Determination of mechanical properties of NiCoCrAlY and (Ni, Pt)-Al based bond coats used in Thermal Barrier Coatings

Abstract

Recent developments include the use of thermal barrier coatings to increase the efficiency of the modern gas turbines. The temperature inside the combustion chamber limits their efficiency. For this reason EB-PVD partial stabilized zirconium oxide coatings are employed for thermal protection of turbine blades. In previous experimental research activities carried out at the Materials Research Institute of DLR, thermal barrier ceramic (TBCs) layers, ca. 200 µm of Partially stabilized zirconium oxide were coated on polycrystalline Ni-superalloy IN100 substrates covered with NiCoCrAlY and (Ni, Pt)Al bond coats (see schema of the multilayer system). These multilayer systems were thermally cycled at temperatures up to 1100°C for 50 min and then cooled down to room temperature in 10 min. The durability of the so coated TBCs was studied. Changes in the thermal treatment of the bond coats had a huge impact in the durability of the TBCs systems. For instance, IN100 + NiCoCrAlY systems treated at 1080°C in vacuum survived for more than 800 thermal cycles compared to the same system treated in Ar-H2 stream, this system withstand only ca. 200 cycles [1]. These differences are related only to the thermal treatment of the bond coat. During these treatments a thermal induced oxide (TGO) between the TBC and the bond coat grows which might be responsible for the durability of the coating system. However, the bond coat itself plays also an important role. Cracks formation in the ZrO2-Al2O3 mixture zone is the first step for the failure mechanism and spallation of the protective layer. Cracks dissipate to the bond coat, which in turn may stop or allow them to propagate or to move in other directions (i.e. parallel or vertical to the surface). Reports in the literature indicate that after ageing and failure the TGO adheres to the metallic bond coat, but further spallation takes place and the top coat takes the whole TGO suggesting a different failure mechanism taking place between the TGO and the bond coat [2]. [1] H. Lau, PhD Thesis, „Mikrostruktur und Morphologie thermisch gewachsener Oxidschichten von Wärmedämmschichtsystemen für Turbinenschaufeln“ 2004. [2] M. Bartsch, B. Baufeld, S. Dalkilic, L. Chernova, M. Heinzelmann, Int. J. of Fatigue 30 (2008) 211.