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Graphite Coated with Glassy Carbon
We offer
graphite coated with glassy carbon, or both impregnated and coated with
glassy carbon. The glassy carbon coating completely seals all exposed surfaces,
while the impregnation seals and penetrates the substrate to more than 0.25².
Benefits
of Glassy Carbon Coating
- Stops out gassing problems
- Stops absorption of contaminants into the graphite porosity
- Dramatically reduces particulate generation common to
graphite materials
- Gives a uniform and continuous carbon surface that enhances
the electrical and thermal uniformity of the graphite substrate
- Withstands fluorine based etch systems significantly
better than SiC coatings
Glassy
Carbon Coating Solves Problems
Typically Associated with Graphite
The
macro structure of graphite is very much like a sponge. About 20% of the
graphite's volume consists of pores and inclusions. Because of the small
sizes of the pores and inclusions for the given pore volume, graphite
actually has a much larger surface area than that dictated by the geometric
surface area of a given part. This allows the graphite to absorb gases
and liquids to a greater extent than continuous solid materials. The porous
nature of graphite will cause problems in processes that require tight
control of the chemical environment. Gases or liquids absorbed in previous
uses of the graphite artifact will out gas over a long period of time
and pollute the critical process environment. The glassy carbon coating
stops absorption and out gassing by sealing off the porosity with a uniform,
continuous carbon coating, thus preventing process complications due to
contamination being introduced from the porosity of the graphite.
Being a porous material, the density of the graphite varies from vicinity
to vicinity with respect to variations in pore sizes and quantity of pores
in a given volume. Changes in density affect the electrical and thermal
conductivity in the associated region. In cases where graphite is being
used as electrodes, variations in density will cause variations in the
electric field. If the electric field is critical in depositing a film
or layer during the process, the thickness uniformity of the deposited
layer will be adversely affected. In a similar manner, if the product
being processed requires uniform heating from the graphite artifact while
in contact with it, optimum process conditions may not be practical due
to variations in thermal conductivity of the graphite surface. When the
glassy carbon coating is applied, a uniform density is obtained in the
surface region of the graphite. This result significantly improves the
uniformity of an electric field being generated and/or of heat being transferred
conductively to the product being processed. If the process being considered
involves the deposition of a film, this means that a more uniform film
thickness can be obtained.
Because grains of graphite have a structure similar to that of mica, graphite
particles are easily rubbed off. This results in unwanted graphite particles
floating around during a process and possibly contaminating the product
being processed. The glassy carbon coating has an amorphous molecular
structure similar to glass. This type of structure does not readily particulate.
By coating the graphite with the glassy carbon coating, the graphite grains
are held in place by the coating and are no longer able to easily rub
off the surface, thus creating a processing environment free of graphite
particles.
Graphite and carbon materials exhibit extraordinary chemical inertness
to over 2,800° C. The exception to this is heating graphite to above 400°
C in the presence of oxygen, water, or other oxygen containing reactants.
Typically coatings on graphite containing silicon, titanium or other non-carbon
components are more reactive to the process chemicals than the graphite
itself. Because the glassy carbon coating is composed only of carbon,
it maintains the same chemical inertness as the graphite substrate. This
reduces complications caused by side reactions with other coating materials.
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