CARBON–CARBON COMPOSITES .1 i nTroducTion

A.3.5 Slower Reaction Controls [C]

5.8 CARBON–CARBON COMPOSITES .1 i nTroducTion

5.8 CARBON–CARBON COMPOSITES

the thermal conductivity along the a-axis of the structure is on the order of 100 W/m-k while in the c-direction it is on the order of 1 W/m-k. This anisotropy in properties can be utilized to optimize the performance of a part such as a rocket nozzle. This is the kind of carbon or graphite deposition that is used in CVI to infiltrate and bond together the carbon fibers in a carbon–

carbon composite.

5.8.4 f

aBricaTionof

c

arBon

f

iBerS

By far, the majority of carbon fibers are used to reinforce polymers to form carbon-fiber- reinforced-polymers, CFRPs. These composites are now used not only in military aircraft but also in civilian aircraft structural parts, as automotive parts, and as high-technology bicycle frames as well. They are produced by many manufacturers by largely proprietary processes. Nevertheless, there are several principles that are fairly common in making fibers. They are all made from carbon- rich polymer resins, an example being polyacrylonitrile (PAN), as shown in Figure 5.24. The poly- mer is mixed with a solvent to form a viscous material that is drawn into fibers. This drawing process helps to align the polymer molecules and, what will become, the graphite rings parallel

Substrate CH₄= C(s) + 2 H₂

c-axis

a-axis

Columnar grains

Growth direction

FIGURE 5.23 Schematic microstructure of a pyrolytic graphite deposit depicting the columnar grain structure from the decomposition of methane. The c- and a-axes of the graphite crystals are shown.

The columnar grains extend through the thickness of the deposited layer that may be as thick as several centimeters. Because of this strong preferred orientation, properties such as thermal conductivity along the a-axis may be as much as 100 times larger than that along the c-axis.

CNn

N N

low T PAN1 to PAN2

300°C, air

−H₂O

2000°C, argon

n n

−N₂, graphite

C C

H

H C

H

N

CN n Polymerization

Acrylonitrile Polyacrylonitrile (PAN)

FIGURE 5.24 The formation of polyacrylonitrile (PAN) and its multistep thermal transformation to graphene sheets in carbon/graphite fibers.

to the long axis of the fiber. The solvent is then removed by extraction in another solvent or by evaporation. The polymer fiber is given a low temperature treatment in oxygen or air to remove hydrogen and finally a high temperature graphitization reaction in excess of 2000°C to remove the nitrogen from the structure and form graphene sheets whose a-axes lie parallel to the fiber axis. During heat treatment, tension is applied to the fiber to enhance the alignment of the graph- ite sheets along the fiber axis increasing the modulus of elasticity of the fiber (Buckley and Edie 1992). The finished fibers are about 8 µm in diameter, have elastic moduli of about 300 GPa (43 × 10⁶ psi) and fracture strengths on the order of 3.1 GPa (443,000 psi) (Matthews and Rawlings 1994). Compare these values to those of mild carbon steel with an elastic modulus about 200 GPa (26 × 10⁶ psi) and a tensile strength around 430 MPa (61,000 psi) (Ashby and Jones 1980). Carbon fibers are sold commercially in bundles or yarns or tows consisting of several thousand individual carbon fibers.

5.8.5 c

^ompoSiTe

f

^aBricaTion

Carbon–carbon composites are made from woven carbon fibers yarns—as bundles or fabric—that are formed into the shape of the part to be made—the prepreg (Savage 1993). One process consists of infiltrating the porous fabric with a phenolic resin (or any number of carbon-rich resins)—a ther- mosetting resin produced by the reaction of formaldehyde, CH2O, and phenol, HOC6H5—cured and pyrolyzed (heated in a nonoxidizing atmosphere to drive all volatile gases leaving only carbon behind) to graphite at some elevated temperature in excess of 2000°C. The graphite forms a rigid bond between the fibers producing a solid piece.

A variant is to use a CVD process to deposit the graphite in the interstices between the carbon fibers, such as in Equation 5.49 (Savage 1993):

CH g4 temperature C s 2 H g);( G 1500 C 103kJ mol.

2 o

( )

^{→}

( )

^{+ ∆}

(

^°

)

^{= − /}

This process is chemical vapor infiltration (CVI) and is a variation of the CVD process in which the conditions are controlled, so that surface reaction is the slower step and is rate controlling. This ensures that the reactant gas concentrations are essentially uniform throughout the fibrous part—

since diffusion is relatively fast—and the deposition rate will be the same throughout the part. This also implies that low pressures are preferred since diffusion in gases varies inversely with the pres- sure as will be seen later. Figure 5.25 shows a schematic microstructure of a typical carbon–carbon composite infiltrated by CVI. Usually, several infiltrations by CVI are necessary as the surface reac- tion is still sufficiently fast to close the surface porosity and stop the reaction. As a result, the part must be cooled and the surface ground to remove the dense graphite layer exposing the porosity in the structure to allow additional CVI.

50 μm Graphite

fiber CVI layer

Braid or tow Porosity

FIGURE 5.25 Schematic microstructure of a carbon–carbon composite showing three braids or tows of car- bon fibers woven together in different directions showing: carbon fibers, CVI carbon layer, and residual porosity.

For the carbon–carbon composites used on the space shuttle, the outer surfaces of the parts are given a CVD coating of SiC by a similar reaction to close any remaining porosity and slow down the rate of oxidation of the carbon–carbon

SiHCl g3

( )

⁺CH g4

( )

SiC s

( )

⁺H g2

( )

⁺3HCl g

( )

; DG^o

(

1200^{° = −}C

)

100kJ/mmol.

Here, diffusion control of the reaction is important, so that all of the reaction takes place at the sur- face and sealing it. Unfortunately, SiC has a larger thermal expansion coefficient than that of graph- ite, and the SiC layer cracks on cooling. The cracks are filled with TEOS—tetraethyl orthosilicate or tetraethoxysilane—Si(C2H5O)4, which reacts with water at room temperature to form SiO2 (glass) that seals the cracks at high temperatures.^* When completed, a typical carbon–carbon composite still contains about 15 volume percent porosity (Buckley and Edie 1992).

5.9 GENERAL OBSERVATIONS ABOUT