Fig. 1. Morphologies and size distributions of agglomerates and nanosized powders [45]: (A) morphology of agglomerates; (B) surface morphology of agglomerates with nanosized powders; (C) size distribution of agglomerates; (D) size distribution of nanosized primary particles (d0.1: 10% particles have diameters smaller than the value 53.7?μm; d0.5: 50% particles have diameters smaller than the value 79.9?μm; d0.9: 90% particles have diameters smaller than the value 152.4?μm).

Fig. 2 shows the polished cross-sections of the samples after progressively increasing the thermal-exposure duration. At the as-deposited state, the conventional sample exhibits a typical lamellar structure with inter-splat pores and intra-splat cracks. In contrast, the nanostructured sample exhibits a bimodal structure, which implies that medium gray nanozones are surrounded by the lighter lamellar zones. Moreover, the lamellar zone in the nanostructured samples is similar to that in the conventional samples. Regarding the structural evolution of the conventional samples, the number of large-sized pores (approximate size of 5-15?μm) observed at the as-deposited state decreases distinctly during the thermal exposure. Regarding the structural evolution of the nanostructured samples, the contrast between the nanozone and lamellar zone is not as distinct as in the as-deposited state. This may be attributed to the sintering occurring in both lamellar zones and nanozones.

Fig. 2. Global structural evolution during thermal exposure: (A) conventional TBCs; (B) nanostructured TBCs.

Fig. 3 shows the changes of the in-plane elastic modulus as a function of the thermal-exposure duration. Common phenomena can be observed for the conventional and nanostructured samples. The elastic modulus exhibits a significant increase, at macroscale and microscale. Moreover, the evolution exhibits an obvious non-linear trend. A very rapid increase occurs at initial short exposure durations, while a gradual increase could be observed at following long exposure durations. Therefore, the increment mainly occurs during the initial exposure. The comparison between the conventional and nanostructured samples reveals that the elastic moduli at the as-deposited states are comparable. However, the increments in the nanostructured samples are lower than those of the conventional samples. This suggests that the nanostructured samples, to some degree, can retard the sintering-induced stiffening.

Fig. 3. Changes of elastic modulus as a function of thermal exposure duration: (A) macroscale; (B) microscale.

Fig. 4 shows the changes in the thermal conductivity as a function of the thermal-exposure duration. A similarity between the mechanical and thermal properties could be observed; the evolution trends of the thermal conductivity are also non-linear. Therefore, the fast increase at initial short durations turns into a significantly slower increase at long exposure durations. The comparison between the conventional and the nanostructured samples reveals that the nanostructured samples exhibit a slightly lower thermal conductivity at the as-deposited state. Moreover, it is clear that the increase in the nanostructured samples is not as significant as that of the conventional ones after long durations. Therefore, the nanostructured coatings weaken the degradation of the thermal insulation during thermal exposure.

Fig. 4. Changes of thermal conductivity as a function of thermal-exposure duration.

Fig. 5. Statistical 2D pore densities during thermal exposure: (A) conventional TBCs; (B) nanostructured TBCs.

Fig. 6. Quasi in-situ healing behavior of an inter-splat pore at lamellar zones: (A) inter-splat pore at 0?h; (B) local region of inter-splat pore at 0?h; (C) local region of inter-splat pore after thermal exposure for 20?h.

Fig. 7. Comparison of decrease degrees of 2D pore density at different stages.

Fig. 8. Interfacial evolution between nanozones and lamellar zones during thermal exposure: (A) as-deposited state; (B) 20?h; (C) 500?h.

Fig. 9. Comparison of apparent porosities between conventional TBCs and nanostructured TBCs during thermal exposure.

Fig. 10. Changes of elastic modulus in double logarithmic plots: (A) macroscale; (B) microscale.

Fig. 11. Comparison of normalized elastic modulus with respect to their as-deposited states.