The TGO formation in overaluminized TBC obtained using plasma spray physical vapour deposition process during cyclic and isothermal oxidation

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. K e y w o r d s : 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)