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Fake diamonds help jet engines take the heat

An airplane's engines draw sand from a runway. (Credit: Image courtesy of the U.S. Department of Defense.)

Ohio State University engineers are developing a technology to coat
jet engine turbine blades with zirconium dioxide – commonly called
zirconia, the stuff of synthetic diamonds – 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.

Nitin Padture, professor of materials science and engineering at
Ohio State, said that he had military aircraft in mind when he began
the project. He was then 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,” he said. “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: When sand hits the hot engine blade it melts – and becomes glass.

“Molten glass is one of the nastiest substances around. It will dissolve anything,” Padture said.

The hot glass chews into the ceramic coating. But 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.

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. 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 said.

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:
the process is energy-consuming.

But if that 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 be able to use more
sophisticated ceramics that boost the heat-resistance of engines.
Eventually, technology could go beyond aircraft and power-generator
turbines and extend to automobiles as well, Padture said.

His coauthors on the Acta Materialia 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.

This research was funded by the Office of Naval Research and Naval Air Systems Command.

Ohio State University (2008, March 19). Fake Diamonds Help Jet Engines Take The Heat. ScienceDaily. Retrieved March 20, 2008, from http://www.sciencedaily.com­ /releases/2008/03/080317123255.htm