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  • 1
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    Defect Clustering and Nano-phase Structure Characterization of Multicomponent Rare Earth-Oxide-Doped Zirconia-Yttria Thermal Barrier Coatings (2004)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Advanced thermal barrier coatings (TBCs) have been developed by incorporating multicomponent rare earth oxide dopants into zirconia-based thermal barrier coatings to promote the creation of the thermodynamically stable, immobile oxide defect clusters and/or nanophases within the coating systems. In this paper, the defect clusters, induced by Nd, Gd, and Yb rare earth dopants in the zirconia-yttria thermal barrier coatings, were characterized by high-resolution transmission electron microscopy (TEM). The TEM lattice imaging, selected area diffraction (SAD), and electron energy-loss spectroscopy (EELS) analyses demonstrated that the extensive nanoscale rare earth dopant segregation exists in the plasma-sprayed and electron-physical-vapor-deposited (EB PVD) thermal barrier coatings. The nanoscale concentration heterogeneity and the resulting large lattice distortion promoted the formation of parallel and rotational defective lattice clusters in the coating systems. The presence of the 5-to 100-nm-sized defect clusters and nanophases is believed to be responsible for the significant reduction of thermal conductivity, improved sintering resistance, and long-term high temperature stability of the advanced thermal barrier coating systems.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2004-212480 , E-14018 , ARL-TR-3014
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  • 2
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    Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3-2SiO2) or mullite + BSAS (1-xBaO-xSrO-Al2O3-2SiO2) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, silica volatility, phase stability, and thermal conductivity. Variants of this family of EBCs were applied to monolithic SiC and melt infiltrated SiC/SiC composites. Reaction between BSAS and silica results in low melting (approx. 1300 C) glasses at T 〉 1400 C, which can cause the spallation of the EBC. At temperatures greater than 1400 C, the BSAS top coat also degrades by formation of a porous structure, and it suffers significant recession via silica volatilization in water vapor-containing atmospheres. All of these degradation mechanisms can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. There was evidence of rapid sintering at temperatures as low as 1300 C, as inferred from the sharp increase in thermal conductivity.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2002-211372 , E-13195 , NAS 1.15:211372
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    NASA TECHNICAL REPORTS
  • 3
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    Development of Advanced Thermal and Environmental Barrier Coatings Using a High-Heat-Flux Testing Approach (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The development of low conductivity, robust thermal and environmental barrier coatings requires advanced testing techniques that can accurately and effectively evaluate coating thermal conductivity and cyclic resistance at very high surface temperatures (up to 1700 C) under large thermal gradients. In this study, a laser high-heat-flux test approach is established for evaluating advanced low conductivity, high temperature capability thermal and environmental barrier coatings under the NASA Ultra Efficient Engine Technology (UEET) program. The test approach emphasizes the real-time monitoring and assessment of the coating thermal conductivity, which initially rises under the steady-state high temperature thermal gradient test due to coating sintering, and later drops under the cyclic thermal gradient test due to coating cracking/delamination. The coating system is then evaluated based on damage accumulation and failure after the combined steady-state and cyclic thermal gradient tests. The lattice and radiation thermal conductivity of advanced ceramic coatings can also be evaluated using laser heat-flux techniques. The external radiation resistance of the coating is assessed based on the measured specimen temperature response under a laser- heated intense radiation-flux source. The coating internal radiation contribution is investigated based on the measured apparent coating conductivity increases with the coating surface test temperature under large thermal gradient test conditions. Since an increased radiation contribution is observed at these very high surface test temperatures, by varying the laser heat-flux and coating average test temperature, the complex relation between the lattice and radiation conductivity as a function of surface and interface test temperature may be derived.
    Keywords: Nonmetallic Materials
    Type: 27th Annual International Conference on Advanced Ceramics and Composites; Jan 26, 2003 - Jan 31, 2003; Cocoa Beach, FL; United States
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  • 4
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    Furnace Cyclic Behavior of Plasma-Sprayed Zirconia-Yttria and Multi-Component Rare Earth Oxide Doped Thermal Barrier Coatings (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Ceramic thermal barrier coatings will play an increasingly important role in advanced gas turbine engines because of their ability to enable further increases in engine temperatures. However, the coating performance and durability become a major concern under the increasingly harsh thermal cycling conditions. Advanced zirconia- and hafnia-based cluster oxide thermal barrier coatings with lower thermal conductivity and improved thermal stability are being developed using a high-heat-flux laser-rig based test approach. Although the new composition coatings were not yet optimized for cyclic durability, an initial durability screening of numerous candidate coating materials was carried out using conventional furnace cyclic tests. In this paper, furnace thermal cyclic behavior of the advanced plasma-sprayed zirconia-yttria-based thermal barrier coatings that were co-doped with multi-component rare earth oxides was investigated at 1163 C using 45 min hot cycles. The ceramic coating failure mechanisms were studied by using scanning electron microscopy combined with X-ray diffraction phase analysis after the furnace tests. The coating cyclic lifetime will be discussed in relation to coating phase structures, total dopant concentrations, and other properties.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2002-211690 , NAS 1.15:211690 , E-13420 , 26th Annual International Conference on Advanced Ceramics and Composites; Jan 13, 2002 - Jan 18, 2002; Cocoa Beach, FL; United States
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  • 5
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    Thermal Conductivity and Sintering Behavior of Advanced Thermal Barrier Coatings (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Advanced thermal barrier coatings, having significantly reduced long-term thermal conductivities, are being developed using an approach that emphasizes real-time monitoring of thermal conductivity under conditions that are engine-like in terms of temperatures and heat fluxes. This is in contrast to the traditional approach where coatings are initially optimized in terms of furnace and burner rig durability with subsequent measurement in the as-processed or furnace-sintered condition. The present work establishes a laser high-heat-flux test as the basis for evaluating advanced plasma-sprayed and physical vapor-deposited thermal barrier coatings under the NASA Ultra Efficient Engine Technology (UEET) Program. The candidate coating materials for this program are novel thermal barrier coatings that are found to have significantly reduced thermal conductivities due to an oxide-defect-cluster design. Critical issues for designing advanced low conductivity coatings with improved coating durability are also discussed.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2002-211481 , NAS 1.15:211481 , E-13249 , 26th Annual International Conference on Advanced Ceramics and Composites; Jan 13, 2002 - Jan 18, 2002; Cocoa Beach, FL; United States
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