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Coatings Could Protect Jets from Ash

Monday, April 18, 2011

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New ceramic coatings originally developed to protect jet engines from airborne sand could also protect against the kind of volcanic ash that brought air traffic over Europe to a standstill last spring.

The eruption of Iceland’s Eyjafjallajökull volcano caused $2 billion in economic losses and the most severe disruption in European air travel since World War II.

But the eruption was not an isolated incident. Volcanic ash poses an “enormous threat” to jet engines, with about 100 aircraft inadvertently flying through ash clouds, “endangering the lives of tens of thousands of passengers and causing millions of dollars’ worth of engine damage,” a team of coatings scientists writes in “Jet Engine Coatings for Resisting Volcanic Ash Damage,” published recently in the journal Advanced Materials.

Wikimedia Commons / boaworm

The ash cloud from Iceland’s Eyjafjallajökull volcano closed most of western and northern Europe's airspace for six days in April 2010.

Coatings Tested

The team, from the Ohio State University, tested two coatings that were developed to protect jet engines from airborne sand alongside a typical jet-engine coating.

One of the new coatings was a new zirconia alumina formula, developed by lead researcher Nitin Padture from OSU’s College of Engineering; the other was a commercially available new formula based on gadolinium zirconate.

The team took metal samples treated with the three coatings, covered them in ash from Eyjafjallajökull, and then heated them in a furnace to simulate the operating temperatures of a jet engine. Internal jet engine temperatures reach up to 2,500°F, melting ingested ash onto the coating. With traditional coatings, the ash forms a brittle glass film as it cools and then flakes off along with the coating.

“Pores give the coating its strain tolerance,” doctoral student Julie Drexler explained to The Engineer, a UK publication. “They make room for the coating to expand and contract as the engine heats up while flying, and as it cools after landing. When all the pores are plugged with ash, the coating can’t adjust to the temperature anymore and it breaks off.”

Protective Layer Develops

In the researchers’ test, the molten ash penetrated all the way through the typical ceramic coating, while the sand-resistant coatings remained intact.

Chemical analysis showed that the ash reacted with the alumina in the first coating to produce a thin layer of the mineral anorthite below the surface, while, on the gadolinium zirconate, it produced a layer of the mineral apatite.

“The chemical reaction arrests the penetration of the ash into the coatings," fellow doctoral student Andrew Gledhill told The Engineer. “The unaffected pores allow the coating to expand and contract.”

Nitin Padture

Top Row: A cross-sectional scanning electron microscopy (SEM) image shows the fate of a typical jet coating after exposure to volcanic ash. The silicate map at right shows penetration of the silicate-based molten ash all the way to the bottom of the coating. Bottom Row: A new gadolinium zirconate-based coating designed to protect against airborne sand also protects against ash penetration.

The research won’t “solve all the problems of ash clouds and jet engines, but we are making progress, and we've learned a lot about the physics of the situation,” Padture told journal publisher Wiley in an interview. “We also learned how to pronounce ‘Eyjafjallajökull.’”