FR2770233A1 - Carbon fiber preforms are produced from a continuous intermediate carbonized carbon precursor fiber yarn - Google Patents

Abstract

Carbon fiber preforms are produced from a thread or yarn of continuous intermediate carbonized carbon precursor fibers by stretching/breaking to obtain discontinuous fibers, slightly twisting the thread or yarn, forming the preform and then completing the carbonization. Carbon fiber preforms are produced from a thread or yarn of continuous fibers made from carbon precursor fibers by intermediate carbonization to a carbon content of 70-90% and a tensile breaking strength of \-3000 MPa such that subsequent complete carbonization can be carried out without tensioning. The process comprises subjecting the thread or yarn to a stretching/breaking operation to obtain a thread or yarn of discontinuous fibers, slightly twisting the thread or yarn, forming the preform from the thread or yarn and subjecting the preform to thermal treatment to complete the transformation of the discontinuous fibers into carbon fibers.

Description

Field of invention
The present invention relates to the manufacture of carbon fiber preforms for the production of parts made of composite material comprising a fiber-reinforced preform densified with a matrix.

 A particular field of application of the invention is the production of preforms for parts made of carbon / carbon (C / C) composite material, that is to say having a preform or reinforcement of carbon fibers densified by a carbon matrix. . Parts made of C / C composite material are used in various fields, in particular that of friction, in the form of brake disks or clutches.

Arnère-plan of the invention
A technique commonly used to obtain a carbon fiber preform is to develop a carbon precursor fiber preform, and then carry out at least one carbonization step to convert the precursor carbon. Various precursors may be used, such as precursors based on pitch, or phenolic, or cellulosic, or polyacrylonitrile (PAN) preoxidized.

 An essential advantage of using precursor fibers is that it is possible to perform textile operations, particularly needling, to develop preforms with the desired characteristics, while needling could have a destructive effect if it were made directly on commercially available carbon threads.

 However, this technique has several disadvantages. when the carbonization of the fibers is carried out after the development of the preform, so without being put in tension, that is to say in the static state, the mechanical characteristics of carbon fibers are much lower and may present a dispersion more extended in comparison with carbon fibers from the same precursors, but carbonized under tension. As an indication, carbon fibers from preoxidized PAN have a tensile strength of between about 1,600 and 2,400 MPa, when the carbonization is carried out in the static state, while the tensile strength at break is between about 3,800 and 4,000 MPa when the carbonization is carried out under tension. As for the module, it goes from a value between 200 and 210 GPa approximately to a value between 220 and 240GPa approximately. Another disadvantage is that carbonization causes shrinkage. It is therefore necessary to take this into account when sizing the preform made with the precursor fibers.

 It was then sought to make preforms from carbon fibers, while offering the possibility of performing textile operations, including needling. A solution proposed in US Pat. No. 5,228,175 consists, by means of continuous carbon filament yarns, of subjecting the yarns to a stretch-cracking operation to transform them into discontinuous carbon fiber yarns arranged substantially parallel to each other. each other and to give at least temporary cohesion to the non-twisted yarns, so that the yarns can be handled and undergo textile operations such as weaving, while needling is possible without damaging the yarns by fiber removal non-twisted carbon staple. The cohesion of the yarns can be provided by wrapping with a thread of fugitive material, for example a soluble yarn which can be removed after preparation of the preform.

 This solution is satisfactory, but remains relatively expensive because of not only the particular treatment of the carbon wire, but also and above all the cost and low titers of commercially available carbon threads. In addition, it remains necessary, at least for certain applications, to heat treat at a temperature higher than that at which the carbon wire was worn during its manufacture. This is the case, for example, in the case where the carbon thread contains undesirable residual impurities removable by heat. An example of an impurity is sodium, which may be present in carbon threads originating from PAN precursor, which has a carbon oxidation catalyst effect and therefore reduces the oxidation resistance of the finally manufactured composite material. . It can also be noted that the manipulation of carbon threads leads to fiber pollution which can be harmful for both men and machines.

Summary of the invention
The object of the invention is to provide a process for producing carbon fiber preforms which makes it possible to combine the advantages of the prior art techniques, while eliminating the main disadvantages.

This goal is achieved by a method according to which
at least one continuous fiber wire or cable made from carbon precursor fibers having undergone an intermediate carbonization is used such that the fibers have a carbon content of between 70% and 90% and will exhibit a tensile strength at less than 3,000 MPa after their carbonization has been completed without necessarily being tensioned,
the wire or cable is subjected to a stretch-cracking operation so as to obtain a wire or cable formed of staple fibers,
the yarn or cable formed of discontinuous fibers is subjected to a slight twisting,
the wire or cable formed of discontinuous and slightly twisted fibers is used as is to produce the preform, and
the preform is subjected to a heat treatment at least in order to complete the transformation of the staple fibers into carbon fibers.

 A characteristic of the process lies in the use of son or cables formed of continuous fibers resulting from a carbonization of a precursor of carbon which is not complete but sufficient to finally give the fibers mechanical properties similar to those of carbon fibers originating from the same precursor but fully carbonized under tension. Indeed, the intermediate carbonization being carried out before the elaboration of the preform, it can be carried out under tension in order to finally obtain optimal mechanical properties, the carbonization being then completed in static, after elaboration of the preform.

 In addition, using wires or cables at an intermediate stage of carbonization, it avoids certain disadvantages already mentioned related to the use of carbon son. In particular, it is possible to use commercial wires or cables having a higher titer than carbon wires and more economical use. Preferably, wires or cables of at least 50 K, that is to say formed of at least 50,000 filaments, are used.

 The heat treatment carried out on the preform may aim not only to complete the conversion of the precursor, by raising the temperature to at least about 1 2000C, but also to remove impurities, by prolonging it to a higher temperature of at least 1 6000C. . Compared to the prior art using carbon fiber yarns, and in which a high temperature heat treatment had to be performed to remove impurities, the process does not introduce any additional step. Compared with the prior art using carbon precursor fiber wires or cables, the method not only makes it possible to obtain a preform in which the fibers have much better mechanical properties, but also to avoid taking into account a subsequent withdrawal of the preform. It can therefore be developed closer to its final dimensions, thus optimizing the duration of the textile operations required for this purpose.

 By slight twisting imposed on the wire or cable formed of staple fibers is here understood to be a twisting sufficient to give the wire or cable the strength necessary to be able to undergo textile operations, in particular weaving, especially high speed weaving, while leaving the possibility of at least one subsequent needling during which staple fibers can be picked up by needles without damaging the wires or cables significantly. The twist may vary depending on the title of the wire or cable. Preferably, it is between 20 tr / m and 120 tr / m approximately.

Brief description of the drawings
In the accompanying drawings,
FIG. 1 shows successive steps of an embodiment of a method according to the invention; and
FIGS. 2 and 3 show very schematically drawing-cracking installations.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
As shown in Figure 1, a first step (10) is to provide fiber wires or cables from an intermediate carbonized carbon precursor. Intermediate carbonization here means an intermediate carbonization between the precursor state and the carbon state. This intermediate carbonization is carried out under tension so as to obtain fibers having optimum mechanical characteristics. The degree of carbonization is preferably chosen so as to reach a level of mechanical characteristics close to or substantially equal to that of the characteristics obtained after complete transformation of the precursor under tension. Such a level of carbonization is achieved when the carbon content is between 70 and 90%, this may vary depending on the carbon precursor used. A carbonization lasting less than that required to achieve complete carbonization.

 For example, in the case of a preoxidized PAN precursor which, during its preparation, was brought to a maximum temperature of about 2500 ° C., a satisfactory intermediate carbonization is carried out by heat treatment under tension at about 900 ° C., whereas that conversion of the preoxidized PAN to carbon is usually carried out at about 1400 ° C.

 Preferably, relatively high-titre wires or cables are used, preferably wires or cables of 50 K or more, i.e. formed of 50,000 or more filaments. Indeed, in general, the son or cables are commercially available at a cost based on the unit mass which decreases as the title increases.

Advantageously, the son or cable proposed under the name "Pyon" is used by the British company SGL Technics Ltd., cables of 320 K to 480 K being commercially available. These wires or cables consist of continuous filaments derived from the precursor PAN of the British company
Courtaulds after intermediate carbonization performed under tension until a carbon content between 70 and 80%.

 In a second step (20), the wire or cable 11 is subjected to a stretch-cracking operation, to transform it into a wire or cable 12 formed of discontinuous filaments substantially parallel to the longitudinal direction of the wire or cable. The stretch-cracking operation is well known and is generally performed by stretching the wire or cable 11 and causing it to break between two pairs of rollers 22, 23 of a drawing gear 21 (FIG. 2). FR-A-2,608,641 and US-A-4,759,985 describe the stretch-cracking of carbon son. It will be noted, however, that in the process according to the invention, the stretch-cracking is carried out without particular coating of the wire or cable. In addition, the draw-cracking is performed to obtain a wire or cable 12 formed of long staple fibers. By long fibers is meant here fibers of average length at least equal to 60 mm.

 Fig. 3 shows a stretch-cracking apparatus in which a plurality of roller stretching trains 31a to 21p are provided for effecting the stretch-cracking of a corresponding number of wires or cables 11a to 11s.

 The son or cables 12a to 12p formed of staple fibers can then be mixed by passing through a strip-drawing device 25. The latter, comprising combs mounted on an endless chain, makes it possible to mix the staple fibers of the different yarns. or cables, while performing a drawing, so that the resulting wire or cable 13 has the same title as each of the wires or cables received by the device 25. Thus, for example, when the number of wires or cables 1o to 12p is equal to 16 (the son having the same title), the device 25 is set to perform stretching with multiplication by 16 of the length.

An installation of the type of that of Figure 3 is particularly suitable for the production of composite son, that is to say son formed of staple fibers of different natures. Indeed, in the context of the invention, the son i la to 1lp may include: - one or more son or cables formed of continuous fibers from fibers of
carbon precursor having undergone an intermediate carbonization such as
fibers have a carbon content between 70% and 90% and will
tensile strength at least equal to 3 000 MPa after their
carbonization has been completed without necessarily being put in tension, - one or more son or cables formed of continuous fibers coming from precursor of
carbon giving fibers having a lower breaking strength, by
example of continuous fibers derived from phenolic precursor, cellulosic or
isotropic pitch, - one or more wires or cables formed of continuous fibers originating from precursors
ceramic, for example precursor of silicon carbide, alumina,
silica, ..., and - one or more wires or cables formed of continuous carbon fibers or
carbon, such as continuous fiber wires or cables from
anisotropic pitch which inherently has a high resistance to fracture.

 The bar drawing device permits intimate mixing of staple fibers from the various yarns after stretch-cracking.

 The son or cables obtained after stretch-cracking undergo a slight twisting (step 30) in order to give them an outfit allowing them to undergo subsequent textile operations. The production of fibrous preforms from yarns or cables may involve various operations such as weaving, unidirectional webbing, winding, needling. Certain operations, in particular weaving, require minimal cohesion of the threads or cables formed of discontinuous filaments, in particular when they are made at high speed, that is to say, for weaving, a speed of up to 400 knots. min or more.

On the other hand, needling, in order to be able to be carried out without significant damage to the wires or cables, requires that discontinuous filaments can be easily removed. Also, the twist must be sufficient to give minimum cohesion to the wires or cables, but limited to allow subsequent needling.

This is why the degree of twisting is preferably between 20 rpm and 120 rpm. It may be chosen at a higher value for a relatively weak title thread (expressed in tex) than for a relatively strong title thread. Thus, the coefficient a giving the ratio between the torsion in r / m and the square root of the title in metric number (Nm), is preferably between 30 and 60.

 The twisting may be carried out, in a well-known manner, for example by means of a spindle bench, or a continuous spinning machine, or a sleeve rubber, the latter producing rather a "scrambling" of the fibers which a real twist.

 The slightly twisted wires or cables can then be used for the preparation of the desired preforms (step 40). For this purpose, operations such as weaving, layering, winding and needling can be carried out, as indicated above.

 For example, a preform can be made by stacking two-dimensional layers, flat or draped on a shape, and binding the layers together by needling. The two-dimensional layers may be fabric layers or unidirectional layers formed of son or cables parallel to each other and superimposed in different directions.

 When needling is performed, very fine needles are preferably used because of the slight twisting of the wires or cables. By very thin needles is meant here excel, needles whose active part has, in section, a triangle shape whose height is relatively low, that is to say less than 0.5 mm.

 After elaboration of the preform, it is subjected to a heat treatment (step 50) in order to complete the transformation of the precursor fibers.

This treatment is carried out at a temperature of preferably at least 12000 ° C., for example about 1400 ° C. After a plateau at this temperature, the heat treatment can be continued by raising the temperature to a plateau, for example at room temperature. less than about 1600 ° C, to remove undesirable impurities present in the carbon fibers, for example sodium. Finally, the desired carbon fiber preform is obtained with fibers having high mechanical properties and without appreciable shrinkage during the heat treatment. example
An exemplary embodiment of C / C composite disc and brake pad preforms and tests performed with discs and brake pads incorporating such preforms will now be described.

 We use 30 g / m linear density cables, a 30ktex title marketed by the British company SGL Technics Ltd under the name "Pyon 15". These are fiber cables from pre-oxidized PAN having undergone an intermediate carbonization under tension such that the fibers have a carbon content of 76%, the remainder consisting essentially of nitrogen.

 The cables are subjected to a stretch-cracking operation to obtain a yarn of discontinuous 1 ktex staple fibers which is made coherent by a slight twist of 35 rpm (<i = 35).

 The yarn obtained is used to produce a fabric (twill weave of 2) having a basis weight of 840 g / m 2 and a thickness under load (50 g / m 2) of 1.8 mm.

 Fabric layers are stacked and needled layer-to-layer, as described in FR-A-2 726013, to bring the volume content of fibers to a value of about 20%. A heat treatment is first carried out at about 1400 ° C to complete the carbonization of the precursor, then the temperature is raised to 1600 ° C to remove residual impurities, especially the sodium present in the fibers. observed mass is about 30%.

 Annular brake disc preforms are cut, as well as brake pad preforms, and then densified by a pyrolytic carbon matrix by chemical vapor infiltration, in a manner well known per se, to obtain disks and platelets. C / C composite brake.

 By way of comparison, reference brake disks and pads made of C / C composite are made in a similar manner, but starting from preoxidized PAN fiber cables which have not undergone any intermediate carbonization, the carbonization being carried out after needling. , therefore not energized.

The reference brake discs and pads and according to the invention are subjected to the same high energy braking tests and the resulting wearings are evaluated by measurement of loss of thickness expressed in mm. The results are given in the table below.

<Tb>

<SEP> Reference <SEP> According to <SEP> the invention
<tb> Wear <SEP> disk <SEP> 1.38 <SEP> 0.85
<tb> Wear <SEP> platelet <SEP> 1.78 <SEP> 1.29
<Tb>
The reduction in wear, with the C / C material according to the invention, is 38% for the discs and 27% for the discs.

Claims (11)

 A process for producing carbon fiber preforms, characterized in that  at least one continuous fiber wire or cable made from carbon precursor fibers having undergone an intermediate carbonization is used such that the fibers have a carbon content of between 70% and 90% and will exhibit a tensile strength at less than 3,000 MPa after their carbonization has been completed without necessarily being tensioned,  the wire or cable is subjected to a stretch-cracking operation so as to obtain a wire or cable formed of staple fibers,  - the wire or cable formed of staple fibers is imposed a slight twisting,  the wire or cable formed of discontinuous and slightly twisted fibers is used as is to produce the preform, and  the preform is subjected to a heat treatment at least in order to complete the transformation of the staple fibers into carbon fibers.  2. Method according to claim 1, characterized in that at least one wire or cable formed of continuous fibers from a precursor which has undergone an intermediate carbonization under tension is used.  3. Method according to any one of claims 1 and 2, characterized in that at least one wire or cable formed of continuous fibers from pre-oxidized polyacrylonitrile having undergone carbonization such that the carbon content is between 70% and 80%.  4. Method according to any one of claims 1 to 3, characterized in that one uses a plurality of different wires or cables chosen from son or cables formed of continuous fibers derived from carbon precursor and son or cables formed of continuous fibers. from ceramic precursor.  5. Method according to claim 4, characterized in that each wire or cable is subjected to a stretch-cracking operation, the son or cables formed of discontinuous fibers obtained are mixed, and a slight twist is imposed on the wire or cable. resulting composite.  6. Method according to any one of claims 4 and 5, characterized in that at least one wire or cable formed of continuous fibers derived from carbon precursor selected from precursors based pitch, phenolic base, based on cellulosic and preoxidized polyacrylonitrile base.  7. Method according to any one of claims 1 to 6, characterized in that at least one wire or cable of at least 50 K.  8. A method according to any one of claims 1 to 7, characterized in that imposes the wire or cable formed of staple fibers a twist of between 20 and 120 tr / m.  9. Method according to any one of claims 1 to 8, characterized in that the manufacture of the preform comprises at least one needling step.  10. Method according to any one of claims 1 to 9, characterized in that the manufacture of the preform comprises at least one high speed weaving step at least equal to 400 strokes / min.  he. Process according to any one of claims 1 to 10, characterized in that the heat treatment is carried out at a temperature of at least 1200 ° C to complete the transformation of the precursor.  12. The method of claim 11, characterized in that the heat treatment is continued at a higher temperature, at least equal to 1600 "C.