Microstructural Change and Fracture Behavior under Different Heat Exposure Conditions on Thermal Barrier Coatings Deposited on TiAl Intermetallic Compound

Air plasma sprayed thermal barrier coatings (APS-TBCs) deposited on the TiAl intermetallic compounds was heat exposed in air at different temperatures and times to evaluate the microstructural change and delamination behavior. The thermal barrier coating (TBC) layer, bond coat (BC) layer and substrate were composed of 4 mol% Y2O3 stabilized ZrO2, CoNiCrAlY alloy (Co-32Ni-21Cr-8Al-0.5Y (mol%)) and TiAl intermetallic compound (Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1 Si (mol%)), respectively. Due to the heat exposure, diffusion of the elements occurred between the BC layer and the substrate, and diffusion layers were formed on both the BC layer and the substrate. A thermally grown oxide (TGO) layer was formed between the TBC layer and the BC layer. The thickness of the TGO layer and the diffusion layer increased with increasing exposure temperature and time. In the TBCs heat exposed at 1273 K for 200 h, a composite oxide of Al2O3 and TiO2 was formed in the BC layer. Regarding the TBCs which were as-deposited and heat exposed at 1073, 1173 K up to 200 h and at 1273 K for 10 h, delamination occurred in the TBC layer near the BC layer. In the TBCs exposed at 1273 K for 50 h or more, delamination occurred at the vicinity of the interface between diffusion layer on the substrate side and the unreacted side of the substrate too. In case that the TBCs were heat exposed at 1073 and 1173 K, the shear strength decreases after reaching the maximum value of the shear strength at 10 h heat exposure. When the TBCs were exposed to heat at 1273 K, the shear strength indicated a constant value after the shear strength increased up to 50 h. This change may be due to the change in crack path after exposure for 50 h at 1273 K.