It is of great significance to search oxide thermal/environmental barrier coatings (T/EBCs) with high working temperatures and thermal expansion coefficients (TECs) matching to different substrates. ABO₄-type oxides have been widely studied due to their high working temperatures, adjustable TECs, and low thermal conductivity. In this work, ABO₄-type (A=Ga, In, Cr; B=Nb, Ta) oxides are studied as EBC candidates based on their relatively low TECs. The influences of crystal structures, distortion degree, types of polyhedrons, as well as the A- and B-site ionic radii and atomic weights on TECs are discussed. It is found out that the TECs of ABO₄-type oxides are not depended on one single factor, and reducing A-site ionic radius may be a good way to decrease their TECs. Based on the TECs, AlNbO₄, InNbO₄, and GaTaO₄ are chosen as EBCs for C-, SiC-, and Al₂O₃-based substrates, respectively. The similar TECs between ABO₄-type oxide EBCs and substrates are beneficial for reducing interfacial thermal stress, which is good for their long-term applications. This work shows that the applications of ABO₄-type oxides can be expanded by effectively regulating TECs.

Rare-earth tantalates RETaO₄ have been extensively investigated as thermal protective materials, but their applications as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) are limited by their high thermal expansion coefficients (TECs≥9.0×10^-6 K^-1). The high-entropy design provides a feasible route to tailor the thermal properties of RETaO₄; however, most reports focus on equimolar RETaO₄ HECs. In this study, a series of non-equimolar monoclinic-prime (m´) RETaO₄ HECs are designed and synthesized to clarify the composition-structure-property relationships. The non-equimolar design is used to regulate configurational complexity, polyhedral distortion, and lattice strain in m´-RETaO4 within the same m´ phase. Atomic-scale TEM and GPA characterizations reveal the associated local distortion and lattice strain. These observations link non-equimolar composition with local structural distortion and macroscopic thermal transport behavior. The lowest thermal conductivity reaches 1.52-2.68 W·m^-1·K^-1 at 25-900 °C, and is mainly associated with RE-site disorder, lattice strain, and polyhedral distortion. Polyhedral distortion also suppresses thermal expansion by restricting atomic anharmonic vibrations. Among the designed compositions, the optimized non-equimolar m´-RETaO₄ HECs (Sc_0.2Y_0.2Tm_0.2Ho_0.2Dy_0.1Gd_0.1)TaO₄ exhibits the lowest TECs of 6.2×10^-6 K^-1 at 1500 °C, closer to SiC-based CMCs than the other compositions, indicating its potential as candidate EBCs.  This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of non-equimolar RETaO₄ HECs, providing guidance for the design of complex oxides.

Thermal barrier coatings (TBCs) with high working temperatures and long service life are indispensable for the hot-end components of gas turbines and aircraft engines. This work designs and verifies that tantalate high-entropy ceramic (HEC) coatings are excellent TBCs with working temperatures reaching 1500 °C. We reveal the structural evolution and failure mechanisms of tantalate HEC coatings synthesized via air plasma spraying (APS). After they are subjected to thermal shock at 1500 °C for 614 cycles, thermal fatigue at 1150 °C for 12,830 cycles, and annealing at 1100 °C for 384 h. The thermal stress caused by the temperature gradient, differences in thermal expansion coefficients (TECs), and mechanical properties between ceramic coatings and bond coat (BC) lead to the spalling of coatings during thermal shock, while the effects of BC oxidation are limited. In thermal fatigue, the accumulative thermal stress between BC and thermally grown oxides (TGO) is higher than the fracture resistance when the h/R ratio is higher than 0.32 (h and R are the TGO thickness and undulation radius, respectively), which mainly leads to the spalling of coatings. Additionally, the effects of coating sintering and stiffness are also considered, which lead to surficial spalling during the high-temperature service. Two different failure mechanisms are proposed based on their microstructural evolution, and synthesized tantalate HEC coatings can be used at temperatures up to 1500 °C, which further promotes the design and application of high-performance TBCs.

Tailoring crystals and microstructures of multicomponent rare-earth tantalate RETaO₄ nano powders can promote their application as thermal protective coating materials because their properties are dominated by the structures. In this work, multicomponent RETaO₄ nano powders are synthesized via the chemical coprecipitation method, and their structures are tailored by changing the annealing temperature. After annealed at 800–1500 °C, the multicomponent RETaO₄ nano powders can be crystallized into metastable tetragonal (t′), monoclinic-prime (m′), and monoclinic (m) phases, and their particle sizes (8–652 nm) gradually increase with increasing temperature. The optimal annealing temperature of RETaO₄ powders is determined to be 1000 °C based on the crystallinity degree and particle sizes, which are 11.3 nm and 89.5%, respectively. High-resolution transmission electron microscopy (HR-TEM) and corresponding energy dispersive X-ray spectroscopy (EDS) mapping have validated the compositional uniformity of each element at the nanoscale, and the interplanar spacing of different phases corresponds to the X-ray diffraction (XRD) Rietveld refinements. This work demonstrates that high-temperature heat treatment can act as an effective mean to tailor the particle sizes and crystal structures of RETaO₄ ceramic, which can be further applied to synthesize nano spherical powders and coatings in future studies.

High entropy engineering has been widely used to optimize properties of various materials, and we improve comprehensive performance of rare-earth tantalates RETaO₄ (RE is rare earth) by changing configurational entropy in this work. Four medium/high entropy RETaO₄ (M/HERT) are successfully prepared, and the variations of disorders and distortion degree of lattices with the increasing configurational entropy are described in detail. It is revealed that M/HERT with the highest configurational entropy does not correspond to the best comprehensive properties. Unexpected variations in properties of M/HERT compared to RETaO₄ are observed. By comparing with values obtained from rule of mixture (ROM), it is believed that the cocktail effect exists in M/HERT. The synergistic optimizations of thermo-mechanical properties are realized, including reducing thermal conductivity, increasing thermal expansion coefficients (TECs), and enhancing mechanical properties. M/HERT exhibit excellent high temperature stability and provide a good thermal insulation gradient, which is significant for high-temperature applications of RETaO₄. This work serves as an important part for thermal barrier coatings materials with high working temperatures and low thermal conductivity.

High fracture toughness, low thermal conductivity, and thermal expansion coefficient (TEC) matching substrate are essential for thermal barrier coatings (TBCs) and abradable seal coatings (ASCs). In this work, TmNbO₄/Tm₃NbO₇ composites are designed and synthesized to increase their fracture toughness (K_IC) and thermal insulation performance. Compared with those of TmNbO₄ (K_IC = 2.2±0.1 MPa·m^1/2) and Tm₃NbO₇ (K_IC = 1.7±0.2 MPa·m^1/2), the increments in fracture toughness are as high as 50.0% and 91.1%, respectively. The highest toughness reaches 3.3±0.4 MPa·m^1/2, which is attributed to the superior combination of grains between TmNbO₄ and Tm₃NbO₇, as well as the simultaneous effects of microcracks and crack bridging and bifurcation. Accurate estimation of the effect of the interfacial thermal resistance on the thermal conductivity at low temperatures was achieved using the minimum interfacial thermal resistance model. A novel method is proposed to inhibit radiative heat transfer by utilizing oxides with glass-like thermal conductivity to suppress thermal radiation. Consequently, the TmNbO₄/Tm₃NbO₇ composite maintains a low thermal conductivity (1.19–2.02 W·m⁻¹·K⁻¹) at 1000 °C. The high TECs (10.4×10⁻⁶–11.8×10⁻⁶·K⁻¹ at 1500 °C) and excellent high-temperature stability ensure that the designed TmNbO₄/Tm₃NbO₇ composites can be used at temperatures reaching 1500 °C. Accordingly, simultaneous enhancement of fracture toughness and thermal insulation in TmNbO₄/Tm₃NbO₇ composites is effective, and the revealed mechanisms are useful for various materials.