NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.
This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.
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This study by the National Materials Advisory Board was conducted under Contract No. MDA-972-92-C-0028.
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Abstract
This report assesses coatings materials and processes for gas-turbine blades and vanes; determines potential applications of coatings in high-temperature environments; identifies needs for improved coatings for performance enhancements, design considerations, and fabrication processes; assesses durability of advanced coating systems in potential service environments; and discusses required inspection, repair, and maintenance methods. Promising areas for research and development of materials and processes for improved coating systems and approaches for increased standardization of coatings are identified, with an emphasis on materials and processes with the potential for either improving performance, quality, or reproducibility or significantly reducing manufacturing costs.
COMMITTEE ON COATINGS FOR HIGH-TEMPERATURE STRUCTURAL MATERIALS
ROBERT V. HILLERY (chair),
GE Aircraft Engines, Cincinnati, Ohio
NEIL BARTLETT,
University of California, Berkeley
HENRY L. BERNSTEIN,
Southwest Research Institute, San Antonio, Texas
ROBERT F. DAVIS,
North Carolina State University, Raleigh
HERBERT HERMAN,
State University of New York, Stony Brook, New York
LULU L. HSU,
Solar Turbines, San Diego, California
WEN L. HSU,
Sandia National Laboratories, Livermore, California
JOHN C. MURPHY,
Johns Hopkins University, Laurel, Maryland
ROBERT A. RAPP,
Ohio State University, Columbus
JEFFERY S. SMITH,
Howmet Corporation, Whitehall, Michigan
JOHN STRINGER,
Electric Power Research Institute, Palo Alto, California
National Materials Advisory Board Staff
ROBERT M. EHRENREICH, Senior Program Officer
CHARLIE HACH, Program Officer
JACK HUGHES, Research Associate
AIDA C. NEEL, Senior Project Assistant
ROBERT E. SCHAFRIK, NMAB Director
ROBERT SPRAGUE, Consultant
JILL WILSON, Program Officer
Technical Advisors
WILLIAM J. BRINDLEY,
NASA Lewis Research Center, Cleveland, Ohio
STANLEY J. DAPKUNAS,
National Institute of Standards and Technology, Gaithersburg, Maryland
Government Liasion Representatives
WILLIAM BARKER,
ARPA, Arlington, Virginia
NORMAN GEYER,
Wright-Patterson Air Force Base, Ohio
DAWN MIGLIACCI,
Naval Air Warfare Center, Trenton, New Jersey
WILLIAM PARKS,
U.S. Department of Energy, Washington, D.C.
ROBERT R. REEBER,
Army Research Office, Research Triangle Park, North Carolina
NATIONAL MATERIALS ADVISORY BOARD
ROBERT A. LAUDISE (chair),
Lucent Technologies, Inc., Murray Hill, New Jersey
G.J. (REZA) ABBASCHIAN,
University of Florida, Gainesville
JAN D. ACHENBACH,
Northwestern University, Evanston, Illinois
MICHAEL I. BASKES,
Sandia-Livermore National Laboratory, Livermore, California
I. MELVIN BERNSTEIN,
Tufts University, Medford, Massachusetts
JOHN V. BUSCH,
IBIS Associates, Inc., Wellesley, Massachusetts
HARRY E. COOK,
University of Illinois, Urbana
EDWARD C. DOWLING,
Cyprus AMAX Minerals Company, Englewood, Colorado
ROBERT EAGAN,
Sandia National Laboratories, Albuquerque, New Mexico
ANTHONY G. EVANS,
Harvard University, Cambridge, Massachusetts
CAROLYN HANSSON,
University of Waterloo, Waterloo, Ontario, Canada
MICHAEL JAFFE,
Hoechst Celanese Research Division, Summit, New Jersey
LIONEL C. KIMERLING,
Massachusetts Institute of Technology, Cambridge
RICHARD S. MULLER,
University of California, Berkeley
ELSA REICHMANIS,
Lucent Technologies, Inc., Murray Hill, New Jersey
EDGAR A. STARKE,
University of Virginia, Charlottesville
KATHLEEN C. TAYLOR,
General Motors Corporation, Warren, Michigan
JAMES WAGNER,
The Johns Hopkins University, Baltimore, Maryland
JOSEPH WIRTH,
Raychem Corporation, Menlo Park, California
ROBERT E. SCHAFRIK, Director
Acknowledgments
A great deal of work has gone into this study from its inception to the finished product. The committee is grateful for all the help it has received and expresses thanks to everyone who has participated. Without the patience and support provided by many individuals and organizations, this report could never have been completed.
The committee is grateful to those people who took time to brief the committee on the latest developments in coatings for high-temperature structural materials. The information and ideas from these briefings were essential to the study. Briefings were presented by Steve Balsone, Wright-Patterson Air Force Base; Andy Culbertson, Naval Air Warfare Center Trenton; Jack Devan, Oak Ridge National Laboratory; Dick Novak, Engineered Coatings; Randy Sands, Naval Air Warfare Center; Fred Soechting, Pratt & Whitney; Joseph Stephens, NASA Lewis Research Center; and Sharon Vukelich, Wright-Patterson Air Force Base.
The staff of the Southwest Research Institute (SWRI) did an excellent job of hosting one of the committee meetings, and the committee thanks Martin Goland, Henry Bernstein, and the SWRI staff for providing a tour of the facilities and accommodating the many needs of the meeting.
In particular, the committee thanks William Brindley of the NASA Lewis Research Center for making information available to the staff. The committee also thanks Stanley Dapkunas of the National Institute of Standards and Technology. Their support as technical advisors to the committee was invaluable.
The government liaisons who served this committee were also of enormous value. The committee thanks William Barker of ARPA, Dawn Migliacci of the Naval Air Warfare Center, William Parks of the U.S. Department of Energy, and Robert Reeber of the Army Research Office.
The chair of the committee thanks the members for their dedication and patience during the course of this study. This report could never have been completed without the diligence and goodwill of the members.
The committee thanks the staff of the National Materials Advisory Board, almost all of whom seemed to have been involved in the study at one time or another. Three program officers helped guide the study. In particular, the committee thanks Robert Ehrenreich for finishing the study. Tom Munns initiated the project and deserves thanks. Jill Wilson's support and guidance during the course of the study is greatly appreciated by the committee, as is Robert Sprague's, whose ideas as a consultant helped shape the report. Charles Hach and Jack Hughes were invaluable in the latter stages of the report. The committee also thanks Robert Schafrik for his support and direction along the way. Finally, the committee gratefully acknowledges the support of Aida C. Neel, senior project assistant.
Preface
The U.S. Department of Defense and the National Aeronautics and Space Administration requested that the National Research Council conduct a study and provide recommendations on future research and development needs for high-temperature coatings systems. This report represents the work of the Committee on Coatings for High-Temperature Structural Materials, established by the National Research Council for this purpose.
Performance improvements in high-temperature mechanical systems have resulted in increasingly severe operating environments for high-temperature structural materials, particularly in gas turbines. This has sparked an increased demand for more reliable coatings that possess predictable failure mechanisms, improve the performance of structural materials, and extend the operating range of applications. With this background, the following objectives were outlined for this study:
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assess the state of the art of coatings materials and processes
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identify potential applications for coatings in high-temperature environments
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identify needs for improved coatings for performance enhancements, design considerations, and fabrication processes
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assess durability of advanced coating systems in expected service environments
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identify required inspection, repair, and maintenance methods
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recommend promising areas for materials and process research and development for improved coating systems and identify approaches to increased coating standardization
To address these objectives, the committee considered (1) propulsion systems for commercial and military aircraft and their marine and industrial derivatives and (2) land-based turbines for power generation and mechanical drives (excluding automotive, diesel engines, and space applications). The committee directed its efforts toward the hot section (combustor and turbine) of these gas-turbine systems, because this represents the most significant materials and coating challenges. To focus the study further, the committee considered a wide range of application and technology areas that might be covered under this broad charter and identified those that would be reviewed in detail. The intent was to concentrate on the materials systems and degradation modes in the hottest section of the identified power-generation systems and to consider the technology and application implications for coatings systems under these most severe conditions. A primary focus was on the needs for advanced machines under development by the U.S. Department of Energy, the U.S. Department of Defense, and the National Aeronautics and Space Administration sponsorship. This deliberation resulted in the list shown in table P-l, which defines the study focus, the areas not considered, and the areas that were only referenced by association with the primary focus.
Through a series of briefings from industry and government experts, the committee reviewed current coating systems, newly developed coating systems, and their implementation in products over the next five to eight years. To evaluate coating needs beyond this time frame, the committee reviewed the substrate materials (e.g., ceramics and intermetallics) being considered for future engine designs. The committee recognized that defining needs for many future systems would currently lack clarity, but a need was perceived to anticipate any fundamental changes that may demand longer-range research, process development, or manufacturing innovations.
This report reviews the state of the art for coating systems based on the following approach. First, the application needs were identified and a description of the domain of use was developed. Second, the environment that currently exists and the substrate materials that are now used in the hot section of gas-turbine engines were examined. This, in turn, led to a more complete definition of the coatings systems required. Third, the application processes, the industrial base, and the repair and overhaul requirements were discussed and the support capabilities (e.g., modeling, testing, and nondestructive evaluation) were assessed. This review provided a baseline for discussion of future trends and indicated how U.S. industry, government, and academia are planning to address the requirements of advanced propulsion systems.
To determine materials and coatings needs, advanced systems were assessed. The assessments on these advanced systems were obtained through presentations and information provided by the program managers for three major
TABLE P-1 Committee Focus
Focus |
Associated/Referenced |
Excluded |
Industrial, marine, power generation, aircraft engine |
Land-based systems |
Auto, diesel engines, space |
Structural materials superalloys (Ni, Co) intermetallics composites (IMC, CMC) monolithic ceramics |
Refractories |
Titanium alloys |
700°C + temperatures (to capture Type II hot corrosion) |
|
|
Oxidation, corrosion, erosion |
Seal systems degradation caused by the service environment |
Tribological wear |
Gas-path coatings and clearance coatings |
Gas-path seals |
|
Combustor, transition piece, high-pressure turbine, power turbine |
Off-line combustion |
Compressor, fan |
Diffusion coatings, overlays, TBC, surface modifications |
Functionally graded materials, claddings, vitreous coatings |
Low observable coatings, fiber coatings |
Thermal spray, CVD, PVD, advanced processes |
Plating, C/S/N/O-resistant coatings |
|
Operating environment (air, fuel, water, particulates) |
Combustion, emissions |
|
Environmental impact: manufacturing, service, overhaul |
|
|
Repair considerations |
Lower-temperature coatings affected (e.g., impact-resistant coatings) |
Repairs involving brazing, welding, etc. |
Nondestructive evaluation (NDE) |
|
|
Standards and standardization |
|
|
Databases, modeling, engine condition sensors |
Controls, intelligent processing of materials |
|
Systems design (coating/substrate integration): advanced concepts |
Material systems that might reduce or eliminate need for high-temperature coatings (e.g., Lamalloy) |
|
Customer-DOD, DOE, NASA, original equipment manufacturers, suppliers |
Airframe manufacturers, airlines, utilities |
|
TABLE P-2 Primary Performance Goals for Advanced Engine Systems
Advanced Engine System |
Requirements |
Integrated High-Performance Turbine Engine Technology (IHPTET) |
Cold section (fan and compressor): 2-3 times specific strength for materials 650-1000°C operating temperature Hot section (the focus of this report; includes the combustor, turbine, augmenter, and nozzle subsytems): 3-5 times specific strength for materials 1650-2200°C with advanced cooling 1550°C uncooled Nonstructural (bearing and lubes) up to 825°C |
High-Speed Civil Transport (HSCT) |
Range: 2 times greater than the Concorde Payload: 3 times greater than the Concorde Economics: 8 times greater than the Concorde Environmental emissions: 8 times lower than the Concorde Noise: 3 times quieter than the Concorde |
Advanced Turbine Systems (ATS) |
High efficiency, clean gas-turbine systems initially based on natural gas; adaptable to coal- or biomass-derived fuels Power generation: >60% system efficiency Industrial systems: >15% improvement Environmental: 8 ppm NOx emissions; CO and HC < 20 ppm Cost competitive: 10% reduction in busbar cost of electricity |
government-sponsored materials efforts representing future military, commercial, and power-generation needs:
-
Integrated High-Performance Turbine Engine Technology—advanced military systems
-
High-Speed Civil Transport—aimed at the advanced supersonic commercial market
-
Advanced Turbine Systems—advanced utility and industrial power generation
Table P-2 summarizes the primary performance goals for each of these advanced systems. For each case, the committee obtained information on mission profile; systems needs; and specifics on time, temperature, and environmental requirements for materials in the propulsion system. This provided a perspective on what were the ultimate materials needs for propulsion systems reaching maturity early in the next century. In all cases, the goals underscored the demand for materials that can withstand significantly higher operating temperatures and service life than today's state-of-the-art devices. The reviews also provided information on significant changes that might be required as a result of new regulatory requirements, such as those that might stipulate permissible emissions from future propulsion systems. In addition, these reviews showed those areas common to aircraft engines and power-generation machines and changes to this commonality that might be demanded by derivative machines as, for example, the increased use of air to combat NOX in the combustor of land-based electric utility turbines. Some fundamental differences exist between aircraft and land-based systems that might cause a divergence in materials (and coatings) technologies. For instance, land-based systems are less affected by weight and can be supplemented with auxiliary systems, such as air supplies, steam supplies, or heat exchangers. The potential use of alternative fuels in nonaircraft systems might be another divergence affecting coatings type. Both aircraft and land-based gas turbines are moving toward higher temperatures and longer service-time requirements; this trend is causing increased emphasis on coatings needs. Finally, the committee heard presentations on the design requirements for coating systems and the engineered materials efforts that may have a bearing on the development and application of advanced coating systems.
In reviewing these briefings, the committee considered the following key questions:
-
Can the goals for the advanced systems be achieved simply by an evolution from today's materials?
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Are programs and efforts in place to address the key potential barriers?
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Are there additional recommendations that can be made to enhance the chances for success in any of the key areas?
The committee then considered future opportunities for developing improved coatings by virtue of evolutionary development and by way of innovative concepts. The committee developed several innovative concepts for advanced coating systems and suggested how a wide variety of ideas could be integrated into coatings development and application advances. The members of the committee continually posed several key questions during their considerations of innovative approaches:
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Are there concepts that have not been explored or that should be re-evaluated in light of recent knowledge?
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Are there ideas or knowledge bases in other industries that can revolutionize thinking in the gas-turbine engine coating industry?
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Are there ideas that can shorten the development cycle of existing efforts and thereby enhance the chance for success?
-
What are the latest developments in modeling, intelligent process manufacturing, and smart materials, and can these technologies be focused in the coatings area?
-
Are there new testing techniques that would be required and will new industry standards and procedures have to be defined?
Finally, the committee provides its conclusions and recommendations for the future as well as a bibliography of cited references and available texts.
Robert V. Hillery, chair
Committee on Coatings for High-Temperature Structural Materials
List of Figures and Tables
FIGURES
1-1 |
Typical ranges of a coating property compared with advanced engine requirements for a property |
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2-1 |
Cross section of a typical modern gas-turbine engine |
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2-2 |
Overview of transportation gas-turbine use |
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2-3 |
Overview of power-generation and mechanical-drive gas-turbine use |
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2-4 |
Approximate increases in firing temperature capabilities from 1980 to 2010 |
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2-5 |
Turbine blade cooling methods |
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3-1 |
Coating compositions as related to oxidation and corrosion resistance |
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3-2 |
Schematic showing the benefit of development and deployment of manufacturing process control for high-temperature coatings |
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4-1 |
Types of high-temperature attack for metallic coatings (aluminide, chromide, MCrAlY, etc.) on nickel-base superalloys with approximate temperature regimes and severity of attack |
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4-2 |
Micrograph of service-exposed CoCrAlY overlay coatings showing internal oxidation of coating and base metal |
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4-3 |
Comparison of the microstructure of EB-PVD and plasma-sprayed TBCs |
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4-4 |
Photomicrograph of a plasma-sprayed TBC |
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4-5 |
Photomicrographs of EB-PVD TBCs before and after failure |
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5-1 |
Tradeoffs between first cost and operating cost |
TABLES
P-2 |
Primary Performance Goals for Advanced Engine Systems, |
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2-1 |
Typical Duty Cycles for Various Gas-Turbine Engines |
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2-2 |
Nickel-and Cobalt-Base Alloys Used in Land-Based and Aircraft Gas Turbines |
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2-3 |
Properties of Ceramic Materials that are Candidates for Hot-Section Use |
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2-4 |
Selected Properties of High-Temperature-Capable Intermetallics Compared to a Conventionally Cast and a Single-Crystal Superalloy |
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2-5 |
Properties of Refractory Metals and an Alloy Compared to B 1900 Nickel-Base Superalloy |
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3-1 |
Coating Functions and Coating Materials Characteristics |
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3-2 |
Types of Coatings Used in Hot-Section Components |
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3-3 |
Generic Information on Coating Types Used in Superalloy Hot-Section Components |
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3-4 |
Summary of the Benefits and Limitations of the Atomistic and Particulate Deposition Methods |
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4-1 |
Environmentally Induced High-Temperature Structural Material Failure Modes |
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4-2 |
Environmentally Induced High-Temperature Coating Failure Modes |
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6-1 |
Survey of Nondestructive Evaluation Techniques |
Acronyms
AISI
American Iron and Steel Institute
ASM
American Society for Materials
ASME
American Society of Mechanical Engineers
ASTM
American Society for Testing and Materials
ATS
Advanced Turbine Systems
CTE
coefficient of thermal expansion
CVD
chemical vapor deposition
DOD
U.S. Department of Defense
DOE
U.S. Department of Energy
EB-PVD
electron-beam physical vapor deposition
FGM
functionally graded materials
HSCT
High-Speed Civil Transport
HVAF
high-velocity air fuel
HVOF
high-velocity oxy fuel
IHPTET
Integrated High-Performance Turbine Engine Technology program
ISO
International Standards Organization
LPPS
low-pressure plasma spraying
NASA
National Aeronautics and Space Administration
NDE
nondestructive evaluation
PVD
physical vapor deposition
SNECMA
Société National d'Etude et de Construcion de Moteurs d'Aviation
STEP
Standard for the Exchange of Product
SVPA
SNECMA vapor phase aluminizing
TBC
thermal barrier coating