Fibres
Fibre choice is important when it comes to toughening a material. Fibres that are strong, will bond sufficiently well to the matrix and have the appropriate dimensions are required. In addition, the cost of processing the fibres needs to be taken into account: if the fibres are too expensive, they will not be practical for widespread use.
Examples of fibres used as reinforcing materials are polyacrylonitrile (PAN), carbon fibres, glass fibres and silicon carbide fibres.
Polyacrylonitrile (PAN) fibres
PAN is similar to polyethylene, but in every other CH2 group, one of the two hydrogen on the backbone is replaced with a nitrile group, -C≡N. Therefore, PAN is represented by the chemical formula (CH2CHCN)n, or, more succinctly, (C3H3N)n. PAN fibres are used in textiles where they are known as acrylic fibres. They are also used as precursor fibres to carbon fibres.
Carbon fibres
Carbon fibres are most often produced from PAN precursors, These precursors are stretched and pyrolyzed, i.e., heated in the absence of oxygen to decompose the fibres into carbon-rich residues – carbon fibres. In these fibres, graphitic planes are preferentially aligned parallel to the fibre axis., This gives the fibres very high stiffness along the fibre axis, but a low transverse stiffness. Depending on the production method, carbon fibres are referred to as either high modulus (HM) or high strength (HS) fibres. Typical fibre diameters are 5 μm.
Glass fibres
These are fibres made of a silica base with oxide additions. They are amorphous. The addition of oxides lowers the glass transition temperature of the silica by disrupting the rigid tetrahedral structure of silica that would otherwise form. This lower glass transition temperature makes the material easier to draw, but it reduces the maximum use temperature of the fibres relative to that of pure silica.
The fibres are made by melting the raw materials together and then pouring the viscous liquid through holes so that the fibres are formed under gravity. These fibres are easily susceptible to surface damage which would reduce their strength because this surface damage introduces surface flaws. To help prevent such surface damage, the fibres are often coated with an emulsifying polymer. This polymer protects the surface of the glass, and also acts as a coupling agent to help bond the fibre with the matrix by providing strong chemical bonds across the fibre-matrix interfaces.
Silicon Carbide
Silicon carbide in its crystalline form has strong sp3 bonding like diamond. Silicon carbide fibres have the desirable features of low density and high stiffness, together with good thermal conductivity. However, because of the way in which silicon carbide fibres are produced from organic precursors for reinforcement of ceramic composites, such fibres are more accurately described as Si-C-O fibres as they contain significant quantities of SiO2 and C. These Si-C-O fibres are microcrystalline / nanocrystalline materials.
Typically, silicon carbide fibres are 5 μm in diameter when used as reinforcements in ceramic composites and 100-150 μm in diameter, and higher purity, when used as reinforcements in metal matrix composites. The higher diameter fibres have a carbon or tungsten core around which the silicon carbide is deposited by chemical vapour deposition, and are purer than the finer Si-C-O fibres.
Carbon Nanotubes
Carbon nanotubes are very thin rolled up sheets of graphene which have very high tensile strengths because of their lack of grain boundaries and lack of large flaws − they cannot have large flaws because their diameter is too small for large flaws to exist. However, they are expensive to produce, and they are not suitable for toughening matrices. Therefore, despite their seemingly attractive mechanical properties, they are not seen in everyday use in composite materials designed for high toughness.
Toughness
The main way in which fibres toughen a material is by fibre pull out, rather than by exploiting the strength of the fibre itself. This means that larger diameter fibres, such as those used to toughen metal matrices, are more suitable than short fine fibres, such as carbon nanotubes, which are not suitable. A simple way of appreciating this is to recognise that larger diameter fibres each have a larger possible process zone volume either side of any propagating transverse crack to help absorb the energy of crack propagation, thereby helping to increase the toughness of the composite material.