Effects of Ca/Mg/Al/Si oxide additions on the phase stability of yttrium-iron garnets

Rare-earth-containing garnets are important phases that evolve during the interaction of thermal and environmental barrier coatings (T/EBCs) with molten silicates (generically known as CMFAS). Notwithstanding their critical role, the chemical complexity of the garnet phase challenges an understanding of the phase equilibria and a description of their underlying thermodynamics. This study systematically examined substitutions of Ca, Mg, and Si into Y 3 Fe 5 O 12 and Y 3 (Fe 0.25 Al 0.75 ) 5 O 12 at 1400 °C. The results are compared to a similar study on substitutions into Y 3 Al 5 O 12 . The collective findings reveal that the Fe:Al ratio of the system is a key parameter for moderating the solubility of Ca and Si in the garnet phase. While the total Mg solubility is generally unaffected by Fe:Al ratio, the latter does alter the partitioning of Mg across the dodecahedral and octahedral sites. These solubility changes generally stabilize garnet, with decreased formation of secondary phases, notably apatite. Implications for T/EBC design are discussed. • Substitution into yttrium-iron/aluminum garnets is studied systematically at 1400°C • Higher Fe:Al ratios increase Ca and Si solubility and promote garnet stability • Mg is favored in octahedral coordination, particularly for high Fe:Al ratios • Apatite, spinel, olivine, Y2SiO5, and melt phases compete with garnet