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Air Cycle Machine Low Friction, Medium Temperature, Foil Bearing Coating


TECHNOLOGY AREA(S): Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA-265, F/A-18 Program Office

OBJECTIVE: Design and develop a durable low friction safe coating, and an application method, for use on foil bearings used in aircraft air cycle subsystem turbomachines.

DESCRIPTION: In many aircraft, air cycle machines (ACM) are used to provide cooling, cabin pressurization, and as part of the system that provides breathing gas to the pilot oxygen system. The foil (air) bearings in the aircraft ACM use coatings to reduce friction during transient conditions such as starts and stops and inadvertent contacts, when hydrodynamic forces are insufficient to support bearing loads. Properties that are desirable in these systems are high lubricity (low friction) and high durability. Post-failure teardown and analysis of current foil bearing ACMs used in U.S. Navy tactical aircraft shows unacceptable (shaft contacts the bearing base metal) coating wear in the form of off-gassing, erosion, and delamination of the bearing coating. In an effort to improve ACM reliability, an alternative coating that will meet the requirements of air cycle machine foil bearings is needed. The coating should provide a low friction contact surface that will not impede rotation during starts and stops and should be wear resistant to provide a suitably long life (goal is a 6000 operational hour bearing) prior to requiring replacement. The coating must not introduce any toxic or hazardous constituents or byproducts to the airflow over the operating temperature range of the unit and may not require redesign of any component. The air bearings are used to support both axial and radial loads (two different bearings). During operation, there is the potential for impact between the rotating shaft and the bearing surface which can generate short term temperature spikes and higher than normal stresses. The bearings are flexible and provide a limited amount of deflection with known spring rates. The normal operating temperature ranges are 250 to 350 F with spikes estimated to be as high as 1400 F.

The application of the coating to the bearing base metal must not affect the base material (high temperature metal, e.g. Inconel) integrity of the substrate and should be able to sufficiently cover the contact surfaces. The friction between the shaft and the bearing should be minimized to allow for starting of the air cycle machine.

PHASE I: Design and develop an innovative coating material, and a means of applying the coating, which is durable, exhibits low friction, and does not produce any toxic or hazardous constituents or byproducts, especially at temperatures above normal operating temperatures. Demonstrate the feasibility of developed technology through limited testing.

PHASE II: Fully develop the coating designed in Phase I into a durable and low friction bearing surface for use in air bearings for an air cycle machine which will include the application process and formulation of the coating. Demonstrate the prototype coating through verification and validation of coated bearings in a relevant naval environment (TRL 6). The technology required for full scale manufacturing will also be developed and verified that it is feasible.

PHASE III DUAL USE APPLICATIONS: Manufacture actual bearings for testing in air cycle machines and develop life estimates. The target result should be a bearing coating that can be qualified for use in aircraft (F-18 or F-35). Complete the transition from TRL 6 to TRL 8 or higher. The developed technology may have applications in coatings for tools, sports equipment, internal combustion engine coatings, rolling element bearings, and kitchen utensils.


    • Agrawal, Giri L, (1997). Foil Air/Gas Bearing Technology - An Overview. R&D Dynamics Corporation, Bloomfield, CT, ASME 97-GT-347.


    • Dellacorte, C., NASA Glenn Research Center, et. al., (2000), Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery, Tribology Transactions Volume 43, Issue 4.


    • Dellacorte, C. and Bruckner R., NASA Glenn Research Center, (2010), Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery, NASA/TM—2010-216762.


    • Howard, S. and Bruckner R., NASA Glenn Research Center, and Radil, K., U.S. Army Research Laboratory, (2010), Advancements Toward Oil-Free Rotorcraft Propulsion, NASA/TM—2010-216094.


  • Barnett, M. and Silver, A. (1970). Application of Air Bearings to High-Speed Turbomachinery, SAE Technical Paper 700720, 1970, doi: 10.4271/700720.

KEYWORDS: foil bearing; air cycle machine; coating; low friction; Turbomachinery; non toxic

  • TPOC-1: 301-342-8964

Questions may also be submitted through DoD SBIR/STTR SITIS website.

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