Thermal barrier coatings: A brief history of evolution and the future challenges

Kurzfassung

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 operating temperatures called Thermal Barrier Coatings. These TBCs are mainly deposited by electron beam physical vapour deposition (EB-PVD) which has inter-columnar porosity and air plasma spraying (APS) which have porosity and cracks generally running parallel to the interface. 7wt.% yttria stabilised zirconia (7YSZ) is the most common and state-of-the art coating which is used as a TBC material. The durability and the functionality of 7YSZ TBC in gas turbines is severely affected by the infiltration of molten deposits such as calcium magnesium aluminosilicate (CMAS) and volcanic ash (VA). CMAS/VA melts at higher temperatures (above 1150°C) and infiltrates through the inter-columnar TBC structure with the help of capillary forces. This infiltration causes severe mechanical stresses within the TBC upon thermal cycling, subsequently leading to crack formation and TBC spallation. The CMAS infiltration depth is governed by several temperature dependent factors such as viscosity, surface tension of the melt, operation temperature, and shape of the inter-columnar gaps. The variation of EB-PVD process parameters allows altering the resulting columnar morphology and porosity of the coating. The effect of morphology and density of EB-PVD 7YSZ columns on the CMAS infiltration behavior was studied in detail. Two different TBC pore geometries were created by varying the deposition parameters and CMAS infiltration experiments were carried out at 1250°C and 1225°C for different time intervals. Novel TBCs such as Gd-Zirconate and Yttria rich coatings have been tested as alternatives to 7YSZ coatings as these coatings promise a scope for mitigation of CMAS infiltration. Gd-zirconate and Yttria rich Zirconia react quickly with CMAS and forms a crystalline by-product of calcium oxy-apatite and seals the capillary channels against further CMAS penetration. The corresponding reaction mechanisms and results are discussed in detail.