Phase equilibria of the Er2O3–Al2O3–SiO2 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.