Within the framework of the VAsCo research project we investigate the interaction of various volcanic ashes with EB-PVD and APS applied TBCs of YSZ and GZO material in order to better understand damaging processes and give hints for further optimization of future TBCs. One of the first questions from the perspective of a volcanologist was to what extent the ground collected ash samples represent the in-air ash, ingested into a jet engine. To answer this question we characterized one volcanic ash sample from Tungurahua volcano, Ecuador, with respect to any grain size depended mineralogical and / or chemical variability. It was found that there is a high probability of a density driven separation, which leads to an enrichment of glass. As glass will be the first phase that softens upon heating of a volcanic ash this enrichment can lead to an enhanced sticking, thus wetting of the turbine blades. Details can be found in the publication Müller et al. (2019) (https://doi.org/10.1016/j.jvolgeores.2019.04.005).
For the ash vs. TBC experiments we extensively characterized the volcanic ashes in a first step by means of XRD (phase state), XFR, (bulk chemistry), EPMA (mineral chemistry), DSC (glass transition), optical dilatometer (melting and spreading behavior) and viscosimetry (determination of melt viscosity). The high temperature ash vs. TBC experiments were conducted within an optical dilatometer, that allows the recording of melting and spreading of the ash on various substrates under air. After the experiments the samples were cut to investigate the chemical reaction between molten ash and TBC. It turned out, that the ashes show significantly different spreading behavior as well as chemical reaction products. Depending on ash and TBC chemistry the coating resisted the chemical reaction or was completely dissolved. First results were shown at the Thermal Barrier Coatings conference in June 2018 in Irsee, Germany. The poster presentation can be seen below.
Viscosimetry of 5 remelted volcanic ashes in their pure state as well as doped with 6.5 wt% YSZ or GZO was done. After dissolution of YSZ or GZO TBC material the viscosity was significantly reduced. For more information check our publication in the American Mineralogist (https://doi.org/10.2138/am-2020-7334).
Peer reviewed journal publications:
Müller, D., Kueppers, U., Hess, K.-U., Song, W., Dingwell, D.B., 2019, Mineralogical and thermal characterization of a volcanic ash: Implications for turbine interaction, Journal of Volcanology and Geothermal Research, 377, 43-52. https://doi.org/10.1016/j.jvolgeores.2019.04.005
Müller, D., Hess, K.-U., Kueppers, U., Dingwell, D.B., 2020, Effects of the dissolution of thermal barrier coating materials on the viscosity of remelted volcanic ash, American Mineralogist, 105, 1104-1107. https://doi.org/10.2138/am-2020-7334
More publications are in preparation and will be listed here in future.