TEM study on microstructure of thermally grown oxide in EB-PVD thermal barrier coatings

The crystalline structure and composition of the thermally grown oxide (TGO) in an electron beam deposited (EB-PVD) thermal barrier coating (TBC) was investigated using transmission electron microscopy. The TBC was composed of a 200-μm Y2O3 (7 wt.%) stabilized ZrO2 (YSZ) top layer and a 100-μm NiCoCrAlY bond coat (BC) layer on a superalloy substrate. For as-deposited samples, a 500-nm mixed oxide zone was formed at the YSZ/BC interface, which was composed of a fine-grained α-Al2O3 matrix and nano-sized t-ZrO2 crystals. After thermal exposure at 1150 °C for 50 h, a 3-μm TGO layer with α-Al2O3 as the dominant component was developed, which consisted of three distinct layers with different crystalline structures and compositions. They were an Al2O3-ZrO2 mixed zone, a well-crystallized α-Al2O3 zone, and a fine-grained α-Al2O3 zone, respectively. Finally, the formation and evolution of TGO was discussed from the viewpoint of rapid inward diffusion of oxygen and slow outward diffusion of metals.