Evolution of hot corrosion resistance of YSZ, Gd2Zr2O7, and Gd2Zr2O7 + YSZ composite thermal barrier coatings in Na2SO4 + V2O5 at 1050 °C
Introduction
Thermal barrier coatings (TBCs) are frequently used on the blades and vanes of gas turbines to provide thermal insulation. By lowering the metal temperature in conjunction with the use of internal cooling and film cooling technology, TBC improves both component durability and engine efficiency.1, 2 TBC is comprised of a ceramic top layer and a metallic bond coat.3, 4 The most common top layer is made of yttria partially stabilized zirconia (YSZ) for reducing the temperature of the substrate, and a typical bond coat is the MCrAlY alloy, for efficiently preventing the substrate from oxidation and hot corrosion. Thermal barrier coatings can be fabricated by various processing techniques such as atmospheric plasma spray (APS), vacuum plasma spray, HVOF (High Velocity Oxygen Fuel) thermal spray, and electron beam physical vapor deposition (EB-PVD).5 Although YSZ based TBC systems have been used widely in gas turbine industry, YSZ is prone to hot corrosion caused by molten salts, such as Na, S and V, contained in low-quality fuels at high working temperatures.6, 7 The search for alternative coating materials other than the well established YSZ system has consisted of two main approaches: (i) alternative materials to ZrO2-based systems, and (ii) alternative stabilizers to Y2O3 for ZrO2-based systems. Significantly, the A2B2O7-type rare-earth zirconate ceramics, such as La2Zr2O7, Nd2Zr2O7, Gd2Zr2O7 and Sm2Zr2O7, have been shown recently to have lower thermal conductivity, higher melting points, relatively higher thermal expansion coefficients (TEC), higher stability, and better ability to accommodate defects than YSZ.8, 9, 10 However, for the hot corrosion behavior of Gd2Zr2O7 and other rare earth zirconates, most of early studies reported a testing temperature range between 650 and 900 °C on hot pressed samples. In this paper, the hot corrosion behavior of APS Gd2Zr2O7, YSZ, and Gd2Zr2O7 + YSZ composite coatings under Na2SO4 + V2O5 mixture is examined at an engine representative temperature of 1050 °C.
Section snippets
Material and methods
Nickel-based superalloy (Inconel 738) disks of ∅ 25 mm × 1.5 mm were employed as the substrates. TBCs with a ceramic top coating and a NiCrAlY bond coat (Amdry 9625, Sulzer Metco, particle size 45–75 μm) were deposited onto the superalloy substrates by the atmospheric plasma spray (APS) process. Three types of top coats, YSZ, 50 wt% YSZ + 50 wt% Gd2Zr2O7, and Gd2Zr2O7 were made using agglomerated powders. The plasma spraying was carried out using a Sulzer-Metco 9 MB plasma spray system with an Ar/H2 gas
Results and discussion
Fig. 2 reveals the X-ray diffraction patterns for the as-received APS YSZ, Gd2Zr2O7 + YSZ, and Gd2Zr2O7 coatings. It can be seen that the major phase of the APS coated YSZ is tetragonal zirconia. Gd2Zr2O7 + YSZ coating includes both tetragonal ZrO2 and Gd2Zr2O7 phases, and Gd2Zr2O7 has a single phase as expected. The cross-sectional microstructure of APS YSZ, Gd2Zr2O7 + YSZ and Gd2Z2O7 TBC specimens are shown in Fig. 3. All layers of the as-sprayed specimens have similar microstructures with a
Conclusions
Under a typical gas turbine metal surface temperature of 1050 °C, the reactions between yttria (Y2O3) and V2O5/NaVO3 produce YVO4, leaching Y2O3 from the YSZ and causing progressive tetragonal to monoclinic destabilization transformation. Based on hot corrosion chemical reaction formulas, the amount of corrosive salt charged in the tests was enough to react with the entire YSZ and Gd2Zr2O7 layers (20 mg/cm2 per cycle). After 20 h (5 cycles) of hot corrosion test at 1050 °C, the failure of the YSZ
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific
Acknowledgments
This material is based upon work supported by the Department of Energy National Energy Technology Laboratory under Award Number DE-FE0004734 and NASA-EPSCoR program (Grant NNX09AP72A).
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