Propagation feature of cracks in plasma-sprayed YSZ coatings under gradient thermal cycling

The cracking behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) during thermal gradient cycling test was investigated. The relationship between the size of the bright spot observed on YSZ coating surface and the size of the crack in thermal barrier coatings (TBCs) was examined by introducing pre-cracks in TBCs assisted with finite element analysis (FEA). The cracking behavior during thermal cycling, and the relationship between the residual lifetime of TBCs and the crack size were examined. Results show that the bright spot observed on YSZ surface corresponds to the crack in the coating and thus the size of bright spot increases with crack size. It was found that the crack size increases linearly with thermal cycle number when the relative thermal cycle number is less than 85% of thermal cyclic lifetime. The crack propagation acceleration at the later stage of lifetime is attributed to coalescence of numerous cracks. The obvious coalescence of cracks in TBCs begins to take place when the residual lifetime becomes approximately 15%, which leads to a rapid decrease of the residual lifetime of YSZ coating with the increase of thermal cycle number.