**4. Results**

## *4.1. Crack Initiation*

The images around the indentation imprints formed by different loading levels on the surface of the TBC are shown in Figure 1. Figure 1A,B is imprints generated by loading levels of 30 and 50 N, respectively, and the white arrows indicate induced cracks. Crack formation initiated from the center of the rhombus through the angular points and edges regardless of direction, and showed larger rhombus-shaped imprints as well as longer crack length in the indentation load of 50 N, compared to 30 N. The as-coated microstructure image and the induced cracks on the cross-sectional area are shown in Figure 2. The typical APS-coated microstructure was observed with some defects like pores and splat boundaries. The red-dotted line designates the interface between the top and bond coats, suggesting the imprint was formed just above the interface. The high-resolution back-scattering emission mode image of the white-dotted box from the normal SEM image of Figure 2B is shown in Figure 2C. The horizontal crack that was parallel to the interface was evidently formed longer than that in the vertical direction.

Figure 3 shows the initial crack lengths before the CTF test with the indented position, loading level, and crack direction. The surface crack length with different loads of 30 and 50 N were 101 ± 17 and 121 ± 30 μm, respectively. On the other hand, the cross-sectional crack lengths induced by 30 N were noticeably different depending on the direction. The vertical crack length was 50 ± 10 μm, while the horizontal crack length was 153 ± 7 μm, which was larger than the 50 N loaded on the surface.

**Figure 1.** Surface images of Thermal Barrier Coatings (TBCs) with cracks induced by indentation: (**A**) 30 N and (**B**) 50 N.

**Figure 2.** Cross-sectional images of TBCs with cracks induced by an indentation load of 30 N: (**A**) as-prepared image; (**B**) image of crack generation; and (**C**) highly-magnified image of the boxed area.

**Figure 3.** Crack lengths measured on the surface and cross-sectional images before cyclic thermal fatigue (CTF) tests.
