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Characterization of surface layers on M-50 steel exposed to perfluoropolyalkyethers at elevated temperatures
Applied Surface Science 135 Ž1998. 169–177
Characterization of surface layers on M-50 steel exposed to perfluoropolyalkyethers at elevated temperatures J.H. Sanders a b
a,)
, J.N. Cutler a , G. John
b
WL r MLBT, Materials Directorate, 2941 P. Street, Suite 1, Wright Laboratory, Wright-Patterson AFB, OH 45433-7750, USA Department of Engineering Physics, AFITr ENP, Air Force Institute of Technology, Wright-Patterson AFB, OH 45433, USA Received 2 February 1998; accepted 10 April 1998
Abstract Complex surface layers have been identified on M-50 steel samples exposed to a linear perfluoropolyalkyether ŽPFPAE. in the presence of air at 2608C. An understanding of the mechanisms that drive the formation of these layers is believed to be crucial for efforts to develop PFPAEs as effective lubricants for advanced high temperature turbine engines. Work presented here shows that the extreme surface region contains physisorbed PFPAE and Fe fluoride under which an Fe 3 O4 layer is observed followed by an FeF2 layer which is adjacent to the substrate. Similar layered structures have been observed for Fe-containing materials exposed to HF but have not been reported for PFPAE systems. X-ray photoelectron spectroscopy ŽXPS. and conversion electron Mossbauer spectroscopy ŽCEMS. were used to unambiguously characterize these layers. ¨ q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mossbauer spectroscopy; X-ray photoelectron spectroscopy; Corrosion; Tribology; Iron; Ethers ¨
1. Introduction The demand for higher performance jet aircraft has brought about the need for increased thrust-toweight ratio in the next generation turbine engine. Increased thrust can be obtained through higher temperature engine operation; however, materials limits still exist that must be addressed. Ongoing research is focused on the need for higher temperature liquid lubricants that are stable up to 3458C in the near term and eventually 3608C in the long term. These lubricants include the base-fluid and additive package. Perfluoropolyalkylethers ŽPFPAEs. show partic)
Corresponding author. Tel.: q1-937-255-9098; fax: q1-937255-2176; e-mail: [email protected].
ular promise as high temperature base-fluids because they have excellent thermal and oxidative stabilities, and viscosity profiles w1–5x. To date, a limiting factor in the use of PFPAEs is the degradation of the fluid and corrosion of surfaces in the presence of Fe containing surfaces at relatively low temperatures, less than 3008C. In comparison, TiN and Ni surfaces have been shown to be corrosion resistant in the presence of PFPAEs at temperatures up to 3758C w6x. Much work has been reported suggesting that FeF3 forms on the surface of Fe-containing samples and act as Lewis acid sites, catalyzing the degradation of the fluid and stimulating corrosion of the substrate w7–17x. Prior work using Mossbauer spectroscopy re¨ vealed the presence of FeF2 as a final product on the
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 7 3 - 6
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J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
surface of Fe-containing samples exposed to high temperature in sealed containers with minimal oxygen present w18,19x. A model was proposed that involved the partial fluorination of surface Fe 3 O4 Žpredominantly through the B-site. with evidence of fluorides and oxyfluorides as intermediates for the conversion to FeF2 . Only under excessive exposure to air where the seal was broken was FeF3 detected as a final product w18x. To effectively select base-fluid and additive formulations, a detailed understanding of the mechanism of corrosion and fluid degradation is necessary. A review of past investigations does not produce clear details as to how the surface degrades or what surface corrosion species are produced. A more comprehensive understanding of the chemical nature of surface corrosion species was sought in this investigation. The work presented in this paper uses Mossbauer ¨ spectroscopy and X-ray photoelectron spectroscopy ŽXPS. to characterize the surface regions of M-50 bearing steel in the presence of a linear PFPAE at high temperatures. A complex layered structure was noted on the surface of M-50 substrates that to our knowledge has not been reported previously. XPS provides depth distribution of reactive elements, however, is somewhat ambiguous with regards to the exact chemistry of Fe fluoride components. Mossbauer spectroscopy provides clear and unam¨ biguous results pertaining to Fe fluoride identification.
2. Experimental The base fluid for these experiments was Demnum S-65 produced by Daikin Industries which has a backbone structure of, CF3 CF2 CF2 ŽOCF2 CF2 CF2 . m OCF2 CF3 and was used without further purification. The surface films were prepared on polished M-50 steel washers using a standard oxidation–corrosion ŽO–C. test which is based on a micro-version of the ASTM D4636 standard oxidation test w20x. Briefly, the oxidative films were prepared by suspending metallic coupons ŽM-50. of a known weight in unformulated fluid. The fluid containing the coupons was bubbled with dry air Ž1 lrh. at 2608C for 24 h. At the end of the 24 h period, the coupons were re-weighed and compared to the pre-test weight.
Generally, a fluid is reported to fail the O–C test when the coupon shows a weight change in excess of 0.2 mgrcm2 . Analysis of coupons presented here passed the O–C test. The M-50 washers were examined pre- and post test using both X-ray photoelectron ŽXPS. and conŽCEMS. spectroscopies. version electron Mossbauer ¨ XPS spectra were recorded using a Surface Science Instruments SSX-100. This system produces monochromatic Al K a X-rays at an energy of 1486.6 eV. The instrument was operated using a 300 m m X-ray spot for all experiments. For high resolution scan data, the electrostatic energy analyzer was operated at a pass energy of ; 55 eV and data was acquired in scan mode. This gave an overall instrumental resolution of ; 0.85 eV based the FWHM of the Au 4f 7r2 peak. For the depth profile data, the energy analyzer was operated at a pass energy of ; 156 eV giving an overall instrumental resolution of ; 1.38 eV. Prior to XPS analysis, specimens were ultrasonically cleaned in 1,1,2-trichloro 2,2,1,-trifluoroethane for 20 min. For depth profiling, ion sputtering was carried out with 3 keV Arq ions. The sputter rate of ˚ sy1 . Although the a SiO 2rSi standard was 0.74 A sputter rates will not be the same for Fe based compounds, past work has indicated that the SiO 2 rate provides a reasonable estimate of the surface erosion rate. All spectral binding energies were referenced to the C 1s line of adventitious carbon, 284.75 eV. Mossbauer spectroscopy was performed in the ¨ conversion electron mode with a constant acceleration electromechanical transducer operated in flyback mode. Spectra were obtained at room temperature with a 57Co source in a rhodium matrix. A gas flow proportional counter operated with a mixture of 3% methane in helium was used to provide information within the first 300 nm of the surface. CEMS spectra were analyzed with a least squares program that could fit theoretical models of up to eight Fe sites. A Monte Carlo simulation routine was used to provide a non-linear error analysis based on fluctuation predicted by counting statistics. a-Fe was used to calibrate the spectrometer and errors are estimated to be less than 0.02 mmrs. Three characteristics of Mossbauer spectra that ¨ identify chemical compounds are, the isomer shift,
J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
electric quadrupole splitting, and magnetic hyperfine splitting. Electrostatic interactions of the nuclear charge with surrounding electrons and ions produce the isomer shift and quadrupole splitting while the magnetic field produced by the crystal at the nucleus causes the hyperfine splitting. Since the electromagnetic environments do differ for different Fe compounds and crystal lattices, unique spectra are possible.
3. Results and discussion The use of PFPAE lubricants in the presence of Fe-based alloys at high temperatures largely depends on the ability to eliminate fluid degradation and surface corrosion problems that currently exist. Todate, work has been focused on the extreme surface as this is the region where catalytic species are formed that interact with the fluid during the decomposition process. The results presented in this paper demonstrate that complex chemistry is active in the
171
subsurface that defines the corrosion process of Fecontaining substrates and is likely very important for a better understanding of the fluid decomposition process as well. 3.1. XPS Fig. 1 and Table 1 show XPS depth profile data of a M-50 coupon exposed to Demnum at 2608C for 24 h. F, O, Fe, and C were monitored during the depth profiles and their atomic concentrations were plotted. Except for the outermost surface, the carbon content was minimal and the peak location was consistent with carbide from the substrate; therefore, for clarity is not shown. Immediately apparent, a layered structure is produced on the surface that extends ; 200 nm into the bulk. Three distinct layers are revealed: Ži. the extreme surface Ži.e., the topmost 5 nm. which consists of very little Fe with a high concentrations of F, O, and C; Žii. the mid-region Ži.e., 5–55 nm. that appears to be primarily Fe oxide with little F or C content; and Žiii. the buried
Fig. 1. XPS depth profile of an M-50 substrate exposed to a linear PFPAE for 24 h at 2608C in the O–C test apparatus. Three distinct layers can be seen Ži. the extreme surface, Žii. a mid-region, and Žiii. a buried region Žadjacent to the substrate.. Iron s open triangles, oxygens filled circles, and fluorines filled squares.
J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
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Table 1 Selected atomic percentages from a depth profile of an O–C coupon produced at 2608C for 24 h Sputter Time Žs.
C
O
F
Fe
0 60 320 720 1100 1800
46.9 7.8 13.9 4.9 4.5 4.6
29.9 56.9 45.4 30.8 12.7 7.1
20.3 7.2 5.9 30.4 48 23.5
2.9 28.2 34.8 33.9 34.7 64.9
region Ži.e., 55–200 nm. that is composed of Fe and F with little O or C content. The thickness of the oxide film found in region Žii. is approximately 50 nm. This oxide film is much thicker than the 5 nm native oxide film observed on
the coupon prior to O–C testing. Region Žiii. reveals the presence of a previously unreported F containing layer underneath the Fe oxide. From the depth profile data, the nature and origin of this buried layer is unclear. Therefore, a series of higher resolution XPS spectra were collected. Figs. 2 and 3 provide high resolution XPS data of the O–C coupon prior to and after 1100 s of Arq ion sputtering Ž; 80 nm of removed material, near midpoint of region iii.. Before sputtering, the C 1s spectra is composed of five peaks as seen in Fig. 2. Most of the C signal originates from the presence of adventitious C Ž284.75 eV. from exposure to air. Four weaker peaks at higher binding energy, 293.5, 292.2, 288.4 and 286.4 eV are related to the interaction of the M-50 coupon with Demnum fluid. The
Fig. 2. Post O–C test narrow scan XPS spectra of C and O before and after Arq sputter removal of ; 80 nm of surface material. Ža. C 1s peaks A, B, C, and D originate from the PFPAE fluid, and peak E originates from adventitious C. Žb. O 1s peak A is associated with PFPAE and peaks B, C, and D arise from Fe oxide, hydroxyl groups, and adsorbed H 2 O, respectively.
J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
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Fig. 3. Post O–C test narrow scan XPS spectra of F and Fe before and after Arq sputter removal of ; 80 nm of surface material. Ža. F 1s peak A is associated with the PFPAE fluid, and peak B is associated with FeF2 andror FeF3 . Žb. Fe 2p 3r2 peak A is characteristic of an Fe oxide and peak B is from Fe metal.
two weak peaks at 293.5 and 292.2 eV are CF3rCF2 O and CF2 , respectively, and originate from Demnum base fluid which is physisorbed to the surface. The peak at 288.4 eV is indicative of C that may be present in carboxylic acid, a carbonyl, or acyl fluoride. The weak shoulder at 286.4 is due to C–O. These species have been shown previously to be productsrintermediates during PFPAE decomposition in the presence of Fe surfaces w17x. After 1100 s of sputtering, all of the carbon species discussed above have been removed from the surface and a weak new line is observed. This line at 282.5 eV is produced by Fe carbide found in the steel substrate. The O 1s spectra ŽFig. 2. prior to sputtering is best fit using four peaks with fixed separations that are consistent with expected species both from prior experience in our laboratory and as reported in the
literature w21x. The peak at 536.1 eV is due to the small amount of physisorbed Demnum on the substrate. The three lower binding energy features are assigned to Fe oxide Ž529.7 eV., surface hydroxyl groups Ž531.4 ev. and adsorbed H 2 O at 532.7 eV. Following sputtering, as seen in the C 1s spectra, all of the adsorbed Demnum has been removed. Also evident in the sputtered O 1s spectra, the adsorbed water has been expunged and the intensity of the surface hydroxyl groups has been substantially reduced. The unsputtered F 1s spectra ŽFig. 3. has two distinct peaks at 684.0 and 689.3 eV. The higher binding energy peak is associated with F found in Demnum which is adsorbed onto the surface. Whereas, the lower binding energy peak at 684.6 eV is consistent with an Fe fluoride, likely FeF3 as this
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would be consistent with the formation of a strong Lewis acid at the surface that is in contact with fluid acting to perpetuate the decomposition process. After sputtering, as seen in the C and O 1s spectra in Fig. 2, the adsorbed Demnum has been completely removed; the peak at 689.3 eV has disappeared. Only one strong peak remains with a chemical shift consistent with Fe–F. Unambiguous identification of the Fe–F compound is not possible based on XPS analysis alone. However, when placed in context with the Fig. 1, the subsurface layer which is evident in region Žiii. can in general be categorized as a thick buried Fe fluoride layer Ži.e., FeF2 , FeF3 , Fe 2 F5 , etc . . . .. As seen in Table 1 and Fig. 1, there is some oxygen detected in this region as well. This may result from the presence of some Fe oxyfluorides Žseen previously in Ref. w18x and consistent with an unidentified FeŽIII. component in the CEMS, shown later., andror a result of smearing from the depth profile process. Limitations in XPS include the inability to detect hydrogen and only semi-quantitative analysis. The unsputtered and sputtered Fe 2p 3r2 spectra are shown in Fig. 3. Prior to sputtering, the Fe 2p spectra shows only one broad band centered at ; 710 eV which is characteristic of an Fe oxide. Whereas, after sputtering, the Fe 2p 3r2 spectra shows a new peak at 706.2 eV which is in good agreement with Fe metal. Along with the new Fe metal peak, a higher binding energy peak at 710.6 is observed. The exact nature of this peak is not clear although it indicates the Fe is present in high valence states Žq2 and q3. similar to that produced in an Fe oxide. Fig. 4 shows a montage of Fe region from the depth profile data of Fig. 1. From these data one can more clearly visualize the layered structure and the distinct Fe species that dominate each layer. From the atomic percentages ŽTable 1, 1100 s. the total Fe content is 34.7%. Further breakdown into Fe o and Feq2 ,q 3 reveal atomic percentages of 10.0% and 24.7%, respectively. Considering that Feq2 ,q 3 encompasses many molecular species including Fe hydroxides, Fe oxides, Fe oxyfluorides and Fe fluorides, and considering that inaccuracies in XPS cross-sections, variations in constituent sputter yields, etc . . . exist in XPS measurements, a definitive stoichiometry of the Fe-fluoride species cannot be determined. However, assuming some of the Feq2 ,q 3 is associated with Fe
Fig. 4. Fe 2p1r 2 – 3r2 region collected during the depth profile shown in Fig. 1. Chemical shifts reveal the distinct nature of the Fe chemical species within each layer on the Fe-alloy surface.
oxide ŽFerO s 0.67., the remaining Feq2 ,q 3 is in the form of an Fe-fluoride species having FerF between 0.3 and 0.5. 3.2. CEMS The CEMS spectrum shown in Fig. 5 provides identification of the Fe species present in the complex layered structure produced in the O–C test apparatus. A table of the fit parameters and relative areas is presented in Table 2. The largest area, 0.59, identified is from the M-50 substrate that is comprised of a sextuplet with a hyperfine field of 33.4 T. It is clear from this observation that the CEMS sampling depth encompasses the entire layered structure and that the other spectral components reside above the M-50 substrate. The asymmetric doublet that dominates the center of the spectrum has an area of 0.16, quadrupole splitting of 2.72 mmrs and an isomer of shift of 1.34 mmrs, clearly indicative of FeF2 . Two other sextuplets are apparent that can be identified with Fe 3 O4ŽA-site.-hyperfine field of 49.8 tesla, and Fe 3 O4ŽB-site.-hyperfine field of 45.0. The ratio of ArB is 0.78 which is slightly higher the 0.5 ratio theoretically expected. As discussed in detail by John et al. w18x, this indicates that F may be substituting for oxygen in some of the octahedral B-sites. There is also a doublet present with an isomer shift of 0.33 mmrs and a quadrupole splitting of 0.81
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nuclei within the sampling depth. Therefore, it is reasonable that the extreme surface contribution falls below the signal-to-noise threshold. The existence of catalytic FeF3 w7–17x in the extreme surface layer is consistent with the XPS results discussed above where the F 1s spectrum was shown to have a component from Fe fluoride in the unsputtered condition. As of yet there is no direct mechanism established for the formation of the FeF2 layer. 3.3. Layered structure formation
Fig. 5. CEMS spectra of a specimen a post O–C test substrate showing the Fe components within the surface region. Parameters are shown in Table 2.
mmrs. These parameters indicate the presence of an FeŽIII. compound that has not yet been clearly identified. This compound was observed previously w18x and is thought to be an Fe oxyfluoride intermediate in the formation of FeF2 . Further clarification of the FeŽIII. compound is underway and will be discussed in a future publication. The presence of FeF3 is not observed Žreference parameters provided in Table 2.. The CEMS data is complimentary to XPS data and clearly show that a complex surface structure forms on the surface of the Fe substrates in the presence of PFPAE at high temperatures. The thickest layers are FeF2 which is adjacent to the Fe substrate and Fe 3 O4 which is near the surface. Identification of these layers was unambiguously determined by CEMS and the location was confirmed by XPS. CEMS cannot resolve the topmost monolayer, because it contains only a small fraction of 57 Fe
The exact mechanism for the formation of complex corrosion layers on Fe-alloys in the presence of PFPAEs has not yet been established. However, the presence of F ions essential for the formation of a buried FeF2 layer is consistent with the various mechanisms of fluid degradation proposed by others w9,22x. Also, similar layered structure has been observed for Fe in the presence of HF solutions w23x. 3.3.1. Fluid degradation mechanisms One proposed mechanism is that strong Lewis acid sites, FeF3 , occur from initial interaction of PFPAEs with weak Lewis acid sites, Fe 3 O4 , which are found on native surfaces w9x. This scenario suggests that a slow decomposition of PFPAE occurs prior to rapid decomposition upon the formation of sufficient surface conversion of Fe 3 O4 to FeF3 . The proposed decomposition process involves the formation of HF at the surface, allowing highly reactive F species to reside within the surface region which they observe to exit the system through the gas phase. Hydrogen is present in the system from many possible sources including impurities in the production of PFPAE fluids and hydroxyl ions on the Fe-alloy and reaction vessel surfaces. Another pro-
Table 2 Mossbauer parameters obtained from the oxidation-corrosion film generated from Demnum at 2608C for 24 h ¨ Compound
Isomer shifts Žmm sy1 .
Quadrupole splitting Žmm sy1 .
Hyperfine field ŽT.
Relative area
FeŽIII. Ž?. Fe 3 O4 ŽB. Fe 3 O4 ŽA. FeF2 M-50 Ž1. FeF3 Ref. w17x
0.33 0.41 0.33 1.34 0 0.49
0.81 0 0 2.72 0 0
y 45.0 49.8 y 33.4 39.5
0.10 0.09 0.07 0.16 0.59 y
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J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
posed mechanism suggests that decomposition proceeds through fluid interaction with hydroxyl ions bound to surface oxides w22x. Of primary importance to the work presented here is that as with the first mechanism mentioned above, a product of the decomposition reaction is the generation of labile F that resides at the surface over a brief time. Neither of the two mechanisms briefly described above probe into the mechanisms of Fe-alloy corrosion in any detail. Much of the work has focused on the topmost surface and the decomposition of the fluid. This paper reveals that F ions produced during the decomposition process of PFPAEs are not just confined to the extreme surface and fluid, but permeate down to the fresh metal interface to stimulate surface corrosion and create complex surface layers. 3.3.2. Layered structure A similar layered structure to that presented here has been observed on Fe substrates in the presence of HF. Crouse and Stander w23x performed a study using CEMS and gravimetric techniques to determine the corrosion mechanisms of Fe surfaces exposed to the vapors above HF diluted in H 2 O up to 40%. They showed that for dilutions greater than 0.1%, the formation of b-FeOOH occurred at the outermost surface of Fe by the oxidation of FeF2 P H 2 O, which formed adjacent to the Fe substrate early in the reaction sequence. Subsequently, the rate of formation of b-FeOOH at steady state was controlled by the diffusion of HF through the FeF2 P H 2 O layer to react with the Fe substrate to replenish the FeF2 P H 2 O layer. Although the work presented here is on PFPAE interaction with Fe-alloys, there are similarities with regards to surface chemistry. The major layer constituents detected during this investigation were Fe 3 O4 Žwith possible F substitution in some B-sites. above FeF2 Žadjacent to the bulk.. The FeF2 was not hydrated Žeasily discerned from CEMS., however, there is H 2 O and O 2 present in the fluid that may play a role in the reaction sequences that form the Fe 3 O4 and FeF2 layers. Also, there is a substantial amount of etching that occurs on the surface of the pyrex reaction vessel that indicate the formation of HF during the O–C test. Although, the exact mechanisms of Fe-alloy corrosion are not yet understood, the determination that a complex layered surface structure is formed during
O–C tests has direct implications with regards to the tribological properties of these systems. Mechanically weak layers or interfaces between layers will adversely affect wear rate and may perpetuate further corrosion through exposure of fresh surfaces to reactive species in the fluid. Presently, mechanistic details for the layer formation are being studied. The influence of parameters such as environment, temperature, and time will be explored.
4. Conclusions Ø A complex layered structure on the surface of Fe substrates in the presence of PFPAEs has been identified with XPS. This structure has three distinct layers, Ži. the extreme surface Ž5 nm. –likely containing the reactive catalytic species FeF3 which has been shown in the literature to decompose the fluid, Žii. mid-region Ž5–55 nm. –predominantly Fe oxide, identified as Fe 3 O4 , and Žiii. buried region Ž55–200 nm. –identified with CEMS as FeF2 . Ø The formation of the subsurface FeF2 layer is likely a result of a decomposition reaction during which F ions are produced and then diffuse into the surface. Various proposed decomposition reactions in the literature involve the production of these F ions which at least in part leave the surface by gas phase species. Results presented here indicate that conditions favor diffusion into the surface and reaction with subsurface Fe as well. Ø An understanding of the mechanism by which these surface layers form and the relationship with parameters such as environment, temperature, and time may prove beneficial to the design of effective lubrication systems involving PFPAEs and Fe-containing surfaces.
Acknowledgements The authors would like to thank Dr. K.C. Eapen, Mr. G. Fultz, and Ms. O. Scott of the University of Dayton Research Institute for sample preparation and helpful discussions. The authors are grateful to Drs. J.S. Zabinski and H.L. Paige for their thought provoking comments. This work was supported by the Materials Directorate of the Air Force Research Lab-
J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
oratory, Wright-Patterson Air Force Base, OH and the Air Force Office of Scientific Research, Bolling Air Force Base, Washington, DC. References w1x W.H. Gumprecht, ASLE Trans. 9 Ž1966. 24. w2x D. Sianesi, V. Zamboni, R. Fontanelli, M. Binaghi, Wear 18 Ž1971. 85. w3x C.E. Snyder Jr., R.E. Dolle Jr., ASLE Trans. 19 Ž1976. 171. w4x W.R. Jones Jr., C.E. Snyder Jr., ASLE Trans. 23 Ž1980. 253. w5x K.J.L. Paciorek, R.H. Kratzer, J. Fluorine Chem. 67 Ž1994. 169. w6x J.S. Zabinski, M.A. Capano, A.A. Voevodin, Proceedings of the 82nd NATOrAGARD Tribology of Aerospace Systems Specialist Meeting, NATO, London, AGARD-CP-589, 1996. w7x W. Morales, NASA TP 2774, 1987. w8x D.J. Carre, ´ ASLE Trans. 29 Ž1985. 121.
w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x
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M.J. Zehe, O.D. Faut, Tribol. Trans. 33 Ž1990. 634. P.H. Kasai, Macromolecules 22 Ž1992. 6791. P.H. Kasai, P. Wheeler, Appl. Surf. Sci. 52 Ž1991. 91. P.H. Kasai, W.T. Tang, P. Wheeler, Appl. Surf. Sci. 51 Ž1991. 201. P.J. John, J. Liang, J. Vac. Sci. Technol. A 12 Ž1994. 199. B. C¸ avdar, J. Liang, P.J. John, Tribol. Trans. 39 Ž1996. 779. S. Mori, W. Morales, Wear 132 Ž1989. 111. S. Mori, W. Morales, Tribol. Trans. 33 Ž1990. 325. P. Herrera-Fierro, W.R. Jones Jr., S.V. Pepper, J. Vac. Sci. Technol. A 11 Ž1993. 354. G. John, J.S. Zabinski, V.K. Gupta, Appl. Surf. Sci. 93 Ž1996. 329. J.N. Cutler, J.H. Sanders, G. John, Tribol. Lett. 4 Ž1998. 149. L. Gschwender, C.E. Snyder Jr., G.W. Fultz, D.A. Hahn, J.R. Demers, Tribol. Trans. 38 Ž1995. 618. G. Kurbatov, E. Darque-Ceretti, M. Aucouturler, Surf. Interf. Anal. 18 Ž1992. 811. P. Li, L.M. Ng, J. Liang, Surf. Sci. 380 Ž1997. 530. P.L. Crouse, C.M. Stander, Corros. Sci. 33 Ž1. Ž1992. 13.
Characterization of surface layers on M-50 steel exposed to perfluoropolyalkyethers at elevated temperatures J.H. Sanders a b
a,)
, J.N. Cutler a , G. John
b
WL r MLBT, Materials Directorate, 2941 P. Street, Suite 1, Wright Laboratory, Wright-Patterson AFB, OH 45433-7750, USA Department of Engineering Physics, AFITr ENP, Air Force Institute of Technology, Wright-Patterson AFB, OH 45433, USA Received 2 February 1998; accepted 10 April 1998
Abstract Complex surface layers have been identified on M-50 steel samples exposed to a linear perfluoropolyalkyether ŽPFPAE. in the presence of air at 2608C. An understanding of the mechanisms that drive the formation of these layers is believed to be crucial for efforts to develop PFPAEs as effective lubricants for advanced high temperature turbine engines. Work presented here shows that the extreme surface region contains physisorbed PFPAE and Fe fluoride under which an Fe 3 O4 layer is observed followed by an FeF2 layer which is adjacent to the substrate. Similar layered structures have been observed for Fe-containing materials exposed to HF but have not been reported for PFPAE systems. X-ray photoelectron spectroscopy ŽXPS. and conversion electron Mossbauer spectroscopy ŽCEMS. were used to unambiguously characterize these layers. ¨ q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mossbauer spectroscopy; X-ray photoelectron spectroscopy; Corrosion; Tribology; Iron; Ethers ¨
1. Introduction The demand for higher performance jet aircraft has brought about the need for increased thrust-toweight ratio in the next generation turbine engine. Increased thrust can be obtained through higher temperature engine operation; however, materials limits still exist that must be addressed. Ongoing research is focused on the need for higher temperature liquid lubricants that are stable up to 3458C in the near term and eventually 3608C in the long term. These lubricants include the base-fluid and additive package. Perfluoropolyalkylethers ŽPFPAEs. show partic)
Corresponding author. Tel.: q1-937-255-9098; fax: q1-937255-2176; e-mail: [email protected].
ular promise as high temperature base-fluids because they have excellent thermal and oxidative stabilities, and viscosity profiles w1–5x. To date, a limiting factor in the use of PFPAEs is the degradation of the fluid and corrosion of surfaces in the presence of Fe containing surfaces at relatively low temperatures, less than 3008C. In comparison, TiN and Ni surfaces have been shown to be corrosion resistant in the presence of PFPAEs at temperatures up to 3758C w6x. Much work has been reported suggesting that FeF3 forms on the surface of Fe-containing samples and act as Lewis acid sites, catalyzing the degradation of the fluid and stimulating corrosion of the substrate w7–17x. Prior work using Mossbauer spectroscopy re¨ vealed the presence of FeF2 as a final product on the
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 7 3 - 6
170
J.H. Sanders et al.r Applied Surface Science 135 (1998) 169–177
surface of Fe-containing samples exposed to high temperature in sealed containers with minimal oxygen present w18,19x. A model was proposed that involved the partial fluorination of surface Fe 3 O4 Žpredominantly through the B-site. with evidence of fluorides and oxyfluorides as intermediates for the conversion to FeF2 . Only under excessive exposure to air where the seal was broken was FeF3 detected as a final product w18x. To effectively select base-fluid and additive formulations, a detailed understanding of the mechanism of corrosion and fluid degradation is necessary. A review of past investigations does not produce clear details as to how the surface degrades or what surface corrosion species are produced. A more comprehensive understanding of the chemical nature of surface corrosion species was sought in this investigation. The work presented in this paper uses Mossbauer ¨ spectroscopy and X-ray photoelectron spectroscopy ŽXPS. to characterize the surface regions of M-50 bearing steel in the presence of a linear PFPAE at high temperatures. A complex layered structure was noted on the surface of M-50 substrates that to our knowledge has not been reported previously. XPS provides depth distribution of reactive elements, however, is somewhat ambiguous with regards to the exact chemistry of Fe fluoride components. Mossbauer spectroscopy provides clear and unam¨ biguous results pertaining to Fe fluoride identification.
2. Experimental The base fluid for these experiments was Demnum S-65 produced by Daikin Industries which has a backbone structure of, CF3 CF2 CF2 ŽOCF2 CF2 CF2 . m OCF2 CF3 and was used without further purification. The surface films were prepared on polished M-50 steel washers using a standard oxidation–corrosion ŽO–C. test which is based on a micro-version of the ASTM D4636 standard oxidation test w20x. Briefly, the oxidative films were prepared by suspending metallic coupons ŽM-50. of a known weight in unformulated fluid. The fluid containing the coupons was bubbled with dry air Ž1 lrh. at 2608C for 24 h. At the end of the 24 h period, the coupons were re-weighed and compared to the pre-test weight.
Generally, a fluid is reported to fail the O–C test when the coupon shows a weight change in excess of 0.2 mgrcm2 . Analysis of coupons presented here passed the O–C test. The M-50 washers were examined pre- and post test using both X-ray photoelectron ŽXPS. and conŽCEMS. spectroscopies. version electron Mossbauer ¨ XPS spectra were recorded using a Surface Science Instruments SSX-100. This system produces monochromatic Al K a X-rays at an energy of 1486.6 eV. The instrument was operated using a 300 m m X-ray spot for all experiments. For high resolution scan data, the electrostatic energy analyzer was operated at a pass energy of ; 55 eV and data was acquired in scan mode. This gave an overall instrumental resolution of ; 0.85 eV based the FWHM of the Au 4f 7r2 peak. For the depth profile data, the energy analyzer was operated at a pass energy of ; 156 eV giving an overall instrumental resolution of ; 1.38 eV. Prior to XPS analysis, specimens were ultrasonically cleaned in 1,1,2-trichloro 2,2,1,-trifluoroethane for 20 min. For depth profiling, ion sputtering was carried out with 3 keV Arq ions. The sputter rate of ˚ sy1 . Although the a SiO 2rSi standard was 0.74 A sputter rates will not be the same for Fe based compounds, past work has indicated that the SiO 2 rate provides a reasonable estimate of the surface erosion rate. All spectral binding energies were referenced to the C 1s line of adventitious carbon, 284.75 eV. Mossbauer spectroscopy was performed in the ¨ conversion electron mode with a constant acceleration electromechanical transducer operated in flyback mode. Spectra were obtained at room temperature with a 57Co source in a rhodium matrix. A gas flow proportional counter operated with a mixture of 3% methane in helium was used to provide information within the first 300 nm of the surface. CEMS spectra were analyzed with a least squares program that could fit theoretical models of up to eight Fe sites. A Monte Carlo simulation routine was used to provide a non-linear error analysis based on fluctuation predicted by counting statistics. a-Fe was used to calibrate the spectrometer and errors are estimated to be less than 0.02 mmrs. Three characteristics of Mossbauer spectra that ¨ identify chemical compounds are, the isomer shift,
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electric quadrupole splitting, and magnetic hyperfine splitting. Electrostatic interactions of the nuclear charge with surrounding electrons and ions produce the isomer shift and quadrupole splitting while the magnetic field produced by the crystal at the nucleus causes the hyperfine splitting. Since the electromagnetic environments do differ for different Fe compounds and crystal lattices, unique spectra are possible.
3. Results and discussion The use of PFPAE lubricants in the presence of Fe-based alloys at high temperatures largely depends on the ability to eliminate fluid degradation and surface corrosion problems that currently exist. Todate, work has been focused on the extreme surface as this is the region where catalytic species are formed that interact with the fluid during the decomposition process. The results presented in this paper demonstrate that complex chemistry is active in the
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subsurface that defines the corrosion process of Fecontaining substrates and is likely very important for a better understanding of the fluid decomposition process as well. 3.1. XPS Fig. 1 and Table 1 show XPS depth profile data of a M-50 coupon exposed to Demnum at 2608C for 24 h. F, O, Fe, and C were monitored during the depth profiles and their atomic concentrations were plotted. Except for the outermost surface, the carbon content was minimal and the peak location was consistent with carbide from the substrate; therefore, for clarity is not shown. Immediately apparent, a layered structure is produced on the surface that extends ; 200 nm into the bulk. Three distinct layers are revealed: Ži. the extreme surface Ži.e., the topmost 5 nm. which consists of very little Fe with a high concentrations of F, O, and C; Žii. the mid-region Ži.e., 5–55 nm. that appears to be primarily Fe oxide with little F or C content; and Žiii. the buried
Fig. 1. XPS depth profile of an M-50 substrate exposed to a linear PFPAE for 24 h at 2608C in the O–C test apparatus. Three distinct layers can be seen Ži. the extreme surface, Žii. a mid-region, and Žiii. a buried region Žadjacent to the substrate.. Iron s open triangles, oxygens filled circles, and fluorines filled squares.
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Table 1 Selected atomic percentages from a depth profile of an O–C coupon produced at 2608C for 24 h Sputter Time Žs.
C
O
F
Fe
0 60 320 720 1100 1800
46.9 7.8 13.9 4.9 4.5 4.6
29.9 56.9 45.4 30.8 12.7 7.1
20.3 7.2 5.9 30.4 48 23.5
2.9 28.2 34.8 33.9 34.7 64.9
region Ži.e., 55–200 nm. that is composed of Fe and F with little O or C content. The thickness of the oxide film found in region Žii. is approximately 50 nm. This oxide film is much thicker than the 5 nm native oxide film observed on
the coupon prior to O–C testing. Region Žiii. reveals the presence of a previously unreported F containing layer underneath the Fe oxide. From the depth profile data, the nature and origin of this buried layer is unclear. Therefore, a series of higher resolution XPS spectra were collected. Figs. 2 and 3 provide high resolution XPS data of the O–C coupon prior to and after 1100 s of Arq ion sputtering Ž; 80 nm of removed material, near midpoint of region iii.. Before sputtering, the C 1s spectra is composed of five peaks as seen in Fig. 2. Most of the C signal originates from the presence of adventitious C Ž284.75 eV. from exposure to air. Four weaker peaks at higher binding energy, 293.5, 292.2, 288.4 and 286.4 eV are related to the interaction of the M-50 coupon with Demnum fluid. The
Fig. 2. Post O–C test narrow scan XPS spectra of C and O before and after Arq sputter removal of ; 80 nm of surface material. Ža. C 1s peaks A, B, C, and D originate from the PFPAE fluid, and peak E originates from adventitious C. Žb. O 1s peak A is associated with PFPAE and peaks B, C, and D arise from Fe oxide, hydroxyl groups, and adsorbed H 2 O, respectively.
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Fig. 3. Post O–C test narrow scan XPS spectra of F and Fe before and after Arq sputter removal of ; 80 nm of surface material. Ža. F 1s peak A is associated with the PFPAE fluid, and peak B is associated with FeF2 andror FeF3 . Žb. Fe 2p 3r2 peak A is characteristic of an Fe oxide and peak B is from Fe metal.
two weak peaks at 293.5 and 292.2 eV are CF3rCF2 O and CF2 , respectively, and originate from Demnum base fluid which is physisorbed to the surface. The peak at 288.4 eV is indicative of C that may be present in carboxylic acid, a carbonyl, or acyl fluoride. The weak shoulder at 286.4 is due to C–O. These species have been shown previously to be productsrintermediates during PFPAE decomposition in the presence of Fe surfaces w17x. After 1100 s of sputtering, all of the carbon species discussed above have been removed from the surface and a weak new line is observed. This line at 282.5 eV is produced by Fe carbide found in the steel substrate. The O 1s spectra ŽFig. 2. prior to sputtering is best fit using four peaks with fixed separations that are consistent with expected species both from prior experience in our laboratory and as reported in the
literature w21x. The peak at 536.1 eV is due to the small amount of physisorbed Demnum on the substrate. The three lower binding energy features are assigned to Fe oxide Ž529.7 eV., surface hydroxyl groups Ž531.4 ev. and adsorbed H 2 O at 532.7 eV. Following sputtering, as seen in the C 1s spectra, all of the adsorbed Demnum has been removed. Also evident in the sputtered O 1s spectra, the adsorbed water has been expunged and the intensity of the surface hydroxyl groups has been substantially reduced. The unsputtered F 1s spectra ŽFig. 3. has two distinct peaks at 684.0 and 689.3 eV. The higher binding energy peak is associated with F found in Demnum which is adsorbed onto the surface. Whereas, the lower binding energy peak at 684.6 eV is consistent with an Fe fluoride, likely FeF3 as this
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would be consistent with the formation of a strong Lewis acid at the surface that is in contact with fluid acting to perpetuate the decomposition process. After sputtering, as seen in the C and O 1s spectra in Fig. 2, the adsorbed Demnum has been completely removed; the peak at 689.3 eV has disappeared. Only one strong peak remains with a chemical shift consistent with Fe–F. Unambiguous identification of the Fe–F compound is not possible based on XPS analysis alone. However, when placed in context with the Fig. 1, the subsurface layer which is evident in region Žiii. can in general be categorized as a thick buried Fe fluoride layer Ži.e., FeF2 , FeF3 , Fe 2 F5 , etc . . . .. As seen in Table 1 and Fig. 1, there is some oxygen detected in this region as well. This may result from the presence of some Fe oxyfluorides Žseen previously in Ref. w18x and consistent with an unidentified FeŽIII. component in the CEMS, shown later., andror a result of smearing from the depth profile process. Limitations in XPS include the inability to detect hydrogen and only semi-quantitative analysis. The unsputtered and sputtered Fe 2p 3r2 spectra are shown in Fig. 3. Prior to sputtering, the Fe 2p spectra shows only one broad band centered at ; 710 eV which is characteristic of an Fe oxide. Whereas, after sputtering, the Fe 2p 3r2 spectra shows a new peak at 706.2 eV which is in good agreement with Fe metal. Along with the new Fe metal peak, a higher binding energy peak at 710.6 is observed. The exact nature of this peak is not clear although it indicates the Fe is present in high valence states Žq2 and q3. similar to that produced in an Fe oxide. Fig. 4 shows a montage of Fe region from the depth profile data of Fig. 1. From these data one can more clearly visualize the layered structure and the distinct Fe species that dominate each layer. From the atomic percentages ŽTable 1, 1100 s. the total Fe content is 34.7%. Further breakdown into Fe o and Feq2 ,q 3 reveal atomic percentages of 10.0% and 24.7%, respectively. Considering that Feq2 ,q 3 encompasses many molecular species including Fe hydroxides, Fe oxides, Fe oxyfluorides and Fe fluorides, and considering that inaccuracies in XPS cross-sections, variations in constituent sputter yields, etc . . . exist in XPS measurements, a definitive stoichiometry of the Fe-fluoride species cannot be determined. However, assuming some of the Feq2 ,q 3 is associated with Fe
Fig. 4. Fe 2p1r 2 – 3r2 region collected during the depth profile shown in Fig. 1. Chemical shifts reveal the distinct nature of the Fe chemical species within each layer on the Fe-alloy surface.
oxide ŽFerO s 0.67., the remaining Feq2 ,q 3 is in the form of an Fe-fluoride species having FerF between 0.3 and 0.5. 3.2. CEMS The CEMS spectrum shown in Fig. 5 provides identification of the Fe species present in the complex layered structure produced in the O–C test apparatus. A table of the fit parameters and relative areas is presented in Table 2. The largest area, 0.59, identified is from the M-50 substrate that is comprised of a sextuplet with a hyperfine field of 33.4 T. It is clear from this observation that the CEMS sampling depth encompasses the entire layered structure and that the other spectral components reside above the M-50 substrate. The asymmetric doublet that dominates the center of the spectrum has an area of 0.16, quadrupole splitting of 2.72 mmrs and an isomer of shift of 1.34 mmrs, clearly indicative of FeF2 . Two other sextuplets are apparent that can be identified with Fe 3 O4ŽA-site.-hyperfine field of 49.8 tesla, and Fe 3 O4ŽB-site.-hyperfine field of 45.0. The ratio of ArB is 0.78 which is slightly higher the 0.5 ratio theoretically expected. As discussed in detail by John et al. w18x, this indicates that F may be substituting for oxygen in some of the octahedral B-sites. There is also a doublet present with an isomer shift of 0.33 mmrs and a quadrupole splitting of 0.81
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nuclei within the sampling depth. Therefore, it is reasonable that the extreme surface contribution falls below the signal-to-noise threshold. The existence of catalytic FeF3 w7–17x in the extreme surface layer is consistent with the XPS results discussed above where the F 1s spectrum was shown to have a component from Fe fluoride in the unsputtered condition. As of yet there is no direct mechanism established for the formation of the FeF2 layer. 3.3. Layered structure formation
Fig. 5. CEMS spectra of a specimen a post O–C test substrate showing the Fe components within the surface region. Parameters are shown in Table 2.
mmrs. These parameters indicate the presence of an FeŽIII. compound that has not yet been clearly identified. This compound was observed previously w18x and is thought to be an Fe oxyfluoride intermediate in the formation of FeF2 . Further clarification of the FeŽIII. compound is underway and will be discussed in a future publication. The presence of FeF3 is not observed Žreference parameters provided in Table 2.. The CEMS data is complimentary to XPS data and clearly show that a complex surface structure forms on the surface of the Fe substrates in the presence of PFPAE at high temperatures. The thickest layers are FeF2 which is adjacent to the Fe substrate and Fe 3 O4 which is near the surface. Identification of these layers was unambiguously determined by CEMS and the location was confirmed by XPS. CEMS cannot resolve the topmost monolayer, because it contains only a small fraction of 57 Fe
The exact mechanism for the formation of complex corrosion layers on Fe-alloys in the presence of PFPAEs has not yet been established. However, the presence of F ions essential for the formation of a buried FeF2 layer is consistent with the various mechanisms of fluid degradation proposed by others w9,22x. Also, similar layered structure has been observed for Fe in the presence of HF solutions w23x. 3.3.1. Fluid degradation mechanisms One proposed mechanism is that strong Lewis acid sites, FeF3 , occur from initial interaction of PFPAEs with weak Lewis acid sites, Fe 3 O4 , which are found on native surfaces w9x. This scenario suggests that a slow decomposition of PFPAE occurs prior to rapid decomposition upon the formation of sufficient surface conversion of Fe 3 O4 to FeF3 . The proposed decomposition process involves the formation of HF at the surface, allowing highly reactive F species to reside within the surface region which they observe to exit the system through the gas phase. Hydrogen is present in the system from many possible sources including impurities in the production of PFPAE fluids and hydroxyl ions on the Fe-alloy and reaction vessel surfaces. Another pro-
Table 2 Mossbauer parameters obtained from the oxidation-corrosion film generated from Demnum at 2608C for 24 h ¨ Compound
Isomer shifts Žmm sy1 .
Quadrupole splitting Žmm sy1 .
Hyperfine field ŽT.
Relative area
FeŽIII. Ž?. Fe 3 O4 ŽB. Fe 3 O4 ŽA. FeF2 M-50 Ž1. FeF3 Ref. w17x
0.33 0.41 0.33 1.34 0 0.49
0.81 0 0 2.72 0 0
y 45.0 49.8 y 33.4 39.5
0.10 0.09 0.07 0.16 0.59 y
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posed mechanism suggests that decomposition proceeds through fluid interaction with hydroxyl ions bound to surface oxides w22x. Of primary importance to the work presented here is that as with the first mechanism mentioned above, a product of the decomposition reaction is the generation of labile F that resides at the surface over a brief time. Neither of the two mechanisms briefly described above probe into the mechanisms of Fe-alloy corrosion in any detail. Much of the work has focused on the topmost surface and the decomposition of the fluid. This paper reveals that F ions produced during the decomposition process of PFPAEs are not just confined to the extreme surface and fluid, but permeate down to the fresh metal interface to stimulate surface corrosion and create complex surface layers. 3.3.2. Layered structure A similar layered structure to that presented here has been observed on Fe substrates in the presence of HF. Crouse and Stander w23x performed a study using CEMS and gravimetric techniques to determine the corrosion mechanisms of Fe surfaces exposed to the vapors above HF diluted in H 2 O up to 40%. They showed that for dilutions greater than 0.1%, the formation of b-FeOOH occurred at the outermost surface of Fe by the oxidation of FeF2 P H 2 O, which formed adjacent to the Fe substrate early in the reaction sequence. Subsequently, the rate of formation of b-FeOOH at steady state was controlled by the diffusion of HF through the FeF2 P H 2 O layer to react with the Fe substrate to replenish the FeF2 P H 2 O layer. Although the work presented here is on PFPAE interaction with Fe-alloys, there are similarities with regards to surface chemistry. The major layer constituents detected during this investigation were Fe 3 O4 Žwith possible F substitution in some B-sites. above FeF2 Žadjacent to the bulk.. The FeF2 was not hydrated Žeasily discerned from CEMS., however, there is H 2 O and O 2 present in the fluid that may play a role in the reaction sequences that form the Fe 3 O4 and FeF2 layers. Also, there is a substantial amount of etching that occurs on the surface of the pyrex reaction vessel that indicate the formation of HF during the O–C test. Although, the exact mechanisms of Fe-alloy corrosion are not yet understood, the determination that a complex layered surface structure is formed during
O–C tests has direct implications with regards to the tribological properties of these systems. Mechanically weak layers or interfaces between layers will adversely affect wear rate and may perpetuate further corrosion through exposure of fresh surfaces to reactive species in the fluid. Presently, mechanistic details for the layer formation are being studied. The influence of parameters such as environment, temperature, and time will be explored.
4. Conclusions Ø A complex layered structure on the surface of Fe substrates in the presence of PFPAEs has been identified with XPS. This structure has three distinct layers, Ži. the extreme surface Ž5 nm. –likely containing the reactive catalytic species FeF3 which has been shown in the literature to decompose the fluid, Žii. mid-region Ž5–55 nm. –predominantly Fe oxide, identified as Fe 3 O4 , and Žiii. buried region Ž55–200 nm. –identified with CEMS as FeF2 . Ø The formation of the subsurface FeF2 layer is likely a result of a decomposition reaction during which F ions are produced and then diffuse into the surface. Various proposed decomposition reactions in the literature involve the production of these F ions which at least in part leave the surface by gas phase species. Results presented here indicate that conditions favor diffusion into the surface and reaction with subsurface Fe as well. Ø An understanding of the mechanism by which these surface layers form and the relationship with parameters such as environment, temperature, and time may prove beneficial to the design of effective lubrication systems involving PFPAEs and Fe-containing surfaces.
Acknowledgements The authors would like to thank Dr. K.C. Eapen, Mr. G. Fultz, and Ms. O. Scott of the University of Dayton Research Institute for sample preparation and helpful discussions. The authors are grateful to Drs. J.S. Zabinski and H.L. Paige for their thought provoking comments. This work was supported by the Materials Directorate of the Air Force Research Lab-
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oratory, Wright-Patterson Air Force Base, OH and the Air Force Office of Scientific Research, Bolling Air Force Base, Washington, DC. References w1x W.H. Gumprecht, ASLE Trans. 9 Ž1966. 24. w2x D. Sianesi, V. Zamboni, R. Fontanelli, M. Binaghi, Wear 18 Ž1971. 85. w3x C.E. Snyder Jr., R.E. Dolle Jr., ASLE Trans. 19 Ž1976. 171. w4x W.R. Jones Jr., C.E. Snyder Jr., ASLE Trans. 23 Ž1980. 253. w5x K.J.L. Paciorek, R.H. Kratzer, J. Fluorine Chem. 67 Ž1994. 169. w6x J.S. Zabinski, M.A. Capano, A.A. Voevodin, Proceedings of the 82nd NATOrAGARD Tribology of Aerospace Systems Specialist Meeting, NATO, London, AGARD-CP-589, 1996. w7x W. Morales, NASA TP 2774, 1987. w8x D.J. Carre, ´ ASLE Trans. 29 Ž1985. 121.
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