Integrated approach for the development and investigation of Al-Si based oxidation protection coatings for γ-TiAl

Kurzfassung

The material class of γ-titanium aluminides (γ-TiAl) is characterized by a low density combined with excellent mechanical properties, such as high-strength and a Youngs modulus as well as good oxidation resistance. This qualified γ-TiAl alloys as high-temperature structural materials in aircraft engines. Nowadays, turbine blades made of γ-TiAl alloys are already used in the low-pressure turbine of modern jet engines where they substitute the much heavier nickel-based alloys. The resulting weight savings increase the thrust-to-weight ratio of the engine, the performance and the fuel economy of the whole aircraft. However, the insufficient oxidation resistance of γ-TiAl alloys at temperatures above 800 °C limits the further use today. The development and application of new oxidation coatings can extend the field of application to higher temperatures. For this purpose, Al and Si are two promising elements to use as coating materials. Both elements form oxidation resistant intermetallic phases with Ti and, thus, have a high compatibility with γ-TiAl alloys. Additionally, by using only two elements, the complexity in the coating system is kept low and tractable. In this work, various aspects of Al-Si based antioxidant coating systems for γ-TiAl alloys are studied in detail. The coating deposition was carried out by magnetron sputtering. This allowed to deposit an exact composition, specific morphologies and the combinatorial deposition of coatings with different compositions in one coating process. For the first time, an Al-Si oxidation protection coating for γ-TiAl alloys was successfully prepared by magnetron sputtering. The Al-18Si (in at.%) coating and its transformation process towards intermetallic phases during a subsequent heat treatment of the coating were investigated in detail. Different heat treatments were carried out in air and in vacuum and the resulting materials were analyzed. It was found that the heat treatment in air had no effect on the oxidation behavior of the coating. Prior oxidation, the heat-treated coating consists mainly of the Ti(Al,Si)3 phase with individual Si containing precipitates. Also, a Si-rich interlayer formed at the interface between the substrate material and the coating. The oxidation tests were performed isothermally at 850 °C for 300 h in lab air. During oxidation, a protective oxide layer predominantly of α-Al2O3 and small amounts of TiO2 were formed. After oxidation, the coating possessed a network-like structure consisting of a network of the Ti5Si3 phase and TiAl2 and TiAl3 phase in between. This structure formed due to the segregation of Ti5Si3 phase at the grain boundaries during the oxidation of the Ti(Al,Si)3 phase. The Si containing interlayer transformed to a broad Ti5Si3 layer. The high content of oxidation-resistant intermetallic phases, i.e., TiAl2, TiAl3, Ti5Si3, after oxidation indicated that Al-18Si coating provides high oxidation protection, even at long oxidation times. In order to find an optimal Si content in terms of oxidation resistance, Al-Si coatings with Si contents of up to 81 at.% were prepared combinatorically. Cyclic oxidation for 5000 cycles of 1 h each at 900 °C showed that Si contents up to 12 at.% provide excellent oxidation resistance and the time the coating resists the oxidation increases with Al-Si film thickness. The effect of Si on the oxidation behavior was attributed to a "Ti getter effect". This effect describes that Si binds the Ti which is “released” during the oxidation of the Ti(Al,Si)3 phase and, thus, enables the formation of Al2O3. The formation of non-protective TiO2 is suppressed in this manner. In addition, the diffusion inhibiting properties of the Ti5Si3 phase were discussed. This prevents depletion of the coating of Al thereby maintaining oxidation protection. Evidence of these diffusion inhibiting properties of the Ti5Si3 phase was established in this work. For this purpose, a Ti5Si3 diffusion barrier was deposited below an oxidation resistant Al-30Ti (in at.%) layer on TNB-V2 as well as TiAl48-2-2 alloy substrates. The coating system was analyzed in detail by high temperature X-ray diffraction (HT-XRD) investigations as well as cyclic oxidation tests at 900 °C to 1000 cycles of 1 h each. It was shown that, compared to an Al-30Ti based protective coating without barrier, the coatings with diffusion barrier exhibit a higher oxidation resistance as well as a 5 to 10 times slower Al depletion. Thus, for the first time, a functional diffusion barrier was developed and its long-term properties were described. In this investigation, effects of the alloying elements from the different γ-TiAl alloys, the TNB-V2 and TiAl48-2-2 alloy, on the diffusion barriers properties were observed and described. For instance, additions of Cr, which diffused from the substrate alloy into the coating, further slows down the diffusion rate of Al through the diffusion barrier. Moreover, observed but not yet published was the effect of different γ-TiAl alloys on the combinatorically deposited Al-Si coatings. However, from preliminary results it is evident that Si increases the oxide scale adhesion on a Cr containing TiAl48-2-2 alloy.   In summary, the present investigations contribute to advance the understanding of the oxidation behavior of Al-Si based coatings and provide important knowledge for the application of Al-Si based coating systems on γ-titanium aluminides. The suggested Si content defines the parameter boundaries for future developments of new coatings. This will allow the further development of oxidation protective Al-Si coatings for γ-TiAl alloys to be performed in a targeted manner. In addition, by using the here pioneered diffusion barrier composition, new coating designs are enabled which may increase the oxidation resistance of γ-TiAl alloys even further.