Module 1: Introduction to Composites
  Lecture 2: Reinforcement: Materials and Forms
 


Carbon Fiber:

The first carbon fibre for commercial use was fabricated by Thomas Edison.

  • Sixth lightest element and carbon - carbon covalent bond is the strongest in nature.
  • Edison made carbon fiber from bamboo fibers.
  • Bamboo fiber is made up of cellulose.
  • Precursor fiber is carbonized rather than melting.
  • Filaments are made by controlled pyrolysis (chemical deposition by heat) of a precursor material in fiber form by heat treatment at temperature of 1000-3000  
  • The carbon content in carbon fibers is about 80-90 % and in Graphite fibers the carbon content is in excess of 99%. Carbon fibre is produced at about 1300 while the graphite fibre is produced in excess of 1900 .
  • The carbon fibers become graphitized by heat treatment at temperature above 1800 .
  • “Carbon fibers” term is used for both carbon fibers and graphite fibers.
  • Different fibers have different morphology, origin, size and shape.
  • The size of individual filament ranges from 3 to 147  .
  • Maximum use of temperature of the fibers ranges from 250 to 2000 .
  • The use temperature of a composite is controlled by the use temperature of the matrix.
  • Precursor materials: There are two types of precursor materials (i) Polyacrylonitrile (PAN) and (ii) rayon pitch, that is, the residue of petroleum refining.
  • Fiber properties vary with varying temperature.
  • Fiber diameter ranges from 4 to10 .
  • A tow consists of about 3000 to 30000 filaments.
  • Small diameter results in very flexible fiber and can actually be tied in to a knot without breaking the fiber.
  • Modulus and strength is controlled by the process. The procedure involves the thermal decomposition of the organic precursor under well controlled conditions of temperature and stress.
  • Cross section of fiber is non-circular, in general, it is kidney bean shape.
  • Heterogeneous microstructure consisting of numerous lamellar ribbons.
  • Morphology is very dependent on the manufacturing process.
  • PAN based carbon fibers typically have an onion skin appearance with the basal planes in more or less circular arcs, whereas the morphology of pitch-based fiber is such that the basal planes lie along radial planes. Thus, carbon fibers are anisotropic.