In particular, the present invention relates to a shaped composite material based on carbon/silicon carbide (hereinafter referred to as"C/SiC") and containing filaments constituted substantially by carbon.

The term"filaments constituted substantially by carbon"is intended to include fibrous materials produced by pyrolysis of various products of synthetic origin, for example, polyacrylonitrile (PAN) and polysilazane, or of natural origin, for example, pitches, or natural cellulose-based sources such as vegetable fibres and wood.

These filaments are normally produced from bundles of filaments comprising groups of filaments variable from 3000 to 50000 units and having a diameter of between 2 and 3 am, combined with one another and impregnated with a resin such as, for example, a polyurethane resin. The bundles are then chopped to a length of less than 30 mm and, finally, are arranged randomly in the mixture.

The bundles of filaments are commonly defined on the basis of the number of units making up the bundle; for

example 3K, 10K, 50K correspond to 3000,10000 and 50000 units, respectively, and so on.

The above-described composite ceramic materials are used in many applications which require good impact and compression strength and good resistance to temperatures generated by friction, which characteristics cannot be ensured by ordinary ceramic materials owing to their intrinsic fragility. An advantageous application is in braking systems, particularly for the production of disk- brake disks.

According to the prior art, these composite materials can be produced in the following manner: the bundles of filaments are mixed with a binding resin, pitches, and other additives and the mixture is placed in a mould in which it is moulded by heating and the application of a pressure to produce a shaped semi- finished product. The semi-finished product is then subjected to a first firing in a furnace at a temperature such as to bring about carbonization or pyrolysis of the resin.

As a result of this firing, the semi-finished product acquires a predetermined porosity owing to the loss of volatile material at the carbonization or pyrolysis temperatures. The fired semi-finished product is then subjected to a second firing in the presence of

silicon at a temperature such as to bring about fusion of the silicon and its infiltration into the pores of the semi-finished product.

The infiltration of the silicon increases the cohesion of the bundles of carbon filaments and, at the same time, some of the fused silicon reacts with the carbon of the semi-finished product in the conditions of the second firing, forming silicon carbides which have the effect of improving the cohesion characteristics of the material.

In spite of its good mechanical characteristics, the composite ceramic material described above has, as a serious disadvantage, an unacceptable loss of free carbon from the braking surfaces, which leads to the formation of surface cavities. This problem is caused by the tendency of the material to undergo surface oxidation such as to cause the superficial loss of carbon-based material, particularly at the high working temperatures.

To solve this problem, disk-brake disks have been proposed, in which the composite ceramic material is coated with a layer of silicon silicates, carbides, nitrides or pure silicon. However, this solution also has disadvantages. In fact, because of the different thermal expansion of the ceramic material in comparison with the coating and because of its thinness, cracks

fissures often arise in the coating, leading to disintegration thereof.

The technical problem underlying the present invention is to provide a shaped composite material for braking applications which does not have the disadvantages pointed out above with reference to the prior art, and the braking characteristics of which remain substantially unchanged over time.

This problem is solved by a shaped composite material as specified in the appended claims.

In particular, the present invention is based on a composite material based on C/SiC containing filaments constituted substantially by carbon, characterized in that it comprises a base layer and at least one surface layer on at least one of the faces of the base layer, and in that, in the surface layer, the filaments constituted substantially by carbon have a reactive contact surface large enough to enable a percentage of free carbon of less than 20% by weight, preferably of between 5% and 8% by weight, calculated on the total weight of the at least one surface layer, to be obtained after the step of treatment with silicon.

According to the invention, this result is achieved by the provision, in the surface layer, of filaments constituted substantially by carbon having lengths of

less than 5 mm. The filaments constituted substantially by carbon in the surface layer preferably have a length of between 2 and 4 mm, even more preferably, a length of about 3 mm.

When compared with the filaments constituted substantially by carbon in the base layer, the normal length of which is less than 30 mm, preferably between 7 and 10 mm, typically about 8 mm, the filaments of the at least one surface layer are considerably shorter.

Without wishing to be bound by any theory, this characteristic would enable a larger surface area per unit of weight to be achieved, consequently favouring reactivity with the silicon and hence the formation of silicon carbide.

Although the length of the filaments constituted substantially by carbon in the base layer is irrelevant for the purposes of the solving of the technical problem specified in the present invention, a length of the order of at least 7 mm and no greater than 30 mmm, typically 7- 10 mm is preferred, so as to produce a disk having adequate mechanical and thermal characteristics.

The thickness of the at least one surface layer, with reference to the finished disk, is preferably between 0.3 and 2 mm, more preferably between 0.7 and 1.5 mm, even more preferably about 1 mm.

The thickness of the base layer, however, depends on the type of application and on the diameter of the disk and, purely by way of example, will be between 24 and 30 mm for high-performance cars.

A characteristic feature of the material of the present invention is that the transition from the base layer to the at least one surface layer takes place without a break in continuity or, in other words, there is no clear demarcation line between the layers. This result, which translates into the substantial advantage of minimizing the cracks and fissures which are characteristic of the disks of the prior art, is achieved by means of the production method which will be described below.

The composite ceramic material according to the invention may also contain reinforcing fibres. These reinforcing fibres extend within the structure of the material, preferably throughout its shape.

Alternatively, the reinforcing fibres may be provided only in some regions of the composite material, according to the regions in which cracks arise and on their propagation paths, both of which can be predicted on the basis of structural calculations. An example of ceramic material comprising reinforcing fibres is that described in EP 1 124 071 published on 16th August 2001, the

description of which, in this connection, is incorporated herein by reference.

The material of the reinforcing fibres will preferably be constituted by carbon fibres. It is, however, possible to use other materials such as SiC, Si3N4, or TiC, as well as metals, for example, platinum, which are suitable for withstanding the temperatures of the interaction with the silicon. The reinforcing fibres may be incorporated in the material according to the invention in various ways. For example, the reinforcing fibres may be arranged in a plurality of bundles which are disposed along predefined axes. These axes may be warp and weft axes, the bundles forming a fabric.

Alternatively, the reinforcing fibres may constitute a non-woven fabric, for example, a felt.

The content of the components of the shaped composite material according to the invention may vary as percentages by volume relative to the volume of the finished material, as follows: filaments constituted substantially by carbon 40- 70%, preferably 50-60%, binder 5-30%, preferably 15-25%, additives 0.5-20%, preferably 1-15%, reinforcing fibres 0-30%.

According to a preferred embodiment of the present invention, the at least one surface layer covers both faces of the base layer of material in the manner of a sandwich.

The composite ceramic material according to the present invention can be produced by a method similar to that described in the above-mentioned published application EP 1 124 071. This method comprises the steps of: a) preparing a first mixture containing a predetermined quantity of filaments constituted substantially by carbon and having a length no greater than 30 mm, and a predetermined quantity of a chemical binder, b) preparing a second mixture containing a predetermined quantity of filaments constituted substantially by carbon and having a reactive contact surface large enough to enable a percentage of free carbon of less than 20% by weight, calculated on the total weight of the at least one surface layer, to be obtained after a step of treatment with silicon, and a predetermined quantity of a chemical binder, c) placing the first mixture and the second mixture in a mould having the shape of the product to be produced in a manner such that the first mixture constitutes a

base layer and the second mixture constitutes at least one surface layer on at least one of the two faces of the base layer, d) optionally incorporating, in the first mixture of the base layer and/or in the second mixture of the at least one surface layer, a plurality of reinforcing fibres which extend along the shape in a manner such as to hinder the propagation of cracks, e) moulding the first mixture and the second mixture in the mould to produce a semi-finished product, f) subjecting the semi-finished product to a first firing at a temperate such as to bring about substantial carbonization or pyrolysis of the chemical binder, g) subjecting the fired semi-finished product to a second firing in the presence of silicon at a temperature such as to bring about fusion of the silicon and its infiltration into the semi-finished product, producing the shaped composite material in which the percentage of free carbon in the surface layer is less than 20% by weight, calculated on the total weight of the at least one surface layer.

According to a preferred embodiment of the invention, the filaments constituted substantially by carbon in the at least one surface layer will have a surface area such as to enable a percentage of free

carbon in the finished product of between 5% and 8% by weight relative to the total weight of the at least one surface layer, to be obtained.

After each of the base layer and the at least one surface layer has been arranged in the mould (step c), it may be necessary to level the surface before the deposition of the next layer so as to ensure a uniform thickness of each layer.

The thickness of the at least one surface layer in the finished product is between 0.3 and 2 mm, more preferably between 0.7 and 1.5 mm, and even more preferably is about 1 mm.

In the method according to the invention, the filaments constituted substantially by carbon preferably have a diameter of from 0.1 to 2 mm, more preferably from 0.3 to 0.5 mm.

The content of filaments constituted substantially by carbon in the starting mixture may vary from 50% to 80% by volume relative to the volume of the mixture and is preferably within the range of 60%-70%.

The filaments constituted substantially by carbon in the at least one surface layer preferably have a length of less than 5 mm, more preferably a length of between 2 and 4 mm, even more preferably a length of about 3 mm.

These filaments, which become part of the composition of

the mixture for the at least one surface layer, may be produced from bundles of filaments by the same methods and with the same apparatus as are used for the longer filaments, simply by suitable setting of the apparatus for cutting the filaments.

The filaments in the base layer preferably have a length of between 7 and 10 mm, more preferably a length of about 8 mm.

The filaments both in the first mixture and in the second mixture and/or the reinforcing fibres, may advantageously be coated beforehand with a protective resin, preferably polyurethane resin, before being used in accordance with the method of the invention.

Alternatively, the filaments and/or the reinforcing fibres may be coated beforehand with the same chemical binder which is used for the preparation of the mixture.

Greater cohesion of the material and a more compact product is thus produced.

During the first firing of the semi-finished product, the resin and the chemical binder carbonize, creating a protective layer on the bundles of filaments and on the reinforcing fibres, preventing any disintegration or even dissolving thereof during the subsequent treatment with silicon. The bundles of filaments, and any reinforcing fibres, therefore retain

their original shape throughout the process, thus producing a material with good cohesion and strength characteristics.

The chemical binder is a conventional binder which may be selected from the group comprising phenolic and acrylic reins, paraffin, pitches, polystyrenes, etc. The binder is preferably selected from the group comprising pitches and phenolic resins.

The binder is preferably added to the mixture in the solid state. For example, phenolic resin may be added in the form of pellets, powder or granules.

The content of chemical binder in the mixture may vary from 5% to 30% by volume relative to the volume of the mixture and is preferably within the range of 20%- 26%.

The mixture may also contain other conventional additives used as fillers and, indirectly, for regulating the porosity and the density of the desired composite material.

These additives are constituted by particles of inorganic materials, preferably such as powders of graphite, silicon carbide, and metal carbides and nitrides. The content of additives in the mixture may vary from 0.7% to 23% by volume relative to the volume of

the mixture and is preferably within the range of 9%- 15%.

Mixing may be performed in conventional manner and with conventional apparatus and the filaments are arranged randomly in various directions.

The optional incorporation of reinforcing fibres in the mixture may be performed by various methods. An example of a method is that specified in the above- mentioned published application EP 1 124 071 (column 5, paragraph 0052), the description of which, in this connection, is incorporated herein by reference.

The quantity of reinforcing fibres incorporated in the mixture depends on the desired fibre content in the final composite material, this content being within the range of 0-30% by volume relative to the volume of the material.

During the moulding step of the method of the invention, the first and second mixtures are heated in the mould to a temperature of from 80°C to 180°C, preferably 100-120°C, and a pressure of between 0.1 N/cm2 and 5 N/cm2, preferably 0.5-1 N/cm2 is applied thereto.

The shaped and compacted semi-finished product thus produced is removed from the mould and is then subjected to a first firing so as to carbonize the chemical binder (step f, pyrolysis).

This firing is performed in a conventional furnace at a temperature which depends substantially on the type of binder used and which is generally within the range of 900-1200°C.

The firing is performed in the presence of a flow of inert gas such as nitrogen or argon and in an extra pressure of 10-100 mbar, preferably 20-30 mbar. This flow also advantageously removes the gases which are released by the pyrolysis of the chemical binder.

During this step of the method, the semi-finished product acquires a greater porosity, which is important in the subsequent firing since it allows the fused silicon to infiltrate therein.

According to one embodiment of the invention, the method may further comprise a step for the finishing of the surface of the semi-finished product produced by the first firing of step f). This advantageously enables any surface deformations of the semi-finished product to be removed by conventional apparatus so as to give it the desired shape.

The finishing operation is preferably performed dry, for example, with diamond.

The semi-finished product fired in accordance with step f) is subjected to a second firing in the presence of silicon (step g).

In order to perform the second firing, the semi- finished product, fired and possibly subjected to finishing, is inserted in the chamber of a container having a volume of approximately twice the volume of the semi-finished product, and the space formed between the semi-finished product and the container is filled with silicon which surrounds the semi-finished product. The quantity of silicon used is thus the amount which is necessary to fill the pores of the semi-finished product, or slightly more.

Pure silicon, or an alloy of silicon and aluminium or copper in granule or powder form, is used to fill the space.

The chamber may be in communication with the exterior by means of suitable holes for the discharge of the gases released during the firing.

After the silicon has been loaded, the container is inserted in a suitable conventional furnace heated to a temperature of 1400-1700°C. At these temperatures, the silicon melts and infiltrates the pores of the semi- finished product (silication step).

The firing is performed under a reduced pressure of from 900 mbar to 300 mbar, preferably from 800 to 500 mbar.

Upon completion of the firing, the composite material is cooled, for example, with argon or, preferably, with nitrogen, so that the residual silicon solidifies into small spheres which are easy to recover from the container.

The composite material according to the invention thus obtained may optionally be subjected to finished operations, for example, surface finishing, which may be performed dry or wet, in conventional manner.

Naturally, the steps of firing in the furnace, that is, pyrolysis and silication, may be performed in a single furnace, reducing production times and the complexity of the apparatus.

The composite material according to the invention may be formed to various shapes according to its final use. In particular, the material according to the invention may advantageously be used in the production of components for vehicle brakes, particularly disk brakes.

In this application, the material may be shaped as a braking ring or band for a disk to constitute the braking component of a disk brake or may be applied to a bell for supporting the braking band. Moreover, the material may also be applied to the caliper body of a disk brake as well as to the braking pad and may be shaped suitably for these applications. So-called ventilated disks such as

those described in International applications No.

PCT/IT00/00543 of 22.12. 2000, No. PCT/IT01/00412 of 27.07. 2001 and No. PCT/IT01/00411 of 27.07. 2001, in the name of the applicant of the present patent application, may also be produced with the composite material of the invention.

The characteristics and the advantages of the present invention will become clearer from the following description of an example of the production of a shaped composite material according to the invention, the description being given by way of non-limiting indication with reference to the following drawing: Figure 1 shows the composite material according to the invention, in section.

EXAMPLE A first mixture containing, as percentages by volume relative to the volume of the mixture, 65% of carbon filaments having a diameter of from 0.3 mm to 0.5 mm and a length of from 7 mm to 10 mm, 23% of dry phenolic resin and 12% of silicon carbide powder was prepared in a mixer known as an Erigh mixer, in conventional manner.

A second mixture containing, as percentages by volume relative to the volume of the mixture, 65% of carbon filaments having a diameter of from 0.3 mm to 0.5 mm and a length of about 3 mm, 23% of dry phenolic resin,

and 12% of silicon carbide powder, was prepared in the same manner.

A portion of the second mixture was then put in the cavity of an annular mould with an inside diameter of 150 mm, an outside diameter of 335 mm and a height of 102 mm, in a quantity such as to form, in the finished product, a layer (surface layer 1 in Figure 1) with a thickness of about 1 mm.

After the surface of this layer had been levelled, a further layer (base layer 2 in Figure 1), constituted by the first mixture, was spread thereon in a quantity such as to produce a thickness of about 30 mm in the final product. This surface was also levelled.

A third layer constituted by the second mixture (surface layer 3 in Figure 1) was deposited on the base layer 2 in a quantity such as to form, in the finished product, a layer with a thickness of about 1 mm, so as to produce a sandwich-like configuration.

The layers of the first and second mixtures were then moulded by heating the mould to a temperature of 100°C and applying a pressure of 1 N/cm2, to produce a rough disk-shaped body.

After the rough disk had been removed from the mould, it was fired in an furnace heated to a temperature of 1100°C for a period of 12 hours.

The firing was performed with a pressure of 30 mbar and in an inert atmosphere in the presence of argon which was conveyed into the furnace with a flow of 30 litres/minute.

After firing, the disk was subjected to dry diamond finishing, in conventional manner, to remove surface deformations.

At this point, the rough disk was placed in a container provided with gas-outlet holes. The container was filled with the quantity of silicon granules required to fill the space formed between the disk and the container. The container was then transferred to a furnace heated to a temperature of 1500°C and was left in the furnace for a period of 8 hours. The firing was performed at a reduced pressure of 700 mbar and was followed by cooling in the furnace with continuous blowing-in of nitrogen.

A disk of the composite material according to the invention was thus obtained and, after cooling, was subjected to diamond finishing, in conventional manner, so as to remove surface deformations and to produce the final shape, with the desired precision and tolerances.

Analysis by weight performed by thermogravimetry (in this example with NETZSCH Mod. STA 409 PC instrumentation) on a portion of surface layer of the

disk determined a percentage of free carbon of 7% by weight.

The composition of the composite material of the disk, as percentages by volume relative to the volume of the material, was as follows: 65% of filaments constituted substantially by carbon, 13% of additives, and 22% of products resulting from the carbonization of the binder.

The disk thus produced was tested as a component of a vehicle disk brake and was found to have optimal characteristics of hardness, impact strength, wear resistance, compression strength, and resistance to the temperatures generated by friction during braking. In addition, after a cycle of 1000 braking operations, the disk of the example showed a loss of carbon, by weight, which was reduced to one quarter in comparison with a disk of the prior art.

As is clear from the foregoing description, the composite material of the invention has good resistance to surface oxidation which also makes it particularly attractive for prolonged uses. A further advantage is that both the base layer and the at least one surface layer are made of the same material-the sole difference being the lengths of the filaments constituted substantially by carbon-and that there is no clear

junction line between the layers (as a result of the production process, which creates a unitary body composed of the layers but prevents mixing of the shorter fibres and of the longer fibres which are introduced into the two starting mixtures). This protects the disk from the formation of surface cracks or fissures which, in the embodiments of the prior art, are due to the different coefficients of thermal expansion of the various materials.

These characteristics, together with economy and simplicity of production suggest the use of the material of the invention even in the large-scale vehicle- production field.

The composite material according to the invention is distinguished by its optimal frictional characteristics, hardness, bending strength, resistance to wear and to temperatures generated by friction, and impact and compression strength.

In order to satisfy contingent and specific requirements, a person skilled in the art may apply to the above-described preferred embodiment of the composite ceramic material many modifications adaptations and replacements of elements with other functionally equivalent elements without, however, departing from the scope of the appended claims.

In fact, it would clearly also be possible to increase the surface area of the filaments constituted substantially by carbon in ways other than that suggested in the foregoing description, for example, by modifying the superficial nature of the filament (greater porosity) either solely in the vicinity of the surface or throughout the body of the material, and hence without the need to provide filaments shorter than those of the prior art. In this case, the advantage of increasing the silicon carbide/carbon ratio and hence rendering the surface of the material less liable to oxidation will in fact also be achieved.