Foil bearings are particularly attractive for systems where conventional oil-lubricated or rolling element bearings are unsuitable due to temperature, speed, working fluid or any combinations thereof. Since foil bearings are oil-free and can operate with ambient air, they are also environmentally friendly solutions for systems where lubricant contamination would be problematic. Applications now possible for foil bearings include aircraft gas turbine engines, auxiliary power units, microturbines, pumps, compressors, cryogenic turboexpanders and turbochargers, to name a few. Recent advances in foil bearing design have resulted in significant improvements in load capacity, damping, stiffness and hightemperature performance. The lack of effective hightemperature solid lubricant coatings and/or alternate materials currently limits the use of foil bearings to operating temperatures below 300 8C. Therefore, reliable, long-life, high-temperature materials and coatings are necessary to transition these advanced bearing and seal technologies from ambient temperature applications to the extreme temperature environments found in gas turbine engines. In order to develop effective high-temperature foil bearings, new coating combinations that exhibit good high-temperature tribological characteristics must be found. Oil-free compliant foil bearings (CFBs) and seals (CFSs) with the ability to operate under large rotor excursions, high temperatures and surface speeds without a liquid lubrication system have the potential to make significant improvements in gas turbine engine performance, fuel consumption, operability and reliability. Compliant foil bearings are already used in and have been evaluated for a wide variety of rotating machinery ranging from turboexpanders and industrial compressors to air handling systems for fuel cells to numerous gas turbine engines. While the most successful foil bearing application to date has been aircraft air cycle machines for cabin pressurization and cooling, their thermal environment has not required high-temperature coating.
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