applications will require new materials with improved thermomechanical and thermochemical properties. CMCs are recognized as having the potential for providing high strength, toughness, creep resistance, notch insensitivity, and environmental stability at temperatures that will meet the anticipated needs of future high-performance turbine engines and power generators.

In addition to turbine engine components, there are several industrial applications for which a comparatively large market could be realized for a broad range of CMC components. For example, hot gas filters for pressurized fluidized bed combustion (PFBC) and furnace hardware, such as pipe hangers for petroleum refining, represent potential near-term applications for CMCs. These relatively low-risk industrial applications could provide the market volume necessary to lower fiber costs (and prices) significantly, as well as to develop experience and confidence in using CMCs. Longer-range industrial uses for CMCs include heat exchangers for externally fired combined cycle (EFCC) power systems and reforming tubes for the chemical processing industry. Substantial improvements will be required, however, in the thermomechanical and thermochemical properties of ceramic fibers and coatings to enable CMCs to meet the service lifetime requirements.

Composite materials derive benefits both from the properties of their constituent phases and from the method of their combination, including the tailoring of interfaces between phases, to achieve properties that none of the constituents exhibits individually. For example, CMCs have well demonstrated damage tolerance, which is attributed to frictional sliding at fiber matrix interfaces (see, for example, Box 1-1). Frictional sliding is enabled by fiber interfacial coatings. Damage tolerance is manifested in ductilities on the order of 1 percent and notch sensitivity comparable to aluminum (Al) alloys. CMCs also have excellent room-temperature fatigue properties, with thresholds (stress below which fatigue does not occur) at about 90 percent of their ultimate tensile strength (UTS). However, fatigue problems are evident at elevated temperatures. The UTS of CMCs (typically 300 MPa [44 ksi]), although not exceptional, is volume invariant because the damage tolerance suppresses the weakest link scaling effects found in monolithic ceramics. That is, because of crack deflection and crack tip blunting mechanisms, CMCs can tolerate cracking that would lead to catastrophic failure in monolithic ceramics. These thermomechanical properties are particularly attractive for large, static, thermally-loaded components.

The CMC market is divided into two classes, oxide and non-oxide materials. Oxide composites consist of oxide fibers (e.g., alumina [Al₂O₃]), interfacial coatings, and matrices. If any one of these three components consists of a non-oxide material (e.g., silicon carbide [SiC]), the composite is classified as a non-oxide composite. These classes have different properties, different levels of development, and different potential applications.

Because most development work has been done on non-oxide materials, particularly SiC fiber-reinforced SiC CMCs (SiC/SiC) with fiber interfacial coatings of either carbon or boron nitride, non-oxide CMCs are more advanced than oxide CMCs. Non-oxide CMCs have attractive high temperature properties, such as creep resistance and microstructural stability. They also have high thermal conductivity and low thermal expansion, leading to good thermal stress resistance. Therefore, non-oxide CMCs are attractive for thermally loaded components, such as combustor liners (see Figure 1-4), vanes, blades, and heat exchangers.

Composite behavior has also been studied in oxide systems (e.g., oxide fiber-reinforced porous oxide matrix composites with no interfacial coatings). Oxide composites have the attractive features of oxidation resistance, alkali corrosion resistance, low dielectric constants, and potentially low cost. Because of these properties, oxide CMCs could be attractive for hot gas filters, exhaust components of aircraft engines, chemical processing equipment, and long-life, lower temperature components.

Both oxide and non-oxide CMCs have demonstrated shortcomings. Embrittlement occurs at intermediate temperatures (~700°C [1,292°F]) in all non-oxide composites, exemplified by SiC/SiC. Embrittlement is most severe with cyclic loading beyond the proportional limit, whereupon matrix cracking occurs because oxygen that ingresses through the matrix cracks reacts locally with the fibers and fiber coatings to form oxide products. These reaction products suppress the internal friction mechanisms that otherwise impart toughness. Although this effect does not occur when the stresses remain below the proportional limit, design studies and end-user experience indicate that stress excursions above the proportional limit must be anticipated. Local embrittlement is, therefore, the dominant life-limiting phenomenon of non-oxide composites. The committee considers the solution to this problem to be imperative for long-life applications of non-oxide CMCs. A systems-level approach that includes considerations of fibers and fiber coatings, as well as ways to diminish oxygen ingress, is discussed in this report.

Oxide CMCs are not subject to oxidative embrittlement but have higher temperature limitations (~1,000°C [1,832°F]) associated with creep and sintering, both governed by the high diffusivities of oxides compared to SiC. Also, fiber coating technologies for oxide materials are less mature than coating technologies for non-oxide materials. In fact, nearly all of the performance data available for oxide CMCs are for systems in which interfacial coatings were not used, and damage tolerance was a result of matrix porosity. Concepts for suppressing the high-temperature degradation mechanisms (e.g.,

Front Matter (R1-R16)

Executive Summary (1-5)

1 Introduction (6-11)

2 Current and Future Needs (12-19)

3 State of the Art in Ceramic Fiber Performance (20-36)

4 Ceramic Fiber Processing (37-48)

5 Materials and Microstructures (49-53)

6 Interfacial Coatings (54-74)

7 Cost Issues (75-82)

8 Recommendations and Future Directions (83-86)

References (87-92)

Biographical Sketches of Committee Members (93-95)