Coating processes – Coating by vapor – gas – or smoke – Carbon or carbide coating
Patent
1996-04-25
1998-08-04
King, Roy V.
Coating processes
Coating by vapor, gas, or smoke
Carbon or carbide coating
427122, C23C 1626, C04B 3583
Patent
active
057890269
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chemical vapor infiltration (CVI) method of infiltrating a pyrolytic carbon matrix into a porous substrate.
The field of application of the invention is more particularly that of manufacturing parts of composite material having fiber reinforcement and a carbon matrix. The fibers are carbon or graphite fibers or fibers of a refractory material, e.g. a ceramic, optionally coated in carbon or graphite.
Chemical vapor infiltration is used to form a deposit of pyrolytic carbon on the fibers of the substrate, throughout the volume thereof, so as to bond the fibers together and fill in the pores initially in the substrate.
To perform chemical vapor infiltration, the fiber substrate is placed in an enclosure. A gas constituting a precursor of carbon is admitted into the enclosure. Under determined conditions of temperature and pressure, the gas diffuses into the substrate and forms the deposit of pyrolytic carbon on the fibers. By way of example, the gas may be constituted by a hydrocarbon or by a mixture of hydrocarbons producing pyrolytic carbon by decomposing on coming into contact with the fibers of the substrate.
Several types of CVI method are in existence: the constant temperature and pressure method, the pressure gradient method, and the temperature gradient method.
In the constant temperature and pressure method, the substrate to be densified is placed in an isothermal enclosure. Heating is provided, e.g. by means of a graphite susceptor or core surrounding the enclosure and itself surrounded by an induction winding. Energy is applied to the substrate essentially by radiation from the enclosure. The temperature inside the enclosure is regulated to the desired value by controlling the current in the winding, while the pressure is adjusted by connecting the enclosure to a vacuum source and controlling the rate at which the gas is admitted into the enclosure. Matrix material is deposited inside the substrate and on the surface thereof. The temperature and pressure are selected to have values that are only slightly greater than those required for a deposit to form, so as to avoid massive deposition on the surface of the substrate occurring immediately on contact with the gas, since that would quickly lead to the surface pores being shut off, thereby preventing densification taking place within the substrate.
Nevertheless, it is inevitable that the surface pores will be closed progressively, thereby stopping the densification process before it is complete within the core of the substrate. It is then necessary to remove surface crust by machining so as to reopen the array of pores and continue densification. Several intermediate crust-removal operations may be necessary on a single piece prior to achieving the desired degree of densification.
By accurately controlling infiltration conditions, that method makes it possible to obtain a matrix of desired quality, and to do so in reproducible manner. It also has the major advantage of enabling a plurality of pieces of various shapes to be densified simultaneously within the same enclosure.
In spite of these advantages which justify its use on an industrial scale, the constant temperature and pressure method suffers from drawbacks of lengthy duration and large cost, in particular when manufacturing composite pieces of great thickness. Densification requires deposition to take place slowly, and thus requires cycles of long duration. In addition, the intermediate machining operations for crust removal give rise to losses of material and contribute to increasing cost price, with alternation between infiltration and crust removal lengthening the total duration of manufacture and increasing its cost. Finally, in particular for pieces of great thickness, it is inevitable that a considerable densification gradient remains within a given piece, with the degree of densification being significantly less deep within the piece than at its surface.
The pressure gradient method uses a forced
REFERENCES:
patent: 5348774 (1994-09-01), Golecki et al.
"A Review of CVD Carbon Infiltration of Porous Substrates", W.V. Kotlensky, 16th National Sampe Symposium, Apr. 21, 1971, Anaheim, U.S.A., pp. 257-265 .
Delperier Bernard
Domblides Jean-Luc
King Roy V.
Societe Europeene de Propulsion
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