[Back]

S. Kagerer:
"Hochtemperaturbeständige Oxidationsschutzschichten auf Basis von Aluminiumoxid und MCrAlY-Legierungen";
Supervisor: H. Riedl, P.H. Mayrhofer; E308, 2018; final examination: 2018-04-19.

New technologies and materials rises the possibilities for the highly requested reduction of CO2 emission in industrial applications and a sustainable usage of resources in general. Especially, companies out of the automotive sector and aerospace industry continuously try to enhance their net zero carbon footprint. For this reason new high temperature, lightweight materials are highly desired. The intermetallic γ-TiAl alloys are an already well-established alternative to Ni superalloys, which are primarily applied in low-pressure turbines (LPT) for high-performance turbines - especially in geared fans. However, the unique thermal properties of these alloys involving low density, high elastic modulus, as well as outstanding creep resistance are in conflict to its poor oxidation resistance - applicable only up to 750 °C in oxygen containing atmospheres. For operation at higher temperatures protective coatings with retarded oxidation kinetics are inevitable. To protect the bulk material against oxidation and particularly Titanium outward diffusion - major issue for γ-TiAl alloys - protective coatings based on crystalline α-Al2O3 would be an excellent choice. Nevertheless, due to kinetic aspects extremely high deposition temperatures for α-Al2O3 (in either CVD or PVD) restrict their usage due to thermally induced bulk material degradation. Therefore, alternative concepts involving PVD based synthesis at low temperatures are extremely interesting. The aim of this study is to design an oxidation resistant protective coating involving alumina and MCrAlY based systems. The two different coating materials should be architecturally arranged as a bilayer, to ensure retarded oxidation kinetics as well as excellent adhesion to the substrate material. The γ-Al2O3 base layer should act as diffusion barrier for oxygen inward and titanium outward diffusion. In contrast, the MCrAlY top layer should operate as an oxygen getter by forming oxide scale on top during application. The γ-Al2O3 layers are deposited in oxygen rich gas atmospheres, which are mainly steered by so-called target poisoning effects. To avoid such effects, Tungsten alloyed Al-targets are proved as an excellent alternative shifting the transition zone between metallic and poisoned region up to higher oxygen partial pressures. Minor amounts of W within the target material are sufficient for this delay, hence not affecting any coating properties. The influence of the ferromagnetic target materials - as Ni and Co are highly important for MCrAlYs - was studied in detail utilizing Ni targets alloyed with Cr as a model system to decrease the curie temperature. Throughout this concept, it is possible to deposit highly dense, columnar Ni-based coatings exhibiting face-centered cubic crystal structures. For the model system NiCr - combined with a metastable γ-(Al1-xWx)2O3 base layer - a dense and adherent, oxide scale (Cr2O3 rich) is maintained even after 30 h annealing at 1000 °C in air. Applying NiCrAlY instead of NiCr - deposited at substrate temperatures of about 160 °C - also leads to a retarded oxidation kinetic. Cross sectional SEM investigations revealed only 600 nm Al-rich oxide scales - as deposited state of the bilayer 3.70 µm, involving 1.0 µm γ-(Al1-xWx)2O3 - after exposing the coated γ-TiAl for 100 h at 900 °C in air. For prolonged annealing periods (tOx = 30 and 100 h) a diffusion induced degradation of the interface between the bulk material but also the single coating layers is obvious. Especially, Ti outward diffusion plays a major role, as the γ-(Al1-xWx)2O3 diffusion barrier needs to be affective in both ways. In depth chemical and structural analysis (Transmission Electron Microscopy - TEM) in the as-deposited state also reveal non-stoichiometric compositions of the (Al1-xWx)2O3-δ base layer as well as a mainly amorphous morphology with small γ-(Al1-xWx)2O3 crystallites. These facts are in strong relation to the observed Ti outward diffusion, as a perfect γ-Al2O3 would have a minor tendency for any diffusion driven processes. In summary, the developed bilayer concept - γ-(Al1-xWx)2O3 base layers with NiCrAlY toplayers - is a highly oxidation resistant protective coating for γ-TiAl alloys. Due to the layered architecture, the oxidation kinetic is extremely retarded by the NiCrAlY layer, constituting an oxygen getter reservoir - paralinear oxide scale growth at 900 °C. In combination with the metastable γ-(Al1-xWx)2O3 layer, this concept promises a high potential for even prolonged oxidation periods.

PVD based thin films; Oxidation resistance

Project Head Paul Heinz Mayrhofer:
MTU Gasturbinen II 13102016