Microstructural evolution at interfaces of thermal barrier coatings during isothermal oxidation

Using transmission electron microscopy, we investigate microstructural evolution at interfaces of a plasma sprayed thermal barrier coating system with an yttria-stabilized zirconia top coat and a NiCrAlY bond coat after isothermal oxidation at 1100 °C for up to 50 h. Interestingly, thermally grown oxide (TGO) layers split into two sub-layers. In principle, one of the two sub-layers comprises a mixture of α-Al2O3, Cr2O3 and Ni(Al, Cr)2O4, while the other one consists of dense α-Al2O3 and Cr2O3. Main microstructural evolution of the TGO layers follows a similar sequence that: original phases → α-Al2O3 + Cr2O3 → α-Al2O3 + Cr2O3 + Ni(Al, Cr)2O4. Particularly, after 50 h oxidation, γ-Ni and NiO form in the upper TGO sub-layer at the top coat-bond coat interface, and γ-Ni and α-Cr form in the upper TGO sub-layer at the bond coat-substrate interface. Moreover, γ-Al and β-Ti precipitate at the TGO-substrate interface after 50 h oxidation.