HK1033158A - Method for making carbon fibre preforms - Google Patents

Description

Method for manufacturing carbon fiber prefabricated blank

no marking

The present invention relates to a method for manufacturing carbon fiber preforms for manufacturing parts made of composite material containing a fibrous preform densified by a matrix.

A particular field of application of the invention is the manufacture of preforms for components made of carbon/carbon (C/C) composite materials, i.e. preforms or reinforcements containing carbon fibres densified by a carbon matrix. Components made of C/C composite materials can be used in various fields, in particular in the form of brake or clutch discs for friction applications.

The current technique for obtaining carbon fiber preforms is to make the preform from carbon precursor fibers, then to carry out at least one carbonization step in order to convert the precursor into carbon. Various precursors may be used, such as precursors based on pitch, phenol, cellulose or even pre-oxidized Polyacrylonitrile (PAN).

The expected advantage of using precursor fibers is that it makes it possible to carry out textile operations, in particular needling, to produce preforms with the desired characteristics, but the needling process can have a destructive effect if carried out directly on carbon yarns available on the market.

However, this technique has some drawbacks. When the fibres are carbonized after the preform has been prepared, so that they are not placed under tension, i.e. carbonized in a static state, the mechanical properties of the former carbon fibres are significantly reduced and show a more extensive dispersion than those obtained from the same precursor but carbonized under tension. As an indicator, carbon fibers obtained from pre-oxidized PAN have tensile breaking strengths in the range of about 1600 to 2400MPa when carbonized under static conditions, whereas tensile breaking strengths in the range of about 3000 to 4000MPa when carbonized under tension. The modulus has a value ranging from about 200Gpa to 210Gpa to about 220Gpa to 240 Gpa. Another disadvantage is that carbonization causes shrinkage. This must therefore be taken into account when sizing preforms made from precursor fibers.

It is therefore desirable to make preforms from carbon fibres, while also offering the possibility of carrying out weaving operations, in particular needling. The solution proposed in US-A-5228175 is to subject the yarn formed of continuous carbon filaments to A drawing/cracking operation, converting it into A yarn of discontinuous carbon fibers substantially parallel to each other, and giving the untwisted yarn at least temporary cohesion, so that the yarn can be treated and subjected to o mutextile operations such as weaving, and needling is also possible since it is not possible to damage the yarn by removing untwisted discontinuous carbon fibers. The yarns may be held together for cohesion by using yarns made of materials that are present for a short period of time, such as soluble yarns that can be removed after the preform is prepared.

This solution is satisfactory but nevertheless relatively costly, not only because of the special treatment of the carbon yarn, but also and in particular because of the low price and weight of the carbon yarns available on the market. Moreover, it is nevertheless necessary, at least for some applications, to carry out the heat treatment at a higher temperature than the carbon yarn produced during its manufacture. This may be applied, for example, where the carbon yarn contains residual impurities that are not intended to be removable by heating. An example of an impurity is sodium, which may be present in the carbon yarn from the PAN precursor, acts as a catalyst for the oxidation of carbon and thus reduces the oxidation resistance of the final manufactured composite. It is also noted that the treated carbon yarn causes contamination by fibers that are harmful to both humans and machinery.

It is an object of the present invention to provide a method for manufacturing carbon fibre preforms which combines the advantages of the prior art while excluding the major drawbacks thereof.

This object is achieved by the following method:

using at least one yarn or tow formed of continuous fibers derived from carbon precursor fibers which have been subjected to intermediate carbonization so that the carbon content of the fibers is in the range of 70 to 90% and which exhibit a tensile breaking strength of not less than 3000Mpa after the carbonization is completed, the carbonization not necessarily being carried out under tension;

making a preform using a yarn or tow;

and heat treating the preform at least to complete the conversion of the discontinuous fibers to carbon fibers.

One feature of this process is the yarn or tow formed of continuous fibers obtained using carbonized carbon precursor fibers, the carbonization being incomplete, but nevertheless sufficient to give the fibers final mechanical properties similar to those of carbon fibers obtained from the same precursor but completely carbonized under tension. Since the intermediate carbonization is carried out before the preparation of the preform, it is advantageously carried out under tension in order to obtain the best final mechanical properties, so that the carbonization can be carried out statically after the preparation of the preform.

Furthermore, by using yarns or tows having intermediate carbonization stages, some of the disadvantages associated with the use of carbon yarns described above can be avoided. In particular, it is possible to use the yarns or tows which are currently available, which are heavier and cheaper to use than carbon fibers. It is preferable to use a yarn or a tow of not less than 50K, that is, a yarn or a tow formed of not less than 50000 filaments.

The heat treatment of the preform serves not only to complete the conversion of the precursor by raising its temperature to at least about 1200 c, but also to remove impurities by prolonging the heat treatment at higher temperatures of not less than 1600 c. For the prior art using carbon fiber yarns and having to be subjected to a high temperature heat treatment to remove impurities, this method does not introduce an additional step. With the prior art using yarns or tows made from carbon fiber precursors, this process not only allows preforms of fibers with significantly improved mechanical properties to be obtained, but also avoids any need to allow the preform to subsequently shrink. The preform thus produced is closer to its final dimensions, thereby optimizing the weaving operation time required to achieve this purpose.

In a first embodiment of the invention, the yarn or tow formed of continuous fibers is subjected to a stretching/cracking operation in order to obtain a yarn or tow formed of discontinuous fibers and to give the yarn or tow formed of discontinuous fibers sufficient cohesion to make it suitable for the manufacture of preforms.

Cohesion may be achieved by applying a small amount of twist to a yarn or tow formed of discontinuous fibers. By "a small amount of twist applied to a yarn or tow formed of discontinuous fibers" is meant a twist sufficient to impart sufficient strength to the yarn or tow to enable it to withstand weaving operations, particularly weaving, and more particularly high speed weaving, while still retaining the possibility of subsequent needling in at least one instance during which the discontinuous fibers can be needled without significantly damaging the yarn or tow. The amount of twist can vary as a function of the weight of the yarn or tow. The amount of twist is preferably in the range of about 20 twists per meter (tr/m) to 120 tr/m.

In one variation, the cohesion of the yarn or tow formed of discontinuous fibers may be achieved by over-coating or over-spinning a layer of, for example, synthetic or non-synthetic filaments.

In another embodiment of the process of the present invention, the preform is made directly in an untreated state using yarns or tows formed from continuous fibers.

In both cases, conveniently manufacturing the preform comprises at least one needling step. Brief Description of Drawings

In the drawings:

FIG. 1 shows the sequential steps of one embodiment of the process of the present invention;

FIGS. 2 and 3 show a highly generalized stretch/cracker unit; and

fig. 4 and 5 show the successive steps of other embodiments of the process of the invention. Detailed description of embodiments of the invention

In the embodiment of fig. 1, the first step (10) of the process consists in providing a yarn or tow made of fibers derived from a carbon precursor which has been subjected to an intermediate carbonization. "intermediate carbonization" refers to the carbonization of an intermediate between a precursor state and a carbon state. The intermediate carbonization is carried out under tension in order to obtain fibres with optimal mechanical properties. The degree of carbonization is preferably such that the level of mechanical properties is close to or substantially equal to the level of mechanical properties obtained after complete conversion of the precursor under tension. Such carbonization levels are achieved when the carbon content is in the range of 70-90%, the level of carbon content varying depending on the carbon precursor used. Intermediate carbonization may be achieved by heat treatment at a lower temperature and/or in a shorter time than is required to achieve complete carbonization.

For example, in the case of a pre-oxidized PAN precursor, which has been raised to a maximum temperature of about 250 ℃ during its preparation, satisfactory intermediate carbonization can be carried out by heat treatment under tension at about 900 ℃, whereas the conversion of pre-oxidized PAN to carbon is usually carried out at about 1400 ℃.

It is preferred to use a relatively heavy yarn or tow, preferably not less than 50K, i.e. not less than 50000 filaments. Generally, as the weight of the commercially available yarn or tow increases, the price per unit mass decreases.

Yarns or tows manufactured by British company SGL technologies Ltd under the trade mark "Peon" are conveniently used, with tows ranging from 320K to 480K being commercially available. The yarn or tow is formed from filaments made from PAN precursor available from british company Courtaulds, which are intermediately carbonized under tension until a carbon content in the range of 70-80% is obtained.

In a second step (20), the yarn or tow 11 is subjected to a drawing/cracking operation to convert it into a yarn or tow 12 formed of discontinuous filaments that are substantially parallel to the longitudinal direction of the yarn or tow. The drawing/cracking operation is well known and is typically carried out by drawing the yarn or tow 11 and by causing it to break between two pairs of rollers 22 and 23 of a drawing system 21 (fig. 2). FR-A-2608641 and US-A-4759985 describe the stretching/cracking of carbon fibers. It should be observed, however, that in the process of the present invention, the drawing/cracking is carried out without special coating of the yarn or tow. Further, drawing/cracking is performed to obtain a yarn or tow 12 formed of long discontinuous fibers. "Long fibers" means fibers having an average length of not less than 60 mm.

Fig. 3 shows a drawing/cracking apparatus in which a plurality of drawing systems including rolls 21a to 21p are provided to perform drawing/cracking of a corresponding number of yarns or tows 11a to 11 p.

The yarns or tows 12a to 12p formed of the discontinuous fibers are then mixed together by a drawing device 25 having a needle plate. The apparatus includes combs mounted in an endless loop to enable discontinuous fibers of various yarns or tows to be commingled and simultaneously drawn so that the resulting yarns or tows 13 have the same weight as each yarn or tow received by the apparatus 25. Thus, for example, when the number of yarns or tows 12a to 12p is equal to 16 (yarns having the same weight), the adjusting device 25 stretches the length by a factor of 16.

The apparatus shown in fig. 3 is particularly suitable for making composite yarns, i.e., yarns formed of various discontinuous fibers. Within the scope of the invention, the yarns 11a to 11p may comprise:

one or more yarns or tows formed from continuous fibers of carbon precursor fibers which have been intermediately carbonized so that the carbon content of the fibers is from 70% to 90% and which exhibit a tensile breaking strength of not less than 3000Mpa after carbonization is completed, the carbonization not necessarily being performed under tension;

one or more low breaking strength continuous fibers derived from carbon precursor fibers, such as yarns or tows formed from continuous fibers based on phenol, cellulose, or isotropic pitch;

one or more yarns or tows derived from ceramic precursors such as silicon, alumina, silica …, carbide formation; or

One or more yarns or tows formed entirely or almost entirely of continuous fibers made of carbon, such as yarns or tows made from continuous fibers having high intrinsic breaking strength of isotropic pitch precursors.

The drawing equipment 25 with needle plates enables good mixing of the discontinuous fibers from the various yarns after drawing/cracking.

The yarns or tows obtained after drawing/cracking are twisted (step 30) with a small amount to impart sufficient strength or cohesion to make them withstand subsequent weaving operations. Various preforms made from yarns or tows require various operations such as weaving, arranging on unidirectional sheets, winding, and needling. Certain operations, particularly weaving, require that the yarns and tows formed from the discontinuous fibers have a minimum amount of cohesion, particularly when they are run at high speeds, i.e., speeds no less than 400 strokes/minute for weaving. In contrast, in order to avoid serious damage to the yarn or filament bundle during needling, it is necessary to use discrete filaments that are easily handled. The amount of twisting must also be sufficient to impart minimal cohesion to the yarn or tow, while at the same time being sufficiently limited to enable subsequent needling to occur. This is why the twist is preferably in the range of 20tr/m to 120 tr/m. Lighter yarns (in tex) are selected to have higher values than heavier yarns. Thus, the ratio of the amount of twist (unit: tr/m) expressed by the coefficient α to the square root of the weight thereof (unit: metric count Nm) is preferably in the range of 30 to 60.

Twisting can be carried out with a roving frame, or a continuous spinning frame, or even a draw frame (ribbon winder) according to well known methods, for example said draw frame will produce a number of "commingled piles" of fibres rather than true twisted threads.

This allows the desired preform to be prepared using a yarn or tow having a small amount of twist (step 40). For this purpose, operations such as weaving, yarn-making sheets, winding and needling, as described above, can be carried out.

For example, the preform can be made by stacking two-dimensional layers, planar layers, or overhanging layers onto a forming card and joining the layers together by needling. The two-dimensional layer may be a fabric layer or a unidirectional laminate layer formed from yarns or tows that are parallel to each other and coincident in each direction.

Due to the small twist of the yarn or tow, it is preferable to use an extremely fine needle for needle punching. "extremely fine needles" are, for example, needles whose active part has a triangular portion, which is of small height, i.e. less than 0.5 mm.

After the preform is prepared, it is heat treated (step 50) to complete the conversion of the fiber precursor. The temperature at which the treatment is carried out is preferably not less than 1200 c, for example about 1400 c. After a period of time at this temperature, the elevated temperature continues with a higher level of heat treatment, for example at about 1600 ℃, in order to remove undesirable impurities, such as sodium, present in the carbon fibers. The resulting desired carbon fiber preform has improved mechanical properties of the fibers without severe shrinkage during heat treatment.

Figure 4 shows another apparatus of the process of the invention which differs from that shown in figure 1 in that the yarn or tow obtained after stretching/cracking (step 20) is sufficiently cohesive by means of a sheath (step 30'), rather than by a small amount of twisting.

The outer covering may be made of synthetic or non-synthetic filaments. The filaments may be made of a material that can be removed, for example by being dissolved before the complete conversion of the discontinuous fibres made of carbon fibres, or by being subjected to a heat treatment before or during said conversion. Filaments of materials that leave carbonaceous residue after complete conversion of the discontinuous fibers made of carbon fibers may also be selected. Examples of materials for making the clad filaments are cotton, viscose, polyethylene, polyester and polyvinyl alcohol.

The coated yarn or tow is used to prepare a preform (step 40) prior to heat treatment (step 50). When the preparation of the preform includes a needling step, the removal of the cladding filaments can be carried out optionally before or after the needling.

Fig. 5 also shows another apparatus of the process of the invention, which differs from that shown in fig. 1 in that preform preparation step 40 and heat treatment step 50 are carried out directly on the yarn or tow made of precursor fibers provided after intermediate conversion (step 10) which omits stretching/cracking and a small twisting step to provide cohesion.

Example 1

The examples described below relate to the manufacture of C/C composite preforms for brake discs and brake linings using the method shown in FIG. 1 and the testing of brake discs and brake linings incorporating the preforms described above.

The tow used has a mass per unit length of 30g/m, i.e. a weight of 30 kilotex, sold by the british company SGL technologies Ltd under the trade mark "Pyon 15". The tow is made of fibers derived from pre-oxidized PAN that has been intermediately carbonized under tension to a carbon content of 76% with the remainder consisting essentially of nitrogen.

The tow was subjected to a drawing/cracking operation to obtain a yarn weighing 1 kilotex and formed from discontinuous fibers that had been twisted into cohesive with a small amount of 35 tr/m.

The resulting yarn was used to make a fabric (double twill) with an areal weight of 840g/m²And load (50 g/m)²) The lower thickness is 1.8 mm.

The layers of fabric are stacked and stitched layer by layer as described in FR- A-2726013 to achieve A fiber volume fraction of 20%. A heat treatment is initially carried out at about 1400 c to complete carbonization of the precursor, and then the temperature is raised to 1600 c in order to remove impurities present in the fibers, in particular sodium. The mass loss observed was about 30%.

Annular preforms for brake discs and brake linings are cut out and then densified by chemical vapor infiltration with a pyrolysed carbon matrix, in a manner known per se, in order to obtain brake discs and brake linings made of C/C composite material.

For comparison, a control brake disc and brake lining were produced in a similar manner from a C/C composite, but the tow used at the outset was made from pre-oxidized PAN fibers without intermediate carbonization, which was carried out after needling and therefore not under tension.

The control brake disc or pad is subjected to the same high-energy braking test as the brake disc or pad of the invention, and the reduction in thickness (in mm) is measured to assess the wear induced. The results are shown in the following table.

	Control	According to the invention
			Disc wear	1.38	0.85
Lining wear	1.78	1.29

The wear of the discs and linings using the C/C composite of the invention was reduced by 38% and 27%, respectively.

Example 2

Following the apparatus flushing procedure of figure 4, the starting material was a tow manufactured by British company SGLTechnics Ltd under the trade mark "Pyon 18". The tow was formed from 320,000 filaments (320K) from pre-oxidized PAN that had been intermediately carbonized under tension to a carbon content of 73%. The weight of the starting tow was 34g/m, i.e., 34 kilotex.

The tow was subjected to a drawing/cracking operation to obtain 833 tex cohesive yarn, with a 14.7 tex heavy cotton filament covering to ensure its cohesion.

The yarn with the covering was used to produce a fabric (8 satin weave) with a mass per unit area of 840g/m²And a thickness under load equal to 1.7 mm.

The fabric was dried in air to pyrolyze the cotton-covered yarns.

The multi-layer fabric was overlapped and stitched without difficulty and the resulting preform was heat treated as in example 1.

Example 3

The same procedure as in example 2 was followed, but without degrading the cotton coating before needling. The needling can be smoothly performed on the covering yarn. When the temperature is raised for the final heat treatment of the conversion precursor, the cotton cladding is degraded.

Example 4

The punching procedure was carried out according to the apparatus of FIG. 5, starting from a yarn of 50k filaments manufactured by British company SGLTechnics Ltd under the trade mark "Peon". The yarn is formed from continuous fibers derived from pre-oxidized PAN that have been intermediately carbonized under tension to a carbon content of 76%. The weight of the yarn is equal to 4.4 kilotex.

The yarn is directly knitted without special technical preparation. The mass per unit area of the obtained fabric was 1.2Kg/m². Although the yarns are formed from continuous filaments, multiple layers of fabric can be overlapped and stitched without difficulty. The resulting preform is heat treated to convert the precursor.

Claims (15)

1. A method of making a carbon fiber preform, the method characterized by:

at least one yarn or tow formed of continuous fibers obtained from carbon precursor fibers which have been subjected to intermediate carbonization under tension so that the carbon content thereof is in the range of 70 to 90% and which exhibit a tensile breaking strength of not less than 3000Mpa after completion of carbonization which does not necessarily have to be performed under tension is used;

using the yarn or tow to make a preform; and

the preform is heat treated at least to complete the conversion of the discontinuous fibers to carbon fibers.

2. A process according to claim 1, characterised in that at least one yarn or tow used is formed from continuous fibres derived from a precursor which is subjected to intermediate carbonisation under tension.

3. A method according to claim 1 or 2, characterized in that at least one yarn or tow used is made of fibres of pre-oxidized polyacrylonitrile, which has been subjected to intermediate carbonization under tension to a carbon content of 70-80%.

4. A process according to any one of claims 1 to 3, characterized in that the yarn or tow formed of continuous fibres is subjected to a stretching/cracking operation to obtain a yarn or tow formed of discontinuous fibres and to give the yarn or tow formed of discontinuous fibres sufficient cohesion to make it suitable for the manufacture of preforms.

5. A method according to any one of claims 1 to 3, wherein the plurality of different yarns or tows are selected from yarns or tows formed from continuous fibres derived from carbon precursors, continuous fibre yarns or tows derived from ceramic precursors.

6. A process according to claim 5, wherein the or each yarn or tow is subjected to a drawing/cracking operation, and the yarns or tows formed from the discontinuous fibres produced are mixed together to give the composite yarn or tow sufficient cohesion to make it suitable for use in the manufacture of preforms.

7. A method according to claim 4 or claim 6, wherein a small amount of twist is applied to the yarn or tow formed from the or each discontinuous fibre.

8. A method according to any one of claims 1 to 7, wherein the amount of twist applied to the yarn or tow formed from the discontinuous fibres is from 20tr/m to 120 tr/m.

9. A method according to claim 4 or claim 6, wherein the yarn or tow formed by the or each discontinuous fibre is given cohesive strength by the cladding.

10. A method according to any one of claims 1 to 9, characterised in that at least one yarn or tow is used which is formed from continuous fibres derived from carbon precursors selected from the group comprising pitch, phenol, cellulose or pre-oxidised polyacrylonitrile-based.

11. A method according to any one of claims 1 to 10, wherein at least one yarn or tow is used which is not less than 50K.

12. A method according to any one of claims 1 to 11, wherein the preform is produced by at least one needling step.

13. The method according to any of claims 1 to 12, wherein the preform is produced by at least one step of high speed weaving with not less than 400 strokes/min.

14. A method according to any one of claims 1 to 13, characterized in that the heat treatment is carried out at a temperature of not less than 1200 ℃ in order to complete the conversion of the fiber precursor.

15. The method of claim 14, wherein the heat treatment is continued at a higher temperature of not less than 1600 ℃.