This invention relates to a method for the manufacture of carbon-carbon composite material and to composite material formed by the method of the invention.

The established techniques for manufacture of carbon-carbon composite material for use in such products as friction discs for aircraft brakes involve long time scales whether the carbon-carbon composite material is formed by a technique involving the charring of an organic material such as a phenolic resin or pitch or by a chemical vapour infiltration technique. In consequence cost of the resulting material is high.

The present invention seeks to provide a method for the manufacture of carbon-carbon composite material which is less expensive and/or less time consuming.

In accordance with one of its aspects the present invention provides a method for the manufacture of a carbon-carbon composite material comprising providing a polymeric material of a kind which yields greater than 60% by weight of carbon when heated to a temperature of at least 600°C at normal atmospheric pressure, providing a mixture of said polymeric material and carbon fibres within an enclosure which provides a non-oxidising environment and heating said mixture in the enclosure to a temperature of at least 600°C whilst the mixture is acted upon by press means to apply a substantially uniaxial compressive mechanical load, and maintaining the applied temperature and compressive mechanical load for a time sufficient to consolidate the mixture whilst in its softened condition, to substantially wholly carbonise the polymeric material, and to densify the material at least during the carbonising process.

The press means may be in the form of a membrane, e g a flexible membrane, acted upon by fluid pressure but which applies a compressive force only to a surface of said mixture and prevents the fluid entering any

interstices within the mixture. More preferably, however, the press means comprises a rigid member, such as a press platen.

Restraining means may be provided to restrain outwards deformation of surface regions of the mixture not acted upon directly by the press means.

Preferably the method comprises application of the compressive mechanical load for substantially the whole of the time for which the mixture is subject to said temperature of at least 600°C. It is envisaged that whilst maintained at an elevated temperature the press means will be operated upon to enable it to move thereby to accommodate any shrinkage of the overall external volume of the mixture and/or to consolidate the mixture in its softened condition.

Means may be provided to maintain substantially atmospheric pressure and vent means may be provided to ensure that gases evolved during carbonisation do not cause an undesirable increase of gas pressure. Alternatively the by products of carbonising may be removed for example by a vacuum system.

Suitable polymeric materials for acting as precursors to produce a carbon matrix around the carbon fibres shall all be of the high char yield type, ie of a type which are capable of yielding by weight of carbon greater than 60% of their original weight when heated to a temperature of at least 600°C. These include thermoplastic polymers and pitches (including pitch solutions) and thermosetting polymers and mixes thereof. Examples of particularly suitable materials are mesophase pitches or green cokes which are pitches processed to a stage just before carbonisation. Whilst pitches are normally products of coal and petroleum industries it is to be understood that suitable pitches may be synthesised from organic materials and this invention includes the use of synthesised organic materials.

The polymeric material selected to be of a kind

which yields by weight of carbon greater than 60% of its initial weight when heated to at least 600°C,- which may be referred to as a high char yield material, preferably yields at least 70% and more preferably 80% or more by weight of carbon when so heated.

The carbon fibres can be of any of the kinds well known to those skilled in the art of forming carbon- carbon composite materials. Examples of suitable precursor materials for the carbon fibres include polyacrylonitrile, pitch and rayon.

The carbon fibres may be discrete staple length fibres or may be of a relatively long length substantially continuous filamentary form. Particularly when of a filamentary form they may be formed into a fabric such as a woven fabric. Alternatively fibres of staple and/or filamentary form may be formed into a non-woven fabric by other techniques e g' by needle punching. The carbon fibres typically are to have a length greater than 10 mm and preferably greater than 25 mm.

The initial combination of polymeric material and carbon fibres for subjecting to compression under elevated temperature conditions may be formed by for example conventional techniques familiar to those skilled in the manufacture of composite materials. These techniques include for example the dipping of carbon fibres into molten pitch, impregnating a fabric of fibres with a solution of the pitch, adding pitch in the form of a powder to a layer of fibres, film stacking techniques in which layers of carbon fibre material are alternated with layers of plastics and combinations thereof. These techniques can be applied also to the use of polymeric materials other than pitch. The initial combination of polymeric material and carbon fibres may also be in the form of a moulding compound.

The actual temperature chosen for carbonising preferably is selected to result in the materials exhibiting plastic behaviour and thereby being

susceptible to mechanical deformation under the action of the press means.

Although the temperature used to carbonise the polymeric material should be at least 600°C it is envisaged that normally it will not exceed 3000°C above which carbon materials start to vaporise.

The mixture of polymeric material and carbon fibres preferably is one in which the carbon fibres are distributed uniformly within the polymeric material. More preferably the mixture of matrix and embedded fibres is substantially devoid of interstices. The fibres may be uniformly distributed and whether or not uniformly distributed may be provided in a random orientation or be preferentially oriented if the resulting material is required to have particular properties in one or more preferential directions.

As a part of or subsequent to the process of forming a mixture of polymeric material and carbon fibres the mixture may be shaped in a mould. Shaping may take place in a mould from which, after cooling of a mixture, the mixture is transferred to a furnace for heating to a higher temperature than that, typically up to 300°C, used during a preliminary moulding stage. Alternatively the preliminary moulding stage and heating up to in the order of 300°C may be performed in the same enclosure as that subsequently used to heat to a higher temperature for carbonisation.

A product formed by the method of the present invention may be re-impregnated with polymeric material and re-heated to a temperature of greater than 600°C and, optionally again under uniaxial compressive mechanical load, to convert that additional polymeric material into carbon and thereby further increase the density of the product. Alternatively the density of the product may be increased by for example a chemical vapour deposition technique.

By way of illustration two examples of the method of the invention will now be described.

EXAMPLE I

20 layers of fabric each weighing 31.4 grams per layer and of a size 50 mm by 100 mm were cut from a carbon fibre fabric which had been produced by aligning and needle punching of carbon fibres by a method as described in GB Patent 1447030. 150 grams of mesophase pitch (ex Ashland Company USA) were ground to finer than 80 mesh (British Standard 410) and spread at the rate of 7.5 grams per layer on each layer. The layers were then laid up in a superimposed manner with the general orientation of the fibres in successive layers being at 90° to one another.

The resulting laid up preform was pressed to a thickness of 23 mm under a pressure of 40 MPa and heated between the platens of the press to 300°C. The press platens were maintained at the spacing of 23 mm whilst the preform was maintained at the temperature of 300°C for a period of 1 hour. Thereafter the preform was cooled, removed from the press and weighed. It had a weight of 136.7 grams, some of the pitch having been lost during spreading and handling leaving a reduced quantity equivalent to an average of 5.3 grams on each layer. The resulting density of the block of preform

3 so formed is 1.27 grams per cm .

The preform block was then transferred to a furnace and heated at the rate of 1.5°C per minute to 1,000°C in an atmosphere of flowing nitrogen at atmospheric pressure to remove the by products of carbonising and prevent oxidation of the carbonised product, and under a constant unidirectional mechanical compression pressure of 0.4 MPa. After attaining the temperature of 1,000°C the furnace was allowed to cool to room temperature with the nitrogen flow maintained therethrough. The resulting product was then removed from the furnace and its density measured, this being

3

1.41 grams per cm .

EXAMPLE II 34 layers of carbon cloth each of a 5 harness

satin weave and grade 0009 (ex Fothergill Engineering UK) were cut to a size of 50 mm by 50 mm. Each layer was then spread with 1 gram of powdered Ashland Type 80 pitch (ex Ashland Company USA) of mesh size finer than 80 mesh (British Standard 410) and the so-treated layers were then laid up in a superimposed manner in which the orientation of carbon fibres in each layer was at 90° to that of a successive layer.

The resulting laid up assembly of layers was then compressed to a thickness of 12.5 mm between hot platens at 260°C using a maximum applied pressure of 120 MPa.

The weight of the resultant preform was 47.4 grams and its density was 1.37 grams per cm . The preform •was then transferred to a high temperature furnace and subject to carbonisation in the same manner as described in Example I and again under a uniaxial mechanical compression pressure of 0.4 MPa which was continuously applied during the entire heating operation. Subsequently after natural cooling the resulting product was removed from the furnace and

3 its density measured, this being 1.56 grams per cm .

The method of the present invention can be expected to enable a carbon-carbon composite to be manufactured in a matter of days rather than weeks or months. Accordingly substantial cost benefits arise particularly in respect of utilisation of expensive furnace equipment. The composite material formed by the method of the present invention is particularly suitable for the manufacture of aerospace products such as the annular friction discs of aircraft brakes but is not limited thereto.