## 2 Thermal Barrier Coating
### Durability According to Structure of Thermal Barrier Coatings  
The microstructures of TBCs contain pores and are largely classified into lamellar structures and vertical structures. These microstructures are known to form various coating structures depending on the size of the powder and the spraying conditions (spray distance, current value, spray rate, etc.) in the coating method [69]. In addition, as the structure is changed, the thermal structure and thermal stress in the coating in the operating environment are altered, which changes the durability performance accordingly. Structural factors affecting durability include thickness, cracks in coatings, pores, and TGO shapes, and studies on the effect of each factor have been conducted. An understanding of the durability performance according to the detailed structural shape of the TBC is necessary for the development of efficient coatings. Figure 13: Cross-sectional SEM of an air-plasma sprayed (APS) TBC that has been subjected to 120 thermal cycles (reproduced with permission from [58], The American Association for the Advancement of Science, 2002). The thickness of the top coating and the bond coating can be adjusted when manufacturing the TBC. In general, as the thickness of the TBC increases, the thermal barrier performance improves and the thermal stress generated between the top coating and the bond coating interface is reduced. Figure 14 shows the results of variations in the top coating-TGO interfacial stress (S11) as the thickness of the top coating changes from 200-800 \(\upmu\)m. However, excessively thick coatings cannot be applied to turbines such as combustors, high pressure turbine (HPT) blades, and HPT nozzles in commercial and military jet engines because of the strong centrifugal forces generated at high speeds of rotations above 3000 RPM. Even if the coating is applied, the durability of the coating is reduced because the residual stress formed by the TBC manufacturing process increases [70; 71; 72; 73]. Cracks present in the top coating may cause the TBC to fail, but vertical cracks artificially inserted in the coating manufacturing process increase the durability of the TBC. DVC and solution precursor plasma spray (SPPS) coatings that form a dense coating layer using small particles and then thermally impact the coating layer using a relatively low temperature plasma have internal vertical cracks. These vertical cracks improve the durability and life of the coating by reducing thermal stresses caused by repeated thermal fatigue expansion and contraction. Figure 15 shows the results of FE analysis, performed by [PERSON] et al. [74], of the stress intensity factor and energy release rate depending on the horizontal and vertical positions and crack lengths of the vertical cracks in the top coating. The stress intensity factor (\(K_{\text{I}}\)) and energy release rate (\(G\)) increased as the vertical crack position shifted from peak to valley to peak (Figure 15a). Also, as the vertical crack approached TGO (y increased), the stress intensity factor and energy release rate decreased because the TGO layer reduces stress concentration (Figure 15b). In addition, as the crack length increases, stress intensity factor and energy release rate increase (Figure 15c), which is consistent with the theory of linear elastic fracture mechanics [74]. Figure 14: Stress variation according to the thickness of TBC. Among the TBC structures, pores exist mainly in the top coating, which is a ceramic material. Increasing the amount of pores decreases the thermal conductivity of the top coating, thereby reducing the heat transfer to the base material and reducing the thermal stress at the bond and top coating interfaces, thus increasing the thermal fatigue life of the TBC. However, if excessive pores are present, reducing the adhesion between the top coating and the bond coating becomes problematic. Therefore, the pores present in the coating layer should be adjusted according to the use environment of the component to which the TBC is applied.