The Effect Of Er₂O₃Doping On The Structures And Properties Of Ta₂O₅-Y₂O₃-ZrO₂Ceramics

In recent years, the thermal barrier coatings have received considerable attention withthe rapid development of aerospace industry. Y2O3partially stabilized ZrO2thermal barriercoatings can not follow the technical requirements because of which will be leaded tophase transformation in the long-term of thermal barrier coatings above1200℃. Inaddition, the phase transformation causes volume expansion. And the volume expansioncoefficient of the ceramics is not matched with the volume expansion coefficient ofbonding coatings. Furthermore, some microcracks are found in ceramics, which result inthe failure of the coatings. Therefore, doping with the rare earth oxide is applied byresearchers to improve the properties of thermal barrier coatings in order to develop newtopcoat ceramics suitable for the technical requirements.In this work, ErxY0.18-xTa0.18Zr0.64O2ceramics were prepared by the solid reactionmethod with Er2O3, Y2O3, Ta2O5and ZrO2as starting materials. The phase structures,phase stabilities, surface microstructures, thermal expansion coefficients and thermaldiffusivity coefficients were characterized by X-ray diffraction (XRD), scanning electronmicroscope (SEM), high-temperature dilatometer and laser-flash apparatus (LFA427),respectively. The bulk densities were tested by Archimedes principle. The specific heatcapacities were calculated by Neumann-Kopp principle. The theoretical thermalconductivities of the ceramic top-coatings were calculated by point defect model andcompared with the experimental thermal conductivities.The XRD results showed that ErxY0.18-xTa0.18Zr0.64O2ceramics were composed of thesingle tetragonal phases and had high tetragonalities after sintered at1600℃for6h. Thephase structures and compositions of the ErxY0.18-xTa0.18Zr0.64O2ceramics are not changedafter heat-treating at200℃for450h and1500℃for310h, which were still composed ofsingle tetragonal phase, indicating that the ErxY0.18-xTa0.18Zr0.64O2ceramics had good phasestability below1500℃heat treatment.The SEM photographs indicated that the grain size of the ErxY0.18-xTa0.18Zr0.64O2ceramics sintered at1600℃for6h were uniform and had minor changes with the increasesof Er3+content. In addition, the pores in the ceramics were little. The experimental densities measured by Archimedean principle were consistent with the theoreticaldensities.The results of thermophysical characteristic showed that the average thermalexpansion coefficients of ErxY0.18-xTa0.18Zr0.64O2ceramics were smaller than that of theTa2O5-Y2O3-ZrO2ceramics from room temperature to1300℃, ranged from9.86×10-6K-1～10.44×10-6K-1. The specific heat capacities of the ceramics increased with thetemperature growing from room temperature to1200℃and decreased with the increasingEr3+contents. The thermal conductivities of the ceramics were changed with temperatureincreasing from room temperature to1000℃, and Er2O3ceramics showed the lowestthermal conductivities when the doping content is2mol%, ranged from1.13Wm-1K-1～1.49Wm-1K-1. The theoretical thermal conductivities were calculated by point defectmodel, and the results indicate that the thermal conductivities are affected jointly by atommass difference and radius difference between submitted atom and matrix atom and thefield of stress around the point defect. And the field of stress around the point defect isconsidered to be more influence on the thermal conductivities.In summary, the ErxY0.18-xTa0.18Zr0.64O2ceramics have the stable structure, the goodphase stability, thermal expansion coefficients and low thermal conductivities when thedoping content of Er2O3is2mol%, which is the best suitable ceramics for thermal barriercoatings for the performance and the cost remain need further investigation.