The cross-sectional micrographs of the nanostructured TBC failed after thermal shock testing at 1 150 ℃ are shown in Fig.1. These SEM micrographs were taken from the coating remained on the substrate. General view of the coating at a low magnification is shown in Fig.1(a). After 35 thermal cyclings at 1 150 ℃, more than 50% of the coating area was dropped, but obvious separation along the interface between the top coat and bond coat was not observed. Cracks formed in the zirconia top coat. Vertical cracks are clearly illustrated in Fig.1(b). The vertical cracks did not penetrate the whole top coat. They stopped in the area just above the top coat/bond coat interface. There appears to be a large horizontal crack in this area. However, under SEM at a high magnification (Fig.1(c)), it was found that it was not a real large horizontal crack. It was resulted from the breaking up of the zirconia top coat and pull out of the broken particles during grinding and polishing of the coating samples.
TGO formed in the bond coat of the nanostructured TBC tested at 1 150℃. Its thickness was about 2 μm. No small cracks formed at the crests of the undulated TGO layer (Fig.1(c)). TGO also formed at the bond coat/ substrate interface, but the TGO layer is not continuous (Fig.1(d)).
Fig.1 Cross-sectional micrographs of nanostructured TBC failed after 35 thermal cyclings at 1 150℃
When thermal shock test was performed at 1 250℃, the nanostructured TBC failed only after 13 thermal cyclings. General view of the coating remained on the substrate is shown in Fig.2. It is seen from Fig.2(a) that the whole top coat appears no cracks. This results from the pull out of the broken particles during polishing of the sample. TGO thickness increases slightly, compared with that formed at 1 150℃. The number of large vertical cracks increases. However, these large vertical cracks did not penetrate the whole top coat (Fig.2(b)). Small cracks inside the so-called large horizontal crack were observed (Fig.2(c)), but small cracks in the top coat area just above the TGO layer were not observed. TGO formed at the bond coat/substrate interface and inside the bond coat (Fig.2(d)).
Fig.2 Cross-sectional micrographs of nanostructured TBC failed after 13 thermal cyclings at 1 250℃
The thermal shock behavior of the conventional TBC was tested only at 1 250℃. Different from what was observed in the nanostructured TBC, large horizontal crack in the conventional TBC was not observed. However, large vertical cracks penetrated the whole top coat and stopped at the top coat/bond coat interface (Fig.3(a)). TGO formed at the interface with thickness of about 3 μm (Fig.3(b)). Unlike what happened in the nanostructured TBC, there was no breaking up of the conventional top coat and pull out of the particles during polishing of the sample. This experimental result tells us that the cohesive strength of conventional TBC is higher than that of nanostructured TBC.
Fig.3 Cross-sectional micrographs of conventional TBC failed after 5 thermal cyclings at 1 250℃
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