Vapor phase chemical infiltration process of a material into a p

Details Vapor phase chemical infiltration process of a material into a p Vapor phase chemical infiltration process of a material into a p

1996-04-25

1997-07-29

Pinalto, Bernard

Coating processes

Direct application of electrical, magnetic, wave, or...

Chemical vapor deposition

427122, 427228, 427237, 427238, 427249, 427294, 427591, C23C 800

Patent

active

056520301

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a chemical vapor infiltration (CVI) method of infiltrating a material into a porous substrate.
The field of application of the invention is in particular that of manufacturing pieces made of composite material comprising a fibrous substrate or "preform" that has been densified by a matrix. In this application, chemical vapor infiltration is used to form a deposit of the matrix-constituting material on the fibers of the substrate and throughout the volume thereof, in order to bond the fibers together and fill in the pores initially accessible in the substrate. Chemical vapor infiltration may also be used to finish off densification performed in part by some other method, e.g. using a liquid process in which the substrate is impregnated with a liquid precursor for the matrix-constituting material, and then the precursor is transformed, generally by heat treatment.
To perform chemical vapor infiltration, the fibrous substrate is placed in an enclosure. A reaction gas is admitted into the enclosure. Under determined conditions of temperature and pressure, the gas diffuses within the substrate and forms the deposit of matrix material by means of the components of the gas decomposing or reacting on making contact with the fibers.
The composition of the gas is selected as a function of the matrix to be made. CVI methods are well known for forming matrices out of pyrolytic carbon or "pyrocarbon", or out of ceramic, e.g. silicon carbide, boron nitride, or refractory oxides.
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

REFERENCES:
Kotlensky, "A Review of CVO Carbon Infiltration of Porous Substrates", 16th National SAMPE Symp., pp. 257-265.14, 21 Apr. 1971.

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