Chemical vapor infiltration method with variable infiltration pa

Details Chemical vapor infiltration method with variable infiltration pa Chemical vapor infiltration method with variable infiltration pa

1997-10-06

1999-12-14

King, Roy V.

Coating processes

Coating by vapor, gas, or smoke

Carbon or carbide coating

42725512, C23C 1600

Patent

active

060014199

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a method for chemical vapor infiltration of a material into a porous substrate.
A field of application of the invention lies in making composite material parts comprising a fibrous reinforcing substrate or "preform" that is densified by a matrix, and in particular parts made of carbon-carbon composite material (carbon fiber preform and carbon matrix) or parts made of ceramic matrix composite (CMC).


BACKGROUND OF THE INVENTION

CMC and carbon-carbon composite materials are used in various fields where their thermostructural properties, i.e. their very good mechanical properties, make it possible to build structural elements that are heavily stressed, and that have the ability to retain these mechanical properties even at relatively high temperatures. This applies, for example, in the field of space, in particular for panels providing thermal protection or nozzles for thrusters, in the field of aviation, e.g. for parts of airplane jets, and in the field of friction, in particular for airplane brake disks.
Chemical vapor infiltration of a material into a porous substrate consists in placing the substrate inside an enclosure, in causing a gas to diffuse within the accessible internal pores of the substrate, which gas contains at least a precursor of the material in the gaseous state, and simultaneously in controlling in particular the temperature and the pressure inside the enclosure so that a deposit is formed from the precursor throughout the volume of the substrate. The precursor for carbon can be an alkane, an alkyl, or an alkene, giving rise to pyrolytic carbon by decomposition. For chemical vapor infiltration of a ceramic material, a gas is diffused that contains one or more gaseous species giving the desired ceramic material by decomposition or by mutual chemical reaction. Thus, for example, chemical vapor infiltration of silicon carbide (SiC) can be obtained by means of a gas containing methyltrichlorosilane (MTS) and in the presence of hydrogen gas (H.sub.2). Gaseous species that are precursors for other ceramics such as carbides, nitrides, or oxides are well known to the person skilled in the art.
Several vapor infiltration methods exist, in particular isobaric methods that are isothermal and isobaric methods that have a temperature gradient.
In isothermal isobaric methods, the substrates to be densified are maintained at all times at uniform temperature throughout their volume and under uniform pressure. A drawback thereof lies in the impossibility, in practice, of obtaining densification that is uniform. The matrix material tends to deposit preferentially within pores close to the outside surface of the substrate. The progressive obstruction of the surface pores makes access for the gas to the inside of the material more and more difficult, resulting in a gradient of densification between the surface and the core of the material. It is of course possible to descale or machine the surface of the substrate one or more times during the densification process in order to open up surface pores. However that requires the process to be interrupted for the time required to extract the substrate from the densification installation, to cool it down, to descale it, to put the substrate back into the installation, and to return to the desired temperature.
With a temperature gradient type method, the above-mentioned drawback of the isothermal method can be limited to a great extent. A temperature difference is established between a hotter inner portion and a cooler surface of the substrate which is exposed to the gas. The matrix material then deposits preferentially in the hotter inner portion. By controlling the temperature of the substrate surface so that it is below the decomposition or reaction threshold of the gas, at least during an initial portion of the densification process, it is possible to ensure that the densification front progresses from the inside towards the surface of the substrate as the process continues. In conventional manner, the t

REFERENCES:
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patent: 5348774 (1994-09-01), Golecki et al.
patent: 5738908 (1998-04-01), Rey et al.
patent: 5789026 (1998-08-01), Delperier et al.
patent: 5846611 (1998-12-01), Christin
patent: 5853485 (1998-12-01), Rudolph et al.

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