Y2Zr2O7 is one of the most promising candidate materials for ultra-high-temperature thermal barrier coatings (TBCs), but it suffers from low fracture toughness and performance degradation caused by high-temperature sintering. Varying content of Y3Al5O12 (YAG) is introduced into Y2Zr2O7 coatings to achieve better mechanical properties and sintering resistance in this study.
(1-x)Y2Zr2O7-xY3Al5O12 (x=0, 0.1, 0.15, 0.2) ceramic coatings are fabricated by atmospheric plasma spraying (APS) with spray-dried agglomerated powders. The powders are synthesized from ZrO2, Y2O3, and Al2O3 precursors, mixed in varying molar ratios to achieve YAG content levels of 0 mol%, 10 mol%, 15 mol%, and 20 mol%. The spray-drying process involves feeding the mixed powders into a spray dryer (LGZ-8) at an inlet temperature of 240 ℃, an outlet temperature of 110 ℃, a nozzle frequency of 35 Hz, and a feed pump speed of 25 r/min. The resulting agglomerated powders are then calcined at 1 000 ℃ for 5 hours to remove organic binders and enhance particle cohesion.
The APS process is conducted using an Oerlikon Metco UniCoatProTM system equipped with an F4MB-XL spray gun. The spraying parameters are optimized to ensure uniform deposition. The (1-x)Y2Zr2O7-xY3Al5O12 coatings are deposited after a bond layer of NiCrAlY and a layer of 8YSZ on 310S stainless steel substrates and graphite substrates. The microstructure and phase composition of the coatings are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The average grain size is determined from SEM images by Image J software. The hardness and fracture toughness is evaluated according to the Vickers indentation method, with a load of 1 000 g and a dwell time of 15 s. The adhesion strength of the coatings is measured with a WDW-5E universal tensile testing machine, at a crosshead speed of 2 mm/min.
The as-sprayed coatings exhibit a typical plasma-sprayed morphology with varying degrees of porosity and unmelted particles. The addition of YAG significantly refines the microstructure, leading to a more uniform distribution of elements and a reduction in unmelted particles. The XRD analysis reveals that the as-sprayed 0YAG coatings primarily consist of cubic ZrO2 and a small amount of Y2O3. In contrast, the YAG-doped coatings contain additional phases such as Al2Y4O9 and amorphous regions. The presence of these secondary phases indicates that YAG doping induces chemical reactions during the spraying process, resulting in a more complex microstructure.
The adhesion strength of the coatings increases with YAG content increases, reaching a maximum value of 43.71 MPa of 0.1YAG. This enhancement is attributed to the improved interfacial bonding between the coating and the substrate, facilitated by the presence of YAG. However, further increasing the YAG content to 20 mol% leads to a decrease of adhesion strength, which is due to the agglomeration of YAG particles, introducing stress concentrations and reducing the overall mechanical integrity of the coating.
The high-temperature sintering behavior of the coatings is evaluated by isothermal treatment at 1 400 ℃ for 24, 48, 72 and 96 hours. The XRD patterns of the sintered coatings show that the Al2Y4O9 phase decomposes, and YAG grains nucleate and grow during sintering. The c-ZrO2 phase remains stable, but its grain size increases more slowly in the YAG-doped coatings compared with the undoped 0YAG coating. The SEM analysis reveals that YAG grains form at the grain boundaries of c-ZrO2, effectively pinning the grain boundaries and inhibiting grain growth. This phenomenon is particularly evident in the 0.1YAG coating, where the c-ZrO2 grain size is increased by only 8% after 96 hours at 1 400 ℃, compared with a 35% increase in the 0YAG coating.
The fracture toughness of the coatings is significantly enhanced by YAG doping. This improvement is attributed to the refined microstructure and the presence of YAG/c-ZrO2 interfaces, which promote crack deflection and bridging. The YAG grains also reduce the porosity of the coatings, further contributing to the enhanced mechanical properties.
The study demonstrates that YAG doping significantly improves the microstructural stability, mechanical properties, and high-temperature sintering resistance of Y2Zr2O7-based ceramic coatings. The optimal YAG content is found to be 10 mol%, resulting in a coating with high bond strength, enhanced fracture toughness, and excellent sintering resistance. The underlying mechanisms include the formation of metastable phases during spraying, the nucleation and growth of YAG grains during sintering, and the pinning of c-ZrO2 grain boundaries by YAG. These findings provide valuable insights for the design and optimization of advanced thermal barrier coatings for high-temperature applications. Future work may focus on further investigation of the performance of Y2Zr2O7-Y3Al5O12 coating under thermal cycling and CMAS corrosion.
Key words
thermal barrier coating /
Y2Zr2O7 /
Y3Al5O12 /
high-temperature sintering behavior /
fracture toughness
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Funding
Anhui Provincial Natural Science Foundation (2308085QE135); Key Research Project of Natural Science Foundation of Anhui Provincial Universities (KJ2021A0389)
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