Carbon Fibres, Carbon-Polymer Composites and Carbon-Carbon Composites

Carbon-Polymer composites consist of carbon fibres, either unidirectional, woven or knitted or 3-Dimensional embedded in a polymer matrix. The polymer matrix can be either a thermosetting resin such as epoxy or a thermoplastic resin such as PEEK. While woven carbon fibre is available un-coated for use in wet lay up manufacturing, the majority of carbon fibre is pre-coated or pre-impregnated (pre-preg) with the polymer. Carbon-carbon composites consist of highly-ordered graphite fibers embedded in a carbon matrix. C-C composites are made by gradually building up a carbon matrix on a fiber preform through a series of impregnation and pyrolysis steps or chemical vapor deposition. C-C composites tend to be stiffer, stronger and lighter than steel or other metals.

PRODUCTION PROCESSES - Carbon Fibres

The production of carbon based composites begins with the production of the carbon fibers. Carbon fibers are long bundles of linked graphite plates, forming a crystal structure layered parallel to the fiber axis. This crystal structure makes the fibers highly anisotropic, with an elastic modulus of up to 5000GPa on-axis versus only 35GPa off-axis. Fibers can be made from several different precursor materials, and the method of production is essentially the same for each precursor: a polymer fiber undergoes pyrolysis under well-controlled heat, timing and atmospheric conditions, and at some point in the process it is subjected to tension. The resulting fiber can have a wide range of properties, based on the orientation, spacing, and size of the graphite chains produced by varying these process conditions.

Precursor material is drawn or spun into a thin filament. The filament is then heated slowly in air to stabilize it and prevent it from melting at the high temperatures used in the following steps. The stabilized fiber is placed in an inert atmosphere and heated to approximately 1500°C to drive out the non-carbon constituents of the precursor material. This pyrolysis process, known as carbonization, changes the fiber from a bundle of polymer chains into a bundle of "ribbons" of linked hexagonal graphite plates, oriented somewhat randomly through the fiber. The length of the ribbons can be increased and their axial orientation improved through further heating steps up to 3000°C, a process called graphitization. Because the graphite ribbons are bonded to each other perpendicular to the fibres only by weak Van der Waals bonds, the ribbons must be reoriented to increase the tensile strength of the fiber to a useful level. This is accomplished through the application of tension at some point in the stabilization or pyrolysis phases, the exact time depending on the precursor material. Increased axial orientation increases the fiber's tensile strength by making better use of the strong covalent bonds along the ribbons of graphite plates.

There are three principal precursor materials for carbon fibers, of which polyacrylonitrile (PAN) and rayon are the most common. PAN is stretched during the stabilization phase, and heated to 250°C in air. The tension is then removed, and the fiber is heated slowly in an inert nitrogen atmosphere to 1000-1500°C. Slow heating maintains the molecular ordering applied by tension during the stabilization phase. Graphitization at temperatures up to 3000°C then follows. Applying tension at 2000°C further increases the proper ordering of graphite ribbons. Rayon, a cellulose-based fiber made from wood pulp, is spun into a filament from a melt, and stabilized without tension up to 400°C. It is then carbonized without tension up to 1500°C, and is stretched in the graphitization phase up to 2500°C. Pitch, as PVC, coal tar or petroleum asphalt, is also used as a precursor. The pitch material is spun into a filament from either a simple melt or from a liquid crystal "mesophase" melt (obtained by heating the melt above 350°C for an extended period). If the precursor filament is obtained from the simple melt, it must have tension applied during the graphitization phase. When a mesophase melt is used, the spinning process imparts a high degree of order to the resulting graphite, so tension need not be applied during the pyrolysis phases. Filaments from pitch are otherwise processed into fibers by the same process as other precursors. Polyvinyl alcohol, polyamides and phenolics may also be used as precursors. Precursors must be able to be carbonized without melting.

Properties of PAN-based carbon fibres - available in 6000 or 12000 fibres/tow

PRODUCTION PROCESSES - Carbon-Polymer Composites

• Wet Lay-up
• Pre-preg (Epoxy)
• Pre-preg (Thermoplastic)
• Pultrusion
• Filament Winding
• Diaphragm Forming
• Injection Moulding

PRODUCTION PROCESSES - Carbon - Matrix

The second phase in carbon-carbon composite production is the building up of the carbon matrix around the graphite fibers. There are two common ways to create the matrix: through chemical vapor deposition and through the application of a resin.

Chemical vapor deposition (CVD) begins with a preform in the desired shape of the part, usually formed from several layers of woven carbon fabric. The preform is heated in a furnace pressurized with an organic gas, such as methane, acetylene or benzene. Under high heat and pressure, the gas decomposes and deposits a layer of carbon onto the carbon fibers. The gas must diffuse through the entire preform to make a uniform matrix, so the process is very slow, often requiring several weeks and several processing steps to make a single part. In the second method a thermosetting resin such as epoxy or phenolic is applied under pressure to the preform, which is then pyrolized into carbon at high temperature. Alternatively, a preform can be built up from resin-impregnated carbon textiles (woven or non-woven) or yarns, then cured and pyrolized. Shrinkage in the resin during carbonization results in tiny cracks in the matrix and a reduction in density. The part must then be re-injected and pyrolized several times (up to a dozen cycles) to fill in the small cracks and to achieve the desired density. Densification can also be accomplished using CVD.

A limiting factor on the use of carbon-carbon composites is the manufacturing expense associated with these slow and complex conventional methods. In response, two less-expensive alternative methods for building up the carbon matrix have been developed. The first is a forced-flow/thermal gradient process developed at the Georgia Institute of Technology in Atlanta, and is a variation on CVD. This method deposits carbon matrix up to 30% faster than conventional methods, and allows thicker items to be produced. Carbon-bearing propylene, propane or methane is forced under pressure through the preform while it is heated in an oven at 1200°C. A temperature gradient in the material forces vapor to flow through the preform, ensuring the even formation of the matrix. Vapor infiltration and carbon deposition are faster with this method, so parts up to 1cm thick can be produced in as little as eight hours. Parts up to 2cm thick (with material properties comparable to CVD-produced parts) have also been produced. Because the process itself ensures uniform vapor infiltration, it can be run under a wider range of operating conditions than CVD -- the process is less dependent on precise heating, pressure and timing conditions. In the future, this flexibility may even allow the addition of graphitization catalysts and oxidation preventers during production, thereby eliminating a separate treatment. T he second alternative method was developed by the Across Company of Japan, and is a variation on pre-impregnated or "pre-preg" materials used to create a preform. Graphite yarns are coated with graphite precursor powders made from coke and pitch, and are then sealed in a flexible thermoplastic sleeve to protect the powder coating during handling and manufacture. The treated yarn can then be woven into sheets or chopped into short fibers and applied to a mold. The laid-up form is then hot-pressed to make the composite part. Yarns can also be processed into tubes, rods, cloth, thick textiles, unidirectional sheets and tapes. Better penetration of the matrix into fiber bundles ensures uniform properties in the composite and higher strength than conventional composites. Fewer densification steps are needed, so manufacturing time and costs are reduced.

GENERAL PROPERTIES

The most important class of properties of carbon-carbon composites is their thermal properties. C-C composites have very low thermal expansion coefficients, making them dimensionally stable at a wide range of temperatures, and they have high thermal conductivity. C-C composites retain mechanical properties even at temperatures (in non-oxidizing atmospheres) above 2000°C. They are also highly resistant to thermal shock, or fracture due to rapid and extreme changes in temperature. The material properties of a carbon-carbon composite vary depending on the fiber fraction, fiber type selected, textile weave type and similar factors, and the individual properties of the fibers and matrix material. Fiber properties depend on precursor material, production process, degree of graphitization and orientation, etc. The tensioning step in fiber formation is critical in making a fiber (and therefore a composite) with any useful strength at all. Matrix precursor material and manufacturing method have a significant impact on composite strength. Sufficient and uniform densification is necessary for a strong composite. Generally, the elastic modulus is very high, from 15-20GPa for composites made with a 3D fiber felt to 150-200GPa for those made with unidirectional fiber sheet. Other properties include low-weight, high abrasion resistance, high electrical conductivity, low hygroscopicity, non-brittle failure, and resistance to biological rejection and chemical corrosion. Carbon-carbon composites are very workable, and can be formed into complex shapes.

Shortcomings

The chief drawback of carbon-carbon composites is that they oxidize readily at temperatures between 600-700°C, especially in the presence of atomic oxygen. A protective coating (usually silicon carbide) must be applied to prevent high-temperature oxidation, adding an additional manufacturing step and additional cost to the production process. The high electrical conductivity of airborne graphite particles creates an unhealthy environment for electrical equipment near machining areas. Carbon-carbon composites are currently very expensive and complicated to produce, which limits their use mostly to aerospace and defense applications.

Properties of Carbon-Carbon Composites

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