Preparation of (Gd0.9Sc0.1)2Zr2O7/YSZ thermal barrier coatings and their corrosion resistance to V2O5 molten salt
Introduction
Thermal barrier coatings are widely used to enable hot-section metallic components of turbine engines to operate at high temperature, which increases the engine thrust-to-weight ratio and improve the engine efficiency [1,2]. Typically, a ceramic topcoat and a metallic bond coat make up a TBC system. The former is used as thermal insulation layer, while the latter has an important role in protecting the underlying substrate from oxidation and corrosion and improving the bonding between the ceramic top coat and the substrate [[1], [2], [3], [4]]. Currently, the widely used ceramic topcoat is made of Y2O3-stabilized metastable tetragonal ZrO2 (t′-YSZ); however, its application faces a limitation, i.e., it is unable to operate for long periods above 1200 °C [3,[5], [6], [7]]. At higher temperatures, YSZ coatings undergo a phase decomposition from the t′ phase to Y-rich and Y-lean phases. The latter transforms to monoclinic (m) phase on cooling, accompanied with a destructive volume expansion, which leads to premature failure of the coatings.
The thrust-to-weight ratio and thermal efficiency of engines scale with the operation temperature. Therefore, alternative TBC materials to YSZ that have even higher phase stability are strongly required. Additionally, for better thermal insulation, the newly developed TBC materials should have lower thermal conductivities. Recently, some high-temperature ceramics are proposed for TBC applications, such as Gd2Zr2O7, LaMgAl11O19, GdPO4, Gd3TaO7 [[8], [9], [10], [11], [12]]. However, their poor mechanical properties, especially low toughness, highly restrict the application. Among these candidates, Gd2Zr2O7 has been widely investigated, and several strategies were reported to improve its toughness. Our previous study has found that 10 mol% Sc2O3 doped Gd2Zr2O7, (Gd0.9Sc0.1)2Zr2O7, has significantly improved toughness compared with Gd2Zr2O7 [13]. Also, (Gd0.9Sc0.1)2Zr2O7 is phase stable up to 1600 °C, and has much lower thermal conductivity than YSZ [13].
Besides the issue of phase instability, hot corrosion from molten salt is a significant life-limiting factor for YSZ TBCs when low-grade fuels with vanadium, sulfur, sodium are used or coatings are operated under marine conditions [[14], [15], [16]]. Molten sodium salts of vanadium and sulfur on the coatings have extreme corrosion at 600–1050 °C [[16], [17], [18]]. At high temperatures, they permeate through the coating, and react with the stabilizer yttria in YSZ, causing the decomposition of the t′ phase [16,19]. As a result, YSZ coatings spall quickly in the presence of molten salts. Newly developed TBC materials also face hot corrosion from molten salt. Although La2Zr2O7 and Gd2Zr2O7 are more resistant than YSZ to hot corrosion, their corrosion resistance is still not satisfactory [17,20]. We have investigated the hot corrosion behavior of Ba2YbAlO5, which reveals a better corrosion resistance than YSZ [21]. Interestingly, when LaPO4 and NdPO4 thermal barrier oxides are exposed to V2O5 molten salt, a RE(P,V)O4 (RE = Nd, La) solid solution is produced causing little damage to the original microstructure, which gives LaPO4 and NdPO4 excellent resistance to molten salt corrosion [22].
Research reported that (Gd0.9Sc0.1)2Zr2O7 is a promising TBC material, and its corrosion resistance to molten salt is better than that of Gd2Zr2O7 [23]. However, the previous study just indicated that (Gd0.9Sc0.1)2Zr2O7 is a TBC candidate; for TBC applications, there are much work needs to do, including how to produce (Gd0.9Sc0.1)2Zr2O7 coatings, how to design the coating microstructure, and what is the performance of the coating in the presence of molten salt. Although (Gd0.9Sc0.1)2Zr2O7 has improved toughness compared with Gd2Zr2O7, it cannot be used as a ceramic top directly to replace YSZ; usually, a concept of double ceramic-layer (DCL) is employed. DCL coatings have been reported to reveal much improved thermal cycling duration compared with their single counterparties and conventional YSZ coating [[24], [25], [26]].
In this study, (Gd0.9Sc0.1)2Zr2O7/YSZ DCL TBCs are fabricated by air plasma spray (APS), and the emphasis is focused on producing the (Gd0.9Sc0.1)2Zr2O7 coating and charactering the phase structure and microstructure of the coating. Then the hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7/YSZ coatings in V2O5 molten salt at 700–1000 °C is investigated, and the corrosion products are analyzed and determined. Results indicated that (Gd0.9Sc0.1)2Zr2O7/YSZ coatings reveal high resistance to molten salt penetration, and the related mechanisms are discussed.
Section snippets
Experimental procedures
The (Gd0.9Sc0.1)2Zr2O7 and YSZ powders were synthesized by chemical co-precipitation and calcination method, the details about which are available in previous reports [23,27]. A spray drying method was used to produce the (Gd0.9Sc0.1)2Zr2O7 and YSZ microscopic particles with better fluidity, which is necessary to achieve the desirable result in the following spray process. The agglomerated particles size distribution was determined using a laser scattering method (MICROTRAC HRA, 9320-X100,
Preparation and characterization of (Gd0.9Sc0.1)2Zr2O7/YSZ coatings
Fig. 1 shows SEM micrograph of (Gd0.9Sc0.1)2Zr2O7 agglomerated particles. All the particles have a spherical shape with a diameter of ~50 μm, which is basically consistent with the result obtained from particles size analysis. Individual particle was observed at a higher magnification, revealing a rough surface and a porous microstructure (Fig. 1b). Note that the microscopic particle consists of many fine particles with sizes smaller of ~100 nm. Fig. 2 shows the XRD patterns of (Gd0.9Sc0.1)2Zr2O
Conclusions
(Gd0.9Sc0.1)2Zr2O7/YSZ double-ceramic-layer TBCs were fabricated by air plasma spray (APS). Typical characteristics of APS coating, such as lamellar microstructure, micro-cracks and pores, were found in the (Gd0.9Sc0.1)2Zr2O7 coating. Also, the (Gd0.9Sc0.1)2Zr2O7 coating had the same phase structure with and a close chemical composition to those of the agglomerated particles for thermal spray. Among the splats in the (Gd0.9Sc0.1)2Zr2O7 coating, there were some porous zones, which are composed
CRediT authorship contribution statement
Lei Guo: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Supervision. Hui Xin: Methodology, Formal analysis, Investigation, Data curation, Writing - original draft. Zhao Zhang: Methodology. Fuxing Ye: Validation.Zheng Yan: Investigation.
Declaration of competing interest
This work is sponsored by the National Natural Science Foundation of China (Grant No. 51971156). There are no potential conflicts of interest include: employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and grants or other funding.
Acknowledgments
This research is sponsored by the National Natural Science Foundation of China (Grant No. 51971156).
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2022, Ceramics InternationalCitation Excerpt :As can be seen in Fig. 7, the V-rich layer also has a large amount of Gd, indicating the formation of GdVO3 corrosion product. This is easy to be understood based on Lewis acid-base rule [30,38]. GZO-Sc can be regarded as a mixture of Gd2O3, Sc2O3 and ZrO2 in appropriate proportions, and V2O5 has a large tendency to react with these basic oxides.