Method for forming a carbon-containing material for a brake disc

The present invention relates to a carbon-containing material for a brake disc .

Disc brakes comprising a caliper and a disc have been widely adopted, particularly for automotive applications. The disc is squeezed during braking between pads of friction material mounted in the caliper. The disc is mounted so that it rotates about an axis parallel to the axis of wheel rotation.

During use, a brake disc will need to withstand considerable stresses, particularly shear, frictional and abrasive forces . A brake disc will often wear due to the abrasive forces over a period of time.

Brake discs that may be subjected to particularly high energy braking forces commonly contain some type of reinforcement, often carbon filaments in some form such as cloth, chopped fibre, woven fibres and the like. The primary purpose of such filaments is to impart high mechanical properties such as strength and rigidity at elevated temperature to the discs so that they can withstand the forces encountered during braking in high performance applications. Such applications are typically, but not exclusively, in the automotive and aerospace industries, for example in the braking systems of high performance sports cars, jet aircraft and the like. Brake discs may contain a carbon material, which may be in the form of a carbon-carbon (C/C) composite, e.g. a brake disc containing carbon cloth

or filament layers contained within a carbon matrix. (The term "carbon" is used in the generic sense and can mean any- type of carbon, including either amorphous carbon or crystalline graphite) . Additionally, brake discs have been made utilizing a bulk carbonaceous layer such as bulk graphite in order to reduce the long fabrication time of the C/C composite laminate and to reduce cost. Of course, brake discs have also utilized combinations of the C/C composite and bulk graphite layers.

In operation disc brakes can reach temperatures in excess of 1000 ⁰ C. Such high temperatures lead to oxidation of the disc (the threshold for oxidation of carbon is approximately 1010 ⁰ C and many users set an upper operation limit of 800 ⁰ C) and results in high wear rates of both the friction and non-friction areas.

The processes currently available for creating carbon- carbon brake discs take a considerable amount of time and require a substantial amount of energy to produce the desired material. All processes currently available have the common feature of providing a carbon preform material (a loosely packed body of carbon fibres) and then densifying the preform by the deposition of carbon within its porous structure. The processes differs in the type of preform used and the way in which the carbon is deposited within the structure .

The preform may comprise, for example, a fabric in the form of a felt, a woven cloth, a knit, a braid, a unidirectional sheet of yarns, tows or strands, or a multilayer fabric wherein each layer comprises unidirectional

sheets of the carbon fibres and the direction of each layer is perpendicular to the adjacent layers, these layers being bonded by needling.

There are two common ways of depositing carbon within the preform: chemical vapour infiltration and liquid infiltration techniques. In the chemical vapour infiltration technique, a preform is placed in an enclosure into which a gas is inserted containing carbon precursors, such as alkanes or alkenes . The gas diffuses into the preform and, at the appropriate temperature and pressure, pyrolytic carbon is deposited within the pores of the preform. In the liquid infiltration technique, a liquid containing a carbon precursor is introduced into the pores of the preform and the preform heated to carbonize the precursor, thereby building up carbon within the preform material .

Commonly, it is necessary to repeat the steps of infiltrating the preform and then depositing the carbon many times to build up sufficient carbon within the preform. This means that the time it takes to create a high-quality carbon-carbon material may be in the order of months.

It is an aim of the present invention to overcome or mitigate at least one of the problems associated with the prior art .

In a first aspect the present invention provides a method for forming a carbon-containing material for a brake disc, the method comprising:

(i) providing a porous body;

(ii) introducing into the pores of the porous body a liquid suspension of particles comprising carbon;

(iii) depositing the particles comprising carbon within the pores of the porous body; (iv) introducing into the pores of the porous body one or more precursor materials for forming or depositing a ceramic material, wherein the precursor material comprises a liquid suspension of ceramic particles and/or acid phosphate; (v) forming or depositing the ceramic material from the precursor material within the pores of the porous body.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

In the method of the present invention, steps (ii) to (v) can be repeated one or more times to increase the amount of carbon and/or ceramic material deposited within the porous body. The method of the present invention may involve the formation of a carbon-containing material by infiltrating the porous body with a plurality of liquid suspensions in succession, only one of which needs to contain particles comprising carbon, and depositing the particles from each suspension within the pores of the porous body, with further processing (if required) to cure the deposited particles. It has been surprisingly found

that by using the method of the present invention, particularly when depositing the particles comprising carbon by charging the porous body, that it does not generally require a curing step or further processing to adhere the particles of carbon to the material forming the porous body.

The suspension may contain a plurality of types of particles, one of the types being carbon. By using such a suspension, two or more types of particles may be deposited from the suspension at the same time.

In a second aspect, the present invention provides a material for a brake disc formable by the method of the present invention.

In a third aspect, the present invention provides a brake disc comprising a material of the present invention.

In a fourth aspect, the present invention provides a clutch plate comprising a material of the present invention.

The particles should be of a suitable size to allow them to pass through at least some of the pores of the porous body. Preferably, the particles have an average diameter of from 25 to 120 nm. "Average diameter" is the arithmetic mean of the diameters of the particles.

"Carbon", unless otherwise stated or required by the context in the description, means elemental carbon. In the particles comprising carbon, the carbon may be in the form of one or more of its allotropes, preferably graphite or in its amorphous form. The particles may comprise carbon black

particles. The particles can be formed from lamp black, furnace black or decomposition of gas or by any other means known to those skilled in the art of making fine Carbon particles. A common use of these materials is in printers inks .

Preferably, the or each suspension is a dispersion of particles in a liquid, and such a suspension will be referred to simply as a "dispersion" . The dispersion may be a Sol. A "Sol" is a colloid in which solid particles are dispersed in a liquid continuous phase (Oxford Dictionary of Chemistry, Fourth Edition 2000) . The suspension of particles comprising carbon may comprise up to 40% by weight of carbon suspended in water. Preferably, the suspension comprises 20-30 wt% of Carbon suspended in water. A suspension (particularly a Sol) can however contain as little as 0.2% carbon. However, it has been found that the higher the wt% of carbon in the suspension, the higher the deposition rate of carbon will be. A surfactant or wetting agent is typically added when making Sols with more than 5 wt% of Carbon. Suitable surfactants and wetting agents are known to the skilled persons. Sols can be prepared by a variety of methods known to the skilled person including, but not limited to, ball milling, mixing and ultrasonic mixing. Sols containing carbon particles in suspension can also be obtained commercially from companies such as Degussa (eg the Derussol family of products) . The suspension can be a mixture of Sols.

Preferably, the or each suspension is an aqueous suspension.

Preferably, the or each suspension has a pH of less than 7, more preferably a pH of from 3 to 5, most preferably a pH of about 4.

Preferably, the or each suspension further contains a surfactant. Preferably, the surfactant is such that the particles in the Sol have a positive charge. Preferably, the surfactant is a cationic surfactant, which may be selected from cetyl trimethylammonium bromide, cetylpyridium chloride, polyethoxylated tallow amine, benzalkonium chloride or benzethonium chloride. Most preferably, the surfactant comprises cetyl pyridium chloride.

The or each suspension may comprise a plurality of types of particles, of which carbon is one type. The suspensions may, for example, comprise particles comprising carbon and particles comprising alumina. Such a Sol can be made by providing an alumina Sol and adding the carbon particles to it, with the Sol then being mixed and agitated as required to distribute the carbon particles throughout the liquid.

The particles of the suspension can be infiltrated into the porous body by one or more processes, including, but not limited to, vacuum infiltration or soaking (i.e. by capillary creep into the pores) . Various processes can be used to assist infiltration such as physical agitation and ultrasonic waves. The particles to be infiltrated can also be placed in a stream of suspension that can also be recirculated.

Once the suspension is infiltrated within the porous body, then the particles can be deposited onto the surfaces of the pores by several processes. These include drying of the porous body and in the case of a sol the destabilization of the Sol using chemical and physical methods (e.g. freezing, control of pH, electrophoresis, interaction with the porous body or with one another) .

One preferred method of deposition is to electrically charge the porous body, preferably to +/- 3 V or more, preferably +/- 10 to 50 V. Preferably, in step (iii) and/or step (v) , the porous body is negatively charged. The body may be electrically charged for a time of 1 minute or more, preferably, 5 minutes or more, preferably 10 minutes or more, most preferably from 10 to 45 minutes to allow the deposition of the particles to occur. The current density during deposition is dependent upon the internal surface area of the porous body and the nature of the suspension. A compromise between deposition rate and evenness of deposition can be arrived at by simple experimentation. If the porous body is negatively charged, preferably the pH of the suspension is less than 7 and/or contains a cationic surfactant as described above .

The porous body may be electrically charged using known techniques in the art, e.g. attaching a power source, such as a battery, to the porous body such that the porous body becomes an electrode, with a second electrode being attached to the other polarity of the power source. The first and second electrodes may then be placed in the liquid Sol and, when the power is switched on, a charge is applied.

Preferably, in step (ii) and/or step (iii) and/or step (iv) and/or step (v) the body and the liquid suspension are exposed to an ultrasonic treatment. The ultrasonic treatment has been found to aid infiltration of the particles through the pores of the porous body and also improve the deposition of the particles in the pores. Preferably, ultrasound waves having a frequency of from 20 to 400 ICHz are applied, more preferably a frequency of from 35 to 80 KHz.

"Introduced into the pores of the porous body" indicates that the suspension is introduced into at least some of the pores of the porous body. Preferably, the suspension is introduced substantially throughout the porous body. The porous body may have first and second surfaces on opposite sides of the porous body and the suspension may be introduced into part of the porous body so that, in that part, the suspension (and hence subsequently the deposited carbon) is present substantially throughout the body from first to second surfaces. This may be achieved by masking part of the first and/or second surfaces of the porous body before step (ii) to allow introduction of the suspension into only part of the body, but allowing passage of the suspension from the first surface to the second surface in that part. The porous body may have first and second surfaces on opposite sides of the porous body and the suspension may be introduced into the porous body so that the suspension (and hence subsequently deposited particles) is present substantially throughout the body from first to second surfaces.

The porous body preferably comprises one or more materials selected from carbon, silicon carbide, silicon nitride, alumina, silica and glass, preferably in the form of fibres. In other words, the matrix of the porous body that defines the pores preferably comprises one or more of these materials. Preferably, prior to introduction of the suspension, the porous body comprises 50 % or more, more preferably 90% or more, by weight of one or more materials selected from carbon, silicon carbide, silicon nitride, alumina, silica and glass. The porous body may comprise a material for use in a brake disc and may be suitable for use as a brake disc prior to carrying out the method of the present invention.

Preferably, in step (i) , at least some of the pores in the porous body are interconnected to one another. Preferably, the pores in the interior of the body are interconnected to a free surface of the body. Such a body is ¹ termed a body having open interconnected porosity. Optionally, the pores on the surface of the porous body may be sealed once the desired amount of material has been deposited within the pores.

The porous body, in step (i) , is preferably permeable to gases, and liquids such as water. The material of the present invention need not be permeable to gases and/or water after deposition of the particles from the suspension within its pores, but it may be permeable to gases and/or water if desired.

The porous body, in step (i) , may be a rigid body. The porous body may be in the shape of a brake disc. The porous

body may be in the shape of a ring. The porous body may- comprise a material for use in a brake disc.

The pores will preferably be of a size sufficiently large to allow the particles comprising carbon and, if present, ceramic particles of the precursor material through the porous body and preferably the porous body will contain pores having a diameter of at least 5 μm, more preferably at least 10 μm, still more preferably at least 100 μm, most preferably at least 300 μm. The infiltrated material can then penetrate from the free surface into the interior of the body.

The porous body, in step (i) , may comprise a collection of particles and/or fibres, which preferably comprise carbon, preferably 50 % or more by weight of carbon, more preferably 90 % or more by weight of carbon, as measured prior to the introduction of the suspension into the porous body. The porous body may be in the form of a preform, preferably a preform for forming a brake disc. The particles and/or fibres may be a loosely bound collection of particles and/or fibres, i.e. the particles/fibres are not chemically bonded together. Such a collection of particles or fibres would not be suitable for use without further processing as a material for a brake disc. The porous body may comprise chopped fibres. In one aspect of the invention the porous body may be a "green body" compact, e.g. a body composed of loosely bonded particles and/or fibres that have been pressed or bonded together. The present inventors have found that by using the process of the present invention, the particles/fibres are adhered together and form a body suitable for use as a friction or structural material, e.g.

for use as a brake disc (i.e. it has sufficient durability under normal braking conditions to be used as a brake disc) .

The amount of energy required to form the material of the present invention has been found to be generally much less than in the methods of the prior art used to form a carbon-carbon brake disc. The method uses less time and resources and therefore reduces cost and environmental impact. Furthermore, the method does not produce any noxious fumes and all of the materials used may be recyclable .

It will also be obvious to one skilled in the art that it is possible to combine conventional carbon-carbon synthetic methods (eg CVI, CVD and liquid pitch pyrolization and others mentioned herein) with the method of the present invention. For example, particles may be deposited in the pores of a porous body using the method of the present invention, following which more carbon may be deposited within the pores using the process of CVI. Using such a combination of deposition techniques, different size fractions of carbon particles can be introduced into the porous matrix in order to ensure maximum packing efficiency and minimum porosity (if that is what is desired within the finished material) .

It has been surprisingly found that a brake disc made according to the processes described in International Patent Application No. PCT/GB2006/002010 greatly benefits from the incorporation of carbon particles either in addition or as a partial replacement for the ceramic particles described therein. In the case of the former, the process produces a

denser brake disc with higher heat capacity, and in the latter case it would lead to a lighter weight disc, relative to a disc in which the equivalent volume of ceramic particles has been deposited. In one embodiment, it has been found to be advantageous to deposit alumina within the pores of a preform prior to the introduction of the carbon Sol as this assists the wetability of the preform for the carbon Sol .

The braking properties of a brake disc formed using the method of the present invention are comparable to the carbon-carbon brake discs of the prior art. Furthermore because of the reduced processing time of the discs compared with the existing technology, huge cost savings can be expected. It is also possible to engineer the properties of the disc more precisely than existing carbon-carbon materials to reflect the braking requirements with a much quicker and more economic turnaround time, greatly benefiting the product.

The body may comprise cloth comprising fibres that comprise carbon. The body may comprise layers of cloth. The body may comprise a cloth in the form of a felt, a woven cloth, a knit, a braid, a unidirectional sheet of yarns, tows or strands, or a multi-layer fabric wherein each layer comprises unidirectional sheets of the carbon fibres and the direction of each layer is perpendicular to the adjacent layers, these layers being bonded by light needling.

The fibres preferably have an average cross-sectional diameter of from 3 to 20 μm. "Average diameter" is the arithmetic mean of the diameters of the fibres. The fibres

are preferably arranged into tows. The fibres may be of any suitable length. The fibres and/or tows may be in a woven or non-woven arrangement. The fibres may be sewn or otherwise arranged to make a three dimensional arrangement. The porous body may comprise a felt-like material comprising fibres, which preferably comprise carbon. "Felt-like material" means a material comprising non-woven, compressed fibres .

A "tow" is an untwisted collection of continuous fibres. A tow may contain many, e.g. hundreds or thousands, of fibres. The tow preferably contains 1000 fibres or more, more preferably 5000 fibres or more. Tows may be grouped together, preferably without twisting, and in this configuration are termed a "roving" .

There is no limitation to the volume percentage of the porosity contained within the porous body. The porous body, in step (i) , preferably has a porosity of from 5 to 40 % by volume.

If the porous body comprises a cloth material, which may comprise carbon, then the aerial density of the porous body, in step (i) , is preferably from 50 to 2500 g/sq m. Aerial density is the weight of a material, typically clo,th, per unit area and is typically expressed in g/sq m.

The suspension containing the particles comprising carbon may further contain the one or more precursor materials for forming or depositing a ceramic material, for example the use of a metal oxide in combination with an acid phosphate. Alternatively, the precursor material may be

introduced into the pores of the porous body separately from the suspension containing the particles comprising carbon. The precursor material, may, for example, be introduced before or after introduction of the carbon suspension. The precursor material may be introduced before, during or following deposition of the particles comprising carbon within the porous body, preferably following the deposition of the particles comprising carbon.

Preferably, the ceramic particles comprise a material selected from one or more of alumina, zirconia, magnesia, yttria, silicon carbide, silica, boron carbide, boron nitride, titanium boride, iron oxides and chromium oxides.

Preferably, the ceramic particles have a size of from 10 nm to 100 μm, more preferably 10 nm to 10 μm, most preferably, the ceramic particles have an average size of from 10 nm to 100 μm, more preferably of from 10 nm to lOμm. Preferably, the ceramic particles have an average size of from 20 nm to 200 nm. The "size" of a particle indicates its largest cross-sectional diameter. The "average size" of the particles is the arithmetic mean diameter of the particles .

Preferably, the precursor material comprises a Sol of ceramic particles. A "Sol" is a colloid in which solid particles are dispersed in a liquid continuous phase (Oxford Dictionary of Chemistry, Fourth Edition 2000) .

Preferably, the precursor material has a pH of from 0.5 to 7.0. Preferably, the precursor material has a pH of from 1 to 5.

Preferably, the precursor material comprises a polar solvent, more preferably water.

The precursor material may comprise from 10 to 30 % by weight of the ceramic particles. Preferably, the precursor material comprises of from 15 to 25 % by weight of the ceramic particles.

The suspension and/or precursor material may be introduced into the pores of the porous body by one or more techniques selected from vacuum infiltration, immersion of the body at least partially into the precursor material, painting the body with the precursor material and spraying the porous body with the precursor material. Introduction of the precursor material into the porous body may be aided by the agitation of the porous body and precursor (s) using an ultrasonic treatment. Preferably, ultrasound waves having a frequency of from 20 to 400 KHz are applied, more preferably a frequency of from 35 to 80 KHz.

The ceramic particles of the precursor material are preferably deposited into the pores by means selected from one or more of electrically charging the porous body, freezing the porous body, introduction of an acidic or alkaline material and introducing into the pores a material containing particles having an opposite charge from the particles in the suspension.

To effect deposition of the ceramic particles, the porous body may be electrically charged to +/- 3 V or more, preferably +/- 10 to 50 V. The body may be electrically

charged for a time of 1 minute or more, preferably, 5 minutes or more, preferably 10 minutes or more, most preferably from 10 to 45 minutes to allow the deposition of the ceramic particles to occur. The current density during deposition is dependent upon the surface area of the carbon fibres and the nature of the suspension. A compromise between deposition rate and evenness of deposition can be arrived at by simple experimentation.

Composite parts of the porous body, e.g. two or more layers of porous material, such as cloth, may be combined before or after deposition of the particles comprising carbon and/or the ceramic material .

The particles of the precursor material may be deposited in the presence of Yttria. Yttria, in the form of a Sol, has been found to aid deposition of certain ceramic particles, in particular particles with a positive charge in suspension, such as alumina particles also suspended in a sol. Yttria has also been found to have the advantage of toughening the deposited ceramics, particularly when the particles are reacted, cured or vitrified by heating.

The particles comprising carbon and/or the ceramic particles of the precursor material may be assisted in depositing within the pores of the porous body by applying ultrasound to the porous body while the suspension and/or precursor material, as appropriate, is within the pores of the porous body. Preferably, ultrasound waves having a frequency of from 20 to 400 KHz are applied, more preferably a frequency of from 35 to 80 KHz.

The particles of the precursor material once deposited in the pores may be reacted, cured or vitrified to form the ceramic material .

The porous body may be moulded in to the shape of a brake disc in step (i) , and/or during or after any of steps (ii) to (v) . Pressure may be applied to maintain the porous body in the shape of a brake disc during steps (ii) to (v) .

If ceramic particles have been deposited within the porous body, the particles may be cured by heating the body during or after their deposition. The body may be heated for a first period at a temperature of 100 to 300 ⁰ C, preferably for a period of 1 hour or more, and then optionally for a second period at a higher temperature, preferably of a temperature of from 350 to 370 °C, preferably .for a period of 1 hour or more .

Preferably, the acid phosphate comprises one or more of potassium acid phosphate, calcium acid phosphate, ammonium acid phosphate and aluminium acid phosphate.

Preferably, the acid phosphate comprises mono aluminium phosphate .

The acid phosphate may be vitrified within the pores of the porous body to form the ceramic material . The curing or vitrification of the acid phosphate may be in the absence of any other reactant, i.e. the acid phosphate alone may cure or vitrify.

Preferably, the precursor material contains a suspension of (i) ceramic particles and (ii) acid phosphate and/or phosphoric acid.

In the method of the present invention, a first precursor liquid containing a suspension of ceramic particles and a second precursor liquid containing acid phosphate and/or phosphoric acid may be introduced separately into the pores of the body. Either the first precursor material and/or the second precursor material may comprise the particles comprising carbon. Alternatively, the particles comprising carbon may be introduced and deposited in the pores of the porous body separately from both the first and second precursor materials, either before or after introduction of the first and/or second precursor materials. Preferably, the particles of the first precursor liquid are deposited in the pores of the porous body prior to introduction of the second precursor liquid. Preferably, the body is dried following deposition of the particles of the first precursor liquid in the pores and before introduction of second precursor liquid.

Preferably, the body is heated to react, cure or vitrify the deposited ceramic particles and the acid phosphate. Typical curing conditions can vary dependent on the acid phosphate used, but preferably the body is heated for a first period of 1 to 2 hours at a low temperature (e.g. 100 to 130 ⁰ C) and then for a second period at a higher temperature (e.g. 350 to 370 ⁰ C).

The acid phosphate may react in the pores in the presence of a metal oxide to form the ceramic material, which comprises a chemically bonded phosphate ceramic.

The metal oxide may be a divalent or trivalent metal oxide .

The metal oxide preferably comprises one or more of aluminium oxide, calcium oxide, iron oxide, magnesium oxide and zinc oxide.

The present invention provides a porous body containing within its pores particles of carbon deposited using the method of the present invention and particles comprising a ceramic material.

The present invention also provides a method for the manufacture of a porous body, which may be as defined herein, the method comprising: carrying out steps (i) to (v) of the present invention and (vi) forming the chemically bonded phosphate ceramic in situ in the pores of the porous body. Step (vi) may be carried out at any suitable point during the method.

"Chemically bonded phosphate ceramic" is a term of the art and includes materials formed from a reaction between an acid phosphate and a metal oxide. Chemically bonded phosphate ceramics and their methods of synthesis are illustrated in a paper entitled Chemically Bonded Phosphate Ceramics, by S. Y. Jeong and A. S. Wagh, published in

Materials Technology, June 2002. The content of this paper, particularly its method of synthesis of magnesium phosphate

ceramics, aluminium phosphate ceramics and iron phosphate ceramics, are incorporated herein by reference. An "acid phosphate" is a chemical species comprising a phosphate ion and at least one hydrogen ion, e.g. HPO ₄ ^2" _. The acid phosphate will normally further include a metal ion. Mono aluminium phosphate has the formula Al (H ₂ PO ₄ ) ₃ .

The chemically bonded phosphate ceramic may partially or completely fill some or all of the pores of the porous body.

Preferably, the chemically bonded phosphate ceramic comprises a chemically bonded divalent or trivalent metal phosphate ceramic. A "chemically bonded metal phosphate ceramic" is a chemically bonded phosphate ceramic comprising the said metal, which may be formed from the acid phosphate of the said metal. Preferably, the chemically bonded phosphate ceramic comprises one or more of a chemically bonded magnesium phosphate ceramic, a chemically bonded calcium aluminate phosphate ceramic, a chemically bonded zinc phosphate ceramic, a chemically bonded aluminium phosphate ceramic and a chemically bonded iron phosphate ceramic. Particularly preferred is a chemically bonded aluminium phosphate ceramic, preferably formed from a reaction between aluminium oxide (alumina) and aluminium hydrogen phosphate .

The chemically bonded phosphate ceramic may contain further materials, preferably ceramic materials, which may be in the form of a powder. In the method of manufacturing the porous body as defined herein, these further materials and the precursors to the chemically bonded phosphate

ceramics may be introduced into the porous body either separately or in combination with each other. These further materials may contribute to the physical or mechanical properties of the material but are not necessarily reactants in the formation of CBPCs although they may be physically interlocked with the CBPC. Preferably but not exclusively these further materials comprise ceramics such as aluminium oxide, silicon carbide, silica, magnesium oxide, boron carbide, boron nitride, titanium boride, yttria, iron oxides and chromium oxides, or any combination thereof _.

The body preferably has a porosity of 5-40% by volume, the porosity being measured prior to the introduction of the chemically bonded phosphate ceramic into or formation of the chemically bonded phosphate ceramic in the pores.

The present invention further provides a brake disc comprising a porous body as defined herein.

As mentioned above, the present invention provides a method for the manufacture of a material of the present invention further including a chemically bonded phosphate ceramic within the pores of the porous body, as defined herein, the method comprising: carrying out steps (i) to (v) of the present invention and (vi) forming the chemically bonded phosphate ceramic in situ in the pores of the porous body.

The reagents and reaction conditions for making the chemically bonded phosphate ceramic may be as disclosed in the paper entitled Chemically Bonded Phosphate Ceramics, by S. Y. Jeong and A. S. Wagh, published in Materials Technology,

June 2002. Within this paper the reaction between sparingly soluble metal oxides and acid phosphates is described and illustrated by means of examples of the most common reactions studied thus far.

The chemically bonded phosphate ceramic is formed from precursors, these precursors preferably comprising an acid phosphate and a metal oxide . The precursors may be in water, either as a solution or a suspension.

Preferably, the acid phosphate comprises one or more of potassium acid phosphate, calcium acid phosphate, ammonium acid phosphate and aluminium acid phosphate. Aluminium acid phosphate (aluminium hydrogen phosphate) is particularly preferred.

The metal oxide is preferably a divalent or trivalent metal oxide, more preferably a sparingly soluble metal oxide. The metal oxide preferably has a solubility product, pK _sp , in water of from 10 to 50. The metal oxide may comprise one or more of aluminium oxide, calcium oxide, iron oxide, magnesium oxide and zinc oxide. Aluminium oxide is particularly preferred.

The metal oxide may be in solution and/or in the form of a suspension of solid particles in a liquid, as herein described.

If a divalent metal oxide is used, e.g. magnesium oxide, preferably the divalent metal oxide is calcined, preferably at a temperature of about 1300 ⁰ C or above so that its grains are well crystallized and micropores from the

grains are substantially removed. This reduces the solubility of the divalent metal oxide, particularly magnesium oxide, to a level at which improved yields of the chemically bonded phosphate ceramic are obtained.

If aluminium oxide is used, preferably the reaction to form the chemically bonded phosphate ceramic is carried out at a raised temperature, preferably at a temperature of from 100 to 200 ⁰ C, more preferably of from 118°C to 170°C, most preferably at about 150 ⁰ C. This raises the solubility of the aluminium oxide so that the yield of the chemically bonded phosphate ceramic is improved.

If Fe ₂ O ₃ is used as a trivalent metal oxide, preferably a small amount of elemental iron is used, since this reduces the trivalent iron to divalent iron, improves the overall solubility of iron in solution and thus improves the yield of the chemically bonded phosphate ceramic.

The precursors to the chemically bonded phosphate ceramic may further comprise one or more of silicon carbide, silica, boron carbides, boron nitrides and titanium borides . The precursors may also further comprise deflocullants or retardant materials such as boric acid.

The acid phosphate may constitute less than 40% by weight of the total mass of the precursors to the chemically bonded phosphate ceramic.

Preferably, the body has open interconnected porosity and the precursors to the chemically bonded phosphate ceramic are introduced into at least some of the

interconnected pores of the porous substrate prior to the formation of the chemically bonded phosphate ceramic in the pores .

At least two of, and possibly all of, the precursors to the chemically bonded phosphate ceramic may be mixed together and the resultant mixture then introduced into the pores of the porous body prior to the formation of the chemically bonded phosphate ceramic. The precursors may be the acid phosphate and the metal oxide.

At least two, and possibly all of, the precursors to the chemically bonded phosphate ceramic may be introduced into the pores of the porous body separately. For instance, the acid phosphate and metal oxide may be introduced separately.

The precursors may be cured by the application of heat to form or strengthen the chemically bonded phosphate ceramic.

The precursors to the chemically bonded phosphate ceramic may be introduced into the pores of the porous body by one or more of : spraying the porous body with the precursors; dipping the porous body into a mixture of the precursors; painting the porous body with the precursors; vacuum infiltration of the precursors into the porous body and pressure infiltration of the precursors into the porous body. Preferably, the precursors are introduced by first spraying the precursors onto at least one surface of the porous body and then drawing the precursors into the interior of the body by vacuum infiltration.

The present invention provides a porous body containing deposited carbon particles and a chemically bonded phosphate ceramic within its pores, the body formable by the methods as defined herein.

The present invention further provides the use of a porous body as defined herein as a material for example for use as a brake disc or clutch plate; or as a structural component for use at elevated temperatures for example for use as a heat shield; engine components; oven furniture; kiln furniture; metal-, glass- or ceramic-processing equipment; heater components; or light fittings.

The present invention further provides the use of a porous body as defined herein in a brake disc.

A combination of two or more different types of chemically bonded phosphate ceramics (CBPCs) may be present in the pores of the porous body.

The properties of the porous body containing the CBPC in its pores will be dependent on the amount of CPBC in the pores and the constitution of the CPBC. The amount and content of CPBC may be determined according to the properties desired in the resultant porous body.

The precursors are preferably in a form so that they have sufficient fluidity to infiltrate the porous body (this is also dependent on the method of infiltration) , for instance in solution and/or a fine suspension in a solvent, such as water. The precursor material and/or the suspension

may also contain deflocullants, dispersing agents and other additives designed to optimize fluidity and stability as is well known to those skilled in the art of controlling aqueous suspensions of fine powders. The dispersing agent may be from the Dispex series of dispersing agents manufactured by CIBA.

The size fraction of the metal oxides is selected to optimize incorporation into the porous body and the speed and extent of the CBPC reaction. If the infiltrated material is composed of several different metal oxides, each powder may be of a different size fraction; similarly if a single metal oxide is used, the metal oxide may comprise powders of two or more different size fractions.

Following infiltration the entire body may be cured using a heating cycle appropriate to the particular CBPC reaction or combination of reactions being used and optimized for the particular combination of particulates and powders used in the infiltrated mixture. Alternatively, the CBPC may be cured at room temperature (e.g. 20 ⁰ C) or below room temperature, including freezing processes.

Preferably, the surface of the material of the present invention has a fine surface finish, and preferably has a mean surface roughness above 5 microns Rz. Preferably, the surface of the body has mean surface roughness of less than 30 microns Rz. Preferably, the mean surface roughness is from 5 to 30 microns Rz. The surface finish may be altered using techniques known -to those skilled in the art.

The surface of the discs may also be finished by techniques such as grooving, cutting or drilling the discs, as is known to those skilled in the art in order to improve braking performance. The degree of impregnation can be controlled in different parts of the disc by processes such as masking and hence the thermal energy flow within the disc can be varied.

The method of the present invention preferably forms a brake disc.

The present invention further provides a material for forming a brake disc comprising a porous body containing within its pores deposited particles comprising carbon and/or a ceramic material, wherein the deposited particles and/or the ceramic material is present substantially throughout at least a portion of the porous body. If the porous body is in the shape of a disc having first and second opposing surfaces in the plane of the disc, the deposited particles and/or the ceramic material may, in at least a portion of the disc (or optionally in the entire disc) be present substantially throughout the disc from the first to the second surfaces. In other words, when a cross section is taken through that portion of the porous body, the deposited particles and/or the ceramic material will be seen to be present throughout that portion from the first to the second surface, preferably in a substantially consistent concentration (in mass of ceramic material per unit volume of the porous body, e.g. g/cm ³ ) , for example preferably the concentration of deposited particles and/or the ceramic material does not vary by more than 20% from the first surface to the second surface.

Preferably, in a portion of the disc where the deposited particles comprising carbon and/or the ceramic material is present at or just below the first and second surfaces, the concentration of ceramic material for a given cubic centimetre (or more preferably a unit volume of 125 mm ³ ) at any point between the first and second surface of the porous body in that portion of the disc is not zero. In preparing such a disc, various parts of the disc may be masked during the synthesis process to prevent infiltration of the particles comprising carbon and/or the ceramic material into the disc.

"Substantially throughout the porous body" includes, but it is not limited to, a porous body having less than 3 mm between pores containing the ceramic material or deposited .particles comprising carbon (as appropriate) , preferably less than 2 mm, most preferably less than 1 mm, between pores containing the ceramic material or particles comprising carbon.

The advantage of the ceramic material being present at a fairly consistent concentration in the pores of the body from the first to the second surface is that, in use, the wear and/or frictional properties of the body remain relatively constant as the frictional surface (i.e. the first or second surface) of the body is worn away in use.

The present invention provides a friction system comprising two engageable frictional components, wherein at least one of the components comprises a material of the present invention. One component may be a brake disc and the other component may be a brake pad, said brake disc

comprising a material of the present invention. Alternatively, one or both components is/are a clutch plate.

Both engageable frictional components may comprise a material of the present invention or only one of the engageable frictional components may comprise a material of the present invention. Preferably, one or both engageable frictional components have a mean surface roughness of 30 microns Rz or less, preferably on the surface with which one component engages with the other component.

The brake pad may comprise material that is not a material of the present invention. The brake pad may comprise a conventional brake pad material, which may include materials such as abrasives, e.g. alumina or silicon, lubricants, e.g. graphite or a sulphide, loads, elastomers, metals, polymeric fibres and bonding resins. The one or more brake pads may each be mounted in a caliper in a conventional manner.

If the body is or forms part of a brake disc, various aspects of the performance of the disc can be enhanced by controlling the surface finishing operations carried out on the disc. In particular the co-efficient of friction, initial bite and bedding-in of the discs can all be enhanced by producing a disc with a fine surface finish (preferably a mean surface roughness of 30 microns Rz or less, preferably from 5 to 30 microns Rz) . Preferably at least the wear surfaces of the disc have a fine surface roughness, preferably of from 5 to 30 microns Rz. The mean surface roughness is measured according to the Rz ISO standard, which is also termed the "Ten Point Average Roughness" . The

mean surface roughness is calculated by averaging the height of the five highest peaks and the depth of the five lowest valleys over the measuring length, using an unfiltered profile.

It has been found that a brake disc produced according to the invention can be used with brake pads made from a variety of different materials including organic, organo- metallic, carbon-metallic, carbon-ceramic and carbon-carbon and other brake pads known to those skilled in the art.

Carbon-carbon brake pads suitable for use on the unprocessed disc (untreated disc) are preferred. Clearly different pad materials give a different braking response that may or may not be desirable.

The material of the present invention may also be used in combination with other materials such as, for example, traditional carbon-carbon, monolithic carbon or carbon fibre reinforced plastic materials. By such use a brake disc may be formed which has high heat capacity in one area, high thermal conductivity in another and high wear resistance in another as desired.

The material of the present invention may be used as a friction material. However, it may also be used as a structural material, particularly in situations where heat resistance is required. The material of the present invention may, for example, be used in rocket nozzles, motor brushes, aircraft nose tips, leading surfaces of space shuttle wings, and in nuclear reactors.

Components that can be made using the material of the present invention include brake discs and similar friction products, heat shields, engine components, oven or kiln furniture, metal-, glass- or ceramic-processing equipment, heater components, light fittings and other high temperature structural components..

The porous body may also be used to form other components for use in environments where the component will be subjected to high stresses and/or high temperatures. Such components can be parts for use in the automotive (including Heavy Goods Vehicles), aerospace, railway, machine tool, medical or construction industries.