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ORNL develops self-lubricating coating for engine parts
OAK RIDGE, Tenn., March 30, 1995 A self-lubricating composite coating that could make engine and other moving parts last longer has been developed at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL).
The self-lubricating coating may be used for high-speed bearings, piston rings, cylinder linings, valve guides, and other parts in jet and internal combustion engines. Coated samples from ORNL are now being tested at the General Electric Company's Aircraft Engines Division in Cincinnati, Ohio.
"Oil-based lubricants break down at high temperatures," says Ted Besmann, a scientist in ORNL's Metals and Ceramics Division, and one of the developers of the new coating. "As a result, components of engines that are run at high temperatures to improve their fuel-use efficiency tend to wear rapidly and require replacement. Our composite coating should help engine parts last longer because it incorporates a solid lubricant that makes them slide more easily."
Metals and Ceramics Division staff members Woo Lee, Peter Blau, postdoctoral scientist Y. Bae, and Besmann developed a composite coating made of titanium nitride, a hard, wear-resistant material at high temperatures, and of molybdenum sulfide, a solid lubricant. Electron micrographs of the composite coating show that separate grains of the solid lubricant are dispersed near the surface of the titanium nitride coating.
To make the coating, the researchers at ORNL used chemical vapor deposition (CVD). In this process, vapors or gases are allowed to flow over a heated solid surface (called a substrate), react, and form a solid coating.
The researchers simultaneously deposited the desired materials in gaseous form on substrates such as silicon and graphite. Then they deposited the composite coating on a titanium alloy used for parts in engines.
"To make the coating for a titanium alloy substrate," Besmann says, "we flowed several gases at a temperature of 800 degrees Celsius and low pressure into a reactor containing the heated substrate. To get the best coating composition, we varied the composition of the gases, which included molybdenum hexafluoride, hydrogen sulfide, ammonia, and argon that carried the titanium organometallic vapor into the reactor."
Researchers elsewhere have used CVD, sputtering, or plasma spraying to deposit films of titanium nitride and of molybdenum sulfide. But the ORNL group is the first to co-deposit the two materials to form a composite coating.
"Solid lubricating films of molybdenum sulfide have been deposited on moving parts, but the lubricant tends to be worn away quickly," Besmann says. "Our composite coating increases the sliding life of the molybdenum sulfide lubricant by incorporating it in a hard material."
Besmann proposed that CVD could be used to develop a low-wear, low-friction composite coating. The development of the coating was carried out primarily by Bae and Lee.
Other division members who contributed to the work are Blau and Charles Yust, who measured the coating's friction and wear characteristics; Karren More, who took electron micrographs of the experimental coatings to reveal their structure; and David Braski, who analyzed the coating structure using Auger electron spectroscopy.
"Through the analytical work," Besmann says, "we are determining which coating structures and compositions are lowest in wear and friction. We have found that the friction of our best coatings containing titanium nitride and molybdenum sulfide is three times lower than a titanium nitride coating alone."
The coating development was supported by the Advanced Energy Systems Program in the Department of Energy's Office of Basic Energy Sciences. ORNL, one of the DOE's multiprogram research laboratories, is managed by Martin Marietta Energy Systems, a Lockheed Martin company, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.