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Reexamination Certificate
1999-06-25
2001-09-04
Cain, Edward J. (Department: 1714)
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Reexamination Certificate
active
06284358
ABSTRACT:
The present invention relates to:
fiber preforms sheathed in a boron nitride coating, said coating (which is generated on said fiber preforms after they have been made) presenting an original structure;
thermostructural composite materials in which the reinforcing fabric is constituted by said fiber preforms; and
preparing both said fiber preforms and said thermostructural composite materials.
For many years, the Applicant has been consolidating and densifying composites by a gaseous process, in particular composites having an SiC matrix, and by using the technique of chemical vapor infiltration (CVI). That technique is very reliable and it enables materials of very good quality to be made and it enables parts of relatively complex shapes to be made. However, in the context of reducing cost and time, certain steps of that technique can be replaced by a step using a liquid process.
In the chemical vapor infiltration (CVI) technique, the fiber preform to be treated is initially placed in an oven while held between graphite tooling so as to be coated with an interphase; said interphase may consist, in particular, in a layer of pyrolytic carbon or of boron nitride, presenting a microstructure that is lamellar. Said tooling is bulky and expensive, is of limited lifetime, and reduces the load capacity of the oven. Secondly, after an initial deposit of matrix material by a gaseous process, the preform is consolidated, so it is possible to remove said tooling for continued densification by a gaseous process (FR-A-2 714 076).
For the purpose of completely eliminating said graphite tooling, the Applicant has developed a method of consolidating fiber preforms by means of a liquid process. In that method, said preforms are preimpregnated with a phenolic type resin; thereafter, while being supported by metal tooling that can be reused for a large number of operations, the preforms are subjected to heat treatment (which cures and pyrolysizes said resin) after which they are in a state in which they are sufficiently consolidated to enable them to be densified by CVI and without using tooling. Unfortunately, the preforms densified with a ceramic matrix as obtained in that way do not present satisfactory mechanical properties insofar as during removal of the polymer (during pyrolysis), the residue thereof does not constitute a high performance interphase. In that method where consolation is implemented by a liquid process, it has therefore been found necessary to make an interphase by a gaseous process:
either prior to impregnation with the resin;
or else on the consolidated preforms (FR-A-2 707 287).
In order to avoid problems of oxidation associated with the presence of carbon, it is necessary to make an interphase and a consolidation matrix with one or more ceramic materials.
In this context, the Applicant has developed an original method which leads to a novel product that is particularly advantageous in certain variants. The method of the invention makes it possible to generate a coating of original structure on the surface of the fibers of the preforms, where said coating, from a certain thickness upwards, of itself performs both the function of an interphase and the function of a consolidating phase. Said film-forming coating coats the fibers of the preform in satisfactory and stable manner (a small amount of volume shrinking is observed at the end of pyrolysis and said generated coating does not become detached from the fibers) and it constitutes an interphase that makes dissipative breaking possible in the final composite material (i.e. a non-fragile mode of breaking).
The results on which the present invention are based have been obtained after research performed in different directions. The Applicant has tested various compounds as precursors for the consolidating phase, and in particular precursors for SiC, such as polyvinylhydrogenosilane (PVS and more precisely PVS 200 from Flamel Technologies) and polysilastyrene (PSS, and more particularly PSS 400 from Nippon Soda), taken separately or in mixtures, and precursors of BN, such as polyborazilene and condensed polyborazine. It is with this latter type of precursor that surprising and most advantageous results have been obtained. A BN coating of original structure presenting very high performance as an interphase (capable of constituting an effective consolidating interphase) and as a consolidating phase has been obtained. In characteristic manner, said BN coating presents a microporous granular structure (a cellular structure).
Thus, in a first aspect, the invention provides fiber preforms in which the reinforcing fibers are sheathed in a boron nitride coating designed to constitute an interphase between said fibers and a matrix for densifying said preforms; said boron nitride coating, made (generated) on the previously-prepared fiber preforms, presents, in characteristic manner, a structure that is granular with micropores between the granules of said structure.
Such a cellular or microporous structure which is the result of small grains or granules being assembled together is entirely original for BN coating of fibers. In the prior art, BN coatings generated by a gaseous process generally present a structure that is dense, of the lamellar type. It is true that J. Am. Ceram. Soc., 74 (10), October 1991, pp. 2482-2488 reports obtaining a microporous BN coating using a gaseous process, but the structure of said coating remained of the stratified, lamellar type. The authors compared the porosity of said resulting microporous structure with that of turbostratic pyrolytic carbon. Within that structure, the stacking of the basic structural units is imperfect, thereby giving said structure its microporous lamellar character.
The original microporous granular structure of the BN coating of the invention has been observed, in particular, by means of micrographs (see the figures accompanied the present description).
For example, the inventors have observed BN granules of diameter lying in the range 40 nanometers (nm) to 50 nm, with the assembly thereof constituting a microporous coating with porosity of about 20%.
A cellular granular structure of the type of the invention is particularly advantageous in that firstly it limits crack propagation under stress and in that secondly it favors decoupling between the matrix and the reinforcement in the final composite materials, thus enabling them to break under stress in a manner that dissipates energy.
The boron nitride which sheaths the fibers of fiber preforms of the invention in regular manner thus presents, in characteristic manner, a microporous granular structure. The pores of said structure are submicron-sized. They thus present, in general, a mean equivalent diameter that is less than 1 micron.
Said boron nitride can be deposited on the surface of said fibers of said preforms to a greater or lesser thickness. In general, the fiber preforms of the invention have their fibers sheathed in said microporous coating (BN) to a mean thickness lying in the range 0.1 micrometers (&mgr;m) to 1.2 &mgr;m.
Advantageously, to constitute a consolidating interphase, said microporous coating (BN) has a mean thickness that is equal to or greater than 0.4 &mgr;m, and thus generally lying in the range 0.4 &mgr;m to 1.2 &mgr;m. The mass of such a coating of the invention, which constitutes a consolidating interphase, thus generally represents about 10% to 15% of the mass of the fiber preform whose fibers are sheathed in said coating. It is specified at this point that the use of a greater mass is not excluded in any way from the present invention; i.e. said microporous BN coating could have a thickness greater than 1.2 &mgr;m. In any event, the thickness of said coating will generally be less than 5 &mgr;m.
Fiber preforms whose fibers have been sheathed in a microporous structure BN coating of smaller thickness e (e≦0.4 &mgr;m, generally 0.1 &mgr;m≦e<0.4 &mgr;m) nevertheless also form part of the first aspect of the present invention. They are novel. Said coating can constitute an interphase having a
Cavalier jean-Claude
Framery Eric
Jouin Jean-Marie
Parlier Michel
Ropars Marcel
Cain Edward J.
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
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