Design

The Potential for CMCs to Replace Superalloys in Engine Exhaust Ducts

Richard Roth, Joel P. Clark, and Frank R. Field III

The Materials Systems Laboratory at the Massachusetts Institute of Technology has conducted research to develop decision tools that can facilitate materials selection and provide a deeper understanding of the design tradeoffs that occur when choosing among advanced aerospace materials for high-tem­perature applications. As an illustration of the use of these tools, this paper describes research done to evaluate the material alter­natives currently under consideration for exhaust ducts in aircraft gas turbine en­gines. Although nickel-based superalloys currently prevail for this application, the increasing temperatures of modern engines are necessitating the usage of higher tem­perature materials.

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INTRODUCTION

Initiatives such as the Integrated High Performance Turbine Engine Technology program (which seeks to "double the capability of today's most advanced turbine engine by the end of the century")! and the national aerospace plane are continuing to drive the development of new materials. As a result of these and other programs, there is a proliferation of new materials and technologies that have the potential for use in a variety of airframe and powerplant applications. One such application is the exhaust duct, which typically experiences temperatures greater than 750°C as well as relatively low externally applied stresses. Candidate materials for this application include superalloys, monolithic ceramics, ceramic-matrix composites, and carbon-carbon composites. The alternatives offer various advantages and disadvantages when compared with one another and existing technology. This leaves the engine designer with difficult decisions concerning the optimal choice of material and processing technique.

In view of this situation, the study described here attempts to establish a framework for consistently determining the optimal materials choice. The techniques employed are well-known methods used in the areas of engineering decision analysis and operations research. The methods employed are multi-attribute utility analysis, subjective probability assessment, and technical cost modeling.

The investigation examined the issue of materials substitution in the exhaust ducts for medium-sized missiles. The question it attempts to answer is "What must each material supplier do to improve or maintain its competitiveness? The materials considered were, as described in the sidebar, superalloys and ceramic-matrix compos­ites (CMCs).

RESEARCH METHODOLOGY

Multi-attribute utility analysis was used to determine a materials-blind comparison of the performance characteristics that are considered to be important for exhaust ducts. Subjective probability assessment was then applied to determine the "believ­ability" of the claims made by materials suppliers for achieving specific performance levels. Finally, a process called technical cost modeling was used to determine the best­case cost that can be achieved for producing the part from a given material by a given process. In this way, materials alternatives can be compared on the basis of what could potentially be achieved in production. This methodology is explained in greater detail in Reference 9.

Five engine designers participated in the study (Subjects 1-5 as indicated in Figure 1). Each one responded separately to the multi-attribute utility analysis questionnaire. Three of the five subjects also participated in the subjective probability assessment portion of this study. One subject chose not to participate in this section because he felt that he was not qualified. Another subject was not questioned about his attitude toward specific materials as he no longer worked as an engine designer and was instead employed by a material supplier.

IMPLICATIONS FOR SUPERALLOY SUPPLIERS

Superalloys capable of enduring 1,090°C while still possessing the favorable me­chanical properties described in the sidebar could cost as much as $2,750 per duct for Subject 3 and more than $10,000 per duct for Subjects 1 and 4. They would have the same utility as current CMC ducts. More importantly, lower cost CMCs-from less than $2,150 per duct for Subject 1 and less than $5,150 per duct for Subject 5-are worth paying for improved service temperatures as this will result in a multi-attribute utility greater than that of superalloys.

Thus, the need for higher-temperature materials in exhaust ducts will likely reduce the cost advantage held by superalloys. Further, it may also have the effect of changing the way engine designers trade operating temperature for other attributes. This may also diminish the competitive position of superalloys in this application.

The main focus of research for superalloy suppliers should be to improve spe­cialized casting techniques with an eye toward cost reductions. While the use of directional solidification and single-crystal casting techniques can result in com-

JOM • January 1994

1994 January • JOM

ponents with good elevated-temperature mechanical properties, the cost of using these techniques may be too great.

IMPLICATIONS FOR CMC SUPPLIERS Slurry Infiltration

Other than cost, the main area in which slurry-infiltrated SiC/SiC composites need improvement is strength at elevated temperatures. While strength was not generally considered to be a major concern when designing exhaust ducts, the reported strength of an SiC/SiC composite produced by this slurry infiltration was near the bottom of the utility scale for most of the participants in this study, being below the minimum for one.

However, the subjective probability portion of the study indicates that engineers involved in the materials selection process believe that slurry-infiltrated CMCs should exhibit much higher strengths. As a result, suppliers of this product should focus on improving the strength of their material. However, this must be done without any large cost increase, which would offset the benefits of the improved mechanical properties. For strengths of 207 MPa and with all other properties being as before, slurry-infiltrated SiC/SiC composite exhaust ducts would have the same utility as superalloy ducts at costs up to $5,000 (which is above its estimated cost) for Subject 3 and up to $1,000 for Subjects 1 and 4.

Innovative superalloy castings have the potential to be used at temperatures greater than those of CMC components made by slurry infiltration.4 Thus, improving the temperature capabilities of the slurry-infiltrated composites would help them main-

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tain a competitive position. Improving service temperature is closely related to the issue of strength, as mechanical properties are limited by operating temperature. Cost reductions for CVI could eliminate the competitive advantage that slurry infiltration currently possesses. At 1,400°C (with all other properties being as before), slurry­infiltrated SiC/SiC composite exhaust ducts costing as much as $3,850 for Subject 5 and $1,000 for Subject 1 would have the same utility as the current superalloy duct. For Subject 4, the slurry-infiltrated material would also have to demonstrate improved strength.

Another advantage that superalloy exhaust ducts have over CMC ducts is that there is no problem with thermal expansion mismatch. However, the use of CMCs through­out the engine might even make their low coefficient of thermal expansion a selling point in applications with tight geometric tolerances.

If the thermal expansion mismatch is eliminated with the incorporation of CMCs into the turbine section of the engine, slurry-infiltrated SiC/SiC composite exhaust ducts costing as much as $6,100 for Subject 5 and $1,000 for Subject 4 would have the same utility as the current superalloy duct.

Under the CMC engine scenario, the introduction of CMCs into engine components should be accompanied by a redesign of the entire engine to accommodate the new material, not just the new component. Therefore, it may be beneficial for CMC suppliers to work toward applying their materials in a number of applications in order to make an overall engine redesign feasible and cost-effective.

A combination of these initiatives could improve the competitive position for slurry infiltrated exhaust ducts. A duct with a strength of 207 MPa at 1,400°C and with no thermal expansion mismatch (density and toughness at baseline values) would have the same utility as the current superalloy duct at a cost as high as $8,500 for Subject 4 and $3,700 for Subject 1. Under a very optimistic processing scenario (i.e., assuming 100 percent process yield, annual production volume of 10,000 units, and a fiber price of $111 per square meter), the cost of a slurry-infiltrated duct is $2,407. This indicates the possibility for slurry-infiltrated SiC/SiC composite exhaust ducts to compete with superalloy ducts. The key for suppliers of this material is to not only cut costs as much as possible, but to improve properties.

Chemical Vapor Infiltration

As for CMCs made by CVI, it must be stated that while performance is valued, it is not valued enough to offset the large costs associated with this technique. Cost reduction is clearly an aspect that is needed to make these materials competitive for use in exhaust ducts. Further, there is still room for improving the properties of these materials, particularly in the areas of strength and, over the long term, operating temperature.

Strength increases to 207 MPa would improve the competitiveness of CVI com­posites as is the case for slurry-infiltrated composites. With all other properties but strength remaining the same as before, CVI exhaust ducts would have the same utility as current superalloy ducts if they cost less than $5,485 for Subject 3 and below $1,000 for Subjects 1 and 4.

Like slurry-infiltrated ducts, the elimination of thermal expansion mismatches between the exhaust duct and the turbine section would also result in an improved competitive position for CVI ducts. In this case, a CVI SiC/SiC composite exhaust duct

1.0

0.9

~ 0.8 '5

Superalloy CV Infiltration Slurry Infiltration

4

Subjects

Figure 1. The utility of the alternative exhaust duct materials based on the responses of the five study participants.

JOM • January 1994

costing as much as $7,950 for Subject 5 and $3,000 for Subject 4 would have the same utility as the superalloy duct.

A combination of these improvements would lead to an even better competitive position for CVI exhaust ducts. A duct with a strength of 207 MPa and no thermal expansion mismatch (all other properties remaining as before) could cost as much as $8,500 for Subject 4 and $3,700 for Subject 1.

The technical cost model suggests that under a best-case technology scenario (Le., 100 percent process yield, annual production volume of 10,000 units, a fiber price of $111 per square meter, and only one infiltration cycle), the lowest potential costfor CVI ducts is $3,647. This indicates the possibility for CVI SiC/SiC composite exhaust ducts to compete with superalloy ducts. Again, CVI composite suppliers must focus on cost reductions; property improvements will make the necessary cost reductions more realistic.

Finally, the same issues concerning engine redesign and the introduction of CMCs into more than one component apply with CVI as well. More importantly, CMC producers using CVI should look toward other engine applications, particularly those where a higher premium placed on performance. Potential applications range from relatively easy mechanical requirements (e.g., combustor liners) to mechanically demanding applications (e.g., turbine components).

CONCLUSIONS

Based upon current requirements, superalloys remain the material of choice for aircraft engine exhaust ducts. Their cost and thermal expansion compatibilities more than offset the high density and adequate high-temperature capabilities.

Infiltrated ceramic components are presently too expensive to fabricate com­petitively for this application. While designers clearly value the elevated-temperature performance and low density of these materials, these attributes do not make up for manufacturing limitations, system incompatibilities, insufficient high-temperature strength, and relatively high raw material costs. For CVI composites, the cost of the manufacturing (a direct consequence of long infiltration times and the need for expensive machining cycles) dominated all other considerations. Process innovations leading to shorter cycle times would improve the competitiveness of CVI composites, given that performance would otherwise exceed that of superalloys and slurry­infiltrated composites.

TMS

References 1. james H. Brahney, "Propulsion Systems for the '90's," Aerospace Engineering (August 1990). 2.CT. Sims and W.C Hagel, The Superalloys (New York john Wiley & Sons, 1972). 3. j.L. Everhart, Engineering Properties of Nickel and Nickel Alloys (New York: Plenum Press). 4. M. Gell et aI., "Advanced Superalloy Airfoils," J. Metals, 39 (july 1987), pp. 11-15. 5. RW. Rice, "Ceramic Matrix Composite Toughening Mecha­nisms: An Update," Ceramic Engineering and Science Proceed­ings, 5 (7-8) (]985), pp. 589-{)07. 6. A. Kelly and S.T. Mileiko, Fabrication of Composites Volume 4 (Amsterdam, Netherlands: Elsevier Science Publishers B.V., 1983). 7. j.5. Reed, Introduction to the Principles of Ceramic Processing (New York john Wiley & Sons, 1988). 8. R. W. Rice, "Ceramic Matrix Composite Toughening Mecha­nisms: An Update," Ceramic Engineering and Science Proceed­ings, 5 (7-8) (]985), pp. 589-{)07. 9. R. Roth, "Materials Substitution in Aircraft Gas Turbine Engine Applications," Ph.D. thesis, MIT, Cambridge, MA, 1992.

ABOUT THE AUTHORS

Richard Roth is currently a research associ­ate with the International Motor Vehicle Pro­gram at the Massachusetts Institute of Tech­nology in Cambridge, Massachusetts.

Joel P. Clark is currently POSCO Professor of Materials Systems at the Massachusetts Institute of Technology in Cambridge, Massa­chusetts. Dr. Clark is also a member of TMS.

Frank R. Field III is currently a director in the Materials Systems Laboratory at the Massa­chusetts Institute of Technology in Cambridge, Massachusetts.

For more information, contact Joel P. Clark, Materials Systems Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Av· enue, Cambridge, Massachusetts 02139-4307.

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