Post on 10-Apr-2018
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UNIT V
ADVANCES IN COMPOSITES
Carbon / Carbon Composites
- B. Anand Ronald
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UNIQUE PROPERTIES OF CARBON MATRIX:
It remains solid @ higher temperatures than anyother material
Its sublimation point at atmospheric pressure isabove 3600rC and its strength increases with
temperature upto this point. It resists radiations, has high thermal emissivity
and excellent high temperature dimensionalstability.
Carbon is exceptionally inert to chemicals andreacts only at high temperatures with H2, O2, S, Cl2and F2.
It is self lubricating (because of graphite).
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C/ C composites can withstand higher temperaturethan other materials, but in OXIDIXING Atmosphere,it oxidizes at much lower temperatures.
In metals, that withstand high temperatures a tough
oxide coating is formed on the surface in oxidizingatmosphere. This oxide layer prevents oxygen fromentering into the material and prevents furtheroxidation. Carbon does not have this ability (asoxidation products are gases CO, CO2). Therefore itmust be protected from oxidation using coatings andimpregnates that block O2 from penetrating.
The process of making C/ C composites is costly andtime consuming.
Unlike other ceramics, carbon powder is nonsinterable and therefore cannot be processed byconventional process (Pressing/ Sintering)
LIMITATIONS OF C/ C Composites:
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Carbon-carbon composites applications in
aircraft and F1 braking
rocket motor nozzle throats and exit cones
nosetips/leading edges Re-entry thermal protection systems
Heat Shield and Nozzle for Rockets andMissiles
Bolts and nuts for graphite refractory lining Gas Turbine engine nozzles
NOTE: For CVD technique refer Chawla For Sol gel , refer my PPT
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FABRICATION OF C/ C composites
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Carbon - carbon exit cones for rocket motors- Snecma Propulsion Solide
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