Selection of materials for carbon-carbon processing (1)- types of reinforcement

Oxidized PAN fiber (opf): A particular advantage of using opf is that a preform can be more readily processed by conventional textile processes than by using the alternative carbon fiber. The blank disks for carbon brakes are produced using modified conventional textile, but are strictly proprietary procedures. Due to the fact that the opf will be converted to a PAN based carbon fiber, the product will be non-graphitic, unsized and untreated.

PAN based carbon fibers: carbon fibers are normally sized for improved handling and if this size is present, it would decompose during the first carbonization stage. Hence it is considered advisable, as a preliminary step, to burn off any size prior to further processing.

Since the matrix and reinforcing fiber in carbon-carbon are both brittle materials, it is n to aim for a weak interface in the composite, permitting cracks to travel through the matrix in order to debond at the fiber/ matrix interface.

The phenolic composites were converted to carbon-carbon from a hydrocarbon. They found that high strength and high strain treated fibers which were used to produce a tough composite in conventional thermoset matrices, gave brittle low strength carbon-carbon composites.

The surfaces of the fibers were examined by X-ray Photoelectron Spectroscopy and a strong correlation was found between the surface oxygen of the fiber and the ILSS of the phenolic resin composite. A relationship was found between the longitudinal flexural strength retained in the carbon-carbon composite and the ILSS of the phenolic composite. Generally, fibers processed above 1500C and untreated yielded stronger, less brittle carbon-carbon composites. The adhesion of the resin char to the fiber affects the densification and with untreated fiber, the resin char shrinks away from the fiber leaving longitudinal fissures around the fiber, which could subsequently be filled with the pyrocarbon. With treated fiber, the resin char is well bonded and shrinks onto the fiber with no longitudinal gaps, but with regularly spaced transverse cracks.

Surface treated fiber can be detreated by removing the phenolic resin from the prepreg, heating in Ar at 1100C, re-impregnating with phenolic resin and making a phenolic composite, which possesses a reduced ILSS, leading to improved carbon-carbon properties.

Pitch based carbon fibers (pbcf): Pitch based carbon fibers are graphitic and will give carbon-carbon composites with higher yields, densities, moduli, thermal conductivities and heat capacities, but will have lower strengths and will be softer than the product made from a PAN fiber. The type of microstructure of the pitch fiber does have an influence on the mode of graphitization of the carbon-carbon composite and a fiber with a parallel sheet-like microstructure pre-stresses the carbon matrix, causing shear and taking up a position in the fiber direction during the graphitization process. Stretching the fibers will result in an expansion of the composite, thus increasing the flexural strength. A pitch based fiber with a sheath and core structure does not expand in the fiber direction, and the flexural strength will decrease.

Cellulose based carbon fibers: Cellulose based carbon fibers to be used for ablative application, which require lower thermal conductivity and enhanced insulating properties than other carbon-carbon materials. The precursor for this product is rayon, which when carbonized, produces rayon based carbon fibers (RBCF). This type of fiber is usually more porous and weaker, which lowers its effective thermal conductivity. At high temperatures, it oxidizes and /or evaporates without disintegration, producing an evaporative cooling effect.

 

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