Overview
Applying Nanostructured Materials to Future Gas Turbine Engines
Maurice Gell
The need for improved materials to provide increased gas turbine engine performance is as great today as at any time in the 50-year history of this field. The emerging technology ofnanostructured materials holds the potential for satisfying the gas turbine industry's requirements with a new generation of materials possessing a quantum improvement in properties. In the laboratory, significant increases in strength and hardness combined with toughness and ductility have been demonstrated. Additionally, desirable physical properties such as enhanced diffusivity and reduced thermal conductivity have been found. In the following article, an aggressive and focused technology development strategy is described that will allow an early assessment of this promising technology for year 2000 gas turbine applications.
INTRODUCTION
Nanostructured materials have structural features, such as grain size, layer spacing, and precipitate size, that exist on the nanoscale, where a nanometer is 1 Q-9 m or 10 A. In general, nanostructured materials will have structural features in the 1 nm to 100 nm range. Current engineering materials derive their properties from structural features in the micrometer size range or greater, where 1 ~mis l~m. Thus, nanostructured materials gain their unique properties from structural features that are 10 to 1,000 times smaller than those found in current engineering materials (Figure 1).
N anostructured materials come in two general morphologies: nanolayered ma-
1 micron (1,000 nm)
terials deposited by physical vapor deposition or electrodeposition processes and nanograined materials, which are usually consolidated from nanostructured powders (Figure 2). As grain size becomes smaller, there are an increasing number of atoms associated with grain boundary sites compared to crystallattice sites. For example, at a grain size of 100 nm, approximately 3% of all atoms are associated with grain boundaries. As the grain size is reduced to 10 nm, the percentage increases to 30; at 5 nm, about 50% of all atoms are associated with grain boundary sites. The unique properties of nanograined materials are associated with the fineness of structure as well as the enhanced solubility and increased atomic mobility associated with grain boundaries.
For a more detailed description of the synthesis, structure, and properties of nanostructured materials, the interested reader is referred to a number of excellent review papers. l -6
FUTURE GAS TURBINE REQUIREMENTS
Since the introduction of gas turbine engines into commercial airline service in the 1950s, there has been continual improvement in gas turbine performance as measured by thrust-specific fuel consumption (Figure 3a). Fuel efficiencies have been obtained by improvements in engine cycle and component efficiencies and by increased materials strength and temperature capabilities. Since fuel costs can amount to as much as 40% of airline
Nanocrystal Materials
100nm 10nm
Figure 1. Relative sizes of commercial ultrafine powders.
30
a
b
~
25 nm
L..........J
25nm
Figure 2. Two common morphologies of nanostructured materials: (a) nanolayered stainless steel/zirconium7 and (b) nanograined titanium dioxide powder.4
operating cost, there are continuing competitive pressures to further improve engine fuel efficiency. Additional fuel efficiency improvements (20% or more) are possible with ultrahigh bypass engines (e.g., the advanced ducted prop or the propfan), in which the major portion of the air entering the front of the engine "bypasses" the engine core. In addition, NASA is sponsoring design studies and critical materials development programs for the demanding requirements of the high-speed civil transport (Figure 3b).
Military gas turbine engines have also shown marked improvements in performance with time, as measured by thrust per unit weight (Figure 4a). These improvements derive from engine cycle and component efficiencies and materials improvements, especially highstrength, lightweight, and high-temperature materials. In recent years, fighter aircraft maneuverability has been greatly improved with the use of thrust-vectoring nozzles (Figure 4b). Despite the end of the Cold War, the u.S. Department of Defense has very aggressive goals and active development programs directed toward advanced engines with significantly increased thrust-per-unit weight.
The modern gas turbine engine uses a
JOM • October 1994
i
Turbo)els
0.9 low-bypass Turbofans
0.8 0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
a
b
Figure 3. (a) Improvements in commercial gas turbine engine fuel efficiency over time. (b) A rendering of NASA's high-speed civil transport.
large number of structural materials and coatings (Figure 5). Structural materials have traditionally been the focus of development for improved performance and coatings for enhanced durability. In the last decade, however, the use of coatings as compressor- and turbine-abradable seals and as thermal barrier coatings has contributed significantly to performance gains.
Gas turbine engine designers have been asked to define how they are going to achieve the aggressive goals established for advanced commercial and military engines (Figures 3 and 4). They have indicated that more than 50% of the performance gains will have to come from improved materials, processing, and coatings (Figure 5). Materials will have to be developed with increased strength,lower density, higher temperature capability, increased toughness and ductility, and lower cost. Both on an absolute basis and on a relative basis to other gas turbine engine technologies, the demand for improved materials is as great now as any time during the history of gas turbine engines.
THE PROMISE OF NANOSTRUCTURED
MATERIALS
Hardness and Strength For structural materials applications,
much of the promise for nanostructured materials has been based on hardness and strength measurements. The studies conducted to date have largely focused on pure metals and relatively simple oxide ceramics.6 Hardness has been found to increase by factors of two to ten as grain size is reduced (Figure 6).
1994 October • JOM
Many materials obey the Hall-Petch relationship, with hardness proportional to the in verse square root of the grain diameter. In metals, when the same sample is annealed to produce increased grain size, both hardening and softening effects have been observed. This varying behavior has been associated with the increasing importance of grain boundary sliding and diffusion-controlled deformation processes as
Polymer Composiles Hi-Temp Steels
Resistant
a
grain size is reduced and to the annealing of nanometer-size pores existing in most as-processed materia1.6
The tensile strength of several binary layer systems, including copper-monel, increases by factors of three to ten as the layer spacing is reduced from the microto the nanometer scale (Figure 7),7 For the Pt-Cr and zirconium-stainless steel systems, strength in excess of 2.8 GPa has been estimated based on hardness measurements.
Ductility and Toughness
To become useful as engineering materials, nanostructured materials must possess not only increased hardness and strength, but adequate levels of ductility and toughness. Most metals are less ductile as grain size is reduced into the nanometer range.6 This is attributed to the difficulty in the generation and multiplication of dislocations, as well as the presence of porosity. While ductility and toughness are reduced, there should be adequate levels for engineering purposes. Thus, for nanostructured metals
JS7
a
b
Figure 4. (a) Improvements in Pratt & Whitney's military gas turbine engine performance as measured by thrust per unit weight. (b) Pratt & Whitney's thrust-vectoring F100 engine.
ErosionReSistant
c <l)
E
~ a. .E
b
Thermal Barriers
low Observable
Figure 5. (a) A cross-section of Pratt & Whitney's F100 military engine showing the wide range of structural materials and coatings employed. (b) Relative improvements of various engine technologies expected by engine designers.
and alloys, there should be higher strength levels combined with adequate ductility and toughness.
Ceramics and intermetallics exhibit both enhanced hardness and ductility with reduced grain size. The enhanced ductility is associated with increased grain boundary sliding and diffusioncontrolled grain boundary deformation. This plasticity will greatly improve ceramic and ceramic-matrix composite consolidation processes and may ultimately help to produce ceramic materials with a reduced flaw size and distribution. Nanostructured ceramics will remain brittle at high strain rates, so their utility as structural materials remains in doubt.
Perhaps the best approach to improved structural materials using nanostructures is to consider current high-strength materials with limited, but useful levels of ductility, where the ductility is controlled by the brittleness of a second phase. By refining the size of the brittle phase, improved ductility and toughness should be possible.
Tungsten carbide-cobalt is example of a limited-ductility material that consists of a high volume fraction of coarse WC particles in a soft cobalt matrix. This material is widely used as a machine tool
31
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10 0
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~ 6 r::: "E '" :c
4
2
6. - Fe o -Fe O -Ni '\l - Ag
• - Cu • - Pd • - Ni
3
• •
0.6
incomplete densification, fiber damage, and fiber-matrix interaction when consolidated at 1,400°C (Figure 11). By contrast, consolidation using nanostructured zirconia powder, at a 360°C lower tempera-ture, produced a fully dense matrix and no fiber damage or fiber-matrix interaction .
The other physical property changes shown in Figure 10 (e.g., reduced density and elastic modulus and increased thermal expansion coefficient) are rea-Figure 6. Hardness versus grain size for a numberof nanophase
metals.s sonablyexplainedbythe
increased grain boundary volume. However, the magnitude of the effects reported may be greater than actual because of inadequate specimen densification.s Reduced thermal conductivity can be explained by enhanced phonon scattering from grain and layer boundaries.
insert and as a wear-resistant coating. The hardness of WC-Co has doubled as the carbide and grain sizes have been reduced from the micro- to the nanometer scale (Figure 8).8 Of significance are scratch-test observations that show a rough surface with extensive cracking of the WC phase in the micrograined material and a smooth surface and no evidence of cracking in the nanograined material (Figure 9).9 It is this apparent high strength combined with toughness in specific nanostructured materials that could produce major engineering benefits.
Physical Properties
Figure 10 summarizes the changes in physical and mechanical properties that have been observed for nanostructured materials.s Just as strength and ductility can be improved, diffusivity can be orders of magnitude higher in nanostructured materials. This is associated with enhanced grain boundary solubility and interface atomic mobility as well as the significantly increased grain boundary volume as grain size is reduced. Thus, diffusion-controlled processes, such as sintering and high-temperature creep, will occur much more rapidly and at lower temperatures.
It has been demonstrated that the enhanced diffusivity of nanostructured materials can be used to facilitate the fabrication of ceramic-matrix composites.t O A composite of alumina fibers in a conventional zirconia matrix showed
4~--~~--------------~ • PVCr as 3 • 304 SS/Zr
fu2 l:l ~1 en
10 100 1,000 Layer Spacing (nm)
Figure 7. Tensile strength versus layer spacing for bi-material systems.7
32
Materials Testing
For the most part, nanostructured materials properties have been generated on simple metal and ceramic systems using subscale specimens that, in some cases, lack full densification. Clearly, there is a need for generating engineering properties on defect-free, full-scale specimens of alloys and compounds with engineering utility.
Fabrication
There are a larger number of processes for generating nanostructured materials. Layered materials can be generated by magnetron sputtering, chemical vapordeposition, electroplating, and physical vapor deposition processes such as
a
b
1,200
SOO~--~--~~~~--~~ 200 500 1,000 SO,OOO
Grain Size (nm) 160,000
L-J 200 nm
Figure 8. (a) Hardness versus grain size for WC-Co. (b) The WC-Co structure viewed via transmission electron microscopy.s
a 10 ~m
10 ~m
Figure 9. Scratch tests of (a) micrograined and (b) nanograined WC-CO.9
electron beam, cathodic arc, and the vapor jet processes.
Nanostructured powder can be made by chemical and gas phase synthesis processes as well as sol-gel processing. These powders are then consolidated using traditional processes, such as hot pressing, hot isostatic pressing, and thermal spray. Processes for making nanostructured powder are described in detail elsewhere.U ,5
Availability and Cost
Even at this early stage of nanostructural materials development, the cost and availability of materials must be given strong consideration. While costs for nanostructured materials are high, emphasis should be placed on the cost-effectiveness of the material. Developmental activities should concentrate on those applications that have the highest payoff and on fabrication processes that have the potential to provide mature production costs that are cost-effective.
Material availability is limited, but improving. Moreover, a number of small companies have been established to supply pure metals, alloys, and ceramics (Table 1). Production quantities are becoming available for WC-Co from Nanodyne, and Nanophase Technologies is supplying alumina and iron oxide.
DEVELOPMENT STRATEGY
Given the need forimproved materials and the considerable (but early) potential of nanostructured materials to satisfy this requirement, how should the
JOM • October 1994
ward increased flexibility and throughput. Fabrication research should define instances where the properties of nanostructured materials will enhance the fabrication of current, hard-to-fabricate materials and components, such as metal- and ceramic-matrix composites. Only in the longer term should large components or temperature-limited applications be pursued (Figure 12).
High-Payoff Applications
Figure 10. Property changes associated with nanostructured materials.
Based on the extensive use of coatings in gas turbine engines (Figure 5), the need for improved, cost-effective coatings should drive the development of nanostructured materials. The property improvements that have been demonstrated (Figure 10) suggest a new generation of coatings can be developed with resistance to wear, erosion, and oxidation; they should also have improved crack and thermal resistance. These coatings can be built up as a series of nanolayers or deposited as nanograined coatings using thermal spray processes. Nanostructured We-Co powder has been deposited as a coating using a high-velocity, oxygen-fuel thermal spray process. The hardness of the deposited coating with a 100 nm grain size
Table I. Sources for Nanostructured Powders
Company Materials Avaliable
Nanodyne WC-Co 19 Home News Row, New Brunswick, NJ 08901
Nanophase Technology A~Oym Fepy Ti02; CeO, Crpy YPy Zr02, AI, Co, Cu, Au, Fe, Ni, Pd, Si, Ag
AI, Cu, Fe Mo, Ni, W, Ti02, TiN, TiCN
8205 South Cass Ave., Darien, II 60561 Ultram International
Three Park Central, 1515 Arapahoe Street, Denver, CO 80202
technical community proceed to assess this new technology in an aggressive, yet prudent, manner? How do we avoid the excesses of the past, where hundreds of millions of dollars were spent on advanced development and transition-to-
a ~
20J,lm
Figure 11. Consolidation of ZrO.jAIP3 fiberreinforced composites: (a) alumina fibers and zirconia matrix with conventional powder (processed at 1,400°C; 110 MPa); (b) nanostructured powder (1 ,040°C; 110 MPa).10
1994 October • JOM
is greater than that obtained using commercial, micrometer-size powder (Fig-
production activities before technical ure 13).11 feasibility was demonstrated? Letus con- Main shaft bearings in gas turbine centrate on a few aspects ;:::===================== of these critical issues.
It is suggested that the broad field of nanostructured materials be focused by defining a series of high-payoff applications and that a complete technology plan be established at the outset that includes basic materials and processing research, development, scale-up, and transition to production activities. Success criteria should be established for each stage of the technology-progression process. One does not proceed to the next, more expensive, stage until all the success criteria have been satisfied for the current stage.
Since near-term nanostructured materials are likely to be costly and limited in supply, highpayoff applications should be selected that use limited quantities of material, such as coatings and small engineering structures (e.g., bearings). Process research should be directed to-
Short-term Long-term
Figure 12. Development strategy for nanostructured materials.
OPH
Bulk Consolidated ~no$truclured Malerlal
HVOF •• Nanoslruclured Powder
1000
Particle Velocity (mls)
Figure 13. Hardness values of conventional and nanostructured WC-Co coatings.11
33
a
b
L......J
40 nm
L-...J
70 nm
Figure 14. Nanostructures of M50 steel: (a) powder and (b) compact. 12
engines are an example of a structural material, limited in size, that would demonstrate a major payoff if a combination of increased hardness and toughness could be obtained by structural refinement. A chemical synthesis process and vacuum hot pressing are being used to fabricate M50 steel with a grain size of 50 nm (Figure 14);12 the engineering properties will be determined.13
Need For Directed Research
As part of the technology assessment effort for nanostructured materials, a high level of basic research is needed to support the applications-focused development. The major property changes that have been observed are from materials" phenomena occurring on the atomistic and nanometer levels. Research tools must be developed and used that permit discovery of these novel mechanisms. Directed research and applications-driven development will accelerate the assessment and ultimate applications of nanostructured materials.
ACKNOWLEDGMENTS
I would like to express my appreciation to Dr. Robert D. Shull of the National Institute of Standards and Technology for the invitation to present this paper in the EM 2000 Symposium at the 1994 TMS Annual Meeting in San Francisco, California, and the encouragement to publish it in JOM, and to Dr. Lawrence Kabacoff of the Office of Naval Research for his encouragement of applications-driven nanostructured materials research. Many thanks go to Steve Meyst of the University of Connecticut' s Precision Manufacturing Center for skillful preparation of the illustrations and to Pratt & Whitney for gas turbine engine historical information and illustrations.
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References 1. RH. Kear et aI., eds., Research Opportunities for Materinls with Wtrafine Microstructures (Washington, D.C: National Academy Press, 1989). 2. H. Gleiter, "Nanocrystalline Solids," J. App!. Cryst., 24 (1991), pp. 79-90. 3. H. Gleiter, "Materials with Ultrafine Microstructures: Retrospectives and Perspectives," NanoStructured Materials, 1 (1992), pp. 1-20. 4. RW. Siegel, "Nanophase Materials: Synthesis, Structure, and Properties," Physics of New Materinls, ed. F.E. Fujita (Heidelberg, Germany: Springer·Verlag, 1992). 5. C Suryanarayana and F.H. Froes, "The Structure and Mechanical Properties of Metallic Nanocrystals," Met. Trans., 23A (1992), pp. 1071-1081. 6. RW. Siegel and G.B. Fougere, "Mechanical Properties of Nanophase Materials," Nanophase Materials: Synthesis·Properties-Applications. ed. GC Hadjipanayis and RW. Siegel (Dordrecht, Netherlands: Kluwer, 1994). 7. Troy Barbee, Lawrence Livermore National Laboratory, unpublished research, 1992. 8. B.H. Kear and L.B. McCandlish, "Chemical Processing and Properties ofN anostructured WC-Co Materials," NanoStructured Materials. 3 (1993), pp. 19-30. 9. T. Fischer, Stevens Institute of Technology, unpublished research, 1993. 10. S. Bose, Pratt & Whitney, B. Hartford, CT, unpublished research, 1993. 11. P.R Strut! and RE Boland, University of Connecticut, unpublished research, 1993. 12. KE. Gonsalves et aI., "Synthesis and Processing of Nanostructured MSO Type Steel," NanoStructured Materials, 4 (1994), pp. 139-147. 13. KB. Gonsalves et aI., "Nanostructured Bearing Alloy Studies," ONR Grant No. NOOO14-94-1-0S79.
ABOUT THE AUTHOR
Maurice Gell earned his Ph.D. in metallurgy at Yale University in 1965. He is currently professor in residence at the University of Connecticut.
For more information, contact M. Gell, Department of Metallurgy, University of Connecticut, Storrs, Connecticut 06269.
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