# Review of Functionally Graded Thermal Sprayed Coatings
## 4 Major Applications of Functionally Graded Coatings
### Thermal Barrier Coatings  
TBCs are the coatings designed to protect the gas turbine parts against intensive heat flux and various eroding and corroding particles contained in air and fuel [287; 288]. The frequent TBC's configuration includes ceramic top coat (TC) and metallic bond coat (BC). Each layer, shown in Figure 13, has different role to play in protecting the gas turbine components. Two goals to reach by TBC are: (i) extension of the turbine lifetime in service; and (ii) increasing the operating temperature and the thermodynamic efficiency. The TC is exposed directly to the thermal, thermomechanical, mechanical, and chemical loads. This role of this coating is to protect the internal ones. The coating is of ceramics, being usually partly stabilized zirconia. The BC is composed mainly of MCrAIY alloy (M is metal as e.g., Ni, Co, Fe). Its role is the reduction of the CTE mismatch between TC and substrate. Another role is to protect the super alloy substrate against oxidation at high temperature. The BC contains aluminum, which diffuses forming a protective \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) which is called thermally grown oxide layer (TGO). The TGO growth results in a generation of stresses at the BC/TC interface. These stresses reduce the time of TBC service [289, 290]. TBC is a very well-known FGC system. The initial studies on the functionally graded thermally sprayed TBCs were performed more in the 1970s [291, 292]. During the last years, because of the progress in spray technology and feedstock materials, two advanced functionally graded TBCs concepts were developed:  
* _First concept_ consists of gradual change from fracture-resistant BC layer to heat-resistant TC layer by initial mixing the feedstock powders or by injecting feedstock by different injectors. It resulted in a significant decrease of residual stresses and in improvement of adhesion between subsequent layers leading to the long-term performance of TBC [249]. The concept could be realized by mixing the zirconia and MCrAIY alloy powders and then spraying the mixtures [293, 294]. The authors found out that some interlamellar discontinuities were formed in the final structure because of different properties of the mixed feedstock. Other authors used the mechanically pre-alloyed MCrAIY and 8 YSZ powders with different ratios. The APS deposition of powders prepared in that way enabled high deposition rate, uniform coating density, and chemical homogeneity to be achieved [249, 295]. The use of two injectors for 8 YSZ and MCrAIY powders separately was another idea [296, 297]. The coatings sprayed in this way were almost crack-free and their chemical composition changed gradually across the thickness without a distinct interface between layers. A new idea is the deposition of functionally graded TBCs using spraying of hybrid powder-suspension feedstock [44]. The authors report about gradual change of TBC microstructure from dense bond coat to columnar top coat. The TBC showed high resistance against oxidation at high-temperature test and high resistance against thermal shocks. * _Second concept_ consists of gradual change of chemical composition in TC. At least two different ceramics are used as layers. The outer one made of ceramics with high phase stability and low thermal conductivity, like e.g., alumina. The bottom ceramic layer should have high toughness, high fatigue performance, and low thermal conductivity. This specification corresponds to e.g., yttria stabilized zirconia such as 8 YSZ [298, 299]. There are several works focused on multi-ceramic TC in which YSZ is used as the bottom layer. The studies concerned:  
* APS and SPS deposited 8 YSZ/La\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\) coatings [45, 300];  
Figure 13: The schematic representation of TBC structure (**left**) and as-sprayed one (**right**) [27]. Reproduced with permission from [Advances in Materials Science Research], Nova, 2016.