Following last April's historic eruption of the Eyjafjallajökull volcano in Iceland, commercial flights were cancelled within most of Europe for several days – it was the largest disruption of air travel since the Second World War. Well, while no one is suggesting that airliners could now merrily fly right through clouds of ash, researchers from Ohio State University (OSU) have developed a coating that they say could allow jet engines to better withstand small amounts of volcanic ash that are ingested over time.
The metallic parts of jet engines are already coated with a ceramic thermal barrier coating, that allows them to withstand the 2500F (1371C) temperatures that occur during regular operations. When volcanic ash is sucked into an engine, however, it melts onto the coating and penetrates it. That ash becomes a brittle glass upon cooling and it flakes off, taking the coating with it.
Because airborne sand can cause similar damage, OSU's Prof. Nitin Padture previously devised a zirconia alumina coating to protect against it. Curious to see how Padture's coating – along with a newer one, based on gadolinium zirconate – would stand up to ash, two of his doctoral students performed an experiment. They took samples of metal that were treated with either one of the two sand-resistant coatings or the conventional heat-resistant coating, covered them with ash from Eyjafjallajökull, then heated them in a furnace to simulate the operating temperatures of a jet engine.
The two sand-resistant coatings came out fine, while the regular one was badly damaged. This was apparently because the molten ash was able to penetrate its pores all the way down to its base. The clogged pores kept the coating from being able to adequately expand and contract while the metal heated and cooled, so it ultimately broke off.
On the zirconia alumina coating, the ash reacted with the alumina to create a thin layer of the mineral anorthite, just below the coating's surface – this kept the ash from reaching the pores underneath. A similar process took place on the gadolinium zirconate coating, although in its case the ash-blocking layer was formed of the mineral apatite.
"This study's not going to 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 said. "We also learned how to pronounce 'Eyjafjallajökull.'"