EP0489637B1 - Procedure for the fabrication of a fibrous preform made of fire-resistant fibres in order to produce a composite material - Google Patents

Description

The present invention relates to the manufacture of fibrous preforms formed of refractory fibers for the production of parts of composite material according to the preamble of claim 1. The invention also relates to a composite yarn suitable for the manufacture of such preforms according to the preamble of claim 8.

By refractory fibers is meant here essentially carbon fibers and ceramic fibers. Among the latter, mention may be made of fibers made of carbide, nitride or refractory oxide, such as carbide or silicon nitride, boron carbide, alumina, etc.

By precursors of refractory fibers is meant here fibers in a state preceding the refractory state, the transition to the latter being most often effected by heat treatment. For example, a carbon precursor is the preoxidized polyacrylonitrile (PAN), or a pitch, while a precursor of the silicon carbon is polycarbosilane (PCS).

A particular field of application of the invention is the manufacture of parts made of composite material consisting of a refractory fibrous preform densified by a matrix. Densification consists in the deposition or infiltration of the matrix material in the porosity of the preform, throughout the volume of the latter.

There are various methods for making a refractory fiber preform. A conventional method consists of superimposing strata of a two-dimensional fibrous texture, in particular a fabric, and, optionally, of binding the strata together, for example by needling.

Document FR-A-2 608 641 describes the production of a fibrous preform from yarn formed of staple fibers of refractory material using conventional spinning equipment.

Document JP-A-2 210 036 discloses a reinforcing thread comprising a plurality of threads, in particular of refractory material, ligated by means of one or more covering threads. These can be of a soluble or easily carbonizable material, or still in a material which, after heat treatment, behaves like a matrix.

A difficulty encountered with known refractory fiber yarns consists in their poor ability to undergo textile operations, such as weaving, especially for ceramic fiber yarns, and, above all, needling.

One way of overcoming this difficulty consists in carrying out the necessary textile operations on yarns, the constituent fibers of which are in the precursor state, better able to undergo these operations. The transformation of the precursor into refractory material is then carried out after the textile operations have been carried out.

Another way of overcoming this difficulty for the needling of superimposed layers of carbon fiber fabric is to interpose layers of felt between the layers. With a fabric formed of threads in which the cohesion of the carbon fibers is ensured by twisting, the attack of the threads by the needles has the effect of breaking the fibers more than detaching the fibers to implant them through the strata. The interposition of layers of felt makes it possible to provide the fibers likely to be entrained by the needles.

Another problem encountered during the production of parts made of composite material is that of the accessibility of the internal porosity of the preform during densification.

There are several densification techniques, including densification by liquid and densification by gas. The liquid method consists of impregnating the preform with a precursor liquid of the material constituting the matrix and then transforming the precursor, generally by heat treatment. The gas process (chemical vapor infiltration) consists of placing the preform in an enclosure where a gas flow is introduced under determined temperature and pressure conditions to form the matrix material in contact with the fibers of the preform by decomposition of one or more constituents of the gas flow or by reaction between constituents thereof.

Whatever technique is used, it is in practice impossible to densify the preform completely. Indeed, the constituent wires of the preform define between them volumes, some of which are almost closed. The densification of these "dead" volumes cannot be achieved, even by chemical vapor infiltration, their possible reduced access being very quickly obstructed.

The object of the present invention is to provide a method for manufacturing a fibrous preform made of refractory fibers, the manufacturing process possibly comprising textile operations, including needling.

Another object of the invention is to provide a process allowing the manufacture of fibrous preforms having practically no "dead" volumes, therefore capable of being easily densified.

• fournir un fil composé essentiellement de fibres discontinues en matière réfractaire ou en un précurseur de celle-ci, et former une préforme fibreuse à partir du fil composé des fibres discontinues,
  procédé dans lequel, conformément à l'invention, le fil est composé essentiellement de fibres discontinues arrangées parallèlement les unes aux autres, en étant non retordues, et l'intégrité du fil est assurée par un fil de guipage en matière fugitive, et
• après formation de la préforme fibreuse, le fil de guipage est éliminé pour permettre aux fibres discontinues de se libérer dans tout le volume de la préforme et grâce à un fil selon la revendication 8.

These objects are achieved by a process for manufacturing a fiber preform comprising the steps of:

• supplying a wire composed essentially of staple fibers of refractory material or a precursor thereof, and forming a fibrous preform from the wire composed of staple fibers,
  process in which, in accordance with the invention, the thread is composed essentially of staple fibers arranged parallel to one another, being untwisted, and the integrity of the thread is ensured by a covering thread made of fugitive material, and
• after formation of the fibrous preform, the covering yarn is eliminated to allow the staple fibers to be released throughout the volume of the preform and by means of a yarn according to claim 8.

Preferably, the covering yarn has a low denier compared to that of the assembled staple fibers, so as not to leave too large voids within the preform after elimination of the covering yarn. The denier of the covering yarn is preferably less than a tenth of that of the assembled staple fibers.

By fugitive material, which constitutes the covering yarn, is meant here any material capable of being eliminated without leaving a residue on the yarn and without alteration of the refractory fibers. For example, the fugitive material is a soluble polymer, such as a PVA (polyvinyl alcohol), or a polymer capable of being completely removed by heat treatment, such as polyvinyl acetate or polyethylene.

The supply of the wire comprises a step of obtaining staple fibers, preferably long staple fibers, parallel to each other, in refractory material or in a precursor thereof. This is achieved for example by controlled stretching and cracking of a multifilament cable, as described in particular in document FR-A-2 608 641.

In the aforementioned document FR-A-2 608 641, the fibers are transformed into yarn by twisting on conventional spinning equipment.

On the other hand, in the thread used in accordance with the invention, the fibers are left parallel to each other, and not twisted, the integrity of the thread being made up by covering with the thread of fugitive material. The covering is carried out by means of a known covering machine, for example the "Parafil" machine from the German company Spindelfabrik Suessen.

The covering gives the yarn the necessary resistance to undergo textile operations, in particular weaving.

After elimination of the covering yarn, the existence of non-twisted, discontinuous parallel fibers allows needling with removal of these fibers by the needles, without it being necessary to provide a texture of the felt type supplying the fibers liable to be entrained. by needles.

Thus, the method according to the present invention can be implemented for all applications requiring the execution of textile operations on the yarn, such as weaving and needling.

The method according to the invention provides an additional advantage in that it allows the elimination of dead volumes which are not completely densifiable. In fact, when the preform has been produced and the covering yarn eliminated, the released fibers tend to occupy the volumes available under the effect of the "swelling" of the yarn. This makes it possible to obtain a preform whose porosity is more easily and more uniformly accessible to the material constituting the matrix. This results in more complete densification and less inhomogeneity of the material.

When the wire used for the manufacture of the preform is a precursor of the desired refractory material, the transformation of the precursor into refractory material is carried out after manufacture of the preform and after elimination of the covering wire. When the covering thread is made of a removable material by heat, this elimination can be carried out during a temperature rise carried out with a view to the transformation of the precursor by heat treatment.

Particular examples of embodiment of fibrous preforms according to the invention will now be described for information, but not limitation.

Example 1 : Manufacture of a carbon fiber preform.

A two-dimensional texture (2D) is formed by weaving a yarn of non-twisted fibers in PAN (polyacrylonitrile) preoxidized gimped with a yarn of PVA (polyvinyl alcohol). The characteristics of the 2D fabric are as follows: - title of the pre-oxidized PAN wire Nm2 - PVA covering wire 45 dtex - weaving weave satin 8 - number of warp threads 10 / cm - number of threads in the weft 10 / cm - surface mass 1050 g / m²

After weaving, the fabric is washed in a water bath at 80 ° C for 10 minutes, then wrung and dried. The PVA covering yarn is completely dissolved and the fibers of the preoxidized PAN yarns expand within the fabric allowing direct needling thereof, without adding a layer of felt.

Several layers of fabric are then superimposed and needled to make a fibrous preform. This is then subjected to a heat treatment (carbonization) to transform the pre-oxidized PAN into carbon. A fibrous carbon fiber preform is then obtained. The fabric described above makes it possible to obtain a needled preform in which the carbon fiber content is approximately 30% (percentage of the apparent volume of the preform actually occupied by the fibers).

The carbon fiber preform can then be densified by a material constituting a matrix, for example carbon or a ceramic, in order to obtain the composite material. desired carbon fiber reinforcement. Densification is carried out by liquid means or by chemical vapor infiltration. The proliferation of fibers within the fibrous texture, resulting from the loosening of the untwisted fibers after elimination of the covering yarn, avoids the formation of dead volumes within the preform and consequently promotes a more complete and more homogeneous densification.

Example 2 : Manufacture of a ceramic fiber preform.

A texture is formed by multilayer weaving of a thread of non-twisted silicon carbide (SiC) fibers wrapped in a PVA thread. The characteristics of the fabric are as follows: - title of the SiC thread Nm 3 - PVA covering wire 45 dtex - weaving weave Interlock - number of layers 5 - number of warp threads 40 / cm - number of threads in the weft 30 / cm - fabric thickness 3 mm

After weaving, the texture is soaked in a water bath at 80 ° C for 15 minutes then wrung and dried. It is noted that the PVA yarn is dissolved and that the SiC fibers expand within the texture. The volume content of fibers in the woven texture as indicated above is approximately 30%.

As indicated in connection with Example 1, the texture obtained is particularly suitable for subsequent densification by the liquid or gaseous route.

A carbon fiber preform can be made directly from carbon fibers, including high strength fibers.

It is also possible to produce a preform in ceramic fibers, such as SiC fibers, from a SiC precursor, such as polycarbosilane (PCS).

Claims (11)

1. Process for the manufacture of a fibrous preform for producing a composite material article, said process comprising the steps of :

   - providing a yarn essentially composed of discontinuous fibers made of a refractory material or a precursor thereof, and

   - forming a fibrous preform from the yarn composed essentially of discontinuous fibers, characterized in that:

   - the yarn is essentially composed of discontinuous fibers arranged in parallel one to the other, while being non-twisted, and the integrity of the yarn is ensured by a covering yarn in fugitive material, and

   - after formation of the fibrous preform, the covering yarn is eliminated to allow said discontinuous fibers to loosen within the bulk of said preform.
2. Process according to claim 1, characterized in that the covering yarn has a denier less than one tenth of that of the assembly of discontinuous fibers.
3. Process according to any one of claims 1 and 2, characterized in that the discontinuous fibers are obtained by controlled stretching and breaking process.
4. Process according to any one of claims 1 to 3, characterized in that the covering yarn is made of a soluble polymer.
5. Process according to any one of claims 1 to 4, characterized in that the covering yarn is made of a material capable of being eliminated by heat.
6. Process according to any one of claims 1 to 5, characterized in that the yarn is essentially composed of discontinuous fibers made of a precursor of a ceramic material, and the transformation of the precursor into ceramic is carried out after the step of eliminating said covering yarn.
7. Process according to any one of claims 1 to 6, characterized in that it comprises a needling step carried out on the fibrous preform after the step of eliminating said covering yarn.
8. A composite yarn essentially composed of fibers made of a refractory material or a precursor thereof, and of a covering yarn in fugitive material,
      characterized in that the fibers are discontinuous and disposed in parallel one to the other, without being twisted, and the covering yarn, which achieves the integrity of the composite yarn, has a denier less than one tenth of that of the assembly of discontinuous fibers.
9. Composite yarn according to claim 8, characterized in that said refractory material is selected from carbon and ceramics.
10. Composite yarn according to any one of claims 8 and 9, characterized in that the fugitive material is a soluble polymer.
11. Composite yarn according to any one of claims 8 and 9, characterized in that the fugitive material is a material capable of being eliminated by heat.