However, in order to attain this property, proper stabilisation techniques need to be implemented. Adhesion to resins can be improved by means of whiskers manufactured via oxidation of carbon surface with nitric acid. A recent invention claims to have avoided the usage of higher temperatures in imparting the stiffness to the fibres.
The method adopted during the manufacture of the polymeric base for the derivation of the carbon fibres and the kind of heat treatment this had undergone, help determine the end use pattern and suitability to the numerous application possibilities. Youngs modulus and the tensile strength are of greatest importance in indicating the fibre characteristics. However, this kind of description becomes difficult when some precursor materials such as the pitch based carbon fibres are used.
Types of carbon fibres
The end users vary on the basis of utilisation of strength, modulus, high value of strain to failure, compressive strength or impact resistance, inter- laminar shear strength or any other property of the matrix. For example, if the fibre is to be used for making some electrical applications, the above said properties do not play any important role in decision.
Few available varieties of carbon fibres are listed below:
o High heat treated
o Low heat treated
o Pitch based
o PAN based
High modulus fibres generally posses low strain to failure. Lower modulus means that the preferred orientation will be imperfect. Measurement methods available for testing tensile strength depend on the sample - either bundle of fibres or a single filament. The former one gives reliable results. Fibres having high strain to failure, need good adhesion between the matrix and the fibre to deliver a high impact resistance. Lower diameter fibres are preferred in order to acquire a controlled and consistent production. The fibres, at least in their precursor stage, must be as pure as possible, in order to have better end use properties and behaviour. A thorough control of the process parameters and production conditions are mandatory to get the best possible results.
High modulus carbon fibres have literally replaced boron fibres, decades ago. Alloying with metals improves the strength. Nevertheless, it reduces or even destroys certain special properties of the metals. For example, copper is known for its use either in its raw form or alloy form or in reinforced form where it is subjected to severe stress conditions especially in electrical applications. Carbon fibres, when used in combination with polyesters, have exhibited Youngs modulus to the levels of 160 GN/m2, flexural strengths of 0.6 GN/ m2, tensile strengths of 0.8 GN/m2, with density around 1.5 G/m2. Mats or felts of short fibres can be used in thermoplastics as well as in compression mouldings. Carbon black is sometimes added as a lubricant or filler in thermoplastics.
Composites
Fibres have a critical volume fraction. This determines the lowest limit at which strengthening can be attained. Below this value, the strengthening effect will not be reached. Volume loading, in case of carbon fibres, must be equal to 18 - 19 per cent for the strengths of the fibres to equal that of copper. However, application arenas and their requirements decide the ultimate parameters. Very fine fibres are generally preferred as they help achieve both reinforcement and hardening characteristics. Carbon coated silica on copper showed fabulous results when conductivity was the required aspect. Here, it is interesting to note that the density of carbon is much lesser than that of silica. Nickel too runs similar to copper, with respect to its association with carbon fibres, as far as the production processes, the end use characteristics etc, are concerned. Electroplating nickel on to a filament with carbon would give useful composites.
The tensile strength of these composites is comparable with that of even the copper-tungsten composites. Impurities or extraneous materials present may have adverse effects. Carbon fibres in aluminium show poor adhesion properties. Low transverse properties too are experienced. This factor becomes responsible for applying the use of aluminium reinforced by carbon to be more dependent upon the properties of the metal and leaves the characteristics of the matrix, rather secondary, at least in usage patterns where the final produces are required to undergo conditions of high temperatures and stiffness to shearing.
Copper clad aluminium is used widely, instead of the individual metals, in combination with carbon fibres. This is true especially where the restrictions posed by the produce on the volume aspects are nil or not so critical. However, the cost factor also plays a role in the selection procedure. Carbon combined with polyimide materials may replace the otherwise widely used aluminium and its alloys, thanks to the farmers flexural strengths. However, the lightness and the high modulus clubbed with the strength retention characteristics play a vital role in determining the use of carbon fibres. The usage arenas become rather limited for carbon fibres due to these as well as economical factors and the inexperience.
Some of the applications of carbon fibres are:
- Chairs for medical applications
- Compressor blades
- Drive shafts
- Fly wheels
- Frames
- Human hip joint replacement aid
- Human knee ligament replacement facility
- Leaf springs
- Off shore oil drilling equipment pipes
- Seats in airplane
- Super conductors
- Turbine blades
- Ultra centrifuges
- Windmill bladesn
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| Posted : 10/26/2005 |
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