LauerTL2009 - Tribol Lett DOI 10.1007/s11249-009-9514-7 1...

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Unformatted text preview: Tribol Lett DOI 10.1007/s11249-009-9514-7 1 ORIGINAL PAPER 4 5 Nicolas Argibay James H. Keith Brandon A. Krick D. W. Hahn Gerald R. Bourne W. Gregory Sawyer 6 7 Received: 5 August 2009 / Accepted: 25 August 2009 Ó Springer Science+Business Media, LLC 2009 OO PR example, there are tremendous efficiency gains that can be realized only if the operating temperatures are raised. The thermal limits of conventional lubrication strategies remain a daunting obstacle to these design concepts. As outline by Lauer and Bunting [1]: ED Abstract Following the pioneering work of Prof. James Lauer, the ability to provide continuous solid lubrication through vapor phase delivery of carbonaceous gases has been successfully demonstrated on a pin-on-disk contact at the temperatures of 650 °C. Results from tribological experiments under 2 N normal load and 50 mm/s sliding speed showed an over 209 reduction in friction coefficient. The samples were silicon nitride (pin) versus CMSX-4 (disk) and the experiments when run in a nitrogen environment with an acetylene admixtures. Two repeat experiments gave average friction coefficients of l = 0.03 and l = 0.02. The process was robust and provided low friction for the entire 500 m of sliding. Using focused ionbeam milling, high-resolution transmission electron microscopy, and confocal Raman spectroscopy, the resulting solid lubricant was found to be oriented microcrystalline graphite. Several possibilities exist for high temperature lubrication. They are each listed here with a major drawback which needs to be overcome. CO RR E CT Synthetic fluids: Solid lubricants: Molten glass: Keywords Vapor phase lubrication Á High-temperature tribology Á Solid lubrication 28 1 Introduction 29 30 High-temperature lubrication continues to be a limitation for a wide variety of applications. In power generation, for A1 A2 A3 A4 A5 N. Argibay Á J. H. Keith Á B. A. Krick Á D. W. Hahn Á W. G. Sawyer (&) Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA e-mail: wgsawyer@ufl.edu A6 A7 A8 G. R. Bourne Á W. G. Sawyer Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA UN Author Proof 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 F 3 High-Temperature Vapor Phase Lubrication Using Carbonaceous Gases 2 500 °C maximum use temperature replenishment Solid at room temperature Only solid lubricants show any promise of operating from ambient to above 500 °C. Solid lubricants would have to be replenished in order to provide long life and reliable operation at high temperatures. Various types of replenishment systems have been suggested, and they include such methods as: 1. 2. 3. 4. Stick or Powder Feed Gaseous or Liquid Suspension feed Incorporation in pockets or retainers Gaseous materials which react at the surface. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 An example of this last method, which is the subject of this paper, would be to chemically form the lubricant directly on the bearing surfaces from a gaseous feed material. 54 55 56 57 Lauer’s pioneering work with vapor phase lubrication began with a hypothesis that the exposure of the nascent surfaces through wear would catalyze reactions with the ambient environment. In the 1988 paper by Lauer and Bunting [1], they were able to show that friction coefficients as a low as l = 0.10 were able to be achieved at 58 59 60 61 62 63 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 31 32 33 34 35 OO Author Proof F Tribol Lett laboratory (b), the admixture of gases such as acetylene would provide a replenishment of the carbonaceous solid lubricant on the wear track during exposure to the environment. The films have recently been observed and modeled as fractional (c) with regions of solid lubricant coverage on top of nascent surfaces 64 65 66 67 68 69 70 71 72 73 74 75 temperatures of 500 °C using ethylene as a feed gas for carbonaceous lubricating films (one of the initial choices of a substrate material was a nickel containing alloy that was thought to promote a ‘‘catalytically generate(d) carbon’’ film. This technique of using chemical reactions to build a solid lubricating film even as the existing film is being worn away became widely referred to as vapor phase lubrication. An illustration of Lauer’s vision for a vapor phase lubrication approach of a low heat rejection (‘‘adiabatic’’) diesel engine is shown in Fig. 1. As Lauer and Dwyer described in a 1991 paper [2], 76 77 78 79 80 81 the authors have been injecting a carbonaceous gas, such as ethylene, continuously into the conjunction region of the tribosurfaces. A well-adhering lubricating carbon film is formed and replenished after wear. The authors’ approach is essentially not destructive of the solid wear surface… The ability to provide low friction in pin-on-disk, rolling four-ball, and combined rolling-and-sliding contacts was a commonplace during the 1990s in the 5th floor laboratory of the Jonsson Engineering Center at Rensselaer Polytechnic Institute, where much of this work was performed. During this time, a number of different high-temperature tribometers were developed and equipped with vapor phase delivery systems, and the carbonaceous gas with the most efficacies for high-temperature vapor phase lubrication turned out to be acetylene. Working with acetylene (which is reportedly an intrinsically unstable compound that will readily decompose explosively) as an efficient, vapor phase additive seems in some ways contrary to Lauer and Dwyer’s [9] discussion in their 1990 manuscript on the topic: 94 95 96 97 98 99 100 101 102 103 104 105 106 107 After all, what could be simpler than injecting a gas,…, and converting it into a solid lubricant (graphite like) right on the wear surface? 108 109 110 82 83 84 85 86 87 88 89 90 91 92 93 Over the years, vapor phase lubrication with a number of different carbonaceous gases and tribological materials has been successfully demonstrated in the laboratory. The range of materials includes high performance ceramics (nitrides [3–6], carbides [2, 3], oxides [2, 3]) and hightemperature metals and alloys (bearing steels [3, 7, 8], stainless steels [3], and nickel super alloys [1, 3]). The range of gas chemistries include a wide variety of hydrocarbons (ethane, ethylene, acetylene, benzene, propane, and 1-propanol) [1, 2, 5, 9–11] as well as mixtures of carbon monoxide and hydrogen, and a simulated rich burn exhaust gas [4, 6]. A glimpse into some of the complexity of this approach can be found in a rather innocuous description of the safety measures [11] that were implemented in the home-built high-temperature tribometer used in these studies. 111 112 113 114 …as illustrated…the present apparatus retains the two chambers…the inner one where the tests are actually carried out and the outer one that provides the safety shield by being filled with argon or another inert gas. The top of the outer chamber is not attached so that it can lift in case of an explosion. 115 116 117 118 119 120 These little explosions were not actually as uncommon as one might hope. During undergraduate research in the 121 122 UN CO RR E CT ED PR Fig. 1 An illustration of Lauer’s high-temperature vapor phase lubrication (VPL) concept. As shown in (a) the injection of an appropriate gas admixture would lead to the deposition of thin and lubricous solid lubricants that could be continuously replenished during operation. In pin-on-disk experiments conducted in the 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 143 2.1 High-Temperature Tribometer 144 145 146 147 148 149 150 151 152 Following the designs of Lauer and co-authors [11] and Blanchet et al. [4], a commercially available high-temperature pin-on-disk tribometer (CSM) was modified to have dual chamber design (the inner chamber was at elevated temperature and contained acetylene and nitrogen gas, whereas the outer chamber acted as a nitrogen dilution/safety chamber). The tribometer schematic is provided in Fig. 2. The outer chamber was made of acrylic and had feedthroughs for environmental control. A F 2 Experimental Procedure OO 142 continuous flux of *0.6 L/s of gaseous nitrogen was supplied to the outer chamber and was sufficient to maintain a positive pressure. An oxygen sensor (Delta-F Corp., model 310) was used to measure and ensure ppm levels of O2 were maintained inside the acrylic chamber during testing. An additional flux of N2 was also injected into the inner chamber. Finally, a stainless steel tube was inserted through the spindle housing lid and pointed at the central region of the disk; it was this tube that carried acetylene to the contact. The sample surfaces for these studies were maintained at 650 °C. Normal load was applied through dead-weights and counterbalances, and the rotary motion was controlled via a belt-driven spindle and servo motor with a coupled rotary encoder. This spindle design could provide rotation speeds in the range 0.3–500 rpm. The friction forces were measured using a calibrated flexure and linear motion transducer sensitive enough to measure into the mN range. The radius of the wear track was controlled and measured with a calibrated micrometer stage, and for all experiments performed in this study were in the range 5–20 mm. Custom data acquisition software was made to operate the tribometer using LabVIEW (National Instruments). The software controlled motor speed, oven temperature, and sample loading and unloading. The program also monitored and recorded analog input voltages for friction force, oven, and sample temperatures, the angular position of the disk, and motor RPM that were then averaged at 1-s intervals and recorded. The program saved the raw data for three full revolutions of the disk every ten or hundred seconds (user defined), which, when coupled with the measured angular position of the disk, made it possible to correlate friction coefficient with a given location on the wear track. PR laboratory, one of the authors of this manuscript (W.G.S.) often set a large adjustable wrench on the lid such that after the explosions the lid would fall back into place on the tribometer and not travel too far; this method enabled experiments to continue (even if the graduate students were a little startled). In later designs of high-temperature tribometers, PTFE seals and baffles were used to prevent leaks and maintain positive pressures within the chambers, which eliminated the occurrence of explosions during testing. Recently we have performed high-temperature vapor phase lubrication on a pin-on-disk high-temperature tribometer with acetylene feed gas. At the conclusion of these experiments, we flooded the inner-chamber with nitrogen gas in an effort to provide rapid cooling and retain the ‘‘graphite-like’’ surface films. This provided an opportunity to perform detailed microscopy characterization and spectroscopic analysis on the surface films and the near surface region of the substrates. CO RR E CT ED 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 Fig. 2 A schematic of the hightemperature tribometer used for this study. The double chamber design described by Lauer and Blanchet was implemented in this study, and was able to provide inert environments that could safely promote in situ continuous lubrication with acetylene feed gas at sample temperatures of 650 °C UN Author Proof Tribol Lett 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 Tribol Lett 3 Tribological Results 212 213 Results from three experiments are shown in Fig. 3. At the top of Fig. 3a, the friction coefficient for the silicon 232 The ability to provide adequate lubrication continuously using a vapor phase delivery of a solid lubricant requires that the deposition rate of the solid lubricant at least balances the removal rate. In the case of vapor phase lubrication with carbonaceous gases, the elevated temperature makes in situ studies of the competitive rates of formation and removal challenging. In competitive rates modeling efforts, Blanchet et al. [4] first treated the process as a thin film growth and removal process where the net rate of 233 234 235 236 237 238 239 240 241 F 211 4 Discussion and Analysis of the Carbonaceous Films OO The pin was a 6.35-mm diameter silicon nitride sphere. The disk was a high strength and high-temperature superalloy of predominately Nickel and Aluminum (CMSX-4) that is intended for use in turbine engines. The disks were Electro Discharge Machined to have 55 mm diameters and thicknesses of 10 mm. The disk samples were carefully polished using standard metallographic techniques and had initial RMS roughness values of better than Rq = 20 nm. The outer chamber was purged using N2 to provide an O2 concentration of less than 100 ppm before commencing heating. The temperature of the disk reached 650 °C before the pin was actuated into contact and sliding commenced. A sliding speed of 50 mm/s and normal force of 2 N were used for all tests. All tests were allowed to run to a total sliding distance of 500 m. The sliding direction was reversed thrice during the test in order to accurately compute friction coefficients [12, 13]. The pin was actuated out of contact upon completion of sliding and the disk and spindle were allowed to cool to below 100 °C before shutting off the nitrogen cover gas flow and opening the chamber to remove the pin and disk. The pin and disk were then placed in a sealed plastic container for transfer to the various characterization equipment. 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 PR 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 nitride versus CMSX-4 in a nitrogen environment at 650 °C is shown over 500 m of sliding. This combination had high friction coefficients, which near the end of the test approach unity. Over the entire duration of the experiment, the friction coefficient had an average value of l = 0.77 with a standard deviation of r = 0.07. In stark contrast, the friction coefficient for two repeat experiments run with acetylene admixtures (with a flow rate of *0.003 L/s) had average friction coefficients of l = 0.03 and l = 0.02 with standard deviations of less than r \ 0.01. Figure 3b shows the traction coefficients for the two experiments run with acetylene admixtures, where the positive and negative signs are for forward and reverse sliding directions, respectively. The friction coefficient remained very steady and low for the duration of these acetylene admixture experiments, and it was clear that the continuous supply of acetylene was sufficient to provide continuous low friction. CT ED 2.2 Samples, Preparation, and Experimental Procedure CO RR E Fig. 3 Results from tribological experiments conducted at 650 °C under 2 N normal load and 50 mm/s sliding speed. a The friction coefficient for the silicon nitride versus CMSX-4 in a nitrogen environment is shown over 500 m of sliding (l = 0.77) and two repeat experiments run with acetylene admixtures (l = 0.03 and l = 0.02). b The traction coefficients for the two experiments run with acetylene admixtures, where the positive and negative signs are for forward and reverse sliding directions, respectively UN Author Proof 187 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 Tribol Lett F CT ED where a and b are constants for the deposition rate and removal rate terms, respectively. The deposition rate is assumed to follow an Arrhenius dependence, with an activation energy Ea, gas constant R, and T is the absolute temperature. The flowrate v of the carbonaceous feed gas was assumed to be responsible for setting the local concentration and thus directly influence the film growth rate. The removal rate is essentially Archard like (i.e., having a linear dependence on normal load Fn), and the details of the contact area, wear rate, and sliding speed are lumped into b. The only reason that sliding speed did not appear in [4] was that the original study was conducted at constant sliding speed. The authors successfully mapped out regimes of adequate (low friction) and inadequate (high friction) lubrication. However, the model predicted either complete coverage or zero coverage, and such a process should show a binary trend in friction coefficient. In practice, the friction coefficient could vary smoothly between the two extremes. This finding of intermediate values in friction coefficient then led to the development of a series of fractional coverage models for vapor phase lubrication [7, 8] and even a model for the removal of a fractional solid lubricant film [14]. During these modeling activities, a number of persistent questions emerged. What is the lubricating film? How thick is? Is it fractional? Some of these questions can be answered with the surface analytical instrumentation available today. Therefore, in an attempt to preserve the solid lubricant film and enable characterization the experimental approach followed here included flooding the contact with dry nitrogen during the cool down period of the experiments. Using a focused ion beam scanning electron microscope (FIB/SEM) samples of the disk run with and without the acetylene atmosphere were imaged. The FIB instrument was used to produce site-specific thin transmission electron microscopy (TEM) samples. Figure 4a shows a solid lubricant film that has developed in the wear scar, it is clear from this image that the film is fractional. At higher magnification, Fig. 4b, there is evidence of delamination of the carbonaceous film, and film chips as seen in the center of the micrograph could be readily found within the wear track and in the debris fields. Such a removal process leaves behind bare or nascent surfaces of the CMSX-4, which are clearly visible as bright areas within the wear scar. These carbonaceous films measure approximately 100–500 nm in thickness. CO RR E 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 ð 1Þ UN Author Proof dC ¼ aveðÀEa =RTÞ À bFn ; dt A sample suitable for TEM was produced by FIB milling a longitudinal section of the wear scar in the direction of sliding from the sample exposed to the acetylene atmosphere. Figure 4c shows a bright field TEM image of the lubricant film, which has a thickness of on the order of a few hundred nanometers. The lubricant film also reveals a columnar-like grain growth normal to the counterface. Figure 4d shows a high-resolution TEM (HRTEM) taken from the sample in Fig. 4c. The very fine scale (\10 nm) of the graphite grain microstructure is apparent at this magnification. The selected area diffraction (SAD) pattern in the upper corner of Fig. 4d also indicates a textured nature of the carbonaceous lubricant film with a layer spacing measuring *0.35 nm, which is consistent with the spacing of graphite. Together the HRTEM and the SAD pattern provide consistent indication that the basal planes are parallel to the sliding surface and that the carbonaceous film is graphite. Similar orientation of other lamellar solid lubricants has recently be shown using HRTEM and FIB sectioning along the direction of sliding [15]. Figure 5a, b is ion channeling contrast secondary electron images produced on trenches that are made by FIB milling along the direction of sliding within the wear tracks. In both figures, the top most layer is an in situ Pt deposition used to protect the area of interest from the milling beam and to provide good edge retention at the top most surface. Figure 5a is from the sample that was run in nitrogen cover gas. This sample had high friction coefficient, and shows a fine-grained region of approximately 1-lm thick that experienced severe plastic deformation. Below the fine-grained area, there is a region of largerscale deformation as indicated by bending of the deformation of grains in the sliding direction. In Fig. 5b, the dark region just below the Pt cover is the graphite solid lubricant film. The grains in the substrate in Fig. 5b below the solid lubricant film show no deformation, as the graphite film has provided a low-friction protective layer for the nickel substrate. The final 100 m of sliding was done unidirectionally, which was sufficient to cause grain orientation in the direction of sliding in the unlubricated test. It is this protection from the solid lubricant layer and the friction reduction that likely led to the 1009 wear reductions commonly reported by Lauer and co-authors in their pin-on-disk studies. Raman spectroscopy was used to examine the graphitic nature of the solid lubricant films. Raman spectral data were recorded using a confocal micro-Raman dispersive spectrometer (LabRam, Jobin Yvon) with a 632.8-nm excitation source (15 mW) and 1009 objective (*5-lm focal spot). All Raman spectra were recorded using a 10-s signal integration time, averaged 25–50 times. Figure 6 shows the Raman spectra recorded within and outside of OO carbon solid lubricant accumulation on the surface could be described by Eq. 1. PR 242 243 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 Tribol Lett CO RR E CT ED PR Author Proof OO F Fig. 4 a SEM image of wear track and solid lubricant film. b High-magnification SEM image showing delaminated graphite flake from the solid lubricant film. c TEM bright field longitudinal cross section of the graphite solid lubricant film. d HRTEM image from same sample as (c) 345 346 347 348 349 350 UN Fig. 5 Ion beam channeling contrast secondary electron image showing grain structures in longitudinal cross section orientation of a sample run in dry nitrogen and b sample run with acetylene the wear track, along with the spectrum recorded for a graphite reference. Both tribological films show the D-bands and G-bands at 1315 and 1597 cm-1, respectively, which are shifted and broadened with respect to the graphite reference spectrum. The G-band is attributed to the in-plane stretching of the hexagonal sheets (sp2), while admixture to nitrogen. The platinum layer is deposited in the microscope in order to protect the carbonaceous solid lubricant film from damage during milling the D-band is associated with increasing bond-angle disorder and decreasing micro-crystallinity [16, 17]. The spectra recorded out of the wear track were all very consistent with respect to peak location and width, whereas the spectra recorded in the wear track showed consistent peak widths and some variation in the location of the D-band 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 351 352 353 354 355 356 Tribol Lett 376 1. Lauer, J.L., Bunting, B.G.: High-temperature solid lubrication by catalytically generated carbon. Tribol. Trans. 31, 339–350 (1988) 2. Lauer, J.L., Dwyer, S.R.: Tribochemical lubrication of ceramics by carbonaceous vapors. Tribol. Trans. 34, 521–528 (1991) 3. Barnick, N.J., Blanchet, T.A., Sawyer, W.G., Gardner, J.E.: High temperature lubrication of various ceramics and metal alloys via directed hydrocarbon feed gases. Wear 214, 131–138 (1998) 4. Blanchet, T.A., Lauer, J.L., Liew, Y.F., Rhee, S.J., Sawyer, W.G.: Solid lubrication by decomposition of carbon-monoxide and other gases. Surf. Coat. Technol. 68, 446–452 (1994) 5. Lauer, J.L., Blanchet, T.A., Vlcek, B.L., Sargent, B.: Lubrication of Si3N4 and steel rolling and sliding contacts by deposits of pyrolyzed carbonaceous gases. Surf. Coat. Technol. 62, 399–405 (1993) 6. Sawyer, W.G., Blanchet, T.A., Calabrese, S.J.: Lubrication of silicon nitride in a simulated turbine exhaust gas environment. Tribol. Trans. 40, 374–380 (1997) 7. Sawyer, W.G., Blanchet, T.A.: High temperature lubrication of combined rolling/sliding contacts via directed hydrocarbon gas streams. Wear 211, 247–253 (1997) 8. Sawyer, W.G., Blanchet, T.A.: Vapor-phase lubrication in combined rolling and sliding contacts: modeling and experimentation. J. Tribol. Trans. ASME 123, 572–581 (2001) 9. Lauer, J.L., Dwyer, S.R.: Continuous high-temperature lubrication of ceramics by carbon generated catalytically from hydrocarbon gases. Tribol. Trans. 33, 529–534 (1990) 10. Lauer, J.L., Vlcek, B.L., Sargent, B.L.: Wear reduction by pyrolytic carbon on tribosurfaces. Wear 162, 498–507 (1993) 11. Vlcek, B.L., Sargent, B.L., Lauer, J.L.: Lubrication of ceramic contacts by surface-deposited pyrolytic carbon. Lubr. Eng. 49, 463–471 (1993) 12. Burris, D.L., Sawyer, W.G.: Addressing practical challenges of low friction coefficient measurements. Tribol. Lett. 35, 17–23 (2009) 13. Schmitz, T.L., Action, J.E., Ziegert, J.C., Sawyer, W.G.: The difficulty of measuring low friction: uncertainty analysis for friction coefficient measurements. J. Tribol. Trans. ASME 127, 673–678 (2005) 14. Blanchet, T.A., Sawyer, W.G.: Differential application of wear models to fractional thin films. Wear 250, 1003–1008 (2001) 15. Hu, J.J., Wheeler, R., Zabinski, J.S., Shade, P.A., Shiveley, A., Voevodin, A.A.: Transmission electron microscopy analysis of mo-w-s-se film sliding contact obtained by using focused ion beam microscope and in situ microtribometer. Tribol. Lett. 32, 49–57 (2008) 16. Cuesta, A., Dhamelincourt, P., Laureyns, J., Martinezalonso, A., Tascon, J.M.D.: Raman microprobe studies on carbon materials. Carbon 32, 1523–1532 (1994) 17. Jawhari, T., Roid, A., Casado, J.: Raman-spectroscopic characterization of some commercially available carbon-black materials. Carbon 33, 1561–1565 (1995) 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 Fig. 6 Raman spectra recorded inside and outside of the wear track, along with a reference spectrum recorded for a graphite rod. The upper two spectra have the same intensity scale peak (1315–1328 cm-1) and the G-band peak (1586– 1604 cm-1). Overall, the Raman data suggest deposition of a microcrystalline, graphitic carbon that is not appreciably altered by the sliding motion of the pin 361 5 Closing Remarks 362 363 364 365 366 367 368 As found previously by Prof. Lauer, Prof. Blanchet, and their numerous students and colleagues, carbonaceous gases provide an effective lubrication strategy for continuous operation of high-temperature tribological contacts. This study has revealed that the solid lubricant films responsible for the friction reduction are thin (below 1 lm), fractional, and graphitic. 369 370 371 372 373 374 375 Acknowledgments The authors gratefully acknowledge Bertrand Bellaton and the team at CSM-Instruments for their help, assistance, and modifications to the high-temperature tribometer used in this study. W.G.S. is also indebted to Prof. Blanchet and Prof. Lauer for introduction into laboratory research, tribology, and high-temperature vapor phase lubrication, which ultimately led to undergraduate and graduate theses on the topic. UN CO RR E CT ED 357 358 359 360 PR Author Proof OO F References 123 Journal : Large 11249 Dispatch : 31-8-2009 Pages : 7 Article No. : 9514 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1088 ...
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