| Abstract: |
The thermal mismatch stress, as well as residual stress, in coating/substrate systems (i.e., thermal barrier coatings (TBCs)) often leads to structural changes and subsequent coating debonding during service conditions. In this study, strain-induced structural changes of yttria-stabilized zirconia coatings (YSZ) dependent with bonding ratio were investigated, with the aim of revealing a foundational state prior to constrained-sintering. To begin with, the YSZ coatings were plasma-sprayed on Ni-based superalloy under various deposition temperatures to obtain different bonding ratios. Subsequently, the structural changes induced by a combination of the thermal mismatch stress and the residual stress were examined. Meanwhile, the changes in multiscale properties (i.e., elastic modulus and ionic conductivity) were also determined. Results show that the structural changes induced by strain are highly sensitive to the bonding ratio. In the case of a lower bonding ratio, dispersive intersplat tearing with intrasplat cracking was observed. Consequently, a lower bonding ratio often corresponds to the structural changes in micro/mesoscale primarily. However, in the case of a higher bonding ratio, large vertical cracks, which were accumulated from partially-concentrated intra-cracking and inter-tearing, appeared under strain. Correspondingly, a higher bonding ratio often results in segmented-structure in macroscale dominantly. The changes in properties exhibited that the macroscale properties are more sensitive to the structural changes as compared to the microscale properties. Moreover, the macroscale elastic modulus presented commensurate decrease despite of the coatings with different bonding ratios. That means the micro-dominant structural changes in coatings with lower bonding ratios present comparable enhanced strain tolerance with respect to the macro-segmented structure in coatings with higher bonding ratios. To sum up, this study revealed an actual starting structure of TBCs prior to sintering. The comparison of multiscale structural changes dependent with bonding ratio also provides a potential method for structural tailoring towards enhanced strain tolerance.
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