Cracking evolution of atmospheric plasma-sprayed YSZ thermal barrier coatings subjected to isothermal heat treatment

It is known that residual stress is the main factor causing the final failure of thermal barrier coatings (TBCs). However, it is still unclear how the stress governs the cracking sequence. In this paper, atmospheric plasma-sprayed (APS) 8 wt% yttria-stabilized zirconia (YSZ) TBCs were subjected to isothermal heat treatment at 1050 °C for different time. The cracking evolution and the underlying mechanical mechanisms for cracking were investigated. Residual stress distribution through the YSZ thickness was evaluated using Raman spectroscopy. It is found that, after thermal exposure, the in-plane residual compression in the top-coat (TC) near the TC/thermally grown oxide (TGO) interface increases as the TGO thickens, inducing tensile stresses normal to the interface in the TC close to the undulation crests. A spherical analytical model is used to evaluate this tensile stress, and the result shows that it rises with the TGO growth. This normal tensile stress is the driving force for cracking of TBCs. Cracking parallel to the TGO/TC interface first initiates in the TC near the interface, and then coalesces with the cracks within the TGO and at the bond-coat (BC)/TGO interface, leading to the final delamination. Because TC has a relatively low fracture toughness, the delamination trajectory is primarily in the TC near the TGO/TC interface and along the pathway that contains inter-splat cracks, pores and weak splat boundaries. The results indicate that the large in-plane compressions play a significant role in the cracking evolution of TBCs because they induce normal tensile stress in the TC or at the TGO/BC interface. A cracking pattern dominated by the stress evolution is established in this work and it will provide an informed basis for improving the durability of APS-TBCs. •The large in-plane compression induces out-of-plane tensile stresses in the TC.•The increasing tensile stresses are the driving force for cracking of TBCs.•The delamination trajectory is primarily within the TC near the TC/TGO interface.