Monocrystalline Ni-based superalloys are the current state-of-the-art materials for blades in jet turbine engines . They are coated with thermal barrier coatings (TBCs) including outer layer (top coat) and inner layer (bond coat) for application in the hot section . The former is made of zirconium oxide stabilised with yttrium oxide (YSZ) with a thickness of 100-300 µm, whereas the latter is based on either metallic MCrAlY (M=metal) interlayer or diffusion aluminide layer with a thickness of approx. 80 µm. TiAl alloys, due to a much lower density than Ni-based super alloys (3.7-3.9 vs. 8.0-8.7 g/cm3) stand out as a very interesting alternative for the lower temperature section of the jet engine. What is more, application of a TBC could allow the use of TiAl turbine blades at higher temperatures. But, when TBCs and TiAl alloys are combined, brittle phases are formed at the with the bonding layer (MCrAlY).
The main aim of this work, is to address aforementioned problem with the application of TBCs on the Ti48Al2Cr2Nb (4822) alloy with a bond coat based on polymer-derived ceramic (PDCs) coatings from the SiAlOC system. In [1], SiAlOC coatings were already shown to improve the corrosion resistance of 4822 alloy. TBCs based on YSZ were obtained at several different temperatures (830, 850, 900 and 950°C) using an EB-PVD process. Microstructural studies (SEM and EDS) along with advanced structural studies (Raman confocal imaging ) revealed that the application of the hollow cathode (HC) plasma in the EB-PVD process enabled the successful formation of columnar ceramic coatings at temperatures up to 200°C lower than typical, while still meeting most of the requirements for TBC applications. The coated and bare TiAl specimens were oxidized in lab air for up to 800 h at 750 °C, the current application limit, and 900 °C, the future application aim. Further cross-sectional analysis was done to study the impact of oxidation and the overall adhesion of the coatings.
Acknowledgment: This work was supported by the National Center for Research and Development (NCBR) under the 35th CORNET (COllective Research NETworking) Project, co-funded under contract number CORNET/35/23/PDC-TBC/2024.
