Competition mechanism of interfacial cracks in thermal barrier coating system

The mechanism of competition between different interfacial cracks is investigated, specifically considering the cracks at top-coat/bond-coat and bond-coat/substrate interfaces in a thermal barrier coating system (TBCs). To assess the cracking process driven by mechanical loading, in-situ three-point bending tests were conducted on TBCs samples having different top coats (TC) and bond coats (BC). The competition between cracks at TC/BC and BC/substrate interfaces was observed for the first time, and then analyzed to elucidate the mechanism. Experimental and numerical results show that the differences in modulus ratio and thickness ratio of TC to BC are the main factors controlling the competition, which eventually leads to different fracture modes, i.e. when TC thickness is large or BC modulus is low, the coatings peel off from the TC/BC interface; while, with the decreasing TC thickness or increasing BC modulus, the delamination location changes to BC/substrate interface. A failure map based on the two ratios is numerically established, which agrees well with the experimental results. It is advised that the crack propagation path can be controlled by adjusting the combination of the two ratios to trigger both TC/BC and BC/substrate interfacial cracks in TBCs, thereby leading to more energy dissipation and better bending-resistance.