Thermal barrier coatings (TBCs) are one of the key thermal insulation technologies for high-temperature components in aero-engines and play a critical role in further increasing turbine inlet temperatures. Among various ceramic materials, 8wt.% Y2O3-stabilized ZrO2 (YSZ) has become the most widely used topcoat material due to its low thermal conductivity, excellent phase stability, and favorable thermal cycling resistance. However, under long-term high-temperature service conditions, YSZ coatings inevitably undergo complex temperature-time coupled evolution (i.e., thermal history), during which their phase composition, microstructure, and mechanical properties change significantly, thereby critically affecting the stress state and service lifetime of the coatings.Previous studies have shown that high-temperature exposure can induce tetragonal-to-monoclinic (t→m) phase transformation, sintering-driven densification of columnar structures, and continuous evolution of pore architecture in YSZ coatings. These processes are often accompanied by redistribution of residual stresses and reduction in strain tolerance, ultimately accelerating coating failure. However, most existing studies have focused on isolated thermal conditions, and a systematic understanding of the structure-property evolution under coupled temperature-time effects remains lacking. Therefore, from the perspective of thermal history, establishing the correlation between temperature-time coupling and coating evolution is of great significance for elucidating the high-temperature degradation mechanisms of YSZ coatings.In this work, a series of representative thermal histories were designed with a controlled variable approach: isothermal exposure at 1 200 ℃ for 10, 20, 30, 40, and 50 h, as well as at 1 250 ℃ and 1 300 ℃ for 10 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation were employed to systematically investigate the effects of thermal history on the phase composition, microstructure, and mechanical properties (hardness and Young's modulus) of YSZ coatings, and to further elucidate the underlying evolution mechanisms. At 1 200 ℃, with the increasing holding time, the monoclinic phase content in the YSZ coatings remained at a low level, indicating good phase stability. Meanwhile, the columnar tops gradually became rounded and blunted, the intra-columnar feathery structures progressively disappeared, and inter-columnar pores evolved into vertical cracks due to the fracture of sintering necks under transverse tensile stress, resulting in an overall porosity reduction to ~11%. Correspondingly, the Youn's modulus and hardness initially decreased to ~38.7 GPa and ~4.25 GPa, respectively, and then increased to ~68.9 GPa and ~5.98 GPa after 50 h. This trend reflected the competition between early-stage pore structure rearrangement and subsequent sintering-induced densification.At a constant holding time of 10 h, increasing temperature led to a significant increase in monoclinic phase content, reaching ~17.49vol.% at 1 300 ℃. Meanwhile, the pores within the coating gradually closed, and no apparent vertical cracks were observed, indicating that higher temperature promoted overall sintering densification. When the temperature increased to 1 300 ℃, the porosity further decreased to ~7.9%, while the Young's modulus and hardness increased to ~67.8 GPa and ~7.01 GPa, respectively. Overall, both increasing temperature and prolonging holding time promoted densification and mechanical property enhancement of YSZ coatings, although their underlying mechanisms differed significantly.In summary, YSZ TBCs exhibit good phase stability during service at 1 200 ℃ for up to 50 h. With intensified thermal history (increasing temperature or holding time), the coatings become progressively densified with reduced porosity, and their hardness and Young's modulus show an overall increasing trend. This work systematically reveals the structure-property evolution of YSZ coatings from the perspective of thermal history, providing theoretical guidance for the design and lifetime prediction of TBC systems under high-temperature service conditions.
Key words
thermal barrier coating /
YSZ /
high-temperature treatment /
microstructure /
Young's modulus /
hardness
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Funding
National Natural Science Foundation of China (52301089); Jiangxi Provincial Key Research and Development Program (20232BBE50007)
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