| Abstract: |
Thermal barrier coating systems (TBC) have been investigated for high temperature applications. In their service under combined heat and oxygen load, a reaction zone forms in the BC/TC interface. The reaction zone reveals formation of different thermally grown oxides (TGO) like (Cr,Al)2O3, (Ni,Co)(Cr,Al)2O4 and NiO. For the prepared CoNiCrAlY/YSZ by atmospheric plasma spraying technique (APS), it is proved that enriching the BC/TC interface with Al is capable to form ± Al2O3 in the BC/TC interface. The close dense structure of ± Al2O3 is beneficial to limit oxygen diffusion into the BC. Accordingly, enhance the oxidation resistance of the metallic BC and extend the life time of TBC system. However, the systematic chemical transformation behavior of Al in the BC/TC interface during cyclic thermal treatment is under study.
In the present study, thin Al layers were deposited by DC-magnetron sputtering on the raw surface of APS sprayed CoNiCrAlY bond coats. Afterwards, YSZ was sprayed on top by APS technique. The specimens were subjected to cyclic thermal load with 5, 30, 60 and 120 min dwell time at 1150 °C. The TGOs formation behavior in the BC/TC interface of TBC systems was investigated by scanning electron microscope (SEM) including energy dispersive X Ray Spectrometry (EDS) analysis and Raman spectroscopy. Point analysis was conducted for the TBC specimens after 5, 30 and 60 min dwell time whereas mapping analysis was conducted for the TBC specimens after 120 min dwell time. The designed metal/ceramic systems with Al interlayer (Al-TBC) were compared to reference metal/ceramic systems without Al interlayers (R-TBC). The Raman-EDX combined investigations show the systematic formation of different types of oxides in the BC/TC interface during thermal cycling treatment. The interface enrichment with Al results in less formation of mixed oxides (e.g. Cr2O3 and (Ni,Co)(Cr,Al)2O4) and high content of ±-Al2O3 within the formed TGO. This allows a higher life time of the Al-TBC systems in comparison to the R-TBC systems for the applied thermal load.
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