High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates (RE3TaO7 and RE3NbO7) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the main shortcomings of RE3TaO7 and RE3NbO7 that limit their applications as TBC materials. To increase the hardness, high entropy (Y1/3Yb1/3Er1/3)3TaO7, (Y1/3Yb1/3Er1/3)3NbO7, and (Sm1/6Eu1/6Y1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7 are designed and synthesized in this study. These high entropy ceramics exhibit high Vickers hardness (10.9–12.0 GPa), close thermal expansion coefficients to that of single-principal-component RE3TaO7 and RE3NbO7 (7.9×10−6-10.8×10−6C−1 at room temperature), good phase stability, and good chemical compatibility with thermally grown Al2O3, which make them promising for applications as candidate TBC materials.


Introduction 
Thermal barrier coatings (TBCs) have widely been used in the gas-turbine engines to improve the energy efficiency and protect the hot structure components from foreign particle impact, water vapor, and molten and toughness, high melting point, close thermal expansion coefficient to that of metal substrate, and low thermal conductivity [10][11][12]. The major disadvantage of YSZ is the phase transformation, which leads to the abrupt volume change and coating cracking at higher temperatures such that the long-term service temperature is restricted to below 1200 ℃ [13]. Moreover, the severe sintering and high oxygen conductivity also limit its application in higher operation temperatures [7,14]. In order to further increase the operating temperatures of gas-turbine engines, several types of new TBC materials such as RE 2 Zr 2 O 7 , RE 3 NbO 7 , and RE 3 TaO 7 (RE = rare earth element) with better thermal stability and lower thermal conductivity have been proposed [15][16][17]. In particular, rare-earth tantalates (RE 3 TaO 7 ) and rare-earth niobates (RE 3 NbO 7 ) have attracted enormous attention in recent years. The crystal structures of RE 3 TaO 7 and RE 3 NbO 7 are variable with the change of the containing REs. For RE 3 TaO 7 , when RE is La-Dy or Y, the crystal structure is ordered orthorhombic, while the crystal structure of the rest (RE is Ho-Lu) is defective fluorite, which is analogous to that of Y 2 Zr 2 O 7 [18,19]. Similarly, with the decrease of ionic radius of REs, the crystal structure of RE 3 NbO 7 changes from orthorhombic weberite (La-Tb) to defective fluorite (Dy-Lu, Y) [19][20][21]. More importantly, due to the combination of many intriguing properties, such as good thermal stability, large thermal expansion coefficients, simple crystal structure but extremely low thermal conductivity, RE 3 TaO 7 and RE 3 NbO 7 have been considered as promising TBC materials for higher temperature applications [16,17]. Nevertheless, previous studies indicate that the mechanical properties of RE 3 TaO 7 and RE 3 NbO 7 , such as Vickers hardness, are significantly lower than those of YSZ [17,[22][23][24]. Moreover, some compounds such as Sm 3 TaO 7 and Sm 3 NbO 7 exhibit phase transition with an abrupt volume change at 942 and 817 ℃, respectively, which is unfavorable to their application as TBC materials [22,25].
It is well known that the pursuits of superior performance, for instance, the combination of higher strength, good phase stability, lower thermal conductivity, and lower sintering rate are of critical interest for developing novel TBC materials. Recently, a new class of ceramics containing multi-principal elements have attracted growing interest, which are known as high entropy ceramics (HECs) because of their high configurational entropy [26][27][28]. Compared with the single principal-component ceramics, HECs exhibit fascinating properties like lower thermal conductivity, sluggish grain growth rate, better water-vapor resistance, and tunable thermal expansion coefficient [29][30][31][32][33][34][35][36]. The unique properties of HECs indicate that there is a new window for developing TBC materials, i.e., designing and synthesizing high entropy (HE) TBC materials with superior performance.
To improve the properties of RE 3 TaO 7 and RE 3 NbO 7 for thermal barrier application and overcome the possible phase transition, HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 , i.e., (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 , (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 , and (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 , are designed and successfully synthesized in this study. The choose of these REs is due to the following reasons. Firstly, the compounds containing these REs possess similar crystal structures. Secondly, the difference of ion radius of these REs is small (< 15%, Table 1), which warrants the easy formation of phase-pure solid solution. Moreover, the selecting compounds (such as Y 3 TaO 7 , Y 3 NbO 7 , Er 3 NbO 7 , and so on) have been studied, which makes the comparison of the properties of HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 with them convenient. The phase composition, microstructure, Vickers hardness, thermal expansion coefficients, and chemical compatibility with Al 2 O 3 are investigated. The high hardness, better phase stability, and good chemical compatibility with Al 2 O 3 indicate that these new types of HECs are promising as high-performance TBC materials.
Dense HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 bulks were prepared by using a spark plasma sintering (SPS) apparatus (SPS-20T-6-IV, Shanghai Chenhua Science and Technology Co., Ltd., China) at 1650 ℃ for 4 min under a pressure of 40 MPa. Details of the preparation process were reported in our previous studies [34,36]. After sintering, the surfaces of as-sintered compacts were ground by diamond to remove the carburized layer. The density of as-sintered compacts was measured by Archimede's method. The phase composition of bulk compacts was analyzed by XRD. The microstructure and element distribution of bulk samples were investigated by a scanning electron microscope (SEM, Apollo300, CamScan, Cambridge, UK) with the attached energy dispersive X-ray spectroscopic (EDS) system (EDS Inca X-Max 80T, Oxford, UK). Before SEM observation, the samples were polished by #2000 SiC sand paper and thermally etched at 1400 ℃ for 2 h to make the grain boundary clear.
Vickers hardness measurement was performed by a micro-hardness tester (HXD-1000TMC/LCD, Shanghai Taiming, China) at a load of 9.8 N with a dwell time of 15 s. The linear thermal expansion coefficients were measured by an optical dilatometer (Misura ODHT 1600-50, Expert System Solutions, Italy) from room temperature to 1200 ℃ using the samples with a dimension of 3 mm × 4 mm × 15 mm. Before the test, the samples were polished by #2000 SiC sand paper and then chamfered at one end of the length. The length change of the samples (ΔL) with temperature (T) was recorded and the thermal expansion coefficient (α) was calculated by Eq. (1): where L 0 is the length of the sample at room temperature and T 0 is room temperature. Good chemical compatibility with thermally grown Al 2 O 3 (TGO) is critical for TBC materials. In order to investigate the chemical compatibility between HE RE 3 TaO 7 /RE 3 NbO 7 /RE 3 (Nb 1/2 Ta 1/2 )O 7 and Al 2 O 3 , the as-synthesized HE powders were mixed with α-Al 2 O 3 powders (99.9% purity; HWRK Chem. Co., Ltd., Beijing, China) in a mass ratio of 1 : 1 by ball-milling, and then annealed at different temperatures for 2 h. For comparison, the chemical compatibility between Y 3 TaO 7 , Y 3 NbO 7 , and α-Al 2 O 3 was also investigated by the same method. The phase composition of the annealed products was investigated by XRD. Figure 1 shows the XRD patterns of the as-synthesized Y 3 TaO 7 , Y 3 NbO 7 , and HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 powders. It can be seen that all the as-synthesized powders are phase-pure. The crystal structures of (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 , (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 , and (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 are defective fluorite despite that Y 3 TaO 7 exhibits an orthorhombic structure. Interestingly, the intensities of the diffraction peaks from all reflections of (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 are significantly lower than those of the others. In addition, the widths of the peaks of (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 are broadened. This is one of the main 306 J Adv Ceram 2020, 9(3): 303-311 www.springer.com/journal/40145 characteristics of high entropy materials and can be explained by the intrinsic lattice distortion caused by the addition of multi-principal elements with different atomic sizes, which leads to the increase of the atomic plane roughness and the decrease of the diffraction peak intensity [37]. Based on the XRD patterns in Fig. 1, the lattice parameters of HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 were refined and their theoretical densities were calculated, which are depicted in Table 2. The refined lattice parameter of (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 is close to that of (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 . However, due to   Fig. 3. No pores and cracks can be observed at the surface of all three bulk materials, indicating that the as-sintered bulk compacts are near fully dense. The densities of (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 , (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 , and (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 measured by Archimede's method are depicted in Table 2. Clearly, the relative densities of all the samples are about 99%. Moreover, all the bulk compacts present equiaxed grains,  but the average grain size of (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 is evidently smaller than those of (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 and (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 ( Table  3). The grain size difference of three near fully dense materials can be explained by the sluggish diffusion effect of high entropy materials. The lattice distortion induced by addition of multi-principal elements hinders the atomic movement and effective diffusion of the atoms and thus makes the grain growth rate slower [38]. The EDS mappings indicate that all the containing principal metal elements of three bulk materials are evenly distributed and thus the homogeneous solid solutions are formed.

2 Vickers hardness
During the service, the thermal barrier coatings usually suffer from the corrosion caused by the impact of foreign particles in the fluid. Thus, Vickers hardness of TBC materials is an important parameter and high Vickers hardness can retard the impact corrosion rate of TBC materials. Table 3 exhibits the Vickers hardness of HE RE 3 TaO 7 , RE 3 NbO 7 , and RE 3 (Nb 1/2 Ta 1/2 )O 7 measured at a 9.8 N load and compared with the Vickers hardness of RE 3 NbO 7 , RE 3 TaO 7 , and YSZ [17,[22][23][24].

4 Chemical compatibility with Al 2 O 3
When gas-turbine engines operate at high temperatures, the interconnected porosity of ceramic coatings always allows the ingress of oxygen from the engine environment to the bond-coat layer, which results in the oxidation of bond-coat layer and the formation of TGO layer between the bond-coat layer and the ceramic top-coat layer [1].