Influence of Composition and Processing on the Oxidation Behviour of MCrAlY Coatings for TBC Applications

The adherence of the thermally grown oxide (TGO) to the bond coat is recognized to be crucial for the lifetime of thermal barrier coatings (TBC) in aircraft engine and gas turbine blades. The stability of the system has to be guaranteed over extended service times (around 25000h) at high operation temperatures. With this aim, it is necessary that the TGO is slowgrowing and adherent. This work studies the influence of different parameters in the oxidation behavior and scale adherence on MCrAlY-bond coats. For this purpose, three MCrAlY-coatings were selected and subjected to oxidation tests at high temperature in the form of TBC systems, overlay coatings and free-standing coatings. The first part of the work treats the influence of the chemical composition on the oxidation behavior of the bond coat. The second part investigates the influence of non material-dependent parameters: starting with geometrical parameters (surface roughness and coating thickness) and followed by pretreatment/ processing parameters (surface processing sequence, heat treatment parameters and amount of O-impurities). Finally, the work deals with the possibility of defining a critical TGO thickness to failure in EB-PVD TBC systems. The results demonstrate that not only the chemical composition of a MCrAlY-coating but also those geometrical and processing parameters selected can have an influence on its oxidation behavior and scale adherence. The studied Co-base coating exhibited better resistance to scale spallation despite faster oxidation kinetics, apparently favored by phase stability over a wide temperature range. Furthermore, it is shown that for a specific material, the oxidation behavior can be considerably influenced by the geometrical and processing parameters mentioned above. In general this influence was achieved due to a variation of the yttrium reservoir, mobility and/or distribution, which in turn determined to which extent this element can act at the surface during oxidation. Only by varying the oxygen partial pressure of the atmosphere could be obtained scales with different oxidation kinetics but identical in morphology and composition. After assessment of these scales it seemed to be that a critical TGO thickness to failure in EB-PVD TBC systems exists