Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties
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REVIEW
Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties Emine Bakan1 • Robert Vaßen1
Submitted: 12 January 2017 / in revised form: 5 July 2017 ASM International 2017
Abstract The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic–extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined. Keywords coatings for engine components corrosion protection segmented coatings thermal barrier coatings (TBCs) zirconia
& Robert Vaßen [email protected] Emine Bakan [email protected] 1
Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), 52425 Ju¨lich, Germany
Thermal Barrier Coatings Thermal barrier coatings (TBCs) are protective coatings applied to the surface of hot metallic sections in gas turbine engines. The major fields of the application of gas turbines in which the TBCs are utilized are aircraft propulsion and power generation. In 2016, the market forecasters estimated an impressive production of nearly 228,000 aviation gas turbine engines valued in $1.232 trillion through 2030 and of 5480 power generation gas turbine engines worth $105.3 billion over the next 10 years (Ref 1, 2). Considering these figures, it is only rational to estimate a rising demand for the protective coating technologies in the near future. The conventional TBCs systems consist of a ceramic top coat (1), a metallic bond coat (2), and a thermally grown oxide ‘‘TGO’’ layer (3) that forms due to oxidation of the bond coat as a result of oxygen inward diffusion through the top coat at TBC operation temperatures. The aluminum-rich bond coat ((Ni, Co)CrAlY or aluminides of Pt and Ni), which forms the alumina (a-Al2O3) TGO layer on top, has the primary function of protecting the substrate from oxidation. Providing the thermal insulation in the TBC system is the main function of the ceramic top coat layer. Since it was introduced in the 1970s (Ref 3), 6-8 wt.% yttria-stabiliz
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