Manufacturing, characterizaton of YSZ and CSZ based thermal barrier coatings and comparison of the thermal cycling performance

Thermal barrier coatings (TBCs) are frequently used on the blades and vanes of gas turbines to provide thermal insulation, thus lowering the metal temperature and consequently improving component durability and increasing engine efficiency. A typical two-layer TBC system consists of an oxidation-resistant MCrAlY (M=Ni and/or Co) bond coat on the superalloy and a thermally insulating stabilized zirconia top coat applied to its surface. Yttria (%wt. 6-8 Y2O3) stabilized zirconia (YSZ) has been usually chosen for the top insulating coat material because of its high thermal expansion coefficient, which closely matches that of the substrate, and low thermal conductivity. Among the TBC systems, ceria (CeO2) stabilized zirconia (CSZ)-based TBCs have been intensively investigated for the YSZ replacement because CSZ has a much lower thermal conductivity and a higher thermal expansion coefficient than those of YSZ. In this thesis, the plasma spray process parameters (F4MB plasma spray gun: Ar, H2, Spray Distance, Current) both of stabilized zirconia coatings with respect to deposition efficiency, porosity, surface roughness and microhardness were investigated using an orthogonal array in design of experiments (Taguchi). Ar and H2 flow rates are the most important parameters in the process. These parameters influence the particle trajectory within the plasma jet and consequently the key parameters of particle temperature and velocity upon impact. Hence these factors decisively influence and determine the coating macrostructure and properties.The kind of failure after thermal cycling in such coatings depends on the thermal loading conditions and on the TBC system characteristics (thickness of bond coat and top coat, porosity of ceramic layer). With surface temperatures above 1300 °C, it is often observed that cracks develop in ceramic layer and spallation occured. Bond coat thickness and porosity of ceramic layer in TBC systems are the most effective factors for thermal cyling life time and durability. As a result, thermal cycling performance of the CSZ coatings higher than the YSZ based coatings at 1350 °C.