HK1023984B - Friction element in composite carbon/carbon-silicon carbide material and method for manufacturing same - Google Patents

Description

Carbon/carbon-silicon carbide composite friction member and method for manufacturing same

The invention relates to a C/C-SiC composite material containing carbon fibre reinforcement elements densified with a composite matrix of carbon and silicon carbide, for use in the manufacture of friction parts such as brake discs and/or brake shoes.

Friction parts made using C/C composites are well known, such composites being manufactured by making a fibrous preform from carbon fibers and densifying the preform with a carbon matrix.

Preforms may be prepared from a mat or a fibrous web such as a woven cloth, a knit, a unidirectional sheet of filaments, strands or cords, or a laminate made of a plurality of unidirectional sheets stacked in different directions and bonded together by light needling (needling). To prepare a preform, a number of layers of base woven and/or felted layers are stacked and bonded together until the desired thickness is achieved. The individual bonding of each layer can be achieved by needle punching, as described in US-A-4790052. The base fabric or mat used is made of carbon fibers formed from precursors or precursor carbon fibers which, when present, are converted to carbon fibers by heat treatment after the preform is formed.

Densification from a carbon matrix is achieved by chemical vapor infiltration or by a liquid phase process.

Chemical vapor infiltration involves placing a preform into an enclosed space, then introducing a gas into the enclosed space, which gas diffuses to various portions of the preform and forms deposits of pyrolytic carbon on the fibers under predetermined conditions of temperature and pressure. Generally, the gas includes one or more hydrocarbons, such as methane, to form pyrolytic carbon by decomposition.

The liquid phase method of carbon densification involves dipping a preform in a liquid carbon precursor, such as a resin having a non-zero coke content, and then converting the precursor to carbon by heat treatment.

In braking applications, the C/C composite is currently used as an aircraft brake disc, and its application in land vehicles is currently limited to F1 racing cars.

For these applications, the C/C composite is generally obtained by densifying the preform with a pyrolytic carbon matrix produced by chemical vapor infiltration. Unfortunately, this method is time consuming and expensive, resulting in a high cost of the C/C composite material, which is not suitable for use in other applications, such as railway cars or mass produced private vehicles. In addition, in these other applications, the requirements for friction parts are also quite different from those in an airplane or an F1 racing car. Although these requirements are generally not as severe, experiments carried out by the applicant have shown that there are still some problems. For example, the braking effectiveness appears to vary, particularly with the strength of the brake, and is relatively low when braking in a wet environment. In addition, wear is significant, resulting in insufficient life.

In order to at least partially solve these problems, in particular to improve the wear resistance, document EP- ｃA-0300756 proposes ｃA process for the preparation of ｃA C/C composite friction part, which is: the preform is densified by chemical vapor infiltration and then is impregnated with molten silicon to perform a final silicide formation process, which reacts with the carbon of the substrate to form silicon carbide (SiC).

However, in general, the chemical vapor infiltration methods used today are still relatively time consuming and costly.

It is an object of the present invention to provide friction parts of C/C-SiC composite material which are manufactured and have properties which make them suitable as braking parts for railway vehicles or mass-produced private or racing vehicles, or for general-purpose or industrial vehicles such as heavy trucks.

In particular, it is an object of the present invention to provide a friction element whose braking effectiveness, whether the braking conditions are strong or not, and whether the braking environment is dry or humid, is constant and reproducible.

It is another object of the present invention to provide a friction member that has minimal wear and is suitable for abrading with various types of materials.

These objects are achieved by a friction member having at least one friction face and made of a composite material comprising carbon fibre reinforced components and a matrix having at least a carbon phase and a silicon carbide phase, in which friction member, at least in the vicinity of the or each friction face, the matrix comprises: a first phase containing pyrolytic carbon in the vicinity of the reinforcing fibers obtained by chemical vapor infiltration; a second phase which is refractory and is obtained at least in part by pyrolysis from a liquid phase precursor; and a silicon carbide phase.

Such a friction part may be used to manufacture a brake disc or at least a friction lining for a brake disc in a disc brake for railway vehicles or mass-produced private cars, or racing cars, or general-purpose vehicles or industrial vehicles.

The term "pyrolytic carbon phase" herein refers to a pyrolytic carbon phase obtained by chemical vapor infiltration using one or more carbon gas phase precursors.

The term "refractory phase" refers herein to a carbon phase or a ceramic phase.

Preferably, at least in the vicinity of the or each friction face, the composite has the following composition, in volume percent:

15-35% carbon fibres;

10-55% of a first matrix phase containing pyrolytic carbon obtained by chemical vapour infiltration;

2-30% of a second matrix phase of refractory material at least partially derived from a liquid precursor;

10-30% silicon carbide.

The matrix phase obtained by chemical vapor infiltration forms a continuous coating of pyrolytic carbon on the fibers with a constant thickness, which is not cracked at least in an initial stage. By completely covering the fibers, the pyrolytic carbon can protect the fibers during the formation of the silicon carbide phase of the matrix. Furthermore, pyrolytic carbon, when obtained by chemical vapor infiltration, has a relatively high thermal conductivity, such that the thermo-mechanical properties of the composite are at least sufficient to dissipate heat generated by friction. In addition to the pyrolytic carbon, the first matrix phase may also include one or more layers of material capable of protecting the pyrolytic carbon and the underlying carbon fibers from oxidation. One material that has a protective effect against oxidation and is suitable for deposition by chemical vapor infiltration is silicon carbide, a ternary system of Si-B-C, or boron carbide. The material may be selected from a variety of precursors for a self-healing glass, i.e. the precursors are adapted to form, when oxidised, a glass which is in a semi-solid state at the temperatures used for the friction component, thereby plugging any cracks which may occur in the first phase of the matrix.

The chemical vapor permeation can be carried out by various methods, particularly by a constant temperature and pressure method, a temperature gradient method, a pressure gradient method, or an evaporation film (vaporized film) method. The temperature gradient method can be carried out by inductive coupling between the induction coil and the core located next to the preform to be densified, or by direct coupling between the induction coil and the preform. The thermostatic method with pressure gradient is carried out by forcing the gas constituting the matrix precursor along a path by means of a directional flow under constant pressure, as described in french patent application 2733254, or by forced flow as described in international patent application WO 96/15288. The evaporative film process involves immersing the preform in a bath and heating the preform to a temperature such that a vapor film of the precursor is formed on the contacting preform, followed by infiltration in the vapor phase, as described in U.S. patent 4472454.

The carbon or ceramic second matrix phase may be formed from: resin coke, or pitch, or a ceramic residue obtained by pyrolyzing a ceramic precursor. The liquid carbon precursor resin is selected, for example, from thermosetting resins such as phenolic resins, furan resins, epoxy resins, thermoplastic resins, pitch or combinations thereof. The liquid ceramic precursor is, for example, a polysilazane resin or polycarbosilane resin or a combination thereof. The matrix phase obtained by the liquid phase process, such as resin coke, has a relatively low thermal conductivity. It is possible that at the start of braking the friction surfaces reach a relatively high local temperature.

When the C/C-SiC composite is used in friction applications, the composite has a relatively low coefficient of friction when it is in a cold state, and the coefficient of friction of the composite becomes greater when it is in a hot state. This rapid transition to a high coefficient of friction makes it possible for the composite material to have good braking effectiveness even at very low speeds and when braking in a wet environment. Furthermore, the refractory phase obtained by the liquid phase process constitutes only a part of the matrix, which is not in contact with the fibres, and which does not unacceptably deteriorate the heat-dissipating function. In addition, the second matrix phase is present in the form of a carbon or ceramic agglomerate and is enclosed in the pores that remain after the first matrix phase is formed. This increases the probability of closing the pores with the final phase silicon carbide of the matrix. Closing the pores in this way makes it possible to avoid the effect of a humid environment on the tribological properties.

It is possible to form the second matrix phase in part by means of a solid filler, for example carbon powder or ceramic powder, or powder of a material having a protective function against oxidation. The solid filler may be injected into the liquid phase precursor in the form of a suspension.

The silicon carbide phase of the matrix effectively reduces wear and, in particular, it increases the hardness of the composite material, thereby enabling the material to be used in conjunction with a wide range of different materials in friction applications. Furthermore, the presence of SiC makes it possible to obtain sufficient braking efficiency with minimum braking energy in a dry environment. Furthermore, SiC prevents oxygen from the environment from entering the core of the composite by forming a barrier layer, and by closing the pores, the oxidation resistance of the composite can be increased at least to a large extent.

According to one feature of the friction member of the invention, the silicon carbide phase in the matrix need only be present within a limited depth range from the or each friction face.

Thus, when the friction member is a brake disc having a friction face, the brake disc has a core portion and at least one friction portion, or wear portion, the core portion of the brake disc being at least partially made of a composite material that does not contain a silicon carbide phase in the matrix. Furthermore, the absence of the silicon carbide phase also makes the core less rigid, so that the core of the disc retains good mechanical properties when subjected to braking forces, which are generally transmitted by mechanical connection along the inner or outer circumference of the core.

It is also possible to manufacture the friction part of the invention in the form of a friction or wear lining which is fixed together with the metal core part of the disc. The friction lining can contain silicon carbide over its entire thickness or only over a limited depth from the friction surface.

In another aspect, it is an object of the present invention to provide a method of making a friction part of a C/C-SiC composite material, the method comprising preparing a carbon fiber preform having accessible internal voids and densifying the preform with a matrix having at least one carbon phase and at least one silicon carbide phase.

According to the invention, the densification of the preform comprises: the first step of chemical vapor infiltration is to fill 10-55% of the preform volume with a first matrix phase containing pyrolytic carbon and forming a continuous coating on the carbon fibers; a second step of densification by dipping the partially densified preform in a liquid composition comprising a precursor of a refractory material and transforming said precursor by heat treatment; the third step is to form a silicon carbide matrix phase at least in the vicinity of the or each rubbing face.

According to a feature of the invention, during the first densification step, a first matrix phase comprising pyrolytic carbon and at least one layer of a material having a protective effect against oxidation may be formed by chemical vapor infiltration.

According to another characteristic of the invention, the second densification step is carried out by impregnating the partially densified preform with a composition comprising a liquid precursor comprising at least one compound chosen from resins and pitches which, after pyrolysis, form carbon residues and resins which, after pyrolysis, form ceramic residues. The composition may also contain a solid filler in suspension, such as a carbon powder, a ceramic powder, or a powder of a material having a protective effect against oxidation.

According to still another feature of the method, the high temperature heat treatment is carried out after the second densification step and before the formation of the silicon carbide phase of the matrix. This heat treatment, carried out at a temperature in the range of about 1800-.

The silicon carbide phase of the matrix may be obtained in various ways:

carrying out a silicide formation treatment by introducing silicon in the molten state and reacting it with carbon of at least one of the first two phases of the matrix;

silicide formation by permeation of a gas containing silicon or silicon vapour at elevated temperature, typically above 1800 ℃;

-carrying out a silicide formation treatment by introducing a solid filler in the form of silicon powder and carrying out a heat treatment to react the silicon with the carbon of the matrix;

chemical vapor infiltration; or

A solid filler in the form of a SiC powder suspension is added to a liquid and infiltrated into the partially densified preform.

It is advantageous to simultaneously subject a number of densified preforms to a silicide forming treatment by alternately stacking a number of densified preforms with silicon sources, each silicon source comprising a majority phase of silicon groups and a minority phase suitable for forming a structure that retains and excludes molten silicon, from each silicon source the molten silicon being able to migrate into the or each adjacent densified preform by heating to a temperature above the melting point of silicon. Such a silicide formation process is described in the french patent application No.9513458 filed by the applicant on 14/11 in 1995.

This method has the advantage that the degree of silicide formation can be controlled. To this end, the amount of silicon introduced into the densified preform by the or each rubbing face is determined as a function of the desired silicide formation depth, so that a silicon carbide matrix phase is formed only within a limited depth range from the or each rubbing face.

In the following, embodiments of the invention will be described in more detail by way of non-limiting description, with reference to the accompanying drawings, in which:

FIG. 1 is a sequence of steps of a method of the invention;

FIGS. 2A, 2B and 2C are diagrammatic representations of the microstructure of the composite material in different steps of manufacturing a friction part;

FIGS. 3 and 4 are graphs of the coefficient of friction over time for one friction member of the present invention at different slip rates, different brake pressures, and during experiments in dry or wet environments.

In the following description, reference will be made more specifically to the preparation of a friction member of C/C-SiC composite material in the form of a brake disc, it being understood that other types of friction members, such as brake pads and friction linings fixed to one or both faces of a disc core having one or both friction faces, can be made of the same material.

A process for manufacturing a brake disc comprising (figure 1):

preparing an annular fibrous preform 10 made of carbon fibers;

the first step is to densify the preform part with a matrix phase formed at least in part by chemical vapour infiltration from pyrolytic carbon;

a second step of partial densification of the preform by means of a carbon or ceramic matrix phase obtained at least in part by a liquid phase process;

a further step of forming a silicon carbide matrix phase;

the final step is machining the brake disc to final dimensions.

The fiber preform 10 is prepared by stacking multiple layers or plies of a fiber fabric 12 or multiple different fabrics and bonding the plies together by needling. The formation of the fiber fabric 12 may include: a felt, a woven, a knitted, a unidirectional sheet of filaments, cords, or strands, or a laminate made of a plurality of unidirectional sheets stacked in different directions and bonded together using a mild needling technique. As described in detail in document US-A-4790052, the layers or sheets are stacked and needled one after the other, each needling having A substantially constant needle pass density and needle penetration depth per unit areA, so that the needling density is substantially uniform per unit volume. The deposition and needling of the layers or sheets is not stopped until the preform has reached the desired thickness.

The fibrous web 12 is made of carbon fibers or carbon precursor fibers, such as pre-oxidized polyacrylonitrile fibers. In this case, the carbon precursor is converted by subjecting the fiber fabric to a heat treatment before or after the preform preparation is completed.

An annular preform is obtained by stacking and needling the planar plies and then by cutting out a portion of the preform obtained at the end of the needling process. Pre-cut annular plies may also be used. Both methods are known and need not be described in detail here.

The volume fraction of carbon fibers in the preform, on average, is preferably about 15-35%. It is a function of the coverage ratio of the fabric used and the strength of the needling, which produces a compacting effect on the fabric. The term "volume fraction" of the fibers as used herein refers to the apparent volume fraction of the preform that is actually occupied by the fibers. It will be seen that the volume fraction of fibres near the friction surface is less than the volume fraction in the preform portion corresponding to the brake disc core, for example, the fibre volume fraction drops to 10% at the friction surface. Too small a volume fraction of fibres at the brake disc core impairs the reinforcing function of the fibres and thus the mechanical properties of the composite brake disc core obtained, while too high a volume fraction of fibres causes a reduction in porosity, hindering densification. The preform thus has an internal porosity of 65-85% of its volume and, in particular because of the way in which it is produced, the pores of the preform are open, i.e. accessible from the outside.

A solid filler is added to the preform to occupy 2-10% of the preform volume before the first densification step is performed. The solid filler is a refractory powder, i.e. a carbon powder or a ceramic powder.

The preform is placed in a treatment chamber of a chemical vapor infiltration apparatus to carry out a first densification step by means of a constant temperature and pressure method. A reactive gas is introduced into the process chamber, the temperature and pressure conditions in the chamber being established according to the principle of favouring the diffusion of said gas into the preform pores. When the gas contacts the surface of the fiber, matrix material is deposited on the fiber by the reaction of the gas. Pyrolytic carbon is typically deposited from a gas containing one or more hydrocarbons, such as methane. The temperature in the closed process chamber is typically maintained at 950-.

Preferably, a plurality of annular preforms are placed in the treatment chamber of the chemical vapor permeation device so that they can be treated simultaneously. An example of how to stack a number of preforms together while introducing a gas stream is described in the above-mentioned french patent application No. 2733254.

As already mentioned, other chemical vapor permeation methods can also be used, for example temperature gradient methods such as heating the preform by direct inductive coupling, or pressure gradient methods, or evaporation film methods.

Chemical vapor infiltration may form a continuous coating 15 of pyrolytic carbon (fig. 2A) that covers the fibers 14, respectively. The first step of densifying the preform is preferably continued until about 10-55% of the preform volume is filled with pyrolytic carbon. The amount of pyrolytic carbon deposited on the fibers must be sufficient to provide the brake disc with the required thermal conductivity to be able to provide the heat dissipation function while providing sufficient mechanical strength. However, the amount of pyrolytic carbon deposited must be limited to retain sufficient porosity for continuous densification.

The first densification step may also include forming one or more layers of a material having a protective effect against oxidation, either over the pyrolytic carbon, or alternating with layers of pyrolytic carbon. The material which has a protective effect against oxidation and which is deposited by chemical vapour infiltration may be silicon carbide, boron carbide or a ternary Si-B-C compound. Preferably, a material is used which is suitable for forming a self-healing glass at the operating temperature of the brake disc in an oxidizing atmosphere.

The second densification step is carried out by: the partially densified preform is immersed in a liquid precursor of carbon or ceramic using a liquid phase process, after which the precursor is converted by heat treatment. A typical carbon precursor is a resin with a coke content different from zero (coke content is the amount of carbonaceous residue obtained after carbonization, expressed as a percentage with respect to the initial mass of the resin), or, indeed, it may also be pitch. Suitable resins include, inter alia, thermosetting resins such as phenolic, furan and epoxy resins, thermoplastic resins, pitch and combinations of the foregoing. The precursor of the ceramic is typically a resin such as a polysilazane resin or a polycarbosilane resin or a combination thereof.

For example, the impregnation of the preform may be carried out by immersing the preform in a bath of an impregnating composition formed from a resin and optionally a solvent, the impregnation being carried out under pressure or vacuum conditions to facilitate the passage of the impregnating composition into the remaining voids in the core of the preform. And drying the soaked preform, and then heating to about 900-1000 ℃ after the resin is crosslinked so as to perform pyrolysis treatment on the preform.

The second densification step may be performed using one or more successive soak-carbonization cycles to fill about 4-40% of the preform volume with the refractory matrix phase. The amount of carbon or ceramic obtained by the liquid phase process must be sufficient so that the final composite contains a low thermal conductivity refractory matrix phase which facilitates a rapid transition to a high coefficient of friction. However, sufficient remaining accessible internal porosity must remain to enable the formation of the silicon carbide matrix phase.

In the illustrated embodiment, the refractory material obtained by the liquid phase process is carbon. In the form of lumps 16 of resin coke or pitch coke present at the partially densified preform voids 17 (fig. 2B).

During the second densification step, the solid filler may be added in suspension to the liquid phase precursor. The solid filler, for example, may be composed of: carbon powder, ceramic powder, or a material having a protective effect against oxidation, such as a powder of a precursor of a self-healing glass.

The heat treatment at a temperature of about 1800 and 2850 c may be carried out immediately after the formation of the second matrix phase, especially when this phase is made of carbon, so that the thermal conductivity of the material is improved.

The silicon carbide phase of the matrix may be obtained by subjecting the preform to a silicide forming treatment, i.e. by adding silicon in the form of molten silicon or vapour to the remaining pores that are accessible and reacting the silicon with the pyrolytic carbon of the first matrix phase and the carbon of the second matrix phase. Various known silicide forming techniques may be used, for example, dipping the preform in a molten silicon bath, or contacting the densified preform with a molten silicon bath through a discharge channel that can draw silicon into the preform by capillary action.

Preferably, a stacked silicide formation process of the type described in the aforementioned french patent application No.9513458 is used. A plurality of densified preforms 10 'are stacked together, with a silicon source 18 interposed between each two preforms, the silicon source being arranged between the two preforms 10' and at the ends of the stack. The silicon source 18 is largely composed of a silicon phase or a silicon-based phase and is, for example, in the form of a powder and contains a few phases suitable for forming a structure capable of holding and discharging molten silicon. The minority phase, for example, is a rigid cellular structure such as a honeycomb structure 18a having cells filled with powdered silicon 18 b. Alternatively, the minority phase may be comprised of a low porosity three dimensional network such as a chopped felt or a non-rigid cellular fabric such as foam, with the minority phase extending throughout the volume of the silicon source.

The stack of preforms 10' and silicon source 18 is subjected to a silicide formation process at a temperature in the range of, for example, 1410-1600 c and a low pressure, for example less than 50KPa, and an inert atmosphere such as argon or vacuum. When the silicon contained in the silicon source 18 reaches its melting point, the silicon migrates along the surface of the preform in contact with the silicon source 18 toward the adjacent preform. Starting from the silicon source 18, this migration takes place under the influence of gravity towards the preform 10 'located therebelow and, by capillary action, towards the preform 10' located thereabove.

Upon infiltration into the remaining porosity in the densified preform 10', the molten silicon reacts with carbon to form silicon carbide (SiC)19, the reacting carbon including both pyrolytic carbon 15 and carbon 16 obtained from a liquid phase process (fig. 2C). Thus, the SiC layer forms just around the already densified preform core, since the pores of the preform have not yet been closed. Depending on the porosity remaining in the densified preform prior to the silicide formation process, the thickness of the SiC layer may range from a few microns to over 10 microns, so long as the silicon source contains sufficient silicon. As a result, the brake disc obtained has the internal protection conferred by SiC19 against oxidation, since SiC19 forms a barrier against the ingress of oxygen in the surrounding medium. In addition, SiC increases the hardness of the brake disc and provides it with resistance to wear in the vicinity of the friction surfaces of the brake disc. It can also be seen that by reacting with the pyrolytic carbon covering the surface of the pores 17 and with the carbon in the particles 16 partially occupying the same pore sites, the silicon forms silicon carbide 19 at least partially closing the pores, as a result of which the composite is sealed, which reduces the effects of a humid environment.

The resulting brake disc 20 is machined, in particular by finishing its friction surfaces and machining a series of grooves (not shown in the figures) on its outer or inner circumference, to obtain a brake disc 20 of final dimensions, said grooves serving to connect the disc 20 to the part rotating at the same speed as it.

It will be appreciated that the brake disc may be machined prior to the siliconising process. The silicon source 18 used in this case has its faces complementary in shape to the faces of the brake disc.

By the silicide formation treatment, 10-35% of the volume of the densified preform should be occupied by silicon carbide. The remaining porosity in the densified preform after the silicide formation treatment is preferably reduced to less than 10% of the preform volume.

The brake disc 20 obtained then comprises, by volume:

15-35% carbon fibers;

5-45% of pyrolytic carbon formed by chemical vapor infiltration but not converted to SiC;

2-30% of carbon formed by liquid phase process but not converted to SiC; and

10-35% SiC.

The density of such a material is very low, at 1600-2100kg/m³Its thermal expansion coefficient is also very low, less than 2X 10^-6and/K, but the heat dissipation performance is better than that of steel. Furthermore, as can be seen in the examples below, the tribological properties are stable and reproducible without sudden abrupt changes and without being significantly affected by environmental difficulties.

Although it is conceivable from the above description that the silicide-forming treatment of the densified preform may extend through the core, it is advantageous to limit the depth of silicide formation from each rubbing face. This limitation can be achieved by using a silicon source that contains an amount that is insufficient to perform the overall formation of silicide. The deficiency of silicon is determined as a function of the depth at which silicide formation is desired. It is thus possible to obtain a brake disc such as 20 '(fig. 1) in which the friction or wear points 21' a and 21 'b are silicide-formed, while the core 22' of the disc does not contain any SiC, at least in the centre of the disc. Thus, the SiC provides the required hardness and wear resistance and also reduces the remaining porosity at the wear sites, while the absence of SiC at least for a large part of the core sites both increases the heat dissipation effect and improves the mechanical strength. The lower rigidity of the core part allows a better transmission of forces between the disc and the fixed or rotating part to which it is connected, which is generally achieved by means of grooves formed on the inner or outer circumference of the core part.

In the above description, the silicon carbide phase in the matrix is formed by introducing silicon in a molten state to undergo a silicide-forming reaction. However, other methods of preparing such a matrix phase are also conceivable, such as silicide formation by introducing silicon in gaseous form, silicide formation by introducing silicon in powder form and subsequently performing a heat treatment, chemical vapor infiltration and the introduction of a solid filler of SiC.

Chemical vapor infiltration of SiC is a known method in which the vapor phase precursor is typically Methyltrichlorosilane (MTS). This method can be carried out under constant temperature and pressure conditions or in the presence of temperature or pressure gradients. In the temperature gradient method, the densified preform may be heated by direct coupling between the preform and an induction coil.

The filler may be introduced in vacuum by impregnating the SiC powder suspended in a liquid. This process can be done in the final step of chemical vapor infiltration.

When chemical vapor infiltration is used and the filler is introduced, there is no reaction with the pyrolytic carbon or with the optional carbon constituting the refractory phase, so the amount of carbon in the final composite is the same as the amount of carbon initially deposited during the first and second densification steps.

Example 1

The disc and tile preparation process in a railway car disc brake is as follows.

A fiber preform was prepared by needling a plurality of carbon fiber mats stacked together, and then the preform was cut to obtain a ring-shaped brake disc preform having a thickness of 60mm, an inner diameter of 235mm and an outer diameter of 660mm, and a rectangular block preform of tiles having dimensions of 15mm x 8mm x 40 mm. The volume fraction of fibers in the preform was 25%.

The first step of densification of the preform is carried out by chemical vapor infiltration, in which the gaseous substance used consists of a mixture of natural gas and propane, the temperature being maintained at about 1000 ℃ and the pressure at about 1.3 KPa. Chemical vapor infiltration continues until about 42% of the preform volume has been filled with pyrolytic carbon.

The partially densified preform is impregnated with furan resin and thereafter heat treated to form a second matrix phase of resin coke. The dipping treatment is performed by dipping the preform in a resin solution in vacuum. After drying and curing the resin, the resin was carbonized at 900 ℃. Resin infusion was carried out in such a manner that 17% of the preform volume was occupied by resin coke.

In preforms densified with carbon in this manner, the residual porosity available was about 16% of its volume.

After machining the densified preform, a silicide formation treatment was carried out using the stacked silicide formation process described in the above-mentioned french patent application No. 9513458.

The amount of molten silicon introduced into the densified preform was selected to obtain a matrix phase of SiC corresponding to about 20% by volume of the composite after silicide formation, corresponding to the deposition of a SiC layer having a thickness greater than 10 microns.

The density of the friction elements (brake discs and tiles) obtained in this way was about 1950kg/m³The final porosity was about 10% and had the following composition by volume:

about 25% carbon fiber;

about 37% of pyrolytic carbon obtained by chemical vapor infiltration;

about 8% of carbon consisting of resin coke; and

about 20% SiC.

In a dry brake bench tester, a brake apparatus composed of a brake disc and a pad tile manufactured in this manner was subjected to an experiment in which the sliding speed was varied in a range of 5 to 100 m/s. The coefficient of friction was measured at various sliding speeds and at different times during the experiment. In fig. 3, the shaded area indicates that all measurements are within this range. Thus, the braking efficiency is substantially constant over a wide range of sliding speeds.

A similar braking device was tested under dry braking conditions at a constant slip speed of 20 m/s. After running in for 3 minutes under the condition of applying the brake pressure of 0.55MPa, the friction coefficients were measured when three different brake pressures were applied, the applied pressure values being 0.25MPa, 0.55MPa and 1MPa, respectively, and the action time of each pressure being 3 minutes. Curve a in fig. 4 shows the measured braking coefficient as a function of time.

A similar brake was tested under the same conditions except that the environment was wet and a hose was used to continuously water the brake. Curve B in fig. 4 shows the measured braking coefficient as a function of time.

Curves a and B demonstrate the excellent performance of C-C composite friction parts made according to the present invention that were subjected to a silicide formation treatment. Firstly, the coefficients of friction at various brake pressures are fairly constant, and secondly the coefficients of friction measured under dry and wet conditions are virtually identical. In this way, a constant braking effectiveness is achieved under different conditions of use.

Example 2

A brake disc of a new mass-produced disc brake device for private vehicles is manufactured as follows.

Several layers of carbon fiber mats stacked together were needled to obtain a fiber preform, and the preform was cut to obtain an annular disk preform having a thickness of 35mm, an inner diameter of 160mm and an outer diameter of 360 mm. The volume fraction of fibers in the preform was 22%.

By alternating deposition of pyrolytic carbon layers and boron carbide B₄A thin layer of C (about 0.5 microns thick) was used for the first chemical vapor infiltration step until 40% of the initial volume of the preform had been filled.

The partially densified preform is impregnated with a phenolic resin and then heat treated at 900 c to form a second matrix phase of resinous coke. The impregnation is carried out so that about 18% of the volume of the preform is filled with resin coke, so that the remaining voids in the preform are only about 10% of its volume.

After machining, a silicide formation treatment was performed in the same manner as in example 1, however, by using a silicon source set to contain an amount of silicon insufficient to achieve silicide formation throughout the entire core, silicide formation was performed only within a limited thickness range from each rubbing face. The obtained brake discs have varying compositions:

within 10mm from each friction face, its composition (by volume) is 22% carbon fiber, 25% pyrolytic carbon + B₄C, 5% resin coke, and about 33% SiC, with a remaining porosity of about 7%;

in the remainder of the disc, in particular in the central part of its core, its composition (by volume) is 22% carbon fibres, 40% pyrolytic carbon + B₄C, 18% resin coke and about 0% silicon carbide, with a remaining porosity of about 10%.

The brake discs obtained have a low apparent density, of the order of 1700kg/m³And lower rigidity, which, however, in terms of wear is comparable to the tribological behaviour of the brake disc in example 1.

Example 3

The manufacturing process of the automobile brake disc is as follows:

a fiber preform was prepared by needling together a plurality of layers of carbon fiber mats stacked up, and the preform was cut to obtain an annular preform of a brake disc having a thickness of 32mm, an inner diameter of 180mm and an outer diameter of 320 mm. The volume fraction of fibers in the preform was 30%.

The first step was chemical vapor infiltration to form a pyrolytic carbon matrix phase 30% by volume of the preform.

The partially densified preform is impregnated with a polycarbosilane resin and then heat treated to form a second matrix phase SiC consisting of pyrolysis residues and occupying 12% of the preform volume.

The preform densified in this way is machined and then, under constant temperature and pressure conditions, a matrix phase of silicon carbide is formed by chemical vapor infiltration until 20% of the volume of the preform is occupied by SiC.

The brake disc obtained had 8% of its volume of remaining porosity.

The brake discs obtained had a lower apparent density, but at their wear sections the tribological properties were comparable to those of the brake discs of example 1.

Example 4

The manufacturing process of the brake disc is as follows:

a fiber preform was obtained by needling together a plurality of layers of carbon fiber mats stacked up, and the preform was cut to obtain an annular preform in which the fiber volume ratio was 23%.

The first step was chemical vapor infiltration to form a pyrolytic carbon matrix phase 45% by volume of the preform.

This partially densified preform was impregnated with a phenolic resin and then heat treated at 900 c to form another matrix phase resin coke that was 10% of the preform volume.

Preforms densified in this manner are heat treated at 2800 ℃ to improve the thermal conductivity of the matrix carbon phase.

Thereafter, a third step was carried out by chemical vapor infiltration to form a matrix phase of silicon carbide (SiC) 15% by volume of the preform.

The brake disc obtained has excellent tribological properties.

Claims (26)

1. A friction member having at least one friction facing and made of a composite material comprising carbon fibre reinforced components and a matrix comprising at least one carbon phase and one silicon carbide phase, said member being characterised in that, at least in the vicinity of the or each friction facing, the composition of the composite material is at least, by volume:

15-35% of reinforcing carbon fibres;

10-55% of a first matrix phase containing pyrolytic carbon obtained by chemical vapour infiltration, located in the vicinity of the reinforcing fibres;

2-30% of a second matrix phase of refractory material at least partially obtained by pyrolysis of a liquid phase precursor;

10-30% of a silicon carbide matrix phase.

2. A friction member according to claim 1, characterized in that the first matrix phase comprises at least one layer of a material having a protective effect against oxidation.

3. A friction member according to claim 1, characterized in that said second refractory phase is made of carbon.

4. A friction member according to claim 1, characterized in that said second refractory phase is made of ceramic.

5. A friction member according to claim 1, characterized in that a silicon carbide phase is present in the matrix within a limited depth range from the or each friction face.

6. A brake disc comprising a core part and at least one wear part, said wear part having a friction surface, characterized in that it is a friction member according to any one of claims 1-5.

7. A brake disc according to claim 6, characterized in that the core of the disc is at least partly made of a composite material without silicon carbide phases in its matrix.

8. Disc brake device for railway vehicles, characterized in that it comprises at least one brake disc according to claim 6.

9. Disc brake device for private, racing, general or industrial vehicles, characterized in that it comprises a brake disc according to claim 6.

10. A method of manufacturing a composite friction member having at least one friction facing, the method comprising: preparing a carbon fibre preform having accessible internal porosity and an average volume fraction of fibres of between 15 and 35%, and densifying said preform using a matrix comprising at least one carbon phase and at least one silicon carbide phase, said method being characterized in that the densification of the preform comprises: the first step is to fill 10-55% of the preform volume by chemical vapor infiltration with a first matrix phase containing pyrolytic carbon and forming a continuous coating on the carbon fibers; the second step is a densification treatment by impregnating a partially densified preform with a composition comprising a precursor of a liquid phase of a refractory material and transforming said precursor by a heat treatment so that 4-40% by volume of said preform is filled with said refractory material; the third step is to form a silicon carbide matrix phase at least in the vicinity of the or each rubbing face so that 5-35% of the volume of the preform is occupied by silicon carbide.

11. A process according to claim 10, characterized in that a solid refractory filler is added to said preform before the first densification step.

12. A method according to claim 10, characterized by: in a first densification step, a first matrix phase is formed by chemical vapor infiltration, which contains pyrolytic carbon and at least one layer of a material having a protective effect against oxidation.

13. A method according to claim 10, characterized by: a second densification step is carried out by impregnating said partially densified preform with a composition comprising a liquid precursor comprising at least one compound chosen from: resins and pitches that can form a carbon residue by pyrolysis, and resins that can form a ceramic residue by pyrolysis.

14. A method according to claim 13, characterized in that the impregnating composition also comprises a solid filler in suspension, said filler being selected from carbon powders, ceramic powders, and powders of materials having a protective effect against oxidation.

15. A method according to claim 10, characterized in that the heat treatment is carried out at temperatures of 1800-.

16. A method according to claim 10 characterised in that the third step is carried out so that residual internal voids in the densified preform are reduced to less than 10% of its volume at least in the vicinity of the or each rubbing face.

17. A method according to claim 10, characterized in that the third step of forming the silicon carbide matrix phase is carried out by introducing silicon in the molten state and reacting it with carbon in at least one of the first two matrix phases to perform a silicide formation treatment.

18. A method according to claim 17, characterized in that a number of densified preforms can be simultaneously subjected to a silicide formation process by: a plurality of densified preforms are stacked alternately with silicon sources, each silicon source comprising a majority phase based on silicon and a minority phase suitable for forming a structure for retaining and excluding molten silicon, the molten silicon being capable of migrating from each silicon source into the or each adjacent densified preform by heating to a temperature above the melting point of silicon.

19. A method according to claim 18, characterized in that in the silicon source used, the majority phase of the silicon base is in powder form.

20. A method according to claim 18, characterized in that in the silicon source used, the minority phase is a three-dimensional structure extending to the respective part of the silicon source.

21. A method according to claim 20, wherein said three-dimensional structure is selected from the group consisting of rigid cellular structures, arrays of fibers, and non-rigid cellular materials.

22. A process according to claim 17 characterised in that the amount of silicon incorporated into the densified preform by the or each rubbing face is determined by the required depth of silicide formation so that a silicon carbide matrix phase is formed over a limited range of depths from the or each rubbing face.

23. A method according to claim 10, characterized in that the third step of forming the silicon carbide matrix phase is performed by chemical vapour infiltration.

24. A method according to claim 10, characterized in that the third step of forming the silicon carbide matrix phase is performed by carrying out a silicide formation treatment by infiltration of a silicon carrier gas at elevated temperature.

25. A method according to claim 10, characterized in that the third step of forming the silicon carbide matrix phase is performed by a silicide forming process in which silicon powder is introduced and heat treated.

26. A method according to claim 10, characterized in that the third step of forming the silicon carbide matrix phase is performed at least partly by introducing a solid filler formed of silicon carbide powder in suspension in a liquid.