Recommendations and Future Directions
The principle focus of this study is on two of the three constituent phases of CMCs, ceramic fibers and the fiber-matrix interface (e.g., fiber coatings). The committee has formulated many specific recommendations regarding ceramic fibers and coatings for overcoming the challenges to implementing these promising materials. These recommendations were presented throughout the body of the report. The key overarching research priorities are presented here along with discussions of their anticipated impact on the field. The committee believes that these priorities are crucial to moving CMC technology forward and, ultimately, to the widespread application of CMCs. Although the focus of this report is on high-temperature ceramic fibers and coatings, the committee believes that a database of CMC properties is necessary for the establishment of research goals and performance criteria to facilitate the development of better fibers and coatings for future CMCs.
Assessing the state of the art of CMCs, including ceramic fibers and coatings, requires a thoughtful definition of performance requirements and a subsequent comparison of these requirements to current capabilities (Chapter 2 and Chapter 3). A general lack of substantiation through field experience has, however, prevented the committee from establishing a clear understanding of how CMCs will perform and how they will age in service. Without operational experience, it has been impossible to calibrate knowledge with analytical performance models. Performance requirements must be made available to those responsible for meeting them, and the evaluation of composite performance must include tests of CMC components in their operating environments. Similar in-service component testing has been done for other structural composite systems, such as Kevlar-49-reinforced epoxy fairings and graphite-reinforced epoxy ailerons. Service experience would enable the establishment of CMC design guidelines that could be included in reference works like the MIL 17 Composite Materials Handbook, Ceramic Matrix Composites (MIL-HDBK-17), which is currently under development.
Recommendation 1. An engineering database for actual (as opposed to model) CMCs should be produced and disseminated. Producing the database will require the following steps:
existing nonclassified data that is not broadly accessible because of its association with classified or restricted studies be made generally available (in particular, the committee recommends that, wherever possible, agency owned engineering data be made accessible and that new programs avoid restricting data)
data that is currently classified be reassessed to determine if it can be declassified and, if so, that it be made more generally available
low risk, government sponsored insertion programs for CMCs be expanded to demonstrate the field performance of CMC components
standardized tests for obtaining engineering data on CMCs be instituted or developed.
The committee anticipates that following this recommendation would facilitate the design of better fibers and coatings while simultaneously increasing confidence in the use of CMCs. Researchers would be able to determine when ceramic fibers and coatings limit CMC properties and thus focus materials research to overcome these limitations. Design engineers would gain confidence in using CMCs when data is available on CMC component performance in operating environments (i.e., field tests). If CMCs are demonstrated to satisfy a need or outperform existing materials, they should begin to gain experience in the market. This would establish a foundation for breaking the market size-cost dilemma that has hindered investment in research and development in production facilities for high-temperature ceramic fibers and coatings.
In addition to facilitating the acceptance of CMCs by designers, field tests will also determine CMC component degradation mechanisms and life-failure modes in operational environments. On the basis of these tests, promising composite systems can be refined, and less viable CMCs can be vetted.
Adequate fiber coatings are requisite for providing damage tolerance (toughness) to CMCs. Fiber coatings also protect fibers from environmental attack during composite fabrication and use. Fiber coatings must demonstrate chemical and mechanical stability in high-temperature corrosive
environments in order to maintain the necessary fiber-coating-matrix debonding characteristics for damage tolerance in the composite. Unfortunately, the development of interfacial coatings has lagged behind the development of ceramic fibers. Therefore, the widespread use of CMCs has been limited, to a large extent, by inadequate fiber coatings.
Current fiber coatings either have inadequate oxidation resistance or are not stable with fibers and matrices at elevated temperatures. Generally, these limitations result in degradation of the strength and toughness of the composites during use. The issues for coatings for non-oxide fibers differ from issues for oxide fibers and are discussed separately below.
Coatings for Non-Oxide Fibers
Although tough, thermally stable, non-oxide ceramic composites have only been demonstrated with carbon or BN fiber coatings, oxidation of the fiber-coating-matrix interface (i.e., pesting) is one of the major life-limiting issues preventing the widespread use of non-oxide ceramic composites. Pesting is particularly prevalent at intermediate temperatures (e.g., 700 to 900°C [1,292 to 1,652°F]) and occurs when the interface is exposed to an oxidizing environment via matrix cracks that allow atmospheric gases to come into contact with fiber coatings.
Recommendation 2a. Concepts should be pursued that enable high durability CMCs with cracked matrices at the temperatures where pesting is prevalent. Future development should be directed toward a system approach that includes improving the oxidation resistance of fiber coatings—in dry and moist atmospheres—and “sealing” matrix cracks as they form. Regenerative in-situ coatings should also be investigated.
The development of oxidation-resistant coatings that maintain the necessary debond characteristic for non-oxide fibers would enable the development of CMCs suitable for applications that demand longer component lifetimes, such as heat exchangers and thermally loaded gas turbine engine components. Furthermore, efficient matrix crack sealing mechanisms (particularly at the “pest” temperature) would also enable the use of CMCs. Finally, regenerative in-situ coatings could provide damage tolerance after initial oxidation of the fiber-matrix interface, provided that the coating has sufficient debond characteristics with SiO2, the oxidation product of Si-based non-oxide ceramics (e.g., SiC).
Coatings for Oxide Fibers
The development of ceramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistance of oxide fibers (compared to SiC fibers) and because of the lack of adequate oxide fiber coatings that promote fiber-matrix debonding. Recent advances in creep-resistant oxide fibers and progress on interface control has improved the potential for oxide ceramic composites in industrial and defense applications. However, an effective coating for oxide fibers that provides a weak fiber-matrix interface (and therefore tough composite behavior) remains to be demonstrated. As was discussed in Chapter 6, all oxide coating concepts discussed in the literature have been demonstrated with model systems rather than actual composite systems.
Recommendation 2b. Coating approaches that promise to provide damage tolerant oxide composites should be evaluated to prove or disprove their viability. Based on the preliminary results discussed in Chapter 6, the committee has concluded that research should be focused on the following areas: weakly bonded, thermally stable oxide coatings (e.g., rare-earth phosphates of the general formula Me3+PO4); and the development of oxide composites that do not require fiber coatings (e.g., porous matrices).
The impact of developing a viable interface for oxide CMCs would be twofold. First, it would enable near-term implementation of CMCs that are not susceptible to oxidative degradation for intermediate-temperature or intermediate-performance applications. Second, the performance of oxide CMCs would no longer be limited by the fiber-matrix interface. Therefore, future advancements in oxide fiber capabilities could be readily utilized in CMCs.
DEVELOPMENT OF OXIDE FIBERS
Ceramic oxide fibers are attractive because they are inherently resistant to oxidation and, therefore, are not susceptible to oxidative embrittlement. They have higher temperature limitations (e.g., T>1,000°C [1,832°F]), however, associated with creep and microstructural instability. This is problematic because oxide fibers must resist creep and maintain microstructural stability in order to be successfully used in high-temperature applications. For example, during CMC component service life, grain growth at the highest use temperature should be less than ~ 20 percent in order to limit strength reduction to less than 10 percent. Furthermore, fiber strains greater than ~ 1 percent are unacceptable because attendant composite distortions compromise attachments and dimensional tolerance. Work to address these performance requirements is ongoing.
Ceramic fibers consisting of mullite and alumina mixtures have demonstrated creep strengths considerably greater than the creep strengths observed in single-phase alumina or mullite fibers (Chapter 3). The mechanism by which multiphase
ceramic oxide fibers resist creep rupture is not understood, however, although it is known that two-phase mixtures (in general) exhibit less grain growth than single-phase materials and, therefore, tend to be more microstructurally stable. The addition of yttrium and other oversized isovalent cations to alumina has also been shown to enhance creep strength. The addition of SiC nanoparticles to bulk samples of alumina has been shown to enhance the creep strength of alumina, particularly when the SiC particles are preferentially located at the alumina grain boundaries.
Recommendation 3. The results discussed in Chapter 3 and Chapter 5 suggest that continued study and increased understanding of dual-phase microstructures, solutes (e.g., yttria in alumina) and nanoparticle reinforcement will lead to improvements in the high-temperature creep resistance, rupture strength, and stability of oxide fibers. Therefore, the committee recommends that research be directed towards: establishing the mechanism(s) by which SiC nanoparticle dispersions inhibit creep in bulk oxide ceramics (including developing oxide nanoparticle systems, analogous to the alumina-SiC nanocomposite system, that can be applied in an oxide fiber); determining the mechanism by which certain solutes lower the creep rate of bulk polycrystalline oxides (e.g., yttria in alumina) and applying them to oxide fibers; and using multiphase microstructures to promote increased high-temperature microstructural stability (e.g., resistance to grain growth) and increased creep resistance in oxide fibers.
The committee believes that oxide fibers with improved creep resistance will enable the use of oxide CMCs in higher temperature applications (e.g., gas turbine combustors and heat exchangers) provided that suitable oxide fiber coatings are developed in parallel. Improved oxide fibers would allow higher operating temperatures and, therefore, greater efficiencies than are attainable with current thermostructural materials.
DEVELOPMENT OF NON-OXIDE FIBERS
Most work on CMCs has been done on non-oxide materials, particularly SiC-fiber-reinforced SiC CMCs (SiC/SiC) with carbon or BN fiber interfacial coatings. These composites have attractive high-temperature properties, such as good creep resistance and microstructural stability. They also have high thermal conductivity and low thermal expansion, leading to good thermal stress resistance. Because of their high thermal conductivity, non-oxide CMCs are attractive for thermally loaded components, such as combustor liners, vanes, blades, and heat exchangers. The most pressing limitation of these materials is oxidative embrittlement at the fiber-matrix interface (addressed in Recommendation 2). Although non-oxide CMCs are not limited by the fibers themselves, some recent improvements in non-oxide fiber performance nonetheless warrant further attention.
One promising development is an amorphous Si-B-N-C fiber produced using a novel polymer precursor technology, for which high strength, high stiffness, and high-temperature strength retention and creep resistance have been reported. In terms of polycrystalline non-oxide fibers, preliminary results suggest that incorporating up to several weight percent boron in a Si-C-O or Si-C-O-(Ti) fiber produces a stoichiometric polycrystalline, high strength, high modulus ß-SiC (or ß-SiC + TiB2) fiber.
Recommendation 4. The key areas of investigation recommended for non-oxide fibers are recently developed amorphous non-oxide fibers, such as Si-B-N-C fibers, to verify their stability and creep resistance and the utility of the reported in-situ coatings, and microstructural refinements to improve performance in crystalline non-oxide fibers.
The development of higher temperature, higher performance fibers will enable the use of CMCs in long service-life, high-temperature applications (e.g., thermally loaded gas turbine engine components and heat exchangers for externally fired combined cycle power systems) if the problems of interface durability are solved in parallel.
The manufacture of fibers has three unusual cost issues: (1) fibers have a very high length per unit mass; (2) the machinery used for fiber production is highly specialized; (3) materials to be formed into fibers require special treatments or additives. The cost of most materials is scaled by cost per unit of mass. Fibers have a tremendous amount of length per unit mass, and processing that length incurs additional cost relative to mass. The cost premium for engineering materials that have been “stretched” to fiber diameters is significant, especially in the linear density range of interest for composite materials.
Fiber prices could be reduced, however, if there was a market-driven increase in production volume, thus lowering the fixed cost per unit. Current fiber manufacturers and independent consultants have conducted studies to predict fiber prices and cost levels. Exact cost predictions have been elusive, however, because of the difficulty of precisely forecasting future yields and process improvements. Furthermore, prices will depend upon the pricing policies of individual manufacturers as much as on actual costs.
The fact that an increase in the market for fibers would reduce the cost of ceramic fibers raises the question of why the ceramic fiber market hasn't grown already. Ceramic fibers are currently available within the price range of commercially available carbon fibers and, by current standards, cannot be labeled expensive. Furthermore, the development costs of specialty materials for jet engines have historically been
readily justified for rather modest increases in temperature capability. But cost alone has not kept ceramic fibers or CMCs from entering the market. Significant efforts must still be directed towards increasing the performance capabilities of these materials. The following guidelines can help reduce development and manufacturing costs.
Using capital non-intensive equipment for fiber production will lower fixed costs, thus lowering cost per unit. Processes that leverage past investments can further reduce fixed costs. The development and use of less expensive fiber precursors also offer a potential reduction in variable costs. Finally, because applying coatings to fibers via CVD adds considerably to the final cost of CMCs, liquid precursor coatings or coatings developed on the fiber in-situ (without a separate processing step) can also lower costs.
Recommendation 5. Efforts to reduce the costs of fiber and coating processing should be directed toward using capital non-intensive equipment; developing less expensive fiber precursors; using processes that leverage past investments; and developing in-situ and liquid precursor coatings.
A broader, more stable vendor base for fibers and coatings could probably be established if costs could be reduced. Lower costs for fibers and coatings would make CMCs more attractive to a larger variety of end-users.
The five recommendations above fall into three categories. Recommendation 1 addresses increasing confidence in existing CMC technology. By providing engineering designers with the information they need to make materials selection decisions, it is anticipated that applications for CMCs will increase. Thus the committee places a high priority on this recommendation.
Recommendations 2, 3, and 4 fall into the second category, performance. As a group, these recommendations, which are also considered to be of high priority, are listed in the order of decreasing priority. The committee found that fiber coatings and other interface technologies for both non-oxide and oxide systems are the major technical limitation to CMC development. That is, improved fiber coatings are needed to enable CMCs to meet the higher temperature performance requirements for many applications. Recommendation 2 has the highest priority in this category because even though the oxidation resistance of oxide fibers is attractive the creep resistance of these fibers must be improved. Recommendation 3 addresses this priority. Recommendation 4 (regarding non-oxide fibers) has a lower priority because for many applications adequate properties have already been attained in non-oxide fibers. Therefore, the committee concluded that resources directed toward improving the properties of fiber coatings, and oxide fibers was more important. However, the committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the work needed to verify them.
Although cost is an important issue, Recommendation 5 is considered lower in priority. The committee concluded that at the current stage of technology performance rather than costs have limited many CMC applications, making improved properties a higher priority than lower costs.