In conventional thermal barrier coatings (TBCs) the major problem is the spalling of a ceramic coating. This could be due to the large thermal stresses which are generated during thermal cycling on the oxidizing environment. One of the ways to improve the life span and overcome mentioned problems is introducing the concept of functionally graded materials (FGM) into the TBC. Functionally graded materials are referred to as a class of advanced materials that are distinguished by variation in their properties with varying their dimensions. Through employing a functionally graded thermal barrier coating (FG-TBC), an intermediate layer with a gradual compositional variation is embedded between the top and the bond coats. This layer(s), composed of ceramic and metal in various ratios, can achieve a gradual composition variation, thereby leading to gradual changes in microstructures and better mechanical and physical properties.
TopIntroduction And Background Of Thermal Barrier Coating
In the past few decades, several industries, e.g., gas turbines, have made considerable progress in the application of ceramic coatings that are used for the protection of the underlying base structures from oxidation, corrosion attack, and mechanical degradation upon exposure to high thermal stresses coupled with fatigue and/or creep phenomena (Lee & Stinton, 1996). These ceramic coatings, generally known as TBCs, currently provide thermal insulation against hot gasses in turbines and engines (Rangaraj & Kokini, 2014).
It is known that TBCs have been employed in different applications for more than 40 years (Miller, R. A. 1997). In the mid-1970s, these coatings were first successfully explored and tested in the turbine engine. In the early 1980s, these were applied in the revenue service on the vane platforms of aircraft gas turbine engines (Widjaja, Limarga, & 2002; Miller, R. A. 1997). In this process, TBCs were applied to metallic surfaces operating at elevated temperature to increase their temperature.
Note that TBCs have already been widely applied to increase the efficiency of high-temperature components, such as aerospace, aircraft, marine automobiles, and heavy-duty utilities (Xiong, Kawasaki, Kang, & Watanabe, 2005). The reliability of turbine components can also be improved by applying the protective coating. The reliability of TBCs can be affected by various factors such as microstructure, porosity, and residual stress. Residual stress has received considerable attention because it can influence the effect of many factors, such as microcrack formation, adhesion strength, and creep behavior, on the performance of TBC (Widjaja, Limarga, & Yip, 2003). The mismatch stress of the coefficient of thermal expansion (CTE) between different layers and substrates should be minimized to increase the spallation resistance (Negahdari. Porda, & Scherm. 2010).