## 2 Thermal Barrier Coating
### Type of TBC According to Manufacturing Process  
The microstructure of TBCs depends on the material used as well as the manufacturing technique. The microstructure of the coating affects the required performances such as the thermal barrier performance and oxidation resistance, and also the coating durability; therefore, it is necessary to understand the coating manufacturing technique. TBCs have different deposition techniques depending on the temperature and material used. When choosing a TBC process, many aspects including performance, capital, process cost per part, thickness, composition, surface roughness requirements, and cooling hole cover should be considered. Such TBC methods include electron beam-physical vapor deposition (EB-PVD), chemical vapor deposition (CVD), and atmospheric plasma spray (APS) EB-PVD process is widely used even though it is an expensive process because it provides excellent thermal barrier performance, well-controlled composition and thickness. When coating with the PVD method, water droplets may be generated on the coating surface due to coating defects, which may negatively affect the surface processing [59]. EB-PVD proceeds as follows. First, the YSZ ingot is evaporated in a vacuum chamber by using an electron beam [60]. Prior to the deposition on the substrate, the substrate is preheated to 1000 \({}^{\circ}\)C under low oxygen partial pressure conditions to form a thin layer of TGO. Next, the surface of the substrate is coated by adjusting the environmental conditions in the vapor cloud. To ensure that the TBCs obtained through EB-PVD have high adhesion, the temperature of the base material during the process is 0.47 times that of the ceramic top coating material (YSZ). The microstructure of the TBC obtained through the EB-PVD process is shown in Figure 11. The EB-PVD process is particularly effective for matching the planar modulus of microstructures. For example, coating through the EB-PVD process makes columnar microstructures, resulting in a low planar modulus. This is an advantage in the generation of strain because of the mismatch in thermal expansion coefficients between the metal base material and the ceramic top coating [61; 62]. Figure 10: Scanning electron microscope (SEM) images of thermal barrier coating (TBC) (reproduced with permission from [58], The American Association for the Advancement of Science, 2002). CVD is a fabrication technique with good coverage and uniform control of the microstructure [64]. It is often classified by the precursor material and energy source and uses a heat source such as an RF induction heater. Because CVD is usually used to coat thin films, deposition rates are usually slow; however, for non-oxides, optimization can be performed by optimizing the process parameters [65]. In contrast, for oxides, it is difficult to increase the deposition rate because of the uniform powder formation in the gas phase; this can be overcome by appropriately selecting the geometrical structural characteristics of the precursor and CVD chamber. Figure 12 shows a cross section of an YSZ coating deposited at 230 \(\upmu\)m/h using laser CVD [66]. APS is a technique used to convert gases such as Ar, He, and N\({}_{2}\) into plasma and discharging them from a nozzle to achieve an ultra-high temperature and high-speed plasma jet as a heat source. Thermal spraying is essentially used for high melting point metals and ceramic coatings, and because of the high plasma jet speed, thermal spraying materials collide with the substrate at a high speed,  
Figure 11: Cross-sectional view of as deposited electron beam-physical vapor deposition (EB-PVD) TBC microstructure (reproduced with permission from [63], Elsevier, 2015). Figure 12: Microstructure of TBC manufactured by chemical vapor deposition (CVD) process (reproduced with permission from [66], Elsevier, 2005). allowing the coating to exhibit high adhesion strength and high density.