**Crack-Growth Behavior in Thermal Barrier Coatings with Cyclic Thermal Exposure**

**Dowon Song 1, Taeseup Song 1,\*, Ungyu Paik 1, Guanlin Lyu 2, Yeon-Gil Jung 2,\*, Baig-Gyu Choi 3, In-Soo Kim 3 and Jing Zhang 4**


Received: 3 May 2019; Accepted: 3 June 2019; Published: 4 June 2019

**Abstract:** Crack-growth behavior in yttria-stabilized zirconia-based thermal barrier coatings (TBCs) is investigated through a cyclic thermal fatigue (CTF) test to understand TBCs' failure mechanisms. Initial cracks were introduced on the coatings' top surface and cross section using the micro-indentation technique. The results show that crack length in the surface-cracked TBCs grew parabolically with the number of cycles in the CTF test. Failure in the surface-cracked TBC was dependent on the initial crack length formed with different loading levels, suggesting the existence of a threshold surface crack length. For the cross section, the horizontal crack length increased in a similar manner as observed in the surface. By contrast, in the vertical direction, the crack did not grow very much with CTF testing. An analytical model is proposed to explain the experimentally-observed crack-growth behavior.

**Keywords:** thermal barrier coating; cyclic thermal fatigue; crack growth; initial crack length; failure
