Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
1999-05-27
2001-09-18
Cain, Edward J. (Department: 1714)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S293700, C428S294100
Reexamination Certificate
active
06291058
ABSTRACT:
The present invention relates to ceramic matrix composite materials, and more particularly to composite materials having fiber reinforcement made of fibers that are constituted essentially by silicon carbide (SiC), with an interphase layer between the reinforcing fibers and the ceramic matrix.
Ceramic matrix composite materials are used in various applications, for example in the fields of aviation and space, where they are used for their thermostructural properties, i.e. their suitability for making structural elements because of their mechanical strength, particularly in bending, in traction, and against shock, which is much better than that of solid ceramics, and because of their ability to conserve such mechanical properties up to high temperatures that may be as much as 1500° C. or more.
Making thermostructural composite materials with an interphase between the fibers and the matrix is known, in particular from documents U.S. Pat. Nos. 4,752,503 and 5,026,604. The interphase used is pyrolytic carbon (PyC) or boron nitride (BN) and its structure is anisotropic being of the lamellar or foliated type so as to encourage the deflection of cracks which appear in the ceramic matrix in order to avoid said cracks reaching the fibers and giving rise to premature destruction of the material by breaking the fibers of the fiber reinforcement.
In ceramic matrix composite materials, cracking of the matrix is practically inevitable, as from manufacture, because of the thermal expansion differences between the reinforcement and the matrix.
The use of a lamellar interphase, which establishes a relatively weak bond between the fibers and the matrix, thus has the advantage of lengthening the lifetime of the material by creating crack deflection zones in which crack-bottom stresses can dissipate by localized decohesion of the lamellar microstructure of the interphase.
Nevertheless, in conditions of use under an oxidizing atmosphere and at high temperature, crack propagation as far as the interphase opens access paths for oxygen. The PyC or BN interphase and even the underlying fiber then oxidizes, leading to a modification of the fiber-matrix bonds and, progressively, to the material being damaged and breaking.
Solutions have been proposed to prevent, or at least retard, oxygen access to the interphase between the fibers and the matrix, in particular by plugging the cracks which appear within the matrix or by slowing down crack propagation within the matrix.
Thus, it is well known to include within the matrix a compound that is capable of healing cracks by forming a glass. The compound is selected so that the glass is capable of plugging the cracks and preventing oxygen passing along them, by taking on a pasty state at the utilization temperatures of the composite material. By way of example, reference can be made to document U.S. Pat. No. 5,246,736 which describes making at least one phase of the matrix out of an Si—B—C ternary system capable of forming a glass, in particular of the borosilicate type, having self-healing properties, and also to document WO-A-96/30317 which describes the formation of a self-healing matrix.
In addition, document U.S. Pat. No. 5,079,639 describes a composite material having toughness improved by sequencing the matrix so that crack deflection zones are generated within the matrix, thereby preventing cracks from progressing directly as far as the interphase. methods using a self-healing phase are effective over a limited temperature range at which the self-healing property is present, whereas methods using a sequenced matrix retard crack propagation but do not prevent them from reaching the interphase.
Thus, an object of the present invention is to provide a fiber and ceramic matrix composite material essentially made of SiC that has improved properties by more effectively preventing cracks from reaching the interphase layer between the fibers and the matrix.
This object is achieved by the fact that the fibers of the reinforcement are long fibers containing less than 5% atomic residual oxygen and having a modulus greater than 250 GPa, and the interphase layer is strongly bonded to the fibers and to the matrix in such a manner that the shear breaking strengths within the interphase layer and at the fiber-interphase bonds and at the interphase-matrix bonds are greater than the shear breaking strengths encountered within the matrix.
Remarkable characteristics of the invention lie in the presence of an interphase capable of providing a strong bond with the fibers and the matrix, and in selecting fibers that are capable of preserving and withstanding such strong bonds with the interphase.
To term “strong” bond is used herein to mean that within the interphase layer and at the interfaces between the interphase and the fibers and between the interphase and the matrix there exist shear breaking strengths greater than those which are to be found within the matrix.
It has been found, in particular, that a strong bond can be obtained with an interphase layer and a material whose microstructure presents little anisotropy. A microstructure is said herein to present “little anisotropy” when it presents anisotropic domains of small size (preferably less than 15 nanometers) which are dispersed within a quasi-isotropic background and which are juxtaposed in random directions.
Examples of materials that can be suitable for the interphase layer are boron nitride and pyrolytic carbon which are formed by chemical vapor infiltration under operating conditions that give them a microstructure which presents little anisotropy.
As mentioned above, it is essential for the fibers used to be capable of preserving the strength of the bond with the interphase and of withstanding this strong bond.
That is why the fibers used are essentially SiC fibers containing little residual oxygen, typically less than 5% atomic, and preferably less than 1% atomic, so as to avoid polluting the composition and/or the microstructure of the interphase by significant migration therein of residual oxygen contained in the fibers.
In addition, in order to be capable of withstanding a strong bond with the matrix, and in particular in order to avoid bonds breaking due to expansion differences of thermal origin between the fibers and the interphase, the fibers used are long fibers which present radial expansion such that the interphase is preferably compressed between the fibers and the matrix.
Fibers are said to be “long” herein when their mean length is greater than 10 cm.
Essentially SiC fibers satisfying these requirements are in particular the fibers sold under the name “Hi-Nicalon” by the Japanese company Nippon Carbon. The use of such fibers for forming unidirectional composites with a silicon nitride Si
3
N
4
matrix is described in an article by A. Kamiya et al., published in “Journal of the Ceramic Society of Japan, International Edition”, Vol. 102, No. 10, October 1994, under the title “Mechanical properties of unidirectional HI-NICALON fiber reinforced Si
3
N
4
matrix, composites”. The Si
3
N
4
matrix is obtained by impregnation by means of a composition containing a precursor organic resin and Si
3
N
4
powder, and then hot pressing. However, the author states that a strong bond is then obtained between the fibers and the matrix in the absence of an interphase, and that the strong bond can be avoided by using fibers that are coated in carbon. In contrast, in the material of the invention, the interphase is selected so as to ensure strong bonding between the fibers and the matrix.
Preferably, when making a composite material of the invention, the fibers can be subjected to pretreatment so as to favor long duration of the strong bond with the interphase layer.
Such pretreatment is, for example, of chemical nature and consists in immersing the fiber fabric that is to form the reinforcement, or indeed the already prepared reinforcement, in a bath of acid so as to eliminate the silica that may be present on the fibers. Such treatment is known and described in document U.S. Pat. No. 5,071,679.
Another pretre
Bertrand Sébastien
Caillaud Alain
Charvet Jean-Luc
Goujard Stephane
Pailler Rene
Cain Edward J.
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
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