Ni-based single crystal superalloy turbine blades with TBC deposited by EB-PVD were supplied after cyclic oxidation test performed under flowing synthetic air in a purpose-built rig. The thermal cycle consisted of a heating period of 30℃min-1 up to 1100℃, followed by a dwell and a cooling ramp with an initial rate of 30 C min-1. The total duration of one cycle was 23 h.
The specimens were cross-sectioned and prepared for microstructural analysis using conventional techniques, which include cutting, grinding, polishing(up to 1200 grit SiC paper), and fine polishing(up to 1 um diamond paste). Before mounting, the samples were coated with a thin epoxy resin layer to avoid damage during metallographic preparation. The cross sections were examined by scanning electron microscopy(SEM) using a FEG-SEM 7001F JEOL equipped with an energy dispersive analyzer Oxford Instruments(Inca Penta FET-x3 energy dispersive spectrometer).
The considered TBC system on 2 mm thick substrate(ZhS32 superalloy with the following chemical composition in wt.%:4.9 Cr,9.0 Co,1.0 Mo,8.5 W,5.9 Al,4.0 Ta,1.6 Nb,4.0 Re) consists of the following layers: the 30 um thick NiCoCrAlY-bond coat, the 60 um thick partially stabilized zirconia(ZrO2-7 wt%Y203) top coat, and a-Al2O3 thermally grown oxide of an average 7 um thickness.Representative back-scattered electron(BSE) images of the TBC cross-section are shown in Fig.1a,1b. The typical irregularities are(1)internal oxidation zones,(2) cracks in TC,(3) cracks in TGO and (4) localised penetration of the TGO into TC and BC layers.
Fig.2 displays a zoomed view of the area (4) that is critical in terms of crack initiation and propagation. The spallation of the top coat near the indicated oxide irregularity was observed. In this area the TGO consists of two clearly distinct zones. The inner dark zone in contact with the metal(marked by blue square in Fig.1c) is predominantly a-Al2O3 and an outer relatively light zone(marked by red square in Fig.1c) was identified as Cr-rich spinel type oxide Ni(Cr, Al)204. The bright spots near the TGO/TC interface are Cr-rich oxide particles. The particles rich in Ta-oxide embedded in alumina were also identified by EDS mapping.
Fig.1. BSE-SEM micrographs of representative TBC cross-sections with TGO irregularities:(1) internal oxidation zones,(2) cracks in TC,(3) cracks in TGO,(4) symmetrical penetration of the TGO into TC and BC.
Fig.2.BCE-SEM micrograph showing magnified views of the localised penetration of the TGO into TC and BC layers.
The cracking along both TC/TGO(1) and TGO/BC(2) interfaces and within TGO(3) was observed(Fig.3a,3b). Indeed, the relation of crack localisation with the TGO geometry was found: for the regular TGO shape the crack initiation sites are located mostly at both interfaces; for the irregular TGO shape the cracks are located predominantly at only one of the interfaces.
Fig.3. BSE-SEM micrographs of observed cracks:(1) along TC/TGO interface,(2) along TGO/BC,(3) within TGO.
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