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High Temperature Solid Lubricant Coatings

PS304 Coating Cross-section

During start-up and shut down, prior to developing a gas film, sliding occurs between the shaft and top (inner) foil surfaces. Solid lubrication must be provided to reduce friction and wear. Traditional solid lubricants (e.g., graphite, Teflon®) readily solve this problem at low temperature. High temperature operation, however, had been a key obstacle. Without a suitable high temperature coating, foil air bearing use is limited to about 300°C (570°F). A new chrome oxide based journal coating, designated PS300, has been shown to provide good friction and wear properties in foil bearings at least to 650°C (1200°F). Subsequent development of PS304 coating has promise of enabling the turbomachinery foil bearing coating objectives: Provide start/stop wear protection for foil bearings Operate from cold start to 815°C (1500°F) No vaporization or emissions Machineability

NASA PS304 - US Patent No. 5866518

U.S. Patent No. 5,866,518 'Self-Lubricating Composite Containing Chromium Oxide', awarded 2 February 1999

• 60% NiCr - Binder
• 20% Cr2O3 - Hardener
• 10% BaF2/CaF2 - Hi-Temp Lube
• 10% Ag - Low-Temp Lube

PS300 Series Development

• PS300 was originally conceived & put into practice in early 1995
• In '96-'99 work continued on the coating tailoring the composition for various substrate alloys
• Oil-Free turbocharger with PS304 demonstrated at 95,000 rpm & 650°C (1,200°F) turbine inlet temperature, 17 March 1999
• PS300 is fully developed 1st license awarded & 3 others in process
• PM300 is being developed (powder metallurgy)

Coating Process Development

• Thermal Spray Technique Evaluation
• Plasma spray
• Low pressure plasma spray (LPPS)
• HVOF
• Environmental Durability
• 1000's of hours of exposure
• Microstructural changes
• Environmental effects (air vs. inert)
• Coating Adhesion Enhancement
• Surface treatment
• Interfacial binder layers
• Adhesion testing

Processing Steps

Plasma spray coating is a well established thermal spray process in which the powder form of the desired coating material is injected into an electrically ionized Argon plasma in a plasma spray gun. The injected powders melt and are accelerated onto the substrate to be coated. Substrate preparation typically consists of undercutting the surface to accept the coating (usually 0.005-0.010" thick). The coating is oversprayed and then finish ground to achieve a specified or desired surface finish and dimension. Plasma sprayed coatings can be polished or put into use in the as ground condition. Porosity levels are low, typically less than 15%. Following repeated rubbing contact, wear debris often "fills in" surface pits and voids yielding mirror smooth surfaces.

Undercut shaft	Sandblast finish	Plasma spray
Finish grind	After initial use

Success Stories

• PS304 coating enabled demonstration of first Oil-Free turbocharger
• PM304 used by ADMA Products, Inc.  to manufacture high temperature bushings as bronze replacement for Lincoln Electric  furnace conveyor system
• PS304 successfully lubricates steam turbine control valves for Elliot Turbines 

Suppliers

• Hohman Plating  (licensee for PS300 coating application)
• ADMA Advanced Materials  (licensee for PS/PM300 powder and powder metallurgy bushing production)

Reports

• Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery, NASA/TM-2000-209187/REV1, Mar 01, 2000
• Particle Size Effects on Flow Properties of PS304 Plasma Spray Feedstock Powder Blend, NASA/TM-2002-211550, May 2002
• Effects of Humidity on the Flow Characteristics of PS304 Plasma Spray Feedstock Powder Blend, NASA TM-2002-211549, May 2002
• Particle Morphology Effects on Flow Characteristics of PS304 Plasma Spray Coating Feedstock Powder Blend, NASA/TM-2002-211206, January 2002

Note: The listings on this page do not constitute an endorsement by the Federal Government (also see the NASA Website Disclaimer Statement). Rather, they are used as a means to foster greater dissemination of information and use of foil bearing technology. Companies wishing to be added should contact the web page curator shown below.