Carbon and Graphite Fiber Composites

The microscopy techniques described for the evaluation of glass fiber composites are widely used to determine the microstructure of carbon and graphite fiber composites. Microscopy of crack propagation in carbon fiber reinforced composites is also very important in understanding mechanical properties. Test specimens and actual composite products are often evaluated to determine the distribution of the fibers 

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J. Dehnonte, Technology of Carbon and Graphite Fiber Composites, Van Nostrand Reinhold Company, New York, 1981, p. 398. 

Asbestos fibers are found worldwide in many products as reinforcement in cement water pipes and the inert and durable mesh material used in filtration processes of chemicals and petroleum, for example. However, asbestos is not the only inorganic fiber in use today. Synthetic inorganic fibers abound. Glass fibers have replaced copper wire in some intercontinental telephone cables. Fiberglas (a trade name) has become the insulation material of choice in construction. Carbon and graphite fiber composites are favored materials for tennis racket frames and golf clubs. Fibrous inorganic materials have become commonplace in our everyday lives. 

High performance composites may be laminates wherein veils of carbon fiber ate treated with an epoxy resin, stacked up to the desired final product thickness, and then laminated together under heat and pressure (see Composite materials Carbon and graphite fibers). Simply mixing together carbon or glass fibers and polymeric resins to form a reinforced plastic leads to a composite material, but this is not a laminate if not constmcted from discrete phes. 

This brief summary of the composition and structural characteristics of glass fibers, whiskers, and carbon and graphite fibers illustrates the ranges of synthetic inorganic fibrous materials. The purposes of the construction of these materials is to capitalize on the physical and chemical advantages of the fibrous morphology, size, and state. 

Graphite fibers [CARBON AND GRAPHITE FIBERS] (Vol 5) [COMPOSITEMATERIALS - SURVEY] (Vol 7) as ceramic composite reinforcements [COMPOSITE MATERIALS - CERAMIC MATRIX] (Vol 7) useofbrominein [BROMINE] (Vol4)... 

Table I. Surface Composition of Carbon and Graphite Fibers...

A large number of studies have been made on a of polymer matrix composites of epoxy, polyimide and other polymers reinforced with carbon and Kevlar fibers [117-123]. The carbon and graphite fibers are characterized by slightly negative (Xi in their axial directions and very large positive aj in their radial directions [122-124]. The unidirectional and bidirectional composites based on them are found to demonstrate highly anisotropic thermal expansion behavior. In unidirectional composites,... 

SEM analysis of both polished and fractured specimens. Applications of microscopy to the study of glass, carbon or graphite fiber composites will be explored, especially relating to the effects of processing, fiber length and interfacial bonding. 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

For friction material appHcations, composite materials (qv) comprising glass or metallic fibers with other minerals have been developed. In such appHcations also, aramid and graphite fibers are effective, although the cost of these materials restricts their use to heavy duty or high technology appHcations (see Carbon fibers). 

Carbon Composites. In this class of materials, carbon or graphite fibers are embedded in a carbon or graphite matrix. The matrix can be formed by two methods chemical vapor deposition (CVD) and coking. In the case of chemical vapor deposition (see Film deposition techniques) a hydrocarbon gas is introduced into a reaction chamber in which carbon formed from the decomposition of the gas condenses on the surface of carbon fibers. An alternative method is to mold a carbon fiber—resin mixture into shape and coke the resin precursor at high temperatures and then foUow with CVD. In both methods the process has to be repeated until a desired density is obtained. 

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