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Tuesday, 19 December 2017
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Finishing and surface treatment Print E-mail
Depending on the hardness of the carbon material electro-corundum wheels with grain size 120-160 μm and hardness P-Z are used.
Due to its manufacture, a carbon artifact contains open pores, which may make up one quarter of its volume. By filling these pores, the density, strength and conductivity of the artifacts can be increased to predetermined levels. Closing the surface pores will also reduce oxidation. Impregnating agents are usually pitch, resins and metals, which are brought into the formed bodies by a vacuum/pressure impregnating cycle. Pitch-impregnated artifacts have to be re-baked in order to carbonize the pitch, whereas resin-impregnated parts are either thermally cured and/or carbonized. The re-baking step also causes new pores to form, so that at least one more impregnating operation is necessary if a high degree of gas or liquid-tightness is requested. Usually, completely gas or liquid-tight grades are manufactured by an impregnation with furan or phenolformaldehyde resins, which are subsequently thermally cured. This resin impregnation, however, reduces the temperature stability of such grades to a maximum of only 200°C or slightly above.
This limit may be increased by approximately 100 degrees by means of impregnating agents that have a higher thermal resistance, e.g. polytetrafluoroethylene waxes. Wax, grease, oil, and salts play an important role as impregnating agents for special applications -in particular for carbon brushes. Not only the physical properties of the grades but also their operating behavior in electrical service can be improved. Upon pyrolysis of gaseous hydrocarbons, so-called pyrocarbon can be deposited in the pores or on the surface of the substrates so that the density, strength and corrosion resistance of the artifacts are also considerably increased. Oxidation resistance up to elevated temperatures of approximately 800°C may be reached by impregnation with borates or phosphates, whereas an efficient protection against oxidation at higher temperatures may only be achieved by coating with silicides, borides, carbides or nitrides.
Superfinishing
is applied to obtain high grade surface finish and may be performed by sundry means like
Honing
Silicon carbide stones of grain size 69-99 um are used. In special cases for hard carbon it is also possible to use diamond coated honing strips depending on the required level of surface finish. Honing is done using the standard honing oils.
Lapping
This process uses lapping powder of aluminium oxide and silicon carbide in grain sizes 1 2-1 6 pm.
Polishing
It is essential to achieve a high degree of lapped finish beforepolishing is carried out. Diamond powder in grain sizes 6-10 pm is used. After machining it is essential to remove the media used (honing oil, lapping and polishing powder) from the work parts.
Ultrasonic dust removal
Where dust-free surfaces are required, ultrasonic dust removal can be carried out. The media used for this are water, distilled water or solvents.

 
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