General considerations of pyrolytic graphite

The production of molded graphite and vitreous carbon, described in other articles, relies on the carbonization of a solid, inorganic substance such as coal-tar pitch, petroleum fractions or polymers. This article is a review of another type of carbon material, produced by a fundamentally different process that is based on a gaseous precursor instead of a solid or liquid. The process is know as chemical vapor deposition and the product as pyrolytic carbon or graphite, some times referred to as pyrocarbon or pyrographite. To simplify, in this article the material will be referred to as pyrolytic graphite, regardless of the degree of graphitization.

Pyrolytic graphite is different from another standpoint: although produced in bulk form, its main use in the form of coatings, deposited on substrates such as molded graphite, carbon fibers, or porous carbon-carbon structures. As such, it is part of a composite structure and is not as readily identifiable as other forms of carbon. It is similar in this respect to CVD diamond and diamond-like carbon.

Pyrolytic graphite is the only graphitic material that can be produced effectively as a coating. The coating can be made sufficiently thick that, after removing the substrate, a free-standing object remains.

Pyrolytic graphite is a key element in the technology of carbon and is used extensively in the coating of specialty molded graphites and in the processing of carbon-carbon composites.

Historical perspective: The CVD of carbon materials is not new. Its first practical use was developed in the 1880’s in the production of incandescent lamps to improve the strength of filaments by carbon deposition and a patent was issued over a hundred years ago, covering the basis of the CVD of carbon.

The CVD process developed slowly in the next fifty years, and was limited mostly to pyro and extraction metallurgy, and little work was done on graphite deposition.

It is only since the end of World War two that the CVD of graphite began to expand rapidly as researchers realized the potential of this technique for the formation of coatings and free-standing shapes. The importance and impact of pyrolytic graphite have been growing ever since.

The Chemical Vapor Deposition process: CVD is now a well-established process that has reached major production status in areas such as semiconductors and cutting tools. It is a vapor-phase process which relies on the chemical reaction of a vapor near or on a heated surface to form a solid deposit and gaseous by-products. The process is very suitable to the deposition of carbon, as reviewed below.

Pyrolytic graphite as a coating: Although, as mentioned above, pyrolytic graphite is used by itself as free-standing structures such as crucibles or rocket nozzles, its major use is in the form of coating on substrates such as molded graphite, carbon foam, carbon fibers, metals, and ceramics.

Composite Nature of coatings: The surfaces of many materials exposed to the environment are prone to the effects of abrasion, corrosion, radiation, electrical or magnetic fields, and other conditions. These surfaces must have the ability to withstand these environmental conditions and/or provide certain desirable properties such as reflectivity, semi-conductivity, high thermal conductivity, or erosion resistance.

To obtain these desirable surface properties, a coating is deposited on the bulk material to form a composite in which bulk and surface properties may be very different.

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