal within few tenth of an eV for all hydroxychinolines. The other at lower binding energy varied from 530.leV for 8-Hydroxychinoline to 530.8eV for both 2-Hydroxychinoline and 4-
Hydroxychinoline. The C Is photoelectron peak was found to consist of one peak located at about 285.4eV for all of the hydroxychinolines and a second peak occurring at higher binding energy for 2-Hydroxychinoline and 4-Hydroxychinoline indicating a different chemical species of carbon. ARXPS showed for 2-Hydroxychinoline that the ratio of the area of the C Is located at 286.8eV to the Cu 2p 3 / 2 had the same characteristics as the N Is and the O Is at 530.8eV indicating the same depth distribution (Figure 2). Alike the angular distribution of the intensity ratios of the C Is at 285.4eV showed the same angular dependence as the O Is intensity ratio at 533.3eV. The intensity ratios of the two different species of carbon had different angular distribution. Where the intensity ratio of the C
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• 8-Hydroxychinoline 500 ppm • 8-Hydroxychinoline 250 ppm • 8-Hydroxychinoline 125 ppm • 2-Hydroxychinoline 500ppm • 2-Hydroxychinoline 250ppm D 2-Hydroxychinoline 125ppm • 4-Hydroxychinoline 250ppm D 5-Hydroxychinoline 250ppm • 6-Hydroxychinoline 500ppm • 6-Hydroxychinoline 250ppm
Figure 8. Film formation of various tested hydroxychinolines. A complete film would act as an insulator. Is located at the higher binding energy became lower at the highest emission angle whereas the other rose constantly. With 4-Hydroxychinoline no specific angular dependence was found. For 8-Hydroxychinoline, the intensity ratios of C Is, N Is and the O Is located at 533.leV showed the same characteristic. They rose steadily whereas the intensity ratio of the O Is located at the lower binding energy declined from 70° to 75° indicating that the compound is located below the other compounds (Figure 3).
100Cr6 steel samples: The survey spectrum of the 100Cr6 (Figure 4) steel samples after half an hour of sputtering shows photoelectron peaks from iron, carbon, nitrogen and oxygen. The positions were taken from peak-fits done with the software mentioned above after Shirley background subtraction using a minimum amount of peaks composed of Gaussian- and Lorentzianpeaks and are listed in Table 2. The peak at 707.3eV was attributed to pure iron [7,8]. The one at 711.3eV was assigned to iron bound to oxygen [9-11]. Other peaks in the 2p 3//2 photoelectron of iron could not be definitely assigned. The peak at about 708.8eV may result from Fe bound to nitro-
gen as assigned by Riviere and co-workers [12] to a peak at 708.2eV. The peak at the highest binding energy was found by Riviere et al. too but was not assigned either [12]. This peak may result from shake-up satellite features, which occur at binding energies 5 to 8eV higher than the respective 2p main core lines [13]. The C Is peak as well as the O Is peak consisted of two peaks. The position of the O Is peak in iron oxides is reported to change only little with the oxidation value and is located at 529.9eV [14-17,8], which lies somewhat lower then the binding energy of 530.2eV we found. The second oxygen peak was ascribed to oxygen in hydroxyl groups [13]. Further nitrogen was found on one of the steel samples. The Fe photoelectron region of 2Hydroxychinoline showed a completely different intensity ratio of the iron and the iron oxide peak (Figure 5). The loss in intensity of the iron peak was due to coverage of the surface with the adsorbed hydroxychinoline, while the iron oxide which overlay the iron was less influenced by the covering layer. At an emission angle of 75° the Fe metal peak vanished in the spectra recorded from the samples with hydroxychinoline
279
Substance
Concentration [ppm]
WS 1.4
av. film [%]
242 92 8-Hydroxychinoline 500 92 280 8-Hydroxychinoline 250 392 71 8-Hydroxychinoline 125 167 91 2-Hydroxychinoline 500 193 92 2-Hydroxychinoline 250 223 90 2-Hydroxychinoline 125 467 19 4-Hydroxychinoline 250 464 17 5-Hydroxychinoline 250 384 72 6-Hydroxychinoline 250 429 6-Hydroxychinoline 500 38 Table 4 HFRR results for various hydroxychinolines at various concentrations.
8-Hvdroxychinoline
4-Hydroxychinoline
2-Hvdroxychinoline
6 -H vdroxvohinoline
5-Hydroxvchinoline
Friction 0.157 0.158 0.19 0.18 0.169 0.182 0.315 0.29 0.213 0.227
The survey spectrum of 4-Hydroxychinoline showed an additional Nls peak resulting from the nitrogen, which was found on the steel sheet to which 4-Hydroxychinoline was adsorbed. Further four different Fe 2p 3/2 and C Is peaks were fitted. Two oxygen peaks, which all hydroxychinolines had in common, showed the same angular dependency as found for the other hydroxychinoline samples. One on top with a depth distribution like that of nitrogen and the other below bound to iron. The two nitrogen species showed a completely different behaviour in their ARXPS spectra. In the spectrum of 8-Hydroxychinoline only two C Is peaks appeared. The intensity of the nitrogen peak and the oxygen peaks behaved like that of the other hydroxychinoline samples.
3.2. HFRR Figure 9. Molecule models for 8-, 4-, 2-, 6- and 5-Hydroxychinoline.
adsorbed (Figure 6). Angular resolved measurements showed that the two oxygen species were situated in different heights. The one located on top appeared to have a similar depth distribution as nitrogen. Whereas the depth distribution of the underlying one corresponded to depth distribution of the peak assigned to iron oxide (Figure 7).
The literature describes 8-Hydroxychinoline as a 'lubricity' additive with an advantageous wear reducing character as well as a high film formation potential [1-3]. In order to investigate the wear relating properties of other hydroxychinolines several tests were carried out (see also the two papers by Kenesey and Ecker, 2003 [18,19]). The aim of these experiments also included investigations on the influence of the location of the hydroxyl group and the nitrogen atom on the behavior of the quinoline compound. The base fluid used for the tests, is a highly refined diesel fuel with a sulphur content of less
280 than lOppm. Measurements of the pure diesel by means of a HFFR showed a WS of 521 pm and 510/im. Density and viscosity at various temperatures for the base fluid is given in Table 3. High purity hydroxychinolines (2Hydroxychinoline with a purity of 99%, 4Hydroxychinoline and 5-Hydroxychinoline with purities of 98%, 6-Hydroxychinoline with a purity of 98% and 8-Hydroxychinoline with a purity of 95%) were dispersed into low sulphur diesel with a concentration of 500ppm for the lubricity tests. The compounds were dispersed and dissolved at a temperature of approximately 60° C for 30 minutes with a magnetic stirrer. For most of the substances, the tests were carried out only once, just in doubtful cases a second or third reference measurement was performed. The average values are given Table 4. The experiments show for 8-Hydroxychinoline and 2-Hydroxychinoline an excellent behavior regarding all tested parameters. In both cases the first test concentration used was 500ppm. The concentration was then gradually reduced to 125ppm. The evaluation indicated for 8Hydroxychinoline as well as 2-Hydroxychinoline still wear reducing properties at this low concentration. In this case 2-Hydroxychinoline presented almost the same low abrasion as experienced with 250ppm. After these outstanding results the further tested hydroxychinolines were mainly investigated at a concentration of 250ppm. However these compounds resulted only in unsatisfactory wear behavior at this concentration. 4-Hydroxychinoline and 5-Hydroxychinoline had a very high wear scar. In contrast to this, 6Hydroxychinoline showed a middle WS 1.4 of 384/wn at a concentration of 250ppm. In order to investigate if the application of a larger amount would improve the results achieved the experiment was repeated at 500ppm. In this case the wear relating properties severely deteriorated. The film formation potential of 6Hydroxychinoline has a value of 72% at a concentration of 250ppm . However, at a level of 500ppm the potential of this property diminishes to 38%. So it could be said that 6-Hydroxychinolin has a clear pro -wear effect at the higher concentration, a fact which is observed in the wear scar value
and as well as in film formation potential. The film formation ability in case of 8Hydroxychinoline decreases only slowly with decreasing concentration. In the case of 2Hydroxychinoline, the potential for film formation stays almost constant at a value of 90% for all three measured concentrations. 4- and 5Hydroxychinoline present with 19% and 17% very poor results and provide consequently almost no 'lubrication film' (Figure 8).
4. DISCUSSION, CONCLUSION AND OUTLOOK The differences in tribological bahaviour of the various hydroxiquinolines have been observed by HFRR and the chemical structure of the film by XPS. Unfortunately, HFRR data could only be taken on steel, while the most conclusive XPS results were obtained on sputtered copper films, since the pure copper and steel samples suffer from residual roughness. Further detailed studies are necessary to conclusively link macrotribological results with information on structural chemistry obtained by XPS. Another method we will apply is atomic force microscopy in the controlled environment of a closed fluid cell. These AFM investigations (performed with a MFP-3D from Asylum Research with top view optics) will permit not only topographic investigations, but also force measurement with pico-Newton resolution in two axes (vertical and one horizontal axis). Furthermore, force spectroscopy will permit determinination of the influence of the monomolecular lubricant layer on attractive and lateral forces. An interesting fact is that it takes more than one hour for the film to strongly adhere to the substrate. Nevertheless, long time needed for strong adhesion is no indication for enhanced lubricity. The question whether the XPS spectra truly reflect microscopic causes for different tribological bahaviour as observed with HFRR can therefore not yet be conclusively answered. The photoelectron peaks and binding energies can easily be assigned for the copper samples. Furthermore, the depth distribution of the com-
281 pounds can be determined. In contrast to the copper samples, the interpretation of the spectra recorded from the 100Cr6 samples is more difficult. Our results indicate that only hydroxychinoline derivates featuring the hydroxyl group next to the nitrogen atom possess good to excellent 'lubricity' enhancing properties (Figure 9). Considering the steric configuration of these substances the conclusion could be drawn that there exists a synergism between the nitrogen atom and the hydroxyl group next to it. This effect leads to an outstandingly good film formation potential and consequently to a lower friction coefficient.
ACKNOWLEDGEMENT This work was funded from the "Austrian Kplus-Program" (governmental funding program for pre-competitive research) and has been carried out within the "Austrian Center of Competence for Tribology" (AC2T research GmbH). REFERENCES 1. D. Wei and H.A. Spikes. The 'lubricity' of diesel fuels. Wear, lll(2):217-235, 1986. 2. D. Wei and H. Song. The wear behaviour of steel lubricated by some oxygen derivatives of heterocyclic nitrogen-containing compounds (hncc) under boundary lubrication conditions, hub. Sci., 4(3):219-232, 1992. 3. D. Wei, X. Han, and R. Wang. The influence of chemical structure of certain nitrogencontaining organic compounds on their antiwear effectiveness: The critical role of hydroxy group, hub. Sci., 2(l):63-87, 1998. 4. H. Stori, R. Kleiner, W.S.M. Werner, R. Kolm, I.C. Gebeshuber, and C. Jogl. Characterisation of monomolecular lubricant films. In Proceedings 14th International Colloquium Tribology, volume III, pages 16631666, Technische Akademie Esslingen, 2004. 5. L. Yu, Y. Lian, and Q. Xue. The tribological behaviors of some rare earth complexes as lubricating additives part 1. an investigation of the friction-reducing behaviors and an
xps study of the tribochemical characteristics. Wear, 214(1) :47-53, 1998. 6. T. Yoshida. Bull. Chem. Soc. Jpn., 53:1327, 1980. Cited in NIST database. 7. G. Ertl and K. Wandelt. Electron spectroscopic studies of clean and oxidized iron. Surface Sci., 50:479-492, 1975. Cited in NIST XPS data base. 8. G.C. Allen, M.T. Curtis, A.J. Hooper, and P.M. Tucker. X-ray photoelectron spectroscopy of iron-oxygen systems. J. Chem. Soc. Dalton Trans., pages 1525-1530, 1974. Cited in NIST XPS data base, Fe, Fe(OH)O. 9. H. Konno and M. Nagayama. J. Electron Spectrosc. Relat. Phenom., 18:341, 1980. Cited in NIST XPS database, Fe(OH)O. 10. J.C. Carver J.C., G.K. Schweitzer, and T.A. Carlson. The use of x-ray photoelectron spectroscopy to study bonding in cr, mn, fe, and co compounds. J. Chem. Phys., 57:973-982, 1972. Cited in NIST XPS database, Fe 2 O 3 . 11. V.V. Nemoshalenko, V.V. Didyk, V.P. Krivitskii, and A.I. Senekevich. Zh. Neorg. Khimii, 28:2182, 1983. Cited in NIST XPS database, Fe 2 O 3 . 12. J.P. Riviere, M. Cahoreau, and P. Meheust. Chemical bonding of nitrogen in low energy high flux implanted austenitic stainless steel. Journal of Applied Physics, 91(10):63616366, 2002. 13. T.-C. Lin, G. Seshadri, and J.A. Kelber. A consistent method for quantitative xps peak analysis of thin oxide films on clean polycrystalline iron surfaces. Applied Surface Science, 119:83-92, 1997. 14. B.J. Tan, K.J. Klabunde, and P.M.A. Sherwood. An xps study of cobalt solvated metal atom dispersed (smad) catalysts. Chem. Mater., 2:186-191, 1990. Cited in NIST XPS database. 15. E. Paparazzo. Appl. Surf. Sci., 25:1, 1986. Cited in NIST XPS database, Fe 2 O 3 . 16. N.S. Mclntyre and D.G. Zetaruk D.G. Anal. Chem., 49:1521, 1977. Cited in NIST XPS database, Fe 3 O 4 , Fe(OH)O. 17. D. Brion. Appl. Surf. Sci., 5:133, 1980. Cited in NIST XPS database, Fe 3 O 4 , Fe(OH)O. 18. E. Kenesey and A. Ecker. How substituents
282 of heteroaromatics influence the lubricity of diesel fuel. In Tribology, Science and Application, Review Conference, April 23-27, pages 378-396, Vienna, 2003. 19. E. Kenesey and A. Ecker. Sauerstoffverbindungen zur verbesserung der lubricity in kraftstoffen. Tribologie und Schmiertechnik, 2:19-24, 2003.
269
Tribochemistry of mono molecular additive films on metal surfaces, investigated by XPS and HFRR R. Kolma*, I.C. Gebeshubera-b, E. Keneseya, A. Eckera, A. Pauschitza, W.S.M. Wernerb, and H. Storib a
Austrian Center of Competence for Tribology AC2T research GmbH., Viktor Kaplan-Strafie 2, 2700 Wiener Neustadt, Austria b
Institut fur Allgemeine Physik, Technische Universitat Wien, Wiedner Hauptstrafie 8-10/E134, 1040 Vienna, Austria In order to see how oxygen and nitrogen containing substances influence the wear relating behaviour of diesel fuels various hydroxychinolines were investigated by means of a high frequency reciprocating rig (HFRR) tribometer. Attention was especially given to the connection between the position of the hydroxyl group at the molecule and the resulting influence on the "lubricity" properties of low sulfur diesel fuel. Monomolecular lubricant films were deposited from the liquid phase onto ultra thin copper films sputtered onto silicon wafers. This substrate serves as a model system for bronze materials. In order to compare the results of X-ray photoelectron spectroscopy (also called electron spectroscopy for chemical analysis, ESCA) measurements with tribological experiments performed with the high frequency reciprocating rig method additional substrates made of 100Cr6 steel were used. X-ray photoelectron spectroscopy investigations were performed using a VG ESCALAB Mk III equipped with a special preparation chamber, permitting the transfer of samples from a fluid cell to the analysis chamber under Helium protective gas. The structure of the molecular film is elucidated using angular resolved X-ray photoelectron spectroscopy. Preliminary X-ray photoelectron spectroscopy investigations of ultra thin layers of 8-Hydroxychinoline on copper samples were performed. Angular resolved measurements demonstrated that, without tribological stress, full coverage of the surface with 8-hydroxyquinoline is not possible. Those results however still need corroboration by atomic force microscopy investigations.
1. INTRODUCTION Additives are of major interest in lubrication technology. They influence the properties of base oils in several aspects. Anti-wear and extreme pressure additives reduce abrasion and wear in boundary and mixed lubrication. Molecules of these additives, which are physisorbed or chemisorbed onto the surface, form films on the surface of the solids. Though some mechanisms of action are understood others still remain unexplained. One of the unexplained mechanisms are different lubrication properties of isomers of hydroxyquinoline. Whereas 2- and 8-hydroxyquinoline exhibit good lubrication properties other isomers do not. In order to acquire knowledge of the behavior of these additives at the interface between * e-mail: [email protected]
solids in contact, a fundamental understanding of the interaction of additive molecules with the surface is necessary. Since little is known in this field of research, exploratory results, as they are presented in this paper, are justified. It is known that isomers of hydroxyquinoline act as lubricants, how and why remains, however, to be investigated. Since the hydroxyquinoline layers are very thin, possible experimental techniques for investigation of their friction reducing properties are limited. The high frequency reciprocating rig technique (HFRR) can provide macrotribological information, atomic force microscopy (AFM) can investigate the tribology on the micro- and nanoscale, and X-ray photoelectron spectroscopy (XPS) can provide information about the structural chemistry. Presently, there is a need to understand the
270
Upper specimen
Fretting flexure lock
Lower specimen
LVDT and flexure housing / /
Electromagnetic vibrator
Counterweight
/
o \
Heater block Main RTD location hole (in far side of block)
Force transducer
Figure 1. Scheme of our High Frequency Reciprocating Rig device. The fretting flexure lock solely enables oscillation of the upper specimen, preventing all other movements. The linear and vertical displacement transducer (LVDT) is mounted in the flexure housing. Binding energy [eV] Cu 2p 3 / 2 2-Hydroxychinoline 933.3 4-Hydroxychinoline 933.2 8-Hydroxychinoline 933.0 Table 1 Binding energies of copper samples for
C Is C Is 286.8 285.4 287.1 285.4 X 285.2
N Is 399.3 399.2 399.0
O Is 533.3 533.2 533.1
O Is 530.8 530.8 530.1
several hydroxychinolines investigated by XPS.
underlying mechanisms of lubricity, as common lubricity promoting substances are removed due to environmental concerns. A prime example is modern, low sulphur diesel fuel. Natural components with 'lubricity' promoting properties are removed from diesel as a result of the severe hydrotreating process. Therefore it became necessary to add 'lubricity' enhancing substances to diesel with very low sulfur-contents. Many substances were tested for their applicability as anti wear additives. Oxygen and nitrogen containing substances show a good suitability as 'lubricity' enhancers. In 1986, Wei and Spikes reported about the
'lubricity' enhancing effect of various nitrogen containing heteroaromatics. Their studies also revealed that 8-Hydroxychinoline already has a notably wear reducing potential at a concentration of 100 ppm. [1] In 1992, Wei and Song investigated the wear behaviour of steel lubricated by some oxygencontaining derivatives. The antiwear activity shows its highest potential with substances containing a hydroxyl group. These results imply that aromatics with an aldo group have a slightly worse 'lubricity' behaviour compared to hydroxyl containing substances. The worst 'lubricity' results were obtained with carbonyl and
271 2 i
1,6 -
c 1s
2-19-1
• c 1s !
•
0,8 -
A
N 1s 0 1s O 1s
0,4 -
10
20
30
40
50
60
70
80
angle [°]
Figure 2. Intensities of various 2-Hydroxychinoline XPS peaks relative to the Cu 2p 3 / 2 peak intensity in the same spectrum, depending on the takeoff angle. In the case of C Is and 0 Is two different chemical binding states were evaluated. alkyloxyl substituents hanging at the ring system. In the case of 8-Hydroxychinoline, these authors experienced an enhancement of the 'lubricity' down to a lower concentration of 125ppm.[2] The results section describes similar results for various hydroxychinoline derivates. In 1998, Wei and coworkers reported 'lubricity ' experiments carried out by means of a four ball wear tester and a HFRR device. The substance class of heteroaromatics with a hydroxyl group represented in this study are 2Hydroxy-benzothiazole, 4-Hydroxybenzene and 8-Hydroxychinoline. The evaluation of the results indicates that the molecules with a hydroxyl group feature a significant improved 'lubricity' behavior and consequently far higher
wear protecting properties than the parent compound without a hydroxyl group. Subsequently, in order to understand the mechanisms and reactions which lead to this improved abrasion character the formed films were analyzed with XPS, Auger electron spectroscopy, ECR and optical microscopy. Wei et al. came to the conclusion that the hydroxyl group enhances the polymerforming tendencies of the parent compound. It also seems that the compounds substituted with a hydroxyl group raise the critical temperature which characterizes the desorption of additives from rubbing surfaces. [3] HFRR is the method of choice to investigate lubricity on a macroscopic scale and is widely used for the engineering of fuels, lubricants and en-
272
0,8 -
y---7*-
ID C 4>
c 0.4 •
0,2 -
10
20
30
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50
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70
80
angle [°]
Figure 3. Intensities of various 8-Hydroxychinoline XPS peaks relative to the Cu 2p3//2 peak intensity in the same spectrum, depending on the takeoff angle. In the case of O Is two different chemical binding states were evaluated. gine parts. In addition, XPS is used to obtain insight into the molecular mechanisms leading to the macroscopic lubricity.
2. MATERIALS AND METHODS Due to technical limitations, different types of samples were used for different experimental methods. While for HFRR the usual steel samples have been used, ultra flat copper samples as well as steel samples were used for XPS studies.
2.1. X-ray (XPS)
photoelectron
spectroscopy
XPS is a surface analytical method. In ultra high vacuum, the sample is irradiated with monochromatic X-rays. This causes the emission
of photoelectrons from the sample. These photoelectrons are separated by means of an electrostatic analyzer and detected. The kinetic energy of the photoelectrons depends on the atomic number and the chemical surroundings of an element. The shift in energy caused by neighboring atoms is called chemical shift and allows assignment of binding partners. Therefore XPS is most suited for compound specific analysis. Furthermore, XPS is highly surface sensitive due to the short decay length (attenuation length) of the signal, which usually is in the range of a few nanometers, and the exponential decay of the signal with sample depth. In angle resolved measurements (ARXPS), the angle between the surface normal of the sample and the analyzer (the emission angle) is varied,
273 Binding energy [eV] Fe 2p 3 / 2 Fe 2p 3 / 2 Fe 2p 3 / 2 Fe 2p 3 / 2 C Is C Is X 711.3 707.3 X X 100Cr6 steel 708.9 707.2 2-Hydroxychinoline 710.8 708.8 287.1 X 713.8 4-Hydroxychinoline 714.0 710.9 708.9 707.3 288.7 286.5 X X 8-Hydroxychinoline 713.7 710.8 708.8 707.2 N Is N Is O Is O Is Binding energy [eV] C Is X 100Cr6 steel 283.5 397.7 531.8 530.2 283.4 399.8 530.2 2-Hydroxychinoline X 531.8 401 4-Hydroxychinoline 283.4 398.7 531.9 530.2 X 8-Hydroxychinoline 283.3 400.1 531.7 530.2 Table 2 Binding energies of steel samples for several hydroxychinolines investigated by XPS. 0.8346g/cm3 0.8146g/cm3 Density at 15°C Density at 40°C 2 2 Viscosity at 40° C 4.7545mm /s Viscosity at 100° C 1.6851mm /s Table 3 Characteristic data of the base fluid used in the experiments therebye providing information about the depth distribution of the different compounds of a sample in their different chemical states. Taking all these into account, XPS is the method of choice to obtain information about organic molecules adsorbed onto solid surfaces, as is the case with additives interacting with tribological surfaces [4]. Copper samples were prepared by cutting about Icm2-sized slides from a silicon wafer and coating with 500nm of copper by sputtering. The surfaces of these slides were investigated with an atomic force microscope (MFP-3D, Asylum Research, Santa Barbara, CA). Then the slides were transferred into the preparation chamber (5 • W~ smbar) of the XPS. There, carbonaceous remnants as well as adsorbed gaseous contaminations were removed by argon ion sputtering. The purity of the samples were checked by XPS analysis prior to preparation of the additive films. 100Cr6 steel samples were prepared from sheets of 8xl2mm 2 , which were cut from a roll taken from a roller bearing. These sheets were ground with silicon carbide abrasive papers with grid sizes from 150 down to 4000 on a Struers LaboPol4 grinding machine. Afterwards, the sheets were polished on the same machine with a MD-Dac disk treated with DP-Spray P containing poly-
C Is 284.8 285.4 285.1 285.6
Density at 100°C
0.7746g/cm3
crystalline diamonds with a grain size of 1/im. After rinsing with deionized water, the sheets were transferred into the preparation chamber ((5 • \Q-&mbar)) of the XPS. There, argon ion sputtering was performed for half an hour with an acceleration voltage of 4kV and the surface was checked by XPS measurements. The additive films were prepared using 5 /zmol/ml solutions of 2-, 4- and 8-hydroxyquinoline in toluene (substances from Sigma-Aldrich). The solutions were transferred into the adsorption device, which is an extension of the XPS and is directly connected to the spectrometer in order to exclude atmospheric contaminations during the preparation and transportation of the samples. The adsorption setup used for all samples consists of two supply reservoirs. The first supply reservoir was used as storage for toluene cleaning the device, the second supply reservoir contained the solution of one of the hydroxyquinoline isomers, a pressure line, which was filled with He 6.0, to provide the pressure to transport the liquids into the adsorption vessel, and an adsorption vessel, where the samples were prepared. The whole device was permanently under a slight overpressure of He in order to keep it from being polluted. Furthermore, the Helium is used to degas the solutions.
274 274
140000 120000 CO r—
ity [cout
••«•
4) C
•*
100000 80000 60000 40000 -
w 20000 -
CO
o _* 1000
800
600
400
L
200
binding energy [eV]
Figure 4. XPS survey spectrum of clean 100Cr6 steel recorded using Mg Ka radiation. The clean copper sheets were transferred from the preparation chamber of the spectrometer directly into the adsorption vessel. There, they were dipped at room temperature for 50 minutes into 5 /imol/ml solutions of the adsorbates. Afterwards, the samples were blown dry with Helium and directly, without exposing them to environmental conditions, transferred into the preparation chamber of the spectrometer. All spectra of the copper samples were recorded using Al KQ radiation with a power of 600W. No monochromator was used. 100Cr6 steel samples were measured using non monochromatized Mg Ka radiation. The source was operated at a power of 300W. The measurements were performed using constant analyzer energy mode and choosing a pass energy of 50eV. The step width was adjusted to leV for survey spectra and to
O.leV for the various compound regions. An angular aperture of the analyzer of 10° was used for ARXPS. ARXPS spectra were recorded at emission angles of 15°, 28°, 47.6°, 60.6°, 70° and 75°. The emission angles were varied by rotating the sample, which was mounted on a sample holder with a 45° slope, around an axis with a fixed angle to the analyzer. To enable comparison of the angular intensity distribution, the intensity ratios were calculated in order to exclude geometric artefacts from the measurements.
2.2. High Frequency Reciprocating Rig (HFRR) The HFRR apparatus is based on a steel ball which is loaded against a stationary steel plate. The plate, which is embedded into the lower specimen holder, is fully immersed into the fuel to be tested. During the measuring process the ball is
275 330000 -
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en
1 230000 o
-100Cr6 2-HCH 4-HCH -8-HCH
u (A
180000 -
130000 -
80000 720
715
710
705
700
binding energy [eV]
Figure 5. Fe 2p 3 / 2 region of the photoelectron spectrum of layers of 2-, 4-, 8-Hydroxychinoline on 100Cr6 steel. For comparison a spectrum of clean 100Cr6 steel is also shown. oscillating with a certain frequency and is in constant contact with the steel plate (see Figure 1). A load of 200g is applied to the upper specimen holder, where the steel ball is embedded. The 200g load used for the HFFR device is given by the norm ISO 12156-1. This norm was introduced for testing the 'lubricity' behaviour of diesel fuel. The test results obtained with these parameters correlate to many fuel/hardware combinations and thereby provide good predictions of the lubricating quality of the fuel. The course of the measurement can be observed online by means of a computer program. This software enables the operator to follow the course of the film formation as well as the changes of the friction coefficient over the testing time. The formation of a lubricating film is indicated by the
electric potential existing between the steel ball and the plate. The built up lubricating film acts as an insulator and separates the test specimen, steel ball and the plate by a thin layer. After the test the abraded area on the ball is evaluated by means of an optical microscope. The observed dimensions are converted into a defined wear scar value by a formula. The WS 1.4 is a standardized measure for the 'lubricity' of diesel fuel systems. Additionally to the wear scar dimensions, the temperature and humidity at the beginning and the end of each testing procedure are also included into the evaluation of the Wear Scar 1.4. This measuring apparatus has many advantages compared to other 'lubricity' testing devices. For example, the low sample amount needed is very favourable if only small sample vol-
276
100Cr6 15° — 100Cr6 75° 2-HCH 15° - 2-HCH 75°
o
716
714
712
710
708
706
704
702
binding energy [eV]
Figure 6. Fe 2p 3//2 region of the photoelectron spectrum of layers of 2-Hydroxychinoline on 100Cr6 steel. The spectra are shown for two different electron takeoff angles. For comparison the spectra of clean 100Cr6 steel are also shown. Especially for the coated surface the different peak shapes for the nearly vertical (75°) and the grazing (15°) takeoffs are visible. umes are available. Hydroxychinolines were dissolved or dispersed into a low sulphur diesel with a sulphur content of less than lOppm. The test solutions with a concentration of 125ppm to 500ppm were consequently tested by a High Frequency Reciprocating Rig (HFRR) following the norm ISO 12156-1. 3. RESULTS 3.1. X P S Copper samples: The adsorption of the Hydroxychinolines onto the sputter coated samples turned out to take longer than expected. We had to put the copper sheets for 50 minutes into the solution of the hydroxychinolines in order to ad-
sorb a measurable amount of the hydroxychinolines onto the atomically flat surfaces. The survey spectra of all samples showed only photoelectron peaks of copper, oxygen, nitrogen and carbon as expected. Peak fitting was done with SPECTRA Presenter, a software provided by Ron Unwin. After Shirley background subtraction a combination of Gaussianand Lorentzian- peaks was fitted to the spectra. The positions resulting for the various compounds of the hydroxychinolines are listed in Table 1. The accuracy of the peak positions is about O.leV. The postion of the Cu 2p 3 / 2 photoelectron peak is nearly constant within few tenth of an eV and is little lower then that found for
277 0,35
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o 0,25
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— A-
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—A-
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—r— N1s
1-
x o
0,15 0,5 -
Z
0,1
0,05
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angle [°]
Figure 7. Intensity ratios of various 2-Hydroxychinoline XPS peaks relative to the 100Cr6 peak intensity in the same spectrum, depending on the takeoff angle. In the case of 0 Is two different chemical binding states were evaluated. copper 8-Hydroxychinoline additives after friction [5]. The lowest binding energy was found for 8-Hydroxychinoline and the highest for 2Hy droxychinoline. Alike behaved the binding energies for the N Is photoelectrons. The position of the N Is peak of 8-Hydroxychinoline is like the one of the Cu 2p 3 / 2 peak 0.6eV lower than that found by Yu and co-workers after friction [5] but is in good agreement with the binding energy measured for N Is in 8-Hydroxychinoline by Yoshida [6]. The 0 Is photoelectron peak was composed of two peaks. The one at a binding energy above 533eV was equal within few tenth of an eV for all hydroxychinolines. The other at lower binding energy varied from 530.leV for 8-Hydroxychinoline to 530.8eV for both 2-Hydroxychinoline and 4-
Hydroxychinoline. The C Is photoelectron peak was found to consist of one peak located at about 285.4eV for all of the hydroxychinolines and a second peak occurring at higher binding energy for 2-Hydroxychinoline and 4-Hydroxychinoline indicating a different chemical species of carbon. ARXPS showed for 2-Hydroxychinoline that the ratio of the area of the C Is located at 286.8eV to the Cu 2p 3 / 2 had the same characteristics as the N Is and the O Is at 530.8eV indicating the same depth distribution (Figure 2). Alike the angular distribution of the intensity ratios of the C Is at 285.4eV showed the same angular dependence as the O Is intensity ratio at 533.3eV. The intensity ratios of the two different species of carbon had different angular distribution. Where the intensity ratio of the C
278
• 8-Hydroxychinoline 500 ppm • 8-Hydroxychinoline 250 ppm • 8-Hydroxychinoline 125 ppm • 2-Hydroxychinoline 500ppm • 2-Hydroxychinoline 250ppm D 2-Hydroxychinoline 125ppm • 4-Hydroxychinoline 250ppm D 5-Hydroxychinoline 250ppm • 6-Hydroxychinoline 500ppm • 6-Hydroxychinoline 250ppm
Figure 8. Film formation of various tested hydroxychinolines. A complete film would act as an insulator. Is located at the higher binding energy became lower at the highest emission angle whereas the other rose constantly. With 4-Hydroxychinoline no specific angular dependence was found. For 8-Hydroxychinoline, the intensity ratios of C Is, N Is and the O Is located at 533.leV showed the same characteristic. They rose steadily whereas the intensity ratio of the O Is located at the lower binding energy declined from 70° to 75° indicating that the compound is located below the other compounds (Figure 3).
100Cr6 steel samples: The survey spectrum of the 100Cr6 (Figure 4) steel samples after half an hour of sputtering shows photoelectron peaks from iron, carbon, nitrogen and oxygen. The positions were taken from peak-fits done with the software mentioned above after Shirley background subtraction using a minimum amount of peaks composed of Gaussian- and Lorentzianpeaks and are listed in Table 2. The peak at 707.3eV was attributed to pure iron [7,8]. The one at 711.3eV was assigned to iron bound to oxygen [9-11]. Other peaks in the 2p 3//2 photoelectron of iron could not be definitely assigned. The peak at about 708.8eV may result from Fe bound to nitro-
gen as assigned by Riviere and co-workers [12] to a peak at 708.2eV. The peak at the highest binding energy was found by Riviere et al. too but was not assigned either [12]. This peak may result from shake-up satellite features, which occur at binding energies 5 to 8eV higher than the respective 2p main core lines [13]. The C Is peak as well as the O Is peak consisted of two peaks. The position of the O Is peak in iron oxides is reported to change only little with the oxidation value and is located at 529.9eV [14-17,8], which lies somewhat lower then the binding energy of 530.2eV we found. The second oxygen peak was ascribed to oxygen in hydroxyl groups [13]. Further nitrogen was found on one of the steel samples. The Fe photoelectron region of 2Hydroxychinoline showed a completely different intensity ratio of the iron and the iron oxide peak (Figure 5). The loss in intensity of the iron peak was due to coverage of the surface with the adsorbed hydroxychinoline, while the iron oxide which overlay the iron was less influenced by the covering layer. At an emission angle of 75° the Fe metal peak vanished in the spectra recorded from the samples with hydroxychinoline
279
Substance
Concentration [ppm]
WS 1.4
av. film [%]
242 92 8-Hydroxychinoline 500 92 280 8-Hydroxychinoline 250 392 71 8-Hydroxychinoline 125 167 91 2-Hydroxychinoline 500 193 92 2-Hydroxychinoline 250 223 90 2-Hydroxychinoline 125 467 19 4-Hydroxychinoline 250 464 17 5-Hydroxychinoline 250 384 72 6-Hydroxychinoline 250 429 6-Hydroxychinoline 500 38 Table 4 HFRR results for various hydroxychinolines at various concentrations.
8-Hvdroxychinoline
4-Hydroxychinoline
2-Hvdroxychinoline
6 -H vdroxvohinoline
5-Hydroxvchinoline
Friction 0.157 0.158 0.19 0.18 0.169 0.182 0.315 0.29 0.213 0.227
The survey spectrum of 4-Hydroxychinoline showed an additional Nls peak resulting from the nitrogen, which was found on the steel sheet to which 4-Hydroxychinoline was adsorbed. Further four different Fe 2p 3/2 and C Is peaks were fitted. Two oxygen peaks, which all hydroxychinolines had in common, showed the same angular dependency as found for the other hydroxychinoline samples. One on top with a depth distribution like that of nitrogen and the other below bound to iron. The two nitrogen species showed a completely different behaviour in their ARXPS spectra. In the spectrum of 8-Hydroxychinoline only two C Is peaks appeared. The intensity of the nitrogen peak and the oxygen peaks behaved like that of the other hydroxychinoline samples.
3.2. HFRR Figure 9. Molecule models for 8-, 4-, 2-, 6- and 5-Hydroxychinoline.
adsorbed (Figure 6). Angular resolved measurements showed that the two oxygen species were situated in different heights. The one located on top appeared to have a similar depth distribution as nitrogen. Whereas the depth distribution of the underlying one corresponded to depth distribution of the peak assigned to iron oxide (Figure 7).
The literature describes 8-Hydroxychinoline as a 'lubricity' additive with an advantageous wear reducing character as well as a high film formation potential [1-3]. In order to investigate the wear relating properties of other hydroxychinolines several tests were carried out (see also the two papers by Kenesey and Ecker, 2003 [18,19]). The aim of these experiments also included investigations on the influence of the location of the hydroxyl group and the nitrogen atom on the behavior of the quinoline compound. The base fluid used for the tests, is a highly refined diesel fuel with a sulphur content of less
280 than lOppm. Measurements of the pure diesel by means of a HFFR showed a WS of 521 pm and 510/im. Density and viscosity at various temperatures for the base fluid is given in Table 3. High purity hydroxychinolines (2Hydroxychinoline with a purity of 99%, 4Hydroxychinoline and 5-Hydroxychinoline with purities of 98%, 6-Hydroxychinoline with a purity of 98% and 8-Hydroxychinoline with a purity of 95%) were dispersed into low sulphur diesel with a concentration of 500ppm for the lubricity tests. The compounds were dispersed and dissolved at a temperature of approximately 60° C for 30 minutes with a magnetic stirrer. For most of the substances, the tests were carried out only once, just in doubtful cases a second or third reference measurement was performed. The average values are given Table 4. The experiments show for 8-Hydroxychinoline and 2-Hydroxychinoline an excellent behavior regarding all tested parameters. In both cases the first test concentration used was 500ppm. The concentration was then gradually reduced to 125ppm. The evaluation indicated for 8Hydroxychinoline as well as 2-Hydroxychinoline still wear reducing properties at this low concentration. In this case 2-Hydroxychinoline presented almost the same low abrasion as experienced with 250ppm. After these outstanding results the further tested hydroxychinolines were mainly investigated at a concentration of 250ppm. However these compounds resulted only in unsatisfactory wear behavior at this concentration. 4-Hydroxychinoline and 5-Hydroxychinoline had a very high wear scar. In contrast to this, 6Hydroxychinoline showed a middle WS 1.4 of 384/wn at a concentration of 250ppm. In order to investigate if the application of a larger amount would improve the results achieved the experiment was repeated at 500ppm. In this case the wear relating properties severely deteriorated. The film formation potential of 6Hydroxychinoline has a value of 72% at a concentration of 250ppm . However, at a level of 500ppm the potential of this property diminishes to 38%. So it could be said that 6-Hydroxychinolin has a clear pro -wear effect at the higher concentration, a fact which is observed in the wear scar value
and as well as in film formation potential. The film formation ability in case of 8Hydroxychinoline decreases only slowly with decreasing concentration. In the case of 2Hydroxychinoline, the potential for film formation stays almost constant at a value of 90% for all three measured concentrations. 4- and 5Hydroxychinoline present with 19% and 17% very poor results and provide consequently almost no 'lubrication film' (Figure 8).
4. DISCUSSION, CONCLUSION AND OUTLOOK The differences in tribological bahaviour of the various hydroxiquinolines have been observed by HFRR and the chemical structure of the film by XPS. Unfortunately, HFRR data could only be taken on steel, while the most conclusive XPS results were obtained on sputtered copper films, since the pure copper and steel samples suffer from residual roughness. Further detailed studies are necessary to conclusively link macrotribological results with information on structural chemistry obtained by XPS. Another method we will apply is atomic force microscopy in the controlled environment of a closed fluid cell. These AFM investigations (performed with a MFP-3D from Asylum Research with top view optics) will permit not only topographic investigations, but also force measurement with pico-Newton resolution in two axes (vertical and one horizontal axis). Furthermore, force spectroscopy will permit determinination of the influence of the monomolecular lubricant layer on attractive and lateral forces. An interesting fact is that it takes more than one hour for the film to strongly adhere to the substrate. Nevertheless, long time needed for strong adhesion is no indication for enhanced lubricity. The question whether the XPS spectra truly reflect microscopic causes for different tribological bahaviour as observed with HFRR can therefore not yet be conclusively answered. The photoelectron peaks and binding energies can easily be assigned for the copper samples. Furthermore, the depth distribution of the com-
281 pounds can be determined. In contrast to the copper samples, the interpretation of the spectra recorded from the 100Cr6 samples is more difficult. Our results indicate that only hydroxychinoline derivates featuring the hydroxyl group next to the nitrogen atom possess good to excellent 'lubricity' enhancing properties (Figure 9). Considering the steric configuration of these substances the conclusion could be drawn that there exists a synergism between the nitrogen atom and the hydroxyl group next to it. This effect leads to an outstandingly good film formation potential and consequently to a lower friction coefficient.
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