Comparison of Single-Phase and Two-Phase Composite Thermal Barrier Coatings with Equal Total Rare-Earth Content
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Comparison of Single-Phase and Two-Phase Composite Thermal Barrier Coatings with Equal Total Rare-Earth Content Amarendra K. Rai1 • Michael P. Schmitt2 • Mitchell R. Dorfman3 Dongming Zhu4 • Douglas E. Wolfe5,6
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Submitted: 2 January 2018 / in revised form: 18 February 2018 ASM International 2018
Abstract Rare-earth zirconates have been the focus of advanced thermal barrier coating research for nearly two decades; however, their lack of toughness prevents a widescale adoption due to lack of erosion and thermal cyclic durability. There are generally two methods of improving toughness: intrinsic modification of the coating chemistry and extrinsic modification of the coating structure. This study compares the efficacy of these two methods for a similar overall rare-earth content via the air plasma spray process. The extrinsically toughened coatings were comprised of a two-phase composite containing 30 wt.% Gd2Zr2O7 (GZO) combined with 70 wt.% of a tougher t0 low-k material (ZrO2-2Y2O3-1Gd2O3-1Yb2O3; mol.%), while a single-phase fluorite with the overall rare-earth content equivalent to the two-phase composite (13 mol.% rareearth) was utilized to explore intrinsically toughened concept. The coatings were then characterized via x-ray diffraction, energy-dispersive spectroscopy, and scanning electron microscopy, and their performance was evaluated & Michael P. Schmitt [email protected] 1
UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432, USA
2
HAMR Industries LLC, 1315 W College Avenue, State College, PA 16801, USA
3
Oerlikon Metco, Danbury, NY, USA
4
NASA Glenn Research Center, Cleveland, OH 44135, USA
5
Materials Science and Engineering Department and the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
6
Engineering Science and Mechanics Department, The Pennsylvania State University, University Park, PA 16802, USA
via erosion, thermal conductivity, thermal annealing (500 h), and thermal cycling. It was shown that the extrinsic method provided an improved erosion and thermal conductivity response over the single phase, but at the expense of high-temperature stability and cyclic life. Keywords air plasma spray (APS) composites durability rare-earth zirconate thermal barrier coatings (TBC)
Introduction Thermal barrier coating (TBC) systems are used to protect hot-section components of gas turbine engines. TBC systems consist of the superalloy substrate, a bond coat that also imparts oxidation resistance, a thin thermally grown oxide (TGO) layer, and an insulating ceramic topcoat. The most prolific topcoat is 6-8 wt.% yttria-stabilized zirconia (YSZ) in the metastable tetragonal (t0 ) phase which exhibits a low bulk thermal conductivity value of * 2.2 W/m K (Ref 1-3), combined with a reasonably high fracture toughness of * 3 MPaHm (Ref 4). Derivatives such as the low-k TBC systems having t0 phase (t0 low k) have also been developed which promote lower thermal conductivity while maintaining mechanical and thermal cyclic dura
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