High-temperature infiltration behavior and phase stability of yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBC) produced by atmospheric plasma spray interacting with volcanic ashes (VAs) are presented here. Three VAs from the Colima, Popocatepetl, and Eyjafjallajökull volcanoes have been used in this work. Previous to infiltration experiments, physicochemical characterization of the VAs was carried out including thermal analyses by DSC, structural studies by XRD, and ICP chemical composition measurements. TBCs' infiltration tests were carried out at 1250 °C for different times. Results showed that infiltration depth as a function of time behaves in a non-linear way. Mainly two important infiltration behaviors were identified corresponding to high-and slow-speed infiltration regimes. Higher infiltration kinetics was detected for VAs with lower SiO 2 content. The extent of chemical degradation of the 7YSZ is directly related to the silica content. For greater SiO 2 values, a higher content of monoclinic ZrO 2 was observed leading to maximum values at intermediate annealing times between 2 and 5 h. This behavior can be correlated with the high-speed tetragonal to monoclinic ZrO 2 transformation at short times (between 2 and 5 h) until a maximum monoclinic content is reached. After that, the reaction follows by the interplay of ZrSiO 4 formation at the expense of previously formed m-ZrO 2 together with the incorporation of Y and Zr to the glass melt.

Rare-earth zirconate (REZ) pyrochlores, specifically (La,Gd)2Zr2O7 are promising alternatives to substitute 7YSZ in thermal barrier coatings (TBC) due to their increased phase stability and resistance to calcium-magnesium-aluminosilicates (CMAS)-based damage at high temperatures. This work analyses the reaction products of isothermal treatments at 1250 °C of (LaxGd1-x)2Zr2O7 powder mixtures in reaction with natural volcanic ashes (VAs) with different chemical compositions (Popocatepetl, Colima, and Eyjafjallajökull). The chemical composition in the (LaxGd1-x)2Zr2O7 system showed a strong influence on the formation of reprecipitation products (Zr,Gd,La oxides), reactive crystallization products (La-Gd apatites, lanthanum silicates, zirconolite, and garnets), and intrinsic crystallization products (hematite-like structures). The formation of garnets, which enhance the mitigation effect against molten VAs/Si-deposits damage, was observed more promptly on (La0.5Gd0.5)2ZrO7 promoted by high contents of Fe, Ca, Ti, and Mg on VAs. Thus, the (La0.5Gd0.5)2ZrO7 system stands as a possible candidate to overcome today's threats for the integrity of current TBCs.

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