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Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
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Executive Summary

High-temperature ceramic fibers are the key components of ceramic matrix composites (CMCs). Ceramic fiber properties (strength, temperature and creep resistance, for example)—along with the debonding characteristics of their coatings—determine the properties of CMCs. This report outlines the state of the art in high-temperature ceramic fibers and coatings, assesses fibers and coatings in terms of future needs, and recommends promising avenues of research. CMCs are also discussed in this report to provide a context for discussing high-temperature ceramic fibers and coatings.

Continuous ceramic fibers in CMCs offer the potential for high performance in high-temperature corrosive environments. CMCs can be fabricated into complex shapes for use in thermostructural environments much the way carbon composites are fabricated for use in less aggressive environments. Unlike monolithic ceramic bodies in which the mechanical strength is determined by the largest flaw in a critical position, CMCs are relatively flaw tolerant because the load is borne by a multiplicity of fibers. Thus, CMCs have toughness and damage tolerance comparable to metals with the added advantages of lower density and greater stability at high temperatures.

For the past 15 years, research and development of CMCs has been sustained because of their potential for military and commercial applications. The applications of interest include (1) aircraft engine components, such as combustors, turbines, compressors and exhaust nozzles; (2) ground-based and automotive gas turbine components, such as combustors, first and second stage turbine vanes and blades, and shrouds; (3) aerospace engines for missiles and reusable space vehicles; and (4) industrial applications, such as heat exchangers, hot gas filters, and radiant burners.

Technical shortcomings must still be overcome, however, before CMCs can be widely used in thermostructural applications. These shortcomings provide research opportunities, particularly for the development of fibers and fiber coatings. The following list describes these opportunities:

  • Fiber coatings for non-oxide composites have demonstrated adequate performance in short-life applications (e.g., rocket nozzles). These fiber coatings have also been demonstrated to be adequate in composite samples (e.g., test coupons) during long-time exposures to stress at high temperatures in laboratory tests. However, fiber coating technologies for long-life applications (e.g., turbine engine components) have not been demonstrated in component testing.

  • Several coatings for oxide ceramic fibers have enabled model composite systems to demonstrate damage tolerant behavior. However, no fiber coatings have been proven to be effective in actual (as opposed to model) oxide composite systems.

  • Non-oxide fibers are generally creep resistant but lack chemical stability; they are prone to oxidation. Consequently, stress oxidation limits the durability of nonoxide composites, especially at intermediate temperatures (i.e., 700 to 900°C [1,292 to 1,652°F]) under cyclic loading conditions. There are no known concepts for producing a coating that can prevent oxidation of the fibers for more than 100 hours, after matrix cracks occur (and remain open). Longer life non-oxide composites will require a combination of oxidation resistant fiber coatings and matrix sealing concepts that protect the fiber from oxidation—particularly when the composite is subject to cyclic thermomechanical loads that can cause sealed cracks to reopen. Such concepts have been developed but have not been tested.

  • Oxide fibers are generally environmentally stable but are subject to excessive creep at high temperatures. Preliminary work indicates that microstructural modifications have the potential for enhancing creep resistance.

A significant barrier to progress is the paucity of engineering data 1 on CMCs, which reflects a lack of access to data generated by classified projects, as well as a general lack of engineering data. Advances have been further impeded by the high cost of coated fibers, which are attributable to low production volumes and, in some cases, to high precursor costs. Consequently, the stability and breadth of the vendor base for fibers, coatings, and coated fibers is questionable.

1  

Engineering data are defined as coupon tests, subelement tests, and component tests, as well as design and life prediction results.

Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×

APPROACH

To conduct this study, the National Research Council (NRC) convened a committee with expertise in ceramic fiber processing, nanoparticle reinforced ceramics research, ceramic fiber research, high-temperature ceramic fiber-matrix interfacial coatings, and synthesis of nanomaterials. The committee also has expertise in ceramic fiber economics, including cost analysis and the determination of the commercial potential of advanced materials. To accomplish the overall objective of identifying research directions to meet the material property requirements of advanced fibers and coatings for high-temperature ceramic composites, the committee took the following steps:

  • characterized the current state of the art in high-temperature fibers and interface materials and identified current domestic and foreign capabilities (both R&D and production capabilities)

  • assessed the capabilities of current fibers to meet future performance needs

  • recommended promising research directions for developing fibers and coatings for improved performance in high temperature applications

  • identified materials processing technologies that have the potential to produce high-temperature ceramic fibers and coatings cost effectively

  • identified incentives for and barriers to the development of commercial-scale high-temperature fibers for low-volume applications

By limiting the scope of this study to fibers and their coatings, independent of CMC processing and matrix materials, the committee was able to focus on issues that limit the strength and toughness of CMCs, particularly at high temperatures. The discussion of composite materials in this report is limited to providing a context for discussions, conclusions, and recommendations regarding ceramic fibers and coatings.

HIGH-TEMPERATURE CERAMIC FIBERS

Non-Oxide Ceramic Fibers

Non-oxide fibers are typically based on silicon carbide (SiC) and are fabricated by several processes: (1) spinning a melt of organometallic precursors (the most favored route); (2) spinning a solution of organometallic precursors (dry spinning); (3) extrusion spinning of a ceramic powder in a polymeric binder; (4) chemical vapor deposition (CVD) of vapor species onto a monofilament core; and (5) conversion of carbon fiber to SiC using Si-containing vapor species.

Spun non-oxide fibers (i.e., those made by the first three processes above) are produced in tows consisting of hundreds of filaments with diameters of 10 to 20 µm. Typical ranges of properties are given in Table ES-1. All of these fibers are based on SiC except for amorphous Si-B-N-C, a promising new fiber.

The best non-oxide fibers have good creep resistance but are susceptible to degradation by formation of an amorphous silica layer upon oxidation. This layer offers some resistance to further oxidation, but prolonged exposure to oxidizing environments results in oxidative embrittlement of the composite.

Oxide Ceramic Fibers

All currently available commercial oxide fibers are based on aluminum oxides. Examples are alumina (Al2O3) yttrium aluminum garnet (YAG) and mullite (3A12O3-2SiO2).

TABLE ES-1 Typical Property Ranges for Ceramic Fibers

Property

Non-Oxide Fibers a

Oxide Fibers b

Tensile Strength (GPa)

1.5–4.0 (220–580 ksi)

1.4–3.0 (260–430 ksi)

Elastic Moduli (GPa)

180–400 (26–58 Msi)

150–380 (22–55 Msi)

Strain to Failure (%)

0.6–1.8

Coefficient of Thermal Expansion (ppm/°C)

3–5

3–9

Thermal Conductivity at 1,500°C (2,732°F) (W/mK)

up to 40 (up to 23 Btu/hr foot °F)

a Representative properties for polycrystalline and amorphous Si-based fibers that contain one or more of the following elements, carbon, nitrogen, or boron

b Representative properites for polycrystalline oxide fibers consisting of predominantly Al2O3

Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×

Commercial polycrystalline oxide fibers are produced by spinning and hydrolyzing precursors. First a fiber precursor solution is filtered and concentrated to remove excess solvent, forming a viscous spin dope. Then, continuous filaments are extruded by spinning. The filaments are pyrolyzed to remove volatile components and then heat treated above 800°C (1,472°F) to crystallize and sinter the fiber. Polycrystalline oxide fibers are produced in tows of 200 to 1,000 fibers with diameters of 10 to 16 µm (0.39 to 0.63 mils). Typical ranges of properties are listed in Table ES-1.

Oxide fibers are inherently resistant to oxidation but have limited creep resistance because of higher diffusivities compared to SiC. Creep rates decrease with increasing grain size, but this advantage is offset by decreasing strength. However, significant improvements have been achieved in the past decade by reducing the amorphous phase content at the grain boundaries of oxide fibers.

FIBER COATINGS

Damage tolerance in a CMC requires a weak interface between the fibers and the matrix; fiber coatings are engineered to provide this weak interface. Fiber coatings also protect the fiber from environmental attack during composite fabrication and use.

Coatings for Non-Oxide Fibers

Most work on coatings has been concentrated on SiC fibers used in non-oxide CMCs. Tough composite behavior has been reported only when coatings consist predominantly of carbon or boron nitride (BN). Oxidation of the fiber/coating/matrix interface is a major limitation for non-oxide composites. This interface may be exposed to an oxidizing environment when cracks develop in the matrix (which otherwise acts as a barrier to oxygen ingress) under load. Oxidation of the fiber/coating/matrix interface degrades the fiber and its debonding characteristics, reducing both the strength and toughness of the composite.

Carbon coatings can be formed by the in-situ decomposition of Si-C-O fibers or applied by CVD. Carbon coatings are typically 0.1 to 0.3 µm (0.004 to 0.01 mils) thick. During oxidation of the fiber coating, carbon is converted to carbon monoxide (CO) or carbon dioxide (CO 2), leaving a gap. The SiC fiber then oxidizes, forming a silicate glass (SiO2), which tends to close the gap. However, if the gap is not closed quickly enough, oxidation may proceed along the fiber/matrix interface, bonding the fiber to the matrix and causing embrittlement of the composite. This phenomenon, sometimes called pesting, is most prevalent at intermediate temperatures.

BN is the only other fiber coating that has been demonstrated to enable “tough” composite behavior. BN coatings are typically deposited via CVD and are 0.3 to 0.5 µm (0.01 to 0.02 mils) thick. Because the oxidation product of a BN coating is a borate (B2O3) glass, which protects against oxidation at intermediate temperatures, the degradation rate of BN-coated fibers is lower than for carbon-coated fibers. As the temperature is increased, the B2O3 reacts with SiO2 (the oxidation product of either the SiC fiber or the matrix) to form a borosilicate glass. However, in wet atmospheres, the B2O3 volatilizes (as boron hydroxides), thus compromising its ability to prevent further oxidation of the fiber, which ultimately leads to embrittlement of the composite.

Coatings for Oxide Fibers

Oxide coatings that are chemically compatible with commercially available oxide fibers have been identified, but adequate debonding and friction have yet to be demonstrated in a composite system. Several coating strategies and materials are being investigated. For example, porous and fugitive coatings have been used, as well as porous matrices (with no coating). These provide toughness as long as sintering between the fibers and particles in the matrix/coatings can be suppressed.

Layered oxides are being studied because of their potential debonding characteristics. A class of sheet silicate minerals known as fluoromicas exhibit easy delamination along crystal planes but are chemically incompatible with current fibers and matrices. Beta alumina (ß-Al 2O3) and magnetoplumbites are compatible with alumina fibers and have sufficiently low fracture energies to provide the weak fiber/matrix interface needed for damage tolerant composites. The magnetoplumbite mineral hibonite, CaAl12O19, has been studied extensively, but Ca tends to diffuse into matrices during hot pressing, which degrades composite properties. Other layered oxides that have been studied preliminarily include perovskites, such as KCaNb3O10 and BaNd2Ti3O10.

“Nonwetting” oxides have shown particular promise. Nonwetting refers to the tendency of the interface between the coating and the fiber to debond readily. The monazite class of compounds (e.g., lanthanide phosphates) and sheelites fall into this category. These compounds have high melting points and are chemically stable (when stoichiometric).

Several coating technologies are used for oxide fibers. CVD can be used, but maintaining stoichiometry is difficult. Solution-based precursors are better for controlling stoichiometry, but maintaining coating uniformity and preventing bridging between fibers in a tow is difficult. The electrostatic attraction between particles in a slurry and the fiber can also be used to deposit fiber coatings. All currently proposed oxide coatings have promising features but have questionable debonding and frictional characteristics, as well as uncertain processing technologies.

Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×

RECOMMENDATIONS AND IMPACTS

The committee formulated many specific recommendations on both fibers and coatings that are presented in the body of the report, the most important of which are listed here. The committee also describes the anticipated impact of these recommendations on the field, which is crucial for moving CMC technology forward. These recommendations are followed by a discussion of the rationale for their prioritization. Although the focus of this report is on high-temperature ceramic fibers and coatings, the committee believes that a database of CMC properties is needed to establish research goals and performance criteria to design better fibers and coatings for future CMCs.

Recommendation 1. Building and disseminating an engineering database for actual (as opposed to model) CMCs is essential. The committee recommends the following:

  • Existing nonclassified data that are not broadly accessible because they are associated with classified or restricted studies should be made generally available. Wherever possible, agency-owned engineering data should be made accessible, and new programs should avoid restricting data.

  • Data that are currently classified should be reassessed to determine if they can be declassified and, if so, they should be made more generally available.

  • Low risk government-sponsored insertion programs for CMCs should be expanded to demonstrate the field performance of CMC components.

  • Standardized tests for obtaining engineering data on CMCs should be developed and instituted.

Impact. Researchers would be able to determine when ceramic fibers and coatings limit CMC properties and focus materials research on overcoming 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). Furthermore, following the recommendations listed above would elucidate composite failure modes, facilitate vetting less viable CMC systems, and establish a foundation for breaking out of the market size-cost impasse that has stymied investments in facilities for the production of fibers and coatings.

Recommendation 2. Coatings for high-temperature ceramic fibers must be improved.

  • Non-oxide fiber coatings research should be focused on:

    • concepts that enable high durability with cracked matrices at the “pest” temperature

    • a system approach that includes concepts for improving the oxidation resistance of fiber coatings in dry and moist atmospheres and “sealing ” matrix cracks as they form

    • investigating regenerative in-situ coatings

Impact. Durable coatings for non-oxide fibers would provide suitable lifetimes for many applications, such as thermally loaded gas turbine components and heat exchangers.

  • Research dedicated to improving the fiber/matrix interface for oxide CMCs should focus on:

    • investigations of weakly bonded, thermally stable oxide coatings for oxide-oxide composites (e.g., nonwetting coatings)

    • approaches that do not use coatings (e.g., porous matrices)

Impact. CMCs that are not susceptible to oxidative degradation could be fabricated for intermediate temperature or intermediate performance applications.

Recommendation 3. Studies should be directed toward improving oxide fiber creep resistance and understanding the underlying scientific mechanisms.

Impact. Oxide fibers with improved creep resistance will allow higher temperature applications (e.g., combustors and heat exchangers) provided that suitable oxide fiber coatings are developed in parallel.

Recommendation 4. The following areas should be investigated for non-oxide fibers:

  • emerging amorphous non-oxide fibers, such as Si-B-N-C fibers, to verify the stability, creep resistance, and utility of the in-situ coatings

  • microstructural refinement to improve performance in crystalline non-oxide fibers

Impact. The development of higher temperature, higher performance fibers will enable the use of CMCs in long service life, high-temperature applications if the problems of interface durability can be solved in parallel.

Recommendation 5. Efforts to reduce the costs of fiber and coating processing should be focused on the following:

  • capital nonintensive approaches

  • processes that leverage past investments

  • in-situ and liquid precursor coatings

Impact. A broader, more stable vendor base for fibers and coatings would probably be established if costs were reduced. Lower costs for fibers and coatings would also make CMCs more attractive to a larger variety of users.

Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×

DISCUSSION OF PRIORITIES

The five recommendations above fall into three categories. Recommendation 1 addresses increasing the knowledge base of, and confidence in, existing CMC technology. The committee anticipates that applications for CMCs will expand if engineering designers have access to the information they need to make materials selection decisions. Thus the committee places a high priority on this recommendation.

Recommendations 2, 3, and 4 (listed in order of decreasing priority) are related to performance, which is also considered to be a high priority. Recommendation 2 is the most important in this category. The oxidation resistance of oxide fibers is attractive, but poor creep resistance is a significant limitation. Thus, Recommendation 3 addresses the need to improve this property. Recommendation 4 (regarding non-oxide fibers) is last in this category because the committee concluded that resources directed toward property improvement in fiber coatings and oxide fibers was more important. The committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the research needed to verify them.

Recommendation 5 is last, not because cost is unimportant but because, at the current stage of the technology, performance rather than cost has limited the use of CMCs. The committee concluded that, at this time, improving properties should be of higher priority than reducing costs.

Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×
Page 1
Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×
Page 2
Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×
Page 3
Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×
Page 4
Suggested Citation:"Executive Summary." National Research Council. 1998. Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. Washington, DC: The National Academies Press. doi: 10.17226/6042.
×
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High-temperature ceramic fibers are the key components of ceramic matrix composites (CMCs). Ceramic fiber properties (strength, temperature and creep resistance, for example)-along with the debonding characteristics of their coatings-determine the properties of CMCs. This report outlines the state of the art in high-temperature ceramic fibers and coatings, assesses fibers and coatings in terms of future needs, and recommends promising avenues of research. CMCs are also discussed in this report to provide a context for discussing high-temperature ceramic fibers and coatings.

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