Failure of the plasma-sprayed coating of lanthanum hexaluminate
Introduction
TBCs find an increasing number of applications to protect high-temperature metallic components. TBCs are deposited on transition pieces, combustion lines, first-stage blades and vanes, and other hot-path components of gas turbines either to increase the inlet temperature with a consequent improvement of the thermal efficiency or to reduce the requirements for the cooling system.1 The selection of TBC materials is restricted by some basic requirements, including high melting point, phase stability between room temperature and operation temperature, low thermal conductivity, chemical inertness, thermal expansion match with the metallic substrate, good adherence to the metallic substrate, and low sintering rate of the porous microstructure.1, 2
No single material satisfies all these criteria. The best compromise among these requirements is presently offered by the partially stabilized zirconia containing 7–8 wt% Y2O3 (i.e. 4.0–4.6 mol% Y2O3) on a MCrAlY bond coat, deposited either by plasma spraying or by electron beam-physical vapour deposition (EB-PVD).1 A major disadvantage of yttria-stabilized zirconia (YSZ) is the limited operation temperature of 1200 °C for long-term application. At higher temperatures, phase transformations from the t′-tetragonal to tetragonal and cubic (t + c) and then to monoclinic (m) occur, giving rise to the coating failure.2, 3 The search for new materials that can withstand higher gas-inlet temperatures was intensified within the last decade. La2Zr2O7 has been proposed as a candidate of TBC material.2, 4, 5, 6 It has a cubic pyrochlore structure which has been discussed in detail by Subramanian et al.7 Compared with YSZ, it has a lower thermal conductivity (1.56 W m−1 K−1 for La2Zr2O7, 2.1–2.2 W m−1 K−1 for YSZ, bulk materials, 1000 °C), lower thermal expansion coefficient ((9.1–9.7) × 10−6 K−1 for La2Zr2O7, (10.5–11.5) × 10−6 K−1 for YSZ, bulk materials and coatings, 30–1000 °C) and lower sintering ability. However, its single layer coating has a very short life due to its low thermal expansion coefficient and low fracture toughness,8 and the life could be improved if a double ceramic-layer coating of La2Zr2O7 with 8YSZ was applied, especially for the high temperature application.9, 10
Lanthanum magnesium hexaluminate (LMA) is an important ceramic material for high temperature applications such as active elements of solid-state lasers,11 combustion catalyst and catalyst support,12, 13 and TBC material.14, 15, 16, 17, 18 It possesses long term structural and thermochemical stabilities up to 1400 °C, and has significantly lower sintering rate than the zirconia-based material. The low thermal conductivity of LMA is caused by its microstructure, i.e. a random arrangement of LMA platelets that build up a microporous coating and the insulating properties of the material with its crystallographic feature itself. A study describing the development of an optimized procedure for the processing, manufacturing and application of LMA as TBC material has been reported by Gadow.16
It is generally accepted that thermal expansion mismatches between the top ceramic coat and metallic bond coat and TGO are the strongest factors for coating failure.19, 20 TGO is composed of mainly alumina (Al2O3), some chromia (Cr2O3) and spinel (NiAl2O4). At the early stage of thermal cycling, the top ceramic coat contacts directly with the bond coat, and the thermal expansion mismatch plays the most important role in determining the thermal cycling life of TBCs. When the bond coat is oxidized and the TGO thickness reaches a critical value (8–10 μm for 8YSZ TBC21), the swelling of TGO would lead to the coating failure.
Gadow and co-workers have reported the basic thermomechanical properties of LMA and the preparation of its coating by plasma spraying.14, 15, 16, 17, 18 In this work, the thermal stability and failure of LMA coating were studied.
Section snippets
Experimental
Main chemicals used in this work were La2O3 (99.99%, Guangdong Chenghai Sanxing Chemicals Co., Ltd.), MgO (99.2%, Wuxi Zehui Chemicals Co., Ltd.) and γ-Al2O3 (99.99%, Tangshan Haigang Huatai Functional Ceramic Materials Co., Ltd.). The starting powder for the plasma-sprayed LMA coating was synthesized by solid-state reaction. The powder mixture of La2O3, MgO and γ-Al2O3 in proper ratio was heated at 1600 °C for 6 h. The as-synthesized LMA powder was mixed with water and Gum Arabic, followed by
Failure of LMA coating
Gadow and co-workers have reported the synthesis of LMA by means of sol–gel.16, 17, 18 In this work, a solid-state method was applied to synthesize LMA on a large scale, which is more convenient than sol–gel. After being heated at 1600 °C for 6 h, the formation of LMA was completed, and the plate-like structure was observable as shown in Fig. 1. The platelet looks very dense and has a thickness of 1–2 μm. The plate-like structure is believed to be the reason for its low thermal conductivity.16
One
Conclusion
The spallation of LMA coating occurs mainly at TGO which is similar to the coating made by EB-PVD. The similar thermal expansion behaviour of LMA with Al2O3 is very helpful to the prolongation of thermal cycling life of LMA coating, and therefore it is suggested to make a thin layer of TGO before the deposition of LMA. On the other hand, even though the plasma-sprayed LMA coating has phase transition during thermal cycling, the plate-like and porous structure of LMA results in a low Young's
Acknowledgement
The authors thank Ms. M.Y. Li for SEM analysis and Mr. G. Li for XRD measurement, Dr. Q.S. Wang of Beijing Polytechnic University for the invaluable assistance in plasma spraying. This work was financially supported by the project of A1320061002.
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