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The manufacturing and design of carbon-carbon (CC) composites are considerable topics that require extensive treatment. This chapter is intended as an overview of the principles involved with carbon-reinforced, carbon-matrix composite materials.
The manufacturing and design of CC composites are founded upon principles common to the design and fabrication of all composites with additional considerations specific to carbon and graphite. Principles common to the design of all composites include the following: the choice of composite preform is driven by the intended application; the reinforcing material and the matrix material must be compatible over the range of temperatures the composite will experience both in processing and intended application; and the properties of the matrix material and the reinforcement material (individually) must be such that the two materials together can interact to provide the desired composite properties. The term preform means the shape and distribution of the reinforcement in the composite. For example, a composite consisting of a uniform distribution of spheroidal silicon carbide particles in an aluminum matrix comprises a metal matrix composite having macroscopically isotropic properties. In contrast, a cured epoxy reinforced with continuous glass fibers all aligned in the same direction comprises a uniaxial composite which results in high anisotropy of properties.
Compatibility refers to chemical and thermal properties. Chemical reactivity or a large mismatch in coefficients of thermal expansion (CTE) between matrix and reinforcement usually will render the materials incompatible if elevated temperatures are experienced by the composite either in processing or in use. Compatibility is typically not an issue with CC composites because of the chemical and physical property equivalence of the resulting matrix with the reinforcement upon completion of processing and because of the very low reactivities between the carbon reinforcement and the matrix precursor(s) during processing.
Considerations specific to CC center on the unique properties of carbon and graphite. Graphite is the most refractory of materials and combines low density with desirable mechanical, thermal, and electrical properties. The low strain capability and susceptibility to thermal shock of monolithic graphite are overcome by CC composites, which retain most of the advantageous characteristics of monolithic graphite. Even though strain capabilities of CC composites are sometimes similar to monolithic graphite, load-carrying capability is significantly higher and catastrophic failure is less likely to occur. Some properties of a CC and a commercial graphite are compared in table I (ref. 1).
The can impart
very low coefficients of thermal expansion exhibited by CC composites iart unparalleled stability for applications such as space structures that experience large temperature excursions. The low CTE, low weight, high thermal conductivity, resistance to spalling, and good strength retention at elevated temperatures have made CC the material of choice for brakes on commercial and military aircraft (ref. 2). Continuous fiber-reinforced CC composites have become increasingly important to the aerospace industry as materials performance requirements for components of missiles, space structures, and aircraft have become ever more demanding.
Table 1. Properties of 3-D CC and Commercial Graphite
The manufacture of CC composites entails high-temperature processing required for the pyrolysis of the matrix precursor (pitch or resin) or for the reaction of gases to form carbon matrix in a chemical vapor deposition (CVD) process. Depending upon the intended maximum use temperature, the composite may be further heat treated to graphitize the carbon matrix and to further graphitize the fiber reinforcement. Such a heat treatment, known as graphitization, chemically and thermally stabilizes the composite for use at temperatures up to the maximum temperature experienced by the composite during graphitization. Pyrolysis and graphitization are processes unique to the manufacture of carbon-matrix composites. Of these, only carbon-reinforced carbon-matrix composites, or CC, have been important engineering materials up to now. The remainder of this chapter will treat CC composites exclusively.
To design a CC composite is to predetermine its woven preform configuration, target density, and degree of stabilization. Matrix morphology (pore structure and degree of process-induced cracking) can also be part of the material design. Density, degree of stabilization, and matrix morphology are determined by selection of the matrix precursor in combination with thermal processing; thus, manufacturing methodology is an important factor in the design of the composite. The most fundamental aspect of CC design, the element which typically is considered first, is the woven preform. Substructure or preform, to a greater extent than any other design consideration, delineates the properties of a CC composite. For continuous fiber-reinforced CC composites, substructure or preform design is referred to as fiber architecture.
Fiber architectures include filament wound patterns, yarn diameters, and spacing in unidirectional tapes, rods, and cylinders, patterns in woven fabrics, stacking patterns of laminates, braid patterns, and multidirectional patterns woven into preform constructions. Multidirectional woven preforms may have an orthogonal three-directional (3-D) fiber architecture, more complex n-D architectures where n > 3. or a cylindrical architecture having reinforcing fibers in the axial, radial, and circumferential directions. The majority of CC applications have typically required continuous fiber reinforcements.
Carbon-carbon composites reinforced with discontinuous fibers have had a very limited range of applications. Perhaps the most important application is that of high-temperature insulation for vacuum or inert atmosphere furnaces. Porous carbon is cost competitive with ceramic furnace insulation and offers an economical alternative to refractor) metal heat shields. Short carbon fibers impart sufficient rigidity and toughness to high (~90 percent) porosity carbon for machining and shape retention. Since thermal conductivity in CC is considered a fiber dominated property, all the fibers should be oriented randomly in-plane or in concentric circles in the case of cylindrical pieces, and should be perpendicular to the thickness of the material. Freestanding CC insulating materials are available in various shapes for use at temperatures up to 3000°C.* A typical component is illustrated in figure 1.
*High-Temperature Insulation. Fiber Materials. Inc.. Biddelord. Maine. I'M.
Figure 1. Porous CC furnace insulation. (Photo courtesy of Fiber Materials, Incorporated.)
Filament Wound CC
Among the continuous fiber-reinforced CC composites, filament wound structures are perhaps the least common. Filament winding was investigated as a means of overcoming a weakness of noninterwoven pattern ply laminates, namely, their tendency to delaminate when subjected to shear loads in thermal environments (refs. 3 and 4). The use of helical winding patterns at intervals along the structure provides a degree of interlocking of the continuous fibers. Filament winding of relatively thin wall bodies of revolution is economical and is commonly done for the manufacture of other types of composites. Alternative approaches, such as tape wrapping, involute construction, or weaving 3-D preforms, have been preferred to filament winding for overcoming delamination problems in CC composites.
One current application of filament wound CC is for hot-pressing molds. The filament winding of continuous yarn into a right circular cylindrical pattern imparts high hoop tensile strength to the composite. Filament wound hot-pressing molds are manufactured under the trademark of Filcarb . These filament wound molds have tensile strengths 5 to 10 times those of monolithic graphite, are resistant to thermal shock, and exhibit a long lifetime compared to bulk graphite molds.
Filament winding of unidirectional tape also has been used to apply circumferential reinforcement to 3-D cylindrical CC structures (ref. 5) and is discussed later in this chapter.
Carbon-carbon laminates find extensive use in the aerospace industry with applications including brake assemblies and structural panels. Various fabrics woven of graphite yam typically comprise the reinforcement, but uniaxial tape may be preferred in certain cases. The fiber architecture of the laminate is determined by the weave pattern in the fabric and by the relative orientation of the fabric layers in the stacking sequence.
Fabrics are available in a variety of styles (ref. 6). Fabric styles vary according to basic weave pattern, the type of yarn, yam spacing, yam packing efficiency, and the percent of yam in each direction. The two weave patterns most widely used in CC laminates are plain weave and satin weave. A plain weave fabric and a five-harness satin weave are illustrated in figure 2.
Uniaxial tape obviously provides the most highly directional properties in a layer, while a balanced plain weave results in the most evenly matched orthotropic distribution of properties within a layer. Satin weaves, which produce smooth fabrics having good drape, are preferred for applications in which the layers must conform to topological features such as depressions and bends in the macroscopic shape of the end product (e.g., a panel with complex curvature).
Laminate structures afford the designer the ability to tailor in-plane properties within the limits of the in situ reinforcement properties. Strength in a given direction is determined by the yam strength and the volume fraction of yam oriented in that direction. In general, off-axis properties are difficult to predict. The assumption underlying such an approach is one of efficient load transfer between yams of different orientations. In CC composites, load transfer between fibers is poor, depending on shear modulus. In some instances, depending upon the processing approach used, the designer will assume no load transfer at all. Not surprisingly, CC laminates exhibit low out-of-plane tensile strength and low interlaminar shear strength. Out-of-plane thermal conductivity is lower than in-plane conductivities, since the carbon fibers comprise the high conductivity "heat pipes" in a CC composite. Anisotropy in thermal expansion can be as high as 15:1 depending on the selection of reinforcing fibers.
Laminate structures generally are termed 2-D CC even though rotation of fabric plies could be termed multidirectional. For example, the adjacent layers of a certain