CERAMIC INNOVATIONS: "Renewable" Zirconia Coating

The zirconia in a new coating for jet engine turbine blades chemically converts sand and other corrosive particles that build up on the blade into a new, protective outer coating.

An airplane’s engines draw sand from a runway. Photo courtesy of the U.S. Department of Defense.

Engineers at The Ohio State University are developing a technology to coat jet engine turbine blades with zirconium dioxide to combat high-temperature corrosion. The zirconia chemically converts sand and other corrosive particles that build up on the blade into a new, protective outer coating. In effect, the surface of the engine blade constantly renews itself. Ultimately, the technology could enable manufacturers to use new kinds of heat-resistant materials in engine blades so that engines will be able to run hotter and more efficiently.

Sand as the Enemy

Nitin Padture, professor of materials science and engineering at Ohio State, said that he had military aircraft in mind when he began the project while he was a professor at the University of Connecticut. “In the desert, sand is sucked into the engines during takeoffs and landings, and then you have dust storms,” said Padture. “But even commercial aircraft and power turbines encounter small bits of sand or other particles, and those particles damage turbine blades.”

Jet engines operate at thousands of degrees Fahrenheit, and blades in the most advanced engines are coated with a thin layer of temperature-resistant, thermally insulating ceramic to protect the metal blades. The coating, referred to as a thermal-barrier coating, is designed like an accordion to expand and contract with the metal. The problem is that when sand hits the hot engine blade it melts and becomes glass. “Molten glass is one of the nastiest substances around,” said Padure. “It will dissolve anything.”

The hot glass chews into the ceramic coating, though the real damage happens after the engine cools and the glass solidifies into an inflexible glaze on top of the ceramic. When the engine heats up again and the metal blades expand, the ceramic coating can’t expand because the glaze has locked it in place. The ceramic breaks off, shortening the life of the engine blades.

Conventional ceramic coating destroyed by molten glass.

Zirconia to the Rescue

In a recent issue of the journal Acta Materialia, Padture and his colleagues described how the new coating forces the glass to absorb chemicals that will convert it into a harmless-and even helpful-ceramic. The key, Padture said, is that the coating contains aluminum and titanium atoms hidden inside zirconia crystals. When the glass consumes the zirconia, it also consumes the aluminum and titanium. Once the glass accumulates enough of these elements, it changes from a molten material into a stable crystal and it stops eating the ceramic.

“The glass literally becomes a new ceramic coating on top of the old one,” said Padure. “Then, when new glass comes in, the same thing will happen again. It’s like it’s constantly renewing the coating on the surface of the turbine.”

Padture’s former university has applied for a patent on the technique that he devised for embedding the aluminum and titanium into the zirconia. He’s partnering with Inframat Corp., a nanotechnology company in Connecticut, to further develop the technology. Padture stressed that the technology is in its infancy. He has yet to apply the coatings to complex shapes, and cost is a barrier as well because the process is energy-consuming.

His coauthors on the paper included Ohio State doctoral student Aysegul Aygun, who is doing this work for her dissertation; former postdoctoral researcher Alexander Vasiliev, who is now at the Russian Academy of Sciences; and Xinqing Ma, a scientist at Inframat Corp. The research was funded by the Office of Naval Research and Naval Air Systems Command.

Ceramic coating designed to resist molten glass, which is in a thin layer at the top.

Widespread Benefits

If the cost eventually came down and the technology matured, the payoff could be hotter engines that burn fuel more efficiently and create less pollution. Manufacturers would also be able to use more sophisticated ceramics that boost the heat-resistance of engines. Eventually, technology could go beyond aircraft and power-generator turbines to extend to automobiles as well, Padture said.

For more information, call (614) 247-8114, e-mail Padture.1@osu.edu or visit www.osu.edu.

Unless otherwise indicated, images are by Aysegul Aygun and Nitin Padture, courtesy of The Ohio State University.


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