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Figure I. Multiformity and general properties of carbon-fiber and carbonmatrix composites.

inert environments. These materials must, however, be protected with coatings and/or surface sealants when used in an oxidizing environment.

The development of CC materials began in 1958 and was nurtured under the U.S. Air Force space plane program, Dyna-Soar, and NASA's Apollo projects. It was not until the Space Shuttle Program that CC material systems were intensively researched. The criteria that led to the selection of CC composites as a thermal protection system were based on the following requirements: (1) maintenance of reproducible strength levels at 1650°C, (2) sufficient stiffness to resist flight loads and large thermal gradients, (3) low coefficient of thermal expansion to minimize induced thermal stresses, (4) oxidation resistance sufficient to limit strength reduction, (5) tolerance to impact damage, and (6) manufacturing processes within the state of the art.

Carbon-carbon composites consist of a fibrous carbon substrate in a carbonaceous matrix. Although both constituents are the same element, this fact does not simplify composite behavior because the state of each constituent may range from carbon to graphite. Crystallographic carbon, namely graphite, consists of tightly bonded, hexagonally arranged carbon layers that are held together by weak van der Waals forces. The single crystal graphite structure is illustrated in figure 2 (ref. 1). The atoms within the layer plane or basal plane (a-b direction) have a covalent bond strength of w524 kJ/mol (ref. 2), while the bonding energy between basal planes (c direction) is ss7 kJ/mol (ref. 3). The result is a crystal that is remarkable in its anisotropy, being almost isotropic within the basal plane but with c direction properties that differ by orders of magnitude. On a larger scale, carbon,

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