Composition effects of thermal barrier coating ceramics on their interaction with molten Ca–Mg–Al–silicate (CMAS) glass

Systematic studies have been performed to investigate the composition effects of ZrO2-based thermal barrier coating (TBC) ceramics on their interactions with molten Ca–Mg–Al–silicate (CMAS) glass. Porous ceramic pellets (∼15% porosity), instead of actual TBCs, are used in these model studies, where the penetration of molten CMAS into the pellets is investigated. This study involves a total of six compositions of ZrO2-based ceramics: two containing low solute (Y3+ or Gd3+) concentration, three with high concentration of solute (Y3+, Gd3+ or Yb3+), and one containing a combined intermediate concentration of Y3++Al3++Ti4+ solutes. While both the type of the solute and its concentration in the ZrO2-based TBC ceramics have been found to influence the extent of molten CMAS penetration into the pellets, the solute concentration has the most dramatic effect. In particular, molten CMAS penetration is almost completely suppressed in porous TBC ceramic pellets containing a high concentration of Y3+ solute (Y2Zr2O7). Possible mechanisms governing these effects are presented, together with a discussion of guidelines for the chemical design of CMAS-resistant zirconate TBC ceramics. Generally, for a TBC ceramic to be highly effective against CMAS attack it must interact vigorously with the molten CMAS, which must result in the rapid crystallization of the refractory oxide phase(s) that form a sealing layer, arresting further penetration of the molten CMAS. The porous ceramic pellets approach used here could be developed into rapid-screening methodologies for the discovery of CMAS-resistant TBC ceramic compositions, and to study the effect of composition of CMAS glasses on their interactions with TBC ceramics of specific compositions.