Abstract
The article presents the results of the microstructural characterization of the newly developed three-layer thermal barrier coating after isothermal oxidation tests. Bond coats were produced by the CVD-overaluminizing of previously low-pressure plasma sprayed (LPPS) MCrAlY coating. The outer ceramic layer was produced by the plasma spray physical vapour deposition method (PS-PVD). For comparison, the MCrAlY bond coat without aluminizing was produced by LPPS. As a result of the overaluminizing process, the formation of the NiAl aluminide layer was observed. Also, the porosity was observed as a result of the Kirkendall effect in the middle zone of the bond coat. Three-layer TBCs microscopic examination showed the formation of a thin and dense TGO layer formed from alumina oxide. The porosity observed on as-deposited three-layer TBCs disappeared during the cyclic oxidation test. A thicker and delaminated TGO layer was formed in a conventional two-layered thermal barrier coating. The isothermal oxidation test at 1100 °C for 1000 h shows that TGO (Thermally Grown Oxides) alumina oxide layer on the overaluminized bond coat was also significantly thinner compared to conventional LPPS-sprayed MCrAlY bond coats. It was concluded that the formation of the TGO layer in three-layer TBC is similar to that observed in diffusion aluminide coatings. The overaluminizing can improve the oxidation resistance of thermal barrier coatings produced by LPPS and PS-PVD methods.

Key words
thermal barrier coatings (TBC), aluminide coating, plasma spraying, low-pressure plasma spraying (LPPS), plasma spray physical vapour deposition (PS-PVD), oxidation, thermally grown oxides (TGO)

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