**6. Conclusions**

Cyclic thermal exposure tests were conducted for the TBCs with cracks induced by micro-indentation to investigate the crack-growth behavior of YSZ-based APS-TBCs as a function of initial crack position and length. The cracks on the surface grew in a similar trend independent of the loading level, while the cracks formed on the cross section showed a different growth behavior with respect to the direction to the interface between top and bond coats. Crack thickening and coalescence were observed together with crack growth during cyclic thermal exposure. The surface showed threshold crack lengths with ranges of 189–392 and 244–381 μm for the 30 and 50 N loads at the failure point, respectively, and the cross section with cracks formed by 30 N was 70–141 and 217–419 μm for the vertical and horizontal cracks, respectively. Therefore, failure criteria in the TBC systems can be proposed in view of crack length on both the surface and cross section.

**Author Contributions:** Conceptualization, D.S., T.S., and Y.-G.J.; Methodology, D.S., T.S., G.L., Y.-G.J., and J.Z.; Software, J.Z.; Formal Analysis, D.S., and G.L.; Investigation, D.S., T.S., G.L., Y.-G.J., B.-G.C., I.-S.K., and J.Z.; Writing—Original Draft Preparation, D.S.; Writing—Review and Editing, D.S., T.S., U.P., Y.-G.J., B.-G.C., I.-S.K., and J.Z.; Supervision, T.S., U.P., and Y.-G.J.; Project Administration, T.S., U.P., and Y.-G.J.; Funding Acquisition, T.S., U.P., and Y.-G.J.

**Funding:** This research was funded by "Human Resources Program in Energy Technology" (No. 20194030202450) and "Power Generation and Electricity Delivery grant" (No. 20181110100310) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), a granted financial resource from the Ministry of Trade, Industry, and Energy, Korea, and by the Fundamental Research Program of the Korean Institute of Materials Science (KIMS, No. PNK5620).

**Conflicts of Interest:** The authors declare no conflicts of interest.
