TEM investigations of TGO formation during cyclic oxidation in two- and three-layered Thermal Barrier Coatings produced using LPPS, CVD and PS-PVD methods

Abstract

In the presented article the three-layer thermal barrier coating (TBC) was developed. The MCrAlY bond coat was obtained using low pressure plasma spraying (LPPS) and aluminized in a low-activity Chemical Vapour Deposition (CVD) process. The ceramic YSZ (Yttria Stabilized Zirconia) columnar layer was obtained using PS-PVD (Plasma Spray Physical Vapour Deposition) method. The two-layered TBC (without aluminizing) was produced for comparison. It was found that the MCrAlY bond coat in the two-layered TBC consists of β-NiAl phase and γ-Ni solid solution. It was also found that additional aluminizing in the low-activity CVD process of the MCrAlY metallic bond coat of the three-layered TBC results in a formation of a layer composed of β-NiAl phase as well as the γ′–Ni3Al phase. In addition, porosity was found as a result of the Kirkendall effect. Microscopic assessment conducted after cyclic and isothermal oxidation tests showed that the three-layered TBC was characterized by higher durability in comparison with a conventional two-layered TBC. This is due to the formation of a thinner layer of TGO, primarily containing Al2O3 oxide. It was also found that in the PS-PVD process, a YSZ ceramic layer with a columnar structure, consisting mainly ZrO2 oxide, is formed in the two- and three-layer TBC.