Reaction mechanisms of Gd2Zr2O7 in silicate melts derived from CAS

Infiltration mechanisms at high temperatures of sand & ashes (CAS) in aircraft thermal barrier coatings (TBC) have been widely studied using the pyrochlore phase Gd₂Zr₂O₇ (GZ) as TBC. Novelty of this study is GZ reactivity with each or a mix of oxides derived from CMAS have been scrutinized in order to better assess the diffusion, stability and role of each reactivity products in mitigation mechanisms across the CAS-GZ system. In particular, Gd^+III, Si^+IV, O^-II diffusion mechanisms at the GZ/SiO₂ interface have been discussed. These mechanisms shed light on why Gd-oxyapatites are the predominant phases at interfaces between silica-rich medium such as CAS and GZ when reactions are not complete, in contrast to thermodynamic data. Moreover, Gd₂Si₂O₇ phases are closely linked to the growth of Gd-oxyapatites and are the seat of the diffusion barrier to Si^+IV species, which further explains why GZ coatings shortly stops CMAS infiltration.