High-temperature composite materials with carbon or carbon-coate

Details High-temperature composite materials with carbon or carbon-coate High-temperature composite materials with carbon or carbon-coate

1998-06-12

2000-05-30

Weisberger, Richard

Stock material or miscellaneous articles

Composite

Of metal

428368, 428378, 428388, 428698, 428902, 428903, 4272481, 427249, 42725512, 4274192, 264 291, 264 297, 264638, 264642, C23C 1600

Patent

active

060689307

DESCRIPTION:

BRIEF SUMMARY
The present invention seeks to improve the ability of thermostructural composite materials having carbon or carbon-coated fiber reinforcement to withstand oxidation.
Thermostructural composite materials are materials comprising fiber reinforcement known as a fiber "preform" in which the fibers are made of a refractory material such as carbon or ceramic, and a matrix that fills in, at least in part, the pores initially present in the fiber reinforcement. Such materials are remarkable for their mechanical properties, enabling them to be used as structural elements, and for their ability to conserve these properties at high temperatures, in particular when the matrix is made of ceramic.
In general, the ceramic matrix of such composite materials is subject to cracking. The presence of cracks can already be observed when the material is made, particularly when the matrix is subjected to thermal stresses because it is obtained by chemical vapor infiltration or by impregnation by means of a precursor liquid with the precursor then being transformed into the ceramic by heat treatment. These initial cracks subsequently multiply under the effect of thermo-mechanical stresses encountered while the material is in use. Nevertheless, because of the presence of the fiber reinforcement, the material-damaging effects of the matrix cracking do not immediately affect its integrity, thus ensuring the thermostructural characteristics of the material.
Thermostructural composite materials find applications in various fields where use is made of their ability to withstand high mechanical stresses at high temperature, particularly in the fields of aviation, space, and braking. In use, these materials are usually exposed to an external environment that is oxidizing, in particular to ambient air.
When the reinforcing fibers are made of an oxidizable material, such as carbon, the combination of a network of cracks in the matrix, an oxidizing environment, and high temperature leads inevitably to the fibers being destroyed and consequently to the mechanical properties of the material collapsing.
A similar problem is encountered when the carbon is present in the material, not as the reinforcing fibers themselves, but as an interphase between ceramic fibers and a ceramic matrix. The making of such a pyrolytic carbon interphase and its usefulness in improving the mechanical behavior of the material are well known in the state of the art. Reference can be made in particular to the Applicant's document EP-A-0 172 082.
In order to improve the ability of a carbon-containing composite material to withstand oxidation, proposals have been made to provide the material with a surface coating that forms a barrier against the oxygen in the ambient medium and that is capable of plugging the cracks as they appear in the matrix of the composite material or in the coating itself. To this end, the protective coating comprises at least one self-healing layer, generally a vitreous layer which, by means of the glass passing to a semisolid state, enables cracks to be plugged at temperatures about the melting point of the glass.
That solution suffers from several drawbacks. Firstly, the self-healing effect can be obtained in satisfactory manner only over a limited temperature range. At lower temperatures, the viscosity of the glass is too high, while at higher temperatures its viscosity becomes too low and the self-healing composition runs the risk of being damaged by friction, shear, or indeed blast, i.e. by being exposed to a strong flow of gas.
Another known solution consists in making a sequenced matrix, with the matrix being built up in layers of material alternating between relatively rigid layers of high stiffness, in particular a ceramic material, and relatively flexible layers of low stiffness in shear, in particular pyrolytic carbon of the rough laminar type, or boron nitride. One such solution is described, in particular, in the Applicant's document EP-A-0 385 869. The sequenced matrix has the effect of complicating the way in which cracks propagat

REFERENCES:
patent: 5552220 (1996-09-01), Goujard et al.
Droillard, C. et al, "Fracture Toughness of 2-D Woven SiC/SiC CVI-Composi with Multilayered Interphases", Journal of the American Ceramic Society, Apr. 1996.
Vandenbulke, L. et al, "Silicon and Boron Containing Components by CVD and CVI for High Temperature Ceramic Composites", Journal De Physique IV (Colloque), Jun. 1995, France.

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