High temperature interaction of volcanic ashes with 7YSZ TBC's produced by APS: Infiltration behavior and phase stability

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 Eyjafjallajokull 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 degrees 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 SiO2 content. The extent of chemical degradation of the 7YSZ is directly related to the silica content. For greater SiO2 values, a higher content of monoclinic ZrO2 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 ZrO2 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 ZrSiO4 formation at the expense of previously formed m-ZrO2 together with the incorporation of Y and Zr to the glass melt.