BaZrO3/Al2O3 composite molds were prepared via sintering at 1 600 ℃ for 8 h with BaZrO3 as a surface coating material and Al2O3 as a backup coating material. The phase composition and microstructure of the interfacial bonding layer between the face and back layers before and after hydration were investigated by optical microscopy, X-ray diffraction, and scanning electron microscopy with energy dispersive spectroscopy. The hydration process and damage mechanism of the interfacial bonding layer in the mold were analyzed. The effect of mold damage on the directional solidification of TiAl alloy was also investigated via directional solidification tests. The results show that BaZrO3 and Al2O3 generate BaAl2O4 at BaZrO3/Al2O3 interface during the high-temperature sintering process; and BaAl2O4 generates Ba(AlO(OH)2)2, Ba2Al4(OH)16 and Ba3(Al(OH)6)2 after hydration, leading to the grain volume change to destroy the intergranular bonding force, the generation of cracks in the mold and the failure of the mold. In the directional solidification process, the generated cracks become a channel for melt penetration of TiAl alloy, resulting in a melt leakage from the mold and the appearance of heterogeneous crystals in the directionally solidified TiAl alloy castings.