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

5 12

10 0

-;;;-s a.. Q.

~ 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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