handbook of carbon, graphite, diamond and fullerenes

210 Carbon, Graphite, Diamond, and Fullerenes
surface treatment on the mechanical properties of carbon-carbon composites
is complex and still not clearly understood at this time. 124 '
Matrix Materials and Processing. In most cases, the starter matrix
material is a high-carbon-yield polymer such as a phenolic (reviewed in Ch.
6, Sec. 2.1). Other polymers are under investigation, including
polyarylacetylene and aromatic diacetylene, the latter giving an unusuallyhigh
charyield of 95%. [25][26 1 Most polymers are considered essentially nongraphitizable
(see Ch. 4, Sec. 3.5).
Processing is lengthy, difficult, and expensive. Typical steps in the
processing of a 20 phenolic-carbon fiber composite are as follows:
1. Vacuum bag/autoclave cure to 150°C in an 8-hour cycle
2. Rough trimming and inspection
3. Post-cure in a restraining fixture to 265°C for 7 days
4. Pyrolysis (carbonization) in a steel retort packed with
calcined coke in an inert atmosphere, the cycle consisting
of 50 hours to reach 820°C, 20 hours at 820°C and 20
hours cool-down
5. Impregnation with furfuryl alcohol in an autoclave, followed
by a 2-hour cure at 150°C and a 32-hour post-cure at
210°C
6. Pyrolysis, repeat of step 4
7. Impregnation, cure, and pyrolysis cycles repeated for a
total of three times
8. Graphitization heat treatment (>2000°C) if required
9. Coating with silicon carbide (SiC) for oxidation protection
(optional)
Processing may vary depending on the characteristics of the starter
materials, the geometry of the part, and other variables. 1271 Pyrolyzation
temperature often reaches 1000°C. The chemistry of the pyrolysis process
is reviewed in Ch. 4, Sec. 2.0.
Chemical Vapor Infiltration: Instead of a carbonized polymer, the
matrix material can be a pyrolitic carbon obtained by the thermal decomposition
of a hydrocarbon gas, usually methane (CH4) or propane (C3H8). This
process is known as chemical vapor infiltration (CVI) and is described in Ch.
7, Sec. 2.6. Various CVI techniques such as temperature and/or pressure