FR2674587A1 - Friction pads made of carbon-carbon composite material with differential texture, methods and devices for manufacturing them - Google Patents

Abstract

Multi-layer friction pad made of carbon-carbon composite material including a structural layer (1) with so-called "coarse" texture comprising segments of tufts (rovings) and at least one friction layer (2) with so-called "fine" texture comprising segments of fibres. Method for manufacturing such a friction pad by preparing a fibrous substrate by introducing segments of tuft and/or of fibres into a mould, preferably with the aid of a dispensing device. Device for automatically obtaining a fibrous substrate having predetermined texture.

Description

FRICTION ELEMENTS OF CARBON-CARBON COMPOSITE MATERIAL
DIFFERENTIAL TEXTURE, METHODS AND DEVICES FOR
MANUFACTURING
FIELD OF THE INVENTION
The invention relates to the field of friction materials made of carbon-carbon composite material and more specifically multi-layer friction materials made entirely of carbon-carbon composite and having a differential texture, as well as a method of manufacturing multilayer friction elements and a device for implementing this method. The invention completes the French application No. 9006488.

REMINDER OF THE PRIOR ART
Composite carbon-carbon materials, which will be referred to hereafter abbreviated as "composites
CC ", have been recognized for many years as particularly interesting friction materials especially in high performance brakes, that is to say more especially when is sought a high braking power per kilogram of friction material, in the field brakes for aeronautics in particular.

In general, the composites C-C consist, as is known, of a fibrous substrate based on carbon fibers and a carbon matrix. The latter can be obtained either by one or more impregnations of the fibrous substrate with a resin or a pitch followed by carbonization, or by chemical vapor deposition of pyrolytic carbon, or by combining the two processes.

In addition, these composites can undergo a heat treatment at very high temperature, called graphitization, typically above 2300 C.

Brake discs having a stack of layers of carbonaceous materials are already known. Thus, numerous patents, such as FR 2 313 601, FR 2 260 726 and US Pat. No. 3,936,552 describe brake discs generally having a core layer, or core, made of carbon or crude graphite on which layers are fixed. DC composite friction system offering better wear resistance than the carbon or raw graphite core layer. One of the motivations invoked to justify this type of stack is the cost of carbon or crude graphite, lower than that of a composite material C-C.

Also known from FR 2,225,654 brake discs resulting from the assembly of several layers of composites
CC by adhesives or by mechanical means so as to have a central layer, reusable, covered with a wear layer, the central layer ensuring the mechanical strength and the wear layer constituting a replaceable wear plate.

However, although FR 2 225 654 describes the general concept and the interest of a multilayer composite DC brake with a layer providing mechanical strength and one or more layers providing the friction function, this patent does not say how to obtain a multilayer brake with high technical performance, nor how to obtain it economically.

OBJECT OF THE INVENTION
The object of the invention is a composite multilayer friction element CC, comprising a structural layer and at least one friction layer, with high technical performance and having characteristics adapted to the functions of each layer, a high mechanical strength and in particular, high resilience for the structural layer, advantageous tribological properties and in particular a low wear rate for the friction layer (s).

The invention also relates to a method for the economical manufacture of multilayer friction materials and a device for carrying out the method.

DESCRIPTION OF THE INVENTION
The first object of the invention is a multilayer friction element comprising a structural layer (1) made of carbon-carbon composite material, at least one face of which is covered with a friction layer (2) made of carbon-carbon composite material. of said structural layer (1) characterized in that, on the one hand, the structural layer (1) is obtained from a so-called "coarse" texture fibrous substrate (5g) having a median upper pore diameter at 100 μm and constituted by carbon fiber or carbon fiber precursor strands and that, on the other hand, the friction layer (2) is obtained from a fibrous substrate (5f) with a so-called texture "Fine" having a median pore diameter of less than 50 μm and consisting of randomly distributed carbon fiber or carbon precursor segments (6).

Indeed, in order to improve the performance of friction elements of multilayer material, the Applicant has studied the influence of the texture of composite materials C-C, and in particular the influence of the fibrous substrate on both mechanical and tribological properties. She had the surprise to make the following observations - as far as the resilience and thus the ability to withstand the shock is concerned, a coarse textured substrate (5g) makes it possible to obtain composites having a resilience of the order of 40 at 50 kJ / m2, whereas a fine-textured substrate (5f) leads to a low-resilience composite, of the order of 3 to 5 kJ / m2, all things being equal.

certain mechanical properties, such as modulus and tensile strength, do not vary substantially with the texture of the fibrous substrate, unlike others, such as the flexural strength which varies from 150 to 200 MPa for a composite to coarse texture and 40 to 120 MPa for a fine textured material.

with regard to the wear resistance, it has been found that a fine-textured fibrous substrate makes it possible to obtain a composite material having a wear rate which could be up to 6 times less than that of a coarse textured fibrous substrate composite material.

The studies carried out by the Applicant have shown the essential role played by the fibrous substrate and have made it possible to find a criterion of texture of the fibrous substrate making it possible either to reinforce the mechanical characteristics of the structural layer by using a coarse-textured fibrous substrate. either to reinforce the tribological properties of the friction layer by using a finely textured fibrous substrate with randomly oriented fiber segments, so that, by combining layers of different texture, it is possible to obtain optimized multilayer brake elements against both mechanical and tribological constraints.

By median pore diameter greater than 100 μm, it is meant that at least 50% of the pore volume consists of pores with a diameter greater than 100 μm. And, similarly, a median pore diameter of less than 50 μm means that at least 50% of the pore volume consists of pores with a diameter of less than 50 μm.

 For practical reasons, the pore diameter is measured not on the fibrous substrate as manufactured according to the invention, but on this fibrous substrate after beginning densification by deposition of carbon, so as to be able to handle the fibrous substrate without the alter and thus have meaningful and comparative measures. As a guide, a fibrous substrate obtained according to the invention and having an apparent density of 0.6 becomes manipulable for a pore diameter measurement when its density passes, by carbon deposition, to about 0.9, knowing that the final apparent density is of the order of 1.8 at the end of densification.

It should be noted that the fineness of the pores and the total porosity (or pore volume) are two different sizes which can vary independently of one another. Thus, in the tests carried out by the Applicant, it was observed that the coarse textured substrate (5g) had a total porosity lower than that of the fine textured substrate ((f), for example, as indicated in Example 1 the coarse-textured substrate (5g) has a porosity of 30% (percent of the total volume), while that of the fine-textured fibrous substrate (5f) has a total porosity of 42%, also determined after beginning of densification.

According to a first embodiment of the invention, the fibrous substrate consists entirely of segments randomly oriented: carbon fiber segments (6) or carbon precursor forming the so-called fine-textured fibrous substrate (5f) of the friction layer (2), - segments of wicks (9) of carbon fiber or carbon precursor forming the so-called coarse texture fibrous substrate (5g) of the structural layer (1).

According to the invention, the length of said wick segment is between 5 and 60 mm and that of said fiber segment is between 0.05 and 60 mm.

the fibrous substrate of the friction layer (5f) may advantageously consist of crushed carbon fibers, comprising fiber segments (6) with an average length of between 0.2 and 2 mm.

The carbon or carbon precursor fibers that can be used for the manufacture of the fibrous reinforcement according to the invention are, as is known, in the form of long strands consisting of a large number of parallel fibers, typically from 1000 to 320000 elementary carbon or carbon precursor fibers, each fiber having a diameter generally between 5 and 12 ym.

According to the invention, the fibrous substrate of a structural layer (1) consists of strands of carbon fibers or carbon precursor. By wick means a large number of elementary fibers which is more or less that of the starting wick, generally greater than 1000 and up to 320000.

By wick segment (9) is meant a wick portion comprising a large number of substantially parallel elementary fibers and of length as defined above.

On the other hand, the fibrous substrate of a friction layer consists of fiber segments (6), each fiber segment being of length as previously defined and ideally corresponding to a single fiber and in practice to a small number of assembled fibers. number typically less than 100.

Thus, in the case of a friction layer (2), the fibrous substrate (su), based on fiber segments (6), can be shown schematically in FIG. 1-4 and FIG. represents an elementary fiber or a small number of fibers (less than 100), while FIG. 1-3 and FIG. 2-1 represent, in the case of a structural layer (1), a fibrous substrate (5g) based on segments of wicks (9), ie based on "packets" consisting of a large number of substantially parallel elementary fiber segments.

A fibrous substrate consisting of only segments of wicks or fibers forms an uninterrupted entanglement of wick / fiber segments and provides a one-piece multilayer friction element without discontinuity of the fibrous substrate between the portion containing a coarse-textured fibrous substrate (5g) and the portion containing a fine-textured fibrous substrate (5f), and without discontinuity of the carbonaceous matrix either. In this case, the fibrous substrate is uninterrupted between the friction layer (2) and the structural layer (1) and continuously varies in texture by mixing and interpenetration of fine-textured and coarse-textured segments at the interface of the two layers which ensures the maximum cohesion between a friction layer and a structural layer.

In this case, there is therefore no specific bonding layer, for example an adhesive layer, between a structural layer and a friction layer, but only at the interface, an uninterrupted entanglement of segments of wicks. and fibers to a greater or lesser thickness.

Similarly, the carbon matrix is continuous over the entire friction element since it results from an overall densification of the entire fibrous substrate, which contributes greatly to the cohesion of the various layers.

According to another embodiment of the invention, the fibrous substrate of the composite friction element CC may also comprise a carbon material in the form of a web - which will be, in the case of the structural layer, an oriented fibrous substrate (13). ), typically based on fabrics, or 3-dimensional woven products obtained from strands of carbon fibers or carbon precursor, to obtain, possibly after compression, a median pore diameter greater than 100 rm.

which will be, in the case of the friction layer, a mat or a felt of carbon or carbon precursor constituted by an entanglement of ideally elementary fibers and making it possible, possibly after compression, to obtain a median pore diameter less than 50) Im.

Finally, according to another object of the invention, the friction element can be obtained from a "mixed" fibrous substrate constituted either by a fibrous substrate consisting of a stack of carbon fabrics or carbon precursor for the structural layer (1) and a fibrous substrate consisting of fiber segments according to the invention, and preferably crushed fibers for the friction layer (2), or by a fibrous substrate consisting of segments of wicks for the layer of structure (1) and a fibrous substrate consisting of a mat or a felt for the friction layer (2).

In general, the invention relates to a brake element whose fibrous substrate has a differential texture that can be adapted to any particular problem. This differential texture is based essentially on the presence of either strands in the structural layer (1) or fibers, ideally elementary, in the friction layer (2), and therefore depends on the number of elementary fibers agglomerated locally, a fabric being considered as wicks, and a felt or mat as fiber. This number is typically at least 10 times greater in the case of a wick (> 1000) than in the case of a fiber (<100) according to the invention.

This texture will generally vary along a Z axis perpendicular to the different layers as indicated above. However, there may be cases where the differential texture may vary in the plane of a layer, radially, for example to strengthen the central portion of a brake disc, as illustrated in FIGS. 4-3 and 4-4 or to strengthen the outer portion, as shown in Figures 8-1 and 8-2.

 According to the invention, the expression "differential texture" thus covers different cases. Some cases have been shown on plates 4, 5, and 8 to illustrate the concept of differential texture.

Whether the coarse-textured fibrous substrate (5g) consists of wicks (fabrics) or wick segments, its apparent density before densification is between 0.3 and 0.7, while that of the fine-textured fibrous substrate is comprised between 0.05 and 0.3. The overall bulk density of the fibrous substrate (5g) and (5f) is between 0.1 and 0.7.

The fibrous substrate (5g) represents from 10 to 50% by volume of the composite CC constituting the structural layer (1), while the fibrous substrate (5f) represents from 3 to 25% by volume of the composite CC constituting the friction layer (2).

According to the invention, the carbon matrix of the multilayered DC composite, whether it be a structural or friction layer, is not specific to the invention and results, in known manner, from the densification of the substrate compressed fiber, either by pyrolytic carbon chemical deposition in the vapor phase, or by one or more impregnation treatments with a resin or a pitch resulting in a high carbon content after carbonization, treatments followed by carbonization, or by combination of the two processes. After densification, the friction element according to the invention is a composite material C-C with a density of between 1.55 and 1.8, which may optionally be subjected to a subsequent graphitization treatment.

The invention also relates to friction elements consisting of an assembly of CC composite structure layers (1) and CC composite friction layers (2) obtained respectively by densification of coarse textured fibrous substrate (5g) and fine-textured fibrous substrate (5f), the assembly being able to be carried out by any known means, in particular by a connecting layer (4).

Preferably, a resin is used as a bonding layer (4) which, by heating, forms a carbonaceous layer, which is generally porous. The friction element obtained by assembly can have a continuous carbon matrix on the whole of the friction element because it is advantageous to assemble structural and friction layers having a residual porosity and to finish the manufacture of the friction element by a carbon deposit so as to densify all the layers and the bonding layer (4).

The invention makes it possible to obtain friction elements that are generally in the form of discs with a diameter that can reach or exceed 600 mm (outside diameter) and that typically can have a central layer with a thickness structure of the order of 10 - 20 mm, covered with two layers of friction of the order of 3 - 7 mm.

As already mentioned, the invention makes it possible to obtain, by virtue of the differential texture of the structure and friction layers of composite DC, high specific performances, ie considered per kilogram of friction element, both in terms of which concerns the mechanical properties, the resilience in particular, that the tribological properties, the wear resistance mainly.

As is known, it is therefore possible to renovate a friction element by replacing the worn friction layer with a new layer obtained according to the invention.

In addition, the friction element according to the invention has, in particular in the case of a fibrous substrate consisting of segments of locks or fibers, the advantage of not having a local absence of substrate at the scale. dimensions of the local substrate, ie within a maximum volume of about 0.05 mm 3 for a fine textured fibrous substrate and about 5 mm 3 for a coarse textured substrate.

The second object of the invention is a process for the economical manufacture of these composite friction elements C-C.

According to a first modality, the method of manufacturing a multilayer carbon-carbon composite friction element consisting of a structure layer (1) and at least one friction layer (2) comprises a step of manufacturing a a fibrous substrate with a desired geometric shape, a possible carbonization step If said fibrous substrate is based on carbon fiber precursor, and a step of densifying said fibrous substrate by depositing carbon to form the carbon matrix, said manufacturing step the desired geometrically shaped fibrous substrate is characterized in that all or part of said fibrous substrate is made by introducing into a mold (10), possibly using a distribution device, segments of carbon fiber locks. or carbon precursor of length between 5 and 60 mm to form the fibrous substrate (5g) of the structural layer (1), carbon fiber segments or carbon precursor of length between 0.05 and 60 mm to constitute the fibrous substrate (5f) of the friction layer (2), and compressing the segments of locks and fibers, so as to obtain a fibrous substrate (5f, 5g) with an overall bulk density of between 0.1 and 0.7 and consisting of segments of wicks and / or fibers entangled and randomly oriented.

The compressed fibrous substrate has the general shape of the mold, which will therefore be cylindrical if the friction element should be in the form of a disc, which is the case generally. However, the method makes it possible to manufacture fibrous substrates with any geometrical shape section.

According to a variant of this first modality, only a portion of the fibrous substrate is formed from segments of locks and / or fibers and its preparation is completed by adding cut elements from carbon or carbon precursor webs, tablecloths. made of fabrics or felt / mat according to whether a fibrous substrate with coarse or fine texture is desired. These different substrate parts can be assembled using adhesives or compression. Once formed, the overall fibrous substrate is subjected to densification treatment by carbon deposition.

According to a second embodiment, the method for producing a multilayer carbon-to-carbon composite friction element consisting of a structural layer and at least one friction layer comprises the manufacture of a structural layer (1). in carbon-carbon composite and at least one carbon-carbon composite friction layer (2), their assembly by any known means, such as a tie layer (4), characterized in that the structural layer is manufactured by depositing carbon on a coarsely textured fibrous substrate (5g) having a median pore diameter of greater than 100 μm and producing the friction layer by depositing carbon on a fine-textured fibrous substrate (5f) having a pore diameter median less than 50 pm, the fibrous substrates being previously carbonized in the case of a fibrous substrate precursor carbon fiber.

The coarsely textured fibrous substrate (5g) is obtained from a web, carbon fiber fabric or carbon precursor, or from carbon fiber or carbon precursor segments, length of between 5 and 60 mm, placed in a mold possibly using a dispensing device and then compressed until an apparent density of between 0.3 and 0.7.

 The fine-textured fibrous substrate (5f) is obtained from a mat, a carbon fiber felt or a carbon precursor, or from carbon fiber or carbon precursor segments, of length between 0.05 and 60 mm, placed in a mold (10), possibly using a dispensing device, then optionally compressed to obtain an apparent density of between 0.05 and 0.3.

This second modality can therefore be advantageously used for the renovation of worn friction elements which in practice have only one structural layer (1) made of composite C-C. But it is also possible to renovate a worn friction element by subjecting a layer of structure (1) of composite CC coated with at least one layer of fibrous substrate (5f) with a fine texture to carbon densification, this modality constituting in a way a mixture of the two preceding modalities.

To implement the invention in the case where the fibrous substrate comprises wick or fiber segments, the applicant has developed a device for distributing wick segments and / or high performance fibers for the implementation of the invention. process, ensuring the automatic filling of the mold.

According to the invention, the wick or fiber segment distribution device comprises at least one distribution head (8) of wick segments and / or fibers, in relative movement with respect to the mold (10) and sweeping the together the horizontal surface of the mold, so as to fill gradually and regularly, in successive layers, the mold segments randomly oriented strands (9) and / or carbon fibers (6) while maintaining a distribution plane (7). ) substantially horizontal, the proportion of segments of wicks and / or carbon fibers distributed by the (es) said (s) head (s) being preferably slaved to the coordinates (z ,, 0) of the distribution plan portion receiving the wick segments and / or carbon fibers, so as to locally have in the mold, and in a predetermined manner, a fibrous substrate of the desired texture that can go, after possible compression, a fine texture with a pore diameter me dian less than 50 μm, with a coarse texture with a median pore diameter greater than 100 μm.

This dispensing device comprises one or more dispensing heads (8), optionally contiguous, provided with continuous feed means or step by step in tow (s) of carbon fibers of great length, possibly sized or pre-impregnated, means for cutting the wick (s) into segments (6, 9) of predetermined length, possible means for facilitating the dispersion of the segments obtained from wick, optionally sized or pre-impregnated so as to obtain fiber segments (6), means of either movement of the head (s) it (s) itself (s) is transporting the segments obtained from the continuous wicks so that the filling of the mold is done in substantially horizontal layers successive (7) and that the segments (6,9) are placed in a precise point of the mold defined by its coordinates finally), enfinde computer means to enslave the proportion of segments of fibers / wicks (defined by the a volume percentage of one of the two kinds of segments) at the position of the segments in the mold so as to locally have in the friction element the desired texture according to a preset filling program.

The device may have two dispensing heads, one fed with carbon fiber wicks sized or pre-impregnated, providing segments of wicks (9), the other supplied with wick of carbon fibers neither sized nor pre-impregnated, providing segments. of fibers (6).

The latter head may, alternatively, be supplied with crushed carbon fibers or carbon precursors, providing fiber segments (6).

According to a modality of the device, it can have a single dispensing head, fed with wick of carbon fiber, sized or pre-impregnated, providing at a given location of the mold and depending on the filling program, wick segments (9) of length at least 5 mm and / or fiber segments (6) less than 5 mm in length.

In this case, the head is provided with a means for cutting the locks to a predetermined length and provided with a means for dispersing the wick segments, which can be activated when fiber segments (6) are to be deposited in the mold.

In order to obtain the segments of wicks (9) according to the invention, it is important that the carbon fiber (or precursor) wicks remain in the form of wicks and have no tendency to disperse into isolated fibers. For this, cut carbon fiber strands (or precursor) preferably sized or pre-impregnated with resin.

To obtain the fiber segments (6) according to the invention, on the contrary, it is important for the fiber segments to consist of isolated fibers, or, failing that, of a small number of associated fibers. For this purpose, carbon fiber (or carbon precursor) locks are preferably cut or ground preferably neither sized nor pre-impregnated.

However, tests have shown that fiber segments (6) can also be obtained from segments of short-length sized or pre-impregnated tows by means of dispersing means sufficient to disperse these segments of locks into fiber segments.

It may be advantageous to feed the head intended to deliver fiber segments (6) not in long strand but directly in fiber segments.

In fact, segments of fibers of short average length, typically less than 1 mm, can be obtained by crushing falls or various residues of fibers, which constitutes a particularly economical source of supply.

The crushed carbon fiber is in the form of a powder that can be conveyed and moved either by gravity or pneumatic drive, or finally in the form of a dispersion in a liquid. In this case, so that the gas or the liquid supporting the powder is evacuated, preferably from the bottom of the mold, the bottom of the mold will be pierced with holes and possibly provided with means allowing the suction of the gas or the liquid to be removed. through the bottom of the mold.

Preliminary tests made it possible to establish, according to the various parameters and in particular the length of the segments of locks or fibers, the correspondence between the height (z) of a portion of fibrous substrate in the mold and the height of the same portion in the compressed fibrous substrate, ie in fact in the friction element itself since the compressed fibrous substrate undergoes few dimensional changes during the subsequent densification treatment.

ADVANTAGES
In addition to the technical advantages already considered of the friction element, the method and device according to the invention also have many advantages.

The device according to the invention makes it possible to reproducibly and automatically reproduce any profile of fibrous substrate with variable texture by "linear comminution" of two materials, one with a fine texture, the other with a coarse texture.

 The invention makes it possible to obtain a three-dimensional fibrous substrate with segments of locks and / or entangled fibers whose texture is predetermined according to the coordinates (z, p, 8) of any point or portion of volume in the space occupied by the fibrous substrate constituting the reinforcement of the friction element.

This process therefore lends itself to complete automation, which also makes it possible to change production, possibly simply by modifying the filling program of the mold.

Plates 4, 5, and 8 illustrate some of these possibilities, particularly the simple case where the fibrous substrate consists of homogeneously textured layers per plane (Fig. 41, 4-2, 5-2).

The invention thus constitutes a means of reproducibly and automatically producing friction elements having a fibrous substrate with any predetermined texture, so that it is possible to obtain both "custom-made" friction elements, competitive prices and respecting the requirements of quality assurance.

Indeed, these friction elements must, in many cases, meet a high level of quality assurance, hardly compatible with manufacturing processes little or not automated, knowing that in the manufacture of a composite material, the matrix continues poses less quality problems than the continuous substrate.

Competitive prices result from several material saving elements since a friction element can be directly obtained in the desired shape, possibility of using cheaper carbon locks than fabrics or crushed carbon fibers, in turn cheaper that carbon wicks, finally economy of the process related to its automation.

DESCRIPTION OF THE FIGURES
Figure 1-1 shows in cross section a multilayer friction element having a structural layer (1) and, on either side thereof, two friction layers (2).

Figure 1-2 is similar to Figure 1-1 but further comprises two connecting layers (4) to make integral the structural layer (1) and the two friction layers (2).

Figure 1-3 schematizes the coarse texture 5g and Figure 1-4 schematizes the fine texture 5f, representations used in Figures 1-1, 1-2, 4-1 to 4-4.

Figure 2-1 shows a coarse texture 5g consisting of segments of wicks (9).

Figure 2-2 shows a fine texture 5f consisting of carbon fiber segments (unitary fibers or small number of associated unit fibers) (6).

Figure 3-1 shows in vertical section, a mold (10) being filled with two associated distribution heads (8), one of fiber segments and one of wick segments. Computer means (not shown in the figure), with a program for filling the mold, ensure the displacement of the heads, whose position is defined by its coordinates (z, p, 8, and the distribution of wicks producing locally the desired texture , provided by the program (fine / coarse / or intermediate by mixing the two textures), so as to ensure the filling of the mold in successive layers (11, 12).

Figure 3-2 is similar to Figure 3-1, but the mold has a central hole.

FIG. 3-3 schematizes the case where the structure layer (1) consists of a stack of oriented fibrous substrate (13) which is introduced into the mold during filling.

Figures 4-1 to 4-4 show sections along the Z axis of different friction elements of geometry and variable texture. Figs. 4-1A through 4-3A show the texture of the fibrous substrate in the AA 'plane perpendicular to the Z axis.

Figures 5-1 to 5-3 comprise for each of them the schematization of the texture of the fibrous substrate in section and a diagram bearing on the abscissa the texture which goes from "f" to "g", "f" representing the fine texture and "g" the coarse texture, and in the ordinate Z the distance z of a layer from the lower part of the fibrous substrate taken as origin.

Figure 5-1 shows a discontinuity of the fibrous substrate.

FIGS. 5-2 and 5-3 show a continuity of the fibrous substrate, with for FIG. 5-2 a zone of mixing of segments of wicks and fibers, of small thickness, whereas in FIG. from the single wick segment layer (12) to the single fiber segment layers (11) is progressively made over a relatively large thickness.

Fig. 6 shows a simplified device which has been used to make examples 1 to 3. A central orifice mold (10) is installed on a rotating drum (14). A dispensing head (8) cuts and distributes segments (9, 6) either of strands or fibers according to the nature of the carbon fiber coil (15) (respectively sized or not).

Figure 7-1 shows the porosity distribution by pore class for the coarse textured compressed fibrous substrate of Example 1. Figure 7-2 shows the same distribution but for the fine textured compressed substrate.

The abscissa shows the diameter of the pores in rm, with the intervals of each class of pores (non-linear scale).

On the ordinate is the percentage of pore volume per pore class, based on the total volume of the sample.

FIGS. 8-1 and 8-2 show, in cross-section along the Z axis, other examples of friction elements, with peripheral reinforcement of the structural layer, contrary to FIGS. 4-3 and 4-4 which have a reinforcement axial.

EXAMPLES
The examples which follow, as well as the figures, illustrate some possibilities of the invention.
Example 1 1- Manufacture of a Fibrous Substrate Using the Device of FIG. 6

In a cylindrical mold (10) 800 mm in diameter and 200 mm in height with a central orifice 180 mm in diameter, a first layer, with a fine texture, was formed to a thickness of 60 mm, consisting of segments of fibers. These fiber segments were obtained from a long length of carbon fiber wick at 12,000 elementary fibers, cutting it into segments of 1 mm in length.

Since this wick is neither sized nor preimpregnated, the segments of wicks are dispersible in elementary fiber segments randomly oriented in the mold.

A second, coarse-textured, 80 mm thick layer of wick segments was formed. These segments were obtained from a tow of carbon fiber long length, 12,000 elementary fibers by cutting into segments of 20 mm. It was observed that the wick segments did not tend to dissociate.

Finally, a third layer was formed identical to the first one.

The total height is about 200 mm.

The fibrous substrate thus obtained was compressed into the mold.

Its height after compression is 26 mm. It consists of 3 layers. Each type of layer, coarse and fine textured, was characterized after densification by carbon deposition to a bulk density of about 0.9
The coarse-textured layer has a porosity (pore volume) of 30%. The diameter distribution of its pores is that of Figure 7-1. By simple calculation of interpolation, it is established that the median pore diameter is close to 140 pm.

 The fine-textured layer has a porosity of 42%. The diameter distribution of its pores is that of Figure 7-2. Its average pore diameter is around 13 μm.

2- Densification of the fibrous substrate obtained
The compressed fibrous substrate, which is manipulable and has substantially the final dimensions of the friction element to be obtained, has been subjected to a step of densification by pyrolytic carbon vapor deposition at a temperature of 10500C.

A friction element of 1.75 density composite material is thus obtained.

Example 2
A friction element was made which differed in the structural layer (1): a wick of 6000 elementary fibers was used instead of 12000 in Example 1. The other parameters remained identical.

Example 3
The fibrous substrate of the coarse texture structure layer (1) is a stack of fabrics obtained from wicks of 3000 elementary fibers. The other parameters remained the same.

Example 4
As the fibrous substrate, fine-textured friction layers of Example 1, crushed carbon fiber powder consisting of fiber segments having a mean length of twist of 200 μm were used.

A perforated bottom mold was used so that the carbon fiber powder could be introduced as a dispersion in an aqueous medium, which limits the nuisance caused by dust.

After compression, a 27 mm thick fibrous substrate was obtained consisting of a coarse-textured core layer 17 mm thick and having an apparent density of 0.5, and consisting of two fine-textured side layers of each 5 mm thick and bulk density of 0.15. Its overall apparent density is 0.175.

The other elements of the test are those of Example 1.

Properties of the friction elements obtained (eg 1 to 4)
On the different layers obtained of type (1) (coarse texture) and type (2) (fine texture), the intrinsic properties of each layer have been characterized.

In particular, the resilience (conventional conditions: shock of a pendulum on a test specimen with impact energy of 7.5 J), the flexural strength and the relative speed of wear under low or high dissipated energy were measured.

Resilience Flexion Relative Wear
kJ / m2 MPa a (*) b (*)
Layer 1 - Ex. 1 50 150 6 20
Layer 1 - Ex. 2 41 200 4 18
Layer 1 - Ex. 3 40 220 4 24
Layer 2 - Ex. 1-3 3.5 115 1 12
Layer 2 - Ex. 4 2.5 45 0.3 10 (*) Condition "a" corresponds to a measurement of wear speed with low energy dissipated, while condition "b corresponds to a measurement with high energy dissipated .

 These comparative results are an illustration of the great interest of a differential texture as defined in the present invention.

It is important to note in particular the low wear observed in the case of the friction layer (2) obtained with crushed carbon fiber powder. The applicant having observed that the orientation of the fiber segments was all the more random as the segments were short and that the growth of the pyrocarbon, during the carbon vapor deposition densification operation, took place parallel to the The orientation of the fiber segments suggested that the wear rate of the friction layer was probably related to the orientation of the fiber segments.

Claims (24)

1 - Multilayer friction element comprising a structural layer (1) of carbon-carbon composite material, at least one face of which is covered with a friction layer (2) of carbon-carbon composite material integral with said structural layer (1 ) characterized in that, on the one hand, the structural layer (1) is obtained from a fibrous substrate (5g) with a "coarse" texture having a median pore diameter greater than 100 μm and consisting of wicks of carbon fiber or precursor of carbon fiber and that, on the other hand, the friction layer (2) is obtained from a fibrous substrate (5f) with a "fine" texture having a diameter of median pores less than 50 μm and consisting of carbon fiber segments (6) or carbon precursor distributed randomly. 2 - friction element according to claim 1 wherein the structural layer (1) is obtained from a fibrous substrate (5g) constituted by segments of wicks (9) of carbon fiber or carbon precursor distributed randomly. 3 - friction element according to claim 2 wherein the length of said wick segment is between 5 and 60 mm and that said fiber segment is between 0.05 and 60 mm. 4 - friction element according to claim 3 wherein the length of said fiber segment is between 1 and 60 mm. 5 - friction element according to any one of claims 1 to 4 wherein the fibrous substrate forms an uninterrupted entanglement of segments, segments of locks (9) in the structural layer (1), fiber segments (6) in the layer of friction (2), segments of wicks and fibers in any proportions in the intermediate portion between said structural layer and said friction layer. 6 - friction element according to claim 1 wherein the fibrous substrate (5g) of the structural layer (1) comprises an oriented fibrous substrate (13), typically based on carbon fiber fabrics or precursor carbon. 7 - friction element according to any one of claims 1 to 4 and 6 wherein the fibrous substrate (5f) of the friction layer (2) is constituted by a mat, a carbon fiber felt or carbon precursor. 8 - friction element according to any one of claims 1 to 7 wherein the structural layer is obtained from a fibrous substrate (5g) compressed bulk density between 0.3 and 0.7. 9 - friction element according to any one of claims I to 8 wherein the friction layer is obtained from a fibrous substrate (5f) of bulk density between 0.05 and 0.3. 10 - Friction element according to any one of claims 1 to 4, 6 to 9 wherein the structural layer (1) and the friction layer (2) are secured by a bonding layer (4). 11 - friction element according to any one of claims 1 to 10 wherein the fibrous substrate (5f) of said friction layer (2) comprises crushed carbon fiber consisting of fiber segments of average length between 0.2 and 2 mm. 12 - friction element according to any one of claims 1 to 11 wherein said locks comprise more than 1000 elementary fibers and said fiber segments comprise less than 100 elementary fibers. 13 - A method of manufacturing a friction element according to any one of claims 1 to 12, comprising a step of manufacturing a fibrous substrate with a desired geometric shape, a possible step of carbonization if said fibrous substrate is based on precursor of carbon fiber, and a step of densifying said fibrous substrate by deposition of carbon to form the carbon matrix, said step of manufacturing the fibrous substrate with the desired geometrical shape being characterized in that all or part of said fibrous substrate is manufactured by introducing into a mold (10), possibly using a dispensing device, carbon fiber or carbon precursor segments of length between 5 and 60 mm to form the fibrous substrate (5g) of the layer of structure (1), segments of carbon fiber or carbon precursor of length between 0.05 and 60 mm to form the substrate fi (5f) of the friction layer (2), and compressing the segments of locks and fibers, so as to obtain a fibrous substrate (5f, 5g) with an overall bulk density of between 0.1 and 0.7 and consisting of segments of wicks and / or fibers entangled and randomly oriented. 14 - Process according to claim 13 wherein only a portion of the fibrous substrate is formed from segments of locks and / or fibers and the substrate is formed by adding cut elements from carbon sheets or precursors of carbon, webs consisting of fabrics or felt / mat according to whether it is desired to form a complementary fibrous substrate with coarse or fine texture. 15 - Process according to claim 13 wherein the segments of carbon fibers or carbon precursor for constituting the fibrous substrate (5f) of the friction layer (2) have a length of between 1 and 60 mm. 16 - A method of manufacturing a multilayer carbon-carbon composite friction element consisting of a structural layer and at least one friction layer, comprising the manufacture of a carbon-composite composite structure layer (1) carbon and at least one carbon-carbon composite friction layer (2), their assembly by any known means, by a bonding layer (4), characterized in that the structural layer is produced by carbon deposition on a coarsely textured fibrous substrate (5g) having a median pore diameter of greater than 100μm and the carbon deposition friction layer is fabricated on a fine-textured fibrous substrate (5f) having a median pore diameter of less than 50 the fibrous substrates being previously carbonized in the case of a fibrous substrate precursor carbon fiber. The manufacturing method according to claim 16 wherein said coarse textured fibrous substrate (5g) is obtained from a web, a carbon fiber fabric or a carbon precursor, or from segments of wicks. carbon fiber or carbon precursor, of length between 5 and 60 mm, placed in a mold possibly using a dispensing device and then compressed to obtain a bulk density of between 0.3 and 0 7.  The manufacturing method according to claim 16 wherein said fine-textured fibrous substrate (5f) is obtained from a mat, carbon fiber felt or carbon precursor, or from fiber segments. carbon or carbon precursor, length between 0.05 and 60 mm, placed in a mold (10) optionally using a dispensing device and optionally compressed to obtain an overall apparent density between 0.1 and 0.7. 19 - The method of claim 18 wherein said fibrous substrate (5f) is obtained from carbon fiber segments or precursor carbon, length between 1 and 60 mm. 20 - Manufacturing method according to any one of the revisions 13 to 19 wherein said device for dispensing strands segments or fibers comprises at least one distribution head (8) of segments of locks and / or fibers, in relative motion by to the mold (10) and sweeping the entire horizontal surface of the mold, so as to progressively and regularly fill, in successive layers, the mold of segments randomly oriented strands (9) and / or fibers (6) of while maintaining a substantially horizontal distribution plane (7), the proportion of segments of wicks and / or carbon fibers distributed by said head (s) being preferably slaved to the coordinates (z, ,) of the distribution plane portion receiving the wick segments and / or carbon fibers, so as to locally have in the mold, and in a predetermined manner, a fibrous substrate of the desired texture that can go, ap very fine compression with a median pore diameter of less than 50 μm, a coarse texture with a median pore diameter greater than 100 μm. 21 - Device for implementing the method according to any one of claims 14 to 20, for distributing the segments of locks and / or carbon fibers in a mold which comprises one or more dispensing heads (8), optionally contiguous, provided with continuous or stepwise feeding means in long carbon fiber tows, optionally sized or pre-impregnated, means for cutting the bit (s) into segments (6, 9) of predetermined length, means possible to facilitate the dispersion of the segments obtained from wick optionally sized or pre-impregnated so as to obtain fiber segments (6), means for moving the head (s) it (s) itself (s) ) for transporting the segments obtained from the continuous locks so that the filling of the mold takes place in substantially horizontal successive layers (7) and that the segments (6, 9) are placed at a predetermined point s of the mold defined by its coordinates (z, p, &commat; ), computerized means to enslave the proportion of fiber segments / wicks (defined by the volume percentage of one of the two kinds of segments) to the position of the segments in the mold so as to have locally in the friction element the desired texture according to a pre-established filling program. 22 - Device according to claim 21 wherein there is used two distribution heads, one fed with wick carbon fiber or preimpregnated pre-impregnated, providing segments of wicks (9), the other supplied with wick carbon fiber or sized or prepreged, providing fiber segments (6).  23 - Device according to claim 21 wherein there are used two distribution heads, one fed with wick of carbon fiber sewn or prepreg, providing segments of wicks (9), the other supplied with crushed fiber carbon or precursor of carbon, providing fiber segments (6). 24 - Device according to claim 21 wherein using a dispensing head, fed with wick carbon fiber or pre-impregnated, providing at a given location of the mold and depending on the filling program, wick segments (9) of length at least 5 mm and / or fiber segments (6) less than 5 mm in length.