Double-layered thermal barrier coatings (DL-TBCs) were developed to meet multiple requirements in service, such as low thermal conductivity, high thermal stability, and high fracture toughness as well. Conventional DL-TBCs were often designed based on equal total-thickness to have long lifespans, which may weaken the thermal insulations. The cause is that the single-scale designed structure often has opposite effects on the thermal and mechanical properties. To co-enhance the thermal insulation and lifespan, this work designed durable DL-TBCs in multi-scales under equivalent thermal insulation. The macroscopic thickness ratio of top layer to bottom layer was tailored to optimize total and single thicknesses, and the microscopic pore size in top layer was tailored to resist sintering. Six groups of samples with different thickness ratios were prepared. The thermal cyclic test shows that the lifespans of DL-TBCs first increase and then decrease with an increase in thickness ratio. The optimized thickness ratio is 2:3 in DL-TBCs, which have the largest lifespan among the six groups. Cross-section morphologies show that the failure modes changed from the spallation of top-layer to the delamination of total double-layers. The long lifespan of the optimized DL-TBCs stems from the co-tailored thickness ratio and porous structure in top layer to lower the total cracking driving force.

The life span for a coating attached to its substrate is basic support for their desired protective function. Therefore, it is necessary to find out the causes responsible for the failure of coatings during service. This paper developed a finite element model to investigate the cracking behavior of plasma-sprayed ceramic coatings induced by the mismatch strain of thermal expansion between coating and substrate. Crack propagation affected by coating thicknesses was realized by the virtual crack closure technique (VCCT). The residual stresses (σ₂₂ and σ₁₂) and the strain energy release rate (SERR) induced at the tip of pre-crack in ceramic coatings are calculated. Results show that the σ₂₂ and σ₁₂ at the tip of the pre-crack increases continuously with the thickening ceramic coatings. The SERRs at the tip of the pre-crack in top-coat (TC) were increased with the thickness of ceramic coatings, resulting in the propagation of cracks. The crack length increases with the thickening of ceramic coatings. The crack propagation and coalescence lead to coating spallation, which is one of the main failure modes for plasma sprayed ceramic coatings during service. Given that, strain tolerant design was developed by inserting vertical pores in coatings. It was found that the SERRs were decreased with the increase in the number of vertical pores, as well as their depth. Moreover, the coatings with vertical pores appear to be crack-resistant, in particular for the thicker coatings. This suggests that the strain tolerant design is helpful to extend the life span of thick coatings, which makes a fundamental contribution to the design and preparation of advanced protective coatings in future applications.

Lifetime is a basic support for the thermal insulation function of thermal barrier coatings (TBCs). Therefore, extending the life span is essential to develop next-generation TBCs. For this objective, the columnar structure formed by vertical cracks appears to make sense. However, the underlying mechanism is still unclear. This work scrutinizes the influence of periodic vertical cracks on cracking behavior in order to tailor high strain tolerant TBCs. A finite element model was evolved to explore the crack behavior influenced by thermal mismatch strain between substrate and coating. The virtual crack closure technique (VCCT) was used to describe the propagation of crack under load. It is found clearly that the space between two vertical cracks (short for SVC) along the in-plane direction has a noteworthy influence on the strain tolerance of TBCs. Results indicate that the strain energy release rate (SERR) and stresses at the pre-crack tip increase continuously with the increase of the SVC, suggesting that the driving force for cracks is increasing. The crack is not propagated when the SVC is very small, whereas the crack grows continuously with the increase of the SVC. The growth of a crack can be prevented by reducing the SVC. A critical value for the SVC was found. When the SVC is less than the critical value, the SERR can be dramatically reduced. Thus, the SVC of periodic cracks can be tailored to obtain TBCs with high strain tolerance.