WO1999022052A1

WO1999022052A1 - Method for making carbon fibre preforms

Info

Publication number: WO1999022052A1
Application number: PCT/FR1998/002298
Authority: WO
Inventors: Renaud Duval; Eric Lherm; Thierry Marjollet
Original assignee: Messier-Bugatti
Priority date: 1997-10-27
Filing date: 1998-10-27
Publication date: 1999-05-06
Also published as: FR2770233A1; CN1092723C; JP2001521073A; CA2307137A1; CN1277642A; FR2770233B1; EP1025295A1

Abstract

The invention concerns a method for making a carbon fibre preform which consists in using at least one yarn or cable formed of continuous fibres derived from carbon precursor fibres previously subjected to an intermediate carbonization such that the fibres have a carbon ratio between 70 % and 90 % and having a tensile failure resistance not less than 3000 MPa's after they have been entirely carbonized without necessarily having been tensioned, and in using the slightly twisted yarn or cable for making the preform, before subjecting it to a thermal treatment to complete the transformation of the continuous fibres into carbon fibres. The yarn or cable can be subjected to a drawing-cracking process to obtain a yarn or cable formed of discontinuous fibres whereof the cohesion is ensured by slight twisting, or by taping.

Description

Title of the Invention

Process for manufacturing carbon fiber preforms

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

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

Invention background

A technique commonly used to obtain a carbon fiber preform consists in preparing a fiber preform of carbon precursor, and then performing at least one carbonization step to transform the precursor into carbon. Various precursors can be used, such as pitch-based, or phenolic, or cellulosic, or even pre-oxidized polyacrylonitrile (PAN) precursors.

A desired advantage of using precursor fibers is that it is possible to carry out textile operations, in particular needling, to develop preforms having 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 drawbacks.

When the carbonization of the fibers is carried out after the preparation of the preform, therefore without being tensioned, that is to say in the static state, the mechanical characteristics of carbon fibers are much lower and may have a more dispersed extended in comparison with carbon fibers from the same precursors, but carbonized under tension. As an indication, carbon fibers originating from pre-oxidized PAN have a tensile strength of between 1,600 and 2,400 MPa approximately, when carbonization is carried out in a static state, while the tensile strength is between 3,000 and 4,000 MPa approximately when 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 240 GPa approximately. Another drawback is that charring results in shrinkage. It is therefore necessary to take this into account for the sizing of the preform produced with the precursor fibers.

It was then sought to produce preforms from carbon fibers, while offering the possibility of carrying out textile operations, in particular needling. One solution proposed in document US-A-5 228 175 consists, from yarns formed of continuous carbon filaments, of subjecting the yarns to a drawing-cracking operation in order to transform them into yarns of discontinuous carbon fibers arranged substantially parallel to each other and to give at least temporary cohesion to the threads without twisting, so that the threads can be handled and undergo textile operations such as weaving, while needling is possible without damaging deterioration of the threads by removal of untwisted staple carbon fibers. The cohesion of the threads can be brought about by covering with a thread of fugitive material, for example a soluble thread which can be removed after preparation of the preform

This solution is satisfactory, but remains relatively high in cost due not only to the special treatment of the carbon thread, but also and above all to the cost and the low titers of commercially available carbon threads. In addition, it nevertheless remains necessary, at least for certain applications, to carry out a heat treatment at a temperature higher than that to which the carbon wire was brought during its manufacture. This is the case, for example, in the case where the carbon wire contains undesirable residual impurities which can be removed by heat. An example of impurity is sodium, which can be present in carbon threads originating from PAN precursor, which has the effect of a catalyst for carbon oxidation and which consequently reduces the oxidation resistance of the composite material finally produced. . It can also be noted that the handling of carbon threads leads to fiber pollution which can be harmful both to humans 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 techniques, while eliminating the main drawbacks. This object is achieved thanks to a process according to which:

- At least one wire or cable formed from continuous fibers from carbon precursor fibers having undergone intermediate carbonization is used such that the fibers have a carbon content of between 70% and 90% and will have a tensile breaking strength at less equal to 3000 MPa after their carbonization has been completed without necessarily being tensioned, - the wire or cable is used to manufacture 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 resides in the use of wires or cables formed of continuous fibers resulting from a carbonization of a carbon precursor which is not complete but sufficient to ultimately give the fibers mechanical properties similar to those of carbon fibers obtained from same precursor but fully charred under tension. Indeed, the intermediate carbonization being carried out before preparation of the preform, it can advantageously be carried out under tension in order to obtain optimal mechanical properties in the end, carbonization then being completed statically, after preparation of the preform.

In addition, by using wires or cables at an intermediate stage of carbonization, certain disadvantages already mentioned linked to the use of carbon wires are avoided. In particular, it is possible to use commercially available wires or cables having a higher titer than carbon wires and of more economical use. Preferably, wires or cables of at least 50 K are used, that is to say formed from at least 50,000 filaments. The heat treatment carried out on the preform may aim not only to complete the transformation of the precursor, by bringing the temperature to at least about 1200 ° C., but also to remove impurities, by prolonging it at a higher temperature at least equal to 1 600 ° C. Compared to the prior art using carbon fiber yarns, and in which a high temperature heat treatment had to be carried out to remove impurities, the method therefore does not introduce any additional step. Compared to the prior art using wires or cables made of carbon precursor fibers, the method makes it possible not only to achieve a preform in which the fibers have far superior mechanical properties, but also to avoid taking into account a subsequent withdrawal of the preform. This can therefore be produced as close as possible to its final dimensions, therefore by optimizing the duration of the textile operations necessary for this purpose. According to a first embodiment of the method according to the invention, the wire or cable formed from continuous fibers is subjected to a drawing-cracking operation so as to obtain a wire or cable formed from staple fibers, and the wire is imparted or cable formed of staple fibers sufficient cohesion for use in manufacturing the preform.

Cohesion can be brought about by imposing on the wire or cable formed of staple fibers a slight twist.

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

As a variant, the cohesion of the wire or cable formed of staple fibers can be provided by covering, for example by means of synthetic or natural filaments. According to another embodiment of the method according to the invention, the wire or cable formed from continuous fibers is used directly as is to manufacture the preform.

In all cases, the manufacture of the preform advantageously comprises at least one needling step.

Brief description of the drawings

In the accompanying drawings,

- Figure 1 shows successive steps of an embodiment of a method according to the invention;

- Figures 2 and 3 very schematically show stretch-cracking installations; and

- Figures 4 and 5 show successive stages of other modes of implementation of a method according to the invention.

Detailed description of embodiments of the invention

According to the embodiment of FIG. 1, a first step (10) of the method consists in providing wires or cables made of fibers originating from a carbon precursor having undergone intermediate carbonization. By intermediate carbonization is meant here 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 optimal 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 reached when the carbon content is between 70 and 90%, this can vary depending on the carbon precursor used. Intermediate carbonization is obtained by heat treatment at a temperature and / or for a period lower than those necessary 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 approximately 250 ° C., satisfactory intermediate carbonization is carried out by heat treatment under tension at around 900 ° C, while the transformation of the pre-oxidized PAN into carbon is usually carried out at around 1400 ° C.

Preferably, wires or cables of relatively high titer are used, preferably wires or cables of 50 K or more, that is to say formed of 50,000 filaments or more. In general, the wires or cables are commercially available at a cost related to the unit mass which decreases when the titer increases.

Advantageously, use is made of wires or cables offered under the name "Pyon" by the British company SGL Technics Ltd., cables from 320 K to 480 K being commercially available. These wires or cables are formed of continuous filaments originating from PAN precursor from the British company Courtaulds after intermediate carbonization carried out under tension until a carbon content of between 70 and 80% is obtained. In a second step (20), the wire or cable 11 is subjected to a stretching-cracking operation, in order to transform it into 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 carried out by stretching the wire or cable 11 and causing it to break between two pairs of rollers 22, 23 of a stretch train 21 (FIG. 2). Documents FR-A-2 608 641 and US-A-4 759 985 describe the stretch-cracking of carbon threads. Note however that, in the method according to the invention, the stretch-cracking is carried out without particular coating of the wire or cable. In addition, the stretch-cracking is carried out so as 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 installation in which several roller stretching trains 21a to 21JD are provided for drawing-cracking a corresponding number of wires or cables 11a to 11β.

The wires or cables 12a to 12β formed from staple fibers can then be mixed by passing through a drawing device with bars 25. The latter, comprising combs mounted on an endless chain, makes it possible to mix the staple fibers of the different wires or cables, while carrying out a drawing, so that the wire or cable obtained 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 12a to 12p. is equal to 16 (the wires having the same title), the device 25 is adjusted to carry out a stretching with multiplication by 16 of the length.

An installation of the type of that of FIG. 3 is particularly suitable for the production of composite yarns, that is to say yarns formed of staple fibers of different natures. In fact, in the context of the invention, the wires 11a to 11JD may include: - one or more wires or cables formed from continuous fibers originating from carbon precursor fibers having undergone intermediate carbonization such that the fibers have a rate of carbon between

70% and 90% and will have a tensile breaking strength of at least 3,000 MPa after their carbonization has been completed without necessarily being tensioned,

- one or more wires or cables formed from continuous fibers originating from carbon precursor giving fibers having a lower breaking strength, for example continuous fibers originating from phenolic, cellulosic or isotropic precursor, - one or more wires or cables formed continuous fibers from ceramic precursor, for example from silicon carbide, alumina, silica precursor, etc., and

- One or more wires or cables formed of continuous carbon or almost carbon fibers, such as wires or cables of continuous fibers originating from anisotropic pitch intrinsically having a high resistance to breakage.

The strip drawing device 25 allows intimate mixing of the staple fibers coming from the different yarns after drawing-cracking. The wires or cables obtained after stretching-cracking undergo a slight twisting (step 30) in order to give them sufficient strength or cohesion allowing them to undergo subsequent textile operations. The production of fibrous preforms from son or cables can involve different operations such as weaving, unidirectional tablecloths, winding, needling. Some operations, in particular weaving, require minimum cohesion of the son or cables formed of discontinuous filaments, in particular when they are produced at high speed, that is to say, for weaving, a speed which can reach 400 strokes / min or more . On the other hand, needling, in order to be able to be carried out without appreciable damage to the wires or cables, requires that discontinuous filaments can be easily removed. Also, the twisting must be sufficient to give minimum cohesion to the wires or cables, but limited to allow subsequent needling. This is why the degree of twist is preferably between 20 rpm and 120 rpm. It can 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 α giving the ratio between the twist in rpm and the square root of the title in metric number (Nm), is preferably between 30 and 60. The twisting can be carried out, in a well known manner, by means for example a bench with spindles, or a continuous spinning machine, or else a sleeve wiper, the latter carrying out rather a "scrambling" of the fibers than a real twist.

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

By way of example, a preform can be produced by stacking two-dimensional, flat or draped layers on a shape, and bonding the layers together by needling. The two-dimensional layers can be layers of fabric or unidirectional layers formed of wires or cables parallel to one another and superimposed in different directions.

When needling is carried out, very fine needles are preferably used, due to the slight twisting of the wires or cables. By very fine needles is meant here, for example, needles whose active part has, in section, a triangle shape whose height is relatively small, that is to say less than 0.5 mm.

After preparation of the preform, it is subjected to a heat treatment (step 50) in order to complete the transformation of the fiber precursor. This treatment is carried out at a temperature preferably at least equal to 1200 ° C., for example around 1400 ° C. After a plateau at this temperature, the heat treatment can be continued by raising the temperature to a plateau, for example at least about 1600 ° C., in order to remove undesirable impurities present in the carbon fibers, for example sodium. The preform is finally obtained in the desired carbon fibers, with fibers having high mechanical properties, and without significant shrinkage during the heat treatment. FIG. 4 shows another embodiment of a method according to the invention, which differs from that of FIG. 1 in that the sufficient cohesion of the wires or cables obtained after stretching-cracking (step 20) is provided not by a slight twist, but by wrapping (step 30 '). The covering can be carried out using natural or synthetic filaments. The filaments can be made of a removable material, for example by dissolution before the complete transformation of the staple fibers into carbon fibers, or by heat treatment or before or during this transformation. It is also possible to choose filaments made of a material leaving a carbon residue after complete transformation of the staple fibers into carbon fibers. Examples of materials which can be used for covering filaments are cotton, viscose, polyethylene, polyester, polyvinyl alcohol.

The wrapped wires or cables are used for the preparation of the preform (step 40), before heat treatment (step 50). When the preparation of the preform includes a needling phase, the possible elimination of the covering filaments can be carried out before or after the needling.

FIG. 5 shows yet another embodiment of a method according to the invention, which differs from that of FIG. 1 in that the steps 40, 50 of preparation of the preform and of heat treatment are carried out directly on the son or cables of precursor fibers supplied after intermediate carbonization (step 10), the steps of stretch-cracking and of cohesion by slight twisting being omitted. Example 1 An exemplary embodiment of preforms of discs and brake pads in composite C / C according to a method of the type of that of FIG. 1, and tests carried out with discs and brake pads incorporating such preforms will now be described.

Cables of linear mass of 30 g / m, that is to say a title of 30 ktex sold by the British company SGL Technics Ltd under the name "Pyon 15" are used. These are fiber cables from pre-oxidized PAN having undergone intermediate carbonization under tension such that the fibers have a carbon content of 76%, the rest being essentially constituted by nitrogen.

The cables are subjected to a stretch-cracking operation to obtain a wire formed of staple fibers of 1 ktex titer which is made coherent by a slight twist of 35 rpm (α = 35).

The thread obtained is used to make a fabric (twill weaving of 2) having a surface mass of 840 g / m ² and a thickness under load (50 g / m ² ) of 1.8 mm.

Layers of fabric are stacked and needled layer by layer, as described in document FR-A-2 726 013, to bring the volume content of fibers to a value of approximately 20%. A heat treatment is carried out first at around 1400 ° C. to complete the carbonization of the precursor, then the temperature is raised to

1600 ° C in order to remove residual impurities, in particular the sodium present in the fibers. The loss of mass observed is approximately 30%.

Annular preforms of brake discs are cut, as well as preforms of brake pads, then are densified by a pyrolytic carbon matrix by chemical vapor infiltration, in a manner well known per se, in order to obtain discs and pads C / C composite brake shoe.

By way of comparison, reference brake discs and pads in C / C composite are produced in a similar manner, but starting from cables made of preoxidized PAN fibers, which have not undergone 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 wear is evaluated by measuring the loss of thickness expressed in mm. The results are given in the table below.

The reduction in wear, with the C / C material according to the invention, is 38% for the discs and 27% for the pads.

Example 2

One proceeds in accordance with the embodiment of FIG. 4 starting from cables marketed by the British company SGL Technics Ltd. under the name "Pyon 18". These are cables formed from 320,000 filaments (320 K) made of fibers from pre-oxidized PAN having undergone intermediate carbonization under tension such that the fibers have a carbon content of 73%. The title of the starting cables is 34 g / m, or 34 ktex.

The cable is subjected to a stretch-cracking operation in order to obtain an 833 tex thread, the cohesion of which is ensured by covering with a cotton filament of 14.7 tex title.

The wrapped yarn is used to make a fabric (satin weaving of 8) with a surface mass equal to 780 g / m ² and a thickness under load equal to 1.7 mm.

The fabric is steamed at 250 ° C in air so as to thermally degrade the cotton covering thread.

Several layers of fabric are superimposed and needled without difficulty and the preform obtained is subjected to a heat treatment as in Example 1.

Example 3

The procedure is as in Example 2, but without degrading the cotton covering before needling. This is done successfully on the wrapped threads. The cotton covering is degraded during the rise in temperature for the final heat treatment of transformation of the precursor.

Example 4 One proceeds according to the embodiment of the figure

5, starting from threads of 50 K filaments manufactured by the British company SGL Technics Ltd. under the name "Pyon". These are yarns formed from continuous fibers originating from pre-oxidized PAN which have undergone an intermediate carbonization under tension such that the fibers have a carbon content of 76%. The title of the threads is equal to 4.4 ktex.

The threads are woven directly, without any special technical preparation. The fabric obtained has a surface mass of 1.2 kg / m ² . Several strata of fabric obtained are superimposed and needled without difficulty, despite the fact that these are yarns formed from continuous filaments. The preform obtained is then subjected to a heat treatment for transformation of the precursor.

Claims

1. A method of manufacturing carbon fiber preforms, characterized in that: - at least one wire or cable formed from continuous fibers from carbon precursor fibers having undergone intermediate carbonization is used such that the fibers have a rate of carbon between 70% and 90% and will have a tensile breaking strength of at least 3,000 MPa after their carbonization has been completed without necessarily being tensioned,

- the wire or cable is used to manufacture the preform, and

2. Method according to claim 1, characterized in that at least one wire or cable formed from continuous fibers from a precursor having 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 from continuous fibers from preoxidized 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 the wire or cable formed of continuous fibers is subjected to a drawing-cracking operation so as to obtain a wire or cable formed of staple fibers and the wire or cable formed of staple fibers is given sufficient cohesion for use in manufacturing the preform.

5. Method according to any one of claims 1 to 3, characterized in that several different wires or cables are used chosen from wires or cables formed from continuous fibers originating from carbon precursor and wires or cables formed from continuous fibers from ceramic precursor.

6. Method according to claim 5, characterized in that the or each wire or cable is subjected to a stretch-cracking operation, mixes the son or cables formed of staple fibers obtained, and the resulting composite wire or cable is given sufficient cohesion for use in manufacturing the preform.

7. Method according to any one of claims 4 and 6, characterized in that a slight twist is imposed on or on each wire or cable formed of staple fibers.

8. Method according to any one of claims 1 to 7, characterized in that it imposes on the wire or cable formed of staple fibers a twist between 20 and 120 rpm.

9. Method according to any one of claims 4 and 6, characterized in that one confers cohesion by wrapping to, or to each, wire or cable in the form of staple fibers.

10. Method according to any one of claims 1 to 9, characterized in that at least one wire or cable formed from continuous fibers is used, originating from a carbon precursor chosen from pitch-based, phenolic-based, base-based precursors. cellulosic and based on preoxidized polyacrylonitrile.

11. Method according to any one of claims 1 to 10, characterized in that at least one wire or cable of at least 50 K is used.

12. Method according to any one of claims 1 to 11, characterized in that the manufacture of the preform comprises at least one needling step.

13. Method according to any one of claims 1 to 12, characterized in that the manufacture of the preform comprises at least one step of weaving at high speed at least equal to 400 strokes / min.

14. Method according to any one of claims 1 to 13, characterized in that the heat treatment is carried out at a temperature of at least 1200 ° C to complete the transformation of the precursor.

15. The method of claim 14, characterized in that the heat treatment is continued at a higher temperature, at least equal to 1600 ° C.