Carbon

Carbon fibre is the reinforcement material of choice for "advanced" composites, particularly following significant price reductions over the past decade. A major advantage of carbon fibres is their higher fatigue resistance compared to glass or Aramid. Unlike these last two materials, carbon fibres do not suffer from stress rupture. Carbon fibres are supplied in tows and may vary from 1000 fibres per tow to hundreds of thousands of fibres per tow.

Untreated carbon fibres do not wet easily, so adhesion to the matrix must be achieved by good mechanical interference coupled with surface treatment of the fibre which allows chemical bonding between the fibre and the matrix. Careful fabrication is required, however, to produce good materials as strong bonds cannot be achieved.

Some polymer composites for electrical purposes such as shielding do not require significant structural strength. The attractive property for such materials is the high conductivity of carbon fibre. Most carbon fibres are derived from polyacrylonitrile, but for even higher conductivity, fibres derived from pitch can have three times the conductivity of copper.

At room temperature the coefficient of thermal expansion of carbon fibres along their axes is slightly negative. Above about 700C the coefficient becomes positive.

Carbon fibre properties depend on the structure of the carbon used. Typically they come defined as standard, intermediate and high modulus fibres. Indicative materials properties are:

Ref ASM Vol 21 Handbook.