On the resistance of rare earth oxide-doped YSZ to high temperature volcanic ash attack

- Volcanic ash reacts with 7YSZ and induces a phase transformation from t to m.
- Volcanic ash penetrates severely through 7YSZ.
- Rare earth-doped YSZ prevents volcanic ash penetration.
- The presence of additional oxides significantly accelerate the formation of ZrSiO4.
- Compared with the volcanic ash, the melting temperature in the VA-7YSZ sample shifted to lower values.
- The melting temperature increased in the VA-rare earth oxide-doped YSZ samples.

Thermal barrier coatings (TBCs), such as 7-8 wt.% yttria stabilized zirconia (YSZ), are widely used to reduce the temperature of coated materials. However, when a turbine operates in a harsh environment, for example a volcanic ash attack, sand and ash particles ingested by the engine could be deposited on the TBC surfaces as molten calcium-magnesium-alumino-silicate (CMAS). CMAS melts and penetrates into TBCs at high temperatures, which causes a loss of strain tolerance and results in the premature failure of the top coat. It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash; therefore, CMAS mitigation solutions are among the top challenges for materials scientists. To some extent, CMAS is the biggest weakness of the traditional 7-8YSZ TBC material. The thermochemical interactions between YSZ and real volcanic ash are investigated in this paper as a means to alleviate the volcanic ash attack by understanding the underlying mechanisms. 7YSZ, 0.5Gd-8YSZ and 3Er-7YSZ powders and free-standing plates were prepared, respectively. The effect of the volcanic ash on the three different samples was investigated using an X-ray diffraction, scanning electron microscopy, a scanning transmission electron microscopy, and a differential scanning calorimetry. The phase transformations and reaction of TBCs materials subjected to a volcanic ash attack were examined and the volcanic ash degradation and mitigation mechanisms were discussed. It was found that rare earth-doped YSZ samples suffered less damage from the volcanic ash attack than the standard 7YSZ. The results demonstrate that rare earth-doped YSZ may be effectively utilized to mitigate a volcanic ash attack.

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