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Robert A. Miller

Bio: Robert A. Miller is an academic researcher from Glenn Research Center. The author has contributed to research in topics: Thermal barrier coating & Coating. The author has an hindex of 36, co-authored 112 publications receiving 4899 citations.


Papers
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Journal ArticleDOI
TL;DR: The state-of-the-art thermal barrier coatings for gas turbine applications are currently a plasma-sprayed ZrO 2 -(6%8%) Y 2 O 3 ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure.
Abstract: The science and technology of thermal barrier coatings has advanced considerably since reports of the first test on turbine blades in a research engine in 1976. Today thermal barrier coatings are flying in revenue service in a low risk location within the turbine section of certain gas turbine engines. The state-of-the-art coating system for gas turbine applications is currently a plasma-sprayed ZrO 2 -(6%–8%) Y 2 O 3 ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure. Although the potential for meeting current and short-term goals is high, longer-range goals may not be attainable with current coating concepts. These longer-range goals will involve high risk designs where coating loss could lead directly to component loss. Several steps must be taken to help meet these goals. Improved understanding of coating failure mechanisms is required. Models are needed to predict lifetimes. Process automation and quality control procedures must be instituted. Finally, new concepts in plasma-sprayed coatings must be developed and alternatives to the plasma- spraying process may be required. The current status of thermal barrier coatings and prospects for future progress in the above areas are summarized.

693 citations

Journal ArticleDOI
TL;DR: In this paper, the failure of a ZrO2-8%Y2O3/Ni-14% Al-0.1% Zr coating system on Rene 41 in Mach 0.3 burner rig tests was characterized.

335 citations

Journal ArticleDOI
TL;DR: In this paper, a procedure for modeling the lives of thermal barrier coatings subjected to high-temperatrue environments is described, based on the assumption that oxidation is the single important time-dependent factor which limits the life of these coatings, and that oxidation-induced strains combine with cyclic strains to promote slow crack growth in the ceramic layer.
Abstract: A procedure is described for modeling the lives of thermal barrier coatings subjected to high-temperatrue environments. The models is used to calculate cycles-to-failure as a function of heating cycle duration. It is based on the presumption that oxidation is the single important time-dependent factor which limits the life of these coatings, and that oxidation-induced strains combine with cyclic strains to promote slow crack growth in the ceramic layer. Good agreement is obtained between calculated and experimental lives for specimens tested in a furnace. This shows that an oxidation-based approach is promising. The importance of reproducible specimen preparation is also discussed.

306 citations

Journal ArticleDOI
TL;DR: The 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), and a BSAS top coat as discussed by the authors.
Abstract: 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.

304 citations

Journal ArticleDOI
TL;DR: In this paper, a laser high heat flux test approach has been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems.
Abstract: Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO2-8% Y2O3 coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.

244 citations


Cited by
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Journal ArticleDOI
12 Apr 2002-Science
TL;DR: In this article, the structure, properties, and failure mechanisms of thermal barrier coatings (TBCs) are reviewed, together with a discussion of current limitations and future opportunities.
Abstract: Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.

3,548 citations

Book
31 Jul 2008
TL;DR: In this paper, the physical metallurgy of nickel and its alloys is discussed and single crystal superalloys for blade applications for turbine disc applications are discussed. And the role of coatings is discussed.
Abstract: 1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

3,067 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present relationships between the durability, the governing material properties and salient morphological features of thermal barrier coatings and show that the failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface.

2,050 citations

Journal ArticleDOI
TL;DR: In this article, the basic properties of ceramic materials for thermal barrier coatings are summarized, showing that they are more resistant to oxidation, corrosion and wear, as well as being better thermal insulators.
Abstract: This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia, other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

1,789 citations

Journal ArticleDOI
TL;DR: In this article, a review of thermal transport at the nanoscale is presented, emphasizing developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field.
Abstract: A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

1,307 citations