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Development of Alternative Engine Materials

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  • DEVELOPMENT OF ALTERNATIVE ENGINE MATERIALS

    M. V. Nathal and S. R. Levine

    NASA Lewis Research Center Cleveland, OH, USA 44135

    Abstract

    The purpose of this paper is to review the current state of development of new materials for advanced aircraft and aerospace engines. The advan- tages and disadvantages of ceramics and intermetallics in both monolithic and composite form as replacements for today's nickel-base superalloys are discussed. Results will be presented for research directed toward overcom- ing some of the problems which restrict the application of these alternative materials, along with some examples of materials which have reached the stage of component demonstration. It is concluded that while notable progress has been made toward solving many technical issues, the state of technology readiness is still immature when placed in a historical perspec- tive with the introduction of new superalloy technology into aircraft engines. In addition to the need for further improvements in material properties, it is expected that a considerable time period will be needed to address issues related to cost, manufacturing, design, and competing tech- nologies.

    Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, R.A. MacKay,

    D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992

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  • Introduction

    Historically, the development of the Ni-base superalloys has been integral to the improvements in efficiency and performance of gas turbine engines. These improvements, as measured by the creep-rupture strength of turbine blade materials, have increased at a relatively steady pace of approximately 1OO'C per decade since the 194O's(1,2). These improvements have been achieved through a synthesis of design, process, and alloy devel- opment combined with a strong need for improved jet engine performance. It has now been 20 years since John Tien speculated at this conference on the "Celestial Limits" of superalloys(3). It is a tribute to superalloys that for the most part the alternatives to superalloys are still discussed in terms of 'potential' and 'barrier problems' rather than with stories of successful introduction into engines. Although improvements in superalloy properties and design concepts continue to increase engine performance and reliability, it is apparent that the physical limits of melting temperature and density of Ni will eventually limit engine performance. The needs for improved performance are still present, and new materials can provide improvements ranging from economic and performance benefits upon direct substitution, to cases where the new materials are enabling technologies, i.e., essential for a vehicle concept to be feasible.

    The DOD and NASA have several programs actively investigating the development of alternative materials. Although these programs are frequent- ly examining similar material systems, they are focussed on different vehicles and thus have different requirements for the engine materials, as summarized in Figure 1. Turbine materials in the Space Shuttle Main Engine are required to withstand severe thermal shocks plus exposure to hydrogen, but desired life is on the order of 50 hours(4). Hypersonic vehicles such as the National Aerospace Plane (NASP) also require high temperature mater- ials with a resistance to a hydrogen environment, but material lifetimes for this demonstration vehicle are also short(5). The needs of advanced mili- tary planes are being addressed in the Integrated High Performance Turbine Engine Technology (IHPTET) program, and tend to emphasize higher tempera- tures and stresses but with relatively short lives, on the order of hundreds of hours(d). In contrast, commercial subsonic aircraft place a heavier emphasis on longer lives and fuel efficiency. A study of 21Se century subsonic transport using ultra-high bypass turbofan engines has predicted significant benefits from the use of advanced materials(7). The schematic diagram in Figure 2 shows alternative materials throughout such an engine. Much current emphasis from NASA is on a new generation supersonic transport, known as the High Speed Civil Transport(HSCT). This vehicle is expected to fly at speeds of Mach 2-3 over a range greater than 6000 nautical miles while carrying 250-300 passengers(8). This represents significant increases in performance over today's supersonic vehicles and advanced materials are required in order for this plane to be feasible(g). For such a vehicle to be viable, it must be environmentally acceptable and economically competi- tive. The former requirement mandates that the emissions of noise be at acceptable levels and that emissions of nitrous oxides (NO,) be minimized in order to avoid damage to the ozone layer. For the noise requirement, the engine contribution can be addressed via exhaust nozzle design. Since a very large nozzle near the tail of the aircraft is required, it is essential that lightweight, advanced high-temperature intermetallic matrix composites and ceramic matrix composites be developed. Such advanced materials could yield a 30% weight reduction compared to the current materials in a mixer- ejector nozzle design. Advanced concepts for control of NO, emissions in an HSCT require innovative combustor designs wherein conventional through-the- wall film cooling is not possible. This results in a requirement for higher temperature materials with reasonably high levels of thermal conductivity. It appears that silicon-based ceramic matrix composites offer the greatest

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  • COMMERCIAL SUPERSONIC

    WCT)

    - MILITARY

    NASP COMMERCIAL

    SUBSONIC -

    SPACE SHUTTLE TURBOPUMP

    EXPENDABLE - TURBINES

    ROCKET THRUSTERS

    I

    1 10 100 1,000 10,000

    Total Life, hours

    Figure 1. Approximate engine life requirements for various vehicles and demonstration programs.

    FAN BLADE FAN FRAME

    c NACELLE . - THRUST REVERSER

    r CWBUSTOR CASE I ANDLININGS

    FAN DISCI RING MM2 --.

    BEARINGS BEARINGS THRUST BHAFf BLADES DlSCS BLADES. DlSCS VERY HlGH LUBRICANTS BEARINGS STEEL CMC IMC CMC IMC LOADING ADVANCED HlGHGN AND

    BEARINGS MMC LININGS AND LUBRICANTS

    PMC, MMC, MC. AND CMC I POLYMER, METAL, INTERMETALLIC, AND CERAMIC MATRIX COMPOSITES, RESPECTIVELY. HPC = HIGH PRESSURE COMPRESSOR, LP 3 LOW PRESSURE, HPT = HIGH PRESSURE TURBINE, IPT = lNTERYEDIATE PRESSURE TURBINE.

    Figure 2. Advanced materials applications for an ultra high bypass engine(4).

    promise for such design concepts. Finally, the issue of economic viability translates not only to production costs, a major problem with advanced materials, but also to extended service lives, on the order of 18,000 hours of hot section life.

    One strength of superalloys is that they possess a balance of the many properties required for engine application, Figure 3. In general, alterna- tive materials show advantages in several of these properties but disadvan- tages in others. For example, ceramics offer large gains in temperature and specific strength capability but at a sacrifice of toughness, processa- bility, and thermal conductivity. In comparison, intermetallics appear to offer smaller gains in performance but their other properties are closer to those of superalloys. This paper will present some of the ceramic and intermetallic materials under current investigation and will discuss their

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  • Specific High Temperature

    Tmax Strength/Life

    I Toughness

    Ia Ceramics lntermetallics

    Oxldatlon Resistance

    I Thermal

    Conductivity

    Processabllity

    SpeClfiC High Temperature

    StrengWLlfe I

    Tmax

    q Si-Base Monolithic q Si-Base CMC

    Oxide-Base CMC

    ETA M&,-Base NIAI-Base

    q Ti-Al-Base

    Oxidation Resistanca

    I Thermal

    Conductivity

    Processability

    High Tkmperature Strength/Life

    Tmax

    Oxi RSS

    Conductivity

    (a)

    (b)

    Figure 3. The balance of properties required for turbine engine application are represented by six axes. Generalizations of the properties of alternative materials are compared to those of superalloys. (a) an overall view of ceramics and intermetallics; (b) major subdivisions of ceramics; and (c) major subdivisions of intermetallics, including IMC's.

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  • potential for use in the various aircraft. Some of the key technical issues that need to be resolved will also be addressed.

    Alternative Materials

    Ceramics

    Since the early 1970% when current generation ceramics efforts were born, the focus has been directed primarily at Sic and Si,N, monolithic ceramics(l0). Intensive efforts on fiber reinforced ceramics (CMC's) did not get underway until about one decade later. Although ceramics remain inherently brittle with low resistance to flaws and overloads, significant progress has been made over this time period, and the benefits and examples of recent successes support the case for continued optimism.

    The primary monolithic structural ceramics for heat engines are Sic and Si,N,. In both materials large gains have been made in net shape fabrication capability, strength, creep resistance and fracture toughness(lO,ll). Sic fracture toughness has been raised from about 3 to about 6 MPa-./m by alloy- ing(12). S&N, fracture toughness has been increased from levels of about 6 to about 10 MPa-Jrn by growth of elongated, interlocking Si,N, grains during the sintering process(l3).

    The primary applications for monolithic ceramics as structural gas turbine engine components are in small automotive and stationary gas tur- bines. In the consumer cost-driven automotive market, it is clear that superalloys cannot be applied from a cost standpoint even if