Spreading and reaction behavior of CMAS-type silicate melts with multiphase Y and Gd aluminate-zirconate T/EBC materials

Thermal and environmental barrier coatings must be robust against attack by calcium magnesium aluminosilicate (CMAS, or CMFAS with iron) deposits, and coatings that produce thin, uniform reaction layers are desirable. The reactions of four multiphase coating compositions in the AlO_1.5–REO_1.5–ZrO₂ systems (RE = Gd or Y) with three model CMFAS deposits were studied. Following CMFAS exposure for 1 h at 1400°C, some samples exhibited thin, uniform CMFAS reaction product layers, while others were less uniform with pockets of deep reactions. Coating materials with higher AlO_1.5 content produced uniform reaction layers, while the most SiO₂-rich CMFAS was most likely to produce a non-uniform response. Apatite was formed in all cases, while the Y was more likely than Gd to form aluminosilicate garnet and cuspidine reaction products. The results are discussed in terms of implications for CMFAS-resistant T/EBC design.