Phase transition behavior of yttria-stabilized zirconia from tetragonal to monoclinic in the lanthanum zirconate/yttria-stabilized zirconia coupled-system using molecular dynamics simulation

Effect of Rare Earth Elements on Stability and Sintering Resistance of Tetragonal Zirconia for Advanced Thermal Barrier Coatings

The effect of dopant species on the sintering resistance of zirconia-based ceramics remains a huge challenge in terms of both experiment and theory. As one of the most popular materials for high-temperature protective coatings, it is still urgent to obtain rare earth-doped ZrO2 with high sintering resistance and good phase stability. Here, the sintering resistance and phase stability of rare earth oxides (La2O3, Nd2O3, Gd2O3, and Y2O3)-stabilized zirconia (ZrO2) were thoroughly studied by theoretical and experimental methods. According to experimental data, ZrO2 doped with rare earth ions with larger radii (La3+, Nd3+, and Gd3+) exhibited improved sintering resistance at reduced tetragonal phase stability. Molecular dynamics simulation results revealed that rare earth ions with larger ionic radii are prone to segregation at grain boundaries, which can more effectively reduce the grain boundary energy in the materials under consideration. Therefore, the proposed approach involving doping of NdO1.5 (~1 mol%) and YO1.5 (YbO1.5, 6 mol%) in ZrO2 is considered to be a promising route for the effective preparation of sinter-resistant ZrO2-based ceramics for refractory and thermal barrier materials.

Oxidation Behavior of the Monolayered La2Zr2O7, Composite La2Zr2O7 + 8YSZ, and Double-Ceramic Layered La2Zr2O7/La2Zr2O7 + 8YSZ/8YSZ Thermal Barrier Coatings

The degradation process of thermal barrier coatings (TBCs) such as monolayered La₂Zr₂O₇, composite 50% La₂Zr₂O₇ + 50% 8YSZ, and double-ceramic layer (DCL) La₂Zr₂O₇/50% La₂Zr₂O₇ + 50% 8YSZ/8YSZ was investigated. Coatings were deposited using the atmospheric plasma spraying (APS) process (ceramic layer and bond-coat) on the Ni-based superalloy substrate with Ni-22Cr-10Al-1Y bond-coat. The thickness of the ceramic top-coats in all cases were 300 µm. In the case of La₂Zr₂O₇/8YSZ, the internal sublayer was built from 8YSZ powder whereas the outer from La₂Zr₂O₇. Between both sublayers’ “composite” a 50% La₂Zr₂O₇ + 50% 8YSZ zone was present. The “composite” 50% La₂Zr₂O₇ + 50% 8YSZ TBC system was sprayed from two different feedstock powders with equal weight ratios. In the first part of the investigation, the microstructural characterization of the TBCs was presented. The main goals were related to the characterization of the degradation processes in different TBC systems with special emphasis on the phenomenon in the thermally grown oxide (TGO) zone related to oxidation, and the phenomenon related to phase stability in ceramic top-coats as related to temperature influence. The oxidation test was carried out in air at 1100 °C for 500 h. In the second step of the investigation, the numerical simulation of the monolayered TBC 8YSZ and La₂Zr₂O₇ systems was analyzed from the stress distribution point of view. Additionally, the two-layered TBC coating of the DCL type was also analyzed.