Search

Gadolinium zirconate is a promising next-generation thermal barrier coating material and its CaO-MgO-Al2O3-SiO2 (CMAS) resistance needs to be further increased. In this study, three gadolinium zirconate coatings with different Gd/Zr ratios are successfully prepared via atmospheric plasma spray using amorphous feedstock. Their mechanical properties and corrosion resistance are investigated. The Young’s moduli and hardness of as-sprayed coatings are comparable with the gadolinium zirconate coatings reported in previous literature. Furthermore, the higher Gd content promotes the formation of the Gd-apatite and the depletion rate of CMAS corrosion. As a result, the infiltration depth of Gd2.3Zr1.7O6.85 coating after 24 h annealing decreases up to 35 % compared with those of Gd2.0Zr2.0O7.0 and Gd1.8Zr2.2O7.1, exhibiting an enhanced long-term corrosion resistance. This work develops a viable fabrication method to produce non-stoichiometric gadolinium zirconate coatings with tailorable CMAS corrosion resistance and is expected to promote the future design of thermal barrier coatings with long service life.

Various excellent properties of rare earth zirconate and stannate pyrochlores are close related with their native point defects. First-principles calculations are performed to systematically investigate the point defect mechanism and the oxygen diffusion behavior of A2B2O7 (A=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd; B=Zr, Sn). The possible defect complexes and their associated reactions under stoichiometric and nonstoichiometric conditions are explored. The O Frenkel pairs are the most stable defect structure in stoichiometric zirconates, whereas the cation antisite defects are the predominant one in stoichiometric stannates. In the case of BO2 excess zirconates and stannates, the BA cation antisite defect with the A vacancy and/or the oxygen interstitial is energetically favorable, whereas the AB antisite defect together with the oxygen vacancy and/or the A interstitial is preferable under the A2O3 excess condition. Meanwhile, the maximum point defect concentrations of zirconates are much higher than those of stannates. Furthermore, the oxygen migration barriers are similar in these compounds, ranging in 0.68 eV∼ 0.80 eV. The predicted point defects and oxygen diffusion mechanisms play the critical role in their engineering applications and are expected to guide the future property improvement of pyrochlores through the control of point defects and/or composition.

Gadolinium zirconate (GZ) is a promising candidate for next-generation thermal barrier coating (TBC) materials. Its corrosion resistance against calcium-magnesium-alumino-silicate (CMAS) needs to be further increased for enhancing its in-service life. As the Gd element plays an important role in the CMAS resistance, three GZ coatings (GZ-0.75, GZ-1.0, and GZ-1.2) with different Gd/Zr atomic ratios are designed and deposited by laser enhanced chemical vapor deposition (LCVD) in this work. It is found that the generated Gd-apatite in GZ-1.2 would block micro-cracks inside the column structure and the inter-columnar gap more efficiently. Thus, the CMAS penetration rate (5.2 μm/h) of GZ-1.2 decreases over 27% comparing with GZ-1.0 and GZ-0.75, which is even lower than the Gd2Zr2O7 coatings fabricated by electron-beam physical vapor depositions (EB-PVDs). This work provides a feasible way to adjust the coating’s corrosion resistance and may guide the development of future coating for long in-service life.

Lanthanum zirconate is a promising thermal barrier coating material owing to its excellent thermophysical properties and La plays the key role in its corrosion resistance. Here, an amorphous precursor is used as raw feedstock material so as to synthesize lanthanum zirconate coatings with tailorable composition by atmospheric plasma spray (APS). Three lanthanum zirconate coatings of La1.7Zr2.3O7.15, La2.0Zr2.0O7.0 and La2.3Zr1.7O6.85 are fabricated. Furthermore, the corrosion resistance of the as-sprayed coatings against CaO-MgO-Al2O3-SiO2 at 1250℃ is investigated. The increased La content promotes the formation of a sealing layer of the crystalline Ca2La8(SiO4)6O2 apatite, which slows down the penetration of molten CaO-MgO-Al2O3-SiO2. Therefore, the infiltration rate of the La2.3Zr1.7O6.85 coating decreased up to 42.6 % compared with the other two coatings. This work develops a feasible preparation strategy to control the La composition for the improved corrosion resistance, which is expected to guide the future coating design and synthesis for the materials with big composition changes during the APS process.

Three lanthanum zirconate (LZO) thermal barrier coatings with designed La/Zr ratios, i.e., 0.7:1 (LZO-7), 1:1 (LZO-10), and 2:1 (LZO-20), are fabricated by laser enhanced chemical vapor deposition and their corrosion resistance in 40 wt% V2O5 + 60 wt% Na2SO4 mixture is studied at 900 °C. The reaction between lanthanum zirconate coatings and NaVO3 produces LaVO4 and ZrO2, which form impediment layer and prevent deeper penetration of the corrosive sulfate-vanadate molten salt. Meanwhile, no severe transverse cracks are found in all the coatings due to the columnar crystalline microstructures. Furthermore, LZO-20 shows the better corrosion resistance than others as the result of the decreased concentration of corrosive molten salts at the contacting interface, which is attributed to the fact that the excess lanthanum consumes maximum NaVO3. Correspondingly, the LaVO4/ZrO2 ratios rise with the increase of lanthanum content. These results uncover the key role of the lanthanum content on the corrosion resistance of the sulfate-vanadate molten salt and are expected to guide the optimization of the future thermal battier coating design by tailoring the composition.