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Monday, March 14, 2011

Carbon-Carbon Composite (CCC)

One of the most advanced and promising engineering material is the carbon fiber reinforced carbon-matrix composite, often termed a carbon–carbon composite; as the name implies, both reinforcement and matrix are carbon. These materials are relatively new and expensive and, therefore, are not currently being utilized extensively. Their desirable properties include high-tensile moduli and tensile strengths that are retained to temperatures in excess of 2000oC (3630F), resistance to creep, and relatively large fracture toughness values. Furthermore, carbon–carbon composites have low coefficients of thermal expansion and relatively high thermal conductivities; these characteristics, coupled with high strengths, give rise to a relatively low susceptibility to thermal shock. Their major drawback is a propensity to high-temperature oxidation.


Application of Carbon Carbon Composites

The development of Carbon Carbon materials began in 1958 and was nurtured under the US Air Force space plane program, Dyna-Soar and NASA's Apollo projects. It was not until the Space Shuttle Program that CC material systems were intensively researched. The criteria that led to the selection of CC composites as a thermal protection system were based on the following requirements:

1. Maintenance of reproducible strength levels at 1650°C (3002°F);
2. Sufficient stiffness to resist flight loads and large thermal gradients;
3. Low coefficient of thermal expansion to minimize induced thermal stresses;
4. Oxidation resistance sufficient to limit strength reduction;
5. Tolerance to impact damage;
6. Manufacturing processes within the state of the art

An example of CC composite applications is a one-piece, bladed turbine rotor that, in service, is coated to prevent oxidation. The rotor offers higher temperature performance without cooling; low weight and use of low-cost, non-strategic materials. Other gas turbine engine applications using CC composites include exhaust nozzle flaps and seals, augmenters, combustors and acoustic panels. CC material systems using coatings, TEOS and additions to the basic CC recipe have improved the oxidation resistance of products made of CC composites by an order of magnitude.

These composites are being used in products such as the nozzle in the F-100 jet engine afterburner, turbine wheels operating at >40 000 rpm, non-wetting crucibles for molten metals, nose caps and leading edges for missiles and for the Space Shuttle, wind-tunnel models and racing car and commercial disk brakes.

Carbon Carbon Composite Plate from SINOTEK MATERIALS

Sunday, March 6, 2011

Gelcoat and Gelcoating - Method, Procedure and Tips

Gel coating is usually polyester resin that formulated to give a protective layer of waterproof color coat. Its main function is to protect the subsequent layer from defects. Some gelcoating resin is colored while it also available in original clear form, and pigment can be added according to users need.

Gelcoating method and procedures

The composite work is started with the preparation of the mould . The mould shall first be cleaned, followed by polishing that normally end up with shiny surface. The suitable wax is then applied to the mold surface that will help to release the finished component when cured. The wax usually applied between two to three times before the application of gel coat. When the release agent is completely dry, the gelcoat is applied by brush or spray. When choosing brush, a wide brush with long soft hairs is preferred. Two coats are generally necessary to prevent brush marks from showing. The second coat being applied once the first has sufficiently cured .

Gel coat is applied to about 0.3-0.5mm (Thickness vary manufacturer to manufacturer and from user to user) one to two layers. The operator must ensure the coat in total contact with the contour before starting the lay-up. Gel time of the gelcoat in bulk should preferably be about 15 minutes.

The condition of the gelcoat can be determined by touching it, once it feel tacky and not easily removed by finger, the next coat resin system is ready. In some cases, surfacing tissue may be used to reinforce the gelcoat.

After the tackiness has been achieved, the first layer of reinforcement (fibre) and impregnation of resin can be start.

Tips on Gelcoating :

1. To avoid uneven colour of the finish product, pigment can also be added to the resin – same pigment colour that being used in the gelcoat. (If you are using it)

2. Please refer to the Materal Data Sheet to determined the ratio of catalyst that will be mix with gelcoat / resin.
That shiny surface is a layer of gelcoat. The person in the image is repairing the gelcoat surface. (Image from http://continuouswave.com/ )