The expected benefits of economical, high-performance civil-aircraft designs that are being considered for the future will be realized only through the development of light-weight, high-temperature composite materials for engine applications to reduce weight, fuel consumption, and direct operating costs.
A major effort underway in this area is the Advanced High Temperature Engine Materials Technology Program (HITEMP) of the National Aeronautics and Space Administration (NASA), which focuses on providing revolutionary high-temperature composite materials: to 425°C for polymer-matrix composites (PMCs); to 1250°C for metal-matrix / intermetallic-matrix composites (MMCs / IMCs); and to as high as 1650°C for ceramic-matrix composites (CMCs).
Composites promise benefits
Numerous conducted studies demonstrate that significant economic and performance benefits can be achieved if lightweight, high-temperature composite materials can reach technology readiness. Based on a preliminary design of a conceptual engine, however, material temperatures approaching 1650°C are anticipated for the turbine inlet, thus requiring extensive use of CMCs throughout the combustor, turbine, and exhaust nozzle.
One benefit of using CMCs is that they allow higher operating temperatures and thus greater combustion efficiency leading to reduced fuel consumption. Thanks to the low density of CMCs, compared with current technology, the use of CMCs in the hot section of the engine along with IMCs in the compressor is resulting in a 50% reduction in engine weight. This translates to an overall reduction in aircraft weight of nearly 40% for an aircraft with four engines, further contributing to lower initial costs, as well as lower operating costs.
The high-temperature composite materials required for these engines will have to operate satisfactorily from 5,000 to 16,000 hours at temperature. Interdiffusion, oxidation resistance, and creep, therefore, are major life-limiting problems that must be solved. Materials research also must include the study of failure modes and joining technology, and a mechanical and thermal-property database must be established. In addition, new, more precise design methods will be needed to address both the application of brittle composite materials and the integration of intricate cooling schemes for a wide range of material thermal conductivities. And finally, low-cost manufacture of the new materials and advanced components will require development of new fabrication processes.
Analytical modeling is being used to investigate the structural behavior of these advanced materials in six distinct areas: micro mechanics, deformation and damage, fatigue, fracture, trade-off studies, and loads definition. In the trade-off studies, coefficient of thermal expansion (CTE) mismatch, compliant layers, and fiber shape/size effects are being investigated using existing analytical tools to develop a physical understanding of advanced-composite development.
The emphasis in the area of loads definition is to develop and verify models to predict the aerodynamic and thermodynamic loads on a composite turbine blade. This is being accomplished by integrating existing aerodynamic, heat-transfer, and structural codes to predict blade response. The results are then calibrated and verified with simplified experiments that also are being defined and conducted under this task.
The results of analysis and experimental verification to date demonstrate the capability to simulate the high thermal gradients associated with engine operating conditions. In the future, this type of analysis will permit evaluation of an advanced-composite material`s performance in a simulated engine component.
Polymer-matrix composites (PMCs) are the lightest of the three types of composite materials under study in the HITEMP program. Recent applications of PMCs in aircraft propulsion systems, such as General Electric`s F-404 engine, have resulted in substantial reductions in both engine weight and manufacturing costs. Unfortunately, the low thermal-oxidation stability of PMCs severely limits the extent of their application. Commercially available state-of-the-art high-temperature PMCs, such as graphite fiber/PMR-15 and graphite fiber/PMR-11-55, are capable of withstanding thousands of hours of use at temperatures between 290 and 345°C).
To realize the full advantages of PMCs in aircraft-propulsion systems, however, new composite materials must be developed with enhanced thermal-oxidative stability permitting their use at temperatures to 425°C. Research on high-temperature PMCs under HITEMP is aimed at achieving this goal. Ongoing work includes:
- Study of the effects of resin/fiber interactions on composite stability and high-temperature performance
- Development of innovative processing techniques
- Exploration of oxidation-resistant coatings
- Synthesis of new polymers having good processability and significantly improved thermal-oxidative stability
Graphite-reinforced composites prepared with one of the new high-temperature polymers, V-CAP, undergo weight losses only about 60% those of comparable PMR-II-base composites after exposure in air at 370°C for 500 hours. An elevated-temperature nitrogen-postcure technique has been developed, which substantially improves the high-temperature (370°C) flexural strength of graphite-reinforced PMR-15 laminates. Application of this postcure method to V-CAP laminates enhances both the high-temperature mechanical properties and thermal-oxidative stability. Thus, the combined use of a higher stability matrix with improved processing yields a PMC with a useful lifetime in air at 370°C double that of a PMR-II-50 composite one of the best high-temperature PMCs currently available.
Continued improvements in the stability of polymer matrices coupled with improvements in polymer/fiber interfaces, composite processing, and oxidation-resistant coatings will yield PMCs for use at temperatures to 425°C.
Intermetallic-matrix composites. Several major problems limit the development of inter-metallic-matrix composites (IMCs), including chemical incompatibility and CTE mismatch between potential reinforcing fibers and matrix materials, poor low-temperature ductility, and marginal high-temperature oxidation resistance of intermetallic materials. Composite fabrication and joining processes that do not result in excessive fiber/matrix reaction or matrix contamination is an additional need.
The initial phase of the IMC program involves investigating available fiber compositions (SiC and Al2O3) in aluminides of iron, titanium, nickel, and niobium. These aluminides are Ti3Al and FeAl for applications to 1000°C and NiAl and Nb-alloy/aluminides for higher temperature applications. Alloying studies of these materials are aimed at increasing toughness, ductility, and oxidation resistance, and promoting longtime stability with the candidate fiber materials. Candidate matrices will be evaluated using tensile, compression, fatigue, creep, and oxidation tests. Measurement of appropriate thermal and physical properties is another planned task.
Powder-cloth fabrication processes have been developed to produce IMC materials, and alternative processing procedures, such as thermal spraying, are being studied. Encouraging results have been obtained on SiC-reinforced TiAl3 + Nb material, based on tensile, thermal-cycle, and strain-controlled fatigue studies for temperatures to 815°C.
The properties of first-generation SiC/Ti-24Al-11Nb composites compare favorably with those of current nickel-base super alloys on a strength/density basis. However, the SiC fiber is too reactive with the matrix material above 815°C, and also with the other candidate matrix materials. Therefore, researchers are focusing on using Al2O3 as the reinforcing fiber for these materials. There is a need for new fibers, however, and new compositions and fiber-processing techniques, such as the laser floating-zone process, have been identified. A project has been initiated to produce experimental quantities of fiber material.
Fiber coatings also are being investigated to function as diffusion barriers to limit fiber/matrix reaction and as compliant layers to lower stresses generated by CTE mismatch between the fiber and matrix. The oxidation resistance of FeAl is adequate for its intended use temperature and the time/temperature oxidation limits have been established for NiAl. Optimized fiber materials coupled with a better understanding of IMC behavior should result in future materials superior to those currently used for aerospace applications.
Ceramic-matrix composites. To meet HITEMP goals, CMC research is aimed at developing the basic and applied technologies needed to fabricate structurally reliable ceramic composites reinforced with long or continuous ceramic fibers. Like monolithic ceramics, these fiber-reinforced ceramics (FRCs) have lower densities, better oxidation resistance, and potential to operate at significantly higher temperatures than super alloys. However, unlike monolithic ceramics, FRCs display metal-like deformation behavior, noncatastrophic failure, and strength properties that is insensitive to processing- and service-generated flaws.
Recent investigations of a NASA-developed SiC/reaction-bonded silicon nitride (RBSN) composite system show that Si-based composite microstructures can be produced that are strong and tough for short times to temperatures.
Fiber development is critical since the development of advanced materials such as high-temperature composites is highly dependent on the availability of high-temperature fibers. If such advanced materials are going to be available for material-critical applications in future civil-transport engines, new fibers must be developed.
The wide range of fiber characteristics needed would require the development of more than one type of fiber. Fibers must have different properties, depending on the composite matrix, as well as the composite end use. In general, a candidate fiber should have low density, high strength, high stiffness, a CTE matching the matrix, chemical compatibility with the matrix, environmental stability, and appropriate fiber diameter.
The selection of appropriate fiber diameter also depends on the composite matrix. A large-diameter fiber (75 to 150 μm) is required for MMCs / IMCs to maximize fracture toughness. Small-diameter fibers ≤ 25 μm are required for CMCs to keep the critical flaw size for these brittle materials as small as possible. The environmental stability of the fiber also is a major factor; fibers must be able to withstand the high-temperature oxidation/hot-corrosion environment of the gas-turbine engine. This requirement emphasizes the need for the development of suitable fiber coatings, in conjunction with the development of the fibers themselves.
Fiber-research efforts begun under HITEMP include fiber fabrication by chemical vapor deposition, physical vapor deposition, polymeric precursors, and laser float-zone methods. Laboratory processes for fiber fabrication, however, are only the first steps toward the development of new high-temperature fibers. It is equally important to consider the scale-up required to produce the quantities of fiber needed for actual composite parts. A great deal of manpower and money is still required to scale-up from the small-size batches of fibers produced in the research laboratory to the vast quantities of fiber that will be needed in the future.
