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Growth of Tribological Films: In Situ Characterization Based on Attenuated Total Reflection Infrared Spectroscopy Federica M. Piras,† Antonella Rossi,†,‡ and Nicholas D. Spencer*,† Laboratory for Surface Science and Technology, Department of Materials, Swiss Federal Institute of Technology, ETH Zu¨ rich, CH-8092 Zu¨ rich, Switzerland, and Department of Inorganic and Analytical Chemistry, University of Cagliari, Cagliari, Italy Received March 20, 2002. In Final Form: May 21, 2002 In this paper we describe the development of a new technique based on attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy and its application to the in situ study of lubricant system behavior under tribological conditions. The lubricated tribological system consisted of a fixed steel cylinder, sliding across the surface of a germanium ATR crystal, coated with a thin iron film in the presence of the well-studied secondary zinc dialkyl dithiophosphate (ZnDTP) lubricant additive. Using this approach, changes in the additive chemistry due to adsorption, as well as reaction film growth, can be studied as a function of sliding time and temperature. The ATR FT-IR spectra reported in this work are fully consistent with the existing ZnDTP literature and show the decomposition of ZnDTP with the formation of P-O-P species following thermal testing at 150 °C, while a simple phosphate film has been detected on the iron surface following tribological testing at the same temperature.
Introduction In most mechanical systems (transport, energy production, manufacturing), lubricants are used to reduce friction and wear between moving parts. Lubricants generally consist of mineral or synthetic oils and contain low concentrations of different additives, including chemical compounds that adsorb on or react with the metallic surface to produce an organic and/or inorganic thin layer, which reduces wear and friction under conditions of boundary lubrication, where sliding speeds are too low or loads too high for a full fluid lubricating layer to be mantained.1 The analysis of these reaction films is of crucial importance for a better understanding of the mechanism of action of the additives. Most of the analytical techniques currently used for investigating the tribochemistry of additive-derived films that are formed in tribostressed systems are performed ex situ, i.e., outside the tribometer and after the friction test. Tribofilms analyzed in this way are not necessarily representative of the films in their active state, and monitoring changes with time, temperature, or other variables becomes more difficult. Although challenging, the in situ and in vivo2 (also called in lubro3) surface chemical analysis of lubricated tribological systems during a friction test is extremely useful, since it provides information on the chemistry taking place under steadystate conditions.4 The few reported studies involving in situ and in vivo analysis have involved optical methods and vibrational spectroscopy in particular.3,5,6 These * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +41-1-6325850. Fax: +411-6331027. † ETH Zu ¨ rich. ‡ University of Cagliari. (1) Ha¨hner, G.; Spencer, N. D. Phys. Today 1998, 51, 9, 22. (2) Donnet, C. Handbook of Surface and Interface AnalysissMethods for Problem-Solving; Rivie`re, J. C., Myhra, S., Eds.; Marcel Dekker Inc.: New York, 1998; Chapter 2. (3) Cann, P. M.; Spikes, H. A. Tribol. Trans. 1991, 34, 2, 248. (4) Martin, J. M.; Le Mogne, Th.; Grossiord, C.; Palermo, Th. Tribol. Lett. 1996, 2, 313.
studies have several limitations: infrared microreflection absorption spectroscopy3 is not a true surface-sensitive technique and is not effective for studies in the boundary lubrication regime, while the use of a diamond anvil cell6 only allows static measurements to be made. Attenuated total reflection (ATR) spectroscopy is one of the most widely used techniques for surface infrared analysis. Although the phenomenon of total internal reflection of light was described by Newton in the early 17th century, it was not until much later that Harrick7 and, independently, Fahrenfort8 were to exploit this phenomenon to obtain absorption spectra and develop the ATR technique. When applied to the study of in situ kinetics of adsorption and reaction of species at liquid/ solid interfaces, ATR spectroscopy can yield valuable surface-chemical data. Such studies have been carried out in a variety of research and technological areas, including biomembranes,9 biofilms,10 thin film structure and reactivity,11,12 and electrochemistry.13 The IR radiation propagating in the ATR element (the optically denser medium) undergoes total internal reflection at the interface with the sample (the optically rarer medium) because of the different refractive indices of the two media (Figure 1). There is, however, an exponentially decaying electromagnetic wave that penetrates into the rarer mediumsthe “evanescent wave”sinteracting with (5) Cann, P. M.; Spikes, H. A. Lubr. Eng. 1991, 48, 335. (6) Westerfield, C.; Agnew, S. Wear 1995, 181-183, 805. (7) Harrick, N. J. Internal Reflection Spectroscopy; Interscience Publishers, John Wiley & Sons Inc.: New York, 1967. (8) Fahrenfort, J. Spectrochim. Acta 1961, 17, 698-709. (9) Fringeli, U. P.; Goette, J.; Reiter, G.; Siam, M.; Baurecht, D. In Fourier Transform Spectroscopy: 11th International Conference, AIP Conference Proceeding; de Haseth, J. A., Ed.; American Institute of Physics: Woodbury, NY, 1998; Vol. 430, p 729. Wenzel, P.; Fringeli, M.; Goette, J.; Fringeli, U. P. Langmuir 1994, 10, 4253. (10) Ishida, K. P.; Griffiths, P. R. J. Colloid Interface Sci. 1999, 213, 513. (11) Chovelon, J. M.; Gaillard, F.; Wan, K.; Jaffrezic-Renault, N. Langmuir 2000, 16, 6228. (12) Fanucci, G. E.; Talham, D. R. Langmuir 1999, 15, 3289. (13) Zhu, Y.; Uchida, H.; Yajima, T.; Watanabe, M. Langmuir 2001, 17, 146.
10.1021/la0202733 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/24/2002
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Figure 1. Scheme of the principle of ATR spectroscopy.
molecules in the vicinity of the surface, and thereby attenuating the reflected intensity and yielding an infrared spectrum similar to that obtained from a transmission experiment.7,14 For radiation in the mid-IR range (4000400 cm-1), the penetration depth (dp), which is a function of the wavelength, typically ranges from 0.1 to 1 µm, and thus standard ATR FT-IR spectroscopy can be regarded as a moderately surface-sensitive analytical technique. As Jakobsen has demonstrated,15 ATR spectra can be obtained from metal-coated ATR crystals to study reactions at the liquid/metal interface. One of the applications reported in his work concerns the investigation of the adsorption of stearic acid onto a thin film of iron, sputter deposited onto a Ge ATR crystal. Applications of “metalcoated” ATR spectroscopy to study the adsorption process and structural changes at the iron/inhibitor interface16 or to perform studies of thin copper films exposed to aqueous solutions17 have also been reported. In the present study, we used this ATR FT-IR approach with tribological experiments to probe the lubricant layer between sliding surfaces by measuring ATR spectra from the underside of a thin metallic film. In this way, the metal surface/lubricant interface can be continuously monitored, in situ, and in a nondestructive manner during a complete tribological sliding experiment. Zinc dialkyl dithiophosphates (ZnDTPs) are widely used as extreme pressure and antiwear additives in many different kinds of engine and industrial lubricants. It is known that ZnDTP forms tribological films on rubbing metal surfaces; it has been proposed that these films consist of amorphous polyphosphates, but the exact chemical composition of the different polyphosphates in the ZnDTP tribofilm is not known, and a generally accepted reaction mechanism has not emerged to date. Most authors believe that thermal decomposition is the major mechanism of ZnDTP tribofilm formation; as a result only tribological experiments conducted at elevated temperatures (60-200 °C) are, typically, reported in the literature.18-22 All the evidence obtained so far for substantiating the amorphous polyphosphate model has been based on ex situ experiments. Our aim in this work was to develop in situ ATR spectroscopy as an analytical method for investigating tribological systems, thus using ATR FT-IR spectroscopy (14) Mirabella, Jr., F. M. Internal Reflection SpectroscopysTheory and Applications; Marcel Dekker: New York, 1993. (15) Jakobsen, R. J. In Fourier Transform Infrared Spectroscopy, Applications to Chemical Systems; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New York, 1978; Vol. 2, p 165. (16) Incorvia, M. J.; Haltmar, W. C. J. Electrochem. Soc. 1986, 133, 8, 41. (17) Ishida, K. P.; Griffiths, P. R. Anal. Chem. 1994, 66, 522. (18) Suominen Fuller, M. L.; Kasrai, M.; Bancroft, G. M.; Fyfe, K.; Tan, K. H. Tribol. Int. 1998, 31, 10, 627. (19) Martin, J. M. Tribol. Lett. 1999, 6, 1. (20) Bec, S.; Tonck, A.; Georges, J. M.; Coy, R. C.; Bell, J. C.; Roper, G. W. Proc. R. Soc. London, A 1999, 455, 4181. (21) Bell, J. C.; Delargy, K. M.; Seeney, A. M. In Proceedings of the 18th Leeds/Lyon Symposium, Wear Particles; Dowson, D., et al., Eds.; Elsevier Science Publishers B.V.: New York, 1992; p 387. (22) Choa, S. H.; Ludema, K. C.; Potter, G. E.; DeKoven, B. M.; Morgan, T. A.; Kar, K. K. Wear 1994, 177, 33.
Figure 2. Schematic (a, top) and photograph (b, bottom) of the ATR tribometer for the in situ chemical analysis of tribological films.
to effectively be able to “see through” one of the rubbing surfaces (present as a thin iron film) and probe the lubricant layer. By means of this approach, changes in lubricant chemistry and the growth of reaction films have been studied in situ and as a function of sliding time and temperature. To determine the usefulness of the in situ ATR tribometry, the experiments presented in this work have been performed in the presence of a well-studied system: ZnDTP on iron/steel.18-22 Of particular interest in the ZnDTP lubricant-additive system studied is a greater understanding of the kinetics of formation of tribofilms and the relative roles of tribochemical and thermochemical processes. Experimental Section ATR Tribometer. An ATR FT-IR spectrometer has been equipped with a special tribometer (Figure 2), designed and constructed in our laboratory in Zu¨rich. Trapezoid ATR elements of monocrystalline germanium with an angle of incidence of 45°, dimensions 72 × 10 × 6 mm (seven reflections), were used in this work. Germanium was chosen as the ATR element because of its favorable mechanical properties, high refractive index, and chemical resistance to many solvents. The crystal can be coated with different metals by magnetron sputtering, constituting one (fixed) sliding partner of the tribological system with a flat surface of 72 × 10 mm. The moving part of the tribological system is a fixed cylinder (diameter 10 mm, width 7 mm) sliding in a reciprocating motion across the metal-coated germanium crystal surface. The line contact between the cylinder and flat surface is thus swept back and forth along the entire length of the crystal. To move the cylinder along the crystal, an electric motor with an extremely high gear ratio (8640:1) is used. The resulting slow circular motion is transformed into sliding by an aluminum bar connected at one end to the plate of the motor and at the other end to the cylinder holder. The average sliding velocity of the cylinder is typically between 20 and 200 mm/min. The normal load on the contact region is applied by applying different weights on the aluminum rig. In this way, variations in the ATR FT-IR spectrum due to tribochemical reactions occurring between the lubricant and the contacting surfaces can be investigated as a function of tribological conditions.
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Table 1. Transmission and ATR FT-IR Experimental Conditions mode
detector
resolution (cm-1)
no. of scans
gain control
acquisition time
scan velocity
transmission ATR (Ge) ATR (Ge/Fe)
DTGS/KBr MCT/A MCT/A
2 2 2
32 32 1024
1 1 8 (auto)
1 min, 9 s 45 s 15 min
0.6329 2.5317 2.5317
Heating of the crystal surface of the ATR tribometer was performed by means of a heatable top-plate for the BenchmarkATR (Portmann Instruments AG, Biel-Benken CH, Switzerland), heatable to 200 °C, with power provided by a temperature controller (FCS-23A, Shinko Technos Co., Ltd., Japan). The temperature was measured with a PT-100 thermocouple. In this way, both purely thermochemical reactions and tribochemical reactions can be studied at elevated temperatures. The experiments in this work were performed with iron, sputter-deposited onto a Ge crystal, a steel cylinder as the sliding part, and commercial ZnDTP as a lubricant. The normal load applied onto the cylinder was 7 N, the Hertzian contact area 0.1 mm2, the average pressure 34 MPa, and the mean sliding velocity 24 mm/min. The tribological tests performed in standard tribometers and reported in the literature have been carried out under a range of tribological conditions, between 26 and 100 MPa of mean pressure, and between 100 and 750 mm/s sliding velocity.18-21 Thermal Tests. Thermal tests were performed in the ATR tribometer, without contact with the slider, under purely thermal conditions at room temperature, 80 °C, and 150 °C for up to 38 h. ATR FT-IR spectra were acquired periodically during the experiment, high-temperature thermal tests requiring cooling of the tribometer to room temperature prior to the spectra being acquired. Tribological Tests. Tribological tests were carried out at room temperature, 80 °C, and 150 °C, while the steel cylinder was slid across the iron-coated germanium ATR crystal surface. Tribological tests at room temperature and 80 °C were performed for up to 90 h, whereas tribotests at 150 °C were performed for up to 20 h. During the tribological tests the ATR spectra were collected periodically, after the tribometer was cooled to room temperature for measurements when the tribotest was carried out at high temperature. Materials. The lubricant additive used was a commercial secondary ZnDTP (Hitec 7169, Ethyl Petroleum Additives International, England), purified by liquid chromatography. The composition of the purified secondary ZnDTP has been checked by both elemental analysis23 and XPS quantitative analysis,24 and found to correspond to the molecular formula C18H40O4P2S4Zn, suggesting a mixture of diisopropyl ZnDTP and hexyl ZnDTP. The thermal and tribological experiments reported in this work were carried out in the presence of the pure ZnDTP. Transmission spectra of diisopropyl ZnDTP (pure, synthesized at the Institute Franc¸ ais du Petrole), FePO4‚xH2O (Alfa, Karlsruhe, Germany), and Zn3(PO4)2‚xH2O (Strem Chemicals, Newburyport, MA) were collected as references. Cleaning of the ATR FT-IR tribometer was performed with cyclohexane p.a. (g99.5%, Fluka, Buchs, Switzerland). The germanium crystals and the top-plates were cleaned with petroleum ether (technical grade) and ethanol p.a. (>99.8%) (Merck, Dietikon, Switzerland). To remove the Fe coating after each test, the germanium crystals were cleaned in a solution of 6 M HCl (HCl fuming 37%, puriss. p.a., Fluka), first with a soaked tissue and afterward by immersion in the HCl solution for at least 15 min. Iron Coating. The iron was coated onto the germanium crystals, by means of magnetron sputtering, at the Paul Scherrer Institut (Villigen, Switzerland). A planar iron magnetron sputtering target (ISO 9001 Certified, target type PK 75) with a metallic purity >99.9% was used. The argon pressure during sputtering was 2.4 × 10-3 mbar, the sputtering rate being about 10 Å/s. The thickness and homogeneity of the iron coating were (23) Piras, F. M.; Rossi, A.; Spencer, N. D. In Proceedings of the 28th Leeds/Lyon Symposium, Boundary and Mixed Lubrication: Science and Applications; Dowson, D., et al., Eds.; 2002, p 72. (24) Piras F. M.; Rossi, A.; Spencer, N. D., submitted.
Figure 3. Transmission FT-IR spectra of diisopropyl ZnDTP and commercial ZnDTP. tested by ellipsometry prior to each run and found to be 12.0 ( 0.3 nm. The purity of the iron surface was checked by XPS, and the presence of a thin iron oxide film detected. XPS analysis was carried out on the iron-coated germanium crystals after each experiment. After sliding at 150 °C on the iron surface, three areas were visible (contact area/wear scar/ noncontact area), differing both in morphology and in chemical composition, as indicated by imaging XPS.24 FT-IR Spectrometer. Transmission FT-IR and ATR spectra were obtained with a single-beam Nicolet Magna-IR System 550 Fourier transform spectrometer, equipped with a Greasby-Specac advanced overhead (specaflow) 1401 Series ATR system. The experimental conditions are listed in Table 1. Sampling in transmission was performed by placing one drop of the pure samples between KBr windows. Data Processing of ATR FT-IR Spectra. A background correction always had to be applied to the experimental spectra, due to the single-beam acquisition mode. The background was acquired at the beginning of each experiment. In the case of the ATR spectra that were to be compared with transmission spectra (only Figure 4), an ATR correction routine was used to allow for the variation in penetration depth by multiplying the sample spectrum by a wavelength-dependent factor to correct the relative peak intensities.25 The standard ZnDTP ATR spectra collected on an iron-coated germanium crystal were corrected for the baseline after the ATR correction (only Figure 4). All the other ATR spectra presented in this work (Figures 5-7) are reported without ATR correction, as acquired.
Results Transmission and ATR FT-IR Spectra. The transmission FT-IR spectra of the diisopropyl ZnDTP reference compound and the commercial ZnDTP (Figure 3) clearly reveal the characteristic peaks of the ZnDTP molecule. The IR peak assignments for the transmission spectra of ZnDTP are reported in Table 2. Four regions can be distinguished: (1) the region at higher wavenumbers (around 2900 cm-1)ssymmetric and asymmetric stretching vibrations of CH3, CH2, and CH; (2) the region at around 1400 cm-1sbending modes of CH3 groups; (3) the region at around 1000 cm-1 with the most intense peak at 978 cm-1 (commercial ZnDTP) and 971 cm-1 (diisopropyl ZnDTP) of the P-O-(C) bond; (25) Nicolet’s OMNIC ESP spectroscopy software, version 4.1a.
functional group
νas(CH3), νas(CH2),ν(CH), νs(CH3) 26,31 δas(CH3)29 δs(CH3)26,29,31,a ν(CO(P))27,28,a ν(PO(C))26-28 ν(CC)28,33 νas(PS)26,27,30 νs(PS)26,27,30
ATR (Ge/Fe, 10 nm)
2974, 2955,2930, 2870 w/m 1463, 1451 w 1383, 1371 w 1176, 1158,b 1140, 1120, 1099b w 1022 vw,961 s 886w
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Figure 4. Transmission FT-IR and ATR spectra of commercial ZnDTP. The ATR spectra have been collected on an uncoated germanium crystal and on iron-coated germanium crystals; the thicknesses of the iron films were 7, 10, 15, and 20 nm.
aCharacteristics
of the isopropyl group.26,28
b
Peaks not yet assigned.
2976, 2958, 2934, 2871 m/s 1467 m, 1454 (sh) 1385, 1373 m 1178,1160,b 1141, 1120, 1103b m 1022 (sh),969 vs 889 m 2976, 2958, 2934, 2871 m/s 1468 m,1454 (sh) 1386, 1373 m 1178, 1160b, 1141, 1120, 1103b m 1022 (sh),978 vs 889 m 658 s 539 m br νas(CH3),ν(CH), νs(CH3)26,31 δas(CH3)29 δs(CH3)26,29,31,a ν(CO(P))27,28,a ν(PO(C))26-28 ν(CC)28,33 νas(PS)26,27,30 νs(PS)26,27,30 2978, 2931, 2872 s/m 1466, 1452 m 1386,1375 s1347 m 1180, 1142,1102 s 1024 (sh), 997(sh), 971 vs 887 s 651, 636 s 546 (sh),532 s
commercial ZnDTP
ATR (Ge) transmission functional group
diisopropyl ZnDTP
transmission
Table 2. IR Frequencies (cm-1) and Functional Groups for the Transmission FT-IR Spectrum of Diisopropyl ZnDTP and for the Transmission and ATR FT-IR Spectra of Commercial ZnDTP
Growth of Tribological Films
(4) the region below 700 cm-1sstretching P-S bands. All vibrational frequencies assigned in Table 2 are in agreement with the literature.26-31 Some minor peaks have not yet been assigned. By comparing the transmission spectra of the model compound diisopropyl ZnDTP with the commercial ZnDTP, it can be observed that all the characteristic peaks of the diisopropyl ZnDTP compound are also present in the spectrum of the commercial product (Table 2). However, differences are observed in the positions and intensities of the peaks, and the commercial ZnDTP spectrum shows more peaks in the regions around 2900 and 1200 cm-1. A comparison of the transmission spectra of commercial ZnDTP with the ATR FT-IR spectra on uncoated and on iron-coated (7-20 nm) germanium ATR crystals are shown in Figure 4. The IR peak assignment for commercial ZnDTP ATR FT-IR spectra are reported in Table 2 (columns 4 and 5). As can be seen in Figure 4, all the characteristic peaks of the ZnDTP transmission spectrum are also present in the ATR mode, with the exception of the peaks in the region below 650 cm-1, where the strong absorbance of germanium obscures all other signals. Thus, the P-S region (Table 2) cannot be analyzed by ATR FTIR if a germanium ATR element is involved. In addition, in the ATR spectrum collected with an uncoated germanium crystal, the peak assigned to the P-O-(C) stretching vibration is shifted to lower wavenumbers (969 cm-1) compared to that in the transmission spectrum (978 cm-1). This shift to lower wavenumbers is even more pronounced in the ATR spectrum collected with the (7 nm) iron-coated germanium crystal (963 cm-1). Furthermore, this peak is skewed toward low wavenumbers in the ATR (Ge/Fe) spectrum, while it is skewed toward high wavenumbers in the other two spectra. All signals in the ATR spectrum collected on the iron-coated germanium crystal show a shoulder at lower wavenumbers, and their relative intensities are different with respect to the ATR spectrum collected on an uncoated germanium crystal. This effect (26) Gallopoulos, N. E. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1966, 11, 21. (27) Harrison, P. G.; Begley, M. J.; Kikabhai, T. J. Chem. Soc., Dalton Trans. 1986, 5, 929. (28) Thomas, L. C.; Chittenden, R. A. Spectrochim. Acta 1964, 20, 489. (29) Nadler, M. P.; Nissan, R. A.; Hollins, R. A. Appl. Spectrosc. 1988, 42, 634. (30) Jiang, S.; Dasgupta, S.; Blanco, M.; Frazier, R.; Yamaguchi, E. S.; Tang, Y.; Goddar, W. A., III. J. Phys. Chem. 1996, 100, 15760. (31) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press Inc.: Boston, 1991; Chapter 16.
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Figure 5. Commercial ZnDTP ATR spectra collected during thermal tests at room temperature (a, top) and at 150 °C (b, bottom) on germanium ATR crystals coated with iron (10 nm). ATR and baseline corrections have not been applied.
has been confirmed by studying the influence of the thickness of the iron coating on the ATR FT-IR spectra of commercial ZnDTP (Figure 4). The intensities of the characteristic peaks of ZnDTP decrease with increasing thickness of the iron coating, and the peak assigned to the P-O-(C) stretching vibration (978 cm-1) is shifted to progressively lower wavenumbers (960 cm-1 for a 20 nm iron coating). Germanium crystals coated with a 10 nm thick iron film have been used to perform the adsorption and tribotests reported in this work. It has been found that the thickness of 10 nm for the iron film is ideal, since it is thick enough to prevent damage to the germanium crystal surface during sliding, but thin enough to allow ATR measurements of the samples deposited on the iron surface. Thermal Tests. ATR FT-IR spectra were collected during thermal tests of commercial ZnDTP on a Ge crystal, coated with 10 nm of iron. At room temperature (Figure 5a), no differences in the spectra in either intensity or peak position were found up to 38 h. The same behavior was found for a thermal test at 80 °C (not shown). Working at higher temperatures becomes increasingly difficult due to the strong reduction in the transmittance of the Ge crystal, since the useful transmission range becomes narrower with increasing temperature. At 150 °C, germanium becomes opaque at wavenumbers below 1600 cm-1. As is shown in Figure 7, the temperature effect is more pronounced when the Ge crystal is coated with an iron film; at 150 °C the transmittance of germanium is almost zero over the whole spectral range. Thus, when adsorption or tribological reactions are studied at temperatures above ca. 150 °C, the whole system has to be
Piras et al.
Figure 6. ATR spectra of commercial ZnDTP ATR acquired during tribological tests at room temperature (a, top) and at 150 °C (b, bottom) on iron-coated (10 nm) germanium ATR crystals. ATR and baseline corrections have not been applied.
cooled to near room temperature to perform ATR FT-IR measurements. In Figure 5b, the ZnDTP ATR FT-IR spectra collected at different times during a thermal test performed at a temperature of 150 °C on a Ge crystal coated with Fe (10 nm) are shown. The spectrum of commercial ZnDTP at the beginning of the experiment (0 h) shows the characteristic peaks of the ZnDTP molecule (Figure 4). The P-O(C) stretching vibration peak (961 cm-1) can be clearly seen. The spectra recorded after 26-38 h at 150 °C show clear differences: the strong ν(P-O-(C)) peak disappears, while new bands appear in the same wavenumber region. The spectrum after heating at T ) 150 °C for 38 h exhibits a broad band in the region around 1100 cm-1, indicating a modification of the molecule. In the functional group region (around 1400 cm-1) the alkane group bending peaks are no longer detected. The peak at ∼916 cm-1 is assigned to ν(P-O-P), and the band around 1100 cm-1 probably results from the overlap of the peaks assigned to ν(PdO) and ν(P-O), which, typically, fall in the regions 13201140 and 950-1060 cm-1, respectively.31-33 At this stage, a complete explanation for the change of the baseline slope over time has not been found. It is probably due to the formation of a further layer (thermal or tribological film) at the iron film/ZnDTP interface, since the same background spectrum was used for all spectra. Tribological Tests. The ATR FT-IR spectra of ZnDTP on an iron-coated germanium crystal, recorded during tribotesting at room temperature, did not reveal any changes up to 90 h of sliding time (Figure 6a), the same (32) Harrison, P. G.; Brown, P. Wear 1991, 148, 123. (33) Thomas, L. C. The Identification of functional Groups in Organophosphorus Compounds; Academic Press: New York, 1974.
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Table 3. IR Frequencies (cm-1) and Functional Groups for the Transmission FT-IR Spectra of Iron Phosphate and Zinc Phosphate and for the ATR FT-IR Spectrum of Commercial ZnDTP after Tribochemical Reaction at 150 °C Zn3(PO4)2‚xH2O
br band:1488-772,1105, 1067 (sh) vs 1015 vs 938 vs
FePO4‚xH2O
br band:1416-820,1117 (sh) s 1019 vs
Figure 7. Temperature effect on the iron-coated (10 nm) germanium ATR spectrum.
result being found for tribological tests performed at 80 °C (spectra not shown). Only in the tribological test performed at 150 °C were clear differences observed (Figure 6b). The spectrum collected at the beginning of the test (0 h) shows the expected characteristic peaks of the ZnDTP molecule on iron (Table 2). After 20 h of sliding in the ATR tribometer at 150 °C the spectrum shows two new bands with maximum intensities at 1102 and 972 cm-1, assigned to PO43- stretching.31-34 Transmission spectra collected on FePO4‚xH2O and Zn3(PO4)2‚xH2O confirmed this assignment (Table 3). XPS analysis collected on the same sample supports the formation of phosphate in the contact area. Ge signals were neither detected in the contact area nor inside the wear scar. Discussion Spectra obtained with the in situ ATR FT-IR tribometer allow changes in the lubricant chemistry to be monitored upon adsorption of additives on iron, as well as the growth of reaction films to be followed as a function of sliding time and temperature in tribological experiments. To allow a correct interpretation, the spectra obtained in the in situ ATR FT-IR tribological apparatus were compared to traditional transmission spectra. Comparison between Transmission and ATR FTIR Spectra. By comparing the transmission and ATR FT-IR spectra of commercial ZnDTP reported in this work (Figure 4), it appears that the ATR spectra collected on the uncoated germanium ATR crystal closely resemble those obtained via transmission. Differences were noticed in the P-O-(C) peak position, however, this peak being shifted to lower wavenumbers in the ATR (Ge/Fe, 10 nm) spectrum (Table 2). A reason for this shift may be the adsorption of the ZnDTP onto the iron surface. Unfortunately the low-wavenumber region, where the Fe-S (34) Shagidullin, R. R.; Chernova, A. V.; Vinogradova, V. S.; Mukhametov, F. S. Atlas of IR Spectra of Organophosphorus Compounds; Kluwer Academic Publishers: Moscow, 1990.
ZnDTP after tribotest at 150 °C
functional group
2955, 2925, 2868 m 1466, 1452 w 1375 vw br band:1241-860,1101 w
ν(alkane groups) δas(CH3) δs(CH3) ν(PO43-)
972 m
interaction may be detected to confirm this result, is not accessible. A shift of the peak positions in ATR spectra is expected to be caused by the wavelength dependence of the penetration depth (dp) and the effective thickness (de) that is analyzed. The concept of de as the interaction of the evanescent field with the sample was introduced by Harrick.7 The effective thickness represents the actual thickness of a film that would be required to obtain the same absorption in a transmission measurement as that obtained by means of an evanescent wave. For bulk materials in which the thickness of the rarer medium is much greater than the penetration depth of the evanescent field, de is proportional to dp. Since dp is in turn proportional to the wavelength, de also increases with wavelength. As a result of the consequent distorting effect on peak shapes, broad absorption bands in ATR spectra of bulk materials show a shift to longer wavelengths (lower wavenumbers).7 The bands at longer wavelengths are also relatively more intense. There is another effect that should be taken into account when ATR spectra are compared to transmission spectra: the variation of the refractive index across an absorption band. This phenomenon is known as dispersion, which affects the spectra for internal reflection, contributing to an increase in band intensity, a slight shift in the band position, and a distortion of the band shape.35 The ATR spectra reported in this work should not be distorted by dispersion, however, since they were obtained at an incidence angle well above the critical angle. Influence of the Thin Iron Coating. ZnDTP ATR spectra collected on a germanium crystal coated with Fe show a further shift of the P-O-(C) absorption band to lower wavenumbers, and the distortion of this band on the lower-wavenumber side. Furthermore, all the bands in these spectra show a distortion at lower wavenumbers and differences in relative peak intensities (Figure 4). From data reported in the literature,36 the optical properties of iron change over the wavelength (wavenumber) region investigated in this work. In particular, at low wavelengths (high wavenumbers), the refractive index of iron is close to that of germanium, while the refractive index increases as the wavelength increases. For wavelengths at which the refractive index of Fe and Ge are the same, these layers are optically indistinguishable. When the refractive indexes of Fe and Ge are different, a three-layered system has to be considered. More theoretical studies (e.g., electric-field analysis) are necessary to understand and determine how optical constants influence ATR spectra in the case of a stratified multilayered system where an iron film is the intermediate layer. In a very recent paper37 on the influence of the optical properties of metals on ATR band shape at the metal-liquid interface, it has been reported that the (35) Belali, R.; Vigoreux, J. M.; Morvan, J. J. Opt. Soc. Am. B 1995, 12, 2377. (36) Ordal, M. A.; Bell, R. J.; Alexander, R. W., Jr.; Nequist, L. A.; Querry, M. R. Appl. Opt. 1988, 27, 6, 1203. (37) Bu¨rgi, T. Phys. Chem. Chem. Phys. 2001, 3, 2124.
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Figure 8. Penetration depth (nm) for the IR beam into the ZnDTP for the three-layered system Ge crystal/ Fe film/ZnDTP as a function of the wavenumber (cm-1). Chart 1
distortion of ATR increases with increasing thickness, increasing the refractive index and decreasing the absorption coefficient of the metal film. To have an initial estimate of the thickness of the film probed when a germanium crystal is coated with an iron film, the penetration depth (dp) of the infrared beam into the ZnDTP for the three-layered system germanium crystal/iron coating/ZnDTP has been calculated according to the approach of Mu¨ller and Abraham-Fuchs.37 The calculated penetration depth as a function of the wavenumber in the mid-IR range (4000-500 cm-1) ranges from 5 to 13 nm (Figure 8). Thermal vs Tribochemical Effect. Of particular interest in the interaction of ZnDTP with iron and steel are the relative roles of tribochemical and thermochemical processes. Some authors have reported that tribochemical reactions are simply due to the high temperature induced during metal-metal contact (by means of plastic deformation, etc.),39 while other authors believe that there is something unique about the high-pressure-high-shear situation existing in tribological systems.19 In fact, the ATR experiments performed in this work did not reveal chemical reactions of commercial ZnDTP with iron during simple adsorption at room temperature (Figure 5a) or at 80 °C. Reactions were neither detected during tribological tests (sliding) at room temperature (Figure 6a) nor at 80 °C despite test durations of up to 90 h. While in the case of the simple thermal experiment, this can be explained by a lack of the necessary activation energy; for the tribological sliding experiments this might be due to insufficient local pressure or an insufficient number of cycles. During a thermal test at 150 °C, the ATR analysis of the ZnDTP surface reaction on an iron-coated germanium crystal indicates a rearrangement of the molecule with the formation of P-O-P bonds. The proposed structure for the thermochemical reaction product, supported by the XPS results collected on the same samples,24 is given in Chart 1. The molecular weight of the reaction product has not been determined at this stage, and thus a real chemical (38) Mu¨ller, G. J.; Abraham-Fuchs, K. Optik 1991, 88, 3, 83. (39) Tysoe, W. T.; Surerus, K.; Lara, J.; Blunt, T. J.; Kotvis, P. V. Tribol. Lett. 1995, 1, 39.
Piras et al.
structure is not proposed, but the schematic of the reaction product contains all the functional groups detected by ATR and XPS analysis.24 The decrease in intensity of the alkane C-H bend peaks, until their disappearance after 38 h of heating (Figure 5b), is presumably due to the elimination of alkenes during the thermal decomposition of ZnDTP. The results of the present study are supported by the mechanism proposed in the literature,40,41 and the ATR results are in agreement with the infrared studies reported by Harrison and Brown.32 After the tribological test performed at 150 °C, bands with maximum intensities at 1102 and 972 cm-1 were present. They are assigned to PO43- stretching, indicating the formation of phosphates (Table 3) as a product of the tribochemical reaction at 150 °C. The tribological reaction seems to be faster than the thermal reaction, since after 20 h of heating only, no significant changes in the ZnDTP ATR spectrum were detected (Figure 5b). In contrast, the spectrum collected after the same time but during sliding at the same temperature shows the formation of an inorganic phosphate film (Figure 6b). These results, in agreement with the X-ray absorption and photoelectron spectroscopic studies of ZnDTP tribofilms formed at 150 °C,42 indicate that the thermochemical and the tribochemical reactions follow two different mechanisms with different kinetics. Some models for ZnDTP antiwear films reported in the literature19-21 propose a multilayered structure where a polyphosphate film is responsible for the wear reduction. The results reported in this work do not indicate formation of P-O-P species, characteristic of polyphosphates, during up to 20 h of sliding at 150 °C. It has to be remembered, however, that the tribological conditions used in this work (normal load, mean pressure, and sliding velocity) are milder than those reported in the literature,19-21 and that pure ZnDTP has been used instead of a solution in mineral oil. It may be suggested that, at the extent of reaction probed in this study, the tribochemical reaction is not complete, and the formation of phosphate is at an intermediate stage. Further experiments at longer sliding times and/or higher normal loads are required to understand more fully the mechanisms of thermochemical and tribochemical processes and their relative roles. Conclusions ATR FT-IR spectroscopy appears to be a powerful and useful technique for the in situ monitoring of tribochemical film formation. By measuring ATR FT-IR spectra from the underside of a thin iron film, the chemistry of the iron/ZnDTP interface has been analyzed as a function of time under both purely thermal and tribological conditions. In situ ATR thermal and tribological experiments have been performed in the presence of a pure secondary ZnDTP at room temperature, 80 °C, and 150 °C. No thermal or tribochemical reactions of ZnDTP with iron were detected during simple thermal tests at room temperature and at 80 °C. The ATR results on the surface reaction of ZnDTP on iron under purely thermal conditions at 150 °C indicate a rearrangement of the molecule, with the formation of P-O-P species. The formation of a phosphate film from (40) Dickert, J. J.; Rowe, C. N. J. Org. Chem. 1967, 32, 647. (41) Coy, R. C.; Jones, R. B. ASLE Trans. 1980, 24, 1, 77. Jones, B.; Coy, R. C. ASLE Trans. 1981, 24, 1, 91. (42) Kasrai, M.; Fuller, M.; Scaini, M.; Yin, Z.; Brunner, R. W.; Bancroft, G. M.; Fleet, M. E.; Fyfe, K.; Tan, K. H. In Proceedings of the 23rd Leeds/Lyon Symposium, Lubricants and Lubrication; Dowson, D., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1995; p 659.
Growth of Tribological Films
ZnDTP after friction at 150 °C (tribotest) is indicated by the ATR results. In the present study we show the utility of the new approach for examining the interface in tribological systems in situ and for understanding the kinetics of formation of tribofilms, as well as the relative roles of tribochemical and thermochemical processes. Current work extends the study to diluted solutions of ZnDTP in mineral oil to simulate real working conditions. Acknowledgment. Financial support of the ETH and Italian MURST (ex 40% grant to A.R.) and Regione
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Autonoma della Sardegna is gratefully acknowledged. Prof. J. M. Martin (Ecole Centrale de Lyon, France) and Dr. H. Camenzind (Ciba Specialty Chemicals, Switzerland) are thanked for supplying the pure additives. Mr. M. Horisberger kindly prepared the iron coatings by magnetron sputtering at PSI (Villigen, Switzerland). Prof. G. W. Stachowiak and Dr. C. Soto are thanked for their help in the mechanical design of the ATR tribometer. Finally, we thank Prof. J. R. Ferraro for his valuable advice and helpful discussions. LA0202733
