The problems caused by the shutdown of European airspace due to the eruption of the volcano Eyjafjallajökull in Iceland in April 2010 cost an estimated US$2 billion and posed a serious threat to both the safety and profits of the travel industry. The ensuing ash clouds posed such a serious hazard that one international team of scientists was prompted to adapt their work on coatings that prevent sand damage to jet engines to tackle the problem of ash penetration.
The team, led by Nitin Padture, whose work has been published in the journal Advanced Materials [Drexler et al., Adv Mater (2011) doi: 10.1002/adma.201004783], revealed that a new class of ceramic coatings could provide modern jet engines with the necessary protection against future volcanic ash damage.
The extent and type of damage that can occur to gas-turbine jet engines as they fly through ash clouds is dependent on factors such as the temperature in the hottest parts of the engine. Due to the extreme heat that builds up inside a jet engine, reaching temperatures of 1600 K, a ceramic thermal-barrier coating (TBC) is required to insulate metallic engine parts. However, when ash lands on the coating during a flight, it can be ingested and adhere to the coating. When it cools, the ash forms a brittle glass that flakes off, taking the coating with it and exposing the bare metal to dangerous hot gases. This can even lead to engine failure when large amounts of ash block engine components.
Previous coatings were only able to offer protection against ash building up gradually, not the large amounts resulting from a volcano erupting, so the team investigated the different effects on a regular jet engine coating and two separate sand-resistant coatings; one containing zirconia and alumina, the other based on gadolinium zirconate. Samples of metal coated with the ceramics were further coated with ash from the Eyjafjallajökull eruption, and were then heated to temperatures similar to those experienced in a jet engine.
Tests revealed that while the regular coating was badly damaged, the other two coatings were able to retain their overall structure. On analyzing the samples, it was found that molten ash had managed to penetrate through the pores of the regular ceramic coating into its base, but that the ash had hardly penetrated the other two at all.
The breakthrough could be applied in TBCs for jet engine hot-section metallic components that are resistant to damage by ingested silicates, including volcanic ash. They could also be relevant to land-based gas-turbine engines that are used to generate electricity, as there is a growing trend for using coal-derived synthetic gas in these engines. The gas can contain silicate impurities, making it a similar problem to that of volcanic ash and sand in jet engines.