Influence of volcanic ash composition on the interaction with thermal barrier coatings of jet engines

Volcanic ash can seriously damage operating jet engines, leading to increased maintenance costs and in worst case to an in-flight outage of the engine itself. Besides the erosion of functional parts (e.g. fan and compressor blades, housing, etc.) the major hazard emerges from the melting of the ash in the combustion chamber. These silicate droplets can easily stick on the surface of turbine blades, leading to a clogging of cooling holes and a destruction of the zirconia-based thermal barrier coating (TBC) through chemical reactions and solidification.

As volcanic ashes from different volcanoes represent a wide chemical variability, it is important to know their different behavior during high temperature interaction with TBCs of various composition. Within the framework of the CORNET research project VAsCo ("Volcanic Ash resistant thermal barrier Coatings for jet engines" - www.vasco-cornet.eu) we have chosen volcanic ashes of basaltic, andesitic, rhyolitic and phonolitic composition, representing the possibly produced ash by explosive volcanic eruptions. As TBC materials we use atmospheric plasma sprayed (APS) and electron-beam physical vapor deposited (EB-PVD) coatings of yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) as state-of-the-art materials. The investigations are based on static and dynamic experiments. Static experiments deploy the heating microscope to study the spreading behavior of the molten ash and muffle furnace experiments to investigate chemical reactions within the coating. For dynamic experiments we thermally spray the ash on the TBC surfaces in order to simulate turbine conditions.

First results of the spreading behavior show a large surface coverage by the basaltic sample and a small coverage by the rhyolitic sample. This indicates that the viscosity is the decisive factor influencing the spreading behavior, as low viscous, SiO₂ lean samples have a higher flowability as SiO₂ rich samples. Looking at the TBC structure a significant difference is observable between APS and EB-PVD applied coatings: APS coatings enhance the spreading of the melt. This is explainable by their horizontally arranged splat-structure, inhibiting the infiltration of the melt, in contrast to the columnar EB-PVD structure that exhibits vertical open channels.