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Unformatted text preview: Tribol Lett DOI 10.1007/s11249-010-9689-y 1 ORIGINAL PAPER Space Tribometers: Design for Exposed Experiments on Orbit 3 Brandon A. Krick • W. Gregory Sawyer 4 5 Received: 24 February 2010 / Accepted: 18 August 2010 Ó Springer Science+Business Media, LLC 2010 OO 27 1 Introduction 28 29 30 31 32 Existing and future moving mechanical assemblies for aerospace applications depend upon the maintenance free, low friction, and low wear operation of solid lubricants and coatings of contacting surfaces. The space environment is harsh, with a wide range of thermal extremes, ultrahigh A1 A2 A3 A4 B. A. Krick Á W. G. Sawyer (&) Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA e-mail: [email protected] PR Abstract Eight pin-on-disk tribometers have been made for testing materials in space on board the International Space Station. They will be exposed directly to the low earth orbit (LEO) environment on board the ‘‘Materials on the International Space Station Experiments’’ platform where they will be exposed to extreme conditions including atomic oxygen, ultrahigh vacuum, radiation (including UV radiation), and thermal ranges from -40 to 60 °C. In order to survive launch and the LEO, these tribometers were designed to be extremely compact, rugged, and reliable. Pin-on-disk tribology experiments are now being performed with a 13.2 mm/s sliding velocity (14 RPM at 9 mm wear track radius) and a 1 N normal load with hemispherical pin of 1.5875 mm radius. Materials tested include MoS2/Sb2O3/Au, MoS2/Sb2O3/C, YSZ/Au/MoS2/ DLC, and SiO-doped DLC coatings, and bulk samples of polytetrafluoroethylene (PTFE) alumina nanocomposites and gold. vacuum, atomic oxygen, solar radiation (including destructive ultraviolet radiation), and micro meteoroids; this was recently reviewed in the January 2010 MRS Bulletin [1]. These environmental conditions collude to make most traditional lubrication strategies and intuitions gained from terrestrial tribology dubious. There remains a desire to better understand the effects of the space environment on materials and the tribological behavior of materials in that environment. Historically, the observations of material degradation as observed on returning spacecraft motivated the studies of the space environment and its deleterious effects on most materials. Over the past half-decade many exposure and flight experiments have been devoted to understand this space environment and explore the possibility of terrestrial laboratory recreation of these conditions. One of the first extensive materials exposure experiments in space was NASA’s Long Duration Exposure Facility (LDEF). Since then, there have been a number of exposure experiments including experiments from the European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and the National Aeronautics and Space Agency (NASA) (recently reviewed by Edwards [2]). The needs for durable and high performance spacecraft are increasing, and tribological materials remain at the forefront of materials development efforts. Eight pin-ondisk tribometers were designed, constructed, and flown as part of the seventh Materials on the International Space Station Experiments (MISSE 7) platform. MISSE 7 is the seventh of a series of experiments designed to provide the community with a platform to perform experiments in the harsh low earth orbit (LEO) environment [3–5]. This ambitious seventh MISSE program includes several active experiments, which actively collect data on orbit and many passive experiments (exposure experiments that are CO RR E CT ED 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Keywords Space tribology Á Tribometers Á Aerospace Á Coating Á Solid lubricants Á Low earth orbit UN Author Proof F 2 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 2 Low Earth Orbit Environment 3 Tribometer Design 121 3.1 Constraints 122 Due to the limited size of the passive experiment carriers used on MISSE 7, the high cost of space missions, and the limited real estate allotted for tribometers, the design and packaging was constrained to two 60 mm9100 mm footprints with a height of 50 mm. In each 60 mm9100 mm space we designed and packaged four rotating pin-on-disk tribometers. One group of four was designated as the ‘ram’ experiments and another group of four was designated as the ‘wake’ experiments. Launch vibrations are over eight g’s during certain periods of the Space Shuttle’s * 8-min ride to orbit. Based on the MISSE 6 vibration conditions, the following random vibration accelerations will be seen by the tribometers: translational accelerations (Nx= ±8.8 g, Ny= ±10.6 g, and Nz= ±8.1 g), and rotational accelerations (Rx= ±195 rad/ s2, Ry= ±60 rad/s2, and Rz= ±80.0 rad/s2) relative to the coordinate system defined in Fig. 1c. The Miles equation, a single degree of freedom solution to random vibration that allows us to approximate the random vibrations with a very conservative 3r design estimate of 60 g’s, was used to simulate these random vibrations applied to our tribometers [14]. Instrumentation for force measurement typically relies on sensible flexibility; thus, these vibrations constrain designs to use light, stiff, and strong components. The entire set of MISSE experiments was allocated 289 W nominal and 410 W maximum electrical power draw. In addition, the human element is not absent in space; the designs were also required to withstand a ‘kick-load’ of 560 N from the astronauts, and not puncture or cut the spacesuits. Sharp corners and edges are not allowed and any edge must have fillets greater than 1 mm or be completely round (any exposed corners must have a radius of 13 mm). 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 3.2 Targeted Experimental Conditions 155 Although the tribometers are capable of operating over a wide range of normal loads and rotational speeds, nominal operational parameters were assigned for this mission. As this is the first opportunity for this type of experiment, parameters were assigned to be somewhat representative of the contact pressures and sliding velocities seen in many space applications and laboratories. The conditions were also picked such that they are repeatable in laboratory tribological testing. The nominal operating parameters are: 156 157 158 159 160 161 162 163 164 CT ED The common perception of space is ultrahigh vacuum and low gravity; however, LEO consists of a particularly aggressive environment. The International Space Station is in LEO*350 km above the earth’s surface and is moving at*27,700 km per hour. At this altitude the environment contains a substantial pressure of atomic oxygen, solar UV radiation (Fig. 1a, b), ionizing radiation, thermal cycling (-40 to 60 8C), and micrometeoroids and debris [1]. The ultrahigh vacuum levels so commonly thought of as the space environment are only expected to be experienced on the ‘wake’ surface (trailing surface) of the experiments. Fig.1c summarizes the conditions of LEO as expected for the MISSE 7 mission. Atomic oxygen is the most prevalent molecular species in LEO and is generated by the breakdown of molecular oxygen (O2) by UV radiation. It has been proven to be highly destructive to surfaces, oxidizing and degrading metals, polymers, and other materials in the LEO environment [9]. Owing to the high orbital speed of the International Space Station, the atomic oxygen (impact energy * 4.5 ± 1 eV) is impinging the ‘ram’ surface (leading surface) at a rate of 1014–1015 atoms/(cm2s) while the trailing surface is in the ‘wake’ and is effectively devoid of atomic oxygen exposure [10]. A much broader frequency and higher spectral intensity of light exists in the extraterrestrial environment, particularly on the UV side of the spectrum [11, 12]. This radiation can be damaging too many materials and in particular is known to degrade polymeric materials [13]. Temperatures of the experiments will continuously change as the experiments cycle into and out of direct exposure to the sun. Based on the positioning of these experiments on the International Space Station, the predicted temperature range of the experiments is from -40 to ?60 °C as given by the MISSE 7 mission constraints. CO RR E 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 F 82 Designing for thermal expansion over this range of temperatures and selecting electronics is a typical design challenge of space. OO analyzed in the lab upon mission completion with no active data acquisition on orbit) including solar arrays, tribometers, optical samples, spectrometers, and others [6]. The development of tribometers for use in space has resulted in a compact and robust tribometer; this tribometer design is versatile and can be used by others in the tribology community to carry out experiments in vacuum and other challenging environments. Further, these tribometers are compact enough to be relatively easily coupled to various in situ surface analysis techniques [7, 8], including Scanning Electron Microscopes (SEM), X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, and others. This manuscript will discuss the design constraints from LEO and the resulting design of the tribometer hardware. PR 68 69 70 71 72 73 74 75 76 77 78 79 80 81 UN Author Proof Tribol Lett 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 Tribol Lett – CT ED PR Author Proof OO F Fig. 1 The MISSE 7 tribometers will experience a variety of harsh conditions while in low earth orbit. They will be exposed to solar radiation, UV radiation (a) and (b), and atomic oxygen while facing the ‘ram’ direction of the International Space Station in contrast to being in ultrahigh vacuum while facing the ‘wake’ direction (c) Normal force * 1 N (varies slightly from sample to sample) Pin radius of 1.6 mm Wear track radius 9 mm Rotational disk speed 14 RPM ± 0.05 Linear sliding speed 13.2 ± 0.1 mm/s – – – – 171 3.3 Tribometer Assembly and Components 172 173 174 175 176 177 178 179 180 181 182 A pin-on-disk tribometer was selected because of the simplicity and ubiquitous nature of the design. The tribometer uses a piezo-electric actuator to pull a monolithic biaxial force transducer into contact with the spinning disk, and to prevent contact with the disk when it is not energized. The flexure pivots and loads a spherically capped pin onto the disk. The disk is rigidly mounted to a spindle with a 157:1 gear head driven by a vacuum compatible DC motor. The normal and tangential forces focus strains at flexure points on the monolithic flexure and are measured via strain gages. This is shown schematically in Fig. 2a. UN CO RR E 165 166 167 168 169 170 3.3.1 Monolithic Biaxial Force Transducer and Loading Flexure (or Flexure) 183 184 The monolithic flexure acts as the normal and friction force transducer, as well as the pin holder. The two orthogonal cantilever beams are instrumented with four strain gages in a full Wheatstone bridge configuration. The flexure was made of Ti6Al4V due to its high strain to failure and strength to weight ratios. The instrumented cantilever beam portions of the flexure are 0.5 mm thick, 5 mm wide, and 8 mm long. A set screw holds a 3.175 mm hemispherical pin at the end of the flexure. The piezoelectric actuator is mounted near the pivot of the flexure to amplify the displacements and accommodate wear during testing. 185 186 187 188 189 190 191 192 193 194 195 3.3.2 Piezo-Electric Actuator Assembly 196 A custom piezo actuator was purchased from Cedrat technologies that can displace a maximum distance of 150 lm (at 0 N load) in the axial direction and apply a maximum force of 18 N at 0 lm displacement. With the 197 198 199 200 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 Tribol Lett CO RR E CT ED PR Author Proof OO F Fig. 2 MISSE 7 Space tribometer schematic. a Single space tribometer schematic with all major components labeled. The tribometer consists of a monolithic biaxial force transducer, loading flexure, and a pin holding assembly that is actuated by a piezoelectric actuator. This applies a normal force to the sample disk, which is turned by a geared motor. b Packaging for four space tribometers onto a single base plate as flown on MISSE 7b tribometer geometry, this yields a 1 N load and 500 lm displacement at the pin/disk interface. The piezo is 9.8 9 12.8 9 9 mm in overall dimensions, has a mass of only 2 g, requires an excitation voltage between 0 and 150 V, has a stiffness of 0.23 N/lm, and a capacitance of 0.25 lF. 206 3.3.3 Motor/Spindle Assembly 207 208 209 210 211 212 213 A flat, coreless DC motor was selected from Maxon motors, due to the compact size, vacuum compatibility, and robustness. The motor was paired with a 157:1 gearbox that was lubricated with vacuum compatible grease. The high torque, slow speed, low power design afforded the opportunity to install eight tribometers on the MISSE 7 platform subject to power constraints. UN 201 202 203 204 205 3.3.4 Baseplate 214 All components of the tribometer are mounted to a common baseplate that is mounted directly onto the MISSE 7 platform. It was designed such that four tribometers fit onto a single 60 9 100 mm area. This base was made of 304 stainless steel, and securely fastened the flexures, motors, piezos, sidewall (a protective aluminum wall surrounding the tribometers as seen in Fig. 2b), and thermocouples. Open areas of the base between the tribometers housed several polymer samples to be exposed to the environment and analyzed and undergo tribological testing upon the return of the experiments. There are also several standoff washers between the base and the MISSE 7 platform that are coated with MoS2/Sb2O3/Graphite; these washers serve 215 216 217 218 219 220 221 222 223 224 225 226 227 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 Tribol Lett For the experimental design and instrumentation used on this tribometer embodiment, the uncertainty in forces was u(F) = 15 mN, and the normal load was nominally FN = 1 N. From these conservative uncertainties, the uncertainty in friction coefficient is u(l) = 0.01 or less; Appendix 1 shows a detailed breakdown of the uncertainty. Subtracting the clockwise and counterclockwise lateral forces allows us to remove any bias errors from the system that arise from uncertain zeroing of the lateral force channel. 284 285 286 287 288 289 290 291 292 293 5 Material Selection 294 With such a rare opportunity for testing, material selection was a large responsibility that was not solely done by the authors; materials were determined by a number of collaborators within the research community and the aerospace industry (all gratefully acknowledged at the conclusion of the manuscript). This collaboration resulted in the selected flight materials as outlined in table 1. Four materials were selected for the ram face (R1–R4), and four materials were selected for the wake face (W1–W4); two materials are on both the ram and wake experiments. Soft metals such as gold, silver, lead, and indium are materials that have been used as tribological materials in space for many years. Gold (R4) will be tested on the ram set of experiments; it is of particular interest as it is inert, oxidation resistant, and has been used as a solid lubricant on orbit. For this experiment we used a sapphire pin, which should also be stable in the LEO environment. Polytetrafluoroethylene (PTFE) is a polymer that is well known for its stability and low friction characteristics. It is widely regarded as a candidate material for use on orbit; however, the high wear rates limit the number of applications. PTFE filled with alpha phase alumina nanoparticles has been shown to provide ultralow wear behavior (the wear rate has been shown to be reduced by almost 10,000 times [15, 16]). These PTFE nanocomposites at 10 wt% alumina loading will be examined on both the ram and wake environments (R1 and W1). MoS2 is a lamellar solid that has proven experience in a vacuum and space [17–25]. On the ram face a commonly used MoS2/Sb2O3/Au coating [21, 26] will be tested in part as a control sample given its proven experience but 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 F Each orbit of the International Space Station lasts approximately 92 min. Periodically over the first half of 2010, the tribometers will run a set of 16 experiments of 1 min duration throughout an orbit. The schedule of experiments is based on the diurnal cycle of the International Space Station. Over the first week, the experiments are attempting to collect data during the following orbits: 1st, 2nd, 5th, 10th, 20th, 50th, and 100th orbits. After this first week on orbit, the experiments then begin to run once at 100 orbit intervals (about once a week) for the duration of the mission. As solid lubricants wear during operation, their lifetime is finite and running continuously may wear through the samples far too early in the mission. The spacing of every 100th orbit was selected as a compromise to study the effects of long-term exposure and provide significant amounts of on orbit data. Due to the limited power constraints the protocol is not to run the tribometers at the same time. Finally, on board memory is limited to 1 GB per tribometer, and we are saving a bolus of kHz data acquisition to the onboard memory along with processed and averaged data that is being telemeter back to ground through available NASA channels. On a selected orbit, each tribometer on a given side (Ram or Wake) will run 16 one minute tests equally spaced throughout the orbit as shown in Fig. 3a. To insure that only one tribometer runs at a time, the tribometers (1, 2, 3, and 4) will run sequential 1 min cycles, followed by a 1 min break as shown in Fig. 3b. After this break, the testing repeats with tribometer 1 to complete the 16 one minute tests for each tribometer distributed over an entire orbit of the earth. Each 1-min test starts with a period of unloading to zero the normal (FN) and lateral (FL) forces and account for any thermal drifts that may occur during the orbit. After zeroing, the experimental protocol is to perform seven complete revolution cycles in the clockwise direction (denoted by the subscript cw) followed by seven complete revolution cycles in the counterclockwise direction (denoted by the subscript ccw); finally the tribometers are unloaded and the force transducers collect data to measure any drift in the signal zeros. A typical voltage trace for a 1-min tribology experiment is shown in Fig. 3c. The sinusoidal behavior of the voltages is the result of the disk test surface not being normal to the axis of the motor causing variations in the height of the disk relative to the transducer which results in a fluctuation in normal force and consequent friction force. OO 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 PR 4 Experimental Protocol The average friction coefficient, l, for each 1-min  experiment is a function of the average normal force (FN ), the average friction lateral force for rotations in the   clockwise (FLcw ) and counterclockwise (FLccw ) directions; it is calculated according to Eq. 1.   FL À FL l ¼ cw  ccw ð 1Þ 2FN UN CO RR E CT ED two purposes: to space the experiments off of the MISSE platform to insure adequate room for the motors and to be used as passive samples that can be tested upon return. 231 Author Proof 228 229 230 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 CO RR E CT ED PR Author Proof OO F Tribol Lett 326 327 328 329 330 331 332 333 334 335 336 of orbit that a single experiment would occupy. c The schedule is programmed to run one minute and a typical friction experimental voltage trace is shown. Each tribometer will run clockwise for 30 s and then counterclockwise for 30 with voltage zeroing at the beginning and end of each experiment unmeasured performance in space. Recently multifunctional nanocomposites have been developed under a hypothesis of adaptive behavior to changing environments [27]. A MoS2/ Sb2O3/C [24] composite coating will be tested on the wake face (W3). The role of carbon in similar films has been the subject of recent study [28, 29], and the susceptibility of attack from atomic oxygen is not a primary concern in higher orbits. The next generation ‘‘Chameleon’’ coatings are using continuous amorphous-nanocrystalline YSZ matrix surrounding nanoscopic inclusions of Au, MoS2, and DLC. These coating are showing promising results in the laboratory and were selected for experiments on both the ram and wake faces (R2 and W2). Hydrogenated diamond-like carbon (DLC) coatings are a class of materials that exhibit extremely low friction and wear with especially good performance in dry and vacuum conditions [30]. However, stability with respect to temperature and oxidation—key challenges for aerospace deployment—are critical limiting factors for conventional DLC’s. Advanced DLC’s formed by adding dopants represent a promising route to ameliorating these concerns. SiO-doped DLC enhances durability and thermal stability UN Fig. 3 Low earth orbit space tribometer on orbit operations and experiment schedule. a The low earth orbit space tribometers are programmed to run one minute experiments at intervals throughout the space station’s orbit. b The datum points (1, 2, 3, and 4) each represent a tribometer on the ram or wake side and shows the fraction 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 337 338 339 340 341 342 343 344 345 346 347 Tribol Lett Table.1 Results from ground testing in the NASA Marshall clean room after final assembly and integration of the tribometers on the MISSE 7b platform. Disk sample/coating Ball material Disk substrate Preflight/cleanroom testing results l (avg) Fn (avg) N Cycles # Distance (m) 87.9 0.12 1 111 304 Stainless Steel 0.15 1.01 111 87.9 440C Ruby Bulk PTFE/Al2O3 Bulk gold 0.16 0.2 1.06 0.73 73 68 57.8 53.8 MoS2/Sb2O3/Graphite 440C 304 Stainless Steel 0.12 0.98 72 57 440C Bulk PTFE/Al2O3 0.12 1.08 70 55.4 YSZ/Au/MoS2/C 440C 304 Stainless Steel 0.12 0.96 71 56.2 DLC/SiO-doped 440C 304 Stainless Steel 0.18 0.8 67 53 OO F 304 Stainless Steel 440C PTFE/nano-Al2O3 The tribometers were run in lab environments of 22C at 24% relative humidity and followed the experimental protocol described in the text. The friction coefficients reported here are averages from the entire testing duration. All experiments had at least 1 h of sliding 354 6 Closing Remarks 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 Eight pin-on-disk tribometers were developed and delivered to the International Space Station, they launched on November 16th 2009 aboard the STS-129 shuttle Atlantis. Four tribometers were installed facing forward (ram) and the other four tribometers were installed facing backwards (wake); forwards and backwards are relative to the space station velocity. These tribometers will run for approximately 8–14 months at which point they are scheduled to return (tentatively summer 2010). The tribometers activated on November 27th 2009 successfully began to run experiments and send data back from the International Space Station. The tribometers are also logging higher resolution data on orbit, and these data are being logged to an onboard non-volatile memory card that will return with the tribometers as part of the STS-134 mission. The experimental result and analysis of the samples from this mission will be presented in future publications after they return to earth. 373 374 375 376 377 378 379 380 381 Acknowledgments This project, designed, developed, and eventually delivered tribometers to the International Space Station. This could not be possible without significant collaborations and support from the research community. Significant thanks are owed to: John Jones, Michelle Ewy, Andrey Voevodin, Shane Juhl, Chris Muratore, Justin Lenoff, Jeffrey Zabinski, and Andy Korenyi-Both, Gary Pippin, Andy Robb, Many Urcia, Miria Finckenor, Phillip Jenkins, David Burris, Robert Carpick, Andy Konicek, Somuri Prasad, Chandra Venkatraman, Jim Keith, Rachel Colbert, Jennifer Vail, Nick Argibay, Jason Steffens, Dan Dickrell, Scott Perry, Linda Schadler, Thierry Blanchet, and Joe Priester. 382 383 Appendix 1: Uncertainty Analysis for the Space Tribometers 384 385 PR as the Si atoms increase the tetrahedral bonding character within the film, and the high oxygen content stabilizes the film against further oxidative degradation. Macro- and nanoscale studies of SiO-doped DLC have demonstrated these desired property improvements [31–33] including through previous space exposure tests [34]. ED 348 349 350 351 352 353 The uncertainty analysis for the friction coefficient follows the methods described by Burris and Sawyer [35] and Schmitz et al. [36]. To a reasonable approximation the uncertainty in the lateral directions is the same as the uncertainty in the normal direction, u(FL) = u(FN) = u(F). In addition, the magnitude of the lateral forces during clockwise and counterclockwise rotations are approximately equal, jFLcw j ¼ jFLccw j ¼ jFL j. Hence,    2 ! 12 FL uðlÞ2 ¼ 2 ð 2Þ þ2 uð F Þ 2 2 FN FN 386 387 388 389 390 391 392 393 Upon simplification, and a substitution for the lateral force magnitude (FL = lFN), Eq. 2 can be simplified to give Eq. 3: 395 396 397 CT CO RR E UN Author Proof 440C PTFE/nano-Al2O3 Gold Wake MoS2/Au/Sb2O3 YSZ/Au/MoS2/C RAM u ð lÞ 2 ¼ 1 þ 2 l2 uð F Þ 2 2 2 FN ð 3Þ Solving for uncertainty in the average friction coefficient is compactly given by Eq. 4. For low-friction materials, the uncertainty in friction coefficient is given by Eq. 5. rffiffiffiffiffiffiffiffiffiffiffiffiffi uð F Þ 1 þ l2 u ð lÞ ¼ ð 4Þ FN 2 399 400 401 uð F Þ uðlÞ % pffiffiffi 2 FN ð 5Þ 403 For the experimental design and instrumentation used on this tribometer embodiment, the uncertainty in forces was u(F) = 15 mN, and the normal load was nominally 405 406 407 123 Journal : Large 11249 Dispatch : 25-8-2010 Pages : 9 Article No. : 9689 h LE 4 h CP h TYPESET 4 h DISK MS Code : TRIL1286 Tribol Lett FN = 1 N. Thus, the uncertainty in friction coefficient is not better than u(l) = 0.01. 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