Processing carbon-carbon matrix materials with thermosetting resin matrices

Low pressure impregnation (LPI):

A carbon fiber tow can be impregnated with a thermoset resin system and chopped to a 25-50 mm staple length to give a compression molding compound that can be placed in a mold, heated in a press and cured. LPI is carried out in two discrete stages- carbonization and impregnation. The randomly reinforced composite is carbonized and then vacuum impregnated with additional resin to achieve densification. The impregnation is assisted by the application of about 2Mpa pressure. About four impregnation/carbonization cycles are undertaken to achieve a successful carbon-carbon composite. Initially, the density falls and then increases with the ILSS as the number of cycles increases.

Alternatively, the carbon fiber reinforcement can be converted to a prepreg using the selected matrix resin system. The advantage of this route is that the architecture of the reinforcement can be selected to give the desired composite properties. When the composite is laid up, the direction of reinforcement can be carefully selected to give the required properties in the composite. The prepreg pack is then cured in a heated press, or an autoclave, prior to subsequent densification. Again, due care must be taken during curing to avoid voids due to evolution of gaseous products.

The composite can be made using the pultrusion process, but this is difficult to operate since the resins cure by a condensation reaction. Filament winding enables the fiber to be placed precisely onto a removeable mandrel and achieve a high fiber content. This technique more suited to handling untreated, unsized fiber and is used to make rocket motor nozzles.

Pressure impregnation and carbonization (PIC):

The prepared carbonized composite is initially subjected to a vacuum and introduced into the liquid resin under a pressure of 1-200 Pa for about 20h and recarbonized at 1000-2880C for some 4-12 cycles.

Hot isostatic pressure impregnation carbonization (HIPIC):

If a pitch impregnation is undertaken at atmospheric pressure, the carbon content of the resultant composite will only be about 50%, but if a high pressure is applied, carbon yields are dramatically increased, producing a product with a high density. As the pressure of a coal tar pitch increases, the matrix microstructure changes from a needle-like structure, in which the mesophase is deformed due to bubble percolation, to finally, a coarser more isotropic form, with the increased pressure suppressing bubble formation. The application of pressure induces the mesophase to form at lower temperatures, but at pressures above 200 Mpa, coalescence of the mesophase does not occur, resulting in inferior mechanical properties.

The application of heat to a pitch causes it to soften and flow and hence requires containment during carbonization. The pitch is processed by employing hot isostatic pressure impregnation carbonization in a hot gas autoclave following the cycle. Since high pressure can deform the original preform shape, it can be beneficial to introduce an initial forming stage using the resin prepreg techinique in conjunction with autoclave molding, followed by the first carbonization stage. Hot pressed composites have flattened pores and 5 Pa pressure can cause vertical cracks. After the HIPIC cycles, the material is removed, surface cleaned and finally graphitized at 2400-2700C, depending on the original fiber architecture and the properties required, in an Ar atmosphere.

HIPIC can be undertaken with a conventional autoclave but specialist equipment has been developed to contain the liquid pitch using a differential pressure system to ensure that gas movement is from the outside towards the inside, avoiding sooty deposits and ensuring control of evolved gases. The pressure ensures that the molten pitch is kept within the pores and also increases the carbon yield. The process is carried out slowly, taking 2-3days and is a one-off process and expensive to run, but gives a product with good yield and superior mechanical properties.

Another practice is to place the preform in a special metal container or can, which is evacuated and sealed by electron beam welding. The metal container acts like a pressure bag transmitting pressure throughout the workpiece.

 

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