Phase equilibria of the Er ₂ O ₃ –Al ₂ O ₃ –SiO ₂ system at 1600°C related to corrosion of environmental barrier coatings

This study presents the first systematic experimental determination of phase equilibria in the Er 2 O 3 –Al 2 O 3 –SiO 2 system at 1600°C in air, employing the high‐temperature isothermal equilibration method followed by rapid drop‐quenching to preserve high‐temperature phase assemblages. The mineralogy and chemical compositions of the equilibrated samples were analyzed by x‐ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe x‐ray microanalysis (EPMA). One single liquid equilibria, five two‐phase equilibria (liquid–SiO 2 , liquid–Er 2 Si 2 O 7 , liquid–Er 3 Al 5 O 12 , liquid–Al 2 O 3 , and liquid–mullite) and seven three‐phase equilibria (liquid–SiO 2 –Er 2 Si 2 O 7 , liquid–Er 2 Si 2 O 7 –Er 2 SiO 5 , liquid–Er 2 SiO 5 –Er 3 Al 5 O 12 , liquid–Er 3 Al 5 O 12 –Al 2 O 3 , liquid–Al 2 O 3 –mullite, Er 2 O 3 –Er 2 SiO 5 –Er 4 Al 2 O 9 , Er 3 Al 5 O 12 –Er 2 SiO 5 –Er 4 Al 2 O 9 ) were observed in the equilibrium ternary system. The 1600°C isothermal section of the Er 2 O 3 –Al 2 O 3 –SiO 2 phase diagram in air ( p O 2 = 0.21 atm) was constructed based on the present experimentally determined equilibrium phase compositions. The liquid area and the primary fields of SiO 2 , Er 2 Si 2 O 7 , Er 3 Al 5 O 12 , Al 2 O 3 , and mullite were constructed. The comparison of the present experimental results deviate significantly from the modelling by FactSage (version 8.3) using its “ FactPS ” and “ FToxid ” databases, suggesting that FactSage underestimated the stability of the liquid phase under the present experimental conductions.