NASA-developed coating earns R&D recognition
The broad temperature range at which this lubricant coating is effective reflects NASA’s commitment to overcoming persistent challenges in aeronautics.
Last month we highlighted the Nanostructured Carbide Derived Carbon technology from Argonne National Laboratory that R&D magazine named as a 2003 R&D 100 award winner. The magazine has now published the full list of winners, and another lubricant technology has joined those elite ranks.
The second lubricant technology named to the R&D 100 is the PS/PM300 High-Temperature Solid Lubricant Coatings and Composites system developed by STLE member Dr. Christopher DellaCorte and his associate, Brian Edmonds, at the NASA Glenn Research Center in Cleveland. The PS/PM300 system coatings were specifically designed for high-temperature applications in foil air bearings, filling a lubrication void that exists between traditional solid lubricants (such as graphite and molybdenum-based systems, which are useful to about 425 C) and exotic narrow-temperature-range glassy lubricants (which work above 800 C). However, even low-temperature applications can take advantage of these chrome oxide-based products, which provide lubrication from – 185 C to about 1,000 C.
The broad temperature range at which this lubricant coating is effective particularly reflects NASA’s commitment to overcoming persistent challenges in aeronautics, says DellaCorte.
“Conventional solid lubricants have been widely available for lubricating components at selected temperatures,” says DellaCorte. “For instance graphite, polytetrafluoroethylene and molybdenum disulfide work well from room temperature to several hundred degrees centigrade. Further, many types of glasses and oxides can be used to lubricate at temperatures over 500 C (1,000 F). However, there are no readily available individual solid lubricants capable of lubricating over a wide temperature range such as would be encountered by an engine that must start at subambient temperatures and may reach 1,000 C, as aircraft engines do.
“So the design philosophy behind PS300 was to make up a composite technology in which low-temperature solid lubricants (silver) and high-temperature lubricants (fluorides) are mixed with a hardener (chrome oxide) and a matrix binder (glue),” says DellaCorte. “This composite marries the properties of all of the constituents and, thus, provides low friction and wear from below room temperature to about 1,000 C.”
In thermal spray coating form (PS300), this composite is comparable in cost to other wear-resistant coatings. As a powder metallurgy (PM300) in a part (e.g., a bushing), it costs about 10 times what porous bronze does but about the same as graphite or polymers. For this reasonable cost, the PS/PM300 technology enables previously impractical engineering systems.
For example, a PS300-lubricated foil air bearing has been proven in stationary, oil-free microturbine generators. Elimination of the oil system reduces the overall engine cost by 15% and reduces annual maintenance to a simple air filter change. PS/PM300 materials also have applications in different types of valves, as well as high-temperature conveyor systems.
The history of the PS/PM300 system goes back several decades. “This concept was pioneered by STLE fellow and life member Harold E. Sliney, my mentor,” says DellaCorte. “He invented the PS100 and PS200 coating systems. The PS300 system is an improvement on the earlier technologies.
“PS100 used the same chemically and thermally stable solid lubricants but used a glass and metal binder,” adds DellaCorte. “It was too soft and wore heavily. PS200 used the same silver and fluoride lubricants as PS100 and PS300 but used a cobalt-chrome carbide binder and was too hard and expensive. It wore the opposing rubbing surface and needed costly diamond grinding finishing. PS300 is midway between the two in hardness and has no high-cost cobalt additive. PS300 also doesn’t need expensive diamond grinding for finishing.”
PS200 also won an R&D 100 award in the 1980s, says DellaCorte, adding that the earlier version of the coating never was fully commercialized because of its cost and other considerations.
As the PS/PM300 family evolves, specific compositions emerge. One of the most successful of these is PS304, a thermal spray coating designed to protect foil bearings during start and stop operations, when sliding between the shaft and top foil surfaces renders them most vulnerable to friction and wear. PS304 operates from a cold start to 815 C, does not vaporize or generate any emissions in use and is machinable. Containing 60% NiCr binder, 20% Cr^sub 2^O^sub 3^ hardener, 10% BaF^sub 2^/CaF^sub 2^ high-temperature lubricant and 10% Ag low-temperature lubricant, the PS304 coating has been demonstrated in the first oil-free turbocharger, a furnace conveyor system, and steam turbine control valves.
For more information about PS/PM300 High-Temperature Solid Lubricant Coatings and Composites, visit the NASA Glenn Research Center at www.grc.nasa.gov/WWW/Oilfree/coatings.htm
Copyright Society of Tribologists and Lubrication Engineers Nov 2003
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