Developing the CMAS (CaO-MgO-Al2O3-SiO2) resistant TBCs (Thermal Barrier Coatings): Alumina (Al2O3) as a candidate for restricting the CMAS infiltration in TBCs.

Abstract

The rise in aero-engine operating temperatures due to economic and environmental issues has generated many technological inventions. Few among them are the development of new generation super alloys which can tolerate higher temperatures and high stresses, and the evolution of ceramic coatings on top of the alloys which can reduce the temperature load. These thermal barrier coatings (TBCs) are mainly deposited by electron beam physical vapor deposition (EB-PVD), which has inter-columnar porosity, and by air plasma spraying (APS), which has porosity and cracks mainly running parallel to the interface. These TBCs are used in protecting combustion chamber walls, turbine blades and vanes allowing higher operating temperatures of aero-engines. CMAS (CaO-MgO-Al2O3-SiO2) attack becomes more dominant in the increased temperature range of current aero-engines. Currently 7 wt. % Yttria Stabilized Zirconia (7YSZ) has been used as a TBC in aero-engines. As CMAS melts at temperature 1200°C and above, it infiltrates through the porous inter-columnar TBC structure with help of capillary forces. This infiltration causes severe mechanical stresses within the TBC upon thermal cycling subsequently leading to crack formation and TBS spallation. As the state of the art 7YSZ TBC coatings could not resist the CMAS infiltration, the development of an EB-PVD alumina layer as an alternative CMAS resistant coating will be part of this work. The fabrication of EB-PVD Al2O3 on top of 7YSZ coatings was achieved by varying the deposition process parameters and the corresponding morphology of EB-PVD Al2O3/7YSZ coatings was studied. The coatings were deposited on alumina substrates in order to conduct long term infiltration tests. CMAS infiltration experiments were performed at 1225ºC and 1250 ºC for different time intervals starting from 1 to 10 hours. In addition, very short term infiltration tests for 5 minutes were performed at 1250°C using a model CMAS source and a real volcanic ash. The results show that the studied EB-PVD Al2O3/7YSZ coatings induced the crystallization of the CMAS melt by forming stable spinel (MgAl1.9Fe0.1O4) and anorthite (CaAl2Si2O4) phases which were characterized by scanning electron microscopy and energy dispersive spectroscopy. These layers exhibit a minimal growth making EB-PVD alumina topcoat a promising candidate for CMAS resistant multilayer TBCs. However, the EB-PVD Al2O3 samples experienced heavy sintering during long time exposure at high temperatures and the formed cracks in the layer have allowed the CMAS to fully infiltrate. Thus, multifunctional reaction-bonded alumina oxide (RBAO) was deposited on top of EB-PVD Al2O3 which acts as a sacrificial layer forming the stable CMAS arresting products and inhibiting further infiltration.