Jet Engine Coatings for Resisting Volcanic Ash Damage

Abstract

The severe disruption in air travel and the economic loss caused by the explosive eruption of the Eyjafjallajokull volcano in Iceland reminds us of the enormous threat posed to aircraft jet engines by volcanic ash clouds over vast airspace. The nature of damage to gas-turbine jet engines flying through ash clouds depends on several factors, one of which is the temperature in the hottest parts of the engine. The maximum achievable operating temperature in modern jet engines has been increasing steadily in response to a strong demand for higher power output which scales with that temperature This is enabled by: (1) improvements in metal-alloys used to make hot-section engine components; (2) advanced air-cooling technologies that cool them; and (3) the use of ceramic thermal barrier coatings (TBCs) to insulate them. Higher operating temperatures also make modern jet engines more vulnerable to ash damage because ingested ash melts and adheres to TBC surfaces that can be as hot as 1200°C. This can result in buildup of a molten-glass deposit that penetrates into the TBCs causing them to spall-off, exposing the bare metal to dangerously hot gases. In extreme cases where the ash concentration is very high, catastrophic engine failure can occur due to heavy ash blockage of engine components. It is recognized that ingestion of some amount of ash by engines may be inevitable. A similar threat to engines is posed by airborne silicate sand particles which, although lower in concentration at flight altitudes, are more pervasive. There is a growing need to build protective measures within modern jet engines against damage from a broad range of undesirable silicate deposits. Jet engine coatings are discussed in this paper.