Interactions between rare-earth zirconates (RE2Zr2O7) and CMAS silicate melts

Rare-earth zirconates (RE2Zr2O7) are potential thermal barrier coatings (TBCs) candidates exhibiting excellent thermal insulation property to protect underlying superalloy substrate. These coating materials, however, are subjected to degradation caused by molten silicon-containing sand dust and volcanic ash whose main compositions are CaO-MgO-AlO1.5-SiO2 (brief to CMAS) at higher operating temperatures (> 1200ºC). Here, we have studied the thermochemical interactions between three RE2Zr2O7 (Sm2Zr2O7, Gd2Zr2O7 and Yb2Zr2O7) and CMAS with three different compositions (referred to as CMAS-1, CMAS-2 and CMAS-3, respectively) at 1300ºC/30 min. Both the diversities of the REO1.5 identity and CMAS composition result in the precipitation of apatite with varying stoichiometry. For an equivalent CMAS case, there is a linear correlation between the apatite content and the RE cation radius. In CMAS-1, the Ca:RE ratio of apatite observed in the Sm2Zr2O7 case is ∼0.31, larger than that of Gd2Zr2O7 (∼0.30) and Yb2Zr2O7 (∼0.20). Additionally, the RE3+ solubility in equilibrium with apatite decreases with the increased CaO content from CMAS-3 to CMAS-1 for a given RE2Zr2O7 material, facilitating the apatite precipitation within a short time. For example, in Sm2Zr2O7 case the Sm3+ solubility decrease from ∼3.18 at % to ∼0.70 at % as the CaO content increases from 19 mol % ( in CMAS-3) to 33 mol % ( in CMAS-1). Also, the composition of residual CMAS melt changes as the interaction proceed, which can impact its flow property and infiltration propensity. These finds may advance the design of current and next-generation TBCs material and development of effective strategies to mitigate the CMAS-induced coating failure.