Phase equilibria in the Ho2O3-SiO2 and Ho2O3-Al2O3-SiO2 systems: thermodynamic insights for environmental barrier coatings design

The Ho 2 O 3 -Al 2 O 3 -SiO 2 system exhibits great application potential for advanced environmental barrier coatings and multifunctional materials, whereas critical high-temperature phase equilibrium data remain scarce. In this work, the 1600 °C isothermal section of the ternary system was systematically constructed using equilibration and quenching experiments, combined with X-ray diffractometry (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). Crucially, the thermodynamic stability of the Ho 2 Si 2 O 7 phase at 1600 °C and above is confirmed, refining the previously reported Ho 2 O 3 -SiO 2 binary phase diagram. At 1600 °C, the Ho 2 O 3 -Al 2 O 3 -SiO 2 system comprises one liquid-phase region, eleven two-phase regions, and nine three-phase regions. The eutectic temperature of the system was measured to be 1359 °C, corresponding to the reaction L ⇋ SiO 2  + Ho 2 Si 3 O 9  + mullite. The solid-solution mechanisms were determined for the two phases Ho 9.33+2 x (Si 1- x Al x O 4 ) 6 O 2 ( x  = 0-0.09) and Ho 4 Al 2(1- x ) Si 2 x O 9+ x ( x  = 0-0.33). Comparison with FactSage 8.3 calculations reveals that current commercial databases exhibit significant deficiencies for the Ho 2 O 3 -SiO 2 binary system. This study further improves the thermodynamic database of rare-earth oxides. The findings not only provide key data for optimizing CALPHAD thermodynamic models, but also offer an important basis for the composition design and phase regulation of environmental barrier coatings intended for extreme environments.